Nd:glass regenerative amplifier with increased bandwidth and high output energy for chirped pulse amplification systems

Nd:glass regenerative amplifier with increasedbandwidth and high output energy for chirped

pulse amplification systems

Paramita Deb,* Kailash C. Gupta, and Jayant K. FuloriaLaser and Neutron Physics Section, Bhabha Atomic Research Centre, Mumbai 400085, India

*Corresponding author: [email protected]

Received 11 November 2009; revised 24 February 2010; accepted 1 March 2010;posted 2 March 2010 (Doc. ID 119606); published 22 March 2010

We demonstrate a large energy output and a wider output spectral bandwidth from Nd:glass-basedregenerative amplifiers. The maximum energy extracted from the regenerative amplifier is 24mJ withan overall gain of 3:4 × 108. The maximum output bandwidth achieved is 4:2nm FWHM when the inputpulse spectral bandwidth is 7nm FWHM. This was made possible by a new cavity optimizationtechnique. © 2010 Optical Society of America

OCIS codes: 140.0140, 140.3280, 140.3530.

1. Introduction

A strong motivation for the development of powerfulultrashort pulse generators exists due to their di-verse applications in areas of fundamental research.In order to achieve very high peak intensities focusedonto gaseous or solid targets, for many high fieldphysics studies, a laser source capable of producingintensities >1018 W=cm2 is necessary. Interest in thestudy of interaction of matter with ultrabright andultrashort light pulses is increasing all over theworld. High-power short pulsed laser systems arecontinually being developed and improved upon asa tool to investigate physics in the extreme condition.Ideally, such a laser system should be compact andreliable. If energy can be extracted from an ultra-short pulse (1ps or less), tremendous peak powerscan be obtained with relatively modest energyrequirements. With the advent of the chirped pulseamplification (CPA) [1] technique , the total laser sys-tem has become more compact. In the CPA techni-que, the oscillator pulse is stretched in time andthen amplified. In the amplification process the re-generative amplifier is the first step, where the

nanojoule (nJ) range energy of the oscillator is ampli-fied up to the millijoule (mJ) range, in one stage.Even multipass amplifiers [2,3] have been used to in-crease the energy to the mJ level, but, here, since allthe energy has to be extracted within 8 or 10 passes,the active medium has to be pumped hard and, due tothe high gain, severe spectral narrowing is seen.Spectral gain narrowing is one of the main issuesin a high-power laser chain because it leads to an in-crease in pulse durations after compression. Multi-pass amplifiers require more optical componentsfor multipasses [4], which require additional spaceand time-consuming and tedious alignment. The ad-vantage of a multipass amplifier is that it does notuse the Pockels cell in its design. Regenerative am-plifiers are relatively simpler but require the use ofthe Pockels cell.

A variety of regenerative amplifiers that have beenreported in the literature till today have a spectrumof output characteristics and operating conditions.The pulse width, energy, and possible repetitionrates of the amplifier depend on the properties ofthe material being used as the active medium, suchas emission bandwidth, thermal conductivity, and ra-diative lifetime. In recent years, many diode pumpedregenerative amplifier systems based on colquiriitecrystals [5–7] have been made, where the emission

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1698 APPLIED OPTICS / Vol. 49, No. 10 / 1 April 2010

bandwidth is large and the extracted energy is a fewtens of microjoules. Ti:sapphire, too, has a largeemission spectral bandwidth, and the extracted en-ergy from a regenerative amplifier reaches the mul-ti-mJ level, and it is the most common material forchirped pulse amplification systems. In many in-tense laser–matter interaction research projects, alaser system capable of producing intensities of1018–1020 W=cm2 uses Nd:glass as the amplifyingmedium [8], because Nd:glass has a high saturationfluence and long upper state lifetime. Utilizing thelarge bandwidth of Ti:sapphire, regenerative ampli-fiers operating in the 1060nm wavelength rangehave been used as a front end for high-power Nd:glass systems [9]. Spectral narrowing, however,shows up in the subsequent Nd:glass power ampli-fiers. With the use of different kinds of glass, suchas silicate and phosphate, with shifted fluorescencecurves, the output bandwidth from the amplifierchain can be increased [10]. If the spectral narrowingcan be limited and more energy extracted in onestage, then the total CPA system could become com-pact. With fewer amplifier stages, the beam qualitydegradation, and also the complexity, will be re-duced. Optical parametric chirped pulse amplifiers(OPCPAs) have been thought of as a replacementfor regenerative amplifiers because the key aspectof OPCPA performance is the ability to amplify thechirped pulse with minimum spectral narrowing.This is because the spectral gain profile of the non-linear material is very large as compared with thebandwidth that has to be amplified. OPCPApreamplifier output in the multi-mJ range has beenreported. With reasonable pump intensities in non-linear crystals, such as β-barium borate, overall gainof about 106–107 is possible, with almost no spectralnarrowing. [11]. In OPCPA, spectral narrowing oc-curs because of the temporal nonuniformity of thepump pulse. The successful operation of OPCPA de-pends critically on the pump laser, which must havea pulse duration comparable with the chirped signalpulse. This pumping system itself may not be conve-nient to set up or easy to use, and the pump to signalconversion efficiency is not very high [12].Therefore, there is an ongoing effort toward im-

proving the emission bandwidth and thermal con-ductivity of Nd:glass-based materials [13]. Besidesthe advantage of the large upper state lifetime that

Nd:glass has, large glass rods of high optical qualitycan be fabricated. Nd:glass regenerative amplifierswith an output energy of 1mJ [14] and spectral widthof 1:6nm have been built. Some papers in the litera-ture describe Nd:glass regenerative amplifiers with asimilar bandwidth [15] but with an enhanced energyof 5mJ output. Two different types of phosphateglass with shifted fluorescent peaks have also beenutilized in the same amplifier cavity for an increasedoutput spectral bandwidth [16,17]. In this paper, weaddress the important aspect of spectral gain nar-rowing in the amplification process because theultimate pulse duration after compression is depen-dent on the spectral bandwidth. We describe aflashlamp-pumped Nd:glass regenerative amplifierthat has a larger output spectral bandwidth andlarger overall gain than reported previously in theliterature.

2. Experimental Setup

The regenerative amplifier assembly is essentially amultipass amplifier with a cavity geometry such thatthe injection of a chirped laser pulse and the ejectionof the amplified pulse can be controlled by two opticalswitches. The regenerative amplifier cavity consistsof two KDP Pockels cells, a quarter-wave plate, a thinfilm polarizer, and an Nd:glass rod (10mm diameterand 150mm length) placed between a curved mirror(6m radius of curvature) and a plane mirror sepa-rated by 1:5m. Figure 1 shows the regenerative am-plifier setup along with the oscillator and stretcher.The oscillator (GLX200 from Time-BandwidthProducts, Switzerland) is a 200 fs, 100MHz Nd:fluorophosphate glass oscillator with a Gaussian pro-filed spectrum, a spectral bandwidth of 9nm at 1=epoint, 7nm FWHM, and a peak at wavelength1056nm. The average output power from the oscilla-tor is 55mW. This pulse train is stretched in a doublepass stretcher. The stretcher layout is based on thedesign proposed by Martinez and uses a pair of holo-graphic reflection gratings (1200 lines=mm) in theantiparallel configuration with a unit magnificationtelescope in between. The stretcher output is a600ps, 100MHz pulse train with an average powerof 7mW and a spectral bandwidth of 7nm FWHM.The spatial beam profile at the end of the stretcheris slightly elliptical. Measurement of the spec-trum across the beam cross section does not show

Fig. 1. Experimental configuration of the regenerative amplifier: PC1 and PC2, Pockels cells; TFP, thin film polarizer; M1 and M2, 100%reflecting regenerative cavity mirrors; M3 and M4, 100% deflection mirrors for seed pulse injection; CL, cylindrical lenses; λ=4, quarter-wave plate; and PD, avalanche photodiode.

1 April 2010 / Vol. 49, No. 10 / APPLIED OPTICS 1699

any spatial chirp. A single pulse is selected from thepulse train by a pulse selector (electro-optic switchfrom Fast Pulse Technology Incorporated, U.S.A.).The single pulse of 600ps duration and 70 pJ energyis then injected into the regenerative amplifier cav-ity. Before injection the pulse is collimated by two cy-lindrical lenses. This is because the spatial profile isslightly elliptical as a consequence of the astigma-tism introduced by the stretcher. This collimation in-creases the intensity of the input pulse and alsoavoids cutoff at the Nd:glass rod in the cavity. Theenergy fluence at this point is 727pJ=cm2, as a resultof beam collimation to a diameter of 3:5mm. The Nd:glass rod is flashlamp-pumped with an electrical en-ergy of 600 J. The Pockels cell, PC1, is triggered, anda quarter-wave voltage of 3:5kV is applied for therequired retardation, to trap a pulse inside the cav-ity. The evolution of the pulse buildup is monitoredon an avalanche photodiode, where a leak pulse fromthe high reflectivity mirror M2 is detected. After thenecessary number of round trips, the pulse is ejectedby switching on Pockels cell PC2, with a quarter-wave voltage of 3:5kV. The ejected pulse is moni-tored and measured after the thin film polarizer withan avalanche photodiode. A pyroelectric joulemeter(Gentec, Canada) was used at this position to mea-sure the energy of the ejected pulse. The synchroni-zation of the various electro-optic switches, firing offlashlamps and single seed pulse selector wasaccomplished with the help of a digital delay/pulsegenerator (DG535 of Stanford Research Systems,U.S.A.).When the seed pulse (600ps pulse duration, 70pJ

energy, Gaussian profiled spectrum, and a spectralbandwidth of 7nm FWHM) is not injected into theregenerative amplifier, the amplified spontaneousemission (ASE) builds up to a Q-switched pulse in-side the cavity. The regenerative amplifier cavitywas first optimized in the Q-switched operation,and then the seed pulse was directed into the ampli-fier. The seed pulse amplification can be detectedfrom the leak pulse, behind mirror M2 (Fig. 1).The seed pulse buildup detected by the avalanchephotodiode is shown in Fig. 2. The seed pulse ampli-fication is seen overriding theQ-switched pulse. Thisis undesirable for a regenerative amplifier. Theoscilloscope trace should show a fully modulatedstructure, giving no indication of a Q-switched back-ground. The cavity has to be adjusted for completeseeding, meaning that there should be noQ-switchedsignal seen underneath the circulating seed pulse. Abackground Q-switched pulse (ASE) reduces the am-plification of the seed pulse, and this reduces energyextraction from the amplifier by the seed pulse.The ASE background of the amplified pulse alsoinfluences the output spectral bandwidth of theregenerative amplifier.

3. Numerical Analysis of Regenerative Amplification

In order to quantitatively estimate the effect of ASEon seed pulse amplification and find the parameters

that largely affect the “amplitude ratio” of seed pulseto ASE and also the overall gain of the regenerativecavity, we did a detailed numerical calculation. Wedescribe the “amplitude ratio” as the ratio of the peakamplitude of the short pulse (originating from theseed pulse) to the peak Q-switched pulse amplitude(originating from the cavity noise) in the regenera-tive cavity. This ratio will give an idea of how muchthe ASE can be suppressed. A higher “amplitude ra-tio” would mean a diminishing Q-switched pulse un-derneath the circulating seed. The amplification canbe described with two coupled recurrence relations,and these give the pulse fluence amplification andgain of each pass. Since the pulse widths under con-sideration here aremuch shorter than the lower leveldecay time (typically 10–100ns, for solid state ampli-fying medium) two levels need to be taken into con-sideration—the upper laser level and the lower laserlevel. Pumping and other relaxation rates are insig-nificant. The two-level nature of the amplifier allowsone to use the standard rate equation model ofFrantz and Nodvik [18] for pulse amplification. It de-scribes the amplification process in terms of thepulse energy. If we assume that the amplifying med-ium can be characterized by a small signal gain coef-ficient, g0ðcm−1Þ, a length LðcmÞ, and a saturationfluence EsðJ=cm2Þ, then Frantz and Nodvik showedthat, for an input pulse energy fluence Ei, the outputpulse energy fluence E0 is given by

E0 ¼ Es ln�1þ expðg0LÞ

�exp

�Ei

Es

�− 1

��: ð1Þ

The saturation fluence and the small signal gain aredirectly related to the population inversion in thetwo-level system. This model by Frantz and Nodvik,with repeated application of the basic equations to amultipass system [19], adequately describes theregenerative amplifier. These equations have the

Fig. 2. Amplification of the circulating seed pulse, detected fromthe leak of mirror M2. The time base is 200 ns/division. The ver-tical line T indicates the time when the Pockels cell is open with avoltage of 3:5kV.


same form as the equations that describe the photondensity and population inversion in a Q-switchedoscillator. Therefore, the regenerative amplifierpulse train envelope has the familiar shape of a Q-switched pulse (Fig. 2). We assume uniform trans-verse beam profiles and use the recurrence relationthat relates the input energy fluence for pass numberk, Ein

k to the output fluence Eoutk. The energy growth

of the seed pulse, as well as the growth of ASE to aQ-switched pulse, is done simultaneously with Einq

k

and Einsk as the input fluence of theQ-switched pulse

and that of the seed pulse, respectively. Therefore,Eoutq

k and Eoutsk, the output energy fluence for the

kth pass, can be written as

Ekoutq ¼ Es ln

�1þ expðgkLÞ

�exp

�Ekinq

Es

�− 1

��;

Ekouts ¼ Es ln

�1þ expðgkLÞ

�exp

�Ek

ins

Es

�− 1

��;

ð2Þ

Ekþ1inq ¼ REk

outq; Ekþ1ins ¼ REk

outs: ð3Þ

For the ðkþ 1Þth pass, the input energy fluence forthe Q-switched pulse and the seed pulse is describedby Eq. (3), which takes care of the reflection and scat-tering losses, andR is that single pass loss. The smallsignal gain coefficient is modified in each pass due tothe energy extracted from the stored energy of theinverted medium. The energy is extracted by thegrowing Q-switched pulse and the growing seedpulse as they circulate in the regenerative cavity.The initial spontaneous emission energy fluence inthe cavity was estimated from the spontaneous emis-sion contribution of all volume elements within asmall solid angle ΔΩ and a spectral interval Δν.The power of this noise can be written as [20]

P ¼ hνKΔνhνKΔνστ

ΔΩS0

4π ðG0 − 1Þ: ð4Þ

Here hν is the energy of the photon, and σ is the sti-mulated emission cross section, KΔν is the part of thespontaneous emission matching the bandwidth ofthe amplifier, and S0 is the area of the beam. The in-itial spontaneous energy fluence in the regenerativecavity was estimated from Eq. (4). This energy flu-ence will grow in the cavity into a Q-switched pulse,but it will be competing with the seed pulse for en-ergy extraction from the gain medium of the cavity.With each pass the stored energy is depleted from themedium by the seed pulse as well as the growing Q-switched pulse, and, therefore, the small signal gaincoefficient is modified. Therefore, g for the ðkþ 1Þthpass can be written as follows:

gkþ1 ¼ gk −ΔEk

LEs; ð5Þ

ΔEk ¼ ðEoutq þ EoutsÞ − ðEinq þ EinsÞ: ð6Þ

From Eq. (1) it is evident that, when the iteration forthe regenerative amplifier is done, the peak energyreached by the injected seed pulse is independentof the initial seed pulse energy fluence. The maxi-mum energy reached remains constant, but thepasses in the cavity required to reach the peak in-crease as initial seed energy decreases. This is thecase when we do not consider the existence of ASEin the cavity. When ASE in the cavity is taken intoconsideration, the amplified seed pulse shows varia-tion in the peak extractable energy as well as a var-iation in the number of passes required to reach thepeak, and also the “amplitude ratio” changes withthe seed pulse injection energy value. In Fig. 3 weshow the output energy fluence as a function of passnumber. To visualize the effect of initial pulse energyfluence variation on the “amplitude ratio,” we havecalculated for three seed pulse energy fluence val-ues of 500pJ=cm2, 700pJ=cm2, and 1nJ=cm2. As ex-pected, we find that by increasing the seed pulseenergy (Fig. 3) the Q-switched pulse diminishes.When the input fluence is 500pJ=cm2, the amplituderatio is about 5 : 1, and with an input fluence of1nJ=cm2, the ratio is 10 : 1. If the input seed pulseenergy fluence is increased to 2:5nJ=cm2, the Q-switched pulse becomes insignificant with the ampli-tude ratio being 23 : 1.

In many experimental cases, a fs range oscillatorpulse, after its transit through a double pass stretch-er, has very little energy that can be injected into theregenerative cavity. Therefore, when the input seedpulse energy is not too large and as a result the Q-switched pulse amplitude is significant, one couldintroduce a differential loss to the seed and theASE as they grow in the cavity. We have taken thevalue for initial seed pulse energy fluence as700pJ=cm2 and worked out the amplification inthe cavity, including this differential loss, and foundthat the Q-switched pulse is almost nonexistent withan amplitude ratio greater than 23 : 1. Murray andLowdermilk [21] worked out the efficiency of cou-pling the pulse into the resonator cavity and foundthat it depends on the alignment along the resonatoraxis and the matching of the resonator modes. Bynumerical calculations, they show that the couplingefficiency depends more strongly on alignment thanon mode matching. In our calculation, we have con-sidered only the angular misalignment, which ischaracterized by the angle α between the beam axisand the resonator axis. The distance between thebeam waist and the point at which the axes cross[21] is denoted as Δ. We also assumed that thereis no translational misalignment between the beamaxis and the resonator axis. The fractional couplingfor misalignment is then given as

T ¼ expf−½ðαΔ=ω0Þ2 þ ðα=ϕÞ2�g; ð7Þ

where ω0 is the waist radii and ϕ ¼ λ=πω0. So the lossin each pass is included in the factor T, and


Ekþ1inq ¼ TREk

outq; Ekþ1ins ¼ TREk

outs: ð8Þ

Equation (7) suggests that the loss due to angularmisalignment is quantified by the parameter α.The value of α used in our computations was inthe range of 0.02 to 0:05mrad. Given this extra loss(T) to the Q-switched pulse as compared to the seedpulse for the initial few passes (approximately 20passes), it is possible to achieve an amplitude ratioof 80 : 1 and an overall gain of 1:9 × 108 at a gain coef-ficient g0 of 0:013 cm−1. Increasing the gain coeffi-cient to 0:015 cm−1, the overall gain of the

amplifier becomes 2:99 × 108, retaining the sameamplitude ratio. When the gain is increased to0:02 cm−1, the overall gain is 6:2 × 108, and the am-plitude ratio remains approximately 80 : 1. Thus,we find tha, by introducing an angular misalignmentinduced differential loss, it is possible to pump moreenergy into the gain medium and obtain a larger out-put, while maintaining an amplitude ratio such thatthe Q-switched pulse is absent.

When the Q-switched pulse (ASE) amplitude isless, then we know that the gain of the amplifyingmedium has been utilized for the growth of the seedpulse predominantly. In all the previous work doneon regenerative amplifiers, optimum coupling ofthe seed pulse was achieved by mode matching. Withthemodematching condition fulfilled, the seed pulse,with its higher initial energy fluence as compared tothe spontaneous emission, is able to extract more en-ergy from the gain medium and, thus, is able to sup-press the growth of the spontaneous emission into aQ-switched pulse. When the pumping in the activemedium is increased, the spontaneous emission inthe active mediumwill bemore, and, therefore, it willhave a chance to extract from the increased gain ofthe medium and grow in the cavity along with theseed pulse. Therefore, increased pumping will notimprove the amplification of the seed pulse. A Q-switched pulse will appear under the circulating seedpulse, thus reducing the amplitude ratio and, there-fore, the overall gain of the regenerative amplifier.While, with an angular misalignment of the ampli-fier cavity, the Q-switched pulse is discouraged fromgrowing, the seed pulse is allowed to grow by inject-ing the seed pulse into the cavity such that it is inperfect alignment with the cavity axis. When the ac-tive medium is pumped harder, the spontaneousemission increases, but it is suppressed becausethe regenerative cavity is misaligned. The injectedseed pulse that is aligned along the cavity axis hasan edge over the spontaneous emission and, there-fore, can grow and extract from the gain mediumat the expense of ASE. This results in a higher over-all gain of the regenerative amplifier and the totalenergy output of the amplifier.

The concept of introducing a differential loss forthe ASE and the seed pulse can be utilized effectivelywhen the seed pulse spectral peak is shifted from theactive medium gain profile peak, and at the sametime keeping a large overlap of the seed pulse spec-trum and the active medium gain profile. The aim ofshifting is to achieve a wider output spectral band-width. The amplified pulse spectrum, IoutðλÞ is deter-mined by IinðλÞGðλÞ, where IinðλÞ is the injected seedpulse spectrum and GðλÞ is the wavelength depen-dent gain (fluorescence curve or gain profile) of theactive medium. Since the peak of IinðλÞ and the peakofGðλÞ are separated, the peak spectral component ofthe seed pulse will experience less gain during am-plification as compared to other spectral componentsðλÞ on the side of the peak of IinðλÞ. The spectral com-ponents of the seed that are on the side of the peak

Fig. 3. Seed pulse energy fluence versus pass number—calculations done with Frantz and Nodvik equations. The figureillustrates the growth of the seed pulse and the ASE into theQ-switched pulse in the regenerative cavity for (a) 500pJ=cm2,(b) 700pJ=cm2, and (c) 1nJ=cm2 initial seed energy fluence. Thecolumns represent the seed pulse as it amplifies in the medium,overriding the growing Q-switched pulse.


experience more gain because they are closer to, orcoincide with, the peak of GðλÞ. Both types of spectralcomponent can extract energy from the active med-ium nearly equally, because one has a larger photondensity and the other experiences a larger gain asthey are amplified. This can lead to the increase ofspectral bandwidth from the amplifier. At the sametime, the ASE also grows in the active medium,mainly due to the peak spectral component of GðλÞ,since it has the highest gain and also because thisspectral component has the largest photon densityin the spontaneous emission, as compared to thewings of the spectrum. The spectral components oneither side of the peak are amplified less, giving riseto gain narrowing in the Q-switched pulse. Since thespontaneous emission can extract from the gain med-ium, the seed pulse growth would be comparativelyless. The seed pulse amplification will be less, mainlyaround the spectral component that coincides withthe peak of GðλÞ, thus preventing the increase inbandwidth. The growth of the Q-switched pulsewould also result in a low “amplitude ratio.” To pre-vent this from happening, the cavity is misaligned(simultaneously ensuring seed pulse injection alongthe cavity axis). The angular misalignment induceddifferential loss introduces a large loss to the ASEand prevents its growth into a Q-switched pulse.Therefore, the seed pulse can grow alone, with littlecompetition from the ASE. This allows the limitingof gain narrowing, leading to a larger output spectralbandwidth from the regenerative amplifier. There-fore, minimizing the ASE or increasing the“amplitude ratio” is important for limiting the gainnarrowing of the amplified pulse.In our experimental setup, we incorporated this

method of misalignment to achieve our goal of a largeoutput energy and a larger output bandwidth fromthe regenerative amplifier. The experimental mea-surements detailed below show to what extent theoutput bandwidth can be increased and also the ex-tent to which the overall gain of the regenerative am-plifier can be increased.

4. Measurements and Discussion

The first step in the operation of the regenerativeamplifier is to optimize the cavity for Q-switched op-eration. After the seed pulse has been injected, theregenerative amplifier cavity has to be adjusted forcomplete seeding where no Q-switched signal is seenunderneath the circulating pulse. The seed pulse am-plification is recorded with the avalanche photo-diode, detecting the leak pulse from mirror M2(Fig. 1). In most previously reported regenerativeamplifier construction, mode matching is carriedout by using intracavity or extracavity aperture orlenses in order to obtain complete seeding. Herewe used a new technique, where the alignment ofthe cavity is changed slightly to partially suppressthe buildup of the spontaneous emission into aQ-switched pulse. Simultaneously, the seed pulseinjection alignment is shifted proportionately by

alignment of mirrors M3 and M4 (Fig. 1) so that itis in perfect alignment with all the components ofthe cavity. Basically, an angular misalignment is in-troduced to produce differential loss for the seedpulse and the cavity spontaneous emission. The re-generative amplifier cavity that is optimized forthe maximum Q-switched output is first misalignedslightly by changing the angular movement of mirrorM1 or mirror M2 (Fig. 1). This angular movement isin the range of 0.02 to 0:05mrad. The avalanchephotodiode behind mirror M2 detects and measuresthe reduction in the Q-switched pulse amplitude.Then the seed pulse is introduced into the cavity,and its alignment along the cavity axis is adjustedby mirrors M3 and M4. In this way the seed pulsehas an initial advantage of exploiting the gain ofthe Nd:glass medium as compared to the cavity spon-taneous emission buildup. The angular movementadjustment of mirrors M1, M3, and M4 can be doneiteratively until the best modulated structure is de-tected by the photodiode. Figure 4 shows the circulat-ing amplified pulse as detected from the leak pulse ofmirror M2. It shows complete seeding with a fullymodulated structure giving no indication of a Q-switched background. The adjustments in the seedpulse injection timing (i.e., triggering the Pockels cellPC1), the energy dumped into the flashlamps and theseed pulse intensity (by adjustment of the cylindricallens pair) were all aimed to get complete seedingwith no Q-switched signal. In the best operating con-dition, we could eject a single pulse with an energy of24mJ. The amplified pulse is ejected after approxi-mately 100 round trips when the amplificationreaches its maximum. Figure 5 is a trace of theejected pulse when the Pockels cell PC2 is switchedon with a quarter-wave voltage. The ejected pulse isdetected with an avalanche photodiode placed afterthe thin film polarizer. The finite extinction ratio ofthe thin film polarizer, the limited contrast ratio of

Fig. 4. Leak signal through the plane mirror M2 of the regenera-tive amplifier cavity, showing the evolution of the seed pulseamplification in the cavity, after the cavity has been adjustedfor complete seeding.


the Pockels cell crystal (PC2), and the long temporalwindow of PC2 allow pulses with the wrong polariza-tion to grow in the cavity and appear as postpulses,separated by the round trip time of the amplifier cav-ity. This can be reduced considerably with a shorttemporal window for PC2, such that the window isclosed when the postpulse arrives. Another methodof removing the postpulses would be to use a pulseselector after the regenerative amplifier cavity.The regenerative amplifier provides an overall gainof 3:4 × 108. The seed pulse that is injected into theregenerative amplifier cavity has a spectral profilethat is shifted away from the Nd:glass rod fluores-cence curve. Various Nd:glass rods were used inthe regenerative amplifier cavity. Nd:silicate glassrods with the peak of the fluorescence curve at1060nm and a bandwidth of 27nm FWHM wereused. Two Nd:phosphate glass rods with the fluores-cence peak at 1054nm and 1053nm were used in theexperiment. The Nd:phosphate glass rods had abandwidth of 22nm FWHM. The Nd:glass rods wereacquired from Kigre, Incorporated, U.S.A., and Crys-taltechno, Limited, Russia. The amplified pulse spec-trum was recorded with the help of the leak pulsesfrom mirror M2, using a McPherson spectrometer.The largest spectral bandwidth that we measuredwas 4:2nm FWHM. This bandwidth of the outputpulse from the regenerative amplifier is much morethan achieved earlier from Nd:glass-based ampli-fiers. The reduction of the spectral bandwidth from7nm FWHM of the seed pulse to 4:2nm of the ampli-fied pulse is due to gain narrowing. The wings of thespectrum are amplified less as compared to the peakof the spectrum, resulting in the narrowing. The Q-switched pulse spectral profile fitted into a Gaussiancurve and the bandwidth (when the seed pulse is notinjected into the cavity) wasmeasured to be 1:3nm atFWHM. Comparing this with the 4:2nm FWHM ofthe amplified seed pulse, we can say that gainnarrowing is mitigated. This was possible due to

the shifted spectral profile of the input pulse (withrespect to that of the fluorescent curve of the activemedium) along with the misalignment method.

With the availability of different kinds of Nd:glasswith shifted fluorescent curves, it has been possibleto enlarge the emission bandwidth from a chain ofamplifiers. Gain narrowing is more pronounced inregenerative amplifiers, and any gain narrowingcaused by the subsequent linear amplifier chain isnegligible. Therefore, getting a very good spectralbandwidth at the regenerative amplifier stage isimportant. Mixing glass media (silicates and phos-phates) with different peak gain wavelengths hasbeen used to increase the amplified pulse spectralwidth in a regenerative amplifier to about 1:8nm[16]. Another possibility that has been tried out isto use a frequency dependent loss during amplifica-tion with greater attenuation at the peak of the gainprofile than in the wings. Spectral filters placed in-side the regenerative amplifier to reshape the ampli-fied spectrum after each pass of the gain mediumhave resulted in larger output bandwidths [22,23].However, intracavity elements introduce higher or-der dispersion that is detrimental to the compressedpulse. Therefore, in our experiment we have tried outa method where no intracavity element is used forspectral shaping. We have tried out a method wherethe seed pulse peak spectral wavelength is shiftedaway from the peak spectral component of the gainprofile [GðλÞ] of the amplifying medium. The seedpulse peak spectral wavelength has a significant in-fluence on the amplification. In one experimentalsetup, the peak spectral wavelength of the seed pulseis 1056nm and the fluorescence peak of the Nd:silicate glass amplifying medium is 1060nm. Withour new technique of alignment (where a varied lossis introduced by an angular misalignment of theregenerative cavity), it was possible to get completeseeding in spite of the 4nm shift from the maxi-mum gain wavelength of the amplifying mediumspectrum. The output spectrum (that fitted into aGaussian profile) of the regenerative amplifier wasmeasured, and the bandwidth was recorded as3:8nm FWHM, with the input seed pulse spectralwidth of 7nm FWHM. The 4nm shift provided a lar-ger input energy fluence at the 1056nm wavelength,but allowed lesser gain as compared to the wave-length 1060nm, where the gain is more but the inputfluence is less. With more initial photons of 1056nm,a good extraction was obtained, though the gain ofthe amplifying medium is less at this wavelength.Though the initial photons of 1060nm wavelengthis less, it uses the larger gain at this wavelengthin the amplifying medium as compared to 1056nm.These two factors balance out, and this has made itpossible to limit the gain narrowing and obtain abandwidth of 3:8nm FWHM. With a lower gain atthe peak wavelength of 1056nm, the probability ofthe Q-switched pulse developing with a peak wave-length of 1060nm was more. The new alignmentprocedure helped in growing the seed pulse only,

Fig. 5. Switched-out amplified pulse detected after the thin filmpolarizer when Pockels cell PC2 is on.


at the expense of the Q-switched pulse, with a littlebit of redshift seen for the regenerative amplifieroutput spectrum. In another of our experimental set-ups, the seed pulse peak spectral wavelength wasretained at 1056nm, but in the amplifying mediuma Nd:phosphate glass rod (with a peak of the fluores-cent spectrum at 1053nm) was used. In this case theoutput spectral bandwidth of the amplifier was mea-sured to be 3:6nm FWHM. In our third experiment,the oscillator pulse used was tuned such that theseed pulse had a spectral peak wavelength at1058:5nm (bandwidth of 7nm FWHM), and the am-plifying medium used was Nd:phosphate glass with apeak spectral gain at 1054nm. Here the amplifiedpulse spectrum had a bandwidth of 4:2nm FWHM.With this spectral bandwidth, the possible tem-poral Gaussian pulse width is about 412 fs afterrecompression.The maximum output energy of the pulse switched

out of the regenerative amplifier was 24mJ when aphosphate rod was used as the amplifying medium.The output energy was reproducible with a variationof about 7%. This variation was mainly due to theshot to shot gain fluctuation of a flashlamp-pumpedNd:glass gain medium. The B-integral calculated[24] for the system is about 0.3, which is small forany nonlinear effects to show up in the Nd:glass med-ium. No self-focusing effects were seen during theoperation of the amplifier. The spatial beam profileof the amplified pulse did not exactly indicate aTEM00 mode, but the profile was elliptical with acircularity of 0:7ðminor axis=major axisÞ. The beamdivergence (full angle) along the x direction is1:1mrad, and the divergence (full angle) in the y di-rection is 0:8mrad. Spectral measurements acrossthe beam cross section do not show any chirp. Withthe present alignment technique, we found that wecould pump the Nd:glass medium with a large elec-trical energy, without having to lose the stored en-ergy in the gain medium to a Q-switched pulsebuildup. The stored energy is extracted by the seedpulse only. This is in accordance with the numericalcalculation of the regenerative amplifier given inSection 3. Thus, the energy output from the regen-erative amplifier was more. Therefore, if the energyextracted and the spectral bandwidth of the outputpulse from the regenerative amplifier (the first stageof the total amplifier chain) is more, then large peakpowers can be obtained from a compact total system.

5. Conclusion

In summary, we built a regenerative amplifier withNd:glass as the active medium. The system produceda maximum output energy of 24mJ and a maximumbandwidth of 4:2nm FWHM. This is, to our knowl-edge, a large pulse energy along with a wide spectralbandwidth, from an Nd:glass, flashlamp-pumpedregenerative amplifier. A novel cavity alignmenttechnique was used, where a differential loss wasprovided to the cavity spontaneous emission andthe main seed pulse to be amplified. This not only

reduced the ASE but also allowed one to pump largerenergy into the gain medium, leading to an increasein the overall gain of the regenerative amplifier. Withan input pulse spectral width of 7nm FWHM to theregenerative amplifier, the output pulse of the regen-erative amplifier narrowed to a spectral width of4:2nm FWHM. Thus, this alignment technique alsohelped in obtaining a wider spectral bandwidth forthe output pulse of the regenerative amplifier. Ingeneral this new cavity optimization technique canbe utilized for any regenerative amplifier with anyactive medium to reduce the gain narrowing and alsoto reduce the ASE substantially.

The authors thank S. Kailas, director of thephysics group of Bhabha Atomic ResearchCentre, for his help during the course of this work.

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Nd:glass regenerative amplifier with increased bandwidth and high output energy for chirped pulse amplification systems