Includes bibliographical references (page 589).We demonstrate the use of a deformable-mirror pulse shaper, combined with an evolutionary optimization algorithm, to correct high-order residual phase aberrations in a 1-mJ, 1-kHz, 15-fs laser amplifier. Frequency resolved optical gating measurements reveal that the output pulse duration of 15.2 fs is within our measurement error of the theoretical transform limit. This technique significantly reduces the pulse duration and the temporal prepulse energy of the pulse while increasing the peak intensity by 26%. It is demonstrated, for what is believed to be the first time, that the problem of pedestals in laser amplifiers can be addressed by spectral-domain correction

Backus, S.

Bartels, R.

Kapteyn, H. C.

Murnane, M. M.

Zeek, E.

English

We demonstrate the use of a deformable-mirror pulse shaper, combined with an evolutionary optimization algorithm, to correct high-order residual phase aberrations in a 1-mJ, 1-kHz, 15-fs laser amplifier. Frequency resolved optical gating measurements reveal that the output pulse duration of 15.2 fs is within our measurement error of the theoretical transform limit. This technique significantly reduces the pulse duration and the temporal prepulse energy of the pulse while increasing the peak intensity by 26%. It is demonstrated, for what is believed to be the first time, that the problem of pedestals in laser amplifiers can be addressed by spectral-domain correction

Backus, S., author

Kapteyn, H. C., author

Murnane, M. M., author

Bartels, R., author

Zeek, E., author

Optical Society of America, publisher

Mountain Scholar (Digital Collections of Colorado and Wyoming)

April 15, 2000 / Vol. 25, No. 8 / OPTICS LETTERS 587Adaptive pulse compression for transform-limited15-fs high-energy pulse generationE. Zeek, R. Bartels, M. M. Murnane, H. C. Kapteyn, and S. BackusJILA, University of Colorado at Boulder, Boulder, Colorado 80309-0440G. VdovinElectronic Instrumentation, Delft University of Technology, 2600 GA Delft, The NetherlandsReceived November 23, 1999We demonstrate the use of a deformable-mirror pulse shaper, combined with an evolutionary optimizationalgorithm, to correct high-order residual phase aberrations in a 1-mJ, 1-kHz, 15-fs laser amplif ier. Frequency-resolved optical gating measurements reveal that the output pulse duration of 15.2 fs is within our measurementerror of the theoretical transform limit. This technique signif icantly reduces the pulse duration and thetemporal prepulse energy of the pulse while increasing the peak intensity by 26%. It is demonstrated, forwhat is believed to be the first time, that the problem of pedestals in laser amplif iers can be addressed byspectral-domain correction.  2000 Optical Society of AmericaOCIS codes: 140.7090, 140.3590, 320.5540, 320.5520.The past five years have seen considerable improve-ments in the capabilities of high-power ultrafast lasers.Ti:sapphire-based oscillator–amplifier systems cangenerate peak powers of 100 TW at 10 Hz, or 0.3–1 TW at kilohertz repetition rates.1 – 3 The pulseduration that is obtainable from these systems hasalso decreased from 100 to 20 fs. However, atvery short pulse durations ,20 fs, pulses from thesesystems often suffer from poor peak-to-backgroundcontrast in the time domain. This poor contrastresults from imperfect correction of the spectral phasearising from large amounts of refractive material inthe laser amplifier system, because compensating forthis material requires that the pulse compressor bemismatched from the stretcher. Imperfect optics andalignment can also contribute to residual dispersion.This results in energy arriving before and (or) afterthe peak of the pulse, and this prepulse reduces theutility of the laser pulses for many experiments,such as ultrafast x-ray generation,4,5 attosecond pulsegeneration, and ultrafast laser plasmas,6 includingsolid-target high-harmonic generation7 and novelabsorption mechanisms.8 Researchers have em-ployed various schemes incorporating prisms, complexstretcher–compressor designs, and ultrahigh-precisionoptics to compensate for up to the fourth-order phasein a Taylor expansion.2,9,10,11 However, very complexphase aberrations can arise from the use of chirpedmirrors12 and intracavity etalons13 in an amplifier sys-tem; to date, these aberrations have been impossible tocompensate for completely. Recently, pulse shaperswith liquid crystals14 have been used to adaptivelycorrect15 for amplifier-induced spectral phase distor-tions.16 – 18 However, in this past work, substantiallyimproved laser-pulse characteristics over what couldbe obtained by use of conventional means were notdemonstrated.Here we present a versatile, low-cost spectral phasecorrection technique that uses a deformable-mirrorpulse shaper.19 This device allows the efficientremoval of high-order phase distortions automati-0146-9592/00/080587-03$15.00/0cally and without introducing additional aberrations,allowing us to generate 15-fs, 1-mJ pulses from akilohertz laser amplifier system that are transformlimited. The pulses were characterized by use ofsecond-harmonic frequency-resolved optical gating,20with a dynamic range of 1024. This technique is ofgreat importance to a variety of high-field scienceexperiments that are sensitive to prepulse. This ap-proach has a very substantial advantage over previoustime-domain techniques21 in that it corrects for, ratherthan filters out, pulse aberrations. Thus the peakintensity of the contrast-enhanced pulses is higher,rather than lower, after correction. This higher inten-sity allows us to generate high-energy pulses that aresignif icantly shorter than previously demonstratedand allows for a new level of precision in the studyof high-field laser–matter interactions by use of lightpulses that are only a few optical cycles in duration.The laser amplifier system used in this work is simi-lar to that described previously,2 with two changes (seeFig. 1). First, the conventional lamp-pump frequency-doubled Nd:YAG pump laser is replaced with a diode-pumped laser. Second, a pulse shaper19 is insertedinto the beam just after the pulse stretcher. Pulsesare first injected into an all-ref lective stretcher be-fore entering the pulse shaper, which consists of a 4-fzero-dispersion stretcher employing a micromachineddeformable mirror (MMDM) at the fold plane. A600-mm radius-of-curvature mirror is used in the pulseshaper, resulting in a spot size of 170-mm diameter forFig. 1. 15-fs amplifier system with a MMDM pulseshaper. 2000 Optical Society of America588 OPTICS LETTERS / Vol. 25, No. 8 / April 15, 2000a discrete color at the focal plane where the MMDM islocated. The 318-gmm grating in the pulse shaperresults in a spectral aperture of 200 nm, mappedonto the 30-mm width of the MMDM. The averagef luence on the mirror is 3 Wcm2, well below thedamage threshold of the mirror. The mirror itself isa 600 nm thick 3 30 mm 3 10 mm silicon nitride mem-brane coated with gold. After it is coated, the mem-brane is suspended over a linear array of 19 electrodes,which forms the controlling structure of the membrane.A voltage applied to an electrode attracts the mem-brane, distorting the surface.High-repetition-rate kilohertz amplifiers are typi-cally pumped by arc-lamp-pumped Nd:YLF lasers, withpulse durations of 160 ns. In high-gain amplifiersystems such as the one used in this work (single-passgain, 83), significant amplif ied spontaneous emis-sion can build up during the pump pulse, depletingthe gain. In this work we use an intracavity-doubleddiode-pumped Nd:YAG laser (Cutting Edge Optronics)that delivers 50-ns pulses. The shorter pump pulseallows for higher gain in the Ti:sapphire amplifier be-fore amplif ied spontaneous emission buildup, increas-ing the overall amplifier efficiency and bandwidth.The pulses injected into the amplif ier have a 125-nmFWHM bandwidth, which narrows to 84 nm after am-plif ication to 1.4-mJ energy for 8-W pump power. Theoptical efficiency of 17.5% compares well with the pre-viously obtained value of 9.2%,22 with a wall-plug effi-ciency of 0.14%. After they are amplif ied, the pulsesare recompressed by use of a standard two-grating(1200-gmm) compressor.22Typical output without the adaptive pulse shaperis an 18–20-fs pulse with 1 mJ of energy. Figure 2shows second-harmonic frequency-resolved optical gat-ing measurements20 of [curve (a)] the pulse spectral in-tensity and [curve (b)] group delay. Since the MMDMcan compensate only for a limited amount of phaseaberration (corresponding to dispersion of approxi-mately 12 mm of material19), it is necessary to modelthe system and adjust it to near optimum before ap-plying the MMDM compensation. In Fig. 2, curves (c)and (d) show the pulse spectrum and the group de-lay, respectively, predicted by a numerical model of thepulse amplifier system.2 The agreement with curves(a) and (b) is excellent.An evolutionary strategy (ES) feedback algorithmwas employed to determine the optimum settingsof the MMDM.23 The feedback signal into the ESalgorithm was the intensity of the second-harmonic-generation frequency-resolved optical gating signal setat zero time delay. The algorithm increased the totalintensity of this signal. The advantage of this typeof optimization is that it requires no calibration ofthe apparatus and is very fast. Even after runningthe optimization algorithm for only 5 min, we findthat the group delay is much improved, as shownby curve (e) of Fig. 2, varying ,15 fs from peakto valley over the entire spectrum. Figure 3 showsthe pulse output in the time domain. As shown inFig. 3, before optimization the pulse FWHM is 18 fs,whereas after optimization it is 15.2 fs—within ourmeasurement error of 60.1 fs of the transform-limitedvalue of 15.1 fs. The pulse contrast (i.e., the peak-to-pedestal intensity ratio) is also improved from the bestcase before optimization. The pulse intensity 25 fsfrom the peak of the pulse is suppressed by morethan an order of magnitude. The resulting measuredpulse shape is near the transform limit down to thenoise f loor of our measurements 1024. The pulse’speak intensity also improves by 26% as a result ofoptimization.The ES algorithm is a powerful technique for gen-eral optimization. It is similar to genetic algorithms,in that it uses evolution as a model for optimization.The ES relies on random mutation of the best solutionsfrom the previous generation to sample new areas inparameter space. We use an adaptive mutation strat-egy to control the generation of new trials. Part of thegenetic code for each trial is a mutation-rate variable.Initial trials are generated randomly. New trials aregenerated by addition of a normally distributed ran-dom variable, with a distribution width characterizedby the mutation rate, to the parents, which includesthe mutation-rate variable itself. From a set of 20 par-ent trails, 100 children are generated in each iteration.As the algorithm settles on an optimum solution, solu-tions with a lower mutation rate naturally become moreFig. 2. Amplif ier output: (a) pulse spectrum before op-timization, (b) group delay before optimization, (c) pre-dicted spectrum before optimization, (d) predicted groupdelay before optimization, (e) measured group delay afteroptimization.Fig. 3. Output pulse on a logrithmic scale: (a) beforeoptimization (18 fs), (b) after optimization (15.2 fs),(c) transform limit of the measured spectrum (15.1 fs).April 15, 2000 / Vol. 25, No. 8 / OPTICS LETTERS 589robust. The response time of the MMDM is in themillisecond range, making a search practical. In oursystem, optimization takes approximately 5–10 min,limited by the speed of the software. The millisec-ond response time of the mirror will allow for furtherimprovements, although it may be limited by pulse-to-pulse f luctuations in the case of 1-kHz repetition-rateamplifiers.This work clearly demonstrates that feedback by useof second-harmonic generation is extraordinarily ef-fective in optimizing pulse characteristics, even in alaser amplif ier in which the pulse-to-pulse f luctuationsare quite substantial, approximately 3% in the laserused here. Yet the second-harmonic-generation feed-back demonstrably improves pulse characteristics evenin the wings, where the intensity is 1024 of the peakintensity. This improvement results from the fact thatthe MMDM serves to redistribute the frequency com-ponents of the pulse, rather than simply filtering outundesired components. A numerical integration of thetemporal profiles of Fig. 3 (assuming no change in to-tal pulse energy, as is the case) shows that the op-timization increased the peak intensity by 26% overthe unoptimized value, to within 6% of the transformlimit. This 26% increase results from the fact that theMMDM optimization transfers energy that was origi-nally spread over hundreds of femtoseconds to the peakof the pulse.We note that previous work on optimization of ampli-fier systems was done with liquid-crystal (LC) modu-lators. Efimov and Reitze demonstrated correction ofhigh-order dispersion to reduce pulse duration in a mil-lijoule multipass amplifier from 32 to 26 fs (Ref. 17);Brixner et al. demonstrated compression of pulses from195 to 103 fs.18 In previous work adaptive compres-sion of low-energy pulses by use of a LC was alsodemonstrated.15 There are distinct advantages anddisadvantages to LC and MMDM systems. The LCmodulates the light in discrete spectral intervals be-cause of the pixelated nature of the device, resultingin the generation of pulse artifacts that will limit thepulse contrast that can be achieved.14,15 The MMDMprovides smooth modulation with no artifacts. How-ever, the MMDM cannot easily provide amplitudemodulation, whereas the LC can. The LC also hasa greater number of actuators, increasing its versa-tility for correcting complex phase aberrations. Thisincreased versatility also increases the number of ad-justable parameters, making it in principle slower andmore difficult for the evolutionary algorithm to op-timize compression. To our knowledge no past workin adaptive pulse compression examined the resultingpulses with high dynamic range. Therefore it is quitepossible that after LC optimization a few frequencycomponents remained unoptimized and created a pulsepedestal. The results presented here are a signif icantimprovement over what has been accomplished withLC technology, at signif icantly lower cost.In summary, we have demonstrated are the highest-contrast sub-20-fs high-energy laser pulses yet gener-ated and the shortest pulses yet obtained from such asystem. Significant improvements in pulse intensity,pulse quality, and prepulse reduction were obtained asa result of optimization.We gratefully acknowledge support for this researchfrom the U.S. Department of Energy and the Na-tional Science Foundation. S. Backus’s e-mail addressis sbackus@jila.colorado.edu.References1. V. Bagnoud and F. Salin, in Digest of Conference onLasers and Electro-Optics, 1999 OSA Technical DigestSeries (Optical Society of America, Washington, D.C.,1999), p. 71–72.2. S. Backus, C. Durfee, M. M. Murnane, and H. C.Kapteyn, Rev. Sci. Instrum. 69, 1207 (1998).3. K. Yamakawa, M. Aoyama, S. Matsuoka, T. Kase, Y.Akahane, and H. Takuma, Opt. Lett. 23, 1468 (1998).4. R. W. 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Yamakawa,Opt. Lett. 21, 219 (1996).14. M. Wefers and K. Nelson, J. Opt. Soc. Am. B 12, 1343(1995).15. D. Yelin, D. Meshulach, and Y. Silberberg, Opt. Lett.22, 1793 (1997).16. A. Efimov, M. Moores, N. Beach, J. Krause, and D.Reitze, Opt. Lett. 23, 1915 (1998).17. A. Efimov and D. Reitze, Opt. Lett. 23, 1612 (1998).18. T. Brixner, M. Strehle, and G. Gerber, Appl. Phys. B68, 281 (1999).19. E. Zeek, K. Maginnis, S. Backus, U. Russek, M.Murnane, G. R. Mourou, H. Kapteyn, and G. Vdovin,Opt. Lett. 24, 493 (1999).20. R. Trebino, K. DeLong, D. Fittinghoff, J. Sweetser,M. Krumbugel, B. Richman, and D. Kane, Rev. Sci.Instrum. 68, 3277 (1997).21. S. Backus, H. C. Kapteyn, M. M. Murnane, D. M. Gold,H. Nathel, and W. White, Opt. Lett. 18, 134 (1993).22. S. Backus, C. G. I. Durfee, G. A. Mourou, H. C.Kapteyn, and M. M. Murnane, Opt. Lett. 22, 1256(1997).23. J. Heitkotter and D. Beasley, ‘‘The hitchhiker’sguide to evolutionary computation (FAQ),’’ USENET:comp.ai.genetic (1997).

Adaptive pulse compression for transform-limited 15-fs high-energy pulse generation

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Adaptive pulse compression for transform-limited 15-fs high-energy pulse generation

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