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).