2114 OPTICS LETTERS / Vol. 20, No. 20 / October 15, 1995

Terawatt Ti:sapphire laser with a spherical reflective-opticpulse expander

Detao Du, Jeff Squier,* Steve Kane, Georg Korn,† and Gerard Mourou

Center for Ultrafast Optical Science, University of Michigan, Room 1006, IST Building, 2200 Bonisteel Boulevard,Ann Arbor, Michigan 48109-2099

Charles Bogusch

Clark-MXR, Dexter, Michigan 48103

Christopher T. Cotton

ASE Optics, Honeoye Falls, New York 14472

Received June 15, 1995

We have developed a novel stretcher–compressor system to produce 45-fs, 75-mJ pulses at a 10-Hz repetitionrate. 1995 Optical Society of America

Multiterawatt Ti:sapphire lasers operating at mod-est energy levels (, 100 mJ or less) have recentlybecome possible as a result of dramatic reductionsin the compressed pulse width.1,2 Barty et al.3 pro-duced 125-mJ, 30-fs pulses, using a cylindrical-mirror-based pulse expander, which allowed for reversiblestretching–compression ratios as large as 10,000 andis sufficient for pulses as short as 10 fs.1 Such largestretching–compression ratios are desirable for sev-eral reasons. First, for efficient extraction of the en-ergy from an amplif ier, the pulse f luence must reachor exceed the saturation f luence of the laser. For in-stance, using a Frantz–Nodvik analysis, Barty et al.3

calculated that for operation at the theoretical extrac-tion efficiency of Ti:sapphire the amplifier must beoperated near a f luence of ,2 Jycm2. However, theintensity must also be kept below the damage thresh-old of the amplif ier materials (#5 GWycm2), which,when combined with the saturation f luence require-ments ($1 Jycm2), dictates that the stretched pulsewidth be at least 160 ps.

The high contrast requirements of these systemsfurther impose a large stretching factor. Perry et al.4

recently demonstrated both theoretically and experi-mentally that even for B-integral values #1, signifi-cant energy appears in the wings of the pulse as aresult of the nonlinear phase distortion. For the caseof B ­ 1 and a 100-fs Gaussian pulse input, theirsimulations show that the wings on the recompressedpulse extend forward many times the pulse duration,even within the first few decades of the peak intensity.Typically, terawatt Ti:sapphire lasers operate with aB-integral of ,1; this value can be reduced (or at leastbe kept to a minimum) by using as large a stretchedpulse width as possible and minimizing the materialpath length in the amplifier.

It is nontrivial to achieve the large stretching–compression ratios described here with femtosecondpulses. The larger the ratio, the more difficult it be-comes to compress the pulse to its transform limit,

0146-9592/95/202114-03$6.00/0

because of the exacting alignment tolerances, aberra-tions induced in the stretching process, high-order dis-persion, finite beam size effects, etc. Several uniquesolutions have been developed to solve this prob-lem, one of which was already mentioned.1 A secondsystem, developed by White et al.,5 uses a refrac-tive achromatic doublet that gives tunable third- andfourth-order phase control.

In this Letter we describe a new approach for a 45-fs,75-mJ Ti:sapphire laser that has several unique char-acteristics compared with previous lasers. First, weintroduce a new expander system that provides signifi-cant stretching factors, enabling us to operate at thesaturation f luence. This single-grating expander em-ploys spherical ref lective optics in a novel, aberration-free configuration.7 Second, the regenerativeamplif ier is designed to provide high pulse-to-pulsestability.8 This design represents an effective newmeans toward attaining stable, sub-50-fs, amplif iedpulses at the terawatt level.

The laser system is illustrated in Fig. 1. Figure 2is a detailed view of our pulse expander. It uses twospherical ref lectors, one with a positive radius ofcurvature (R ­ 120 cm) and the other with a negativeradius of curvature (R ­ 260 cm). The mirrorsare arranged concentrically. In this configuration,all the third-order aberrations are zero, and a unit-magnif ication aberration-free telescope is obtained(this configuration is known as Offner’s design9). Themirrors and the grating are arranged collinearly. Theinput beam lies in the plane perpendicular to the grat-ing. The diffracted light is also in this plane. Theconcave mirror is slightly tilted up so that the ref lectedbeam passes over the grating. The convex mirror isslightly tilted down and ref lects the beam toward theconcave mirror again. The concave mirror ref lects thebeam to the grating at a lower height. After the beamhits the grating for the second time, it is diffracted par-allel to the input beam. A f lat mirror is used to ref lectthe beam back to the expander with a little tilt (up), so

1995 Optical Society of America

October 15, 1995 / Vol. 20, No. 20 / OPTICS LETTERS 2115

Fig. 1. Schematic of the 10-Hz laser system. TFP’s, thin-film polarizers; M’s, mirrors.

Fig. 2. Schematic of the aberration-free pulse expander.The concave and convex mirrors are arranged with theircenters of curvature coinciding at a single point. Theradius of curvature of the convex mirror is half that of theconcave mirror.

that the stretched pulse is displaced (down) from theinput beam. 1200-lineymm holographic gratings areused in both the stretcher and the compressor. Thesegratings are designed to be most eff icient near theLittrow angle; therefore the expander grating is usedat an angle of incidence less than Littrow (22.00± angleof incidence) so that the compressor can be used nearLittrow (25.17± angle of incidence). This simulta-neously maximizes the compressor efficiency andachieves third-order dispersion compensation. Thestretched pulse intensity was measured with an ul-trafast photodetector (Picometrix Model PXD7), andthe pulse width measured 220 ps FWHM. Because ofthe use of the Offner image system in the stretcher, theeffective distance of the stretcher is much longer thanthe physical size of the system. The overall geometryis quite compact (60 cm in length), and the pulse width

expands to 220 ps after only a single round trip throughthe stretcher. It should be noted that there are limita-tions to this expander design. Although it is compactand easily aligned, this system provides no means forcompensation of quartic phase, which will ultimatelylimit the achievable pulse width. Our numericalsimulation shows that for a sech2 pulse of 45 fs, theuncompensated fourth-order dispersion causes wingsat ,1024. However, by choice of a different gratingpair (1450 linesymm) for pulse compression, our simu-lation shows that we should be able to compensate thefourth-order dispersion and suppress the wings downto 1027.10 To date we have used seed pulses in the45–50-fs regime, which are the spectral narrowinglimits of the present amplifier. We have stretchedand compressed 36-fs pulses without amplif ication byusing this geometry but do notice an increase in thewings of the pulse at this level.

A pulse selector located between the oscillator andthe stretcher is used to ensure that only a single pulseseeds the regenerative amplifier. The regenerativeamplif ier is identical to that used in our kilohertz con-figuration.11 The absorbed pump energy is 3.8 mJ,resulting in an amplif ied output of 1 mJ. The peak-to-peak f luctuations are 62%. A similar resonatordesign has been shown to produce peak f luctuations of#1%,12 but at much greater pump levels. The outputof the regenerative amplifier goes through a sec-ond pulse selector, which reduces the inevitablesatellites that precede and follow the main pulsein this type of regenerative amplif ier system. Thepulse next passes through a double-pass ampli-fier, resulting in an output energy of 10 mJ foran incident pump energy of 55 mJ. Both the re-generative amplif ier and the double-pass amplifierare pumped by the same pump laser (Contin-uum Surelite). This amplifier combination pro-duces more energy than our previously reportedsingle-stage regenerative amplif ier, with virtuallyidentical efficiency. However, the long-term ampli-fied pulse stability is five times greater than beforebecause of the better-matched pump mode and theoptimized regenerative amplifier gain-to-loss ratio.After the double-pass amplifier the beam is furtherupcollimated and sent through a four-pass bow-tieamplif ier. This amplifier uses a 12-mm-diameter,20-mm-long, 0.1% doped Ti:sapphire rod (UnionCarbide). The rod is pumped from both ends at af luence of ,1.4 Jycm2. The pump beam profile isimage relayed onto the crystal to prevent the in-evitable hot spots that are present on large-volume,frequency-doubled Nd:YAG lasers. The amplif iedbeam diameter is 4 mm (FWHM). For a net absorp-tion of 500 mJ, the amplified pulse energy is 150 mJ,resulting in an extraction efficiency of 30%.

The amplified spectrum is shown in Fig. 3. Thecompressed pulse width is 45 fs (Fig. 4). We deter-mine this pulse width by fitting the autocorrelation aswell as using the square root of the measured amplif iedspectrum to calculate an autocorrelation width. Bothagree within the experimental error. Note that thispulse width is identical to that obtained by Barty et al.1

without the use of masks to reshape the input spectrumof the seed pulse. The overall eff iciency of the com-

2116 OPTICS LETTERS / Vol. 20, No. 20 / October 15, 1995

Fig. 3. Amplified spectrum. The center wavelength is780 nm, and the FWHM is ,18 nm.

Fig. 4. Single-shot autocorrelation trace. The solid curveis the sech2 fit.

Fig. 5. Focal spot measurement at full power. Projec-tions onto the x–z and y –z planes are the lineouts throughthe center of the focal spot.

pressor is 50%, and the energy of the compressed pulseis 75 mJ. A background-free second-harmonic scan-ning autocorrelator was used to measure the contrastof our system. To the limit of our instrumentation, wefound that the contrast of the laser is $105:1 in a 10-pswindow.

One important parameter of a high-peak-power lasersystem is the beam focusability. We measured the

spatial beam profile at the full power of the systemto determine the beam divergence in both the tan-gential and sagittal planes. A wedge was employedto ref lect a few percent of the energy of the ampli-fied beam, and the attenuated beam was focused byan off-axis parabola. The focal spot was imaged by amicroscope objective lens to a CCD camera (Photomet-rics Star1 CCD camera). The focused spot is shownin Fig. 5. We have measured 5.8 mm FWHM (1.05times the diffraction limit) in the tangential plane and7.2 mm FWHM (1.31 times the diffraction limit) in thesagittal plane. Thus the intensity at the focal spot canbe as high as 2 3 1018 Wycm2.

To conclude, we have demonstrated a 45-fs, 75-mJ laser system with an aberration-free sphericalref lective-optic pulse expander. This system providesa large stretching factor to maximize extraction effi-ciency while minimizing B-integral effects. We havedemonstrated that a simple, compact pulse stretcheris sufficient for the amplification and compression of50-fs pulses to the terawatt level.

We thank J. Workman for his assistance in thebeam profile measurement. This research was sup-ported by the National Science Foundation throughthe Center for Ultrafast Optical Science (contractPHY8920108). J. Squier and S. Kane acknowledgethe support of the Torrey Foundation.

*Present address, Department of Electrical and Com-puter Engineering, University of California, San Diego,Urey Hall, Mail Code 0339, La Jolla, California 92093.

†Present address, Max-Born-Institut fur Nichtlin-eare Optik und Kurzzeitspektroskopie, 12474 Berlin,Germany.

References

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J. P. Chambaret, P. Rousseau, P. Curley, G. Cheriaux,G. Grillon, and F. Salin, in Conference on Lasers andElectro-Optics, Vol. 15 of OSA Technical Digest Series(Optical Society of America, Washington, D.C., 1995),p. 388.

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