Single-grating laserpulse stretcher and compressor

Ming Lai, Shui T. Lai, and Casimir Swinger

Stretching and compressing of laser pulses is demonstrated with a single-grating apparatus. A laserpulse of 110 fs is stretched to 250 ps and then recompressed to 115 fs. The apparatus exploits a two-levelstructure: one level for stretching and the other for compressing. This single-grating configurationshows significant simplification in structure and alignment over existing multiple-grating systems.Such a stretcher-compressor is particularly suitable for use with chirped-pulse amplification in whichlaser wavelength tuning is desirable. Only one rotational adjustment is rquired to restore the alignmentof the entire stretcher and compressor when the laser wavelength is changed.

A laser pulse stretcher and compressor is a keyelement in a chirped-pulse amplifier for amplifyingsubpicosecond laser pulses to an energy level greaterthan 100 .LJ per pulse. In this application, a stretcheris used to lengthen the laser pulses before amplifica-tion, and a compressor is used to restore the pulseduration after amplification. In this way, the peakpower inside the amplifier cavity can be kept suffi-ciently low to avoid optical damage to the opticalelements and to avoid nonlinear distortion on thepulse shape and beam profile.

Using a pair of diffraction gratings to compressoptical pulses was first proposed by Treacyl in 1969.Grating-based laser pulse stretcher-compressors wereinvestigated by Martinez2 and demonstrated by Pes-sot et al.3 in 1987. In the early design by Pessot etal.,3 four identical diffraction gratings were used.Two of the gratings were used in the stretcher tolengthen ultrashort laser pulses by introducing apositive group-velocity dispersion to the pulses. Theother two gratings were used in the compressor toreverse precisely the stretching process by introduc-ing a negative group-velocity dispersion.

Recent advances in self-mode-locked Ti:sapphirelasers and regenerative Ti:sapphire amplifiers haveled to the rapid development of laser pulse stretcher-compressors. Modified designs of the Pessotstretcher-compressor use two or three gratings.4 5

The authors are with NorthStar Scientific Instruments, 1223Orchard Glen Circle, Encinitas, California 92024.

Received 22 February 1993; publication delayed at authors'request.

0003-6935/93/306985-03$06.00/0.© 1993 Optical Society of America.

Although the basic mechanism of phase modulationremains the same, these new designs greatly simplifythe structure of the instrument and reduce thedifficulty in alignment.

However, a major problem remains in all multiple-grating stretcher-compressors. Namely, all of thegratings require precise readjustment when the laserwavelength is changed. These readjustments areextremely inconvenient and time consuming whenfrequent tuning of the laser wavelength is desirable.In addition, strictly matched grating pairs are re-quired in the stretcher and the compressor for main-taining good beam profiles and obtaining a goodpulse-stretching-pulse-compressing ratio.

In our experiment, we eliminated the above prob-lems by using only one grating for the entire stretcher-compressor. Our apparatus uses a two-level struc-ture. Figure (a) is a side view of the design: theupper level is the stretcher and the lower level is thecompressor. Figures 1(b) and 1(c) are top views ofthe stretcher and the compressor, respectively.

As shown in Fig. 1(b), the stretcher consists of thetop half of a diffraction grating (1), 5.08 cm x 10.16cm, 1800 grooves/mm, and gold coated; an achro-matic lens (2), 50-cm focal length, 2.54 cm x 8.89 cm,and AR coated; a flat mirror (3) 5.08 cm x 5.08 cm;and a roof-mirror reflector (4), 1.27-cm x 7.62-cmaperture, for vertical displacement. This layout issimilar to that described in Ref. 4.

As Fig. 1(c) shows, the compressor is composed ofthe bottom half of the same grating used for thestretcher; a roof-mirror reflector (5), 2.54-cm x7.62-cm aperture, for horizontal displacement; and aroof-mirror reflector (6), 1.27 cm x 7.62 cm, forvertical displacement. This layout is a folding of the

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space between grating 1 and roof-mirror reflector 5 isapproximately 26 cm.

Figures 2(a)-2(c) show an autocorrelation trace ofthe initial pulses, a sampling-scope trace of thestretched pulses, and an autocorrelation trace of thecompressed pulses, respectively. After each tuningof the laser wavelength, only a coarse rotationaladjustment to the grating is required to recover thealignment of both the stretcher and the compressor.By a slight adjustment of the distance from roof-mirror reflector 5 to grating 1, a minimum durationof the compressed pulses is obtained. The instru-ment shows no noticeable change (< 5%) in thestretching-compressing ratio over a tunable range of35 nm.

In conclusion, a single-grating, double-layer laserpulse stretcher-compressor is demonstrated showing

(C)

(a)

Fig. 1. Schematic diagram of experimental apparauts: (a):Side view of the single-grating, two-level laser pulse stretcher-compressor: 1, grating; 2, lens; 3, mirror; 4 and 6, roof-mirrorreflectors for vertical displacement; and 5, roof-mirror reflector forhorizontal placement. (b): Top view of the layout of the stretcher.(c): Top view of the layout of the compressor.

compressor described in Ref. 3, achieved with a roofmirror (5) for horizontal displacement.

A beam of laser pulses to be stretched is directed tograting 1 at an angle of 11° away from Littrow. Theinput beam of the compressor is parallel to the inputbeam of the stretcher. In the stretcher, the spacingbetween lens 2 and mirror 3 is equal to the focallength of the lens. The distance from grating 1 tolens 2 controls the amount of positive group-velocitydispersion, which determines the ratio of pulse stretch-ing. In the compressor, the path length from grat-ing 1 to roof-mirror reflector 5 controls the amount ofnegative group-velocity dispersion, which determinesthe ratio of pulse compression.2 All roof-mirrorreflectors used in the experiment are assembled inour laboratory and have an accuracy better than 20arcsec.

To test the single-grating stretcher-compressor, aKerr lens mode-locked Ti:sapphire oscillator is usedas a laser source. The laser produces 200 mW of 110fs pulses at a repetition rate of 90 MHz. The laserwavelength of the self-mode-locking operation is tun-able in the range 810-845 nm. The laser output iscollimated to a spot size of 1.5 mm before it is sentinto the stretcher-compressor. In the test, the 110fs pulses are stretched to 250 ps and then fed into thecompressor to be recompressed back to 115 fs. Thedistance from grating 1 to lens 2 is 21 cm, and the

(b)

(c)

Fig. 2. (a), Autocorrelation trace of the laser pulses prior to thestretcher. The pulse duration is 110 fs. (b): Sampling-scopetrace of the laser pulses at the exit of the stretcher. The pulsewidth is 250 ps. (c): Autocorrelation trace of the recompressed

pulses. The pulses are of 1 fs il duratioln.

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(a)

(b)

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significant simplification in structure and alignmentover existing multiple-grating versions. In particu-lar, this single-grating configuration makes it pos-sible for one to tune the stretcher and the compressorto accommodate a new wavelength in a matter ofseconds, and therefore it becomes easy for one to fullyappreciate the high pulse energy and broad-bandtunability of Ti:sapphire oscillator-amplifier systems.In addition, our design reduces the number of grat-ings to the minimum, consequently reducing the costof the instrument and making the device more com-pact.

References and Notes

1. E. B. Treacy, "Optical pulse compression with diffractiongratings," IEEE J. Quantum Electron. QE-5, 454-458 (1969).

2. 0. E. Martinez, "3000 times grating compressor with positivegroup velocity dispersion: application to fiber compensation in1.3-1.6-p~m region," IEEE J. Quantum Electron. QE-23,59-64(1987).

3. M. Pessot, P. Maine, and G. Mourou, "1000 timesexpansion/compression of optical pulses for chirped-pulse am-plification," Opt. Commun. 62, 419-421 (1987).

4. Y. Beaudoin, C. Y. Chien, J. S. Coe, J. L. Tapie, and G. Mourou,"Ultrahigh-contrast Ti:sapphire/Nd:glass terawatt laser sys-tem," Opt. Lett. 17, 865-867 (1992).

5. For examples, see Spectra-Physics product catalog, 0.1 TWTi:sapphire Amplifier System (Spectra-Physics Lasers, 1330Terra Bella Ave., Mountain View, Calif. 94039), and Quantronixproduct catalog, Model 4820 stretcher/compressor (Quantro-nix, 49 Wireless Boulevard, Smithtown, N.Y. 11787).

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