1442 OPTICS LETTERS / Vol. 19, No. 18 / September 15, 1994

Multiterawatt 30-fs Ti:sapphire laser system

C. P. J. Barty, C. L. Gordon III, and B. E. Lemoff

Edward L. Ginzton Laboratory, Stanford University, Stanford, California 94305

Received April 21, 1994

A near-diffraction-limited transform-limited multiterawatt laser system that produces -30-fs 125-mJ 800-nmpulses at a repetition rate of 10 Hz has been constructed. Methods for the control of femtosecond time-scalephase and amplitude distortions have been developed and implemented.

Chirped pulse amplification (CPA) has become acommon technique for circumventing optical dam-age and nonlinear effects during the amplificationof short optical pulses in solid-state laser media.'Terawatt-level pulses with durations ranging from-1 ps to -100 fs have been produced by a numberof systems2 -9 and have been used in a variety ofhigh-field applications such as diagnostic x-ray gen-eration and high-order harmonic generation.'l"' Al-though much shorter pulse oscillators exist,'11'6 theimplementation of multiterawatt CPA with pulsesof the order of 10 fs has not been accomplished.'7In conventional CPA systems, uncompensatablephase errors in the expansion and recompressionprocess and distortions that are due to refractiveoptics produce broadenings that are much greaterthan 10 fs. To alleviate these problems, a novel ex-pander-compressor design has been developed and alens-free amplification system has been constructed.In addition, gain narrowing during amplification se-verely limits the final pulse duration. To control thepulse spectrum before amplification and thus min-imize this problem, spectral masks have also beendeveloped and implemented.

A schematic of the laser system is given in Fig. 1.This system is capable of producing -125-mJ -30-fs800-nm pulses with near-diffraction-limited beamquality at a 10-Hz repetition rate. To our know-ledge, it is the first such system to produce sub-50-fspulses with multiterawatt peak powers and thefirst to control both femtosecond time-scale phaseand amplitude distortions during multiterawattamplification.

To produce 30-fs multiterawatt pulses, a regener-atively initiated self-mode-locked Ti:sapphire laserthat is capable of producing 20-fs 800-nm 5-nJ pulsesat a repetition rate of 100 MHz is used.' 5 Efficientamplification of these pulses is constrained in threeways: the desire to avoid damage of the optical coat-ings in the amplification chain, the desire to avoidtemporal and spatial nonlinear phase distortions,and the desire to achieve fluences near the satura-tion fluence of the final amplifier material. The firstconstraint implies that the peak intensity should belimited to -5 GW/cm2, whereas the last constraintimplies that the peak fluence should be greaterthan -0.8 J/cm 2 if Ti:sapphire is used. To sat-isfy these criteria safely, the pulse must be tem-porally stretched to significantly greater than 150 ps

before amplification. Stretching to -300 ps is ac-complished with a unique cylindrical-mirror-basedexpander-compressor system.18 This system is ca-pable of temporally stretching a 20-fs pulse by afactor of 15,000 and recompressing it to within 0.1 fsof the transform limit. A ray-tracing analysis showsthat with the correct placement, orientation, and sep-aration of the expander and compressor elements andthe correct amount of optical material in the amplifi-cation chain, it is possible to eliminate linear phasedistortions up to the fifth order. It should be notedthat a major benefit of a design that compensatesfor cubic and quartic phase distortions is that largeamounts of optical material may be incorporated intothe amplification chain without adversely affectingthe pulse quality. In the system depicted in Fig. 1there are 55.6 cm of e-sapphire, 70.6 cm of BK7 glass,58.0 cm of potassium dihydrogen phosphate, 2.2 cmof M18 glass, and 3.3 cm of calcite material path afterthe laser oscillator and the output of the compressor.

The actual settings for the grating and the mirrorangles and separations are also constrained bythe desire to achieve the highest throughput inthe pulse compressor. Measurements of efficiencyversus incident angle for the 1200-grating/mmAu-coated ruled gratings (Milton Roy Corporation,MR136-1-1-4) used in the system indicate that adiffraction efficiency, -90%, is obtained at an angle ofincidence of 48.50. With this constraint, the angle ofincidence of the gratings in the expander is56.700. The angle of incidence of the cylindricalmirrors is 27.000. The 1-m-radius mirrors have aseparation of 89.14 cm between each other, 19.00 cmbetween the first mirror and the first grating,and 15.00 cm between the second mirror and thesecond grating. The arrangement is designed fora central wavelength of 781.7 nm. Straightforward

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Fig. 1. Laser system schematic.

0146-9592/94/181442-03$6.00/0 © 1994 Optical Society of America

September 15, 1994 / Vol. 19, No. 18 / OPTICS LETTERS 1443

alignment procedures produce separations accurateto -100 /,km and grating and mirror rotationsaccurate to -1 arcmin. In the compressed resultsthat follow, the only adjusted parameter is theseparation of the compressor gratings.

The pulse energy after the expander is approxi-mately 500 pJ. Amplification to 235 mJ is accom-plished in two stages. The first stage is a 14-passTi:sapphire regenerative amplifier that is pumpedwith -50 mJ of Gaussian-spatial-profile 532-nmradiation.2 This permits the gain medium to actas a Gaussian spatial filter and results in excel-lent output-beam quality. The output energy of theregenerative amplifier is -9 mJ. The beam diam-eter is then increased from -1.5 to -5 mm withan off-axis spherical mirror telescope. It shouldbe noted that refractive telescopes should not beused for this application. Such telescopes lead tocurved pulse fronts with center-to-edge beam delaysof the order of 10 fs and thus to increases of fo-cused interaction times of the same order.'1920 The5-mm-diameter beam is then amplified in a four-pass arrangement consisting of a 13-mm-diameterX 13-mm-long Ti:sapphire crystal. The crystal ispumped with -850 mJ of 532-nm radiation, whichalso has an approximately Gaussian spatial profile.The resulting 235-mJ pulse is spatially expandedto a -25-mm-diameter with a final off-axis spheri-cal mirror telescope. Grating diffraction and mirrorcoating losses in the pulse compressor result in afinal compressed energy of 125 mJ.

The duration of the amplified pulse is limitedby the seed pulse spectral content and by gainnarrowing during amplification. A typical amplifiedspectrum corresponding to an -25-fs oscillator pulseis given in Fig. 2(a). The corresponding single-shotautocorrelation is given in Fig. 3(a). Superimposedon the measured data is a calculated autocorrelationthat corresponds to the Fourier transform of thesquare root of the measured spectrum,15 which hasa duration of 45.5 fs. The agreement indicates thatthe pulse is transform limited and that dispersioncontrol has been achieved. To produce shorterpulses, the seed pulse spectrum is modified beforeamplification. This is done when a spatially varyingfilter is placed in the focal plane of the first cylin-drical mirror of the pulse expander.21 Two typesof variable-amplitude constant-phase-delay filterwere used. The first consisted of multiple adjacenttransmission gratings etched in Cr on a 2-mm-thickfused-silica substrate. Each grating had a trans-verse width of 25 jam and a period of 10 grooves/mm.Transmission into zero order for any wavelength wasdetermined by the duty cycle of individual gratings.Diffraction from higher orders does not couple intothe regenerative amplifier, which is approximately3 m away. A filter of this type was designed toproduce an approximately flat seed spectrum from765 to 812 nm. The spectrum of this pulse afteramplification is illustrated in Fig. 2(b), and themeasured autocorrelation is shown in Fig. 3(b).The result corresponds to a 36.5-fs transform-limited pulse. The second type of filter was lesssophisticated but simpler to vary. It consisted of

a circular card that was placed at a transverseposition corresponding to the peak of the pulsespectrum. The card blocked approximately 50% ofthe vertical extent of the beam. A typical amplifiedspectrum and corresponding autocorrelations areshown in Figs. 2(c) and 3(c), respectively. The pulseduration of the transform limit in this case is 28.5 fs.The card filter does not provide high spectral resolu-tion and does not result in a spatially uniform seedbeam with respect to spectral content. As a result,the amplified spectrum is a function of the alignmentinto the regenerative amplifier. The seed energy isalso lower in the case of the circular card, and thusthe amplified spontaneous emission (ASE) content ofthe amplified pulse is greater.

._

CA

(IL

750 770 790 810

Wavelength (nm)830 850

Fig. 2. Amplified pulse spectra with (a) no spectral fil-tering, (b) transmission grating masks, (c) circular cardmasks.

a)

._

.A

._

2._.2

0

Ec:Ia)

Cn

-200 -150 -100

- (b)

... nsor Limt =4S. f

(C)

_ / \ ~~~~~~Transform Limit = 28.5 fs

7 / =y

-. ,..,.I , . I . .. I. . . . . .I .. .. I. ,,1,,I....I

-50 0 50 100 150 200

Delay (fs)

Fig. 3. Single-shot autocorrelations of amplified pulseswith (a) no spectral filtering, (b) transmission gratingmasks, and (c) circular card masks. Superimposed oneach measured trace is the calculated autocorrelationof the transform-limited pulse as calculated from themeasured spectrum.

1444 OPTICS LETTERS / Vol. 19, No. 18 / September 15, 1994

100 I (a) -90 -80s Horizonta Waist7 1.04 x Difraction L1mit70 - I

50 -

4030

180 - (b)170 -

160 - Vertical Waist = 1.89 x Diffraction Limit 150:

140130120110 . . . _

0 5 1t0 1 20 25

recently developed shorter-duration oscillators. 12-14

In addition, laboratory-scale, petawatt-peak-powersystems (-15 J in -15 fs) are now feasible with cur-rently available optical components and materials.

The authors thank P. T. Epp for construction ofthe grating masks. This work was supported by theU.S. Air Force Office of Scientific Research, the U.S.Army Research Office, the U.S. Office of Naval Re-search, and generous equipment loans from LawrenceLivermore National Laboratory.

30

Longitudinal Position (mm)

Fig. 4. Longitudinal dependence of beam waist: (a)horizontal, (b) vertical. Solid curves are parabolic fits tothe square of the beam waists.

Measurements of the leading-edge ASE content ofthe pulse were performed with a 50-ps response-timefast photodiode. It was estimated that 3% and6% of the energy exists in a -5-ns-long pedestalbefore the arrival of the main pulse for the gratingand the card filters, respectively. This correspondsto an intensity contrast ratio of 105, which issufficient for high-field gas density experimentsand for hard-x-ray production experiments.'0'22

Higher contrasts may be achieved with liquid- orsolid-state saturable absorbers after amplification,but before pulse compression.

A measurement of the amplified beam quality wasmade when the compressed output was focused with a1-m focal-length spherical mirror used at an angle ofincidence of 4°. Spatial profiles of the focused beamwere obtained with a computer-controlled scanningslit and photodiode and were recorded as functionsof longitudinal position. From these data, the vari-ance (o-r2) and the standard deviation (o-x) of eachtransverse profile were calculated and used to de-fine a beam waist, w,(z) = 2o-r. Because the spatialvariance of any beam follows a parabolic dependencewith distance23 and is transformed by conventionaloptics in exactly the same manner as an ideal Gauss-ian beam, measurements of this type may be used todetermine the focal spot size under different focusingconditions. The results of these measurements areshown in Fig. 4. The beam waists of the horizon-tal and the vertical profiles are 1.04 and 1.89 timesthat of a Gaussian beam with the same far-field di-vergence and size, respectively. The decreased beamquality in the vertical dimension may be due to clip-ping during the spatial multiplexing that occurs inthe four-pass power amplifier. With f/i optics, fo-cused intensities of >1 x 1020 W/cm2 are possiblewith this beam.

In conclusion, a 4-TW 30-fs 10-Hz laser system hasbeen constructed. Pulses from this system are neardiffraction limited and transform limited. Theseresults are achieved despite the presence of largeamounts of dispersive material and significant gainnarrowing during amplification. The extension ofthese results is straightforward. The generation ofhigh-energy -10-fs pulses should be possible with

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