Efficient high-energy pulse-train generation using …

1 Efficient high - energy pulse -traingeneration using a 2n- pulse Michelson interferometerCraig W. Siders, Jennifer L. W. Siders, Antoinette J. Taylor, Sang-Gyu Park, andAndrew M. WeinerWe demonstrate a novel, Michelson-based, ultrafast multiplexer with a throughput approaching 100% fora polarization-multiplexed train and 50% for a linearly polarized train , which is compatible with ahigh- energy pulse train and shaped- pulse generation . The interpulse spacings in the resultant 2n-pulsetrain can be adjusted continuously from multinanoseconds through zero. using this interferometer, wealso demonstrate generation of a 16- pulse train of terahertz pulses. 1998 Optical Society of AmericaOCIS , , , , , IntroductionNumerous applications currently exist that require atrain of high - energy ultrafast pulses. Such applica-tions include multiple-laser- pulse excitation schemesfor high -gradient plasma accelerators,1photocathodeinjectors for conventional accelerators, multiple- pulse excitation of atoms, molecules, and solids,2andhigh-fluence terahertz wave- train generation for ra-dar and microwave op-timization of the efficiency of high -harmonicgeneration when short pulses are used may requirethe use of fast-rise-time flat-top pulses.

2 Such super-Gaussian pulses can be generated when several shortpulses are stacked together with an interpulse spac-ing of the order of the initial short- pulse techniques in which phase and am-plitude masks are used in the focal plane of a zero-dispersion pulse stretcher are typically used forproducing low- energy ~ , oscillator-level!ultrafastshaped pulses and pulse such tech-niques to produce trains of high - energy ~ , ampli-fied!pulses is limited because of damage to the maskresiding in the focal plane of the stretcher. Pulseshaping of 200-mJ pulses by using an acousto-opticmodulator has recently been demonstrated;6how-ever, the setup exhibits 2 orders of magnitude loss,resulting in only a 2-mJ output. A further increasein the diffraction efficiency causes significant acousticnonlinearity in the acousto-optic modulator. Pulseshaping before amplification has been demonstrat-ed,7but care must be taken to avoid nonlinear effects,such as nonlinear temporal diffraction,8that occurwith amplitude masking.

3 Even in the case of phase-only masking, amplifier output fluences are typicallyreduced compared with that from an unshaped , the experimental necessity of a single shortpulse synchronized to the pulse train for cross-correlation measurements ultimately dictates thatpulse shaping occur after schemes have appeared in the literaturethat are variations on a Michelson interferometerusing multiple beam splitters and optical delay linesto produce pulse trains of 2npulses wherenis apositive ,10 These schemes do not propa-gate the beam through a focus and therefore are com-patible with pulse shaping high - energy pulses. Then-fold application of a single Michelson interferome-ter would appear at first glance to reduce the totalfluence of the generated train to 22nbecause eachunit has a single beam on the input and two beams onthe output, one of which is not used. Similar con-clusions on generation efficiency are reached forpulse- train generation when a series of beam split-ters is both beams from each beamsplitter were used, the generation of two outputbeams that contain sequences of 2npulses has , interpulse spacings inthis device are limited by optical elements ps,C.

4 W. Siders, J. L. W. Siders, and A. J. Taylor are with theMaterials Science and Technology Division, Los Alamos NationalLaboratory, MS D429, Los Alamos, New Mexico 87545. and A. M. Weiner are with the School of Electrical andComputer Engineering, Purdue University, West Lafayette, Indi-ana 15 December 1997; revised manuscript received 26 March $ 1998 Optical Society of America5302 APPLIED OPTICSyVol. 37, No. 22y1 August 1998and therefore it is unsuitable for Michelson InterferometerWe present here ann-fold application of a Michelsoninterferometer, which, by using both beams fromeach beam splitter and then by polarization multi-plexing the two output beams, results in a singlepulse train of 2npulses with nearly 100% the frequency domain this interferometer repre-sents, for each polarization, a highly oscillatory am-plitude and phase filter. Our device design for thegeneration of a train of 16 pulses is shown in Fig.

5 The delay arms can be easily positioned forequal lengths, interpulse spacings from zero to mul-tinanoseconds are achievable. The first eight pulsesare orthogonally polarized to the final eight; however,one can easily polarize the output along a single axisby using a high -damage-threshold thin-film polar-izer, resulting in a 50% throughput. In some cases,for example, a laser-drivene1ye2collider, the twolinearly polarized output trains can be individuallyused. In general, for a 2npulse train the designshown in Fig. 1. requiresnbeam splitters,~4n12!mirrors, 2nlinear translators~to set and vary theinterpulse spacing!, one half-wave plate, and onethin-film polarizer, leading to a cost that scales asnand an overall area that scales characterize this device, we use the 800-nm,150-fs, 1-mJ output of a chirped- pulse amplified Ti:sapphire laser system operating at 1 kHz. For a16- pulse interferometer 18 mirrors and 4 beam split-ters are needed.

6 For reduced cost, protected gold-coated mirrors and dielectric beam splitters are used,resulting in a measured throughput of 73%, as ex-pected. If we were to use high -damage-thresholddielectric mirrors, a throughput of 98% would be ex-pected. The interferometer is initially aligned sothat the pulses overlap temporally two at a temporal overlap is determined first with spa-tial fringes and is optimized with second-harmonicgeneration in a doubling crystal. The desired pulsesequence is then set. For equally spaced pulses outof the interferometer in Fig. 1, arm 2 is lengthened byhalf a unit, arm 3 by one unit, arm 6 by two units, andarm 8 by four units. For the data shown in Fig. 2 theinterferometer is configured with 3-ps pulse separa-tions and the output is cross-correlated with a singlegate pulse , split off before the interferometer, in KDP crystal. When individual armsare blocked in the interferometer, various pulse se-quences can be formed as shown in Fig.

7 2, includingthe elimination of every other pulse in the pulse trainas well as double- pulse and quadruple- pulse pulse - train characterization can beachieved by blocking appropriate arms within theinterferometer to allow the passage of each of the 16pulses individually, allowing the linear measurementof individual energies as well as the nonlinear cross-correlation profiles of each pulse . Note that the en-Fig. 1. Interferometer design for generation of a train of 16pulses. For equally spaced pulses, arm 2 is lengthened by half aunit, arm 3 by one unit, arm 6 by two units, and arm 8 by 2. With the 16- pulse inter-ferometer configured for 3-pspulse separations, blocking par-ticular arms produces these andsimilar pulse trains. For thesedata the output of the interferom-eter is cross-correlated with a sin-gle gate pulse , split off before theinterferometer, in a August 1998yVol. 37, No. 22yAPPLIED OPTICS5303ergy of each pulse in the train is constant for alloutput sequences.

8 Figure 3 demonstrates such acharacterization of the device with linear powermeter~Molectron PM3!measurements of the pulseenergies~top!and nonlinear cross correlation of eachpulse separately as well as a cross correlation of thefull 16- pulse train ~bottom!. The small subpulsespresent in the cross-correlation measurements resultfrom a reflection off a beam splitter placed before theinterferometer. We measure a variation of,20% inboth pulse output energy and intensity over the en-tire 16- pulse pulse Stacking: Dark- pulse and Super-GaussianPulse GenerationThis interferometer can also be used to stack severalshort pulses together with an interpulse spacing com-parable with or less than the input pulse width. Inthis case the output pulse structure depends on therelative phase shift between the overlapping multi-plexed pulses. Since the phase difference betweenthe reflected and the transmitted pulses from a di-electric beam splitter is approximatelyp, a darkpulse, in which a nearly constant high -intensity fieldrapidly goes to zero and returns on an ultrafast timescale, can be produced by stacking a series of pulsesof one phase next to a series of pulses with the oppo-site phase, as shown in Fig.

9 4~top!. Moreover stack-ing several pulses together in-phase with pulseseparations comparable with their pulse widths re-sults in a super-Gaussian pulse @shown in Fig. 4~bot-tom!#, potentially useful for Efficient Terahertz Wave- train generation in DASTAs a final application of this technology, we gener-ated a 16- pulse train of terahertz pulses throughopticalrectificationindimethylami no4-N-methylstilbazolium tosylate~DAST!. DAST is anelectro-optic material and an Efficient unbiased tera-hertz emitter, exhibiting saturation in its terahertzoutput11only for optical fluences total excitation fluences below this value, theterahertz output is expected to depend linearly on theoptical pulse train generated by the detection of the terahertz wave form byusing a radiation-damaged silicon-on-sapphire pho-toconductive detector with a 2-mm-wide gap, gatedby a single optical pulse split off from the pulse trainbefore the interferometer, results in a temporal res-olution of 1 ps.

10 Figure 5 reveals the terahertz pulsetrains resulting from multipulse~1, 2, 4, 8, and 16!excitation of DAST with a pulse separation of 3 sign of saturation is present in the wave 3. Linear power meter~Molectron PM3!measurements ofthe pulse energies~top!and cross correlation of each pulse sepa-rately as well as a cross correlation of the full pulse train ~bottom!.Note the good agreement between the linear measurements ofpulse energy and the full- train 4.~Top!Output of the interferometer configured to producea dark pulse . To produce this output, four pulses separated by200 fs were stacked together with the second pair of pulses out ofphase from the first pair byp.~Bottom!Output of the interferom-eter configured to produce a super-Gaussian pulse . To producethis output, four pulses separated by 200 fs were stacked togetherin 5. Terahertz pulse trains resulting from a multipulse~1, 2, 4,8, and 16!optical excitation of DAST with a pulse separation of3 ps.