Barc/2011/E/022 Barc/2011/E/022 Development of the Chirped

BARC/2011/E/022 BARC/2011/E/022

DEVELOPMENT OF THE CHIRPED PULSE AMPLIFICATION TECHNIQUE FOR HIGH PEAK POWER PRODUCTION WITH Nd: GLASS LASER SYSTEM by Paramita Deb, Kailash C. Gupta and Jayant K. Fuloria High Pressure & Synchrotron Radiation Physics Division Physics Group

2011 BARC/2011/E/022

GOVERNMENT OF INDIA ATOMIC ENERGY COMMISSION BARC/2011/E/022

BHABHA ATOMIC RESEARCH CENTRE MUMBAI, INDIA 2011 BARC/2011/E/022

BIBLIOGRAPHIC DESCRIPTION SHEET FOR TECHNICAL REPORT (as per IS : 9400 - 1980)

01 Security classification : Unclassified

02 Distribution : External

03 Report status : New

04 Series : BARC External

05 Report type : Technical Report

06 Report No. : BARC/2011/E/022

07 Part No. or Volume No. :

08 Contract No. :

10 Title and subtitle : Development of the chirped pulse amplification technique for high peak power production with Nd: glass laser system

11 Collation : 33 p., 20 figs.

13 Project No. :

20 Personal author(s) : Paramita Deb; Kailash C. Gupta; Jayant K. Fuloria

21 Affiliation of author(s) : High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai

22 Corporate author(s) : Bhabha Atomic Research Centre, Mumbai - 400 085

23 Originating unit : High Pressure and Synchrotron Radiation Physics Division, BARC, Mumbai

24 Sponsor(s) Name : Department of Atomic Energy

Type : Government

Contd... BARC/2011/E/022

30 Date of submission : September 2011

31 Publication/Issue date : October 2011

40 Publisher/Distributor : Head, Scientific Information Resource Division, Bhabha Atomic Research Centre, Mumbai

42 Form of distribution : Hard copy

50 Language of text : English

51 Language of summary : English

52 No. of references : 18 refs.

53 Gives data on :

60 Abstract : There has been a large amount of development in the field of ultra short pulsed lasers and high peak intensity laser systems. Therefore the objective of this work was to develop the concept of chirped pulse amplification (CPA), that is essential for any high-peak-power laser technology. The design of the CPA system using Nd:glass as the active medium has been developed in this laboratory at B.A.R.C. for the first time. The beginning is with a 100MHz train of 200fs pulses, that is supported by a 7nm FWHM bandwidth and a central wavelength at 1056nm. A single pulse of 70pJ energy is selected from this train after stretching the pulse to 266ps. The purpose of stretching is to decrease the instantaneous intensity, so that no non-linear effect or damage is introduced in the amplifying medium (Nd:glass) during amplification. This stretched pulse is amplified to about 40mJ and then compressed to a 1.5ps pulse having a bandwidth of 3.8nm. Pulse width expansion and compression is achieved by means of conjugate grating pairs. A real-time autocorrellator set up measures the stretched as well as the compressed pulse.

70 Keywords/Descriptors : NEODYMIUM LASERS; PULSE AMPLIFIERS; SPECIFICATIONS; SYNCHROTRON RADIATION; SYNCHROTRON OSCILLATION; PERFORMANCE; GRATINGS

71 INIS Subject Category : S46

99 Supplementary elements :

Abstract

There has been a large amount of development in the field of ultra short pulsed lasers and high peak intensity laser systems. Therefore the objective of this work was to develop the concept of chirped pulse amplification ( CPA) , that is essential for any high-peak-power laser technology. The design of the CPA system using Nd:glass as the active medium has been developed in this laboratory at B.A.R.C for the first time. The beginning is with a 100MHz train of 200fs pulses , that is supported by a 7nm FWHM bandwidth and a central wavelength at 1056nm. A single pulse of 70pJ energy is selected from this train after stretching the pulse to 266ps . The purpose of stretching is to decrease the instantaneous intensity, so that no non-linear effect or damage is introduced in the amplifying medium (Nd:glass) during amplification. This stretched pulse is amplified to about 40mJ and then compressed to a 1.5ps pulse having a bandwidth of 3.8nm. Pulse width expansion and compression is achieved by means of conjugate grating pairs. A real – time autocorrellator set up measures the stretched as well as the compressed pulse.

Contents

Page

CHAPTER 1.

1. Introduction 1

2. Schematic of the Chirped Pulse Amplification Set-up 2

CHAPTER 2.

1. Femtosecond range Oscillator 7

2. The Stretcher 9

3. Pulse selector 12

4. Regenerative Amplifier 13

5. Pulse Cleaner 19

6. Double pass Amplifier and Single pass amplifier 20

7. The Compressor 22

CHAPTER 3.

1. Synchronized triggering of the laser system 24

2. Conclusion 27

CHAPTER 1.

1. Introduction The chirped pulse amplification (CPA) concept is almost a quantum jump in the techniques of high peak power generation in laser systems. In the conventional master – oscillator power amplifier (MOPA) laser system, a tiny laser pulse is passed through a series of optical amplifiers. This pulse is amplified until it begins to incur one of the several nonlinear problems associated with intense light. An important condition for efficient extraction of stored energy from the laser amplifiers is the operation at fluences close to the characteristic saturation fluence for the given laser amplifier medium. This demand is easily met by nanosecond pulse amplifiers. In Nd: glass laser systems built earlier for amplifying nanosecond pulses, each amplifier stage was pumped with more and more energy, so that extraction could be high. Each stage of the amplifier rod had to be of greater diameter than the previous one so as not to cause intensity dependent damage or nonlinear effects in the amplifier rods. With huge installations came problems of space and operation logistics too. The advent of Chirped pulse amplification [1,2] technique (CPA), has made the entire laser system more compact , so that lasers of even tens of terawatts of peak power can be accommodated in a single room. What would happen if femtosecond laser pulses were amplified in a chain of Nd: glass amplifiers? Femtosecond laser pulses pack very high peak powers and high associated electric fields even in modest energy pulses. Therefore this would result in induced beam distortions and optical damage in components through which they propagate. At high intensities materials begin to explicitly show the dependence of their index of refraction on intensity, and therefore intense light begins to suffer phase delays relative to less intense light. The accumulated phase lag suffered by light in traveling through a medium is given by the B-integral. Thus a beam with a Gaussian intensity profile in the cross-section, passing through glass suffers a delay of phase at the centre of the beam which differs from that at the beam edge. This alteration of phase brings about self-focusing [3], filamentation and self phase modulation and birefringent effects. If self focusing occurs in a high power Nd: glass laser system, it can cause the beam to focus within the laser amplifier rod causing catastrophic results. In filamentation instead of the whole beam focusing, it is possible that the beam is not perfectly smooth in its spatial intensity profile and will break up into beamlets and each of these beam lets will focus , causing damage to optical components. Laser induced birefringence also depends on the laser beam intensity. Pulse propagation in MOPA chains (Master oscillator and power amplifiers), is effected severely when the bi- 1

refringent effect [4] causes large beam divergence as compared to well formed circular cross sectioned beam. With birefringence, conoscopic patterns are formed in one amplifier output, which propagates into the next amplifier, is amplified and then moves into the regime of a larger divergent beam, until it is impossible to propagate it further. Self – phase modulation causes the laser pulse to acquire new frequencies and therefore becomes optically chirped. Other than the described nonlinear optical damage, maximum power or fluence of a laser system may be limited by the small absorption of laser energy by components of the laser system. This leads to a damage threshold for laser mirrors and other components which is seen ultimately to depend on wavelength and duration of laser pulse. The solution to the above problems is the CPA technique where the femtosecond range laser pulse is time stretched to lower – peak intensity pulse of the same energy. This pulse can be amplified safely to a higher energy and afterwards reconstituted or recompressed as a very short pulse of enormous peak power. Generally a pair of diffraction gratings are used to stretch the pulse. In proportion to the time expansion of the pulse the peak intensity of the pulse drops. The pulse can be amplified to substantial energies without encountering intensity-related problems. When amplification is complete a complementary arrangement of diffraction gratings can recompress the pulse but now with orders of magnitude greater peak power than would have been possible without the CPA technique.

2. Schematic of the Chirped Pulse Amplification Set-up. The idea of chirped pulse was initially proposed for a different issue. It was a solution for the conflicting requirements of simultaneous long range and high resolution performance in a radar system. The technique of chirp was introduced because resolution depends on transmitted pulse bandwidth. In the radar system the transmitted pulse is a wide pulse in which the carrier frequency is modulated and then by proper signal processing a time compression of the received signal to a much narrower pulse is achieved of higher effective power. Gerard Mourou [1,2] suggested that this technique can also be applied in the optical domain with revolutionary consequences for laser science and technology. Figure 1 depicts the idea that it is possible to manipulate the temporal characteristics of an ultra short pulse , to obtain high peak powers. The inherent property of an ultra short pulse is the broad spectral bandwidth. Therefore using two complementary devices called the stretcher and compressor, the spectral characteristics can be tailored and therefore the temporal characteristics too.

Ultra short pulse

Chirped and stretched pulse

Fig1. The Principle of Chirped Pulse Amplification

Amplified chirped pulse

Recompressed pulse

Time axis

In chirped pulse amplification (CPA) the oscillator pulse is a transform limited pulse with sufficient bandwidth to support the desired pulse duration. This pulse is temporally stretched in a pulse stretcher set up ,where the pulse is chirped or in other words there is a variation in frequency with time . The duration of the stretched pulse is adjusted, so that the pulse intensity in the amplifier remains below the limit imposed by nonlinear distortions in the amplifier medium. The stretched pulse is amplified to the desired value and then compressed back almost to the original pulse duration by folding back all the spectral components of the pulse. The compressor too works by the same principle of large time delay for the different spectral components, so that the dispersion closely matches that of the stretcher. The CPA process can result in amplitude and phase distortions preventing the optimal recompression of the laser pulse. The spatial and temporal distortions are introduced not only in the stretcher

and compressor but also in the amplifying medium. Therefore a careful design of the total system has to be done in order to obtain the best possible peak powers from a laser chain. The components of the CPA system that were assembled and built in house at BARC are the following- (a)The femto-second range Oscillator, (b)The double pass holographic grating stretcher, (c) The single pulse selector, (d) The Regenerative amplifier, (e)The double pass linear amplifier, (f) The single pass linear amplifier and (g) The double pass holographic grating compressor. Figure 2 shows the schematic arrangement and Figure3 is the photograph of the total setup. Each of these components will be described in details in the next chapter. The electronics that drives the total laser system consists broadly of power supplies and trigger systems. The amplifier flash lamps need energy storage capacitors which are charged to a desired voltage with the help of charging circuits, and then discharged during the firing of the flash lamps. Then the Pockel’s cells of the system are driven by high voltage and high speed switching circuits, thereby functioning the pulse selector, pulse cleaner and the regenerative amplifier. A trigger generation system has to be in place with precise timing signals of low jitter and variable delays . The best performance of a CPA system requires the optimization of three key factors. They are, energy extraction, pulse duration and pulse contrast ratio. The practical limit of maximum energy extractable is determined by the damage threshold of the compressor gratings available commercially. The pulse duration is determined by the output spectrum and the ability of the compressor to reconstruct the Fourier transform of the spectrum properly. Therefore the methods of reducing gain narrowing is important. The pulse contrast ratio needs to be controlled so that as the main pulse intensity is increased, the pre-pulse intensity should be minimized. Therefore amplified spontaneous emission during the amplification process needs to be reduced. These factors have been discussed in the following chapter. This technological know how, paves the path towards the building of Multi-Tera-Watt laser systems. With high peak powers and large focused intensities it is possible to experiment with plasma wake-field acceleration and laser driven photo-transmutation. This work is part of the planned project “Nuclear Reactions & Accelerator Physics with Ultra Intense Pulsed Lasers”.

Mode Locked Nd:Glass Oscillator

Stretche r GR GR LL Pulse Select M3 or CL

λ/4 Aperture

Nd:Glass Rod TFP M1 PC1 PC2M2 PD Regenerative Amplifier

Glan Polarizer PC3 PD1

QWP Nd:Glass Rod M3 TFP1 M4

Double pass Amplifier

M6 GR Compressor Nd:Glass Rod M5

Single Pass Amplifier M7 GR

Auto Correlator

Fig 2: Schematic layout of the complete Chirped Pulse Amplification set up, with the laser beam path through the oscillator, stretcher, pulse selector, regenerative amplifier , linear amplifiers and pulse compressor.

Laser Oscillator & Stretcher

Auto-correllator Regenerative Amplifier

Double Pass Linear Amplifier

Compressor

Fig3. Photograph of the complete CPA laser system.

CHAPTER 2.

1. Femtosecond range Oscillator: The oscillator is a passively mode locked system with an Nd:glass (fluoro – phosphate) active medium and a saturable absorber providing an intensity dependent loss in a cavity. When a saturable absorber is inserted into a laser cavity, the natural, small fluctuations of the laser are continually enhanced until a pulse is formed after a build up time. The system relies on diode pumping of the active medium (Nd: glass) and uses a SEmiconductor SAturable Absorber Mirror (SESAM) [5] to start and stabilize the pulse forming process. The propagation of the pulse inside the oscillator cavity can be described by the non linear Schrodinger equation. A pulse with a sech2 intensity profile is an exact stable solution of the non-linear wave equation.

M6 OC SESAM M1

BC P2

M5 M2 P1

M4 Nd:Glass M3

Diode Pumped module 1 Diode Pumped module 2

Schematic cavity layout for the GLX-200

Fig4. Schematic layout of the oscillator with a bow-tie arrangement . The red line is the beam path within the cavity. The active medium is Nd:phlorophosphate glass. SESAM-semiconductor saturable absorbe mirrorr, P1,P2- prisms, OC- output coupler, M1,M2,M3,M4,M5 and M6- mirrors.

OC M2 P2 SESA M6 M1 M5 Diode Pumped P1 dl 2 M3 M4

Nd:Glass Diode Pumped dl 1

Fig5. Photograph of the 200fs, 100MHz oscillator

This is a soliton pulse and it propagates without changing its duration. Soliton propagation occurs when the nonlinearity in the medium is balanced by the dispersion in the cavity. These pulses form a pulse train of mode locked , transform limited 200femtosecond pulses with a centre wavelength of 1056 nm. The spacing between the pulses is set by the length of the oscillator cavity [6], resulting in a repetition frequency near 100MHz. The oscillator cavity is aligned in the “bow-tie” mode. The average power output from the oscillator is about 45mW. Figure 4 is a schematic of the oscillator showing the “bow-tie” arrangement of the optical components of the resonator cavity. Figure5 is the photograph of the oscillator (GLX-200 – Time-Bandwidth products) and Figure 6 is the spectrum of the output beam. The FWHM is measured to be 7nm.

Fig6. Spectral profile of the oscillator beam.

2. The Stretcher. For the setting up of a stretcher system , estimation of group velocity dispersion plays the most important role in the design. Group velocity dispersion (GVD) [7] is the result of different fourier components of a pulse traveling at different phase velocities. The optical path length of different wavelengths varies either because an optical material actually disperses the wavelengths, or because the wavelength components are made to travel different path lengths. One observable consequence of GVD is that different frequency components of an optical signal will propagate at different speeds through a dispersive medium and this leads to changes in the temporal profile of an optical signal though it does not change the spectral profile. Since the time Martinez [8] has shown that a pair of gratings can generate both positive and negative second order GVD, gratings have generally been used in chirped pulse amplification , both for stretching and compression of pulses. We will be interested in the extent to which a pulse can be stretched in time. The physical origin of group velocity dispersion can be attributed to angular dispersion. Gratings are used to generate the angular dispersion. The transit time dispersion increases as the pulse propagates away from the first grating of the pair. After the desired dispersion is obtained a second grating is used to re collimate the beam. An anti-parallel grating pair with a telescope between can produce positive GVD and therefore a stretched pulse is produced in time.

f f

Gr 2f Gr

Z 1 L1 L2 Z2

Focal Plane Focal Plane M1

Fig7: Schematic of a double pass stretcher. L1,L2- Achromatic lenses forming the unit magnification telescope, f-focal length,Gr- Holographic Gratings, M1-100% reflecting mirror.

The extent of pulse stretching is determined by the orientation of the gratings and the distance between them. In the case of a stretcher where a unit magnification telescope is used, the respective distance of each grating from the nearest focal point, the sum of the focal lengths and the effective distance between the gratings needs to be optimized to get the best results. Fig7 shows the schematic of a double pass stretcher. Generally the stretcher assembly is a double pass configuration , that , not only gives a larger dispersion but also eliminates spectral walk-off at the end of the double pass. The gratings are placed in near Littrow arrangement, therefore optimized value for angle of incidence has to be calculated.

Fig.8. Lay out of the stretcher using two holographic gratings. Two achromatic lenses of focal length 60 cm forms the unit magnification telescope. The 200 fs , 100MHz oscillator (GLX – 200) is seen in the backdrop.

The parameters of the configuration are, incident pulse width, wavelength of incident beam, the bandwidth, number of grooves on the grating used, angle of incidence on the first grating and focal length of the lenses used in the stretcher. Changes in pulse length may be accomplished by adjusting the beam incident angle on the grating and the distance between

the gratings. Fig.8 is the stretcher setup in our laboratory. The incident pulse is a transform limited pulse of 200 fs, incident at an angle which is 80 more than Littrow angle, on the holographic gratings. Two types of gold coated holographic gratings were used to setup the stretcher [9] . In case of the 1800 lines/mm grating the stretched pulse is about a nanosecond long. Here the angle of incidence is large, and then not only do we get spectral clip off at the lenses, but we also have spectral clip off at the grating. This of course can be avoided by using large sized lenses and longer gratings. Another way to avoid spectral clip off is to use gratings with 1200 lines / mm grooves. Here the angle of incidence need not be large and so the grating size can be smaller, and at the same time because the spectral spread is less , the lenses of the telescope too can be smaller in diameter. The spectral bandwidth is important and maximum effort is put to preserve the oscillator bandwidth , because we know that retaining the entire bandwidth ensures that the compressed pulse can be reduced to a minimum possible. The performance of the stretcher is also governed by finite beam size, divergence , lateral walk off of different components and the aperture of the telescope. Though in an ideal double pass stretcher there should be no spectral walk off at the end of two passes, a real stretcher always gives rise to an elliptical beam due to introduction of astigmatism and spectral walk off.

Fig9. The near field and far field spatial intensity profile after a double pass stretcher setup. The beam has a major axis of 3.5mm and a minor axis of 2.6mm and with a circularity of 0.7. The elliptical profile is a consequence of the beam passage through the stretcher assembly.

Therefore alignment techniques have to be directed towards the reduction of ellipticity. Figure9 show the near field and far field spatial profile of the pulse after the stretcher. Before

it enters the regenerative amplifier some corrective measures on the elliptical beam needs to be done. Dispersion is defined as pulse broadening per unit bandwidth. Therefore if the dispersion is known and the spectral band width is known then the pulse broadening can be estimated (Δt / Δω is proportional to β). The parameter β describes the angular dispersion. If we consider the input temporal pulse with a Gaussian profile and with a linear chirp then the width of the pulse at the end of the double pass stretcher will be 1 2 2 2 Here β=λ / (2πcdcosθ0) and τ2 = 4(2kβ (z1+z2)). Here z1+z2 are the respective distances of each grating from the nearest focal point (fig7), ‘d’ is the grating groove spacing and θ0 is the angle of diffraction. Tx is the FWHM. A pair of gratings with 1200lines/mm ,a pair of achromatic lenses with focal length of 60 cm and a reflecting mirror was assembled and optimized. The gratings were placed such that 0 the angle of incidence is 8 away from the Littrow configuration and z1+z2 is 75cm. The theoretical estimate for the stretched pulse is about 74 ps. The stretched pulse was measured with an autocorrellator set up, and the measured pulse length is 44ps. Another configuration used a pair of gratings with 1700lines/mm , pair of achromatic lenses and a reflecting mirror. In this case the gratings were placed such that the angle of incidence is 60 away from the littrow position and z1+z2 is 50cm. The theoretical estimate for the stretched pulse in this case is 266ps. The stretched pulse was measure with an autocorrellator set up and the measured pulse length is 119ps.

3. Pulse selector. From the 100 MHz train of stretched pulses a single pulse is selected before injection into the regenerative amplifier. Each pulse has an energy of about 70pJ. The pulse selector is based on the principle of the electro-optical switch . The in house built pulse selector consists of a pair of KDP crystals, placed in series, between two crossed Glan Polarizers. The input polarizer is parallel to the polarization of the oscillator pulse (p-polarized) and the output polarizer is perpendicular to it. The output polarizer allows the pulse to pass through only when the pair of Pockels cells rotate the p-polarized pulse to a s-polarized pulse. Therefore the Pockels cells are associated with a high voltage driver. When the voltage is applied the p- polarized pulse rotates to an s-polarised pulse. The transmission and the extinction ratio of

the pulse selector is 90% and 100:1 respectively. The Pockels cells are driven by a high voltage pulser made of a series of avalanche transistors. It gives a 10 ns pulse with a rise time of 4ns and the voltage applied to the cells is 3.5kV. The half wave voltage for complete rotation of a p-polarized pulse to a s-polarized pulse is 6.5KV. In order to keep the voltage lower two Pockels cells were placed in series in the optical path and parallel electrically, so that they could be driven by a 3.5kV voltage individually. The voltage pulsar is triggered from a PIN photodiode signal inside the GLX-200 oscillator. The triggering of the Pulse selector has to be accurate , because the separation of the optical pulses is also 10 ns. Therefore the 3.5kV voltage has to be applied to the Pockels cells, just when one pulse from the 100MHz train enters the pulse selector. In this way the rest of the pulses are blocked out. A single s-polarized pulse of energy 70 pJ is ready for injection into the regenerative amplifier.

Fig10. The single pulse selector with two Glan polarizers on either end and two KDP crystals between the polarizers. The power supply, generating a 10 ns trigger pulse of quarter wave voltage to the l 4. Regenerative Amplifier. The regenerative amplifier is a crucial component of the CPA system because its design and working defines important aspects of the laser pulse that will be amplified further in a chain of linear amplifiers. This is the component of the CPA system where the key issues of amplified spontaneous emission (ASE), and gain narrowing have been addressed [10,11]. The regenerative amplifier design is essentially a multi-pass amplifier with a cavity geometry such that the injection of a chirped laser pulse and the ejection of the amplified pulse can be controlled by two optical switches. Such a system is necessary to reach the milli-joule level of energy with one amplifier stage [12] . The regenerative amplifier cavity consists of two KDP Pockels cell, a

M3 CL Oscillator StretcheC Pulse M4 Selector

λ/4 Nd: Glass Rod TFP M1 PC1 PC2 M2 PD PD Regenerative Amplifier

FIG 11. Configuration of the regenerative amplifier.PC1 &PC2 are the Pockels cell, TFP is the thin film polarizer, M1&M2 are the 100% reflecting regenerative cavity mirrors, M3&M4 are the 100% deflection mirrors for seed pulse injection, CL are cylindrical lenses, λ/4 is the Quarter wave plate and PD is the avalanche photodiode.

quarter wave plate , a thin film polarizer and a Nd: silicate glass rod (10 mm diameter ,/ 150 mm long) placed between a curved mirror (6 m radius of curvature ) and a plane mirror separated by 1.5m. Fig11 shows the regenerative amplifier setup schematically and Fig12 is a photograph of the assembly of the opto-mechanical components that form the regenerative amplifier . The single pulse of 266 ps ( or 74ps) duration and 70 pJ energy after the pulse selector is injected into the regenerative amplifier cavity. Before injection the pulse is collimated by two cylindrical lenses. This is because the spatial profile is slightly elliptical (Fig9) as a consequence of the astigmatism introduced by the stretcher. This collimation increases the intensity of the input pulse and also avoids cutoff at the Nd:Glass rod inside the cavity. The Nd:silicate glass rod is flash lamp pumped with an electrical energy of 600J. The Pockels cell, PC1 is triggered and a quarter wave voltage of 3.5KV is applied for the required retardation, to trap a pulse inside the cavity. The evolution of the pulse is monitored on an avalanche photodiode, where a leak pulse from mirror M2 is detected. After the necessary number of round trips the pulse is ejected out by switching on Pockels cell PC2, with a quarter wave voltage of 3.5KV. The ejected pulse is monitored and measured after the thin film polarizer with an avalanche photodiode. A pyroelectric joulemeter (Gentec, Canada) was used at this position to measure the energy of the ejected pulse. The synchronization of the various electro-optic switches, firing of flash lamps and single seed pulse selector is an important part of the regenerative amplifier design and will be described in chapter3.

PC2 Nd:glass rod

Cylindrical PC1 lenses

Fig12 .Photograph of the regenerative amplifier assembly.

4.1. Optimization of the regenerative amplifier. The regenerative amplifier cavity was first optimized in the Q- switched operation and then the seed pulse was directed into the amplifier. The cavity had to be then adjusted for complete seeding i.e no Q- switched signal noted underneath the circulating pulse recorded on the avalanche photodiode, detecting the leak pulse from the mirror M2. In most previously reported regenerative amplifier construction, mode matching is carried out by using intra cavity or extra cavity aperture or lenses in order to obtain complete seeding. In this case we have used a new technique [13] where the alignment of the cavity is changed slightly to partially suppress the buildup of the spontaneous emission into a Q- switched pulse. Simultaneously the seed pulse injection alignment is shifted proportionately (by mirror M3) so that it is in perfect alignment with all the components of the cavity. In this way the seed pulse has an initial advantage of exploiting the gain of the Nd: silicate glass medium as compared to the cavity spontaneous emission buildup. Fig 13 depicts the different stages of the regenerative amplifier working by monitoring the amplified pulse, with two photo diodes (PD). One PD shows the circulating amplified pulse as detected from the leak pulse of mirror M2 and the other PD is placed just after the thin film polarizer (TFP), to detect the ejected pulse as well as leaks from the TFP. Fig13a shows the regenerative cavity working in the Q- switched mode when PC1 is triggered and no seed pulse has been injected. The two pulses are the leak pulses from M2 and TFP. Once the seed pulse has been injected into the regenerative cavity, it is amplified and the circulating seed pulse can be detected by the PD after mirror M2 (fig13b). This figure shows that there is amplified spontaneous emission (ASE) or a Q-switched background, and complete seeding has not been achieved. Fig13c

shows complete seeding with a fully modulated structure giving no indication for a Q- switched background. The adjustments in the seed pulse injection timing ( i.e triggering the Pockels cell PC1.) , the energy dumped into the flash lamps and the seed pulse intensity ( by adjustment of the cylindrical lens pair ) were all aimed to get complete seeding with no Q- switched signal. In the best operating condition we could eject out a single pulse with an energy of 15 mJ. The amplified pulse is ejected out after 100 round trips when the amplification reaches its maximum. Fig13d indicates the ejected pulse as measured by PD after the TFP, when the Pockels cell PC2 is switched on with a quarter wave voltage. The Fig13d also shows the time at which the amplified pulse has been ejected out and that is when the seed has been amplified to the maximum value. Fig13e depicts the ejected pulse with some leak from the TFP and Fig13f depicts the ejected pulse with some post pulses. The regenerative amplifier provides a net gain of 2.1X108 . The amplified pulse spectrum is also recorded with the help of the leak pulses from mirror M2, using a McPherson spectrometer. The spectral bandwidth was measured to be 3.8 nm FWHM. This bandwidth of the output pulse from the regenerative amplifier is much more than achieved earlier from Nd:glass based amplifiers. The reduction of the spectral bandwidth from 9 nm FWHM of the seed pulse to 3.8nm of the amplified pulse is due to the well known gain narrowing where different spectral components experience different gain. The availability of different kinds of glass with shifted fluorescent curves opens the possibility of enlarging the emission bandwidth. Mixing two or three types of Nd:glass rods (silicates and phosphates) in a chain of amplifiers provides a feasible solution for limiting gain narrowing [14] and getting shorter pulses after compression. The seed pulse wavelength has a significant influence on the amplification .In the present set up the centre wavelength of the seed pulse is 1056nm and the fluorescence peak of the Nd: silicate glass gain medium is 1060nm. With the new technique it was possible to get complete seeding in spite of the 4nm mismatch in peak wavelength. With more initial photons of 1056nm ,a good extraction was obtained , though the gain of the amplifying medium is less at this wavelength. Though the initial photons of 1060 nm wavelength is less, it uses the larger gain at this wavelength in the amplifying medium as compare to 1056nm. These two factors balance out and this has made it possible to limit the gain narrowing to 3.8 nm. Gain narrowing is more pronounced in regenerative amplifiers and any gain narrowing caused by subsequent linear amplifier chain is negligible. Therefore getting a very good spectral bandwidth at the regenerative amplifier stage is important.

With the present alignment technique we found that we could pump the Nd:glass medium with a large electrical energy, without having to lose the stored energy in the gain medium to a Q- switched pulse buildup. The stored energy is extracted by the seed pulse only. Thus the energy output from the regenerative amplifier was more. Therefore if the energy extracted and the spectral bandwidth of the output pulse from the regenerative amplifier (the first stage of the total amplifier chain.) is more, then tremendous peak powers can be obtained with modest total energy requirements of the whole system. This makes the total system compact too. The best output energy was obtained with the active medium as Nd:phosphate glass and when the rod ends are anti-reflection coated. Due to the hygroscopic nature of phosphate glass, Nd: silicate glass was used as the active medium for a longer duration of use since it is not hygroscopic. Since silicate glass has a lower gain coefficient than phosphate glass the output energy from the regenerative amplifier was about 4 mJ. This stretched pulse of 4mJ energy was further amplified in linear amplifiers.

a b

c d

e f

50ns/div

Fig13. (a) The Q-switched pulse of the cavity when seed pulse has not been injected, as detected by the two avalanche photodiodes (PD). A trace of the fast rise time pulsar applied to Pockels cell PC1. (b). Seed pulse amplification with a background ASE seen at the base of the circulating seed pulse., as detected by PD after mirror M2. (c) Trace of circulating amplified seed pulse with very little ASE background indicating complete seeding. (d) Trace of the fast rise time pulsar applied to the Pockels cell PC2 ,that rotates the polarization of the amplified pulse and enables it to be ejected out . Trace of the ejected pulse when the circulating seed pulse has reached the maximum amplification. (e) The ejected out single amplified pulse as detected by PD after TFP and also the leak pulses. (f) Single amplified pulse along with a few post pulses too.

5. Pulse Cleaner. The amplified pulse is ejected out after approximately 100 round trips when the amplification reaches its maximum . Fig13 e&f is a trace of the ejected pulse when the Pockels cell PC2 is switched on with a quarter wave voltage. The ejected pulse is detected with an avalanche photodiode placed after the thin film polarizer. The finite extinction ratio of the thin film polarizer, the limited contrast ratio of the Pockels cell crystal (PC2) and the long temporal window of PC2 allows pulses with the wrong polarization to grow in the cavity and appear as the post pulses, separated by the round trip time of the amplifier cavity. This can be reduced considerably with a short temporal window for PC2, such that the window is closed when the post pulse arrives. Another method of removing the post pulses would be to use a pulse selector after the regenerative amplifier cavity. The finite extinction ratio of the thin film polarizer (TFP) and the limited contrast ratio of the Pockels cell crystal allows some energy to leak out of the cavity through the polarizer during the pulse buildup time. The train of pulses is separated by the round trip time in the amplifier resonator. Now if the temporal gating of the Pockels cell is perfect then only the main ejected out pulse will be seen at the output. In our experiment PC2 temporal window is long. This is because the quarter wave voltage applied to the Pockels cell has a fast rise time but a slow fall time , thereby increasing the temporal window. In an ideal case , (i.e a good extinction ratio for the TFP and a very good contrast ratio for the Pockels cell crystal ) this would not matter because when PC2 is active the pulse polarization changes and is ejected out of the cavity and no part of the pulse would remain in the cavity. But it is not an ideal case therefore the train of pulses. Typically the temporal window of a PC is 5-10ns. If it is more than 5 ns then more than one ejected out pulse will be seen. Generally the “ brute force” method of an external pulse cleaner is used . While a Pockels cell can improve the pulse train situation it does little to reduce the ASE. in the temporal window. The pre and post pulses from the regenerative amplifier can create serious problems in laser plasma experiments and also in the efficiency of the amplification process of the following amplifiers. Therefore it is necessary to clean up the pulse before it enters the linear amplifiers. This is done using the pulse cleaner. This system is essentially a pulse selector with two crossed polarizers and a Pockels cell. When a half wave voltage is applied for a duration of 5ns , the polarization of the incoming pulse rotates by 900 . The second polarizer allows the light to go through only during this 5ns window. At all other times the incident light on the pulse cleaner is rejected and not allowed to pass through. Therefore the pre and the post pulses are eliminated. Only the unwanted ASE during this window time is also allowed through. When an input energy of about 4mJ is introduced into 19

the pulse cleaner the out pulse energy was measured to be 2 mJ. The pre pulse to main pulse ratio at the exit of the regenerative amplifier was 200:1. After the pulse cleaner the pre pulse to main pulse ratio improved to 1000:1.

Fig14. Laser pulse devoid of pre pulses or post pulses, as detected by a photo diode after the pulse cleaner.

6.Double pass Amplifier and Single pass amplifier. The clean pulse that is free of pre-pulses and post pulses is amplified further in a linear amplifier that can be operated in a single pass configuration and a double pass configuration [15]. The main pulse to pre pulse ratio measured at this stage was 1000:1. Fig15 is the schematic of the double pass configuration linear amplifier . The active medium is a 10mm diameter and 150mm long Nd: silicate glass rod [16] that is pumped by four flash lamps .The Nd:silicate glass rod, mirrors M3, M4, thin film polarizer TFP, and the quarter wave plate QWP form the double pass configured amplifier. The synchronization of the flash lamp firing and Pockel’s cell triggering is well adjusted with the help of delay generators, so that the out put pulse of the pulse cleaner encounters maximum gain in the amplifier. The output beam is reflected off TFP1 after the double pass, and the energy is measured with a Gentec make energy meter. The amplified pulse trace is detected by photodiode PD2 . Fig16(a) depicts the variation of amplification in a double pass amplifier, as a function of the electrical energy pumped into the flash lamps and fig16(b) is the trace of the amplified pulse trace. At 2.7 KJ of electrical energy pumped into the flash lamps an amplification of 7 was obtained and the output energy was measured to be about 15mJ energy. Next the beam is introduced into a single pass amplifier, having the same characteristics of the double pass amplifier active medium. The amplified pulse was measured to have an energy of 40mJ.

Glan Polarizer PC3 PD1

QWP M3 Nd:Glass Rod M4 TFP1

Glass Plate Double pass Amplifier

Energy Meter PD2

Fig15. Passage of the beam through a pulse cleaner and then a double pass amplifier. PC3 is the Pockels cell,M3&M4 are mirrors,TFP1 is the thin film polarizer,PD1&PD2 are photodiodes.

7 a b 6

Gain 3 2 1

0 0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 Electrical Pump Energy (KJ)

Fig16. (a) Amplification in a double pass amplifier as a function of electrical energy dumped into the four flash lamps. (b) Oscilloscope trace of an amplified pulse after a

double pass configured linear amplifier.

7.The Compressor The CPA process can result in both amplitude and phase distortions, preventing the optimal recompression of the laser pulse. Recompression over many orders of magnitude pulse duration is a process that requires high accuracies in design and manufacturing of optical components in the construction of the compressor. After the amplifiers the laser beam is directed into the pulse compressor. A pair of gratings arranged with their faces and rulings parallel, has the property of producing a time delay with wavelength. Therefore the grating pair can compress a positively chirped incident pulse. The compressor [17] consists of two holographic gratings placed parallel to each other and a mirror that facilitates the double pass in the compressor (schematic in figure 17). Just as in the stretcher the double pass is needed for more dispersion and also to eliminate spectral walk off. At the end of the double pass the laser beam has a nearly circular spatial profile.

Fig17. Schematic of the compressor. Gr- Grating, M- mirror and z +z is the distance between gratings whose values are those taken 1 2 for a compatible stretcher.

Generally if a pair of 1200lines / mm grating has been used in the stretcher then the compressor too should contain a pair of 1200lines/ mm grating in order to get the best compressed pulse. In case of a stretcher with a pair of 1700lines /mm grating, the compressor too consists of a pair of 1700lines/mm grating. Here again ,as in case of the stretcher, the orientation of the grating and the distance between them play an important role in the compressed pulse shape. If we consider the input temporal pulse with a Gaussian profile then the width of the pulse at the end of the double pass compressor will be given by the following expression.

1 2 When the term (δωm/δω0)(τ2/T) is –ve the pulse is compressed and the pulse is compressed 2 to its narrowest possible width of Tx=τ2 /T. If one considers a pair of 1200lines/mm grating with a distance of 60 cms between them and the angle of incidence being 80 away from Littrow angle, then the pulse can be compressed to about 415fs, by theoretical estimates. This estimate does not consider additional chirp or phase distortions introduced due to the pulse passage through various optics in the CPA system. The pulse width is measured with an autocorrelator when two laser pulses overlap temporally and spatially in a BBO crystal generating a second harmonic beam. The autocorrelation trace is detected by a high resolution CCD array and the trace can be read directly on an oscilloscope. The width of the autocorrelation trace (Tao) and the width of the 1/2 compressed pulse (Tx) are related as Tx = Tao/(2) . With the 1200lines/mm grating the measured pulse width was 2 pico seconds and with a 1700lines / mm grating pair the measured pulse width is 1.5ps.

CHAPTER 3 1. Synchronized triggering of the laser system. For efficient operation of the laser system, the maximum energy from the three amplifiers have to be extracted. To achieve this, synchronized triggering of all the related devices have to be perfect and with very low jitter. Now the maximum energy can be extracted by the seed pulse, if it arrives in the amplifier cavity when the population inversion is maximum. Figure 17 is the flash lamp profile as captured by a photodiode when it is triggered and the capacitors are discharged . The duration of the pulse is about 700 μs , including the long tail. The peak of the pulse occurs around 200μs and one expects the population inversion in the active medium to be maximum at this point. Therefore the seed pulse has to arrive at the regenerative amplifier around this time in order to be able to extract the maximum energy. The amplifiers work in the single shot operation mode, therefore the pulse selector (described in Chapter 2 ) selects a single optical pulse from the train of 100 MHz pulses. In order to synchronize the oscillator (GLX-200) with the single shot operation of the amplifier chain, the single trigger pulse generator was made. This module has an input from the oscillator and from the amplifier

200μs

Fig18. Flash lamp pulse profile.

the Pockels cell PC1 is triggered with a delay “b” of 80. 011μs from the DG535. This means that that the triggering of PC1 takes place 300.016μs after the flash lamps have fired. Similarly the Pockels cell PC2 is triggered with a delay “c” of 81.011μs when the optical seed pulse is amplified enough to be extracted out of the regenerative amplifier cavity. PC1 & PC2 are driven with high voltage pulsars with a pulse rise time of 4 ns (fig13b&13d). High voltage pulsars use either a series string of avalanche transistors or a Marx bank type design. The Marx bank type design has the advantage of a lower power supply voltage, and the advantage of a single long string of avalanche transistor is that it reduces the total stored energy of the system. Next the electro-optic modulator of the pulse cleaner is driven by a half wave voltage of 6 KV and is triggered with a delay “d” of 81.005μs, as indicated in fig17 &18. The clean optical pulse is then amplified in the linear amplifiers consecutively.

Conclusion A Nd: glass based MOPA chain using the CPA technology has been built. An ultra short pulse of duration 200fs has been first stretched to 266ps, with the help of a pair of holographic gratings and a unit magnification telescope. The temporally stretched pulse has then been amplified in a regenerative amplifier, a double pass amplifier and a single pass amplifier consecutively. The active medium of the amplifiers is Nd: silicate glass. The initial pulse had an energy of 70 pJ and this was amplified to 40 mJ. After the amplification the pulse was compressed , using a pair holographic gratings with a groove density of 1700lines /mm. The gratings of the compressor are matched with that of the stretcher in order to obtain the best possible compressed pulse. The pulse after compression had a FWHM of 1.5ps.

Acknowledgement The authors thank Dr. S. Kailas, Director, Physics Group, B. A. R. C for his help during the course of this work and for supporting this project. The authors also thank Dr. S.M.Sharma, Head, HP&SRPD for his support.

References

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