We demonstrate the stretching of 100- fs pulses by a factor of 12,000, using standard-size opticalelements. These stretched pulses are then amplified and recompressed to their initial duration.
To date, the best way to amplify short pulses insolid-state laser media has been to use the techniqueof chirped-pulse amplification.',2 The principle of thismethod is to stretch a short pulse temporally beforeamplification. After amplification, the pulse is recom-pressed to its Fourier-transform limit. This proce-dure makes it possible to avoid safely any nonlineareffect that could otherwise result in catastrophicdamage. The pulse is stretched by introducing afrequency chirp. This can be done by taking advan-tage of the group-velocity dispersion of any materialor by using the dispersing properties of prisms,3gratings,4 or Gires-Tournois interferometers. 5 Thestretching has to be reversible because the pulse hasto be recompressed after amplification. In fact, onlygratings can provide large and reversible stretchingfactors. Using either two gratings in an antiparallelconfiguration separated by an even number of lensesor two gratings in a parallel configuration separatedby an odd number of lenses, Martinez6 showed thatlarge positive dispersion, and therefore large stretch-ing factors, can be obtained. This positive dispersioncan be compensated for exactly by using similargratings in a parallel configuration.4 The stretching-compression factor is then limited only by the gratingsize. For instance, a transform-limited Gaussian pulsecan typically be stretched to 500 ps by using 10-cm-wide gratings. This value is independent of the input-pulse duration or spectrum. Only the distance be-tween gratings will change with the input-pulsebandwidth. Although the ultimate stretching factordepends only on the grating size, working with nar-row input spectra leads to long distances between
The authors are with the Ultrafdst Science Laboratory, Univer-sity of Michigan, 2200 Bonisteel, Ann Arbor, Michigan 48109-2099.
gratings. In this case, the stretching is limited bypractical considerations and not by the grating size.We present here a stretcher that uses only relativelysmall optical elements in an optimized design thatallows us to stretch 100-fs pulses to 1.2 ns. The broadspectra of the femtosecond pulses lead to a compactsystem.
The design is based on the stretcher described byMartinez.6 Two 1700-lines/mm gratings separated bytwo 0.5-m focal-length lenses in an afocal arrange-ment provide a dispersive delay line (Fig. 1). Thedistance between gratings 1 and 2 and the closest lensis 5 and 18 cm, respectively. The first grating is 3 cmx 3 cm, and the second one is 11 cm x 3 cm. They areused 15 deg off Littrow incidence. Because the twogratings are identical and the magnification of thetelescope is unity, the beam leaving the second grat-ing is collimated. It is then retroreflected by a roofprism. After retroreflection, the beam leaves the firstgrating, shifted below the input beam. This is one ofthe schemes presented by Martinez.6 Although onecan use telescopes with magnification larger than 1 x,we found experimentally that the symmetrical afocalsystem is the easiest to align. Furthermore, the onlyrequirement in this design to obtain a perfectlycollimated beam after the stretcher is that the lenssystem be perfectly afocal. In a stretcher that usesmagnification, the positions of the input-beam waistand of the retroreflecting prism are crucial and, ingeneral, difficult to control. Optimization of thestretcher design can be described as follows. First wechoose a focal length for the lenses. It has to be largeenough to avoid small f-numbers and small enough tokeep the system compact. We then calculate thedistances between the gratings and the lenses so thatthe diffracted beam covers the second grating. Carehas to be taken with a Gaussian spectrum so as not toclip the wings of the spectrum. The general rule isthat the stretching is largest when the grating-lensdistance is at the minimum. The minimum grating-
Fig.1. Schematic of a double-pass stretcher. Ml is a plane mirror.
lens distance is given by the facts that the input beammust not be clipped by the edge of the first lens andthat the diffracted beam leaving the second gratingmust not be clipped by the edge of the second lens. Alast free parameter is the angle of incidence, althoughit has to be as close to the Littrow angle as possible tolimit astigmatism. For a given set of optical elements,there are, in general, several configurations that givethe same stretching factor, and the final design is a
trade-off between practical considerations and perfor-mance.
To double the pulse stretching, we reflected theoutput beam back through the stretcher, using mir-ror Ml (see Fig. 1). After one round trip in thestretcher, the beam emerges exactly collinear to theinput beam. To separate the input and the output, weintroduced a Faraday rotator, a half-wave plate, and apolarizer in front of the first grating (see Fig. 1). The45 deg of rotation of the polarization plane introducedby the Faraday rotator is canceled by the wave platefor the input beam but is doubled for the outputbeam. After the wave plate, the output-beam polariza-tion is then at 90 deg to that of the input beam. Apolarizer is then used to discriminate between thetwo.
We have demonstrated this device in a Ti:sapphireoscillator-amplifier system.7 This laser consists of aself-mode-locked Ti:sapphire oscillator8 producing100-fs pulses tunable from 730 to 800 nm (Fig. 2).These pulses are stretched, and then one of them isinjected into a Ti:sapphire regenerative amplifierpumped by a frequency-doubled Nd:YAG laser. Thisamplifier uses a single Pockels cell for injection andcavity dumping. The input and output beams arethen collinear, and a Faraday rotator is used to
Fig. 2. Amplifier setup. The input comes from the oscillator: P's, thin-film polarizers; FR's, Faraday rotators; G's, gratings; PC, Pockelscell; RP's, roof prisms; Ml, M2, plane mirrors; X/2's, half-wave plates.
separate the input and the output pulses. We foundthat, as the stretcher is introduced between theoscillator and the amplifier, we can take advantage ofthis Faraday rotator for double passing the stretcher
2ns/div
2mVodiv
-6.
-44. mV77.65ns Ins/div 87.65ns
Fig. 3. Pulse shape after a, one and, b, two passes through thestretcher. c, The resolution of the detection system is 104 ps.
and for discrimination between the output and theinput of the amplifier.
The pulse duration at the oscillator output istypically 100 fs. After one pass through the stretcherit increases to 600 ps. After a second pass, the pulseduration becomes 1.2 ns. Figure 3 shows the pulseafter one and two passes through the stretcher. Thepulse shape was recorded with a fast photodiode and asampling oscilloscope. The time resolution of thissystem, measured by using the 100-fs pulses from theoscillator, is approximately 100 ps (Fig. 3c). The finalpulse duration corresponds to a stretching factor of12,000, which is the largest ever obtained for opticalpulses.
These pulses were amplified up to 10 mJ in theTi:sapphire regenerative amplifier (see Fig. 2). Theyare then sent to a double-pass alexandrite amplifier.As alexandrite is a birefringent material with signifi-cant gain only on one axis, we have to use a Faradayrotator and a half-wave plate to discriminate betweenthe input and the output of the amplifier. The pulsesare then sent to a compressor consisting of twoparallel 1700-lines/mm gratings. As with the stretcherand the regenerative amplifier we can take advantageof the Faraday rotator to double pass the compressor.Using such an arrangement, one should be able torecompress the pulse from 1.2 to 100 fs. As thegratings that we were using were not designed for thewavelength at which we were running (765 nm), theirefficiency was poor. The throughput for a single passthrough the compressor was approximately 15%. Todate, we have demonstrated the recompression pro-cess by using the stretcher and the compressor insingle-pass configuration only. In that case the pulsewas stretched to 600 ps and recompressed to 105 fs.We measured the pulse duration with a single-shotautocorrelator, and a sech2 shape was assumed. Be-cause the dispersion of the compressor compensatesfor that of the stretcher, we do not expect any changein the final pulse duration as a result of doublepassing both the stretcher and the compressor. Wefound experimentally that the stretcher and thecompressor gratings have to be used under the sameangle of incidence with a tolerance smaller than 1 deg.If the angles are not the same, a large part of thecompressed pulse energy lies in a broad background.
In some applications, the pulse-peak power aftercompression is too high to permit the pulse to passthrough any thick optical element without severespatial deformation of the beam. In that case one canstill use the stretcher in a double-pass configurationand use the compressor with gratings that are twiceas large in a single-pass configuration.
In summary, we have demonstrated that a largestretching factor of femtosecond pulses can be ob-tained by using relatively small optical elements anddesigning the stretcher properly. This is importantfor the production of high-peak-power femtosecondpulses. With our system, we have already obtained10-GW pulses. The large stretching factor that we
obtained will allow us to use the full potential ofalexandrite amplifiers safely. We expect 0.5-1-TWpulses to be produced with a single alexandrite ampli-fier.
This research is supported in part by the Commis-sarist a I'Energie Atomique/Centre d'Etude Limeil-Valenton and the U.S. Air Force Office of ScientificResearch, University Research Initiative, under con-tract AFOSR-90-0214. J. Squier acknowledges thesupport of a fellowship from Allied Signal Corpora-tion. F. Salin is on leave from the Institut d'OptiqueTh6orique et Appliqu6e, Unit6 Associ6e au CentreNational de la Recherche Scientifique (France).
References
1. D. Strickland and G. Mourou, "Compression of amplifiedchirped optical pulses," Opt. Commun. 56, 219-221 (1985).
2. P. Maine, D. Strickland, P. Bado, M. Pessot, and G. Mourou,
"Generation of ultrahigh peak power pulses by chirped pulseamplification," IEEE J. Quantum Electron. 24,398-403 (1988).
3. R. L. Fork, 0. E. Martinez, and J. P. Gordon, "Negativedispersion using pairs of prisms," Opt. Lett. 9, 150-152 (1984).
4. E. Treacy, "Optical pulse compression with diffraction grat-ings," IEEE J. Quantum Electron. QE-5,454-458 (1969).
5. F. Gires and P. Tournois, "Interferombtre utilisable pour lacompression d'impulsions modul6es en frequence," C. R. Acad.Sci. (Paris) 258, 6112-6115 (1964).
6. 0. Martinez, "3000 times grating compressor with positivegroup velocity dispersion: application to fiber compensation inthe 1.3-1.6 pm region," IEEE J. Quantum Electron. QE-23,59-64 (1987).
7. J. Squier, F. Salin, G. Mourou, and D. Harter, "100 fs pulsesgeneration and amplification in Ti:AIO," Opt. Lett. 16, 324-326 (1991).
8. D. E. Spence, P. N. Kean, and W. Sibbett, "60-fsec pulsegeneration from a self-mock-locked Ti:sapphire laser," Opt.Lett. 16, 42-44 (1991).
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