CHARACTERISATION AND APPLICATION OF A MODE-LOCKED

(MODE-LOCKED/Q-SWITCHED) C.W. NdrYAG LASER

Martin David Dawson, B.Sc., A.R.C.S.

A Thesis submitted for the degree of

Doctor of Philosophy of the University of London

and for the Diploma of Membership of Imperial College

Optics Group

Blackett Laboratory

Imperial College of

Science and Technology

M a r c h 1985 London SW7 2BZ DEDICATION

To Mam, Dad and Pam ABSTRACT

A synchronously operated (Synchroscan) picosecond streak camera has been used in a direct time-resolved study of laser emission from a

GaAs/(GaAl)As double heterostructure laser pumped by 514-nm Ar ion laser pulses of duration close to the ^ 60ps Fourier-transform limit.

Semiconductor laser pulsewidths as short as 20ps were recorded and the dependence of the temporal characteristics of these pulses on average pump power was investigated.

Optical pulses of similar wavelength (532nm) to those obtained from the Ar ion laser, but having considerably shorter duration

(^30ps), have been generated by frequency doubling the output of a mode-locked continuous wave (c.w.) Nd:YAG laser using Type II phase­ matching in a KTiOPOi* (K.T.P.) crystal. High doubling efficiencies

(a.10/6 average power conversion) were achieved.

These pulses have been used to synchronously pump a Rhodamine 6G jet-stream dye laser, whose performance is compared to its Ar ion

514nm-pumped counterpart.

The mode-locked c.w. Nd:YAG laser itself has been thoroughly characterised and various changes made to improve the short and long term stability of the output.

Simultaneous Q-switching of this laser at repetition rates ^ 1kHz, both with and without prelasing, has been investigated in detail using streak cameras. In addition to single-shot streak camera measurements, the usefulness of the Synchroscan camera as a ,,real-timen diagnostic of the Q-switched/mode-locked (QSML) pulses has been demonstrated and has allowed the optimum regime of short pulse, high peak power operation to be clearly established.

The QSML c.w. Nd:YAG laser has been used to generate a forward­ travelling picosecond phase conjugate (at 1,06um) by degenerate four- wave mixing in a silicon wafer and organic dye saturable absorber solutions (DNTPC, 97^0). Efficiencies of up to a few percent were achieved and the temporal characteristics of the conjugate beam quantitatively determined. CONTENTS

INTRODUCTION

Introduction 1

R e f e r e n c e s 5

ULTRASHORT PULSE GENERATION AND MEASUREMENT

2.1 Laser Action in Nd:YAG, Organic Dyes and 7

Semiconductors

i) N d 3 + :YAG 7

ii) Organic Dyes 8 iii) Semiconductors 10

2.2 Ultrashort Pulse Generation 11

2.3 Ultrashort Pulse Measurement 14

i) Nonlinear Methods - SHG Autocorrelation 15

ii) Linear Methods - The Electron-Optical 18

Streak Camera

R e f e r e n c e s 24

SHORT PULSE GENERATION IN AN OPTICALLY PUMPED

GaAs/(GaAl)As LASER

3.1 Introduction 27

3.2 The Mode-Locked Ar Ion Laser Pumping Source 29

3.3 The Semiconductor Laser Structure and 32

M o u n t i n g

3.4 Continuous Wave Optical Pumping of the 34

Semiconductor Laser

3.5 Short Pulse Generation from the GaAs Laser 37

3.6 Discussion

3.7 Conclusions

R e f e r e n c e s P a g e No

CHAPTER IV: THE MODE-LOCKED C.W. Nd:YAG LASER

4.1 Introduction 51

4.2 The Neodymium in YAG Laser 53

4.3 The Commercial NdrYAG Laser - Description 55

4.4 The Acousto-Optic Modulator 57

4.5 Characteristics of the NdrYAG Laser 61

4.6 Improvements to the NdrYAG Laser 69

4.7 3rd Harmonic Mode-locking and the 72

Anti-resonant Ring

4.8 Conclusions 78

References 79

CHAPTER V: THE NdrYAG LASER-PUMPED SYNCHRONOUSLY MODE-LOCKED

C.W. DYE LASER

5.1 Introduction 83

5.2 Extracavity Frequency Doubling in KTiOPO^ 85

(KTP)

5.3 Design of the Doubler Unit 87

5.4 Characteristics of the Second Harmonic 89

G e n e r a t i o n

i) Phase Matching and Angle-tuning 89

ii) Power and Pulsewidth Characteristics 92

5.5 The Rhodamine 6G Synchronously Pumped c.w. 97

Dye L a s e r

5.6 Discussion 102

5.7 Conclusions 103

Refer e n c e s 104 CHAPTER VI: THE Q-SWITCHED/MODE-LOCKED C.W. Nd:YAG LASER

6.1 Introduction 107

6.2 Q-Switching of Pulsed- and c.w.- Pumped 108

L a s e r s

6.3 The Travelling Wave Acousto-Optic Q-switch 111

6.4 Q-Switched (un-Mode-locked) operation 113

6.5 Simultaneous Q-Switching and Mode-locking 116

6.6 Conclusions 128

References 131

CHAPTER VII: PICOSECOND PHASE CONJUGATION IN THE FORWARD

DIRECTION

7.1 Introduction 133

7.2 Phase Conjugation by Degenerate Four-Wave 133

M i x i n g

7.3 The Experimental Set-Up 137

7.4 Picosecond DFWM in Silicon 139

7.5 Picosecond DFWM in Organic Dyes at 1.06um 150

7.6 Streak Camera Results 155

7.7 Conclusions 165

A p p e n d i x I 167

Appendix II 169

Appendix III 171

References 177

CHAPTER VIII: GENERAL CONCLUSIONS

General Conclusions 179

References 184

ACKNOWLEDGEMENTS 185

PUBLICATIONS 1

C H A P T E R I

INTRODUCTION

Ultrashort light pulses, of duration a few tens of picoseconds and below, are being increasingly used in many areas of both pure and applied scientific research. The texts edited by Shapiro (1977)0),

Shank et al (1978)(2), Hochstrasser et al (1980)(3), Eisenthal et al

(1982)(4) and Auston and Eisenthal 0984)(5), illustrate the wide- ranging interest shown in ultrafast pulse techniques and their application in areas such as carrier kinetics in semiconductors, molecular dynamics in photochemistry and photobiology, laser fusion and high speed electronics and communications. The latter two texts, in particular, document the recent extension of these techniques into the

(tens of) femtosecond time domain. Generation of picosecond and sub­ picosecond laser pulses is most commonly achieved by '’mode-locking", where the oscillating longitudinal modes of a laser resonator are forced into fixed phase and amplitude relationships. This results in a repetitive train of discrete optical pulses (of duration ^ 1/bandwidth of the laser) temporally separated by the laser cavity round-trip time

(typically a few nanoseconds). A comprehensive review of laser mode­ locking has recently been given by New (6).

The major methods of achieving mode-locked operation in a large variety of laser systems have now been identified and are quite well understood theoretically. In particular, they involve externally applied loss/gain modulation synchronised to the laser cavity round- trip frequency - so called "active" techniques, "passive" techniques utilising an intra-cavity saturable absorber without external

intervention, and "hybrid" combinations of the two. As a result it is - 2 -

now possible (6) to reliably generate Fourier-transform limited pulses down to a few picoseconds duration from pulsed mode-locked lasers and picosecond and shorter duration pulses from mode-locked continuous wave

(c.w.) laser systems, advanced versions of both of which are now commercially available.

The shortest pulses (o,1ps and below) have typically been provided

(either directly, or indirectly using nonlinear optical frequency- shifting techniques) by mode-locked c.w. organic dye lasers pumped by noble gas ion lasers. Most commonly, these have involved the 5l4nm wavelength line of an Argon ion laser to pump either a synchronously

(7) or passively (8) mode-locked c.w. Rhodamine 6G (R6G) dye laser.

Recently, an alternative laser system has become available - the mode- locked c.w. neodymium in YAG laser - which has been shown to be capable of producing pulses of similar duration via a variety of techniques.

This laser can be acousto-optically mode-locked, using an intracavity active loss modulator, to produce optical pulses of 'v 100 picoseconds duration at the fundamental wavelength (1.06 ym)(9), suitable for synchronous mode-locking of dye (10) and colour centre (11) lasers

operating at wavelengths beyond 1ym. Incorporation of a length of

single mode polarisation preserving optical fibre into the feedback

loop of a Nd:YAG laser-synchronously pumped colour centre laser, operating at ^ 1.5ym wavelength, has recently led to the first demonstration of soliton laser action (12). Fourier-transform limited pulses of duration as short as 130 femtoseconds at this wavelength have

been produced in this manner. The mode-locked Nd:YAG laser output can also be frequency doubled (to 532nm), using the nonlinear optical

crystal KTiOPO4 (KTP), with high enough efficiency for synchronous

pumping of visible-wave length dye lasers. Mourou and coworkers have claimed the generation of 70fs pulses by YAG laser-pumped synchronous mode-locking of a R6G/DQ0CI hybrid dye laser (13). - 3 -

The development of optical fibre/diffraction grating pulse compression techniques has further extended the versatility of short pulse generation using mode-locked c.w. Nd:YAG lasers (and other types of mode-locked laser). Johnson et al have produced 0.4lps duration

532nm pulses by compressing the second harmonic of the Nd:YAG laser using the fibre/grating method (14). These pulses were of suitable average power (150mW at 100MHz) for synchronous mode-locking of a R6G dye laser and allowed direct generation (i.e. without the use of a saturable absorber) of 300fs dye laser pulses. Kafka et al have similarly compressed the YAG pulses at the fundamental wave length to

1.8ps (15).

An additional attractive feature of the c.w. Nd:YAG laser is its facility to be simultaneously mode-locked and Q-switched (16). The high peak power (VIMW), short (o.100ps) mode-locked pulses thus produced enable the second (17) and third ( 1 8 ) optical harmonics to be efficiently generated, suitable for pumping, for example, high repetition rate (500Hz) amplifiers for femtosecond dye laser pulses

(19,20).

This thesis is mainly concerned with a detailed investigation of the characteristics of the c.w. Nd:YAG laser in both mode-locked and simultaneously mode-locked and Q-switched operation, and its applications in synchronous pumping of c.w. dye lasers and in phase conjugate wavefront generation. Work has also been performed on ultra- short pulse generation from a GaAs/GaAlAs double heterostructure semi­ conductor laser, gain-switched by 5l4nm mode-locked pulses from an

Argon ion laser. Consideration is given as to how the mode-locked c.w.

Nd:YAG laser could be used to extend investigations of the latter type.

In the following chapter, we provide the necessary background for the work to be described. A brief summary of laser action in organic dyes, semiconductors and Nd:YAG is followed by an overview of ultra­ - 4 - short pulse generation and measurement techniques. No attempt at a comprehensive treatment has been made, and emphasis has been placed on those techniques which are directly related to the work described in greater detail later in the thesis. - 5 -

REFERENCES

(1) "Ultrashort Light Pulses" Ed: S.L. Shapiro; Topics in Applied

Physics _1_8 (Springer-Verlag, 1977)

( 2) "Picosecond Phenomena" Ed: C.V. Shank, E.P. Ippen and

S.L. Shapiro; Springer Series in Chemical Physics 4_

(Springer-Verlag, 1978)

(3) "Picosecond Phenomena II" Ed: R.M. Hochstrasser, W. Kaiser and

C.V. Shank; Springer Series in Chemical Physics J_4

(Springer-Verlag, 1980)

( 4) "Picosecond Phenomena III" Ed: K.B. Eisenthal, R.M. Hochstrasser,

W. Kaiser and A. Lauberau; Springer Series in Chemical

Physics £3 (Springer-Verlag, 1982)

(5) "Ultrafast Phenomena IV" Ed: D.H. Auston and K.B. Eisenthal;

Springer Series in Chemical Physics _38^ (Springer-Verlag,

1984)

(6 ) G.H.C. New; Rep. Prog. Phys. ^ (1983) 877

(7) R.K. Jain; Proc. SPIE 322 (1982) 2

(8) D.J. Bradley in Chapter 2 of reference 1

(9) M.G. Cohen; Proc. SPIE 322 (1982) 44

( 10) A. Seilmeier, W. Kaiser, B. Sens and K.H. Drexhage; Opt. Lett. 8_

(1983) 205

( 11) L.F. Mollenauer and D.M. Bloom; Opt. Lett. _4 (1979) 247

( 12) L.F. Mollenauer and R.H. Stolen; p.2 of reference 5

(13) G.A. Mourou and T. Sizer II; Optics Comm. 4J_ (1982) 47

(14) A.M. Johnson, R.H. Stolen and W.M. Simpson; Appl. Phys. Lett. _44

(1984) 729

(15) J.K. Kafka, B.H. Kolner, T. Baer and D.M. Bloom; Opt. Lett. 9_

(1984) 505

( 16) D.J. Kuizenga, D.W. Phillion, T. Lund and A.E. Siegman; Optics

Comm. 9 (1973) 221 (17) K.A. Nelson and M.D. Fayer; J. Chem. Phys. 72 (1980) 5202

(18) K.C. Liu, G. Vaillancourt and M.G. Cohen; Paper ThC3, Conference

on Lasers and Electro-optics (June 1984, Anaheim, California)

Technical Digest

(19) T. Sizer II, J.D. Kafka, I.N. Duling III, C.W. Gabel and

G.A. Mourou; IEEE J. Quantum. Elect. QE-19 (1983) 506

(20) J.-M. Halbout and D. Grischkowsky; Appl. Phys. Lett. 45 (1984) - 7 -

C H A P T E R II

ULTRASHORT PULSE GENERATION AND MEASUREMENT

2.1 Laser action in Nd:YAG, organic dyes and semiconductors

A typical laser oscillator consists of a suitable amplifying medium placed between mirrors which form an optical resonator. The resonator provides feedback of photons which stimulate emission of further photons from the gain medium, allowing the optical flux to build up until laser threshold is attained. Laser output then occurs, usually via one of the resonator mirrors which is chosen to exhibit partial transmission at the photon wavelength.

In order for stimulated emission to take place, the gain medium must be maintained in a state of population inversion. This is achieved by excitation or "pumping" of the medium by an independent source of energy which may be, for example, optical or electrical or of other form.

The operational characteristics of a laser are determined to a large extent by the particular active medium and means of pumping used, and in this section we will briefly examine those relevant to the laser systems described in subsequent chapters.

i) Nd3+:YAG

The active medium in a Nd:YAG laser consists of trivalent neodymium ions doped into a yttrium aluminium garnet (YAG = Y3A15012 ) host crystal (1). The energy level structure thus produced is considered in detail in Chapter 4, but may be thought of as a four- level laser system with the major emission line at room temperature occuring at a wavelength of 1.064ym. The long (x ^ 230ys) lifetime of - 8 -

the upper level of the laser transition, combined with four-level operation, leads to high gain and low threshold for laser action. For continuous wave laser emission, pumping of the Nd:YAG rod is normally achieved optically. Commonly, a d.c. krypton arc lamp at one focus of a gold-coated elliptical pump enclosure is used to excite the rod which is situated at the other focus. Typical laser thresholds occur at an input power to the pump lamp M k W (1) and continuous wave laser outputs at 1.06ym of several hundred watts have been obtained in this manner (2).

ii) Organic Dyes

The discovery in 1966 that organic dyes can make excellent laser active media (3), with their large gain-bandwidth offering wavelength-tunable pulsed, continuous wave and mode-locked operation, has led to much interest being shown in these systems. Dye laser action has now been demonstrated, with appropriate choice of dye, from

308.5nm in the ultraviolet (4) to 1.8ym in the near infrared (5), with mode-locked continuous wave operation extending from 420nm (6) to

1.32ym (7).

The complex energy level configuration of organic dye molecules may be considered to form a four-level laser system (8). Excitation of a dye molecule by a suitable source results in an electronic transition from the lowest vibrational levels of the ground state (SQ) into the continuum of states associated with the first excited singlet level

(S1 ). Excess vibrational energy is rapidly (x ^ ps) lost via collisions with the solvent and the molecule subsequently remains in the upper level of the laser transition for up to a few nanoseconds.

Stimulated emission then returns it to the ground state continuum where it rapidly thermalises, again via collisions, back to the lowest vibrational levels. It is the continua of closely spaced rotational - 9 -

and vibrational levels (broadened by collisional and electrostatic perturbations through interaction with the solvent) associated with each electronic state which result in the large gain - bandwidth and consequent tunability of organic dye lasers. Two major types of transition can compete with the radiative (fluorescence) process - that of internal conversion from the excited to the ground state and, more importantly, non-radiative transitions to the triplet state ( t ^) , a l s o known as "intersystem crossing". Owing to the Tj S 0 transition being spin-forbidden, the phosphorescence decay time of the T j_ level

( x ^10_5s) is much longer than the fluorescence lifetime, so this state can act as a trap for excited molecules, inhibiting laser action.

In addition, triplet-triplet transitions of appreciable absorption cross-section can overlap the S 1 - S Q fluorescence band to provide a further impediment to the laser process. In continuous wave dye lasers these effects are reduced by rapidly flowing the laser dye through the active region, which physically removes the triplet population.

Practical c.w. dye lasers are optically pumped, usually by noble gas ion lasers, the approach used in the first demonstration of c.w. dye laser action (9). In Chapter 5 we describe the performance and operation of a mode-locked c.w. R6G dye laser pumped by an alternative source - the second harmonic of a mode-locked c.w. NdrYAG laser. The dye was flowed in an ethylene glycol solvent to form a jet-stream (10) at the common focus of a pair of curved mirrors (constituting a folded section), one of which served the additional purpose of focussing the pump light into the active medium. An intracavity Brewster-angled quartz prism was used for tuning purposes.

The particular cavity design employed is described in greater detail in Chapter 5. Alternative cavity configurations and further details of the principles of dye laser operation can be found in reference (8). 1 0

iii) Semiconductors

Owing to the overlap between electronic wavefunctions in a semiconductor, electron transitions take place between bands of states

- the valence and conduction bands - rather than between two well- defined energy levels (11). In a similar manner to dye lasers, this gives the semiconductor laser a wide amplification bandwidth CM4nm for

GaAlAs (12)) and thus leads to the possibility of (tunable) ultrashort pulse generation. Laser action takes place only in direct bandgap semiconductors, where electronic transitions between the bands do not require the participation of another quantum (usually a phonon) to conserve crystal momentum. If an electron is excited into the conduction band in such a material, then it thermalises rapidly

( t ^0.1ps for GaAlAs (13)) to the band edge via phonon emission. The spontaneous emission lifetime is ^1ns (14) and, following emission of a photon, the electron returns to the valence band and subsequently thermalises rapidly to the valence band edge. The energy level scheme may thus be regarded as forming a four-level laser system when a means of obtaining population inversion is provided, together with an optical resonator for feedback. Semiconductor lasers have been operated in external cavities, allowing spectral tuning/line-narrowing elements or active modulators to be inserted to control the spectral (15) or temporal (16) characteristics of the laser output. This is not a necessary requirement, however, as the high reflectivity (R ^ 30% in

GaAlAs) due to the refractive index step at the semiconductor/air interface is sufficient for laser oscillation to take place (11).

Feedback is thus provided by cleaving the semiconductor along natural crystal planes, leading to highly parallel reflecting surfaces.

Various means of pumping semiconductor lasers have been employed.

Optical pumping, using another laser source of high enough photon energy to induce direct band-to-band transitions, has the particular 1 1 -

advantage that virtually any direct band-gap semiconductor can be used

(17). This is the pumping technique used in the work described in the

following chapter. Electrical injection semiconductor lasers, on the

other hand, require the formation of p-n junctions, which is possible

only in certain semiconductors (11). For room temperature continuous wave operation, semiconductor lasers must be in a form known as a

double heterostructure (18), where the active region is sandwiched

transversely to the laser direction by layers of lower refractive index

material. This provides both carrier confinement and waveguiding of

the light within the active region leading to greatly improved

efficiency, but lattice matching requirements etc. further restrict the

choice of materials available. At present, room temperature c.w.

injection laser action has only been observed in double hetero­

structures of GaAlAs and InGaAsP alloys.

More details concerning laser action in semiconductors can be

found in the book by Thompson (11).

2.2 Ultrashort Pulse Generation

In the previous section, laser action in Nd:YAG, organic dyes

and semiconductors was described. The gain-bandwidth in all these

laser media is wide enough to allow the generation of optical pulses of

picosecond duration, together with some limited tunability in the case

of semiconductors and wide tunability from organic dye lasers. In

order to produce these short pulses, the laser radiation must be

modulated, and in this section we describe the basic principles of the

(mode-locking) modulation techniques used.

To each transverse mode of a laser cavity, these being the

different transverse electromagnetic field distributions that the

cavity will support, there corresponds a discrete set of longitudinal

or axial modes. It is usually possible, by suitable cavity design and/ - 12 -

or the use of apertures, to discriminate against all higher order transverse modes, so that only the fundamental or TEM Q0 mode can oscillate. However, many longitudinal modes corresponding to the TEMQ0 mode will in general lie within the laser gain-curve with high enough gain to oscillate.

These longitudinal modes, being standing waves of the laser resonator, are separated in frequency by (19):

Av = c/2L (2.1)

for a non-dispersive medium, where L is the (optical) cavity length.

The appropriate expression for the case of a dispersive medium is given in the next chapter.

The technique of "mode-locking" is the means by which the otherwise unrelated phases of these axial modes are locked together.

This results in short duration light pulses separated by the cavity round-trip time. The duration of these individual pulses depends on the number of modes oscillating and on how close to the ideal mode- locked situation the laser is operated. For perfect mode-locking, when all the modes oscillate in phase, the duration of the pulses (At) is related to the gain bandwidth (Av) by (19):

At = k/Av (2.2)

where k is a constant of order unity, whose exact value is dependent on the shape of the pulse intensity profile (see table 2.1). The pulses are then said to be bandwidth (or Fourier transform) limited. The product of the pulsewidth and bandwidth is an important measure of the degree of mode-locking in a particular laser system.

The two basic methods of obtaining mode-locked operation are known - 13 -

as "active” and "passive” mode-locking respectively. Active mode­ locking is achieved by loss or gain modulation synchronised to

(i.e. equal to, or a multiple or sub-multiple of) the laser cavity round-trip frequency. The effect of this modulation may be viewed as creating sidebands on each cavity mode at the modulation frequency. If this frequency is matched to the cavity round-trip frequency, the side­ bands will overlap adjacent cavity modes and thus, if the modulation is sufficiently strong, couple them together. Active loss modulation using an intracavity acousto-optic modulator is the means by which mode-locking of the c.w. NdrYAG laser is achieved (20), producing pulses of duration 50ps and below (at 1.06 pm). The particular modulator used in work reported here and its operation is described in more detail in Chapter 4. Kuizenga and Siegman have provided (21) a detailed theoretical treatment of mode-locking in NdrYAG lasers by active loss modulation. The relevant expressions governing pulse duration and evolution derived by these authors are given at suitable points in later chapters.

Active gain modulation, known as "synchronous mode-locking", is the technique described in Chapter 5 for the generation of ultrashort dye laser pulses. Here, a mode-locked laser is used to drive a dye laser whose cavity length is closely matched (to ^ ym) to the pumping laser. Modulation occurs via the fast creation of gain due to the short duration pumping pulses, followed by rapid gain depletion due to stimulated emission by the dye laser pulses. Synchronous mode-locking is the most versatile technique used for the production of tunable picosecond and sub-picosecond pulses and covers most of the spectral range available to c.w. dye lasers. Reviews of synchronous mode­ locking are given in references (22,23).

The major alternative technique used for the generation of ultra- short light pulses is that of passive mode-locking. This is achieved 14 -

by insertion of a saturable absorber, exhibiting a nonlinear absorption as a function of intensity, into the laser cavity. This absorber provides reduced loss for light of high enough intensity to bleach it.

Thus, if the initial intracavity field distribution consists of noise fluctuations, intense noise spikes are preferentially selected and less intense background fluctuations progressively discriminated against.

Over many cavity round-trips this leads to stable ultrashort pulse generation (24). The shortest duration pulses produced directly in work performed to date ( 27fs (25)) have been achieved in this way, using a ring R6G c.w. dye laser and DODCI saturable absorber in a technique known as ’’colliding pulse mode-locking” (CPM)(26). Here, counterpropagating pulses in a bi-directional ring cavity collide in the saturable absorber jet-stream, bleaching it more effectively than in the case of a single pulse. An attempt to produce pulse-shortening by colliding pulse mode-locking in a NdrYAG laser is described in

Chapter 4. The major disadvantage of the passive mode-locking technique is, however, that operation is limited to the wavelength region near the peak of the absorption profile of the saturable absorber and that suitable saturable absorbers have not been identified for many c.w. laser active media.

2.3 Ultrashort Pulse Measurement

Techniques capable of measuring the durations of picosecond and sub-picosecond laser pulses may be broadly divided into two

categories; indirect methods involving nonlinear optical processes and direct methods having linear response. In this section, we describe

those techniques which were used for the work described in the

following chapters. Of the former type, second harmonic generation

autocorrelation was used in the measurement of synchronously mode-

locked dye laser pulses described in Chapter 5. Emphasis is, however, 15

given to the direct electron-optical streak camera technique with which all other ultrashort pulse measurements were performed.

i) Nonlinear Methods - SHG autocorrelation

Optical second harmonic generation (SHG) (27) occurs when two

photons of equal energy combine in a nonlinear material (not possessing

inversion symmetry) to produce a single photon having twice the energy of each original photon. Efficient SHG of the fundamental wavelength

(1.06 ym) of a mode-locked c.w. Nd:YAG laser, using the nonlinear optical crystal KTP, is described in Chapter 5.

Second harmonic generation also forms the basis of one of the commonest methods of ultrashort pulse measurement - that of SHG auto­

correlation (28,29,30). Briefly, in this technique, the optical pulse

to be measured is divided into two beams which are directed into an SHG

crystal via a variable relative optical (and thus temporal) delay. The

intensity of the second harmonic signal, which is maximum when the two

pulses are completely temporally overlapped, is then measured as a

function of the delay. By having the beam polarisations orthogonal

(28,30), or by making them non-collinear in the crystal (29), it can be arranged that no SHG occurs when either beam is blocked, or when there

is no temporal overlap between the pulses arriving at the crystal. For

orthogonal polarisations SHG of the second kind is said to take place, whereas for beams with identical polarisation SHG of the first kind

takes place and, if collinear beams are used, a background signal is present for all delay times. The detected correlation signal is

related to the temporal intensity profile of the input pulse I(t) in

the latter case by (31).'

= c 1 + 2 G ( 2 ) (t ) (2.3) 16 - where:

G (2)(t ) = / I(t) I(t + x)dt (2.4)

— CO

is the second order correlation function (c being a constant and x the

time delay). This is the method used in this work, and it provides

useful information on the degree of mode-locking via the peak-to-

background contrast ratio. For perfect mode-locking contrast ratios of

3:1 are expected, although it has been shown that a contrast ratio of

2.9:1 can be obtained for pulsetrains where only 50% of the modes are

locked in phase (32). This clearly illustrates the particular

disadvantage of the alternative "background-free" technique, now in

common usage, with which no contrast ratio is obtained.

Table 2.1 Time-bandwidth product and Ax/At for various pulseshapes

The duration of the pulse intensity profile, At, calculated from - 17 -

the width of the autocorrelation function A t , is dependent on the particular pulse-shape assumed. The second-order correlation function, however, which is by definition symmetric, does not specify the pulse- shape uniquely, and so in practice the pulse shape is inferred by fitting the experimental autocorrelation shape to the calculated curve for a variety of pulse shapes. Table 2.1 gives the ratio Ax/At for various pulse-shapes and also the relevant time/bandwidth product (k) for transform-limited pulses.

FROM MODE-LOCKED LASER

CHART REC.

Fig 2.1 Autocorrelation set-up. B/S is a beamsplitter, F is an appropriate filter

The experimental set-up used here for SHG autocorrelation is shown in fig. 2.1. A Michelson interferometer arrangement provided the calibrated time delay between the two pulses, varied by translating mirror M 1 using a motor-driven micrometer. A 5cm focal length plano-convex lens focussed the two pulses into an ADP (Ammonium 18 -

Dihydrogen Phosphate) crystal, cut for angle phase-matched second harmonic generation for ^600nm wavelength at normal incidence. The temporal resolution of the method is limited by the phase-matching bandwidth and beam walk-off in the crystal. A 0.5mm-thick ADP crystal provided adequate resolution for the 'v'lps duration pulses measured here but shorter pulses (the shortest pulses recorded to date - 12fs (33) have been measured by this technique) require either the use of much thinner crystals or the non-collinear background-free technique (where the spatial region of overlap can be much less than the crystal l e n g t h ) .

Kodak Wratten 18B filters (F) were used to block the transmitted fundamental signal and the second harmonic was detected by a Mullard

TUVP photomultiplier. The output of the PMT was fed into an oscilloscope for real-time monitoring, or a chart recorder for recording the autocorrelation trace.

Details of higher-order correlation techniques and other nonlinear methods of pulse measurement (e.g. two photon fluorescence) not used in this work may be found in references (31*3*0.

ii) Linear Methods - The electron-optical streak camera

Nonlinear pulsewidth measurement techniques can have excellent temporal resolution (<0.1ps), but require fairly high intensities due to the relative inefficiency of the nonlinear processes involved. In addition, they do not uniquely define the pulseshape, and the presence of substantial inter-pulse background energy may not be detected.

Measurements using electron-optical streak cameras, on the other hand, provide direct information about the temporal intensity profile.

Streak cameras exhibit high sensitivity and linearity of response over a wide spectral range (X-ray - near infra-red) and also possess the facility for simultaneous display of spectral/spatial information

versus temporal variation in intensity.

STREAK TUBE

ANODE SCREEN

_A_A_ 0 DETECTOR SLIT P/C

GENERATOR

Fig 2.2 Schematic of Photochron II Streak camera

Picosecond chronoscopy using a streak camera was first proposed by

Zavoiskii and Fanchenko in 1956 (35). The operation of the device is

best illustrated with reference to figure 2,2, which shows the basic

design of the Photochron II (36) streak image tube used throughout this work. The optical pulse to be studied is imaged onto the streak camera

input slit, and the resulting slit image is focussed onto the photo­

cathode (P/C) of the streak tube. Photoelectrons are thus liberated by

the optical pulse and are rapidly accelerated and focussed by the mesh,

cone and anode electrodes. The resulting electron beam current, as a

function of time, closely resembles the temporal profile of the light

pulse incident on the photocathode. As the photoelectrons pass between

the deflection plates, a fast, linear voltage ramp streaks them

transversly across the phosphor screen. Electrons arriving at the

deflection plates at different times thus experience different

transverse deflections and hence the temporal distribution of the

electrons is effectively transformed into a transverse spatial

distribution. The resulting phosphorescence emitted by the screen at 20 -

any spatial position is proportional to the electron beam current at

that position and thus to the incident light intensity at a particular

point in the temporal intensity profile. The resultant phosphorescent

spatial streak is intensified and subsequently photographed or

electronically recorded, (Some streak tubes incorporate microchannel

plate image intensifiers before the phosphor screen, such that further

intensification is unneccessary.)

The temporal resolution of the streak camera is determined by the

photoelectron time dispersion and also the technical time resolution of

the streak tube. Photoelectron time-dispersion occurs because pair-

production and lattice scattering mechanisms in the photocathode (37)

lead to electrons being emitted with a distribution of velocities.

This results in a spread in the transit-times of the photoelectrons

through the streak tube given by (38):

t, = 2.34 x 10-8 /Ke (2.5) a ------E

w h e r e Ae is the energy spread in electron-volts and E, in Volts per

centimetre, is the extraction field at the cathode. For a particular

cathode, A e is wavelength dependent. Prior to 1970, photoelectron

time-dispersion effects were a major limiting factor in streak tube

performance, restricting temporal resolution to ^ 50ps. Following

incorporation of a high-potential mesh electrode near the photocathode,

however, providing a significantly increased extraction field

(typically ^20kV), resolutions of <10ps were achieved for the first

time (39).

The technical time resolution, tt, is determined by the streak

speed of the image across the phosphor (v) and the dynamic spatial

resolution of the streak tube. It is given by the expression (37): 21 -

t = l/6v (2.6)

which is essentially the time taken to sweep over one resolution element. For a typical dynamic spatial resolution 'MOlp/mm and deflection velocities approaching the speed of light, this technical time limit is in the sub-picosecond range.

In the Gaussian approximation, taking into account the technical time resolution and transit-time dispersion, the recorded pulsewidth t of a laser pulse of width tp is given by:

fcr = (tp2 + fcd2 + (2.7)

although a more realistic evaluation of tr , based on modulation transfer functions (40), has recently been introduced. Recorded streak images having sub-picosecond duration (0.9ps (41)) have been obtained using a Photochron II streak camera (the type illustrated in figure 2.2 and used throughout this work) in single-shot operation, although typical camera instrumental functions are in the few picoseconds range.

An alternative mode of operation of streak cameras, useful in conjunction with mode-locked c.w. laser systems, is that of Synchroscan

(42). Synchroscan streak cameras have been reviewed by Sibbett (43).

In this technique, the single linear, time-varying voltage ramp of the single-shot camera is replaced by a synchronised series of streaking ramps at the laser pulse repetition rate (or a multiple or submultiple thereof). In practice, the sequence of streak ramps is obtained using a high-voltage sinusoidal signal which is linear to within 5$ for approximately one sixth of the period.

This sinusoidal signal is derived either from a fraction of the laser pulse train via a photodiode/tunnel diode oscillator arrangement 22 -

(see figure 2.3) or, alternatively, it can be derived from a fraction of the modulator r.f. drive in the case of active-loss modulated laser systems (in which case it usually needs to be frequency doubled). The latter approach leads to a slight reduction in temporal resolution when recording picosecond and sub-picosecond duration (dye laser) pulses (43), but gives equivalent performance for longer pulses.

Chapter 6 describes its use in the first demonstration of Synchroscan in conjunction with a simultaneously Q-switched and mode-locked c.w.

Nd:YAG laser (where the mode-locked pulse durations were 'MOOps).

Fig 2.3 Synchroscan streak camera arrangement

Following amplification (to M0-20 Watts) the synchronised sine- wave is then coupled into the deflection plates of the streak camera via a high Q impedance matching network, giving a peak-to-peak voltage across the plates of a few kV. For sufficiently good synchronisation, each streak image is precisely superimposed on the previous one, allowing the use of very low light levels and thus improving the dynamic range (44) and negating the use of intensifiers while retaining high temporal resolution. This resolution is governed by the pulse to pulse timing jitter characteristics of the particular mode-locked c.w. 23 -

laser pulse train used. Jitter is particularly evident in synchronously mode-locked laser systems and has limited the best resolution in this case to ^6ps (45). Passively mode-locked c.w. (dye) lasers, on the other hand, have been shown to have potentially sub­ picosecond jitter and have produced the optimum Synchroscan resolution yet obtained - 1.2ps (46).

A convenient read-out system for this type of streak camera is an optical multichannel analyser (e.g. B&M Spectronik OSA) which can provide a real-time image of the streaks, together with a facility for storage and processing of the data. The 500 channel silicon intensified target vidicon used in this work had a readout time ^30ms giving an integrated image of VI0 5 pulses.

The basic disadvantage of the streak camera measurement technique, as compared to the nonlinear correlation methods, is that the optimum resolution obtained (vips in both Synchroscan and single-shot mode) is not high enough to directly resolve the shortest (tens of femtoseconds) duration pulses now being generated.

Improved performance has, however, been predicted for a new class of streak image tube, designated the Photochron IV (47). The theoretical resolution for a streak speed of 3 x 1010cm s"*1 and an initial photoelectron energy spread Ae = 0.6eV is less than 0.5ps (40).

For wavelengths near the long-wavelength cutoff of the photocathode, a time resolution ^0.2ps has been predicted (40).

A first experimental version of the tube has been operated with an extraction field about half that of the design optimum (which is

50kV/cm). The predicted single-shot resolution of 0.7ps at 600nm wave­ length for this tube has been demonstrated experimentally and initial

Synchroscan resolutions of 1.7ps have been achieved using this tube

(48). - 24 -

REFERENCES

(1) W. Koechner, "Solid State Laser Engineering" (Springer-Verlag,

1976)

(2) W. Koechner; Rev. Sci. Inst., 4l_ (1970) 1699

(3) P.P. Sorokin and J.R. Lankard; IBM J. Res. Dev., (1966) 162

(4) F.-G. Zhang and F.P. Schafer; Appl. Phys. B. , 2(3 (1981) 211

(5) H.J. Polland, T. Elsaesser, A. Seilmeier, W. Kaiser, M. Kussler,

N.J. Marx, B. Sens and K.H. Drexhage; Appl. Phys. B. , _32_

(1983) 53

(6) J.N. Eckstein, A.I. Ferguson, T.W. Hansch, C.A. Minard and

C.K. Chan; Optics Comm., 27_ (1978) 466

(7) A. Seilmeier, W. Kaiser, B. Sens and K.H. Drexhage; Opt. Lett.,

8^ (1983) 205

(8) "Dye Lasers", Topics in Applied Physics J_ Ed. F.P. Schafer

(Springer-Verlag 1973)

(9) O.G. Peterson, S.A. Tuccio and B.B. Snavely; Appl. Phys. Lett.,

J_7 (1970) 245

(10) P.K. Runge and R. Rosenberg; IEEE J. Quantum Elect., QE-8 (1972) 910

(11) G.H.B. Thompson, "Physics of Semiconductor Laser Devices"

(John Wiley, 1980) Chapter 2

(12) Chen Jianguo; PhD Thesis, University of London (1984)

(13) Y. Nishimura; Jap. J. Appl. Phys., _13_ (1974) 109

(14) H. Kressel and J.K. Butler, "Semiconductor Lasers and Hetero­

junction LEDs" (Academic Press, 1977)

(15) R. Wyatt and W.J. Delvin; Electron. Lett., _1_9 (1983) 110

(16) J.I. Vukusic (Imperial College); unpublished

(17) M.M. Salour in "Lasers; Physics, Systems and Techniques"

Proceedings of 23rd Scottish Universities Summer School in

Physics (Scottish Universities Summer School in Physics,1983) 25

(18) Zh I. Alferov, V.M. Andreev, V.I. Korol'kov, E.L. Portnoi and

D.N. Tret'yakov; Sov. Phys. Semicond., _2 (1969) 843

(19) D.J. Bradley in "Ultrashort Light Pulses" Ed. S.L. Shapiro;

Topics in Applied Physics _1_8 (Springer-Verlag, 1977) Ch. 2

(20) D.J. Kuizenga and A.E. Siegman; IEEE J. Quantum Elect., QE-6

(1970) 709

(21) D.J. Kuizenga and A.E. Siegman; Ibid QE-6 (1970) 694

(22) R.K. Jain; Proc. SPIE 322 (1982) 2

(23) M.C. Adams, D.J. Bradley, W. Sibbett and J.R. Taylor; Phil.

Trans. R. Soc. Lond., A298 (1980) 217

(24) G.H.C. New; Proc. IEEE 67 (1979) 380

(25) J.A. Valdmanis and R.L. Fork in Laser Focus (December 1984) 8

(26) R.L. Fork, B.I. Greene and C.V. Shank; Appl. Phys. Lett., _38

(1981) 671

(27) P.A. Franken, A.E. Hill, C.W. Peters, G. Weinreich; Phys. Rev.

Lett. , ]_ (1961) 118

(28) H.P. Weber; J. Appl. Phys., 38 (1967) 2231

(29) M. Maier, W. Kaiser and J.A. Giordmaine; Phys. Rev. Lett., J7_

(1966) 1725

(30) T.A. Armstrong; Appl. Phys. Lett., _1_0 (1967) 16

(31) E.P. Ippen and C.V. Shank in Chapter 3 of reference (19)

(32) R.H. Picard and P. Schweitzer; Phys. Lett., 29A (1969) 415

(33) J.-M. Halbout and D. Grischkowsky; Appl. Phys. Lett., _45^ (1984) 1281

(34) D.J. Bradley and G.H.C. New; Proc. IEEE 62 (1974) 313

(35) E.K. Zavoiskii and S.D. Fanchenko; Sov. Phys. Doklady J_ (1956)

285

(36) P.R. Bird, D.J. Bradley and W. Sibbett; Adv. in Electronics and

Electron Phys., 40A (1976) 51

(37) S.F, Bryant; PhD Thesis, University of London (1978) - 26 -

(38) V.V. Korobkin, A.A. Malyutin and M.Ya. Schelev; J. Photog. Sci.,

_1_7 (1969) 179

(39) D.J. Bradley, B. Liddy and W.E. Sleat; Optics Comm., 2_ (1971) 391

(40) H. Niu, W. Sibbett and M.R. Baggs; Rev. Sci. Instrum., 53, (1982)

563

(41) R.S. Adrain, E.G. Arthurs and W. Sibbett; Optics Comm., J_5 (1975)

290

(42) M.C. Adams, W. Sibbett and D.J. Bradley; Adv. in Electronics and

Electron. Phys., 5£ (1979) 265

(43) W. Sibbett; Proc. 15th Int. Cong. High Speed Photography and

Photonics (1982) 15

(44) W. Sibbett; PhD Thesis, Queen's University Belfast (1973)

(45) K. Smith (Imperial College); private communication

(46) J.P. Willson, W. Sibbett and W.E. Sleat; Optics Comm., 42 (1982)

208

(47) W. Sibbett, H, Niu and M.R. Baggs; Rev. Sci. Inst., 53_ (1982) 758

(48) M.R. Baggs, R.T. Eagles, A.E. Hughes, W. Margulis, C.C. Phillips,

W. Sibbett and W.E. Sleat; Proc. 16th Int. Cong. High Speed

Photography and Photonics (1984) - 27 -

CHAPTER III

SHORT PULSE GENERATION IN AN OPTICALLY PUMPED GaAs/(GaAl)As LASER

3.1 Introduction

Generation of high O^GHz) repetition rate ultrashort light pulses from semiconductor lasers (1) has been of considerable interest in recent years, particularly from injection heterostructure diode lasers which are the preferred sources for practical high-bandwidth optical fibre communications and ultrafast optoelectronic signal processing systems (2).

Two basic approaches have yielded picosecond pulse generation from injection lasers - that of mode-locking (either active or passive or a combination of both) and that of direct excitation. Of these, mode­ locking has produced the shortest pulses ('v. 1ps in the passive case (3,

4), 'VIOps in the active case (5,6)) which in many instances have been transform-limited (3,7,8), a desirable property for minimising pulse dispersion in optical fibre applications and thus allowing the highest bit-rate communication and increased repeater spacings. However, a cumbersome external cavity a significant fraction of a metre in length is required, which needs rather critical optical alignment and may be sensitive to mechanical vibration. In addition, it is not easy in the mode-locked case to directly vary the pulse repetition rate (1).

The simpler approach is that of direct excitation, either by pumping the laser (with or without a d.c. bias) using intense, short

(< 100ps) electrical pulses (9,10,11) - a method known as ’gain- switching1 - or by large amplitude sinusoidal (injection) current modulation (12,13,14). Both techniques have resulted in pulse durations on the order of a few cavity round-trip times (i.e. ^15-35ps) - 28 -

which, particularly in the former case, are virtually independent of the driving frequency. One disadvantage is that these pulses tend to show multi-longitudinal mode spectra (1), which may limit their use­ fulness in optical fibre communication (as transform-limited pulses are not generated) although they should be excellent sources for laser ranging, detector response measurements etc. Further investigation of techniques or conditions under which single longitudinal mode output may be obtained with direct pulsed excitation are required (see e.g.

(15) and (16)). Distributed feedback diode lasers (44) and the recently proposed semiconductor diode soliton laser (45) show promise in this area.

Ultrashort light pulses have also been produced by optically pumped semiconductor lasers (17-23). The motivation here has been two­

fold. Firstly, these lasers are useful laboratory sources in their own right, some of which (21,22) have been shown to exhibit wide

tunability, particularly in the wavelength region beyond Mum not readily accessible to dye lasers. Secondly, insights gained using the more versatile optical pumping technique (where pump pulse duration, power etc. can be more readily varied) should be relevant to the design and optimisation of the injection lasers.

In this chapter we describe the results obtained by optically gain-switching a simple GaAs/GaAlAs double heterostructure laser at 77K with < 90ps 5l4nm pulses from a mode-locked Ar ion laser (24). A

synchronously operated picosecond streak camera allowed direct time-

resolved measurements of the stimulated and spontaneous emissions to be

obtained. Semiconductor laser pulsewidths down to 20ps FWHM were

recorded and the dependence of the temporal characteristics of these

pulses on average pump power was investigated. The results are

compared with those of Duguay and Damen (25) who reported the use of a

picosecond sum-frequency technique to measure the stimulated and - 29

spontaneous emission from a transverse-junction (GaAl)As laser gain- switched by 90ps electrical pulses. At the end of the chapter, consideration is given to how a mode-locked c.w. Nd:YAG laser could be used to extend the investigations reported here.

3.2 The mode-locked Ar+ laser pumping source

The source used to optically pump the semiconductor laser was a c.w. argon ion laser (Spectra-Physics Model 164) acousto-optically mode-locked on the 5l4.5nm line. This produced pulses of typical duration 70 - 90ps during the work reported here at a repetition rate of 140MHz and average powers of up to >600mW. In this section, we describe a measurement of the spectral bandwidth of this laser when it produced the shortest pulses, having a duration of 6lps, and show that these pulses are Fourier transform limited. This serves two purposes - first to show that the pulses used in this work were near the transform limit, which is important as we find that the shortest semiconductor laser pulsewidths are obtained using the shortest pumping pulses.

Secondly, these pulses are the shortest reported in the literature (26)

from such a laser and are likely to be near the limiting duration directly obtainable from a mode-locked c.w. Ar ion laser on the 514.5nm line. In the context of this thesis therefore, it is useful to have

this information in order to make a direct comparison with the

frequency-doubled mode-locked c.w. Nd:YAG laser.

A home-made 4mm gap Fabry-Perot interferometer was used in the bandwidth measurements. The interference fringes obtained by

directing a fraction (^4$) of the Ar ion output beam through the Fabry-

Perot were directly recorded on photographic film (Ilford FP4) in a

lensless camera back (Exakta). A neutral density filter (0.3 ND) was

placed over half of the camera aperture to provide an intensity half­ maximum calibration of the recorded fringes. 30

(a)

440 mA------H

Fig 3.1(a) Microdensitometer trace of Fabry-Perot fringes showing

spectral resolution limit of 4mm etalon at 514.5nm

w a v e l e n g t h

Fig 3.1(b) As in (a) but with 61ps 514.5nm pulses - 31 -

Figure 3.1(a) shows a microdensitometer trace of the fringes obtained with the Ar ion laser operating un-mode-locked near threshold.

The spectrally narrow output thus obtained, consisting of only a few o individual longitudinal modes of spacing '\z1.3mA (corresponding to the

^1 metre length cavity), allowed this preliminary check on the finesse of the Fabry-Perot at 5l4.5nm, to ensure that we had adequate spectral resolution to perform the mode-locked bandwidth measurement.

N o w (27):

Free Spectral Range (FSR) = X2 (3.1) 2yt

where X is the wavelength, y is the etalon refractive index (= 1,5) and t is the etalon spacing (4mm). Thus we obtain an FSR of 220ml.

Because of the nonlinearity of the fringe spacing (the spatial fringe separation decreases with higher angular orders) we average over two adjacent free spectral ranges (i.e. we consider the spacing between a o fringe and its next nearest neighbour to be 440mA) and measure the width of the central fringe. Using the 0.3 ND half intensity calibration we see that the recorded fringe full width at half maximum

(FWHM) is 10mA.

Thus the finesse (27) is:

Finesse = FSR = 220mA = 22 (3.2) o fringe width 10mA

o and 10mA is the minimum spectral feature resolvable by this Fabry-Perot at this wavelength. This will be shown to be entirely adequate for the following measurement.

In figure 3.1(b) we show the microdensitometer trace of the fringes obtained when the Ar ion laser was emitting 6lps pulses. Again - 32 -

averaging over 2 adjacent FSR's we obtain a FWHM for the central fringe o of 70.7mA or a frequency bandwidth (Av) of 8GHz« The time-bandwidth product for these pulses is thus AvAt = 0.49, which compares well with the transform limited product of 0.44 for Gaussian pulses. Averaging over a series of independent measurements we obtain a maximum bandwidth spread of 7 ± 1GHz which would imply a transform-limited pulse duration of 64 ± 10ps i.e. the 6lps pulses are effectively transform-limited.

An intensity profile of the recorded 6lps streak images is shown in figure 3.2.

Fig 3.2 Synchroscan streak intensity profiles of 61ps Ar ion laser pulses

3.3 The semiconductor laser structure and mounting

The n-isotype GaAs semiconductor laser structure used in this work was grown by molecular beam epitaxy (MBE) in a non-commercial, fully automated system at Philips Research

Laboratories, Redhill, England (28). HBE (29) is one of several growth techniques used to produce semiconductor laser structures but - 33 -

has the particular advantage of producing abrupt interfaces, allowing precise control of dopant impurities and yielding semiconductor layers of uniform quality with thickness ranging from a few atomic spacings upwards. The laser, which was especially grown for optical pumping (as described below), consisted of a 0.25 micron thick GaAs active layer between a 0.05micron thick top cladding layer of (GaAl)As (30$ Al) and a 1.5ym-thick bottom layer of (GaAl)As (30$

Al) grown on a semi-insulating Cr-doped GaAs substrate. The thin

0.05 pm overlayer allowed for efficient optical excitation of the

GaAs active region (28). This sandwich-type structure (30), where the active region is enclosed transversely to the lasing direction by layers of lower refractive index material, is known as a double heterostructure. It provides both waveguiding of the light and also carrier confinement within the active region, thus minimising losses and providing greatly improved efficiency. It is the only type of semiconductor laser structure to be operated c.w. at room temperature (30).

After growth, the substrate was thinned to 100ym using chemo- mechanical polishing and etching to ensure good thermal contact between the laser structure and the cold finger of a liquid N 2 optical cryostat and then cleaved into 200ym-wide bars for mounting, i.e. the cavity length of the laser was 200ym.

The laser sample was then contacted to an indium-wetted copper block which formed part of the cold finger of the cryostat. The cryostat was a home-made system with a 3 x 10-2 torr vacuum (obtained with a rotary pump) providing thermal isolation for the liquid nitrogen-cooled cold finger. Vibration problems were minimised by using a long length of nylon flexible tubing between the vacuum pump and the cryostat, clamped at suitable points, and by mounting the rotary pump on vibration-damping shock absorber springs. Infr a s i l 34 -

windows arranged axially around the cold finger allowed for optical

excitation of the sample and transmission of the semiconductor laser

output to be detected. The laser was pumped transversely through the

top layer as shown in figure 3.3. The optical pumping geometry is

shown in more detail in a later figure (fig 3.6).

\ ✓' PUMP BEAM „ j

STIMULATED EMISSION CLADDING ACTIVE LAYERS LAYER SUBSTRATE h*—200nm—-j

Fig 3.3 Optical pumping geometry of semiconductor laser

3.4 Continuous wave optical pumping of the semiconductor laser

Before pumping the laser structure with mode-locked optical

pulses, some preliminary measurements were made using c.w, 5l4,5nm excitation. Figure 3.4 shows the integrated single facet output

intensity (measured with an S20 photomultiplier attached to the output of a Monospek 1000 Spectrograph) versus average power in the pump beam.

The pump beam was chopped at a rate of 200Hz (at a point after the power measurements were taken) to minimise heating of the laser sample.

A distinct "knee" in the output intensity associated with the onset of laser action can be seen to occur at a pump power level ^l60mW which is - 35 -

Fig 3.4 Single facet output intensity vs. average pump power

the pump power threshold obtained by extrapolating the output above

threshold back to the abscissa. Typical values of this intercept for repeated measurements were closer to l40mW average pump power.

Following Scott et al (28) we can estimate the pump power absorbed

by the active region at threshold. The pump beam was focussed to a

circular spot estimated to be ^200 ym in diameter i.e. of area

3.14 x 10“4cm2. Taking into account reflection loss at the sample

surface (^30%) and using the absorption coefficient data of Sell and

Casey (31) to show that effectively all of the remainder is absorbed by

the 0.25ym-thick active region, we calculate an absorbed power density 36 -

at threshold ^1/3 kW/cm2. The injection rate of electron-hole pairs resulting from this absorbed power density is 8 x 10 sec cm , giving an equivalent current density of 130 A/cm . This is a low value, but

is not unreasonable for a 0,25vim active region laser considering that room temperature thresholds of heterostructure injection lasers are

typically ^1000 A/cm2 and the threshold usually shows an increase by a

factor of 6 - 10 between 77 and 300K (43). (Reference (43) gives 77K

thresholds of below 200 A/cm2 for injection double heterostructures.)

Using the Monospek 1000 scanning monochromator, spectra of the

laser emission both above and below threshold were also recorded.

Figure 3.5 shows a spectrum of the luminescence below threshold taken

with an average pumping power ^80mW. The spectrum spans the range

8110 - 8350$ with a FWHM > 100A, The rise of the peak intensity

o (centred at 8198A) with pump power is included in figure 3.4.

Fig 3.5 Below threshold semiconductor luminescence spectrum

The spectrum of the laser emission above threshold (average pump

power ^ 200mW) is shown in a later figure (fig 3.8(a)). This spectrum, o centred at 8275A, occurs to the long wavelength side of the

luminescence peak, as we expect from self-absorption. Similar spectra - 37 - o o were also obtained by pumping the laser sample with the 4880A and 4760A lines of the Ar ion laser.

The measured spacing of the discrete longitudinal modes in the

o spectrum shown in fig 3.8(a) is ^3.3A which is apparently inconsistent with the cavity length of 200ym, if we use a refractive index of 3.6 for GaAs in the equation:

Mode spacing, AX = X2 (3.3)

2nL where n is the refractive index and L is the cavity length. This is due, firstly, to the dispersive nature of the medium, i.e. n is not wavelength independent. Hence we need to use the expression:

Mode spacing, AX = ______X^______(3.4)

(n - Xdn/dX)2L which applies for a dispersive medium. In addition, due to the wave- guiding nature of the device, n needs to be replaced by which is the effective refractive index for the guided wave within the laser resonator. These effects are discussed by Thompson (30) who gives a value of 4.5 for the term in brackets, resulting in a calculated mode o spacing of AX = 3«8A, close to the measured value. Other authors however, e.g. Au Yeung (12) give this group index a value of 5 which o then gives very close agreement, AX = 3.4A. Using Au Yeung's value we can calculate the round trip time for the 200ym cavity to be 6.7ps

(Thompson's value gives 6ps) which will be referred to later.

3.5 Short Pulse generation from the GaAs laser

Duguay and Damen (25) reported the first direct measurements with picosecond resolution of stimulated and spontaneous emission from a GaAlAs injection laser gain-switched by 90ps electrical pulses. - 38 -

They used a sum frequency sampling technique (32)» where a synchronised 1ps pulse from a passively mode-locked dye laser samples the temporal profile of the infra-red semiconductor emission by up- conversion to the u.v. in a nonlinear crystal. The u.v. signal is detected with a photomultiplier and recorded as a function of relative delay between the sampling pulse and the signal. Using this technique, they observed that below threshold the luminescence had a rise time of

3 0 0 ps, the luminescence continuing to increase for about 200ps after the termination of the current pulse. This phenomenon was considered

to be due to the time taken by the carriers in diffusing to the

junction (see fig 3.12). The work reported here is somewhat

complementary to that of Duguay and Damen in that gain-switching is

performed by optical pulses (of similar duration to the electrical

case) and pulse width measurements are performed using the Synchroscan

streak camera. One essential difference is the real-time nature of the

diagnostic system, allowing the effects documented to be observed

directly. At suitable points, a comparison of the results obtained

using the two techniques will be made.

Fig 3.6 Arrangement for pulsed pumping experiments

Figure 3.6 shows the experimental arrangement. An extra-cavity 39 -

attenuator (variable N.D. wheel) allowed the average pump power to be adjusted continuously in the range 30 - 300mW without distortion of the

Ar ion laser beam profile or changing the pump pulse length. Infra-red semiconductor laser emission (827nm) and Ar ion laser pumping light scattered from the surface of the sample were collected and collimated by lens L2 and focussed by lens L3 onto the input slit of a Photochron

II streak cameraj which was in turn imaged onto the camera photocathode

(S 20 with spectral sensitivity extending up to 900nm). Appropriate filters F were inserted in front of the slit to transmit preferentially the scattered green pump light or infra-red luminescence or to suitably attenuate the light such that the pulses at both wavelengths could be displayed simultaneously on the streak camera.

An ^ intensity component of the Ar ion laser pulse train was directed by a beamsplitter onto a photodiode/tunnel diode oscillator arrangement and the resulting signal, when amplified, provided the sinusoidal deflection voltage for the streak camera in synchronism with the repetitively excited photoluminescence emission. Processing and recording of the streak images were provided by an optical multichannel analyser (OMA, P.A.R. model 1205D) optically coupled to the streak image tube. Integrated streak profiles were displayed on the storage oscilloscope (SO) or hard copies taken using a chart recorder (CR).

Calibration of the streak images was performed by accumulating and storing an equal number of scans with and without a 5cm glass block between lenses L2 and L3. The storage capability of the OMA permitted the sequential recording of the streak image before and after the insertion of the optical delay (corresponding to a 2.5cm path difference in air) with the additional facility for the simultaneous display of the temporally separated intensity profiles. An independent calibration of the Ar ion laser pumping pulses was performed before the start of the experiment using the usual optical delay line arrangement 40 -

a) b)

Fig 3.7 Synchroscan streak intensity profiles of (a) shortest and

(b) typical semiconductor laser pulses

composed of mirrors M2, M3 and a beam splitter (33).

Above threshold O' 60mW pump power for the mode-locked case) with the system optimised, the infrared semiconductor laser pulses were observed to have a reproducible characteristic intensity profile consisting of a sharp rising edge and a longer trailing edge. For the intensity profile of the streak image that is reproduced in fig 3.7(a) the pulsewidth is 21ps (FWHM) obtained with the shortest (< 70ps) pumping pulses under optimum conditions, but a more typical example is included in fig 3.7(b) where the pulsewidth is 32ps. Infra-red pulse rise times were generally in the range 10 - 20ps. It is considered likely from the multimode spectral data shown in fig 3.8(b) and the short cavity round-trip time of 6 - 7ps derived in the previous - 41 -

824.5nm 830.5nm

a) b)

Fig 3.8 Semiconductor laser spectra recorded under (a) c.w. (197mW average pump power) and (b) pulsed (150mW average pump power) pumping conditions

section, that these pulses consisted of a few subpulses not resolved by

Synchroscan, which in this experiment had a resolution of ^ 10ps.

Some other interesting features can be observed by comparing the i.r. spectra under c.w. and pulsed excitation shown in fig 3.8. These are, respectively, the broadening of the individual longitudinal modes and the shift of the centre of the spectrum towards shorter wavelengths when going from c.w. excitation to pumping with mode-locked laser pulses. Both features have been observed by a number of authors previously, and are both probably mainly due to thermal effects (10,

34-37). In semiconductor lasers, both the cavity modes and the gain curve are temperature dependent due to the temperature dependence of - 42 -

the refractive index and bandgap respectively. In GaAlAs lasers, the gain curve shifts more rapidly to longer wavelength than the cavity modes as the temperature is raised, so a higher temperature results in a spectral shift to longer wavelength. This is consistent with longer wavelength operation occuring with the larger heating incurred under c.w. excitation (the c.w. spectrum was taken with a higher average pump power than the pulsed one). It has been speculated that the shift of the spectrum could also be augmented by the band-filling effect transiently shifting the gain spectrum to higher energies (35) under pulsed excitation. Similarly, broadening of the individual longitudinal modes has been attributed to transient heating during pulsed excitation (36), where the spectral shift incurred will be seen as mode broadening in the time-average mode spectrum which is recorded.

There is possibly also a contribution from carrier-induced frequency chirping, since the carrier density changes dramatically during the pulse (37).

Fig 3.9 Evolution of relative delay and pulse narrowing with average pump power - 43 -

A significant part of the present study was concerned with the determination of the dependence of the temporal characteristics of the

infra-red laser pulses on the power level of the optical pumping

pulses. A trend of decreasing pulsewidth with decreasing pump power was observed, fig 3.9. In addition (figures 3.9 and 3.10) the pulses were found to show an increasing delay with respect to the pumping

pulse as the power was decreased. A slightly larger delay and longer

i.r. pulses for a given pump power were observed with longer pump

pulses. The evolution of the delay and pulse narrowing is most

probably due to threshold being reached progressively earlier in the

higher power pump pulses where gain switching occurs. The short photon

Fig 3.10 Recorded Synchroscan streak intensity profiles showing increasing delay between pumping pulse an i.r. pulse as average pump power decreases. The i.r. pulse occurs later in time in each case 44 -

lifetime in the cavity (^2ps (18)) ensures that the laser pulse "shuts off" near the end of the Ar ion pumping pulse. An examination of these scattered Ar ion pump pulses showed no variation in duration or shape as the power was varied, rise times of 55ps were typical for the 70 -

90ps duration pulses used.

in c ZJ

n o

Fig 3.11 Luminescence temporal profile from the semiconductor laser pumped below threshold (30mW pump power) and its delay from the pumping pulse

Using the high sensitivity of the synchronously operated streak camera to low intensity repetitive signals, luminescence profiles of the below threshold emission were also recorded. Figure 3.11 shows a typical example and includes the scattered Ar ion laser pulse profile.

The luminescence profile at 77K had a rise time of 100ps and a 1/e decay time ^350ps. Other measurements of the spontaneous emission decay time at 300K taken at Philips Research Labs, using a photon counting technique (38) gave an exponential decay time of 600ps. This - 45 -

minority-carrier lifetime is probably dominated by nonradiative recombination at the interfaces and we can calculate interface recombination velocities of M x 104cm/s at 77K and 2 x 104cm/s at 300K respectively using the equation (39):

(3.5)

2T rad

(s = recomb, velocity, T , = radiative lifetime, d = interface spacing rad

= 0.25ym) which applies when Trad « Tnonrad.

The interface recombination velocity is a measure of the quality of a semiconductor heterojunction and the calculated values are felt to be reasonable for this structure by the laser manufacturer (Philips)

(38). No delay was observed between the end of the pumping pulse and the luminescence peak in contrast to the case of electrical injection reported by Duguay and Damen (25). This reinforces their conclusion that what they observed was the finite time for the carriers to diffuse to opposite sides of the heterojunction before recombination occurred, which is not necessary for luminescence produced by optical pumping.

3.6 Discussion

The time profiles of the spontaneous and stimulated emission obtained by Duguay and Damen are shown in fig 3.12. They claim that the luminescence curve shows the variation of the laser gain with time, and explain their increase of laser pulse duration nearer to threshold by the relatively slow change in the gain near the luminescence peak giving rise to less rigorous gain-switching. In contrast, we see for our case from fig 3.11 and 3.9 that the i.r. pulses are generated at times corresponding to the rising edge of the luminescence, where the gain is changing very rapidly but uniformly because no carrier diffusion is required. Thus the rise-time of the pulses remains rapid - 46 -

LASER PULSES

Fig 3.12 Temporal behaviour of stimulated and spontaneous emissions from an electrical pulse gain-switched semiconductor laser with varying bias. From Duguay and Damen (25)

for all delays, which, combined with the short photon lifetime, ensures

that shorter pulses are obtained closer to threshold.

Thus carrier diffusion may not only limit the delay time between

the excitation pulse and the emitted pulse and thus provide a

fundamental limit on modulation speed as proposed by Duguay and Damen,

but may also be a factor limiting pulse durations obtainable by gain­

switching. This may explain why Gobel and coworkers (16) have not

observed substantial pulsewidth reductions when exciting an injection

laser with shorter electrical pulses obtained from an Auston switch.

An investigation into ways in which this diffusion time could be

reduced (narrower active region?) may thus lead to the generation of

shorter optical pulses.

3.7 Conclusions

A powerful technique for studying short pulse dynamics in

gain-switched semiconductor lasers has been demonstrated. Pulses as

short as 20ps have been observed and it has been shown that information - 47

relevant to the optimisation of gain-switched injection lasers, which are the simplest source of short light pulses for optical fibre applications etc. can be obtained using the more versatile optical pumping method. This technique could be applied to semiconductor lasers emitting at 1.3 - 1.55ym, using a mode-locked c.w. Nd:YAG laser as pump source and an extended response S1 photocathode streak camera

(40) for detection purposes. The advent of fibre/grating pulse compression techniques has allowed the generation of YAG laser pulses of 'v 1ps duration (41), recently demonstrated (46) to have low time- jitter characteristics,which could allow a study of gain-switching with high time resolution in semiconductor lasers, using pump pulses much shorter than those currently available from electrical pumping. A further area of study would be the effect of variation in cavity length on the duration of the optical pulses produced. Shorter cavity lengths have already been shown to. generate shorter gain-switched pulses

(through the shorter photon lifetime) in a preliminary investigation

(42) with injection lasers. - 4 8 -

REFERENCES

(1) J.C. AuYeung and A.R. Johnston; Proc. SPIE 322 (1980) 25

(2) D. Botez and G.J. Herskowitz; Proc. IEEE 6S_ (1980) 689

(3) E.P. Ippen, D.J. Eilenberger and R.W. Dixon; Appl. Phys. Lett.,

37 (1 9 8 0 ) 267

(4) J.P. van der Ziel, W.T. Tsang, R.A. Logan, R.M. Mikulyak and

W.M. Augustyniak; Appl. Phys. Lett., 39_ (1981) 525

(5) J. Chen, W. Sibbett and J.I. Vukusic; Optics Comm. 48^ (1984) 427

(6 ) J.C. AuYeung and A.R. Johnston; Appl. Phys. Lett., ^ (1982) 112

(7) D.J. Bradley, M.B, Holbrook and W.E. Sleat; IEEE J. Quant. Elect.

QE-17 (1 9 8 1 ) 658

(8 ) P.T. Ho; Electron. Lett., J5. (1979) 526

(9) C. Lin, P.L. Liu, T.C, Damen, D.J, Eilenberger and R.L. Hartman;

Electron. Lett., J_6 (1980) 600

(10) R.A. Elliott, H. DeXiu, R.K, DeFreez, J.M. Hunt and P.G. Rickman;

Appl. Phys. Lett., 42 (1983) 1012

(11) E.0. Gobel, G. Vieth, J. Kuhl, H-U. Habermeier, K. Lubke and

A. Perger; Appl. Phys. Lett., j\2_ (1983) 25

(12) J.C. AuYeung; Appl. Phys. Lett., ^38 (1981) 308

(13) S. Tarucha and K. Otsuka; IEEE J. Quant. Elect., QE-17 (1981) 812

(14) H. Ito, H. Yokoyama, S. Murata and H. Inaba; IEEE J. Quant.

Elect., QE-17 (1 9 8 1 ) 663

(15) T. Andersson, S. Lundqvist and S.T. Eng; Appl. Phys. Lett., 4_1_

(1 9 8 2 ) 14

(16) G. Vieth, J. Kuhl and E.0. Gobel; Electron. Lett., _1_9 (1983) 385

(17) A. Passner, H.M. Gibbs, A.C. Gossard, S.L. McCall,

T.N.C. Venkatesan and W. Wiegmann; IEEE J. Quant. Elect,,

QE-16 (1980) 1283

(18) T.L. Koch, L.C. Chiu, Ch. Harder and A. Yariv; Appl. Phys. Lett.,

41 (1 9 8 2 ) 6 - 49

(19) W-L Cao, A.M. Vacher and C.H. Lee; Appl. Phys. Lett., ^ (1981)

653

(20) C.B. Roxlo and M.M. Salour; Appl. Phys. Lett., ^I3 (1981) 738

(21) R.S. Putnam, C.B. Roxlo, M.M. Salour, S.H. Groves and M.C. Plonko

Appl. Phys. Lett., 4£ (1982) 660

(22) J. Stone, J.M. Wiesenfeld, A.G. Dentai, T.C. Damen, M.A. Duguay,

T.Y. Chang and E.A. Caridi; Opt. Lett., 6^ (1981) 534

(23) M .A. Duguay, T.C. Damen, J. Stone, J.M. Wiesenfeld and

C.A. Burrus; Appl. Phys. Lett., _37 (1980) 369

(24) M .D. Dawson, W. Sibbett, J.I. Vukusic, P. Dawson, G. Duggan and

C.T. Foxon; Appl. Phys. Lett., 43 (1983) 226

(25) M .A. Duguay and T.C. Damen; Appl. Phys. Lett., 40^ (1982) 667

(26) P .G. May, W. Sibbett, K. Smith, J.R. Taylor and J.P. Willson;

Optics Comm., 42_ (1982) 285

(27) A .P. Thorne; "Spectrophysics" (Science Paperbacks, 1974)

Chapter 6

(28) G .B. Scott, J.S. Roberts and R.F. Lee; Appl. Phys. Lett., _37

( 1980) 30

(29) A .Y. Cho and J.R. Arthur; Prog. Solid State Chem., _10_ (1975) 157

(30) G .H.B. Thompson "Physics of Semiconductor Laser Devices" (John

Wiley, 1980) p.117

(3D D .D. Sell and H.C. Casey, Jr.; Appl. Phys., 45 (1974) 800

(32) H . Mahr and M.D. Hirsch; Optics Comm., J_3 (1975) 96

(33) M .C. Adams, W. Sibbett and D.J. Bradley; Optics Comm., 26^ (1978)

273

(34) J.C. Camparo and C.H. Volk; IEEE J. Quant. Elect., QE-18 (1982)

1990

(35) P-L. Liu, C. Lin, I.P. Kaminow and J.J. Hsieh; IEEE J. Quant.

Elect., QE-17 (1 9 8 1 ) 671

(36) I.. Figueroa, D. Botez and S. Wang; IEEE J. Quant. Elect., QE-13 (1977) 612 -SO­

OT) C. Lin, T.P. Lee and C.A. Burrus; Appl. Phys. Lett., 42 (1983)

141

(38) P. Dawson (Philips Research Laboratories); private communication

(39) G.W. t'Hooft, C. van Opdorp, H. Veenvliet and A.T. Vink;

J. Crystal Growth., 55 (1981) 173

(40) X. Hou, W. Sibbett and B. Weekley; Rev. Sci. Instrum., 52 (1981)

1487

(41) J.D. Kafka, B.H. Kolner, T. Baer and D.M. Bloom; Opt. Lett., 9_

(1984) 505

(42) G.J. Aspin, J.E. Carroll and R.G. Plumb; Appl. Phys. Lett., 39_

(1 9 8 1 ) 860

(43) H. Kressel and J.K. Butler; "Semiconductor Lasers and Hetero­

junction LEDfS" (Academic Press, 1977) p.278

(44) G.H.B. Thompson; "Physics of Semiconductor laser devices" (John

Wiley, 1980) Chapter 8

(45) L.F. Mollenauer and R.H, Stolen; MUltrafast Phenomena IV"

Ed. D.H. Auston, K.B. Eisenthal (Springer Verlag, 1984) p.2

(46) A.S.L. Gomes, W. Sibbett and J.R. Taylor; to be published - 51 -

CHAPTER IV

THE MODE-LOCKED c.w. Nd:YAG LASER

4.1 Introduction

The continuous-wave (c.w.) Nd:YAG laser (1) was first mode- locked as early as 1966 (2 ) and was characterised in detail both experimentally (3) and theoretically (4) by Kuizenga and Siegman in

1970. Pulses of duration as short as a few tens of picoseconds were obtained at 1.06ym wavelength using both acousto-optic loss modulation and electro-optic phase modulation (see also (5)), with average TEMoo powers of up to a few watts. Although various attractive features, such as the capability of simultaneous mode-locking and Q-switching

(6 ), were identified, true exploitation of this laser as a picosecond source began, however, only in the late 1970’s. Possible reasons for this were problems associated with mode-locked Nd:YAG laser technology mentioned in this chapter, including ’’thermal runaway" effects in modulators which were highly acoustically resonant. There was also the lack of efficient harmonic generating crystals and the attractiveness of other available picosecond systems. Pulsed, passively mode-locked

Nd:glass and dye lasers, for example, are capable of directly generating high peak power pulses of substantially shorter duration (a few picoseconds) (7). With the development of c.w. synchronously and passively mode-locked dye lasers capable of producing sub-picosecond pulses (7), noble gas ion lasers (Kr ion and Ar ion) were identified as the preferred pumping sources, for reasons discussed in the next chapter.

Initial interest in mode-locked c.w. Nd:YAG lasers was mainly stimulated by research in optical fibres and by colour centre lasers - 52 -

tunable at wavelengths beyond 1ym which required the Nd:YAG fundamental as a pump source (8,9). However, the commercial mode-locked c.w.

Nd:YAG lasers available at the beginning of the work described here were still essentially industrial tools as opposed to scientific laser

systems. Indeed, work reported until quite recently probably used

these lasers in a comparatively poor state of operation. In this

chapter, the characteristics of a commercial mode-locked c.w. Nd:YAG

laser (Quantronix Model 116) and the work undertaken to improve its

performance and stability are described. The recent availability of

efficient, high-damage threshold frequency doubling crystals (as

described in Chapter 5), has allowed these lasers to generate

sufficient second harmonic power to synchronously pump visible-wave­

length dye lasers, extending their 1.06ym pumping capabilities. The

consequent commercial interest has recently prompted laser manufacturers to make similar improvements in their commercial mode-

locked c.w. Nd:YAG lasers to those documented in this chapter.

In this and subsequent chapters, the performance of the laser as a

picosecond source in both mode-locked and simultaneously mode-locked and Q-switched operation is described, with emphasis on streak camera measurement techniques. The particular advantages of the system are

identified as, firstly, synchronous pumping (using the mode-locked

fundamental) of colour centre lasers (9 ), dye lasers (1 0 ) and optically

pumped semiconductor lasers (1 1 ) tunable at wavelengths greater than

1pm. This is of special interest to studies of short pulse propagation

in optical fibres near the minimum dispersion and attenuation wave­

lengths ('vl . 5 ym) and in studies of semiconductor materials. Secondly,

the high power harmonics that can be generated using simultaneously mode-locked and Q-switched operation can be used to pump dye lasers and high repetition rate (500Hz) optical amplifiers for subpicosecond dye

laser pulses (12). Finally, there is the recent development of fibre/ - 53 - grating pulse compression techniques, where the c.w. mode-locked pulses at both the fundamental (13) and second harmonic (14) wavelengths can be compressed extracavity to M p s duration with high enough power for synchronous pumping. In addition to short pulse pumping of dyes and colour centre lasers, this should extend the fluorescence and luminescence lifetime measurement capabilities of the linear streak camera technique.

4.2 The Neodymium in YAG laser

Since the development of the first Nd:YAG laser by Geusic et al in 1964 (1 ), considerable progress has been made in improving both the quality of the active material and the efficiency of pumping.

Nd:YAG possesses uniquely favourable properties as a laser medium (15) including the mechanical characteristics, high optical quality and thermal conductivity of the host and the narrow fluorescent linewidth, resulting (together with 4-level operation) in high gain and low threshold for laser action. This has ensured that Nd:YAG is now the most commonly used solid state laser material, particularly due to its capability of efficient c.w. operation. Continuous wave output powers of several hundred watts, for example, have been achieved from a single laser rod (1 6 ).

Detailed review articles and books on Nd:YAG have been published

(1 5 ,1 7 ,1 8 ) but a brief summary of its properties is given here. Pure

Y3AI5 0 12 (YAG) is a colourless isotropic crystal having a cubic structure. Nd:YAG is usually composed of ^1$ substitution by mass of

Nd for Y. The NdrYAG laser is a four-level system as shown in the simplified energy level diagram of figure 4.1. The major lasing transition at X = 1.064ym originates from the R 2 component of the ^13/2 level and terminates at the Y3 component of the 4I n /2 level. There are a number of pump bands, those at 0 .8 lym and 0 .75ym being the - 54 -

strongest, which may be suitably pumped by, for example, high pressure krypton arc lamps. The terminal laser level is at 2111cm” 1 above the ground state (the 4I g /2 level) and therefore the thermal population is a factor of exp(-AE/KT) i.e. ^ exp(-10) of the ground state level.

Since the terminal level is thus not populated thermally, the threshold condition is easy to obtain.

The 4F 3/2 laser level has a fluorescent efficiency of 99.5% and a radiative lifetime ^230ys. At room temperature the main 1.06ym line is homogeneously broadened by thermal lattice vibrations. It has a stimulated emission cross-section of ^8 .0 x 10“l9 cm 2 (1 8 ),

r 7 7 7 / / / / l

Fig 4.1 Schematic energy level diagram of Nd:YAG. After Koechner (15)

Unlike Ruby, which has only two potential laser transitions,

Nd:YAG has a large number from the ^ 3/2 to the 4Ig/2 > ^ 11/2 and 4 Ii3/2 levels with output wavelengths from 0.94 ym to 1.36 ym. Although in - 55

practice the 1.06pm transition is hard to suppress, the 1 .3 1 9 pm wave­ length line has been successfully mode-locked to produce pulses of 'MOO picoseconds duration with TEMoo power M - 1.5 Watts in continuous operation and has also been simultaneously Q-switched to give peak pulse powers of tens of kilowatts (1 9 ).

4.3 The commercial Nd:YAG laser - description

—------PHYSICAL LENGTH=142 cm------—

Fig 4.2 Schematic of Quantronix 116 Nd:YAG laser showing relative positions of mirrors, aperture and YAG rod

A schematic of the Quantronix model 116 laser system used in this work is given in figure 4.2. The active medium was a 4mm diameter rod of Nd:YAG with a length of 76mm. The rod was located at one focus of a gold-coated elliptical pump cavity and was continuously pumped by a high pressure krypton arc lamp (EG & G type FK216C-3.44) located at the other focus. A 6kW SCR phase-controlled D.C. power supply drove the krypton lamp with a current ripple of less than 1/6 peak to peak.

Typical operating currents were 35A with a total power dissipated in the lamp of approximately 3.8kW. The arc lamp and rod were both water cooled using a closed cycle cooling system containing a de-ioniser and - 56 particulate filter, which maintained coolant water resistivity (>1Mftcm) and purity, respectively. This assured reliable triggering and operation of the pump lamp. Regulation of the secondary cooling water flow held the water temperature in the primary cooling loop through the laser head constant to within a. 1°C (at 24°C).

The approximately 1.5 metre-long laser cavity employed a concave

(R = +100cm) high reflectivity mirror and a convex (R = -120cm) output coupler. This type of resonator provides efficient extraction of the energy in the active medium due to the large beam diameter at the laser rod - even with the addition of the intracavity elements, the 12£ transmission output coupler produced an % 10W linearly polarised TEMqo beam. The resonator was also relatively insensitive to thermal lensing effects in the NdrYAG rod - although the cooling water removes heat from the rod efficiently, a radial thermal gradient is established by the typical absorbed pumping power. This gradient causes the crystal to act like a positive lens with an effective focal length inversely proportional to the pump power (15), which dominates the optical characteristics of the laser unit (see later). In mode-locked lasers it is important to eliminate sub-cavities, which requires that the surfaces of all optical components in the cavity be wedged with respect to each other. In addition, these surfaces were coated with a low-loss anti-reflection coating to reduce power losses to a minimum. The aperture A (V^" diameter) restricted laser operation to the fundamental

(TEMqo ) transverse laser mode and the intracavity Brewster-angled quartz plate polariser P produced a highly linearly polarised beam

(>95/6). This resulted in a high quality vertically polarised spatially

Gaussian beam of diameter at the output of ^ 0.7mm with a beam divergence (full angle) of 2 .2mrad (2 0 ).

Mode-locking of the laser was achieved using a low-Q 50MHz fused quartz acousto-optic Bragg modulator (AOM) driven at high r.f. power - 57

(7.5W) which is described in more detail later in the chapter. In the commercial system, as initially installed, the modulator drive was provided by a variable frequency solid state oscillator tunable over

0.2MHz; cooling of the modulator was carried out by diverting a small fraction of the coolant entering the laser head into the modulator, which was subsequently fed back into the coolant return pipe. No cavity length adjustment was available in the commercial laser, all components being locked to the extruded aluminium optical rail, so the mode-locking was optimised by tuning the oscillator frequency.

Problems associated with this approach to mode-locking are discussed further in the following sections.

As regards system maintenance, the arc lamps were replaced on average approximately every 150 hours after which the laser output had normally fallen by ^ 2 Watts. This is less than the specified lamp lifetime of 200 hours (2 0 ), but the lamps were operated at their highest output throughout. Periodic maintenance (every 500 hours or so) was required for the cooling system, involving a change of the deionised water and replacement of the in-line particulate filter and deioniser tubes.

4.4 The Acousto-Optic Modulator

We first consider the characteristics of the acousto-optic mode-locker, as commercially supplied (Quantronix model 302)(21), in more detail. Mode-locking by intracavity acousto-optic loss modulation has been treated by Kuizenga and Siegman (3,4), with particular reference to homogeneously broadened laser systems.

The modulator consisted of a fuzed quartz substrate in which the acoustic standing wave grating was formed. Its dimension along the direction of laser beam propagation was ^35mm (with a slight taper).

Perpendicular to the beam direction it measured 10mm x 46mm, 10mm being - 58 -

the "height” of the substrate. The acoustic wave was generated in the

46mm direction by a LiNbO 3 transducer bonded to one of the corresponding faces. The r.f. drive was coupled into the modulator

unit via an LC network (utilising the capacitance of the quartz block) matched into 50ft around the desired drive frequency of ^ 50MHz.

Fig 4.3 Plot of optimum SWR (resonances) vs. frequency over the matched range of frequencies

The substrate acts as a resonator for acoustic waves, and the

width and separation of the resonances in the vicinity of the drive

frequency were measured using a (variable) frequency sythesizer (RACAL

DANA 9081) and an in-line Standing Wave Ratio (SWR) power meter (COAX

NDK-200). A plot of optimum SWR (the resonances) versus frequency is

shown in figure 4.3. The average measured frequency separation of

these resonances is 65kHz. This is consistent with the resonance

separation calculated using (2 2 ):

(4.1) res - 59 where D is the substrate dimension in which the standing wave is formed

(= 46mm) and c g is the speed of sound in quartz for a longitudinal acoustic wave (= 5.95 x 105cm/sec (15)). The width of the individual resonances were found to be ^ 20kHz which are broadened from the substrate acoustic resonance width due to the lossy bond between the transducer and the substrate. This broad width and relatively close spacing meant that the resonances were not completely separated out, so that the changes in SWR with frequency within the broad matched frequency range illustrated in figure 4.3 were minimised. The modulator was thus designed to contain a high-amplitude standing wave relatively independently of the drive frequency (21). The idea was hence to simplify the mode-locking procedure by making cavity length adjustment unnecessary and optimising the mode-locking by frequency­ tuning of the r.f. drive, using a variable frequency source (tunable

49.66 to 49.89MHz) together with Bragg-angle adjustment of the modulator. In addition, the broad resonances were chosen to reduce problems associated with "thermal runaway" (2 3 ) where the high laser intracavity powers (^100W average) can cause heating of the acousto­ optic modulator, thus causing the frequency of the resonances to drift.

The problems associated with this approach include the high r.f. drive power (^7.5W) needed to obtain the necessary diffraction efficiency for good mode-locking, thus leading to considerable heating of the block and so slow establishment of equilibrium following turn-on. There is also the fact that variable frequency r.f. drivers rarely have the long-term stability associated with those of fixed frequency (22). Indeed, the observed progressive drift in the driving frequency of the commercial oscillator (measured using a Philips PM664 automatic counter) was found to be 1-2 Hz/minute (quoted stability

10“ 5/8 hour), which was not improved when the oscillator was run from a

Variac, allowing it to remain on when the laser was not operating. 60 -

This we will show leads to a significant mode-locked pulsewidth detuning over a period as short as hour (even though the diffraction efficiency of the modulator with such broad resonances is almost unaffected). Cooling of the modulator utilised water tapped from the coolant flow into the laser head. Although a heat exchanger in the primary water loop maintains the coolant temperature to a claimed value of ^ 1 0 C once stabilised (20), the time taken for stabilisation was another source to affect the establishment of equilibrium in the modulator.

Finally, the modulation depth of the mode-locker (0 ) was measured, to obtain an idea of the diffraction efficiency and a value

for use in the pulsewidth expression of Kuizenga and Siegman (4) (see later in this chapter). The expression for single pass amplitude

transmission of an acousto-optic modulator operating in the Bragg

regime is (4):

m(t) = cos (0 mmsin 2Trf t) (4.2)

where f is the applied r.f. frequency (50MHz) and 0m is the modulation

depth (in radians). The transmitted intensity thus varies as m 2 (t).

By inserting a nominally 100$ reflecting mirror into the cavity before

the acousto-optic modulator, the leakage laser light from this sub­

cavity (when aligned for laser action) followed the normal intracavity

beam path through the modulator. The average powers in the emerging

diffracted and undiffracted beams were then measured and used to

calculate 0m using the method of Kuizenga and Siegman (6 ),

The ratio of the diffracted beam average power to that transmitted

can be shown to be:

(1 - cos2 0 )/(l + cos2 0 ) (4.3) m m - 61 - from consideration of the previous equation, which with the measured ratio of 7:12 allows us to calculate 0 m = 1.03 (radians). This high value (the maximum 6 possible is 1 .6 ) is indicative of an efficient m modulator, and agrees well with the optimum value 9m = 1.15 measured for a similar Quantronix modulator by Harvey et al (24). The pulse- widths theoretically calculated using this modulation depth are given later.

4.5 Characteristics of the Nd:YAG laser

We now consider the characteristics of the operating laser system. Typical maximum average c.w. output powers at 1.06ym were 8-

10 W in the TEM00 mode throughout this work, as measured by an optical power meter (Coherent Model 210). Average mode-locked c.w. output

LAMP CURRENT (D.C. AMPS) ------

Fig 4.4 Graph of output power vs. krypton arc lamp current for mode-locked (•) and un-mode-locked (+) operation - 62 - powers (at 100MHz pulse repetition frequency) were only slightly reduced from this, being typically in the 7-8 Watt range. A graph of output power versus krypton arc lamp current is shown for both mode- locked and unmode-locked operation in figure 4.4. At the highest currents the output peaks and then falls off rapidly, due to the thermal lensing action in the rod resulting in an effective focal length so short that the laser resonator becomes optically unstable.

Mode-locked pulsewidth measurements were carried out using a synchronously operated streak camera in the standard configuration described in Chapter 2. An ^4$ intensity component of the mode-locked pulsetrain was used to trigger the tunnel diode oscillator and thus provide the synchronous sweep for the camera. The camera utilised a

Photochron II streak image tube with an S1-type photocathode, so that optical harmonic generation was not required for detection purposes.

Fig 4.5 Shortest (a) and typical (b) recorded Synchroscan streak intensity profiles of 1.06ym pulses from the Nd:YAG laser - 63 -

The shortest (a) and typical (b) recorded Synchroscan streak intensity profiles are shown in figure 4,5. The full width half maxima of 47ps for (a) and 7 7 ps for (b) should be compared with those at the second harmonic (532nm) given in the following chapter (figure 5.4). Pulses of duration 5 0 ps or less were recorded on a number of occasions, the shortest being estimated from the second harmonic pulsewidth (see fig.

5.4(a)) to be 44ps. These pulses are among the shortest ever reported directly from mode-locked c.w. Nd:YAG lasers and are the shortest so far achieved from a high power, long cavity length laser of the type used here. In addition, it should be pointed out that the shorter pulsewidths (^3 0 ps (4,5)) were in general reported in 1970 and before, when the nonlinear diagnostics had not reached their full state of development. These shortest pulses were, however, notably less stable than those recorded typically, and seemed to occur at a rear mirror alignment which was extreme in the sense that further angular adjustment resulted in a rapid fall in output power.

Kuizenga and Siegman give the following pulsewidth expression for

AM modulation (4):

'l 1 Tp = (2 £n go| 1 2 (4.4) IT 0 2 m *-*m^ where f is the applied r.f. modulation frequency (= 50MHz), Af is m the atomic linewidth (180 x 109Hz for Nd:YAG (18)) and 0m is the modulation depth, which was previously established to be M . O radian, g is the saturated roundtrip amplitude gain, which is given by (4):

(4.5) (1-L) J

where L is the fractional loss per roundtrip. Using an output coupling loss of 0.12 and an internal roundtrip loss due to other sources o f ^ . l 64 -

(estimated from the thresholds with a 12>6T and 0$T output coupler), we thus estimate a pulsewidth of ^TOps. This in fact compares quite reasonably with that typically obtained.

In an attempt to measure the corresponding spectral bandwidth of the mode-locked pulses, we used a home-made 1.6mm - gap Fabry-Perot etalon, partially silvered (R ^ 80%) on each face. This was because no suitable commercial instrument (e.g. scanning Fabry-Perot

interferometer) was available, and the bandwidth was at the resolution

limit of our 1m spectrograph. Fringe-measurements were obtained (in

the manner of section 3 .2 ) indicating a mode-locked bandwidth of 0.3 -

0.4$. This is consistent with results obtained by other authors (19)

and indicates operation close to the Fourier-transform limit. The

finesse of the etalon, however, was not felt to be high enough for an

absolute bandwidth measurement to be deduced.

Fig 4.6 Effect of frequency detuning on recorded output pulsewidth at Nd:YAG fundamental wavelength - 65

Measurements were taken, using the Synchroscan streak camera, of the variation of mode-locked pulsewidth as the r.f. drive frequency was detuned from optimum (as measured using the frequency counter). The results are shown in figure 4.6. The high sensitivity of the pulsewidth to detuning is apparent from this figure, a significant broadening occuring for a detuning (either positive or negative) from optimum of less than 100Hz in ^50MHz. The pulsewidth effectively doubles for a detuning of 300-400Hz (see the cavity length detuning effects on the second harmonic in the following chapter). This characteristic curve is more sensitive for the mode-locked c.w. Nd:YAG laser than that which has been measured for mode-locked Argon (25) and

Krypton (26) ion lasers, where typically a few kHz detuning in a similar driving frequency is required to double the pulsewidth. It is worth considering the reason for this.

If the Kuizenga and Siegman approach to AM mode-locking (3,4) was valid for both Nd:YAG and noble gas ion lasers, then the major reason would be purely the larger bandwidth (Aw) of Nd:YAG (c.f. the bandwidth measurement for Ar ion lasers given in Chapter 3). As is shown most clearly by New et al (27), the detuning characteristic scales as 1/Aoo.

However, the Kuizenga-Siegman theory is not fully valid for noble gas ion lasers where gain saturation plays a role in pulse formation. This may offset the inverse bandwidth dependence slightly.

Nevertheless, we see immediately the additional problems posed in stable mode-locking of the c.w, Nd:YAG laser, which is possibly another reason why interest in these lasers has been delayed. The frequency stability of the commercial r.f. driver (1-2Hz/min) is obviously not adequate, because significant pulse broadening will occur over intervals ^2 hr to 1 hour. in addition, the cavity length must be much more stably maintained against thermal variations than in the case of noble-gas ion lasers - pulsewidth doubling would occur for changes - 66 -

M O y m in the 1.5 metre cavity length. The poor thermal expansion characteristics of the commercial extruded aluminium rail did not satisfy this. Finally, if a fixed driving frequency oscillator is used and mode-locking optimised using a cavity length adjustment, we see that differential micrometer control is required.

With optimised mode-locking, the output pulsetrain could exhibit a high amplitude stability. Peak-to-peak fluctuations in the pulse amplitude were regularly observed to be as small as 3-^/6. This, however, depended fairly critically on the laser alignment. Optimum alignment was achieved by directing a HeNe beam along the laser beam path in the cavity and adjusting the mirror alignments to suitably direct the reflected beams. Due to the thermal lensing effect, it was important that the beam travelled along the central axis of the Nd:YAG rod. This was achieved by ensuring that the HeNe spot transmitted through the rod and imaged on a distant white card expanded uniformly and symmetrically as the lamp current was increased.

The pulse amplitude fluctuations were found to have two major elements, these being random fluctuations and, in addition, a regular ripple observed on an^ms timescale. This ripple is most clearly seen in figure 5.6 (next chapter) particularly on the dye laser pulsetrain of figure 5 .6 (c), where it is magnified due to the quadratic nature of the harmonic generation process and running the dye laser relatively close to threshold.

Two components can be seen in this ripple, the major one having a

20ms (50Hz) period and the minor one a 3«3ms (300Hz) period. The ripple was 2% peak-to-peak in the fundamental pulsetrain. It was found that this ripple mirrored directly the modulation on the krypton arc lamp line drive, which was measured by imaging the diffuse light from the laser head (no lasing) onto a vacuum photodiode and using the

1Mft plug-in of a Tektronix 783^ storage oscilloscope. The resulting - 67 - ripple measurement is shown in figure 4.7, and was found to be peak-to-peak which is close to the specification for the Electronic

Measurements Inc EMKI 150-40 supply used (28). The two components are identified as mains pick-up in the power supply unit (50Hz) and residual 3~phase ripple (300Hz) due to incomplete SCR regulation.

Fig 4.7 Ripple on current line drive to krypton arc lamp. Horizontal scale is 5ms/major division. Ripple peak-to-peak amplitude

is ^1% of total

The power supply manufacturer (29) confirmed the suggestion that addition of a suitable LC filtering circuit at the power supply output, doubling up on the inductor and capacitor already there, could reduce the current ripple to perhaps peak-to-peak, thus reducing that on the fundamental to ^1$. A choke mimicking that already in the power supply filter circuit was thus designed and mounted in the power supply unit. It was designed to have a resistance (.033ft) much less than that of the operating lamp (a few ohms) at 40A D.C. and an inductance M O m H

(at 50Hz). At the time of writing, however, the resulting improvement had not yet been evaluated owing to the inconvenience of installation as regards work in progress. The improvement is mainly important in dye-laser pumping applications, however, which have not been the major concern of the work to date. 6 8 -

Additional amplitude fluctuations, due to relaxation oscillations, became noticeable when the cavity length was detuned from optimum,

Nd:YAG lasers are known to be particularly susceptible to instabilities of this type because of the long upper state lifetime (^230ys) compared to the M O n s cavity roundtrip time (39). Two distinct types of relaxation oscillations were obtained, consistent with results reported previously (3D. Damped sinusoidal oscillations were observed for detunings (either positive or negative) of a few hundred Hz, of period

^20 ys and occuring in ^200 us bursts. For larger detunings regular spiking of the output was observed, the spikes being typically ^5 us wide and separated by 15-20ys. Both types of oscillation are shown in the oscillograms of figure 4.8(a) and (b). Relaxation oscillations were particularly evident following turn-on of the laser, before thermal equilibrium had been established. Although relaxation fluctuations are likely always to be present to some degree, it should be pointed out that they can be made negligibly small by suitable optimisation of the mode-locking.

Fig 4.8 (a) Damped sinusoidal and (b) regular spiking relaxation oscillations from detuned Nd:YAG laser. Timescale 50us/minor division in (a), 5ys/minor division in (b)

Occasionally, an additional sinusoidal modulation of period ^ 70ns was observed. This frequency is in the range for beating between lower - 69 - order transverse modes (15) and could be eliminated by realignment.

4.6 Improvements to the Nd:YAG laser

Pulses from the mode-locked laser, as commercially supplied, were observed to broaden significantly from optimum on a timescale

M 5 mins, to M hour using the direct streak camera measurement technique. Reasons for this - frequency drift of the modulator driving frequency and thermal variation of the cavity length - have been previously mentioned.

Our approach to mode-locking the laser was to replace the commercial variable frequency driving oscillator by a highly stable

(see below) homemade fixed-frequency crystal oscillator, utilising the commercial power amplifier to provide the 7.5W of r.f. drive required.

Mode-locking was then achieved and optimised by cavity length adjustment using a dual micrometer and differential micrometer translation stage (giving coarse adjustment over 25mm with fine length adjustments ^ 1 pm), rigidly attached to the optical rail, and on which the rear mirror mount was placed. The translation stage used was

Microcontrole type MRL 80-25 with a claimed sensitivity of 0.1ym. In addition, the commercial rear mirror mount was replaced by Micro­ controle type SL 25,4. This mirror mount was compatible with the translation stage and provided micrometer and differential micrometer precision rotational movement, having more sensitive adjustment and also being more stable than the Quantronix mount. Facility was made for the laser beam tube to fit this mirror mount and thus fully enclose the beam from dust and draughts.

The circuit diagram for the crystal-controlled oscillator is given in figure 4.9. The oscillator frequency was chosen to be 49.8417 MHz, corresponding to a resonance near the centre of the matched band of frequencies, for which the S.W.R. was ^ 1.1. Short term ( ^hours) 70 -

Fig 4.9 Circuit diagram of 50MHz crystal-controlled oscillator. PS is a power splitter, providing outputs to the power amplifiers of both the acousto-optic mode-locker and Q-switch (when operational). FB are ferrite beads

frequency stability was observed to be ± 5Hz without temperature stabilisation being required and the long term drift (weeks) was

± 30Hz. Both measurements were made using the Philips PM 664 automatic counter, giving a conservative frequency stability estimate of better than one part in a million. The spectral purity of the output was examined using a spectrum analyser (Hewlett Packard Model 8566A) and it was seen that the noise was reduced by 70dB with respect to the carrier at a 200Hz offset frequency. The spectral profile of the carrier signal at 49.8415 MHz is shown in figure 4.10(a) and a corresponding profile for a variable frequency synthesizer (RACAL 9081) at 50MHz is included in figure 4.10(b) for comparison. The fixed- frequency oscillator can be seen to exhibit much higher spectral purity and gave comparable results to a high quality Hewlett Packard commercial oscillator.

With these changes, an improvement in mode-locked pulsewidth stability was observed, but re-optimisation of the cavity length was still found to be necessary over periods M hour. The cavity length always needed to be decreased for re-optimisation, suggesting that thermal expansion of the cavity was responsible. Super invar or - 71 -

Fig 4.10 Spectral purity of (a) home-made 50MHz crystal oscillator and (b) commercial frequency synthesizer (RACAL 9081) tuned to 50MHz, recorded using a spectrum analyser (HP 8566 A)

graphite cavity length stabilisation (3 2 ) has been shown to be capable

of providing the low thermal expansion characteristics required. We

chose, however, to mount the complete laser system on a large cast iron

optical rail. These well-set rails of large thermal mass have been

shown previously to give satisfactory thermal stability for

synchronously pumped dye lasers, which exhibit similar cavity length

sensitivity (33)• With these improvements, pulses of typical width

(70-80ps) became stable a few hours after turn-on. The final 72 - improvement was to fully establish equilibrium constantly in the mode-lock modulator itself. This was achieved by use of a separate power supply, allowing the r.f. drive to be continuously applied to the modulator (even with the laser switched off), together with an independent temperature-stabilised water cooling loop (also continuously operated). With this improvement, pulses of duration

70-80ps could be stably maintained shortly after the laser was switched on. When shorter pulses (^50ps) were achieved, however, some detuning was still observed to take place. This was felt to be of mechanical origin as these pulses occurred nearer to an extreme of the alignment.

Further improvements such as using more flexible tubing for the cooling water to the laser head and acoustically damping out coolant flow instabilities, would hopefully reduce this effect.

An additional improvement that proved useful, particularly when checking the stability of the output mode-locked train before focussing into the second harmonic crystal (considered in Chapter 5), was the fast photodiode pulse monitor. A passage was hollowed out from the mount holding the Brewster-angled quartz intracavity polariser so that the small reflected beam intensity was directed upwards from the laser cavity. A glass cover slide was placed over the polariser enclosure to transmit this beam but isolate the laser cavity from dust and draughts.

The beam was then directed onto a BPW 28 (Telefunken) photodiode and the pulsetrain directly displayed on a Tektronix 7904 oscilloscope.

4.7 3rd Harmonic mode-locking and the Antiresonant Ring

Two techniques were used in an attempt to obtain shorter, more stable pulses than those achieved with the laser set-up as previously described, motivated by the fact that the linewidth of

Nd:YAG should support pulses of <10ps duration (34). These were, respectively, harmonic mode-locking and passive mode-locking in an 73 - antiresonant ring cavity. From the Kuizenga and Siegman expression for

AM mode-locking in the homogeneously broadened laser (4) given previously in the chapter, we see that:

x oe (1/f )^ (4.6) p m

where is the mode-locked pulsewidth and fm is the applied modulation frequency. Thus assuming constant gain, modulation depth and band­ width, driving the acousto-optic modulator at, for example, the 3rd harmonic of the fundamental (50MHz) r.f. frequency should result in pulse-shortening by a factor of Previous demonstrations of harmonic mode-locking are given in references (35,36).

A similar matching network was used to that described earlier, except that the ^5 metal strip inductive turns (constituting the inductance in the LC matching circuit) were replaced by a single loop to match the modulator unit into 50ft at 150MHz. An S.W.R. of less than

1.5 at 149.545 MHz was achieved (as measured with the in-line SWR meter) with an r.f. driving power of 8W. The r.f. was derived from a frequency synthesizer (RACAL) and a power amplifier tuned to 150MHz.

Two types of mode-locked pulse generation were observed, with the mode-locked pulsetrain having either a repetition rate of 100MHz or

300MHz. Small changes (^10ym) in the cavity length caused the output pulsetrain to change frequency from one repetition rate to the other.

The average power was found to remain the same in both types of operation - 6.5 Watt at 100MHz and 300MHz, This is different to results reported for noble gas ion lasers mode -locked at the fundamental and 3rd harmonic frequencies (36), where increased power has been reported at the higher harmonic due to the reduced spontaneous emission loss resulting in lower loss of gain between pulses. In

Nd:YAG lasers, this does not occur due to the long upper state lifetime 74 -

230us).

>1 l< 100 ps ■I 2,1 l<: 100 ps

Fig 4.11 Pulsetrains at (a) 100MHz and (b) 300MHz obtained with r.f. 3rd harmonic mode-locking and corresponding Synchroscan streak intensity profiles (of the optical second harmonic pulses). FWHM is 54ps in (a), 44ps in (b). Oscillograms are lOns/div and 5ns/div respectively

Similar behaviour of the average power and operation at two different frequencies has been observed for second harmonic mode­ locking of Nd:YAG lasers, in work performed at about the same time by

A.M. Johnson and W.M. Simpson of AT & T Bell Laboratories (37). It is not clear whether this dual frequency operation for a given drive

frequency has been observed in previous reports.

Pulsetrains at 300MHz and 100MHz are shown in figure 4.11 with the

corresponding Synchroscan streak intensity profiles (of the optical 75 - second harmonic 532nm pulses). We observed shorter pulses at 300MHz than at 100MHz (44ps FWHM at 532nm as compared to 54ps), (The 44ps pulses are quite short, corresponding to 62ps at 1.06um and were remarkably easy to obtain.) This is in contrast to Johnson and Simpson

(37), who reported equal pulse durations at the two frequencies, but do not identify whether they determined this by autocorrelation or sampling oscilloscope techniques. They did not use the direct linear streak camera technique. It may have been, however, that our modulation depth (which was not measured) at 150MHz was less than theirs at the 100MHz drive frequency.

The technique looks promising, particularly driving the modulator at higher harmonics while maintaining output pulses at 100MHz so that there is no reduction in pulse peak power. A Brewster-angled quartz modulator with a 150MHz fundamental drive frequency has been designed in our laboratory to further evaluate this method.

We now consider the second pulse-shortening technique investigated

- that of colliding pulse passive mode-locking in an anti-resonant ring

(A.R.R.) cavity. At the time this work was performed, no reports of passive mode-locking of c.w. (as opposed to pulsed) Nd:YAG lasers had been published. Haus had justified this theoretically (38) by showing that steady-state mode-locking is prevented in (c.w.) Nd:YAG lasers by relaxation oscillations. (By contrast, in the pulsed case, mode-locked pulses are established before relaxation oscillations can set in, thus transient mode-locking is assured.) Recognising this it was felt, however, that actively initiated passive mode-locking (39), where the

saturable absorber is used to shorten pulses formed by active mode­

locking using an acousto-optic modulator, should be possible. With the

constraint, then, that the overall cavity length remain the same as in the linear arrangement (to match the acousto-optic modulation

frequency), we attempted active-passive mode-locking in a modified 76 - laser resonator, where the highly reflecting mirror was replaced by an anti-resonant ring.

This type of cavity, proposed for mode-locking and other purposes by Siegman (40,41), has been used to produce a train of ^15ps duration pulses from a flashlamp-pumped pulsed Nd:YAG laser (42), The resonator configuration is shown in figure 4.12. The ring consists of a 50/50 power division ratio beam splitter and two highly reflecting mirrors

(R = 100%) arranged as shown. A pulse entering the ring from the external arm is divided into two equal parts which travel around the ring in opposite directions, recombining at the beam splitter (due to the phase difference between their corresponding frequency components) so that all the signal re-emerges in the external arm. That is, the ring acts effectively as a 100% reflecting mirror. If a saturable absorber is located at the mid-point of the ring, as shown, the two counter-propagating pulses collide at the absorber, bleaching it more effectively than in the case of a single pulse. This type of

"colliding-pulse" passive mode-locking (C.P.M.) has been shown to produce sharply improved performance in passively mode-locked c.w.

Rhodamine 6G dye lasers, allowing the first generation of optical pulses shorter than 0.1ps (43).

\ \

Fig 4.12 Antiresonant ring oscillator arrangement 77 -

Our design of antiresonant ring was essentially a modified folded focussing section from a horizontal jet-stream dye laser. This allowed the saturable absorber jet-stream to be Brewster angled for the vertically polarised Nd:YAG laser beam. The mirror mounts holding the two 100% mirrors were mounted on translation stages (Microcontrole type

MR80-25) for adjustment of the focussing and the mirrors used were both

10cm radius of curvature, leading to a focal spot power density ^40MW/ cm2 at the power levels obtained. The jet-stream holder allowed vertical and horizontal fine movements of the jet and the 50/50 beam­ splitter was mounted on a versatile mount allowing rotational as well as vertical and horizontal adjustments. The beamsplitter (CVI

Corporation) was measured to be exactly 50/50 at the angle used and the rear surface had a high quality anti-reflection coating for 1.06 ym wavelength. The complete unit was enclosed in a blue perspex box

(transparent at 1.06ym for alignment purposes) which provided isolation of the intracavity beam from dust and draughts, while not impeding mirror mount and beamsplitter adjustments. Optimisation of the mode­ locking was achieved by mounting the laser 12%T output coupler on a differential micrometer translation stage. The saturable absorber used was Kodak 9740 dye, which has been used widely to passively mode-lock pulsed Nd:glass and NdrYAG laser systems (7), flowed in an ethylene glycol/benzyl alcohol jet-stream.

We found the ring to function extremely well as an optical element, with mode-locked average output powers similar to the linear cavity configuration being achieved (6W average mode-locked power).

Without a saturable absorber, stable pulses of duration less than 1G0ps were recorded using the Synchroscan streak camera. However, with addition of small amounts (10”5M) of 9740 solution spiking output, like that of figure 4.8(b), always resulted. Similar behaviour was also observed using a solid state (irradiated LiF colour centre crystal) 78 -

saturable absorber.

Our attempt to reduce the pulse duration in this way was thus not successful owing to relaxation oscillations, although Soviet workers

(44) subsequently claimed that steady state purely passive mode-locking of a c.w. NdrYAG laser had been achieved using a weak solution of saturable absorber in a linear cavity arrangement. It is possible that a very careful study of the parameters of our laser system could allow this result to be reproduced. There seems no reason, however, why colliding pulse mode-locking in an antiresonant ring should not work in the simultaneously Q-switched and mode-locked Nd:YAG laser (which is essentially similar to the pulsed flashlamp pumped systems as regards passive mode-locking) and further work to determine this should be undertaken.

4.8 Conclusions

A detailed characterisation of a commercial mode-locked c.w.

Nd:YAG laser has been given, and it has been shown to be capable of generating 1.06 ym pulses of duration less than 50 picoseconds with average TEMqo powers (at 100MHz) of 7.5 Watts. The high sensitivity of the laser in terms of mode-locking detuning effects and amplitude instabilities has been established and improvements necessary to achieve the performance required of a scientific laser system have been described. Areas of interest regarding mode-locked c.w. NdrYAG lasers as pump sources have been identified and in subsequent chapters we will investigate some of these areas in more detail. 79

REFERENCES

(1) J.E. Geusic, H.m. Marcos and L.G. Van Uitert; Appl. Phys. Lett.,

_4 (1964) 1 8 2

(2) M. DiDomenico Jr., J.E. Geusic, H.M. Marcos and R.G. Smith; Appl.

Phys. Lett., 8^ (1966) 180

(3) D.J. Kuizenga and A.E. Siegman; IEEE J. Quant. Elect., QE - 6

(1970) 709

(4) D.J. Kuizenga and A.E. Siegman; ibid QE - 6 (1970) 694

(5) L.M. Osterink and J.D. Foster; J. Appl. Phys., _39_ (1968) 4163

(6 ) D.J. Kuizenga, D.W. Phillion, T. Lund and A.E. Siegman; Optics

Comm., 9. (1973) 221

(7) See for example D.J. Bradley in "Ultrashort Light Pulses", Topics

in Applied Physics Jj3, Ed. S.L. Shapiro (Springer-Verlag,

(1 9 7 7 ) and references therein

(8 ) L.F. Mollenauer, R.H. Stolen and J.P. Gordon; Phys. Rev. Lett.,

45 ( 1 9 8 0 ) 1095

(9) L.F. Mollenauer and D.M. Bloom; Opt. Lett,, 4_ (1979) 247

(10) A. Seilmeier, W. Kaiser, B. Sens and K.H. Drexhage; Opt. Lett., 8^

(1983) 205

(11) R.S. Putnam, C.B. Roxlo, M.M. Salour, S.H. Groves and M.C. Plonko;

Appl. Phys. Lett., 4D (1982) 660

(12) T. Sizer II, J.D. Kafka, I.N. Duling III, C.W. Gabel and

G.A. Mourou; IEEE J. Quant. Elect., QE-19 (1983) 506

(13) B.H. Kolner, D.M. Bloom, J.D. Kafka and T.M. Baer in "Ultrafast

Phenomena IV", Ed. D.H. Auston and K.B. Eisenthal (Springer-

Verlag, 1984) p . 19

(14) A.M. Johnson, R.H, Stolen and W.M. Simpson; ibid p.16

(15) W. Koechner "Solid State Laser Engineering" (Springer-Verlag,

1976)

(16) W. Koechner; Rev. Sci. Instrum., 4_1_ (1970) 1699 80 -

(17) D. Findlay and D.W. Goodwin in "Advances in Quantum Electronics"

J_ (Academic Press, 1970)

(18) R.B. Chesler and J.E. Geusic in Laser Handbook J_ (North Holland

1972)

(19) M.G. Cohen; Proc. SPIE 322 (1982) 44

(20) Quantronix Series 100 Open resonator laser operation manual

(21) Quantronix Model 302 Acousto-Optic Mode-locker operation manual

(22) G.F. Albrecht, L. Lund and D. Smith; Appl. Optics 22 (1983) 1276

(23) D.J. Kuizenga; IEEE J. Quant. Elect., QE-17 (1981) 9694

(24) G.T. Harvey, C.W. Gabel and G. Mourou; ^ 6 (1981) 213

(25) K. Smith (Imperial College); private communication

(26) J.P. Willson; PhD Thesis, University of London (1982)

(27) G.H.C. New, L.A. Zenteno and P.M, Radmore; Optics Comm., _48

(1983) 149

(28) Electronic Measurements Inc., N.J.; EMKI 150-40 power supply

m a n u a l

(29) N. Bownik (Electronic Measurements Inc.); private communication

(30) W. Koechner; IEEE J. Quant. Elect., QE - 8 (1972) 656

(3D J.H. Waddington; PhD Thesis, Queens’ University, Belfast (1973)

(32) Specification sheets for Quantronix Model 41 6 and Spectra-Physics

Series 3000 Nd:YAG lasers

(33) P.G. May, W. Sibbett, K. Smith, J.R. Taylor and J.P. Willson;

Optics Comm., 42 (1982) 285

(34) B.P. Boczar and M.R. Topp; Appl. Optics 22_ (1983) 1611

(35) M.F. Becker, D.J. Kuizenga and A.E. Siegman; IEEE J. Quant.

Elect., Q E - 8 (1972) 687

(36) A.I. Ferguson; Optics Comm., _38^ (1981) 387

(37) A.M. Johnson and W.M. Simpson; Opt. Lett., 8_ (1983) 554

(38) H.A. Haus; IEEE J. Quant. Elect., QE-12 (1976) 169 - 81 -

(39) K. Washio, K. Koizumi and Y. Ikeda; IEEE J. Quant. Elect., QE-13

(1977) 47

(40) A.E. Siegman; Opt. Lett., 6^ (1981) 334

(41) A.E. Siegman; IEEE J. Quant. Elect., QE-9 (1973) 247

(42) H. Vanherzeele, J.L. Van Eck and A.E. Siegman; Appl. Optics 20

(1 9 8 1 ) 3484

(43) R.L. Fork, B.I. Greene and C.V. Shank; Appl. Phys. Lett., ^ 8

( 1 9 8 1 ) 671

(44) L.S. Kornienko, N.V. Kravtsov, V.A. Sidorov and Yu. P. Yatsenko;

Sov. J. Quant. Elect., _13. (1973) 943 - 82 - 83 -

C H A P T E R V

THE Nd:YAG LASER-PUMPED SYNCHRONOUSLY MODE-LOCKED C.W. DYE LASER

5.1 Introduction

Much interest has been shown in the technique of mode-locking

by synchronous pumping, where a mode-locked laser is used to pump another

laser of equal (or a multiple or sub-multiple) cavity length. It has

been established as the most convenient method of generating frequency

tunable picosecond (and in some cases sub-picosecond) optical pulses,

particularly from organic dye lasers ( 1 ,2 ).

Frequency-doubled mode-locked neodymium lasers (Nd:glass) were among

the first pump sources used in synchronous mode-locking - being used to produce tunable picosecond pulses from Rhodamine 6 G (R6 G) and Rhodamine B

dye lasers as early as 1968 (3,4). These systems were, however, pulsed

i.e. the mode-locked pulses occured in trains of^microsecond duration.

Interest in recent years has concerned continuously working (c.w.) synchronously pumped systems due to their relatively high stability, reliability and interfacability with sophisticated signal procesing equipment, as well as their ability to generate shorter light pulses than their pulsed counterparts. The pumping sources in this case have typically been acousto-optically mode-locked c.w. noble gas ion (Kr ion and particularly Ar ion) lasers. These have been shown to produce o-IOOps pulses of suitable power (MOOW peak) for synchronous pumping on many of their stronger lasing lines throughout the visible and u.v. (5 - 8 ). The a d v e n t (9 ) of the remarkable nonlinear optical material potassium titanyl phosphate (K.T.P.) and its commercial availability within the last two years, has led to the mode-locked c.w. Nd:YAG laser becoming a serious alternative to these traditional pumping sources. It has allowed 84 -

extracavity second harmonic generation with high enough efficiency for synchronous mode-locking of visible wavelength dye lasers, to extend the

1.06ym pumping capabilities. The availability of KTP has coincided with the appearance of the new Styryl laser dyes (10,11), with large Stokes shifts to extend the wavelength coverage of this essentially single visible line pump source. The original pumping of these dyes was performed using pulsed frequency-doubled YAG laser excitation (12) which is closer to the absorption peak than 514.5nm pumping. Recognition of the potential of this system has recently prompted laser manufacturers to make the necessary improvements in their commercial mode-locked c.w.

Nd:YAG lasers of the type detailed in the previous chapter (13)*

The capabilities the system offers for Q-switching and synchronous mode-locking at wavelengths greater than 1 ym are described in other chapters. As regards using the second harmonic, various advantages over noble gas ion laser pumping have been claimed (14,15). These include increased system ruggedness, lower cost, shorter pumping pulses giving rise to shorter dye laser pulses of lower temporal jitter characteristics and 532nm - pumping being closer to the absorption peak of R 6 G t han the

5l4nm Argon ion laser line, thus leading to lower thresholds. There is also the improved synchronisability of the pulses to Nd:YAG laser-pumped dye amplifier systems. The extent to which these advantages have been realised so far will be detailed later in the chapter.

In this chapter we describe the design and construction of a frequency doubling unit incorporating a 3 x 3 x 5mm crystal of KTP which, in conjunction with the Quantronix 116 YAG laser, produced typically

5 0 0 - 600mW of average 5 3 2 nm pump light with 5 0 - 6 0 ps duration pulses at a repetition rate of 100MHz. This high quality spatially Gaussian beam was used to synchronously mode-lock a prism-tuned R 6 G jetstream dye laser in an astigmatically-compensated linear cavity. Dye laser pulses of

^ 1 - 2 ps duration and ^ 3 0 mW average power at the peak of the tuning curve - 85 -

(590nm) were achieved, A comparison with Ar ion - pumped dye laser

performance will be made at suitable points.

5.2 Extra-cavity Frequency Doubling in KTiOPOi+ (KTP)

Intracavity frequency doubling of mode-locked c.w, Nd:YAG lasers can provide the average powers ( M W at MOOMHz) at 532nm necessary for synchronous mode-locking of dye lasers (16,17). It has not proved popular, however, due to the long pump pulse width (typically ^OOps) resulting from the competitive effects of mode-locking and harmonic generation - the harmonic generation represents a power-dependent loss to the mode-locked pulse, imposing largest loss to the centre of the pulse and thus broadening it (16,18). Also, this inclusion of additional intracavity elements can lead to a reduction in stability and the crystals used - Ba 2 N a Nb 5 O15 a nd L H O 3 - have low damage thresholds

(25MW/cm2 (19) and 10MW/cm 2 (20) respectively) among other disadvantages.

Ba 2 Na Nb 5 O 15 has also been used successfully for extracavity

frequency doubling to provide ^ 0.5 Watt average power with considerably

shorter pulses ( 50 - 60ps)(14,19,21), Extracavity doubling has the advantage of a / 2 pulsewidth reduction from the fundamental (see later)

as well as avoiding any destabilisation of the pump laser due to the

intracavity harmonic generation process. The conversion efficiency is

limited by the low damage threshold of Ba2 Na Nbs O 15 , however, allowing

only weak focussing and, in addition, temperature phase-matching for 0 normal incidence at 100 C is awkward and the sensitivity to self-heating

effects can lead to instabilities in the second harmonic beam.

The original synchronous mode-locking of c.w. R 6 G dye lasers using

frequency doubled Nd:YAG lasers was performed in this manner however (14,

21) and it was used by Mourou et al in a hybrid mode-locking technique to

generate pulses which were claimed to be as short as 70fs (22). Indeed,

it seems possible that Murray and coworkers (19) could have used their 86

Ba 2 N a Nb 5 0 15 -doubled YAG laser for synchronous pumping as early as 1974,

Nevertheless, the extracavity frequency doubled YAG laser has only recently come into its own following the availability of crystals of the relatively new (9 ) nonlinear optical material potassium titanyl phosphate

(KTi0P04 or KTP), which possesses a combination of properties which make it a unique frequency doubler for neodymium lasers (23). KTP is a biaxial crystal with an orthorhombic structure, transparent over the spectral range 0,35 - 4,5ym. Crystals of optical quality can be grown in sizes up to 5 x 5 x 5 m m 3 and are nonhydroscopic and chemically stable up to at least 900°C, It possesses a very high damage threshold - 250MW/cm 2

almost comparable to those observed in KDP and its isomorphs,

KTP also exhibits a non-linear coefficient comparable to that of

B a 2 Na N b 5 O 15 , which has one of the highest known, and can be phase-matched at 1.06ym for second harmonic generation using either the

Type I or Type II interactions. In particular, the angled-tuned Type II interaction shows a wide thermal bandwidth (^25° C-cm) allowing room temperature phase matching with minimal self-heating effects, and a large angular bandwidth (15-68 mrad-cm) which, combined with the high damage threshold, allows very tight focussing for maximum second harmonic generation. The spectral phase-matching bandwidth is estimated to be 0 5,6 A-cm (see ref. (23))* For the optimum phase-matching operation for

SHG at 1.06ym as calculated by Yao and Fahlen (24) and used in this work

(see later) the effective nonlinear coefficient is d ££(II) = 17.7 x 10“ 9 ert e.s.u. and the beam walk-off angle is minimal (0 .2 6 2 °).

This combination of properties has allowed (25) average second harmonic powers (at 100MHz) as high as 2,5 Watt in 50ps pulses to be obtained from a 10W, 70ps fundamental (average power conversion of 25%).

Intracavity doubling in a kHz repetition rate Q-switched YAG laser has resulted (26) in 5.6W of second harmonic power being generated, limited 87 -

only by the laser power supply. In the following sections we describe

the construction of a doubler unit designed to utilise these properties,

and evaluate its performance in conjunction with the Quantronix 116 mode-

locked c.w. Nd:YAG laser.

5.3 Design of the Doubler unit

Fig 5.1 Photograph of KTP frequency doubling unit showing micrometer rotational adjustments and focussing and collimating lenses

The 3mm x 3mm x 5mm crystal of KTP (Airtron/Litton Systems) was supplied by the manufacturer cut for Type II angle phase-matched second harmonic generation with the input face at 26° to the crystallographic 88 -

X-axis. Details of the optimum phase matching parameters for Type II frequency doubling in KTP (24) did not become available until the completion of this work, so the home-made crystal mount was designed to be versatile, allowing independent micrometer-controlled rotation about 3 perpendicular axes - one along the beam propagation direction and two at right angles to it. An additional micrometer adjustment allowed alteration of the height of the crystal from the optical rail. The crystal itself was mounted in a rotatable perspex cylinder, allowing direct passage of the fundamental and second harmonic beams, and was lightly held in place by a nylon spring-loaded plunger within the body of the cylinder.

The completed mount and emerging second harmonic beam are shown in the colour photograph (fig 5 .1 ) together with the focussing and collimating lenses. Each lens had an independent height adjustment and was mounted on a translation stage (Micro-controle type MR 50.16) for optimisation of the focussing. The whole assembly was mounted on a dural base-plate, on the same optical rail as the Nd:YAG laser for stability.

A pair of dichroic harmonic separator plates (JK Lasers Ltd) on independent mirror mounts in a "dog-leg" arrangement (see experimental set-up in fig 5 .7 ) allowed for separation and direction of the emerging fundamental and second harmonic beams.

The lenses chosen for focussing and collimating were plano-convex of

50mm focal length (Ealing Optics) mounted with the curved surfaces away from the crystal. This was important, in the case of the input lens, in minimising the possibility of feedback into the YAG laser cavity, which can seriously degrade the mode-locking (27). Both the input and output lenses were anti-reflection coated on both sides - the input lens for

1.06 ym (< 1 % reflectivity each face) and the output lens for 5 3 2 n m ( < 1 £ reflectivity each face).

The 50mm focal length input lens was estimated to produce a ^50 ym - 89

diameter focussed spot when the input lens was positioned at ^ O c m from the YAG output coupler (YAG laser beam divergence = 2.2mrad, beam diameter at lens 1,4mm), This focal length was chosen to give a power density at focus (up to ^50MW/cm2) safely within that which would cause optical damage ( 250MW/cm2 ) and was consistent with the beam being focussed well within the crystal acceptance angle with a near-optimum confocal beam parameter (optimal or ’’confocal” focussing (2 8 ) occurs when

Z o = confocal beam parameter = irW02 n / X = 1/2 w h e r e Z is the crystal length, W0is the beam waist, n is the refractive index and X is the wavelength. Our Z/2 is 2,5mm and Z q -3*3mm). We will show in the next section that the estimated focal spot diameter is consistent with the value of the constant obtained by other workers (2 3 ) in the small signal expression for phase-matched second harmonic power. Subsequent published work (15) has shown that we were slightly over-cautious in choosing these lenses, and could safely have used a 40mm focal length instead, to produce focal power densities M00MW/cm 2 with an increase in generated harmonic power (using KTP) to %1W.

The fast diode laser pulse monitor, built into the laser head as mentioned in the previous chapter, proved very important in ensuring that the laser' output was free from relaxation oscillations before being focussed into the crystal, which could otherwise have led to catastrophic damage as we were working within an order of magnitude of the damage threshold.

5.4 Characteristics of the Second Harmonic Generation

i) Phase-matching and angle tuning

As mentioned previously, details of the optimum phase-matching parameters for type II angle-tuned second harmonic generation in KTP were not published until the completion of the work described here, although phase-matching information had been included with other physical - 90 -

parameters of the material in previous publications (9,23). The crystal mount was designed to be suitably versatile, therefore, that we could determine the best phase-matching conditions "empirically" for ourselves.

\ \

BEAM

Fig 5.2 Optimum phase-matching parameters for type II angle-tuned SHG of 1.06pm wavelength in KTP, as calculated by Yao and Fahlen (24). Crystal cut is that specified by the manufacturer

Fig 5.2 shows the optimum phase-matching parameters, as subsequently obtained theoretically by Yao and Fahlen (24), to be Type II operation in the crystallographic X-O-Y plane with 0 (angle between wave normals and the Z-axis) = 90 0 and (angle from the X-axis in the X-Y plane) = 21.3°.

The input (linear) 1.06pm polarisation is at 45° to the plane of the paper in the plan view at left. The crystal cut is that described in data supplied by the manufacturer ( 2 9 )(Litton/Airtron), the entrance and exit faces being perpendicular to the X-Y plane with the normal to the 91

input face being at 26 ° to the crystallographic X-axis as measured in the

X-O-Y plane. The experimental optimum we had previously determined

o o differed from this value by no more than 3 an d f r o m 0 by less than 2

(measured as angles internal to the crystal), however the crystal was oriented spatially so that the input polarisation from the Nd:YAG laser was maintained vertical. The polarisation of the emerging second harmonic beam was thus horizontal, suitable for pumping a dye laser with a vertical jet-stream, A detailed investigation of the effect of phase mismatching on the second harmonic generation efficiency was not undertaken, and both detailed experimental (23) and theoretical (24) results on this topic are now available. Figure 5,3 will, however, give an indication of the sensitivity of the second harmonic power to angle tuning (measured external to the crystal).

ANGLE FROM HORIZONTAL OF INPUT FACE NORMAL! •)

Fig 5.3 SHG efficiency vs. external tuning angle - 92 -

ii) Power and pulsewidth characteristics

During the course of this work the average second harmonic power (measured with a Coherent Model 210 Optical Power Meter) obtained from the doubler unit was in the range 500-600mW (at 100MHz) for a typical average pump power in the range 6£-7W. With new krypton arc lamps in the laser this increased to 650-700mW for a pump power 7-7. 5W.

The best result obtained was 800mW for a pump power of 7-5 Watt. Average power conversion efficiencies were thus usually in the range 8 - 1 0 $ w h i c h compares well with that typically obtained with KTP by other workers (15)

(although the highest conversion efficiency reported is 25$ for a 10W fundamental with tighter focussing (25)).

■100 ps

—H200psf-—

Fig 5.4 Synchroscan streak intensity profiles of (a) the shortest and (b) typical second harmonic pulses from the mode-locked c.w. Nd:YAG laser. Average power was 600mW in (a),

550mW in (b) - 93 -

Pulsewidth measurements for the second harmonic were performed using an S20 photocathode Photochron II Synchroscan streak camera in its standard configuration, where a fraction (^4$) of the unconverted

fundamental was used to trigger the tunnel diode oscillator and thus provide the synchronous repetitive scan for the camera. The shortest (a) and typical (b) recorded second harmonic pulse Synchroscan streak intensity profiles are shown in figure 5.4, The streaks in 5.4(a) are the shortest 532nm pulses obtained to date directly from a mode-locked

c.w. Nd:YAG laser, with average power (600mW in this case) high enough to synchronously pump a c.w, dye laser. They represent a factor of two reduction over the transform-limited 6lps duration 5l4nm Ar ion laser

pulses of similar average power considered in Chapter 3* Production of

these short pulses is aided by the ^2 pulsewidth reduction (with Gaussian pulses) of the second harmonic as compared to the fundamental, incurred

in extracavity second harmonic generation when the spectral phase­ matching bandwidth of the frequency-doubling crystal is not a

. 6 restriction. The fundamental bandwidth in this case Cv-0,3 A, see previous chapter) is well within the spectral band-pass of the crystal o o which has been shown to be 5.6 A-cm (23) (i.e. 11.2 A for a 5mm crystal).

The ^2 pulsewidth reduction is a result of the quadratic nature of the

second harmonic generation process (shown in the small-signal expression

for SHG conversion efficiency given later). The streaks in fig 5-4 are

reduced in width by close to this predicted factor if compared to the

shortest and typical streaks of the fundamental (1.06ym) pulses shown in

the previous chapter (Fig 4.5).

Auschnitt and Jain (30) claim that the dye laser pulsewidth in

synchronous mode-locking scales as the square-root of the pump pulsewidth

(with other factors - pump energy, pulseshape, cavity length etc. being

equal), so we might readily expect to generate sub-picosecond duration

dye laser pulses with our short pumping pulses. In addition, Clemens et - 94 -

al (15) have shown that a reduction in pulse-timing jitter should result from the use of shorter pump pulses (claimed to be proportional to the pump pulsewidth other factors being equal). It should be noted, however, that the pulse-width typically obtained from the doubled YAG laser is not significantly smaller than that obtained from a well-optimised noble gas ion system (31 ) (with particular reference to the Ar ion 5l4nm line).

Next we consider the effect of Nd:YAG laser cavity length detuning on the second harmonic conversion efficiency. Average second harmonic powers and Synchroscan streak intensity profiles showing the recorded frequency-doubled pulsewidth are shown as a function of the cavity length detuning from optimum (in iim) in figure 5.5. The fundamental pump power was 6.5W average in each case.

200 ps I-

Fig 5.5 Effect of cavity length detuning on second harmonic pulse duration and average power

We see that a cavity length detuning of as little as 10ym is enough to double the pulsewidth and decrease the average second harmonic power by roughly a factor of two. It is thus very important to maintain the - 95

pump laser in a stable mode-locked condition. We note that pulsewidth doubling for a 10ym detuning (in a 1,5 metre cavity) corresponds to a mode-locked driving frequency detuning of 'v-BSOHz in 50MHz doubling the

pulsewidth, which is consistent with the result obtained in the previous chapter (fig 4.6),

The small-signal expression for phase-matched second harmonic peak power P(2io) as a function of fundamental peak power P( uj ) is (28):

P(2w) = K£2P(aQ2 (5.1) A

where £ is the crystal length and A is the focussed spot area. We see

that this is consistent with our results, because the average fundamental

power stays constant as the cavity length is detuned by 10ym, whereas

(from the measured widths of the second harmonic pulses) the pulsewidth

doubles i.e. P( oj) is halved. Therefore we expect, from above, the peak

power at the second harmonic to be reduced by (£)2 = |, which it is

because the pulsewidth doubles and the average power halves. With our knowledge of the peak powers at the fundamental and second harmonic and of the crystal length (£ = 5mm) and using K = 1.1 x 10“8W -1 which is the appropriate value obtained by Liu et al (25) for this phase-matching condition in KTP, we estimate a focal spot diameter of 45ym, which

compares well with that obtained previously (with an estimated

fundamental power density at focus of 48MW/cm2).

Finally,we consider the stability of the second harmonic pulses. On a timescale up to tens of milliseconds, the laser was highly stable.

Figure 5.6 shows a storage oscilloscope (Tektronix 7834) traces of the

(a) fundamental, (b) second harmonic and (c) dye laser pulsetrains as monitored with a fast (Telefunken BPW 28) photodiode with 20ms/major

division. All three pulsetrains can be seen to mirror directly the - 96 -

Fig 5.6 M o d u l a t i o n o n (a) fundamental, (b) s e c o n d h a r m o n i c and

(c) dye laser pulsetrains. Modulation amplitude is 2%

peak-to-peak in (a), 4% in (b) and 8% in (c)

ripple on the YAG laser lamp line drive (fig 4.7). Peak-to- peak - 97 -

amplitude variations are 2% in (a), 4 % in (b) and Q% in (c) ((c) typically 8-10/6). The doubling in fluctuation amplitude from (a) to (b) is consistent with the dependence of second harmonic peak power on the square of that at the fundamental. This modulation is magnified in (c) due to the dye laser operating (relatively) close to threshold.

Inclusion of the choke described previously to reduce the ripple in the line drive would be expected to result in 1$ peak to peak fluctuation for

(a), 2% for (b) and ^4$ for the dye laser.

On a timescale ^ tens of seconds, however, there was considerable

fluctuation in the second harmonic power resulting from irregular

"dropping out" of the pump laser (occasional large variations in output

intensity). This was probably of mechanical origin because the optimal harmonic generation and shortest pulses occurred nearer an extreme of the

alignment (see previous chapter). This led to considerable fluctuations

in dye laser output over the timescale in which autocorrelation traces were recorded ( ^ minutes), as will be seen in the next section, and

probably also prevented proper optimisation of the dye pulse duration.

5.5 The Rhodamine 6G synchronously pumped c,w, dye laser

A schematic of the experimental dye laser arrangement and pulse

diagnostics is shown in figure 5.7. The dye laser system employed a

conventional astigmatically compensated linear cavity with a Brewster

angled quartz prism tuning element (P). This arrangement was chosen to

facilitate direct comparison with our Ar ion-pumped (Spectra Physics Ar

ion Model 164) synchronously mode-locked c.w. dye laser, which has been well characterised previously (32,33) and shown to be readily

rearrangeable into a high performance ring dye laser (34) (generating

pulses as short as 0.7ps) essentially by slight tilting of the end _ 3 mirrors. The vertical jet- stream consisting of a 10 M solution of

Rhodamine 6G in ethylene glycol flowed at the common focus of mirrors M2 - 98 -

and M3 (both of 10cm radius of curvature), and mirror M2 served the dual purpose of focussing the horizontally polarised 532nm pump beam into the

Fig 5.7 Schematic of dye laser arrangement and pulse diagnostics

jet. The dye was circulated from a cooled temperature - stabilised

reservoir (T = 15°C) by a micropump, through an in-line filter (Balston

microfibre, 2ym pore size), a surge reservoir (to minimise pressure

fluctuations) and out through a stainless steel nozzle (Coherent

Radiation Inc.), to produce a stable MOOym thick jet with flow velocity

up to 10ms-1. All mirrors used were Broad Band High Reflectivity for the

visible (reflectivity, R ^ 100^, 500-700nm) except for the plane output

coupler M4 which had R ^ 95% near 590nm. The other plane mirror M1 was

mounted on a translation stage (Microcontrole MRL 80-25) permitting

differential micrometer cavity length adjustments ^lym to be made. A

perspex enclosure housing the jet-stream and folded section helped to

minimise instabilities due to air draughts. With a typical 532nm pump

power ^ 500mW, the average dye laser output power at 585nm was in the - 99 -

range 25-30mW. The complete mode-locked tuning curve is shown in figure 5.8. This is similar to that obtained with the Model 164 Argon ion laser synchronously pumped system with a 5% transmission output coupler (33), except that it cuts off slightly sharper than expected at long wavelengths (which usually extends beyond 620nm) most likely due to the transmission characteristic of the particular output coupler used.

Fig 5.8 Mode-locked c.w. R6G dye laser tuning curve

The shortest recorded Synchroscan streak intensity profiles of the output dye laser pulses are shown in the next figure (fig 5.9). With a measured full width half maximum of 7.7ps, these are the shortest streaks obtained from a synchronously pumped c.w. dye laser where the pump pulses are used to trigger the streak camera drive electronics (compared with

9-1 Ops in the Ar ion case (3D) and are close to the optimum so far observed in conjunction with the synchronous pumping (6.5ps (35) using dye laser pulse triggering). It is also noteworthy that this high temporal resolution appeared markedly easier to obtain than in the noble 100 -

gas ion laser pumped case. This effect is likely to be due to the shorter pump pulsewidth ( ^50ps) and the high stability over the ^second recording time more accurately defining the temporal registration of the camera sweep, and reducing the pulse timing jitter characteristics of the dye laser. (Dye laser pulse triggering did not result in an improvement due to the instabilities mentioned previously.) The overall timing jitter characteristics of synchronously pumped c.w. dye lasers however, prevent a direct measurement of the ^1ps pulse duration using the

Synchroscan streak camera technique.

100 ps

Fig 5.9 Shortest recorded Synchroscan streak intensity profiles of the dye laser pulses. Streak camera drive electronics triggered by pump laser second harmonic

We therefore used second harmonic generation autocorrelation in a

0.5mm thick crystal of ADP cut for angled-tuned phase matching at 600nm, as previously described (Chapter 2). The pulsewidths were calculated 1 0 1

from the autocorrelation full width half maximum assuming a Gaussian pulse shape. A typical autocorrelation showing the necessary contrast ratio for complete mode-locking is shown in figure 5.10(a) together with

the corresponding spectral bandwidth (b)(obtained using a Monospek 1000 spectrograph and recorded on a photographic plate, spectrally calibrated using a mercury lamp). The measured pulse duration is 1.6ps and the

. o bandwidth 4.2A giving a time-frequency bandwidth product AvA t = 0.6 i.e. operation above the Fourier transform limit (= 0.44 for Gaussian pulses). Typical pulse durations were in the range 1 - 2ps, the shortest recorded being 1.15ps, which is similar to those obtained from the Ar ion-pumped dye laser system.

Fig 5.10 (a) Autocorrelation trace of 1.6ps pulse from dye laser

and (b) c o r r e s p o n d i n g spec t r a l b a n d w i d t h

The noisy wings of the autocorrelation were believed to be due to the instabilities mentioned earlier occuring over the long ( a. minutes) 1 0 2 -

recording time. On a cautionary note regarding the autocorrelation measurements, Van Stryland (38) has shown that a fluctuating signal in

second harmonic autocorrelation favours the shortest pulses and can lead

to "over optimistic” inference of the pulsewidths achieved.

5.6 Discussion

Similar performance characteristics to an Ar ion-pumped dye

laser system have been demonstrated using frequency-doubled YAG laser

pumping, but the potential improvements have not been shown conclusively.

This work should probably be regarded as a preliminary investigation

however, and the timing-jitter reduction, for example, leading to a

shorter recorded Synchroscan streak duration looks promising. Despite

the improvements described in the previous chapter, the stability of the

pump laser was still felt to be the factor limiting the dye laser

performance. The pump laser must be maintained more stably than its Ar

ion laser counterpart for equivalent performance because of the

exaggeration of fluctuations that is inherent in the second harmonic

generation process. In addition, in an attempt to obtain the highest

harmonic conversion in our collinear doubling arrangement the pump laser

rear mirror tended to be used for alignment of the pump beam, and this

led to instabilities. A future improvement would be the direction of

the pump beam into the doubling crystal using an independent beam

steering mirror. Tighter focussing into the crystal (up to power

densities ^100MW/cm ) should result in second harmonic average powers

>1.0 Watt. We have already demonstrated acousto-optic mode-locking of

the pump laser using the 3rd harmonic of the r.f. drive frequency (see

Chapter 4) and the short pulses produced with good alignment when this

system is optimised should help in producing these high optical second

harmonic conversion efficiencies.

Johnson and Simpson (36) have recently demonstrated average second 103 -

harmonic powers of 1.5 Watt for a 6 Watt fundamental using second harmonic acousto-optic mode-locking. This power is higher than that obtained from the larger model mode-locked Ar ion lasers ('MW at 5l4nm) and has allowed fibre/grating pulse compression (37) of 532nm pulses to

0.4ps with enough average power (150mW) to synchronously pump a c.w. dye laser. Short (0.3ps) dye laser pulses were produced directly by this method where no improvement over Ar ion-pumping had been observed previously using the uncompressed (33ps) pumping pulses (36).

5.7 Conclusions

Attractive features of frequency doubled YAG as opposed to Ar ion laser pumping of dye lasers have been identified, particularly the synchronisability to Nd:YAG laser-pumped amplifier systems (14) and the high average mode-locked pump power available. The latter allows, for example, high power synchronised dual dye laser pumping and, especially, fibre/grating compressed pulse synchronous pumping of c.w. dye lasers.

It is certainly true, as other authors have claimed (14,15), that shorter pulses can be obtained from the doubled mode-locked c.w. Nd:YAG compared with those from the 5l4nm line of the Ar ion laser. However, the second harmonic pulsewidths typically obtained (50-70ps) are in the range available from a well-optimised ion laser system. We should therefore not expect a dramatic improvement in pulsewidth from the YAG-pumped system (because it scales only as the square root of the pump pulsewidth

(30)) but would perhaps be more likely to see an improvement in the time-jitter characteristics of the pulses (15).

Noble gas ion lasers have an advantage in allowing, in addition, c.w. pumping, particularly of passively mode-locked dye lasers. An investigation into the c.w. power produced by intracavity second harmonic generation using KTP in the un-mode-locked Nd:YAG laser however, could possibly provide similar c.w. pumping capabilities in the green wavelength region. - 104 -

REFERENCES

(1) M.C. Adams, D.J. Bradley, W. Sibbett and J.R. Taylor; Phil. Trans.

R. Soc. Lond. A298 (1980) 217

(2) R.K. Jain; Proc. SPIE. 322 (1982) 2

(3) W.H Glenn, M.J. Brienza and A.J. DeMaria; Appl. Phys. Lett. _12_

(1968) 54

(4) B.H. Soffer and J.W. Linn; J. Appl. Phys. 39_ (1968) 5859

(5) J.N. Eckstein, A.I. Ferguson, T.W. Hansch, C.A. Minard and

C.K. Chan; Optics Comm. _27 (1978) 466

(6) S.J. Heising, S.M. Jarrett and D.J. Kuizenga; Appl. Phys. Lett. J_8

(1971) 516

(7) J. Kuhl, H. Klingenberg and D. von der Linde; Appl. Phys. Lett. 25

(1979) 514

(8) G.W. Fehrenbach, K.J. Gruntz and R.G. Ulbrich; Appl. Phys. Lett. ^33

(1978) 159

(9) F.C. Zumsteg, J.D. Bierlein and T.E. Grier; J. Appl. Phys. _47_

(1976) 4980

(10) J. Hoffnagle, L.Ph. Roesch, N. Schlumpf and A. Weis; Optics Comm.

42 (1982) 267

(11) P. Bado, C. Dupuy, K.R. Wilson, R. Boggy, J. Bowen and S. Westra;

Optics Comm., 4_6 (1983) 241

(12) K. Kato; IEEE J. Quant. Elect. QE-16 (1980) 1017

(13) Technical Specifications for Quantronix Model 416 and Spectra-

Physics Model 3000 NdrYAG lasers

(14) T. Sizer II, G. Mourou and R.R. Rice; Optics Comm. 37 (1981) 207

(15) J.M. Clemens, J. Najbar, I. Bronstein-Bonte and R.M. Hochstrasser;

Optics Comm. 47 (1983) 271

(16) C.B. Hitz and L.M. Osterink; Appl. Phys. Lett. 18 (1971) 378 105 -

(17) Yu.D. Golyaev, S.A. Grodskii, V.G. Dmitriev, P.G. Konvisar,

S.V. Lantratov, V.Yu. Mikhailov and S.R. Rustamov; Sov. J.

Quant. Elect. 12 (1982) 1360

(18) A.E. Siegman and J-M. Heritier; IEEE J. Quant. Elect. QE-16 (1980)

324

(19) J.E. Murray, R.J. Pressley, J.H. Boyden and R.B. Webb; IEEE J.

Quant. Elect. QE-10 (1974) 263

(20) M. Bass and H.H. Barrett; IEEE J. Quant. Elect. QE-8 (1972) 338

(21) T. Sizer II, J.D. Kafka, A. Krisiloff and G. Mourou; Optics Comm.

39 (1981) 259

(22) G.A. Mourou and T. Sizer II; Optics Comm. 4J_ (1982) 47

(23) Y.S. Liu, L. Drafall, D. Dentz and R. Belt "Nonlinear Optical

Phase-matching properties of KTiOPOi/’; G.E. Technical

Information Series Rep. 82 CRDO 16 (Feb. 1982)

(24) J.Q. Yao and T.S. Fahlen; J. Appl. Phys. 55 (1984) 65

(25) K.C. Liu, G. Vaillancourt, M.G. Cohen and R.N. Fleming; Paper ThB3

Conference on Lasers and Electro-Optics (June 1983, Baltimore,

Maryland) Technical Digest

(26) Y.S. Liu, D. Dentz and R. Belt; Opt. Lett. £ (1984) 76

(27) M. Hassinger and F.E. Lytte; Optics Comm. 4j3 (1983) 125

(28) See for example A. Yariv "Introduction to Opticl Electronics"

2nd Ed. (Holt Rinehart and Winston, 1976) p .211

(29) Litton/Airtron Data Sheet on KTP crystal cut (supplied with the

crystal)

(30) C.P. Auschnitt and R.K. Jain; Appl. Phys. Lett. 32_ (1978) 727

(31) P.G. May, W. Sibbett, K. Smith, J.R. Taylor and J.P. Willson;

Optics Comm. 42 (1982) 285

(32) J.P. Ryan; PhD Thesis, University of London (1978)

(33) D. Welford; PhD Thesis, University of London (1980)

(34) P.G. May, W. Sibbett and J.R. Taylor; Appl. Phys. B 26_ (1981) 179 106 -

(35) K. Smith (Imperial College); private communication

(36) A.M. Johnson and W.M. Simpson; Opt. Lett. _8 (1983) 554

(37) A.M. Johnson, R.H. Stolen and W.M. Simpson "Ultrafast Phenomena IV"

Springer Series in Chemical Physics 3.8 Ed. D.H. Auston and

K.B, Eisenthal, (Springer Verlag, 1984)

(3 8 ) E.W. Van Stryland; Optics Comm. 31 (1979) 93 107 -

C H A P T E R VI

THE Q-SWITCHED/MODE-LOCKED C.W. Nd:YAG LASER

6,1 Introduction

Simultaneous Q-switching and mode-locking of a c.w.-pumped

Nd:YAG laser was first reported by Kuizenga et al in 1973 (1). In this technique, the laser is Q-switched in a repetitive manner while at the same time being mode-locked by an active modulator. This results in repetitive Q-switched envelopes, containing short mode-locked pulses temporally separated by the laser cavity round-trip time. The highest peak power and the shortest and most stable mode-locked pulses are obtained when the laser is allowed to prelase at a level close to threshold prior to emission of each Q-switched pulse. In this manner, mode-locked pulses of M OO picoseconds duration with peak powers M M W at the maximum of the Q-switched train have been achieved, at repetition rates up to MkHz (2).

This type of laser has recently found application as a pump source for fibre-Raman lasers (3) and high repetition rate (500Hz) amplifiers for sub-picosecond light pulses (4). It has also been used to produce picosecond pulses in the 1200-l600nm wavelength range, within which silica optical fibres show attenuation and dispersion minima, by difference frequency mixing with mode-locked dye laser pulses (5).

Both the second (532nm)(6) and third (355nm)(7) optical harmonics can be efficiently generated, showing promise as pump sources for moderate energy, tunable picosecond dye lasers covering a wide spectral range.

In the next chapter, we describe an additional use of the Q-switched and mode-locked (QSML) c.w. Nd:YAG laser, in generating phase conjugate waves via degenerate four wave mixing in silicon and organic dyes. 108 -

In view of these and other applications, a detailed study of the temporal characteristics of the QSML pulses using the linear streak camera technique has been undertaken, and the results are described in this chapter. Various aspects of the behaviour of the QSML system, previously only inferred from auto-correlation data, have been directly confirmed and the applicability of the Synchroscan streak camera as a

"real time" diagnostic of the QSML pulses has been demonstrated (29).

6.2 Q-switching of pulsed- and c.w.-pumped lasers

Q-switching (8,9) is a method which has been widely used for the generation of short (10's - 100's of nanoseconds typically) high peak power laser pulses, particularly from solid state laser systems

(10). It is so-called because the optical Q of the resonant cavity, defined as the ratio of the energy stored in the cavity to the energy loss per round-trip, is altered when this technique is used. The principle of the technique is as follows. If the cavity Q is lowered to prevent laser emission, then energy stored in the amplifying medium by optical pumping can reach a level much higher than the threshold for normal laser action. The time for which the energy may be stored is approximately the fluorescence lifetime of the upper level of the laser transition ( ). This is the reason for the particular applicability of this technique to solid state laser systems, where this lifetime is often long (10) ( - 230 ys for Nd:YAG). When a high cavity Q is restored, the laser has a gain far in excess of the losses and the stored energy is discharged very quickly. Provided that the high Q is restored rapidly compared to the build-up time of the laser pulse (fast

Q-switching) then a single, short, high peak power "giant pulse" r e s u l t s .

Q-switching has been achieved in practice by a variety of techniques, including electro-optic shutters (11,12), mechanical 109 -

methods involving rapid rotation of one laser end mirror (13,1*0, and passive shutters using organic dyes showing saturable absorption at the laser wavelength (15,16). For continuously-pumped repetitively Q- switched systems, such as the one considered in this chapter, travelling wave acousto-optic Q-switches have been preferred (17).

This is due to their low passive insertion loss and the high single pass dynamic loss achievable. Such Q-switches may also be operated at high repetition rates (>50kHz), The particular Q-switch used in our system is described in more detail later in the chapter.

The c.w.-pumped Nd:YAG laser repetitively Q-switched by an acousto-optic loss modulator was first described by Chesler et al (17).

Alternative techniques for repetitive Q-switching are considered in ref. (18).

In these laser systems, the population inversion varies in a cyclic manner with time as shown in fig. 6.1(a)(10). Between successive Q-switches the population inversion rises from a value n ^ to a value n^, under the influence of continuous pumping and spontaneous decay. The inversion saturates at a value n^ dependent on the pump input power, only at repetition rates small compared to 1/ t ^. The temporal development of the Q-switched pulse is illustrated in fig. 6.1(b)(19).

The theory of laser Q-switching, using a rate equation approach, has been given by Wagner and Lengyel (20) and has been extended to repetitively Q-switched c.w.-pumped Nd:YAG lasers by Chesler et al(17).

They showed that at repetition rates below ^800Hz the Q-switched pulse peak power is independent of repetition rate because there is sufficient time between pulses for the inversion to build up to n^ .

The width of the Q-switched pulse also remains constant within this range, so the average power increases in proportion to the repetition rate. 110 -

As the repetition rate is increased in the range 800Hz - 10kHz the peak power falls off rapidly because the inversion cannot build up to n^ , and the pulsewidth also begins to increase due to the reduction of gain. The average power thus increases rapidly and approaches the rated c.w. power above approximately 10kHz. Confirmation of this behaviour for our laser system is given in section 6.4. Chesler et al also showed that the optimum output coupler transmission in repetitively Q-switched c.w,-pumped lasers is close to that for purely

c.w. operation. Thus if the Q-switch exhibits low enough passive

insertion loss, it can be retained in the cavity during c.w. operation allowing convenient change from c.w. to Q-switched regimes.

Fig 6.1 Population inversion (a) and pulse evolution (b) in a c.w.-

pumped repetitively Q-switched laser. After Koechner (10) Ill -

6.3 The travelling wave acousto-optic Q-switch

The travelling wave acousto-optic Q-switch (Quantronix Model

351) used in this work was similar to the acousto-optic mode-locker described in Chapter 4, operating in the Bragg regime, except that it was a single-pass device. That means that instead of the quartz sub­ strate forming an acoustic resonator, it was tapered opposite the face at which the acoustic waves were generated and terminated by an aluminium block. Thus the acoustic wave produced by the transducer was absorbed after travelling across the interaction medium. This allowed rapid switching of the resonator Q value and provided no constraints on

Q-switch repetition rate (10). The r.f. power (up to 10W) used to drive the Q-switch was derived from the (50MHz) crystal oscillator described in chapter 4, amplified using the commercial Q-switch electronic control and power amplifier unit. The power splitter

(figure 4.9) provided suitable isolation between the oscillator outputs to the mode-locker and Q-switch such that the mode-locker drive was unaffected by the Q-switch being either on or off. The active diffraction loss introduced by the Q-switch was estimated to be 5% in a single pass per r.f. Watt applied (21), for the optical polarisation perpendicular to the direction of acoustic wave propagation. An r.f. drive typically > 6W provided enough dynamic loss to hold off laser action.

The control unit allowed a repetitive modulation signal to be applied to the r.f. driver to periodically turn off the r.f., thus removing the loss and allowing a Q-switched pulse to develop. This modulation signal could be operated in a variety of modes, including single-shot (where a single Q-switched pulse is produced) and externally gated operation. Throughout most of the work reported here the modulation signal was triggered internally by the control unit, which had a digiswitch to allow the Q-switch pulse repetition rate to - 1 1 2 -

be adjusted in the range 0-50kHz (in units of 0,1kHz). The r.f. off- time of the modulation signal could be varied from 1-11ys. Optimum operation at the repetition frequencies used in this work (up to ^20kHz usually) was achieved with an off-time of 7 ys. The low passive

insertion loss (estimated to be <0.5% per pass (21)) and the minimal

effect of the undriven Q-switch on mode-locked c.w. performance of the

laser is illustrated in figure 6.2. The Synchroscan streak intensity

profiles of the optical second harmonic pulses have a full width at

half maximum of 56ps. This is equivalent to that typically obtained

without insertion of the Q-switch and the corresponding average

fundamental power was measured to be 7W, which compares well to typical

results reported in Chapter 4.

200 ps ^ -----

532 nm

Fig 6.2 Synchroscan streak intensity profiles showing 56ps pulses at 532nm obtained with the Q-switch in the laser cavity (undriven). Power in the fundamental was 7W 113 -

6.4 Q-switched (un-mode-locked) operation

With the acousto-optic mode-locker undriven, the performance of the laser was first evaluated in purely Q-switched operation. To first order, the Q-switching and mode-locking processes are independent

Fig 6.3(a) Peak and average power versus Q-switch repetition rate for Q-switched operation only (no mode-locking). No prelase present in this case

Fig 6.3(b) Q-switched pulsewidth vs. repetition rate - 114 -

of each other (22), so in this section we are essentially studying the

b e h a v i o u r of the envelope of the QSML pulses considered later.

The Bragg angle of the Q - s w i t c h was adjusted for optimum

diffraction efficiency and the r.f. power increased (to > 6W) to hold

off laser action between Q-switched pulses. Semi-logarithmic plots of

the Q-switched peak power, average power and pulsewidth obtained as a

function of Q-switch repetition rate are given in fig. 6.3(a) and (b).

These results are in excellent agreement with the predictions of

Chesler et al (17) given in section 6.2. Below 800Hz we see that the

pulsewidth and peak power remain (approximately) constant due to there

being enough time between successive Q-switches for the gain to recover

to saturation. Thus in this region the average power increases

approximately linearly. At higher repetition rates where the gain

cannot recover, the peak power falls off rapidly and the pulsewidth

increases, so that the average power rises quickly and approaches the

c.w. output at MOkHz.

The maximum peak power obtained (MOkW) with minimum pulsewidth

(170ns) is close to specification for the laser system (22). The

single pulse energy in this case was ^2mJ. An oscillogram of such a

pulse (recorded using an S1 photocathode vacuum photodiode and

Tektronix 7904 oscilloscope) is given in figure 6.4(a). The pulses

were observed to have a peak-to-peak stability of better than 5%.

When the r.f. drive to the Q-switch was reduced (to typically

^5.5W) such that laser action was not completely held off between Q-

switched pulses, some interesting effects were observed. The "pre-

lasing” thus occuring between Q-switched pulses began typically ^250ps

after the preceding Q-switched pulse with rapidly damped relaxation

oscillations, followed by c.w. emission close to threshold (see fig.

6.7, inset (c)). The Q-switched pulse obtained with prelasing at 800Hz

repetition rate is shown in figure 6.4(b). Even though the acousto- - 115 -

Fig 6.4 (a) Shortest recorded Q-switched pulse, FWHM = 170ns.

(b) "Mode-locked" substructure appearing when prelase

present.

Timescale lOOns/div in (a), 50ns/div in (b).

Repetition rate 800Hz in (a) and (b)

optic mode-locker was not driven, sub-nanosecond "mode-locked" pulses

separated by the 10ns laser cavity round trip time can be seen. It was

felt that possibly either the Q-switch was acting as a surrogate active

modulator or that "self-locking" effects were taking place during the

long K ms) prelasing period, producing incompletely mode-locked pulses

which then subsequently appeared within the Q-switched envelope. This was supported by the fact that this substructure disappeared at

repetition rates ^4kHz, which were high enough such that the next Q-

switched pulse appeared before the commencement of prelasing following

the previous pulse. Some modification to fig. 6.3(a) was also observed with prelasing, where the peak power did not begin to fall off until

^3kHz repetition rate, probably due to some of the gain which would

otherwise appear as relaxation oscillations and low level c.w. lasing helping to maintain the peak power as the repetition rate was

increased. In addition, the Q-switched envelope was slightly broader

at a given repetition rate, and the average power did not fall off so

rapidly at lower repetition frequencies. 116 ~

6.5 Simultaneous Q-switching and mode-locking

We begin this section with a brief review of the results obtained by Kuizenga et al (1) in their treatment of simultaneous Q-

switching and mode-locking in the c.w.-pumped Nd:YAG laser. They

examined the transient build-up of the mode-locking process from the

time the Q-switch is opened up until the time the actual Q-switched

pulse train occurs. It was shown that a number of roundtrips M were

required for the mode-locked pulse to evolve to within 5% of a steady

state pulsewidth, where:

M > ' “ ' ‘ ' * (6.1 )

Af being the atomic linewidth (180GHz for Nd:YAG), 0m the mode-locker modulation depth, f the applied r.f. drive frequency and g the round-

trip amplitude gain during the prelase period.

The dominant term in the above expression is Af/f , which for m

Nd:YAG is M 0 3-104, so that it takes on the order of a thousand to ten

thousand roundtrips for the mode-locked pulsewidth to approach the

steady state valuer or typically 10-100ys. The Q-switch pulse build-up

time (typically <10ys) without prelasing is shorter than this, so long

pulses are obtained with a lot of substructure. The pulses do get

shorter and less noisy as the Q-switch repetition rate is increased,

though, because the lower gain leads to a longer pulse build-up time.

However, we must use the prelasing technique described subsequently in

order to evolve short, stable, structureless pulses.

The experimental arrangement used to examine the QSML pulses is

shown in figure 6.5. This permitted simultaneous observation of the

pulse characteristics using a fast S1 vacuum photodiode/oscilloscope

(Tektronix 7904) combination and both single-shot and Synchroscan

streak cameras. Both cameras incorporated Photochron II streak image 117 -

tubes with S20 photocathodes (requiring the optical second harmonic to be generated for detection purposes) and a preset optical delay line was used for calibration of all streak data.

PRE POWER CALIBRATED AMP AMP OPTICAL DELAY I SYNCHROSCAN FREQUENCY OMA 0> STREAK — n DOUBLER CAMERA XI V . RAMP XTAL OSC P/DIODE jJ|GENERATOR f

QSML CW t . 1.06um Nd:YA0 LASER D GS SHG 5B2nm n XTAL ’v

Fig 6.5 Experimental arrangement used for examining the QSML pulses. GS is a g ating shutter used in single-shot measurements

The two distinct regimes of operation of the QSML c.w. Nd:YAG mentioned earlier can be obtained by fine adjustment of the active loss per pass introduced by the Q-switch (1). If the laser is maintained below oscillation threshold between Q-switched pulses, near oscilloscope limited "mode-locked” pulses are produced up to Q-switch repetition rates beyond 10kHz. Typically in this case (for our system) the Q-switched envelopes contained 35-40 mode-locked pulses having a round-trip time of 10 nanoseconds as shown in fig. 6.6(b).

Alternatively, in a preferred mode of operation, the laser is allowed to pre-lase at a level close to threshold prior to emission of the Q-switched pulse. This allows the continuously operating active mode-locker to establish short mode-locked pulses approaching the steady state (i.e. un-Q-switched) pulsewidth in the period preceeding - 118 -

the growth of the Q-switched pulse, which then proceed essentially unchanged in width through the subsequent Q-switching process.

Approximately 50 pulses were usually contained within the Q-switched

envelopes under these conditions, as shown in fig. 6.6(a).

Fig 6.6 QSML pulsetrains (a) with and (b) without prelasing at

800Hz QS frequency. Timescale 50ns/div in (a),

20ns/div in (b). Vertical calibration 50mV/div for (a) -

see calculation of peak power given later

The form of the appropriate prelase signal (fig. 6.7 inset (a))

consists of exponentially damped relaxation oscillations beginning

250-500ps after the Q-switched pulse in our laser (depending on the

loss generated by the Q-switch) followed by low level c.w. mode­

locking. (The limitations on repetition rate imposed by this technique

are further discussed later in the chapter.)

Kuizenga (23) has demonstrated that the form of this prelase

signal is a very good indicator of how well the laser is mode-locked.

With optimum mode-locking, the relaxation oscillations decay in a

smooth, regular manner as mentioned above (fig. 6.7(a)). For small

changes of cavity length from optimum (longer or shorter) the

relaxation oscillations become less regular and at a certain detuning

(^60pm in our laser) they become driven and continue through the entire

prelase period (fig. 6.7(b)). The form of the prelase waveform thus 119 - provides a relatively straightforward means of optimising the mode- locked pulses.

QS

h-~900ns — H

Fig 6.7 Measured amplitudes of QSML pulsetrain, relaxation

oscillations and c.w. mode-locked (un-QS) operation

relative to c.w. mode—locked prelase level.

Inset are prelase waveforms under various conditions

(a) optimum, (b) d r i v e n o s c i llations, (c) m o d u l a t o r off

and (d) higher order transverse modes (aperture removed).

Timescale lOOys/div in (a), 0.2ms/div in (b)-(d)

With the modulator off (as is appropriate to the previous section,

6.4) the relaxation oscillations are quite irregular (fig. 6.7(c)) which is explained by the fact that with no mode-locking all the axial modes above threshold are randomly phased and go through the relaxation

oscillations separately. Other laser problems can be diagnosed from

the prelase signal. For example (fig. 6.7(d)) when higher order

transverse modes are present (achieved with the aperture removed from 1 2 0 -

the laser cavity), the build-up time is longer for successively higher modes (due to the lower gain). Thus successive sets of relaxation oscillations appear on the prelase, as each mode goes above threshold.

Fig. 6.7 also gives the relative amplitudes of the Q-switched train,

relaxation oscillations and c.w. mode-locking, calibrated relative to

the prelase c.w. mode-locked level.

H

Fig 6.8 Microdensitometer traces of the second harmonic pulses from the Q-switched/mode-locked laser without prelase, showing the (a) 12th, (b) 25th, (c) 35th pulseshape in the train respectively and (d) a well-formed single pulse generated late (35th pulse) in the output train 1 2 1 -

We first consider the results obtained by examining the Q-switched mode-locked pulses with a single shot streak camera. Details of this streak camera system have been given in chapter 2. The micro­ densitometer traces of single-shot streaks which were obtained without prelasing at a 500Hz Q-switch rate are reproduced in figures 6.8(a)

(d). They confirm that the ’’mode-locked" pulses are really noise- bursts, with considerable substructure at these lower repetition

frequencies as had been inferred from auto-correlation data (1).

Single, double and multiple spikes were all observed, some of which had

quite short (<50ps) duration, the overall width of the noise-envelope

being a few hundred picoseconds (typically 200-300ps). This behaviour

is to be expected, because the build-up time of the Q-switched pulse is not long enough for short, structureless, steady state pulses to be

evolved, as discussed at the beginning of the section. However, by

examining pulses in the latter part of the train it could be seen that

there was less tendency for multiple spiking to occur and,

occasionally, well-defined pulses were established in this region (see

fig. 6.8(d)). Synchroscan streaks (see later) confirmed the random

nature of the pulse formation in time, because broad noisy pulses,

which were essentially many integrations of fig. 6.8(a)-(d), were

recorded.

Under the condition of prelase for the 500Hz Q-switch rate,

shorter (^70ps at 532nm), stable, structureless pulses were produced as

evidenced by the microdensitometer traces in figure 6.9(a)-(c).

Moreover, examination at different points in the train (fig. 6.9(a),

(b), (c) correspond to the 10th, 25th and 30th pulses of the train)

showed that the pulsewidths were relatively constant throughout the

train. This validates our pulse measurements using the Synchroscan

streak camera technique where the mode-locked pulses in each Q-switched

envelope are temporally overlapped and integrated on the streak camera - 1 2 2 -

phosphor to give a "real time" streak image.

H 200 ps

Fig 6.9 As in Fig. 6.8 but with prelase showing the (a) 10th, (b) 25th and (c) 30th pulse

In the usual Synchroscan technique, as described in Chapter 2, a repetitive deflection voltage is applied to the streak plates in synchronism with the mode-locked c.w. laser pulsetrain or laser-induced luminescence to be time-resolved. For these studies, however, the camera had a repetitive scan at the laser cavity round trip frequency

(100MHz) but luminous information was only accumulated during each QSML 123 -

pulsetrain at the repetition rate of the Q-switch. Thus the individual

pulses within each Q-switched envelope (and also during the prelase, where present) were superimposed with picosecond precision on the

streak camera phosphor and the information integrated at repetition

rates 'MkHz. When used with an optical multichannel analyser readout

(B & M Spectronik OSA) this provided a "real-time” linear picosecond diagnostic of the QSML pulses, together with a facility for direct

display and recording. The low level of illumination of the streak camera slit required to produce the streak images of figure 6.10 showed

that significant phosphor integration occurs at these 100’s Hz-kHz repetition rates such that image intensification is unnecessary, which

is not true for 10Hz streak camera operation (24). Careful measurements using calibrated neutral density (ND) filters showed that

the prelase makes an insignificant contribution to the recorded streaks at Q-switch repetition rates above 100Hz. This was important, because although the prelase is much less intense, it lasts for much longer and is therefore integrated to a higher degree on the camera phosphor. It may be seen from figure 6.7 that the initial relaxation oscillations were approximately four times as intense as the un-Q-switched mode- locked pulses. ND filters were placed over the streak camera slit and the beam suitably attenuated, such that pulses could barely be seen in c.w. mode-locked operation with the O.S.A. at its highest sensitivity.

Further ND’s were then used to reduce the intensity incident on the camera slit by a factor of a,4, and the laser adjusted for QSML operation. Clear, intense, streak images were then obtained showing the minimal contribution of the prelase.

A series of streak images and the corresponding prelase waveforms at various Q-switch frequencies are shown in figure 6.10. The larger spike or spikes at the appropriate repetition rate in the oscillograms are the Q-switched pulse envelopes. (Although not adequately - 124 -

Fig 6.10 Synchroscan streak intensity profiles for the second

harmonic of the QSML output with inset prelase waveforms

at v a rious Q— switch rates (a) 500Hzj (b) 1kHz, (c) 2kHz,

(d) 3kHz and (e) 3.3kHz. Time calibration is lOOys/div

for the oscillograms. Pulsewidth is 'WSps (a)-(d) and

a-9Ops for (e) w h i c h occurs at the first r e l a x a t i o n

oscillation 125 -

displayed, their full amplitudes are much larger than those of the

relaxation oscillations.) It should be noticed that the mode-locked

pulsewidth (^75ps at 532nm) only begins to increase at a frequency where the next Q-switched pulse occurs at the first relaxation

oscillation following the previous pulse. The precise frequency at which this occurs depends on the delay to the first relaxation

oscillation and therefore on the loss introduced by the Q-switch, but was typically ^3kHz (3.3kHz for the case shown in fig. 6.10(e), where

the Q-switched pulses are superimposed on the first relaxation

oscillations which occur slightly earlier). Therefore, sufficient

round-trips of the laser cavity occur even during this time before the

second relaxation oscillation so that the shortest pulses can evolve,

(The laser does not go below threshold between these initial relaxation

oscillations so that the mode-locked pulses do not have to evolve from

noise.) We can see that this is not unreasonable by substituting the

appropriate values 0 = 1 radian, f = 50MHz and Af = 180GHz into the m m

Kuizenga and Siegman expression given at the beginning of the section

for the number of round-trips required for the mode-locked pulses to

evolve to within 5% of the steady state (eqn. 6.1). In eqn. (6.1), g

is now the round-trip amplitude gain during the prelase period and is

given by (see Chapter 4):

6 2 g 5 £n Ul-DJ ( . )

where L now includes the cavity losses plus the active loss in the Q-

switch. We obtain a value 10 - 20ys from this, depending on the exact value of g chosen. From the oscillograms, the base-width of the first

relaxation oscillation has been measured to be MOps which is in

reasonable agreement with this estimate. 126 -

The steady state QSML pulses are, however, longer than those obtained from the laser without Q-switching (see fig. 4.5), although pulses as short as 83ps at 1.06ym have been observed, mainly because they develop in a regime of higher (active) loss. We have confirmed that the short pulses recorded at 532nm are reduced in width by a f a c t o r of ^2 compared to the fundamental by repeating the measurements with an S1 photocathode streak camera. The 1.06pm QSML pulsewidths typically obtained were MOOps. Substitution of the appropriate values for 0m> Af, fm and g into the Kuizenga and Siegman pulsewidth expression (25) given in chapter 4:

T (6.3) P

gives a typical pulse duration 100-120ps at the fundamental wavelength, which compares well to that obtained.

When the frequency was further increased (from that shown in figure 6.10) such that the next Q-switched pulse occured before the first relaxation oscillation, the pulses became broad and less regular.

We have directly confirmed, as concluded in (1), that these recorded integrated pulse envelopes become narrower and less noisy as the Q- switch frequency is increased (fig. 6.11) as a result of the lower gain giving rise to a longer build-up time for the Q-switched pulse, thereby allowing the mode-locking process to proceed closer to the steady state. An integrated profile of the noise-bursts occuring at sub-kHz repetition rates without prelasing is also included (fig. 6.11(a)) for comparison with the single-shot results shown previously.

Manufacturers data (2) set a typical upper Q-switch frequency limit ^800Hz for the optimum regime of short pulse high peak power

( ^ 1MW in 100ps at the peak of the Q-switched train) performance achievable with prelasing in a system of this type. This peak power 127 -

value has been confirmed as follows for our system using a calibrated

S1 vacuum photodiode. From fig. 6.6(a) we measure 175mV of signal at

h= 200 ps

/ \

V

b 218 ps

\

Fig 6.11 Synchroscan streak intensity profiles for QSML pulses

produced without prelase at (a) 700Hz, (b) 4kHz and (c) 25kHz

repetition rates. Second calibration pulse not included in

this case - 128 -

the maximum of the Q-switched train, with of the total output beam directed onto the diode. The sensitivity of the photocathode was measured to be 35yA/Watt at 1.04ym. Using Ohm's law therefore, we find our recorded pulse amplitude corresponds to a peak power of M k W and thus the total beam to MOOkW. If we then multiply by a factor M O for the 1ns response of the photodiode/oscilloscope combination to 100ps pulses, we obtain the peak power value MMW.

It is certainly true that the most stable output occurs at repetition rates up to 800Hz (measured to be ^ 1 peak-to-peak fluctuation at the peak of the Q-switched train). However, our streak results have shown directly that short pulses can be maintained at repetition rates exceeding 1kHz and we have observed that the corresponding decrease in stability need only be small (to as little as

3-4/6 peak-to-peak at the maximum of the train) if a stable prelase has been established before the repetition rate is increased.

We have found, in addition, (using the calibrated vacuum photo­

diode) that the power at the peak of the Q-switched train does not fall

off significantly at the higher repetition rates (up to MkHz), as was

also shown to be the case for purely Q-switched operation when

prelasing is allowed. Thus the high peak power is maintained, possibly

due to some of the inversion which would otherwise produce relaxation

oscillations and low-level c.w. oscillation helping to maintain the

peak power in the QSML pulses as the repetition rate increases.

6.6 Conclusions

Over a decade ago, Kuizenga et al (1) reported the first

investigation of simultaneous Q-switching and mode-locking in the c.w.-

pumped Nd:YAG laser. They concentrated on the situation where

prelasing is not present, and showed that the Q-switch build-up time is

never long enough for steady-state mode-locked pulses to be evolved. 129 -

The prelasing method to obtain short, stable, structureless pulses was suggested, but few experimental details given. In view of the recent wide-ranging interest shown in this system, a thorough characterisation of the system both with and without prelasing has been performed using

the linear streak camera technique. We have directly confirmed some of the conclusions reached by Kiuzenga et al, and the operation of the

Synchroscan streak camera as a "real-time” linear picosecond diagnostic of the QSML pulses has been demonstrated.

The main conclusion of this work is that short mode-locked pulses

('v lOOps duration) can be maintained at Q-switch repetition rates >1kHz without substantial loss of peak power or stabilty. QSML c.w. Nd:YAG- pumped optical amplifiers, for example, may thus be operated at repetition rates higher than 1kHz (albeit with some small reduction in stability over 500Hz operation).

The mode-locked pulses obtained with Q-switching are, however, substantially longer than those obtained without. This means, for example, that long, structured pulses are likely to result from dye lasers synchronously pumped by the harmonics of the QSML YAG laser.

Further investigation of methods of shortening the pulses is thus required. As previously mentioned, the anti-resonant ring cavity and mode-locking using higher r.f. drive harmonics look promising in this respect, particularly the latter because of its active nature and the fact that it may be used to shorten the prelase without reducing the power in the Q-switched pulses.

Another possibility is that of extracavity fibre/grating pulse- compression. As mentioned in other chapters, compression ratios MOO have been achieved using this technique for both the fundamental (26) and second harmonic (27) of the mode-locked c.w. Nd:YAG laser. Damage to the fibre is of greater consideration in the simultaneously Q- switched case, but high compression ratios and reasonable efficiencies 130 -

should still be obtained. In recent work performed in our laboratory

(30), compression of the QSML pulses from 85ps to 2.9ps with a consequent increase in peak power of x7 has been demonstrated using the fibre/grating method. It is even possible that some pulse-width reduction might result from simply passing the QSML output through a grating pair or fibre, as the pulses are likely to already be chirped due to self-phase modulation effects in the laser cavity.

Simultaneous Q-switching and mode-locking of Nd:YAG lasers has also been reported for the 1.318pm (4F 3/2 * 1+11 3 / 2 ^ transition (28) and some of the conclusions from this chapter should be relevant to this case also. 131 -

REFERENCES

(1) D.J. Kuizenga, D.W. Phillion, T. Lund and A.E. Siegman; Optics

Comm. 9 (1973) 221

(2) Technical Specifications for Quantronix series 100 and Spectra-

Physics series 3000 Nd:YAG lasers

(3) K. Kitayama, Y. Kato, S. S e i k a i and M. Tateda; Appl. O p t i c s 2 £

(1981) 2428

(4) T. Sizer II, J.D. Kafka, I.N. Duling III, C.W. Gabel and

G.A. Mourou; IEEE J. Quant. Elect. QE-19 (1983) 506

(5) D. Cotter and K.I. White; Optics Comm. 49. (1984) 205

(6) K.A. Nelson and M.D. Fayer; J. Chem. Phys. 72. (1980) 5202

(7) K.C. Liu, G. Vaillancourt and M.G. Cohen; Paper ThC3, Conference

on Lasers and Electro-optics (June 1984, Anaheim, California)

Technical Digest

(8) R.W. Hellwarth in "Advances in Quantum Electronics" (Columbia

University Press, New York) 1961, p.334

(9) F.J. McClung and R.W. Hellwarth; Proc. IRE 51 (1963) 46

(10) W. Koechner; "Solid State Laser Engineering" (Springer-Verlag,

1976) Chapter 8

(11) I.P. Kaminow and E.H. Turner; Appl. Optics 54. (1966) 1374

(12) C.L. Hu; J. Appl. Phys. 38 (1967) 3275

(13) J.E. Geusic, M.L. Hensel and R.G. Smith; Appl. Phys. Lett. 6_

(1965) 175

(14) D. Findlay and A.F. Fray; Opto-Electronics 2_ (1970) 51

(15) H.W. Mocker and R.J. Collins; Appl. Phys. Lett. 7. (1965) 270

(16) B.H. Soffer and R.H. Hoskins; Nature 204 (1964) 276

(17) R.B. Chesler, M.A. Karr and J.E. Geusic; Proc. IEEE j>8 (1970)

1899

(18) D. Findlay and D.W. Goodwin in "Advances in Quantum Electronics"

Volume 1 (Academic Press, 1970) pp 78 - 125 132 -

(19) G.D. Baldwin; IEEE J. Quant. Elect. QE-7 (1971) 220

( 20 ) W.G. Wagner and B.A. Lengyel; J. Appl. Phys. _34_ (1963) 2040

( 21) Operation manual for Quantronix series 300 Q-switches

( 22) D.J. Kuizenga; IEEE J. Quant. Elect. QE-17 (1981) 1694

(23) D.J. Kuizenga; Optics Comm. £2 (1977) 156

(24) R. Yen, P.M. Downey, C.V. Shank and D.H. Auston; Appl. Phys. Lett.

44 (1984) 718

(25) D. J. Kuizenga and A.E. Siegman; IEEE J. Quant. Elect. QE-6 (1970)

694

( 26) B.H. Kolner, J.D. Kafka, D.M. Bloom and T.M. Baer in "Ultrafast

Phenomena IV" Ed. D.H. Auston, K.B. Eisenthal (Springer-

Verlag, 1984) p .19

(27) A.M. Johnson, R.H. Stolen and W.M. Simpson ibid p.16

(28) E. D. Jones and M.A. Palmer; Optical and Quantum Electronics 7_

(1975) 520

(29) M.D. Dawson, A.S.L. Gomes, W. Sibbett and J.R. Taylor; Optics.

Comm., 52 (1984) 295

(30) A.S.L. Gomes, W. Sibbett and J.R. Taylor; to be published 133 -

CHAPTER VII

PICOSECOND PHASE CONJUGATION IN THE FORWARD DIRECTION

7.1 Introduction

In the previous chapter, we described a thorough character­

isation of the simultaneously Q-switched and mode-locked (QSML) c.w.

Nd:YAG laser using streak camera techniques. In this chapter we

consider the application of that system, working in the optimum regime

of short pulse (MOOps), high peak power performance, to the generation

of picosecond phase-conjugated optical pulses by degenerate four-wave mixing in the forward configuration. Conjugation efficiencies of up to

a few percent have been achieved at 1.06ym using both a silicon wafer and saturable absorbing dyes (DNTPC, 97^0), limited by the self-imposed

criterion that the spatial quality of the phase conjugate beam be maintained. The temporal characteristics of the phase conjugate pulses

have been investigated using the Synchroscan streak camera technique,

particularly with regard to solvent effects for the dyes.

7.2 Phase Conjugation by Degenerate Four-Wave Mixing

Optical phase-conjugate wavefront generation (1-3) has been of considerable interest in recent years because of its applications in

aberration correction and real-time processing of electromagnetic waves. In this technique, some nonlinear optical process (most

commonly stimulated Brillouin scattering (SBS)(4,5) or degenerate four- wave mixing (DFWM)(6)) gives rise to a wave that is the "conjugate

replica" of an input so-called "probe" wave, i.e. it contains the

complex conjugate of only the spatial part of the probe leaving the

temporal part unchanged. The conjugate wave thus corresponds to a wave 134 - whose phase is reversed relative to the input probe wave. If, for example, a plane wave were to travel through a distorting medium, becoming aberrated, it could therefore be restored to its original distortion-free state by phase-conjugate reflection followed by re­ propagation through the aberrator. Detailed reviews of the applications, experimental demonstrations and theory of nonlinear optical phase conjugation are given in (1-3).

The nonlinear process used in this work to generate phase- conjugated waves was that of degenerate four-wave mixing (DFWM)(6-8).

Four-wave mixing involves the incidence of three input waves onto a medium showing a third order nonlinear optical susceptibility, •

The polarisation thus produced is of the form (9):

PNL = * X^3)e .1 (£)E2 (r)Ep*Cr) . exp{i [(ojx+o^ - w ) t “ (k! + k 2- k p ) .r] }

+ c.c. (7.1) w h e r e E 1j2 (r) are called the (strong) "pump waves" and E^(r) is the

(weak) probe wave whose conjugate replica is sought. The nonlinear polarisation therefore radiates a conjugate wave (E ) of frequency: c

03=0)1+ too - 0) (7.2) C ^ p

If all three input waves have equal frequency, o> say, then E c will thus be of the same frequency as the input fields and we have "degenerate four-wave mixing".

The two standard experimental configurations used for DFWM are shown in Fig. 7.1. Figure 7.1(a) illustrates the configuration that has been of principal interest in work reported so far - that of

"backward" DFWM (10). Here the two pump waves (labelled f for forward and b for backward) are exactly counterpropagating (]i£+ii^= 0, the k's being the respective wavevectors) and the probe wave (labelled p) 135 - makes a small non-zero angle 0 (0 < 0 << 1) with respect to the forward pump wave. Phase matching (j£c = + Jk^ - jcp) then requires that the generated wave (being the phase conjugate of the probe and labelled c) be radiated in the reverse direction to the probe wave. In practice, backward DFWM can be performed with either equal or nonequal

(amplitude) pump waves. In the latter case, the second pump is typically provided by retroreflecting the forward pump transmitted through the nonlinear medium, using a mirror (11).

An alternative configuration, used principally for thin, highly absorbing samples, is that of ’’forward" DFWM (10), illustrated in

Fig. 7.1(b). This may be thought of as a four-wave mixing process in the sense that the strong pump beam (f) may be considered to constitute two copropagating forward-travelling pumps. The weak probe beam (p) then provides a third input wave at a small angle 9 so that, for thin samples, the four wave interaction is nearly phase-matched in the -0 direction (i.e. k, c = 2]C£-_kp). This latter configuration was the one used in the work described here.

Q) NONLINEAR b)

Fig 7.1 Schematic of the two common configurations for DFWM. The backward configuration (a) has counterpropagating pumps labelled f and p, respectively. In the forward configuration (b) the forward pump is regarded as constituting two copropagating pump beams - 136 -

We now consider, briefly, relevant results reported previously concerning phase conjugation in silicon and saturable organic dyes. In early self-diffraction type experiments using the forward configuration,

Woerdman (12) performed transient holography studies in silicon, examining the imaging properties of the free carrier gratings formed by the interference of the pump and probe. The phase conjugate nature of the self-diffracted beam was not appreciated in this early work, however. Woerdman's investigations were subsequently extended to a detailed study of the semiconductor carrier parameters by Jarasiunas and Vaitkus (13). A thorough investigation of phase conjugation by degenerate four-wave mixing in silicon using the backward configuration with nanosecond duration pulses, was performed by Jain and Klein (14) and Jain et al (15). They provided a theoretical treatment of DFWM near the bandgap of semiconductors, leading to expressions for the steady-state and transient third-order susceptibility (these are given later). The applicability of these expressions in the case of silicon was established and peak phase conjugate reflectivities of over 15056 at

1.06um were measured for a 0,5mm thick sample. (In degenerate four- wave mixing, unlike stimulated Brillouin scattering, it is possible for the conjugate wave intensity to exceed the probe intensity, thus allowing phase conjugate reflectivities in excess of unity.) The only known report of picosecond phase-conjugation in silicon is that of

Van Stryland et al (16). They used the forward configuration but provided little experimental information as they were interested only in the spatial quality of the emerging beams, in their search for "weak wave retardation". (The weak probe beam was shown to exhibit considerably more self-defocussing than the pump beam.)

As regards DFWM using organic dye saturable absorbers, several demonstrations have been reported. Of particular relevance is the work of Moses and Wu (17), Tocho et al (11) and Silberberg and Bar-Joseph - 137 -

(18). Related earlier work was published by Mack (32) and Carman et al

(33). Moses and Wu (17) reported amplification (600$) in phase

conjugate reflection by DFWM at 1.06ym wavelength (using a Q-switched

ns-duration pulse Nd:YAG laser) in a saturable solution of the dye BDN

in toluene. This dye was found to be rather a special case, with a metastable state nonlinearity being proposed to account for these very high reflectivities, and the absorber was modelled as a three-level

system. Tocho et al (11) used 5ps duration mode-locked pulses (at

605nm) in their investigation of DFWM in the saturable absorber dye

DODCI. Considering the dye as a two-level system, they confirmed the expected (squared) dependence of reflectivity on pump power and also measured the reflectivity versus dye concentration. The high efficiencies (up to 50$) achieved may be explained theoretically by the work of Silberberg and Bar-Joseph (18). They showed that if the pulse duration is considerably shorter than the lifetime of the relevant transition involved, then the transient reflected conjugate wave produced can be much stronger than that expected from a steady state analysis. In a subsequent publication (19), Tocho et al extended their work by investigating the role of thermal gratings in phase conjugation by organic dyes.

Direct examination of picosecond phase-conjugated pulses using streak cameras has not been reported, and in this chapter we combine such an examination with a detailed study of DFWM in silicon and two organic dye saturable absorbers (DNTPC, 9740) which have not previously been used as phase conjugate media.

7.3 The experimental set-up

The experimental arrangement used throughout the work described in this chapter is shown in Figure 7.2. The QSML Nd:YAG laser was operated in the optimum regime of stable, short pulse, high 138 -

peak power performance as described in the previous chapter. Mode- locked pulses 'MOOps in duration (at 1.06ym wavelength) were produced,

Q-switched at a repetition rate of 800Hz. The peak power in the mode- locked pulses occuring at the maximum of the Q-switched train was ^1MW, as measured at the laser output. The pulsetrain was split into two parallel beams, as shown, by a beam splitter with its rear surface anti-reflection coated for 1.06 urn. The beam splitter had a power division ratio of^10:1, the weak beam constituting the probe and the

strong beam constituting the pumps, for the four-wave mixing process.

Fig 7.2 Experimental set-up. The pump and probe are derived from the same input beam via a beamsplitter

A variable optical delay was introduced into the pump beam such that

the pump and probe pulses could be temporally overlapped at the sample.

It consisted of a pair of quartz 45 ° prisms (whose reflecting surfaces

were aluminium coated), one of which was mounted on a linear

translation stage (Microcontrole type MR 50-16). A spherical lens, of

large enough aperture to collect both beams, focussed the pump and

probe pulses onto the sample. By varying the length £ (see figure 7.2)

between 50 and 80cm, the spot diameter at the sample surface could be

varied in the range ^1.5mm to ^0.4mm. Typically, £ was 70cm giving a 139

o spot diameter O.omm and an angle 9 = 2.5 . This resulted in a full pump intensity (1^) at the sample of 'viOOMW/cm2. The 100% reflectivity mirror directing the probe beam through the lens was on a mirror mount, allowing spatial overlap of the pump and probe to be achieved at the sample surface. The incident and transmitted pump and probe and the resulting phase conjugate pulsetrains could all be monitored using an

S1-photocathode vacuum photodiode or the S1-photocathode Synchroscan streak camera system. The synchronous sweep for the camera was provided by splitting off, frequency doubling and amplifying a signal derived from the 50MHz mode-locker drive oscillator.

7.4 Picosecond DFWM in Silicon

The silicon sample used was near-intrinsic p-type (light boron doping) with a measured resistivity '\/l7kficm (20). It was in the form of a thin wafer, 0.36mm thick, polished on both sides, chosen to satisfy phase-matching requirements (see later) and to allow adequate coupling and transmission of the beams. Figure 7.3 shows a spectro­ photometer trace (taken with a Varian model 2300 Spectrophotometer) of the silicon slice. The net low light level transmission for 1.064ym wavelength at normal incidence can be seen to be 32.5%. We can use this to obtain a value for the intensity absorption coefficient. The refractive index for silicon at this wavelength is 3*56 (14) and we take account of the reflection loss at both the front and rear surfaces of the wafer contributing to the measured net transmission.

Working back from the net low light level transmission using the reflectivity expression (21):

R nl (7.3) ni + n 2 y

(where R is the fractional reflected intensity for propagation at 140 -

normal incidence from a medium of refractive index n ± to one of refractive index n£), we find that of the light intensity I incident on the front surface of the sample, a fraction 0.685 I propagates into the sample and a fraction 0.475 I reaches the rear surface (before reflection).

Thus, using Beer's law, I = Io exp(-ax), we calculate an intensity absorption coefficient a = 10.2cm_l. Measured values in the literature give (14) a MOcm”1 for silicon at this wavelength, so we may take this to be an indicator of the high quality of the sample used.

Fig 7.3 Spectrophotometer trace of 0.36mm silicon wafer. Net low light level transmission for normal incidence at 1.06ym wavelength is 32.5% 141 -

It is convenient also at this point to calculate the lifetime of the free-carrier grating formed in the silicon by the interference of the pump and probe beams (for the particular geometry used). This will be referred to later in the chapter. Two equal wavelength beams interfering at angle 9 with respect to one another generate an interference pattern, for near-normal incidence on the sample, of period (14):

d = X/2 sin(0/2) (7.4)

where X is the wavelength.

Using the values appropriate to our geometry (0 = 2.5°), this works out to give d = 24.3ym. The electron-hole recombination time in silicon is of at least a few microseconds duration (14), so we may neglect this contribution to the decay of the free-carrier grating and consider the decay to take place solely by carrier diffusion.

Now, the decay time due to carrier diffusion ( x^) is (14):

x = d2/4ir2D (7.5) D a

where D & is the ambipolar diffusion coefficient, with a value for silicon of 15cm2 /sec (13), and we therefore calculate a grating lifetime of 10ns. The angle between the interfering beams was thus chosen to be large enough, consistent with phase-matching constraints,

such that the resulting grating lifetime was not .longer than the temporal separation between successive mode-locked pulses in the Q-

switched train (10ns).

Figure 7.4 is a log-log plot of the phase conjugate % reflectivity

((Ic/Ip ) x (100/1)) for the silicon sample versus pump intensity (I^ ).

These measurements were obtained directly from the amplitudes of - 142 -

Fig 7.4 Log-log plot of phase conjugate % reflectivity vs. pump

intensity for the silicon wafer, derived from the inset phase

conjugate (a) and probe (b) pulsetrains. Oscillograms both

5 0 n s / h o r i z o n t a l di^v., (a) is 2 0 m V / v e r t i c a l div. and (b) 0.2 V /

vertical div. with ^18% of probe

corresponding pulses in the oscillographs of the phase conjugate and probe pulsetrains, shown as insets (a) and (b) respectively. The corresponding pulses in the pump and probe pulsetrains had previously been shown to be proportional, as is expected when they are both derived from the same input beam via a beam splitter. This also served as an additional check on the linearity of the vacuum photodiode over the range of intensities recorded. From equation (7.1), the reflectivity (I /I ) is proportional to I 2 , so we expect a slope of c p £ 143 -

value 2 for a log-log plot of reflectivity versus pump intensity. The measured value is 2.3 which agrees to within experimental error.

The maximum reflectivity, R ^ 2%, was observed for a pump intensity of 110MW/cm2. Higher reflectivities were achieved at higher pump intensities - up to a maximum reflectivity of ^ 6 % with the smaller spot sizes - but at these high intensities, distortions in the pulsetrain envelope were evident and considerable self-defocussing of the beams was observed to take place. This "self-focussing" problem is a basic one associated with high efficiency DFWM where there are large intensity variations in the transverse profiles of the beams and is independent of the specific nonlinearities involved in the four-wave mixing process (22). (Hopf et al (23) have recently used an interferometric technique to examine the "quality" of the conjugate wave in forward DFWM.) In addition, no saturation of the phase conjugate signal due to the comparatively large intensity of the probe relative to the pump (^10%), was observed when a variable neutral density filter wedge was used to attenuate the probe beam. This was probably due to the relatively low overall reflectivities achieved.

(The probe was chosen to be this fraction of the pump so that the absolute phase conjugate signal could be as large as possible.)

From figure 7.4, we can calculate a value for the third order nonlinear susceptibility, , for the four-wave mixing and compare the result to that obtained using the theoretical expression of Jain and Klein (14).

Jain and Klein show that in the transient case, where the laser pulse duration (t l ) is much less than the lifetime of the grating formed by the interference of the pump and probe (t ), (which applies in this case because = 100ps whereas t = 10ns as calculated previously) an effective x^3^ for the process can be obtained using: 144 -

X^ = nance2TT (7 .6 ) 6 16inn ^a)3^ eh

Here n is the quantum efficiency of carrier generation ('vl in intrinsic semiconductors illuminated above the band edge at relatively low intensity), a is the intensity absorption coefficient

(= 10cm-1 = 1000m-1 at this wavelength as shown previously), n is the refractive index (3«56 at 1.06ym) and m * is the effective optical eh mass = 0.16 m Q (14), where m Q is the electronic mass. Substitution of these values and that of the angular frequency (w) of the beams at

1.064ym (= 1.778 x 10l5s_l), together with the fundamental constants di, e and c gives = 6.29 x 10 19m2 /V2. It is customary to express eff

however, in electrostatic units (cm2/ statvolt2). Using the fact eff that 1cm = 10”2m and 1 statvolt = 299.8 volt, we obtain v(3) (e.s.u.) = 5.66 x 10-10. eff For degenerate four-wave mixing in the forward direction, it may be shown from the work of Maruani (3D (see appendix I) that the reflectivity obeys the expression:

R = Ir/Ip = 9 to2£2 x (;S) s i n c Z ‘Ak£j If2 (7.7) £2 ^ n 4 4 ~ o

assuming low reflectivity ( I£ << I ) and neglecting pump depletion.

Here, £ is the thickness of the silicon wafer (= 0.36 x 10”3m), £q is the permittivity of free space (= 8.85 x 10~12 Fm-1) and the other terms

0 are as given previously. The sine expression is the phase-mismatching term, where (24) (as may be shown from a k-vector diagram):

Ak = k02 (7.8)

w i t h k = 2 tt/X and 0 being the angle between the pump and probe beams, 145 -

as described already (6 is expressed in radians). For X= 1.06 x 1(T6in and 0 ^ 2.5°= 44mrad, we obtain Ak = 112.9cm”1 and thus a sine (Ak£/2) value of 0.441. Thus we did have some phase-mismatch in the geometry used, resulting in suppressed reflectivities, which could have been reduced by arranging for 0 to be smaller. However, the grating life­ time would then have considerably exceeded the mode-locked pulse separation (10ns) and complicated the analysis somewhat.

Using the reflectivity expression given above (eqn. (7.7)), we obtain, by taking the log of both sides:

3 oj l v O ) sine ^ Ak& y log R = 2 log 1 ^ + 2 log xeff n2 c2 2

From our graphical plot (Fig. 7.4), R = 1 % = 0.01 at If = 88MW/cm2, which on substitution with the other values gives x eff = 1,37 X 10_18 tii2/V2. Thus we obtain an experimental value for

X (3 > of x (©«s*u.) = 12.3 x 10"10 in reasonable agreement with the eff eff theoretically determined value of Xg|f = 5.66 x 10 “ 10e.s.u. (Jain and

Klein claim their theoretical result to be correct only to an order of magnitude or so.) This value is approximately two orders of magnitude smaller than that obtained using 15nsec laser pulses by Jain and Klein

(14), because of the proportionality between and x^ ( xL << x ) expressed by equation (7.6). However, the higher intensities available with mode-locked pulses ensure that near-comparable reflectivities can be achieved, although we might expect self-defocussing of the beams to become a problem at lower reflectivities in the mode-locked case.

Figure 7.5 shows a plot of the peak phase conjugate pulse amplitude (measured using the vacuum photodiode/oscilloscope combination) versus relative time delay between the pump and probe pulses (achieved by varying the optical delay translation stage in the pump beam). The full-width at half maximum effectively gives the - 146 -

coherence time of the QSML Nd:YAG laser pulses. The value obtained,

^50ps, is approximately half that expected for 100ps bandwidth -

limited pulses. This is explained by Eichler et al (25) by the fact

that a transient coherence function, which is less than the coherence

time, applies in this case. There may also be some frequency chirping

in the mode-locked pulses due to self-phase modulation in the laser

cavity at the high Q-switched intensities. As mentioned at the end of

the previous chapter, this may have relevance to extracavity

compression of the QSML pulses using a grating pair or optical fibre,

depending on the sign of the chirp.

AMPLITUDE OF PEAK R C. PULSE (arb. units)

Fig 7.5 Plot of amplitude of peak phase conjugate pulse (from the oscilloscope) for Silicon, vs. relative delay between pump and probe. The pump arrives relatively earlier going from right to left (i.e. from -ve to +ve delay) and the zero is chosen to correspond to the peak phase conjugate signal on the streak camera 147 -

To confirm directly that phase conjugation was taking place in our

experiment, we used the method of Heer and Griffen (24). A small wire,

0.5mm in diameter, was placed in the probe beam just behind the

focussing lens, giving a transverse beam profile of the type

illustrated in figure 7.6. The lens (see fig. 7.2) focussed the pump and probe beams at a distance ^30cm behind the silicon wafer, i.e. ^ 1m away from the lens. The phase-conjugate beam was observed however, to expand uniformly with distance from the silicon, as expected, and not to go through a focus. In addition, at ^70cm behind the wafer (i.e. the same distance behind as the wire was in front of it) the wire

a)

b)

Fig 7.6 Reticon photodiode array display of transverse beam profiles of (a) phase conjugate (taken v70cm behind sample) and (b) probe (taken just behind lens) beams when a 0.5mm diameter wire was placed in the probe beam after the lens (see text). Scale 50ys/minor div. in each case 148 -

"shadow" in the phase conjugate beam was found to be the same size as just behind the lens. This is shown by figures 7.6(a) and (b), which are the probe (recorded just after the position where the wire was placed, 70cm in front of the silicon) and phase conjugate (recorded

70cm after the silicon sample) transverse beam profiles, respectively, as recorded using a Reticon photodiode array and Tektronix 7834 storage oscilloscope. From a calibration of the Reticon scan using the oscilloscope, we found the total scan duration to be 1.5ms. Now, the spatial channel separation on the Reticon is I6ym (26), which was found to correspond to 2ys temporal separation on the oscilloscope. Thus the total spatial extent of the active Reticon elements is ^12mm. The temporal width of the wire "shadow" in both fig. 7.6(a) and 7.6(b) is approximately 50 ys (i.e. one small square), thus we estimate the spatial width of the shadow in both cases to be

12mm x (0.05ms/1.5ms) - 0.4mm, which is close to 0.5mm, considering the approximation made in estimating the "width" of the wire shadow. An additional piont to notice in comparing figures 7.6(a) and (b) is the narrowing in width of the phase conjugate beam. This is consistent with the fact that we have a Gaussian probe transverse beam intensity profile and the intensity of phase conjugation varies as the cube of the probe intensity (24).

Finally, in this section, we consider phase-conjugation by DFWM in the forward direction using purely Q-switched (i.e. unmode-locked) pulses. The phase-conjugate pulse and (a fraction of) the corresponding probe pulse are shown in figure 7.7(a) and (b), respectively. We are now in the long pulse regime, as defined by Jain and Klein (14), i.e. the laser pulse duration ( t l ^ 170ns) is much greater than the free carrier grating lifetime ( t ^ 10ns as before).

Thus we need to use the steady state expression for , given by ncxnce2T (7.9) xsteady (3) „ state 87nn * -ft oj3 eh

where the terms are as defined previously. Substitution of the

appropriate values, with t = 10ns, gives = 1.26 x 10 ”16 m 2 /V2 or

X^3)(e.s.u.) =1.13 x 10 “7 , which is to be compared to 12.3 x 10“10

(e.s.u.) for 100ps pulses. From the oscillograms, we find a

reflectivity ^2$, which is comparable to that obtained using the QSML

pulses. This is not unreasonable because, from equation (7.7),

reflectivity (R) « [x^3^] I^ 2 and x^3^ has increased by approximately

two orders of magnitude over the QSML case, while 1^ has decreased by

approximately two orders of magnitude (100MW/cm2 ^ 5kW in 0.8mm

diameter spot =* 1MW/cm2). Note also the pulsewidth reduction of the

phase conjugate - 90ns compared to 170ns for the probe. This

reduction is close to the 1/>/J factor we would expect by approximating

the Q-switched envelopes to Gaussians and remembering that I * I 3. c p

Fig 7.7 Oscillograms of (a) phase conjugate and (b) probe pulses

obtained with the Q-switched Nd YAG laser (no mode-locking

operating at 800Hz. Scale 50ns/div. in both cases, vertical

scale lOmV/div. for (a) and 0.2 Volt/div. for (b) with ^18%

of the total probe beam. Note the pulsewidth reduction in (a) 150 -

7.5 Picosecond DFWM in organic dyes at 1.06ym

In this section, we discuss phase-conjugation at 1.06ym wave­ length using the organic dyes DNTPC (27) (3, 3*-diethyl-9,11,15,17 - dineopentylene - thiapentacarbocyanine perchlorate) and Eastman-Kodak

97^0, in a variety of solvents. Both dyes have been used extensively as saturable absorbers for Q-switching neodymium lasers (27,28). The experimental arrangement was essentially unchanged from that described earlier in the chapter, except that a quartz flow cell, having a 0.5mm- thick flow channel, was used for the dye solutions. The dye was flowed through the cell, using a micropump in a PTFE-tubing closed-loop circulation system, in order to minimise thermal distortion effects in the solvent. The typical volume of dye solution required was MOOcm3.

Phase conjugation was observed using solutions of DNTPC in both

1,2-dichloroethane and propylene carbonate. The reflectivity for a particular dye concentration was, however, found to be less in the latter case and, in addition, rapid deterioration of the dye (over a period of hours) was observed. Most of the results reported here were therefore taken using a 1,2-dichloroethane solution of DNTPC.

The optimum concentration of DNTPC was found to be M . 5 x 10~3M for the experimental configuration used, where peak reflectivities ^ 2% were achieved. A plot of phase-conjugate intensity versus molar concentration of the dye is given in Fig. 7.8. Further data points at higher concentrations could unfortunately not be taken due to the limited amount of dye available. It is possible to show (see Appendix

II), that for resonant excitation of a 2-level absorbing medium below saturation, we expect the phase conjugate intensity to follow a curve of the form:

I « exp(~ad) [l - exp(~ad)]2 (7.10) 151 -

Here d is the dye cell thickness (i.e. the thickness of the dye stream) and a is the intensity absorption coefficient, given by:

a = ne (7.11)

where n is the molar concentration and e is the extinction coefficient.

Using the spectrophotometer (Varian Model 2300), a 5mg solution of

DNTPC in 50cm 3 of 1,2-dichloroethane was found to exhibit 33^

transmission at 1.064 ym in a 0.25cm thick cell (not the 0.5mm flow

cell). Together with knowledge of the dye molecular weight (= 705g)

♦

Fig 7.8 Semi-logarithmic plot of peak phase conjugate intensity in

arbitrary units vs. molar concentration of dye for DNTPC in

1,2-dichloroethane (obtained using the photodiode/oscilloscope

combination). Dotted line shows theoretical fit using

e = 1.4 x 10I+M” 1cm_1 for the 0.5mm-thick cell 152 -

and the Beer's law expression I = IQ 10”en^} this allowed us to obtain a measured value for the extinction coefficient of e = 1.36 x 104 litre mol^cm-1 (or, equivalently, M”1 cm""1 ). The theoretical curve of equation (7.10) plotted in figure 7.8 using this value can be seen to be a close fit to the experimental points. (The form of this curve is explained by the fact that at dye concentrations below optimum, the coupling constant for nonlinear mixing is low, giving lowered reflectivities. At concentrations higher than optimum, the reflectivity falls due to self-absorption.)

Figure 7.9 shows a log-log plot of the phase conjugate intensity as a function of pump intensity, obtained directly from the amplitudes of the corresponding pulses in the oscillograms of the respective pulsetrains (shown as insets (a) and (b)). We know that, in our experimental arrangement, the pump and probe intensities are proportional to each other, and also (from equation (7.D) that

p Ic a If 1^ . Hence, in our case, we expect the phase conjugate intensity to vary as the cube of the pump intensity (the reflectivity

I /I varies as I 2 whether the probe intensity is held constant or is c p r proportional to 1^) and thus to have a slope of 3 on a log-log graph.

The measured value is 3*2 which is in good agreement, but we note a slight saturation at the highest pump intensities.

The maximum phase conjugate intensity plotted corresponds to a reflectivity of 2.1$, comparable to that achieved with silicon, obtained at a pump power density of 'M10MW/cm2. The main limitation on this reflectivity was found to be the thermal distortions occuring in the solvent at higher pump power densities. It is felt likely to be this effect, and not a saturation of the phase conjugate reflectivity as such, which led to the deviation from cubic dependence at the highest intensities mentioned above. Possible evidence for this is - 153 -

Fig 7.9 Log-log plot of normalised phase conjugate intensity vs.

normalised pump intensity for DNTPC in 1,2-dichloroethane,

obtained from the (a) phase conjugate and (b) probe pulse-

trains. The peak phase conjugate intensity corresponds to

a reflectivity of 2.1% and the pump intensity is normalised

to 110MW/cm2 total probe intensity. Horizontal scale 50ns/

div. for the oscillograms, vertical scale lOmV/div. in (a)

and O.lV/div. in (b) with ^14% of total probe

given by figure 7.10, which shows the linear increase of phase conjugate intensity with probe intensity at constant I (110MW/cm2

total pump power density). A variable neutral density filter wedge was used to vary the probe beam intensity over the range of values shown.

No saturation of the phase conjugate signal is evident, even at the - 154 -

highest probe intensities (corresponding to ^10% of the total pump

intensity) which, as in the case of silicon, is probably due to the

relatively low phase conjugate efficiencies achieved.

Fig 7.10 Log-log plot of phase conjugate intensity vs. probe intensity

for constant (= 110MW/cm2) taken using a variable neutral

density wedge in the probe beam. The peak phase conjugate

reflectivity is 2.1%

Both population and solvent thermal grating effects are likely to

play a role in the phase conjugation process in DNTPC and are discussed

further in the next section. DNTPC has been used previously as an

active laser medium (29) and it was felt that the population grating

lifetime in the solvents used would be at least of the order of the

100ps Nd:YAG laser pulse duration, giving broadly comparable behaviour

to the case of silicon. The other saturable organic dye in which phase 155 -

conjugation was achieved at 1.06pm, Eastman Kodak 97^0, is known to

have a rapid recovery time (11ps in dichloroethane (30)), however, so

we expected lower efficiencies than for DNTPC, but more interesting

solvent-dependent effects. The streak results of the next section show

these effects, phase conjugation being observed using 97^0 dye in

propylene carbonate, 1,2-dichloroethane and a benzyl alcohol/glycerol

mixture. The maximum phase-conjugate reflectivity achieved was ^ 0.2 %

which was, however, too small to permit the molar concentrational

dependence and the dependence on pump intensity to be examined.

(Solution concentrations of typically ^3 x 10_4M gave the highest

reflectivities.)

Finally, in this section, it should be pointed out that no self-

diffracted (i.e. phase conjugate) beam was observed with any of the

pure solvents used in this work.

7.6 Streak Camera Results

We now describe the results obtained using a synchronously

operated S1-photocathode streak camera to directly examine the phase

conjugate pulses for the various nonlinear media. The synchronous

sweep for the camera (at 100MHz) was obtained by frequency doubling and

amplifying (to MOW) a signal derived from the 50MHz mode-locker drive

oscillator. Real-time streak images of the phase conjugate, pump and

probe pulses were displayed on the O.M.A. (B & M Spectronik OSA) and

hard copies of stored streak data were taken using a chart recorder.

Before describing the results obtained in detail, two preliminary

points are mentioned. Firstly, neither the pump nor probe pulses were

observed to change in width on transmission through the nonlinear medium. This indicates that only weak (if any) saturation effects were

taking place. Secondly, in using the Synchroscan camera to monitor Q-

switched mode-locked pulsetrains, we are integrating over pulses which - 156 -

vary considerably in intensity. In the case of the QSML pulsetrain obtained directly from the Nd:YAG laser, it was shown (by the single­ shot streak results of the previous chapter) that only small pulse to pulse variations in duration and shape occurred throughout the train.

Thus we have confidence that the pulsewidth and shape of the integrated

Synchroscan streak images accurately reflects that of the individual mode-locked pulses. It is not immediately obvious, however, that this is the case for effects induced by the QSML pulsetrains, e.g. the generation of a phase-conjugate train, where the pulse width and shape could possibly be intensity dependent. That we may ascribe meaning to the Synchroscan streak intensity profiles of the phase conjugate pulses, is supported by the fact that the recorded streaks are very much dominated (due to intensity dependent effects at the streak tube phosphor) by the most intense pulses, occuring at the peak of the train, for which the intensity is relatively constant. In addition, it is mainly the shape of the pump and probe pulses and not the intensity

(for a particular temporal delay between the two in a given nonlinear medium) which governs the nature of the phase conjugate temporal profile. Evidence for this is given by the fact that when a variable neutral density wedge was used to vary the probe beam intensity in the case of silicon, only the intensity and not the character or width of the recorded phase conjugate Synchroscan streak image was found to vary.

The streak intensity profiles obtained with the various nonlinear media are shown in figures 7.11 - 7.16. The left hand profile in each pair is that of the phase conjugate, while that at right is the probe

(providing time calibration for the streaks). Time increases from left

to right for each streak pair. In all figures, results are given for a

range of relative time delays between the pump and probe pulses, 157 -

0-36mm SILICON WAFER

88ps 101ps 82ps 96ps 81ps 100ps -40-2ps DELAY -26-8ps DELAY -13-4ps DELAY

ZERO DELAY +6-7ps DELAY +20-1 ps DELAY

Fig 7.11 Synchroscan streak intensity profiles of the phase conjugate

(left in each pair) pulse vs. relative delay between pump

and probe pulses, for Silicon. The pulse at right in each

pair is the probe, "zero delay" is chosen to correspond to

the maximum phase conjugate signal on the streak camera and

the pump arrives relatively earlier from -ve to +ve delay.

Time increases from left to right for each pulse pair 158 -

achieved by varying the translation stage in the pump beam optical delay line (see figure 7.2). The pump pulse arrived relatively earlier in time in going from negative to positive delay, and the maximum

"positive delay" in each case was limited by the geometry of the optical delay line. "Zero delay" was chosen, in each case, to be the

DNTPC in 1,2-0ICHL0R0ETHANE

75ps 98ps -4 0 -2 ps DELAY - 2 6 8 ps DELAY -13-4ps DELAY

109ps 108ps 106 ps 109 ps ZERO DELAY +6-7ps DELAY +20-1 ps DELAY

Fig 7.12 As in Fig. 7.11, but for DNTPC in 1,2-dichloroethane 159 -

point at which the maximum phase conjugate signal was observed on the streak camera (and is not meant to indicate complete temporal overlap between the pump and probe pulses at the sample). Notice, however,

DNTPC in PROPYLENE CARBONATE

63p s 9 7 ps 6 4 p s 100ps 7 9 p s 99p s -4 0 -2 ps DELAY -2 6 8 ps DELAY -13-4ps DELAY

84ps KBps 81 ps 92ps 106ps 99p s ZERO DELAY +6-7 ps DELAY -*-20-1 ps DELAY

Fig 7.13 As in Fig. 7.11, but for DNTPC in propylene carbonate 160 -

66ps 105ps 69 ps 105ps 75ps 100ps -13-4 ps DELAY -6-7ps DELAY ZERO DELAY

86ps 105 ps lOOps 106ps 95ps 106ps +6-7ps DELAY +13-4ps DELAY +20-1 ps DELAY

Fig 7.14 As in Fig. 7.11, but for 9740 in propylene carbonate

that the vertical scales of the streak pairs have been adjusted for display purposes in each figure, such that the streaks have similar amplitudes for all relative delays. The maximum phase conjugate signal may not, therefore, be displayed as such in a particular diagram. 161 -

9740 in 1,2 DICHLOROETHANE

75ps 99p s +6-7 ps DELAY +13*4ps DELAY

Fig 7.15 As in Fig. 7.11, but for 9740 in 1,2-dichloroethane

The results can be briefly summarised as follows. In the case of silicon, which has a long lifetime (10ns) grating, the phase conjugate profile is symmetrical and of approximately the same width (^84ps) for all delays. The maximum phase conjugate signal occurs for the pump pulse arriving ^20ps later than the largest positive delay achievable by the delay line. For the dyes DNTPC and 9740 in the solvents propylene carbonate and 1,2-dichloroethane (figures 7.12 - 7.15), we observe broad (^ pump pulsewidth), symmetrical pulses for positive 162 49ps 99ps ZERO DELAY ZERO 60ps 94ps ♦40 2ps DELAY 49ps lOlps 53ps 106ps 80ps ps 93 75ps 99ps ♦26 8psDELAY 8psDELAY ♦26 0 3 5ps DELAY -13 4ps DELAY -134ps DELAY ~67ps DELAY 44 ps44 ps 104 -2 0 1ps DELAY 9740 in BENZYL ALC0H0L/GLYCER0L MIXTURE 77ps 104ps ♦13 4 ps DELAYps 4 ♦13 ♦201ps DELAY 57ps 96ps ♦67ps DELAY ON •n H* As in Fig. 7.11, but for 9740 in benzyl alcohol/ 0Q

glycerol mixture (6:1 by volume) 163 -

delays, becoming narrower and developing a slight "decay tail" trailing

edge at negative delays. The maximum phase conjugate signal in these

cases occurs, to within experimental error ( ^15ps), at the same

relative temporal delay between pump and probe pulses as for the case

of silicon.

Figure 7.16 shows the results obtained using 97^0 dye in a benzyl

alcohol/glycerol mixture, where an attempt was made to increase the

grating lifetime of a short recovery time dye (97^0 has a recovery time

^11ps in dichloroethane (30)) by using a high viscosity solvent. Here,

anomalous behaviour can be seen to occur for negative delays, where a

double pulse conjugate profile is observed, each sub-pulse being of

short duration (the earlier sub-pulse FWHM is ^50ps). In this case,

the maximum phase conjugate signal is observed for the pump delayed an

extra ^20ps compared to the other media.

To a first approximation, it is possible to interpret the main

features of these results in the following manner. An optically-

induced grating is produced only in the region of overlap between the

pump and probe pulses. It will then decay with a characteristic

lifetime determined by the particular nonlinearity and medium involved.

If the grating lifetime is much shorter than the temporal pulsewidth,

the energy of the conjugate pulse depends only on the area of overlap,

and is thus a maximum for complete overlap between the pump and probe

pulses. In addition, we would expect the phase conjugate profile to be

symmetrical for this complete overlap. However, as the pump pulse

becomes progressively delayed relative to the probe, any pump light

following the region of overlap will continue to self-diffract from a

rapidly decaying grating. In the case of a grating lifetime short, but not negligible, with respect to the pulsewidth, this could result in an asymmetric phase conjugate pulse, showing a decay tail. This is a reasonable explanation for the streak results of figures 7.12 - 7.15, 164 -

if we consider positive delays to correspond to near complete pulse overlap, resulting in symmetrical pulses of the order of the pump pulsewidth. As the pump is progressively delayed (negatively) the pump pulse can scatter from a decaying grating, leading to the asymmetry and pulse shortening observed for negative delays. This asymmetry appears to be more pronounced for 9740 dye, which we expect to have a shorter grating lifetime than DNTPC.

For a grating lifetime much longer than the pulsewidth, on the other hand, we might reasonably expect the conjugate profile to be symmetrical and not to vary in duration or shape over the range of delays examined in these experiments. This is because of the lack of grating decay occurring over these timescales. A variation in pulsewidth would only be expected for large positive delays, where the pump pulse arrives much earlier than the probe, giving only a small region of overlap. We might also expect the maximum phase conjugate signal to occur for the pump pulse slightly delayed relative to the complete overlap position, where a large fraction of the pump energy can arrive late to self-diffract from a pre-formed grating. This would be in disagreement with the observation that the maximum phase conjugate occurs at the same delay in all cases (except 9740 in benzyl alcohol/glycerol). However, if the extra delay in the silicon case is small, it could be accounted for by the M5ps experimental error in the delay measurements.

The double-pulse behaviour shown in Figure 7.16 is more difficult to explain. It is possible, however, that this case involves two different grating lifetimes - one short and one long. The fast grating would self-diffract only for the region of overlap between pump and probe pulses. For the relatively large negative delays this overlap could be quite small, leading to the narrow (^50ps FWHM) first pulse in each pair. The subsequent pulse might then occur by self-diffraction 165

of the large fraction of the pump pulse following the region of

overlap, if a second, longer lifetime grating was involved. Thermal

gratings have been shown to play a role in phase conjugation using

organic dye saturable absorbers (19,32) and it is possible that this

long lifetime grating could be thermal in nature, i.e. due to thermally

induced refractive index changes in the solvent occuring as a result of

collisons between dye and solvent molecules. It should also be noted

that the large range of delays (-33.5 to +40.2ps) over which results were obtained for this solvent mixture, is due to higher phase

conjugate efficiency (^0.2%) being achieved in this case as compared to

9740 in the other solvents.

Considering the phase conjugation as a two-step process, where the pump and probe interfere to "write" a grating which is subsequently

"read" by the pump, a simple theoretical model of the transient scattering process has been developed. The numerical solutions to this model predict some of the features observed in results described in this section, especially for the case of silicon, and details are given in Appendix III.

7.7 Conclusions

Phase conjugate wavefront generation has been investigated in a silicon wafer and saturable absorbing dyes (DNTPC, 9740), using a

QSML c.w. Nd:YAG laser and Synchroscan streak camera system.

Conjugation efficiencies of up to a few percent have been achieved and the characteristics of the phase conjugate signal examined in detail.

Thinner samples of silicon and a narrower channel flow cell in the case of the dyes, would have resulted in more complete phase-matching for the forward configuration, and further work to evaluate the resulting improvement in phase conjugate efficiency is required.

As regards the streak camera results, forward phase conjugation 166 -

has been shown to be a possible method for pulsewidth compression of high power picosecond laser pulses. In addition, in the case of organic dye solutions, the streak results have shown directly how the choice of solvent can affect both the character of the phase conjugate pulses and their efficiency of generation. Optical fibre/grating pair extracavity compression of the QSML pulses could allow experiments of this type to be repeated with shorter duration pump and probe pulses. 167 -

A P P E N D I X I

Phase Conjugate "reflectivity" in the forward direction

For forward DFWM, Maruani (31) gives the propagation equation:

d E c + a - ioox^3) 3|E |2+ 6 | E f |2+ 6 |Ep |2]' E c = ioJX^3^ 3 E f 2 dz .2 2nc JJ 2nc

xEp* exp(iAkz) where z is the direction of propagation of the fields and the other terms are as defined previously. Assuming low reflectivity, Ec << Ep , we neglect 2nd term on LHS,

dEP = 3E^2Ep* exp(iAkz) dz 2nc

2 . In the absence of pump depletion, Ef is a constant

Z Z .’. E c = / d E r .dz = ioox^3^ 3 E ^ 2E * J e x p ( i A k z ) d z 0 dz 2nc ^ o

for sample thickness £.

Ep = 3icox(3) Ef2Ep* exp(iAkz)

2nc iAk -* o

E c = 3ia)x^3^ E 2E * Z. 1 exp(+iAk£/2)-exp(-iAk£/2) f P 2nc iAk£

x exp(iAk£/2) 168 -

Now sine 9 = sin 9 = e - e 9 2i0

E c = i3(ux^3) E f 2E * £ exp(+iAk£/2) sinc(Ak£/2) C 2nc P

E E * = 9co2 E *E £2 s i n c 2 (Ak£/2) c c 4 n 2 c 2

Using intensity 1^ = (eQnc/2) |E^|2 and introducing R reflectivity = I /I , we obtain the resu l t c p

2 R I c 9 a)2£ 2 sine2 (Ak£/2) 2 4 4 eo n c 169 -

APPENDIX II

Derivation of the ooncentrational dependence of phase conjugate intensity in the forward configuration

The propagation equations.for the pump (f) and probe (p) beams in an absorbing medium are:

dEf = -a E f = > Ef (z) = Ef (o) exp(-az/2) dz 2

a n d

-a E p = > E p (z) = Ep(o) exp(-az/2) dz 2

where a is the intensity absorption coefficient, +z is the direction of propagation of the beams and d is the sample thickness. Now, the growth of the phase conjugate (c) is given by:

d E c = - a E c (z) + i 3 E f 2 (z)Ep *(z) dz 2

where the 2nd term on the R.H.S. comes from the polarisation expression of equation (6.1), 3 being a constant.

Thus, combining the above equations,

d E P (z) + a Ec(z) = i3Ef2(o)Ep*(o) exp(-3az/2) dz 2

d Ec(z) exp(+ az/2) = i3^ exp(-az) dz 170 -

w h e r e

(T = 3E 2 (o) E *(o) f P

Thus

d E (d) exp(ad/2) - E (o) = i3" / e x p ( - a z ) d z ^ c o

With the boundary condition Ec(0) = 0, we have

Ec (d) = i3^ exp(-cxd/2) £(exp(-ad)-l) /-aj

which implies that

I c (d) « (13'l2 / a 2 ) exp (-ad) J^l-exp (-ad)J2

A similar concentrational dependence can be shown to hold for backward

phase conjugation with equal intensity pumps. In the backward case with nonequal pumps, where the second pump beam is obtained by

retroreflection of the other transmitted pump, the dependence can be

shown to be

I c (d) « ( 1 3^ I 2/ot2) exp (~2ad) jexp (-ad )- l j 2

This is in slight disagreement with Tocho et al (11) w ho g ive

I « exp(-2ad) J~exp(-2ad)- ll2

but still gives a close fit to their experimental points. 171 -

APPENDIX III

i) A General Formalism of Forward Four-wave Mixing

In the normal form of degenerate four-wave mixing (DFWM), three input beams are incident on a nonlinear material with a third- order susceptibility X ^ i n order to generate a fourth beam, the so- called phase conjugate beam. In this arrangement two (pump) beams counter-propagate to one another while another (probe) beam is incident at an angle to these beams. The phase conjugation mechanism is achieved by a grating induced in the medium by interference between the probe beam and one of the pump beams that is diffracted from by the counter-propagating pump field to produce the phase conjugate of the probe field. Since the induced grating can be transient and if one uses short duration laser pulses, it is also possible for the counter- propagating pump pulse to arrive at a later time to the other two pulses and still be diffracted if the grating has not decayed away completely. This provides a useful technique for the measurement of the decay time of induced gratings in nonlinear media.

As outlined in Chapter 7, one can also achieve forward four-wave mixing in which only two input beams are incident at a small angle in a nonlinear sample (see figure 7.1(b)). This technique can be considered as a self-diffraction process in which the beams exciting the grating are simultaneously diffracted. If the incident probe field (Ep ) is at an angle 0 to the forward pump field (E^), then the phase conjugate signal (E ) is produced at an angle -0. For the degenerate case in c which the pump and probe are of the same frequency the four-wave mixing process is only partially phase-matched with a phase-mismatch given by

Ak.£ - k02£, that restricts the process to small angles (0) and short interaction length samples (*•).

A mathematical model of this process can be realised if equations 172 -

describing a) the grating production and b) the self-diffraction of light by this induced grating can be produced. A general differential equation describing the induced grating is given by:

dn(t) = -n(t) + aEf(t) Ep*(t) (Al) dt tD

where n is the amplitude of the grating and t^ is its decay time, which may depend on parameters such as the spatial period of the grating.

The parameter a is determined by the physical properties of the medium that characterises the nonlinear coupling of light to induce, for instance, a refractive-index grating.

The light diffraction equation can also be expressed as:

d E r (z,t) = cn(t) E f (t) - a E c (z,t) (A2) dz 2

where £ is a coupling constant proportional to and a is the intensity absorption coefficient. Equation A2 neglects phase mismatch and pump depletion. These effects can be important in certain circumstances, but are not required for the modelling of the temporal characteristics of the results in Chapter 7 on ultrashort pulses.

One familiar result can immediately be produced from equations A1 and A2 when the grating lifetime is very short (i.e. 3ri/dt = 0):

E (£,t) E f2 (t) E *(t) (A3) c x p

It displays the nature of the phase conjugate FWM (Ec a Ep*). However, when one uses pulses of duration shorter than or comparable to that of the grating lifetime a more complex solution is produced for the grating amplitude:

t n(t) = / Ef(t") Ep*(t') exp {-(t - Q }dt (A4) tD 173 -

ii) Analytical Modelling of Experimental Arrangement of Chapter 7

In the following analyses the experimental arrangement of

Chapter 7 will be modelled. A mode-locked train of Gaussian-shaped pulses 100ps duration) is incident on the sample. The probe beam is produced by inserting a beamsplitter into the main beam. By using a variable delay arm in the pump beam path, the probe pulses can be delayed with respect to the pump beam by a variable time t (Figure A1).

Fig A.l Geometry and .labelling for forward-travelling DFWM

We will be looking for solutions for the output phase conjugate signal (I (t)) for different relative delay times x between the pump and probe fields, where:

I c (t) « |n(t)|2 I f (t)

If(t) « exp{-( / tf)2} (A5)

I (t) « I.C (t + x) « Pp r

a) Instantaneous Grating (tD << t^)

When the grating lifetime (t^) is very short compared to the laser pulse duration (t^) the grating will respond almost instantaneously to the driving fields without any significant transient build-up time i.e. dp/dt = 0. Combining equations A1 and A2 gives the 174 -

normal FWM solution:

ic(t> « if2(t) i (t)

for Gaussian pulses:

I c (t) ^ exp - (3t2 + 2xt + x 2 ) / t ^ 2 (A6)

One can deduce several important points from this equation about the nature of the conjugate signal. Firstly, it has its peak value for a given probe delay t at a position displaced from that of the pump pulse by a time duration:

t = t /3 (A7) m a x

Secondly, it is reduced in pulse duration from the original pulses and assumes a FWHM:

t , = t i = 0.577tf (A8) 2 2

Thirdly, the peak conjugate intensity falls off from zero delay as:

Tm a x ____ (t ) (A9) ^.max t c ( =0)

As a numerical example, for an input pulse duration (FWHM) of

100ps, the conjugate pulse duration would be 57.7ps and the FWHM of the peak conjugate intensity against t would be 122ps.

b) Intermediate Grating

In common experimental media using laser pulses ^ 100ps duration it cannot usually be assumed that the grating lifetime is 175 -

instantaneous. Instead, one needs to consider the transient build-up of the grating such that its amplitude does not monotonically follow that of the pump pulse. Instead, the peak amplitude of the grating is

attained at a time after the peak of the pump pulse and the maximum

phase conjugate signal attained if the probe pulse is slightly delayed

from the pump pulse. The form of the solution for the conjugate signal

is readily deduced using equations A4 and A5 to be:

/ exp{ - ( I t ' 2- + 2 t 't + x2)/2t|} exp - d t ‘ — 00 l ^

x exp -(t/tf) (A10)

For a full comparison of this analytical solution to the

experimental results for any particular grating lifetime tD and pulse

duration t^ one needs to computationally integrate equation A10.

The trends of the solution compared to the steady-state can in

general be observed. Firstly, the conjugate pulse duration is

increasingly broadened as one increases the grating lifetime tQ. For

example with 100ps (FWHM) inputs the output conjugate duration varies

as 59ps at tD = 10ps; 70ps at tD = 100ps up to a maximum duration of

about 75ps as the grating lifetime becomes infinite compared to the

duration of any single pulse in the mode-locked pulse train.

The pump-probe delay time x at which the maximum conjugate signal

is achieved is also increased as the grating lifetime increases. At

t = 10ps the maximum signal occurs when t = 4ps, s i m i l a r l y at

tQ = 100ps, t = I4ps and as tQ becomes very large compared to the laser

pulse duration, t ^ 20ps.

c) Long Lifetime Gratings

In the experiments of Chapter 7 a mode-locked train of pulses

was used as the pump and probe and the conjugate output was temporally - 176 -

resolved using a Synchroscan streak camera that integrated over the entire pulse train. It is also important therefore, to consider the regime in which the grating lifetime is comparable to the pulse to pulse repetition time ( t 'MOns). This is the case in silicon in which the grating lifetime t D was approximately 10ns.

To model this regime one can make the assumption that because the envelope of the mode-locked pulse train is slowly varying, the grating acquires a quasi-equilibrium in which the grating decay between each pulse is compensated for when the next pulse arrives. A solution for the grating amplitude during a particular pulse is therefore:

t n(t) = n + / aEpE * dt^ (All) eq J f p

where n is the grating amplitude just prior to the arrival of the pulse. It may also be shown that the value of n for any given pump- eq probe delay t is given by:

00 n = a / E^E * (x=0) dt.exp (“l2/4tf2) (A12) exp (tc/tc) - 1

One can then readily solve for the conjugate signal using equation A5.

As a numerical example, consider the experimental case in Chapter

7 in which silicon was used as the nonlinear medium, in which tn = 10ns, t = 10ns and t ^ (FWHM) = 100ps. It is found that the conjugate signal has an output duration of about 85ps (FWHM) varying little with delay t . The peak conjugate signal occurs at a value of t of about 20ps. This peak conjugate signal falls to its half-value in

±65ps (FWHM 130ps). The experimental results are consistent with the above pulse duration and optimum delay time. However, the fall-off of the peak signal is much less O^50ps) than the theoretical value and can be interpreted by the pulses being non-bandwidth-limited. 177 -

REFERENCES

(1) A. Yariv; IEEE J. Quant. Elect., QE-14 (1978) 650, and references

t h e r e i n

(2) "Optical Phase Conjugation" Ed. R.A. Fisher, (Academic Press,

(1983) and articles therein

(3) Special issue on "Nonlinear Optical Phase Conjugation", Optical

Engineering 2J_ (March/April 1982)

(4) B. Ya. Zel'dovich, V.I. Popovichev, V.V. Ragul'skii and

F.S. Fasisullov; J.E.T.P. Lett., J_5 (1972) 109

(5) O.Y. Nosach, V.I. Popovichev, V.V. Ragul’skii and F.S. Fasisullov;

J.E.T.P. Lett., J_6 (1972) 435

(6) R.W. Hellwarth; J. Opt. Soc. Am., 67_ (1977) 1

(7) A. Yariv and D.M. Pepper; Opt. Lett., _1_ (1977) 16

(8) D.M. Bloom and G.C. Bjorklund; Appl. Phys. Lett., 3_1_ (1977) 592

(9) D.M. Pepper in ref (2), page 161

(10) R.K. Jain in ref (2), page 200

(11) J.O. Tocho, W. Sibbett and D.J. Bradley; Optics Comm., 34. (1980) 122

(12) J.P. Woerdman; Optics Comm., (1970) 212

(13) K. Jarasiunas and J. Vaitkus; Phys. Stat. Sol., A44 (1977) 793

(14) R.K. Jain and M.B. Klein; Appl. Phys. Lett., 35. (1979) 454

(15) R.K. Jain, M.B. Klein and R.C. Lind; Opt. Lett., _4 (1979) 328

(16) E.W. Van Stryland, A.L. Smirl, T.F. Boggess, M.J. Soileau,

B.S. Wherrett and F.A. Hopf in "Picosecond Phenomena III",

(Springer-Verlag, 1982)

(17) E.I. Moses and F.Y. Wu; Opt. Lett., 5 (1980) 300

(18) Y. Silberberg and I. Bar-Joseph; IEEE J, Quant. Elect., QE-17

(1981) 1967

(19) J.O. Tocho, W. Sibbett and D.J. Bradley; Optics Comm., 37. (1981)

67 178 -

(20) W. Margulis (Imperial College); private communication

(21) See for example "Optics” E. Hecht and A. Zajac, (Addison-Wesley,

1974) page 80

(22) R.K, Jain in ref (2), page 214

(23) F.A, Hopf, A. Tomita and T. Liepmann; Optics Comm,, 31_ (1981) 72

(24) C.V. Heer and N.C. Griffen; Opt. Lett., 4_ (1979) 239

(25) H.J. Eichler, U, Klein and D. Langhams; Appl, Phys,, 2_1_ (1980)

215

(26) Reticon photodiode array data sheet

(27) B.H, Soffer and R.H. Hoskins; Nature 204 (1964) 276

(28) W, Koechner "Solid State Laser Engineering" (Springer-Verlag,

1976) page 439

(29) W. Kranitsky, B. Kopainsky, W. Kaiser, K.H. Drexhage and

G.A. Reynolds; Optics Comm., _36 (1981) 149

(30) D. von der Linde and K.F. Rogers; IEEE J. Quant, Elect., QE-9

(1973) 960

(31) A. Maruani; IEEE J, Quant. Elect,, QE-16 (1980) 558

(32) M.E. Mack; Phys. Rev. Lett., 22 (1969) 13

(33) R.L, Carman, R.Y. Chiao and P.L. Kelley; Phys. Rev. Lett., _17_

(1966) 1281 179 -

CHAPTER VIII

GENERAL CONCLUSIONS

This thesis has been mainly concerned with a detailed characterisation of the continuously-pumped Nd:YAG laser in both mode- locked and simultaneously Q-switched and mode-locked operation, with emphasis on streak camera measurement techniques and application in dye laser pumping and phase conjugation. Interest in this laser as a pico­ second source has evolved primarily during the course of this work, with KTP crystals becoming available for efficient second harmonic generation (1) and with the recent development of extracavity pulse compression using optical fibre/diffraction grating combinations (2,3).

These advances, together with improvements in commercially available

Nd:YAG lasers similar to those undertaken in work described in

Chapter 4, have now established this laser as a genuine competitor to noble gas ion lasers in some applications. These applications, and certain unique capabilities of the system not available to ion lasers, are summarised in this chapter, which also contains a discussion of the major results obtained in this work and the identification of possible areas of future development.

In mode-locked c.w. operation, a commercial Nd:YAG laser has been shown to be capable of producing pulses of duration <50ps at the

fundamental wavelength (1.06ym), in a linearly polarised TEMQ0 beam, with average powers ^7.5 W (at 100MHz). Measures designed to reduce

the various amplitude instability/pulsewidth detuning problems of this

laser have been introduced and the improvements evaluated. Resulting

amplitude fluctuations of less than 3% peak-to-peak and lack of detuning of pulses having typical durations 70-80ps over several hours 180 -

has been achieved. These fundamental (1.06ym) pulses are useful for

synchronous mode-locking of dye and colour centre lasers tunable at wavelengths >1ym (4,5), which cannot be directly pumped by noble-gas

ion lasers. The resulting tunable ultrashort pulses have applications,

particularly, in studies of pulse propagation in silica optical fibres

at the wavelengths of minimum attenuation (1.3, 1.55ym) and dispersion

(1.3pm), relevant to high bit-rate digital optical fibre communications

systems. Soliton laser action of a YAG-fundamental synchronously

pumped colour centre laser, leading to the generation of bandwidth-

limited pulses with durations as short as 130fs (at 1.55ym), has

recently been demonstrated (6), and there will undoubtedly be more widespread use and development of this technique in the future.

In Chapter 3, optical excitation of short-cavity semiconductor

lasers using mode-locked pumping pulses, leading to "gain-switching", was shown to be a useful means of generating ultrashort-duration semi­

conductor laser pulses. Pulses as short as 20ps at 828nm wavelength were produced in this manner using 70-90ps 5l4nm-Argon ion laser pulses

to excite a GaAs/GaAlAs double heterostructure laser (7). A synchronously operated streak camera, used in conjunction with this system, allowed a detailed investigation of the short-pulse dynamics of the semiconductor laser, and was shown to be capable of providing information relevant to the optimisation of gain-switched electrical injection lasers. Work of this type could be extended to InGaAsP alloy semiconductor lasers emitting in the 1.3-1«55ym spectral region, by use of the mode-locked c.w. Nd:YAG fundamental pulses for excitation and an extended response S1-photocathode Synchroscan streak camera (8) for detection purposes.

The nonlinear optical crystal KTP has been demonstrated to be an efficient, high damage threshold, frequency doubler for the mode-locked c.w. Nd:YAG laser. The resulting second harmonic wavelength (532nm) - 181 -

pulses, of durations in the range 30-60ps, are generated with high enough efficiency ('v. 10%) to allow synchronous mode-locking of visible wavelength dye lasers. A frequency doubled YAG-pumped synchronously mode-locked c.w. Rhodamine 6G (R6G) dye laser in a prism-tuned linear cavity has been shown to have similar performance characteristics to its Ar ion 5l4nm-pumped counterpart. Dye laser pulses of duration

1-2ps, tunable over the range 570-615nm, were achieved from this system, with average powers ^ O m W (at 100MHz) at the peak of the tuning curve. The typical duration 532nm pumping pulses ( 50-60ps) were, however, in the range available from a well-optimised Ar ion laser, so shorter dye laser pulsewidths did not result, owing to the weak scaling

(ref. (9) claims a square-root dependence) of dye laser pulse duration with pump pulsewidth. Improvement in dye laser pulsewidth and interpulse jitter characteristics would be more likely if the shortest

532nm pumping pulses obtained (^30ps) could be maintained as stably as those typically achieved (50-60ps). Acousto-optic mode-locking of the

Nd:YAG laser at the third harmonic of the fundamental r.f. modulator drive frequency, preliminary results of which are described in Chapter

4, is a possible means by which these short, stable pulses could be produced.

One particular potential advantage of frequency doubled YAG, as opposed to Ar ion, synchronous pumping, is the improved synchronis- ability to YAG laser-pumped dye amplifier chains. Mourou and coworkers have demonstrated two such systems (10), one where a fraction of the mode-locked c.w. Nd:YAG output was used to seed a YAG regenerative amplifier and another where two mode-locked c.w. Nd:YAG lasers were used, one of which was simultaneously Q-switched, both driven from the same frequency synthesizer. The former provided subpicosecond amplified pulses of a few hundred microjoules energy at 7Hz, while the latter provided subpicosecond pulses up to a few hundred nanojoules at 182 -

500Hz. Techniques of this kind have not yet become widespread, but further development in the near future is expected. Noble gas ion laser pumping of dye lasers offers the advantages of operation both c.w. and mode-locked on many strong lasing lines throughout the visible and u.v. However, dyes are now available (the Styryl dyes (11)) to extend the wavelength coverage of frequency-doubled mode-locked YAG laser pumping well into the near infra-red (up to Mym) and continuous wave powers at 532nm of up to a few watts might result from intracavity frequency doubling of the YAG laser using KTP.

The c.w.-pumped Nd:YAG laser has the facility, not available to noble gas ion lasers, to be simultaneously mode-locked and Q-switched, producing pulses (at 1.06ym) of duration MOOps with peak power up to

1MW at repetition rates MkHz (12). In Chapter 6, the first demonstration of the Synchroscan streak camera as a real time linear picosecond diagnostic of such pulses was described (14) allowing direct confirmation of various aspects of the behaviour of the system previously only inferred from autocorrelation data. In addition, the regime of optimum, stable, short pulse/high peak power performance was easily determined, and it was shown that short mode-locked pulses could be maintained at Q-switch repetition rates >1kHz, without significant decrease in stability or peak power.

The application of the QSML c.w. Nd:YAG laser and Synchroscan streak camera in a detailed investigation of forward-travelling pico­ second phase conjugation (at 1.06ym) in silicon and various organic dye saturable absorber solutions was described in Chapter 7. Phase conjugate reflectivities of up to a few percent were achieved and the temporal characteristics of the pulses were examined using the streak camera. This self-diffraction technique using a suitable nonlinear medium offers the potential for pulsewidth compression of high peak power laser pulses. - 183 -

Finally, it must be mentioned that nearly all the uses and

applications of the mode-locked c.w. Nd:YAG laser mentioned here could

be augmented by the recently developed optical fibre/diffraction

grating pulse compression techniques. Pulses of duration as short as

1.8ps at 1.06ym have been obtained by Kafka et al (3)» by compressing

80ps pulses from a mode-locked c.w. Nd:YAG laser using a single-mode

optical fibre and a grating pair dispersive delay line. These pulses

could be used, for example, in short pulse synchronous pumping of

colour centre lasers operating at wavelengths beyond 1ym. These

authors also reported frequency-doubling slightly broader compressed

pulses, using KTP, resulting in 500mW average power (at 100MHz) 2.8ps

532nm pulses, suitable for dye laser synchronous pumping. Johnson et

al (2) achieved shorter pulses at 532nm (<0.5ps) by compression of the

YAG laser pulses after frequency doubling, and pulsewidths of 0.3ps

were obtained directly from a R6G dye laser synchronously pumped by

these pulses. In addition to the generation of shorter pulses from dye

lasers, indications are that the compressed pulses also offer low

interpulse jitter characteristics and thus high temporal resolution in

conjunction with Synchroscan streak camera systems. Synchroscan streak

intensity profiles (at 532nm) of duration 4ps have recently been

obtained in our laboratory (13) corresponding to the integration of

M O 8 pulses having durations (estimated from autocorrelation

measurements) of 2.8ps.

A further application of the fibre/grating technique is in the

compression of the QSML c.w. Nd:YAG laser pulses. The second and third

harmonics of these pulses may be used, for example, to provide

picosecond dye laser pulses tunable over a wide spectral range.

i 184 -

REFERENCES

(1) F.C. Zumsteg, J.D. Bierlein and T.E. Grier; J. Appl. Phys., 47

(1976) 4980

(2) A.M. Johnson, R.H. Stolen and W.M. Simpson; Appl. Phys. Lett, 4J[

(1984) 729

(3) J.D. Kafka, B.H. Kolner, T. Baer and D.M. Bloom; Opt. Lett., 9.

(1984) 505

(4) A. Seilmeier, W. Kaiser, B. Sens and K.H. Drexhage; Opt. Lett., ^

(1983) 205

(5) L.F. Mollenauer and D.M. Bloom; Opt. Lett., 4_ (1979) 247

(6) L.F. Mollenauer and R.H. Stolen; Opt. Lett., 9_ (1984) 13

(7) M.D. Dawson, W. Sibbett, J.I. Vukusic, P. Dawson, G. Duggan and

C.T. Foxon; Appl. Phys. Lett., _43 (1983) 226

(8) X. Hou, W. Sibbett and B. Weekley; Rev. Sci. Instrum., 5£ (1981)

1487

(9) C.P. Auschnitt and R.K. Jain; Appl. Phys. Lett., _32_ (1978) 727

(10) T. Sizer II, J.D. Kafka, I.N. Duling III, C.W. Gabel and

G.A. Mourou; IEEE J. Quant. Elect., QE-19 (1983) 506

(11) P. Bado, C. Dupuy, K.R. Wilson, R. Boggy, J. Bowen, S. Westra;

Optics Comm., 4^ (1983) 241

(12) D.J. Kuizenga, D.W. Phillion, T. Lund and A.E. Siegman; Optics

Comm., 9 (1973) 221

(13) A.S.L. Gomes, W. Sibbett and J.R. Taylor; to be published

(14) M.D. Dawson, A.S.L. Gomes, W. Sibbett and J.R. Taylor; Optics

Comm., 52 (1984) 295 - 185 -

ACKNOWLEDGEMENTS

I would like to thank my supervisor, Dr Wilson Sibbett, for his excellent guidance throughout this work and Dr Roy Taylor for his endless advice and assistance. I am grateful to all other members of the Laser Group for help and useful discussions, particularly

Drs John Vukusic, Bill Sleat and Kevin Smith.

I am indebted to Dr Mike Damzen for his interest in the phase conjugation experiments and also to all the technical staff of the

Optics Group.

Special thanks are due to Sarah Webb for the skillful typing of this thesis.

I would also like to thank Urvi, Beng and Colin for keeping me well-fed and convincing me that curries aren’t so bad.

Finally, I am indebted to the Science and Engineering Research

Council for financial support during this work. PUBLICATIONS Streak camera study of short pulse generation in an optically pumped GaAs/(GaAI)As laser M. D. Dawson, W. Sibbett, and J. I. Vukusic Optics Section, Blackett Laboratory, Imperial College, Prince Consort Road, London, SW7 2BZ, England P. Dawson, G. Duggan, and C. T. Foxon Philips Research Laboratories, Redhill, Surrey, RH1 5HA, England

(Received 21 March 1983; accepted for publication 25 Ma y 1983)

A synchronously operated picosecond streak camera has been used in a direct time-resolved study of luminescence and laser emission from a GaAs/(GaAl)As double heterostructure laser pumped by < 9 0 ps, 514-nm optical pulses from a mode-locked Ar ion laser. Semiconductor laser pulse widths down to 20 ps were recorded and the dependence of the temporal characteristics of these pulses on average pu m p power was investigated.

P A C S numbers: 42.55.Px, 78.45. + h, 78.55.Ds, 07.68. + m

W e have studied the temporal characteristics of a The experimental arrangement is shown in Fig. 1. The GaAs/(GaAl)As laser at 77 K pumped by pulses (< 90 ps) sample was mounted on a cold finger at 77 K in an optical from an argon ion laser. The time-resolved measurements of cryostat and pumped transversely through the top layer by the stimulated and spontaneous emissions were obtained (in pulses having durations [full width at half-maximum real time) using a Synchroscan1 streak camera system. ( F W H M ) ] of less than 90 ps produced by an acousto-optical- Our results are compared with those of Duguay and ly mode-locked argon ion laser (Spectra Physics Model 164). D a m e n 2 wh o reported the use of a picosecond sum-frequen­ A n extra-cavity attenuator allowed the average pu m p power cy technique to measure the stimulated and spontaneous to be adjusted continuously in the range 30-300 m W without emission from a transverse-junction (GaAl)As laser gain distortion of the beam profile or changing the pulse length. switched by 90-ps electrical pulses. They observed that be­ Infrared semiconductor laser emission (827 nm) and Ar ion low threshold the luminescence had a rise time of 300 ps, the laser pumping light scattered from the surface of the sample luminescence continuing to increase for about 200 ps after were collected and collimated by lens L2 and focused by lens the termination of the current pulse. This phenomenon was L3 onto the streak camera slit (S) which was in turn imaged considered to be due to the time taken by the carriers in onto the camera photocathode (S20 with spectral sensitivity diffusing to the junction. extending beyond 900 nm). Appropriate filters F were insert­ The w-isotype laser structure was grown by molecular ed in front of the slit to transmit preferentially the scattered beam epitaxy in a noncommercial, fully automated system at green pu m p light or infrared luminescence or to attenuate Philips Research Laboratories.5 It consisted of a 0.25-//- suitably the light such that the pulses at both wavelengths thick Ga A s active layer between a 0.05-/z-thick top cladding could be displayed simultaneously on the streak camera. De ­ layer of (GaAl)As (30% Al) and a 1.5-/z-thick, bottom layer tails of the Synchroscan system used here have been given of (GaAl)As (30% Al) grown on a semi-insulating Cr-doped elsewhere.5,6 Briefly, an ~ 4 % intensity component of the G a A s substrate. The thin 0.05-// layer allowed for efficient optical excitation of the Ga A s active region. The structure 83ps was grown using the arsenic molecular species As 2 which has been shown to increase the minority-carrier lifetime in G a A s grown by MB E 4 when compared with material grown 83ps from the molecular species As4. After growth the substrate was thinned to 100 // to ensure good thermal contact between the laser structure and the cold finger of the cryostat and then cleaved into 200-/z-wide bars. c 3 21 ps

O O

> , i/lc a> 4-* c J\Jl (a Time (ps) FIG. 2. (a) Shortest IR pulses, 21-ps FWHM, recorded with shortest (~ 75 ps) pumping pulses under optimum conditions, (b) More typical IR pulse FIG. 1. Schematic of experimental arrangement. profiles 32-ps FWHM; pump power ~ 150 mW in each case.

226 Appl. Phys. Lett. 43 (3), 1 August 1983 0003-6951/83/150226-03$01.00 © 1983 American Institute of Physics 226 FIG. 5. Recorded profiles showing increasing delay between pumping pulse (a) (b) peak and IR pulse peak as average pump power is decreased. The IR pulse FIG. 3. IR spectra under (a) cw excitation, (b) ~ 90 ps Ar + pulse excitation. occurs later in time in each case. Pump power — 150 mW in each case.

Above threshold, with the system optimized, the in­ A r ion laser pulse train was directed by a beamsplitter onto a frared semiconductor laser pulses were observed to have a photodiode/tunnel diode oscillator arrangement. Wh e n the reproducible characteristic intensity profile consisting of a voltage output was amplified it provided the sinusoidal de­ sharp rising edge and a longer trailing edge. For the intensity flection voltage for the streak camera in synchronism with profile of the streak image that is reproduced in Fig. 2(a) the the repetitively excited photoluminescence emission. Thus pulse width is 21 ps (F W H M ) for a pumping pulse less than streak images associated with each luminous event were pre­ 90 ps, but a more typical example is included in Fig. 2(b) cisely superimposed on the camera phosphor and data were where the pulse width is 32 ps. Infrared pulse rise times were accumulated at the frequency of the mode-locked pumping generally in the range 10-20 ps. It is considered likely from pulses. Processing and recording of the streak images were the multimode spectral data shown in Fig. 3 and the short provided by an optical multichannel analyzer (O M A , P A R cavity roundtrip time of ~ 6-7 ps, that these pulses consisted model 1205D) optically coupled to the streak image tube and of a few subpulses not resolved by Synchroscan, which in this the integrated streak profiles were displayed on the storage experiment had a resolution of 10 ps. A more detailed inves­ oscilloscope (SO) or hard copies taken using a chart recorder tigation of these temporal characteristics is planned using a (CR). Photochron 11A Synchroscan streak camera which had a Calibration of the streak images was performed by ac­ resolution of 1 ps when used in conjunction with a passively cumulating and storing an equal number of scans with and mode-locked cw ring dye laser.7 without a 5-cm glass block between lenses L2 and L3. The A significant part of the present study was concerned storage capability of the O M A permitted the sequential re­ with the determination of the dependence of the temporal cording of the streak image before and after the insertion of characteristics of the infrared laser pulses on the power level the optical delay (corresponding to a 2.5-cm path difference of the optical pumping pulses. A trend of decreasing pulse in air) with the additional facility for the simultaneous dis­ width with decreasing pu m p power was observed, Fig. 4. In play of the temporally separated intensity profiles. An inde­ addition (Figs. 4 and 5) the pulses were found to show an pendent calibration of the Ar ion laser pumping pulses was increasing delay with respect to the pumping pulse as the performed before the start of the experiment using the usual optical delay line arrangement composed of mirrors M2,M3, and a beam splitter.5

Average A r4 laser pump power (mW) FIG. 6. Below threshold semiconductor laser emission showing decay to 1/ FIG. 4. IR pulse width and delay between pumping pulse peak and IR pulse e of peak intensity in 346 ps at ~ 30 mW average pump power. Rise time is peak vs average pumping power. 97 ps. Pump pulse is dotted in for reference.

227 Appl. Phys. Lett., Vol. 43, No. 3,1 August 1983 Dawson eta/. 227 power was decreased. A slightly larger delay for a given clusion that what they observed was the finite time for the p u m p power was observed with longer pu m p pulses. The carriers to diffuse from opposite sides of the heterojunction evolution of the delay and pulse narrowing is most probably before recombination occurred, which is not necessary for due to threshold being reached progressively earlier in the luminescence produced by optical pumping. higher power pu m p pulses where gain switching occurs. It is In conclusion, we have demonstrated the suitability of a emphasized that these effects were observed directly in real synchronously operated picosecond streak camera as a diag­ time using the Synchroscan streak camera system. A n ex­ nostic tool for the observation of the temporal characteris­ amination of the scattered Ar ion pu m p pulses showed no tics of short pulse behavior in semiconductor lasers when variation in duration or shape as the power was varied; rise emitting spontaneous or stimulated emission. It is proposed times of 55 ps were typical for the 70-90-ps duration pulses to extend these studies to other types of semiconductor laser used. including electrical injection of stripe lasers mode-locked in Using the high sensitivity of the synchronously operat­ an external cavity. ed streak camera to low intensity repetitive signals, lumines­ The authors express their thanks to Dr. J. R. Taylor for cence profiles of the below threshold emission were also re­ assistance with the streak camera system. The overall finan­ corded. Figure 6 shows a typical example and includes the cial support for the work at the Blackett Laboratory and the scattered Ar ion laser pulse profile. The luminescence profile C A S E studentship to one of us (M D D ) from the UK SE R C is at 77 K had a rise time of 100 ps and a l/e decay time of gratefully acknowleged. ~ 3 5 0 ps. Other measurements of the spontaneous emission decay time at 300 K using a photon counting technique8 gave 'J. R. Taylor, M. C. Adams, and W. Sibbett, Appl. Phys. Lett. 35, 590 an exponential decay time of 600 ps. This minority-carrier (1979). lifetime is probably dominated by nonradiative recombina­ 2M. A. Duguay and T. C. Damen, Appl. Phys. Lett. 40, 667 (1982). 3G. B. Scott, J. C. Roberts, and R. F. Lee, Appl. Phys. Lett. 37, 30 (1980). tion at the interfaces leading to interface recombination ve­ 4C. T. Foxon, P. Dawson, G. Duggan, and G. W. ’t Hooft, Proc. Int. Symp. locities of 4X 104 cm/s at 77 K and 2x 104 cm/s at 300 K on MBE-CST-2, 81, 1982. respectively which are reasonable values for this structure 5M. C. Adams, W. Sibbett, and D. J. Bradley, Opt. Commun. 26,273 (1978). grown under nonoptimized conditions. N o delay was ob­ 6M. C. Adams, W. Sibbett, and D. J. Bradley, Adv. Elect. Electron. Phys. 52, 265(1979). served between the end of the pumping pulse and the lumi­ 7W. Sibbett, W. E. Sleat, J. R. Taylor, and J. P. Willson, Proc. XV Int. Conf. nescence peak in contrast to the case of electrical injection High Speed Photography 1982, San Diego. reported by Duguay and Damen. This reinforces their con­ 8B. Z. Bachrach, Rev. Sci. Instrum. 43, 34 (1972).

228 Appl. Phys. Lett. 43 (3), 1 August 1983 228 Volume 52, number 4 OPTICS COMMUNICATIONS 15 December 1984

CHARACTERISATION OF THE OUTPUT FROM A Q-SWITCHED/MODE-LOCKED CW Nd:YAG LASER

M.D. DA W S O N , A.S.L. GO M E S , W. SIBBETT and J.R. TA Y L O R Photonics Group, Optics Section, Blackett Laboratory, Imperial College, London SW7 2BZ, UK

Received 4 September 1984

Both single-shot and repetitively-operating streak cameras have been used for direct studies of the optical pulses produced by a simultaneously Q-switched and mode-locked cw Nd:YAG laser. Without prelase the mode-locked pulses are observed to be merely highly structured noise-bursts having durations of a few hundred picoseconds while with prelase, short (~100 ps) stable pulses can be obtained at Q-switch repetition rates in excess of 1 kHz. These pulses are compared to those obtain­ able from the mode-locked laser without Q-switching.

The simultaneously Q-switched and mode-locked (also continuously operated) ensured that full equilib­ (QSML) cw Nd : Y A G laser [1] has recently been of rium was constantly maintained in the modulator it­ interest as a pump source for fibre-Raman lasers [2] self. and high repetition rate (500 Hz) amplifiers for sub­ In the absence of Q-switching, the pulses obtain­ picosecond light pulses [3]. It has been used to pro­ able from the laser have been measured to be as short duce picosecond pulses in the 1200— 1600 nm wave­ as 47 ps at the fundamental wavelength (1.064 jum) length range by difference frequency mixing with and 30 ps at the second harmonic (532 nm) using a mode-locked dye laser pulses [4] and both the second synchronously operated (SI-type) streak camera in its [5] and third [6] optical harmonics can be efficiently standard configuration [7] (see fig. la, b). This ob- generated, showing promise as pump sources for mo d ­ erate energy, tunable picosecond dye lasers covering a wide spectral range. In view of these, and other ap­ plications, a detailed study of the temporal character­ istics of the QS M L pulses using the linear streak cam­ era technique has been undertaken. Our QS M L cw Nd : Y A G laser was essentially a commercial system (Quantronix 116 with model 351 Q-switch) which has been modified by stabilisation of the cavity length against thermal variations, differen­ tial micrometer cavity length adjustment and replace­ ment of the 50 MH z mode-lock modulator driver by a home-made crystal oscillator arrangement of high fre­ quency stability (~1 pt in 107/hr) and spectral purity. Use of a dedicated power supply allowed the rf drive to the mode-locker to be applied continuously (even Fig. 1. Shortest recorded Synchroscan streak intensity pro­ with the laser switched off) which together with an in­ files of (a) fundamental and (b) second harmonic pulses from dependent temperature-stabilised water cooling loop the mode locked cw Nd:YAG system (without Q-switching).

0 030-4018/84/503.00 © Elsevier Science Publishers B.V. 295 (North-Holiand Physics Publishing Division) Volume 52, number 4 OPTICS COMMUNICATIONS 15 December 1984

relaxation oscillations beginning 250— 500 (is after the Q-switched pulse in our laser (depending on the loss generated by the Q-switch) followed by low level cw mode-locking. (The limitations on repetition rate im­ posed by this technique are further discussed later in this paper.) The microdensitometer traces of single-shot streaks which were obtained without prelasing at a 500 Hz Q-switch rate are reproduced in figs. 3(a)— (d). They confirm that the “mode-locked” pulses are really noise-bursts, with considerable substructure at these Fig. 2. Experimental arrangement. GS is a gating shutter used lower repetition frequencies as had been inferred from in single shot measurements. autocorrelation data [1]. Single, double and multiple spikes were all observed, some of which had quite short ( < 5 0 ps) duration, the overall width of the served reduction in pulsewidth of the green pulses is noise-envelope being a few hundred picoseconds (typ­ dose to the factor of y/2 expected for extracavity ically 200— 300 ps). This behaviour is to be expected, second harmonic generation. because the build-up time of the Q-switched pulse is The experimental arrangement used to examine the not long enough for short, structureless steady state Q S M L pulses is shown in fig. 2. This permitted simul­ taneous observation of the pulse characteristics using a fast SI vacuum photodiode/oscilloscope (Tektronix l>00 ps 400 ps 7904) combination and both single-shot [8] and Syn­ chroscan streak cameras. Both cameras incorporated Photochron II streak image tubes with S20 photo­ cathodes (requiring the optical second harmonic to be generated for detection purposes) and a preset optical delay line was used for calibration of all streak data. Two distinct regimes of operation of the QS M L cw Nd : Y A G are possible by fine adjustment of the ac­ tive loss per pass introduced by the Q-switch [1]. The laser can be maintained below oscillation threshold between Q-switched pulses producing near oscilloscope- limited “mode-locked” pulses up to Q-switch repeti­ tion rates beyond 10 kHz. Typically in this case (for 400 ps our system) the Q-switched envelopes contain —35— -40 mode-locked pulses having a round-trip time of 10 ns. Alternatively, in a preferred,mode of operation, the laser is allowed to pre-lase at a level close to threshold prior to emission of the Q-switched pulse. This allows the continuously operating active mode-lock mo d ­ ulator to establish short mode-locked pulses approach­ ing the steady state (i.e. un-Q-switched) pulsewidth, in the period preceding the growth of the Q-switched pulse. Approximately 50 pulses were usually contained Fig. 3. Microdensitometer traces of second harmonic pulses from the Q-switched and mode-locked laser without prelase, within the Q-switched envelopes under these condi­ showing the (a) 12th, (b) 25th, (c) 35th pulse shape in the tions. The form of the appropriate prelase signal (see train respectively and (d) a well formed single pulse generated inset of fig. 5(a)) consists of exponentially damped late (35th pulse) in the output train. 296 Volume 52, number 4 OPTICS COMMUNICATIONS 15 December 1984 pulses to be evolved [1]. However, by examining occur and occasionally well-defined pulses were es­ pulses in the latter part of the train it could be seen tablished in this region (see fig. 3(d)). Synchroscan that there was less tendency for multiple spiking to streaks (see later) confirmed the random nature of the pulse formation in time, because broad noisy pulses, which were essentially many integrations of figs. ------—| 200 ps | ------3(a)— (d), were recorded. Under the condition of prelase for the 500 Hz Q-switch rate, shorter ( ~ 7 0 ps at 5 3 2 nm), stable, structureless pulses were produced as evidenced by the microdensitometer traces in figs. 4(a)— (c). More­ over, examination at different points in the train (figs. 4(a), (b), (c) correspond to the 10th, 25th and 30th pulses of the train) showed that the pulsewidths were relatively constant throughout the train. This validates our pulse measurements using the Synchroscan streak camera technique where the mode-locked pulses in each Q-switched envelope are temporally overlapped and integrated on the streak camera phosphor to give a “real-time” streak image. In the usual Synchroscan technique [7], a repeti­ tive deflection voltage is applied to the streak plates in synchronism with the mode-locked cw laser pulse- train or laser-induced luminescence to be time-resolved. For these studies, however, the camera had a repetitive scan at the laser cavity round-trip frequency (100 MHz) but luminous information was only accumulated dur­ ing each QS M L pulse train at the repetition rate of the Q-switch. Thus the individual pulses within each Q- switched envelope (and also during the prelase, where present) were superimposed with picosecond precision on the streak camera phosphor and the information in­ tegrated at repetition rates ~1 kHz. When used with an optical multichannel analyser readout (B & M Spec- tronik OSA) this provided a “real-time” linear pico­ second diagnostic of the QS M L pulses, together with a facility for direct display and recording. The low level of illumination of the streak camera slit required to produce the streak images of fig. 5 showed that sig­ nificant phosphor integration occurs at these 100’s Hz— kHz repetition rates such that image intensifica­ tion is unnecessary which is not true of 10 Hz streak camera operation [9]. Careful measurements using calibrated neutral density filters showed that the pre­ lase makes an insignificant contribution to the recorded streaks at Q-switch repetition rates above 100 Hz. A series of streak images and the correspond­ Fig. 4. As in fig. 3 with prelase showing the (a) 10th, (b) 25th, ing prelase waveforms at various Q-switch frequencies and (c) 30th pulse. are shown in fig. 5. The larger spike or spikes at the

297 Volume 52, number 4 OPTICS COMMUNICATIONS 15 December 1984

Fig. 5. Synchroscan streak intensity profiles for the second harmonic of Q-switched mode-locked outputs with inset prelase wave­ forms at various Q-switch rates (a) 500 Hz, (b) 1 kHz, (c) 2 kHz, (d) 3 kHz and (e) 3.3 kHz. Time calibration is 100 ^/division for the oscillograms. Pulsewidth is ~75 ps (a)—(d) and ~90 ps for (e) which occurs at the first relaxation oscillation. appropriate repetition rate in the oscillogramsticed are thatthe the mode-locked pulsewidth (~75 ps at 532 Q-switched pulse envelopes. (Although notnm) adequatelyonly begins to increase at a frequency where the displayed, their full amplitudes are much largernext than Q-switched pulse occurs at the first relaxation those of the relaxation oscillations.) It shouldoscillation be no­ following the previous pulse. The precise

298 Volume 52, number 4 OPTICS COMMUNICATIONS 15 December 1984 frequency at which this occurs depends on the delay to the first relaxation oscillation and therefore on the >1 l< 20 0p s loss introduced by the Q-switch, but was typically ~3 kHz (3.3 kHz for the case shown in fig. 5 — see fig. ; 5(e) where the Q-switched pulses are superimposed on j \ the first relaxation oscillations which occur slightly 3 / earlier). Therefore, sufficient round-trips of the laser / cavity occur even during this time before the second/ \ relaxation oscillation so that the shortest pulses can r evolve. (The laser does not go below threshold be­ v . tween these initial relaxation oscillations so that the mode-locked pulses do not have to evolve from noise.) These pulses are, however, longer than those obtained from the laser without Q-switching (see fig. 1), al­ ^ • * . 'J though pulses as short as 83 ps at 1.06 qm have been r ■'% observed, mainly because they develop in a regime of higher loss. (We have confirmed that the short pulses recorded at 532 nm are reduced in width by a factorb > ' f 1 218 ps ofy/2 compared to the fundamental by repeating \ these measurements with a SI photocathode streak / camera.) When the frequency was further increased such that the next Q-switched pulse occurred before the first relaxation oscillation, the pulses became broad and less regular. We have directly confirmed, as con­ /S cluded in [1] , that these recorded integrated pulse / \ | envelopes become narrower and less noisy as the Q- switch frequency is increased (fig. 6) as a result of the \< 160 ps lower gain giving rise to a longer build-up timec for 7 \ i the Q-switched pulse, thereby allowing the mode­ / \ locking process to proceed closer to the steady state. An integrated profile of the noise-bursts occurring_____ at A sub kHz repetition rates without prelasing is also in­ cluded (fig. 6(a)) for comparison with the single shot results shown previously. Fig. 6. Synchroscan streak intensity profiles for Q-switched Manufacturers data set a typical upper Q-switchmode-locked pulses produced without prelase at (a) 700 Hz, frequency limit ~800 Hz for the optimum regime(b) 4 kHzof and (c) 25 kHz repetition rates. Second calibration short pulse high peak power (~1MW in 100pulse ps not at includedpeak in this case. of the Q-switched train) performance achievable with prelasing in a system of this type. It is certainlylase truehas been established before the repetition rate is that the most stable output occurs at repetitionincreased. rates In addition, we have found (using measure­ up to this value (~l|% peak-to-peak fluctuationments with at a calibrated vacuum photodiode) that the the peak of the Q-switched train) but these streakpower re­ at the peak of the Q-switched train does not sults have shown directly that short pulsesfall can off be significantly at these higher repetition rates maintained at repetition rates exceeding 1 kHz(up and to ~3 kHz), as is also the case for purely Q- we have observed that the correspondingswitched decrease operation in when prelasing is allowed (in con­ stability need only be small (to as little as 3—4%trast peak- to when it is not, in which case the peak power to-peak at the maximum of the train) if a stablebegins pre- to fall off at approximately 1 kHz). Thus the

299 Volume 52, number 4 OPTICS COMMUNICATIONS 15 December 1984 high peak power is maintained, possibly due to some References of the inversion which would normally produce relax­ ation oscillations and low-level cw oscillation helping [1 ] D.J. Kuizenga, D.W. Phillion, T. Lund and A.E. Siegman, to maintain the peak power in the QS M L pulses. Optics Comm. 9 (1973) 221. In conclusion, we have demonstrated the useful­ [2] K. Kitayama, Y. Kato, S. Seikai and M. Tateda, Appl. Op­ tics 20 (1981)2428. ness of streak cameras as real-time diagnostics of mode- [3] T. Sizer II, J.D. Kafka, I.N. Duling III, C.W. Gabel and locked pulses Q-switched at repetition rates ~1 kHz. G.A. Mourou, IEEE J. Quantum Elect. QE-19 (1983) 506. This has allowed the optimum regime of short pulse [4] D. Cotter and K.I. White, Optics Comm. 49 (1984) 205. high peak power operation to be clearly established [5] K.A. Nelson and M.D. Fayer, J. Chem. Phys. 72 (1980) for the Q-switched and mode-locked cw Nd r Y A G laser. 5202. [6] K.C. Liu, G. Vaillancourt and M.G. Cohen, Paper ThC3, Conference on Lasers and Electro-optics (June 1984, Anaheim California) Technical Digest. Acknowledgements [7] W. Sibbett, Proc. 15th Inter. Conf. on High speed photo­ graphy and photonics (1982) p. 15, and references therein. W e would like to acknowledge useful discussions [8] D.J. Bradley, Ultrashort light pulses, Topics in applied physics, Vol. 18, ed. S.L. Shapiro (Springer, 1977) p. 17, with M.G. Cohen and G. Vaillancourt of the Quan- and references therein. tronix Corporation. Postgraduate support for one of [9] R. Yen, P.M. Downey, C.V. Shank and D.H. Auston, us (M.D.D.) by SERC (UK), another (A.S.L.G.) by Appl. Phys. Lett. 44 (1984) 718. the CNPq (Brazilian Agency) and overall financial sup­ port by the Science and Engineering Research Council are gratefully acknowledged.

I

300