Proceedings of Ihc 6lh International Symposium on Advanced Nuclear Energy Research

-INNOVATIVE LASER TECHNOLOGIES IN NUCLEAR ENERGY -

ULTRASHORT-PULSE LASERS AND THEIR APPLICATIONS GCrard A. MOUROU

Center for Ultrafast Optical Science University of Michigan 1006 l.S.T. Bldg., 2200 Bonistccl Blvd. Ann Arbor, MI 48109-2099 USA

A revolution has occurred over the past ten years in our ability to generate, manipulate, and amplify ultrashort pulses. Laser pulses can be as short as a few femtoseconds (few optical cycles) and possess extreme power up to several tcrawatts. The applications of these pulses arc numerous in physics, chemistry, and biology, where they can be used to time resolve ultrafast events. We review in this article the state of the art in short pulse generation and amplification.

Keywords: Ultrahigh-peak-power lasers, Ultrashort-pulse lasers, Chirped pulse amplification

1. INTRODUCTION

Over the past few years a revolution has occurred in our ability to produce extremely high-power and high-intensity pulses. Whereas ten years ago a tablctop system with a beam size on the order of a centimeter squared could typically produce gigawatts the same size system can now produce 1000 to 10,000 times this peak power. Today the laser power is well into the tcrawatl regime with focused intensity greater than 1018 W/cm2. This vast improvement in laser peak power and intensity is the direct result of the technique of Chirped Pulse Amplification (CPA).1,2 Figure 1 describes the evolution of laser peak power since its inception. The different power jumps correspond to Q switching, mode locking, and finally CPA. Note that with CPA a dramatic improvement of three to four orders of magnitude has been obtained and should permit in the near future the generation of pulses limited by their theoretical peak power. This technique makes possible efficient energy extraction from superior energy storage materials by short pulses, without inducing undesired nonlinear effects in the amplifier system. This technique has been applied to (1) a wide variety of amplifying materials, i.e., Nd:glass,1-3 alexandrite,6 Ti:sapphirc,7-9 and LhSAF,10'11 with good energy storage characteristics, and (2) existing, large-scale Nd:glass systems, built for nanasccond-pulsc amplification as at the CEA Limcil in France12'13 or at Osaka in Japan.14 Record peak power of 55 TW (25 joules, 400 fs) were obtained at CEA Limeil. More ambitious performances arc contemplated at Lawrence Livermorc National Laboratory (LLNL), where the construction of a pctawatl (1 KJ/ps) system is underway. CPA had not only a major impact on the peak power of short-pulse systems, but also a similar one on their average power. As illustrated in Fig. 2, before CPA, short-pulse amplification systems had an average power of typically 10 mW. With CPA their average power is now on the order of 1 W, a factor-

— 117- of-100 improvement. Here again the average power limit has not been reached and we expect that in the near future femtosecond amplification systems will produce an average power at the 10-W level. High average power is essential for high harmonic generation,15 ulirafast spectroscopy,lh medical17'18 and imaging19 applications. The production of a high-pcak-powcr pulse is a necessary but not sufficient condition to producing a high-irradiancc pulse on target. The brightness and ultimate focused irradiancc achievable with a laser pulse is determined by both the peak power and the spatial quality (divergence) of the pulse. By paying careful attention lo both linear and nonlinear aberrations in the laser and beam transport systems, it is now possible to produce nearly diffraction-limited, mullitcrawalt pulses. When focused, such pulses can achieve irradiancc between 1018 and 1019 W/cm2. Figure 3 illustrates the dramatic impact of CPA on focused intensity over the years. Scaling these systems up to the 100-TVV or even pctawatt (1000-TW) level will enable the study of laser-matter interaction at 1021 W/cm2and beyond. As the pulse duration decreases from the nanosecond to the picosecond and femtosecond regime it becomes increasingly difficult to extract the stored energy without causing unwanted nonlinear effects. This stems from the fact that the input flucncc (J/cm2) necessary to extract the optical stored energy has to be of the order of the saturation flucncc, Fs= hv/o, where h is the Planck constant, v the laser frequency and a the emission cross section. Trying lo extract the energy with picosecond-femtosecond pulses leads to wave-front distortion and filamcntalion20 due to the intensity-dependent index of refraction n = na+ ntf, where I is the beam intensity. The wave-front distortion, B, is equal to

n2l elz (1) o To keep a diffraction-limited beam the value of B should not exceed 0.6, corresponding to wave-front distortion of TJT.21 The conditions for energy extraction and beam propagation can be simultaneously satisfied only with mediocre amplifying media possessing large emission cross sections on the order of 10" ^cm 2. These media, like dyes or excimers, have poor energy storage characteristics and require, for picosecond-femtosecond pulses, an input fluencc of the order of a mJ/cm2to extract their stored energy. 2 For femtosecond-picosecond pulses, this corresponds to an intensity /vof the order of a GW/cm , that is, below the threshold set by optical nonlincaritics. Consequently, before the advent of CPA they were the only choices possible for direct amplification of picosecond-femtosecond pulses. To reduce the size of laser systems, one wants to use good energy storage media, such as Nd:glass, alexandrite, Ti:sapphirc, and Li:SAF. These media have an emission cross section a ~ lO'^-lO"20 cm2 a thousand times smaller lhan dyes and excimcrs. Consequently they have a thousand limes better energy storage characteristics. However, these large saturation flucnccs, typically of a few joules, leads to extremely high saturation intensities /,.= FJT( T is the pulse duration), in the TW/cm2 range for pulses in

- 118 — the picosecond-femtosecond regime. The B associated with these large intensities is a thousand times- above the recommended level, forbidding direct short-pulse amplification. To get around this seemingly insurmountable problem, that is, to keep the input flucncc as high as possible while maintaining the pulse intensity to the lowest level, the pulse is stretched by a large amount (100 to 10,000 x) prior to amplification. It is then amplified by a factor of 106-1010 and rccomprcsscd to its initial value. This technique was first demonstrated in 1985 and was called Chirped Pulse Amplification.

2. THE CHIRPED PULSE AMPLIFICATION TECHNIQUE

The CPA technique in the optical regime involves some impressive manipulations (Fig. 4). First, a very short (ps-fs) and clean (over six to ten orders of magnitude) pulse is generated at the subnanojoulc level. Second, this pulse is stretched by a factor up to 104. Third, it is amplified by 10 to 11 orders of magnitude to the joule level. Fourth, it is rccomprcsscd by a factor of H)4 to its initial value. The end result must be a diffraction-limited pulse with an extremely high contrast ratio. These two attributes arc absolutely required to perform quality experiments at an intensity level greater than 1018 W/cm2. We will now describe the key elements that had to be developed over the past eight years which made Chirped Pulse Amplification the technique of choice for ultrashorl-pulsc amplification today. A. Generation of Short and Ultraclcan Pulses It is now possible to produce ultrashort pulses, using the Kcrr-lcns effect in Ti:sapphirc oscillators22 10-100-fs pulses arc routinely produced. Although these pulses arc extremely short, they have the tendency to exhibit a tail extending over a few hundred femtoseconds before the peak of the pulse. If this pedestal is greater than 1010W/cm2, it can produce a plasma on the target before the main pulse arrival, leading to questionable results. Before stretching, it can be necessary to further clean the oscillator pulse in a pulse cleaner. A high-contrast pulse cleaner has been demonstrated. It uses cross-phase modulation in a single-mode fiber located between two cross polarizers. A contrast enhancement of six orders of magnitude23 can be obtained. B. Stretching and Compression CPA demands the largest possible stretching/compression ratio to efficiently extract the stored energy without causing undesirable nonlinear effects, wave-front distortion, filamentation, and optical damage in the optical system. The first CPA] demonstration used the group velocity dispersion in fiber to stretch the pulse. The recompression was performed by using the negative group velocity dispersion provided by a diffraction grating pair. These stretcher/compressors arc not matched over all orders due mainly to the large third-order dispersion in the grating pair.24 The maximum compression ratio, R, that wc can produce is of the order of -r- where AX is the pulse bandwidth and "k is the laser wavelength.

When R exceeds this value, incomplete recompression is pcrfomicd producing unwanted ripples before or

— 119- after the main pulse. As an example, a picosecond pulse at 1.06 (.tin with a A\ of 20 A cannot be stretched and compressed by a factor R greater than 500. It quickly worsens for shorter pulses in the 100-fs range where R can not exceed 50. Using this embodiment, CPA seemed limited to R of the order ofl 00. In 1987, for optical communication applications, O. Martinez proposed a compressor with positive group velocity dispersion to compress pulses at 1.5 (im.25 In this regime, the fiber has negative group velocity dispersion and the pulses exhibit a negative going chirp. It is then necessary to rccomprcss the pulses with a device with positive group delay dispersion. To perform this operation Martinez proposed a compressor with positive group delay dispersion. This compressor is composed of a telescope with magnification of one between two antiparallcl diffraction gratings. It was recognized and demonstrated by M. Pcssot a a/.26 that this compressor at 1.5 |im could be, for wavelengths in the visible, a matched stretcher over all orders of a Tracy's compressor. Pcssot et al. ^ stretched and compressed an 80-fs pulse with R greater than 1000 without introducing any temporal distortion in the pulse. The discovery of this matched stretcher/compressor was a real breakthrough. It has been the key to ultrahigh-pcak-powcr generation and now equips all the Chirped Pulse Amplification systems. Extensive work is being done by a number of groups27 to further improve the stretching/compression ratio to 104 to 105. This very large ratio will be needed to produce pulses limited to their theoretical peak power. The high-/? stretcher/compressors arc based on low groove densities, i.e., 300 to 12(X) per mm, and refleciivc optics. C. Gain Narrowing Short pulses have a large Fourier spectrum. Because of the large overall gain involved in CPA, typically 1010 in Ihc small signal regime, Ihc pul.se .spectrum is reduced to a value ^^=^^m^r (2) where Acoa is the material gain bandwidth and Gdb((.oa) the gain. From this expression we sec how important it is to work with materials having a large gain bandwidth, such as Nd:glasscs, alexandrite, Ti:sapphirc, LiSAF, or a combination of those. A reduction in gain bandwidth has two undesirable effects. First, it reduces the stretched pulse duration, reducing the amount of cxiractable energy. Second, a narrower spectrum will lead to a longer compressed pulse. In first approximation ihc system output power is reduced according to the square of the gain narrowing.

3. THEORETICAL PEAK POWER

Although formidable peak power well in the terawatt regime has already been obtained, it is interesting to note that we arc far from the theoretical limit. This limit can be estimated as the ratio of the maximum cxtractablc energy given by Fs, ihc saturation llucnce, over the minimum pulse duration imposed by the

— 120 — material gain bandwidth. The peak power limit Ah per unit area, assuming a Au.T = .5 can be estimated as:

This power also represents thai needed to produce a Rabi oscillation in the amplifying medium. The theoretical peak powers for different materials arc estimated in the following Tabic 1. As mentioned above, the generation of this power will require a stretching/compression ratio in the 105 regime. We can predict that in the near future 100-TW pulses will be achievable with very compact systems. It is also conceivable to think about cxawalt (1()18-W) pulses with largc-apcrturc (1-m) systems based on mixed Nd:glasscs.

Table I: Theoretical peak powers

Laser Type Cross Section A\ T ^tl. K)-20 cm 2 (nm) (fs) (TW/cm2)

Nd:Glass Phosphate 4 22 80 60 Nd:Glass Silicate 2.3 28 60 100 Nd:Glass Combination 1.5 60 30 4000 Ti:Sapphirc 0.3 120 8 120 Alexandrite 1 100 10 2000 GvLiSAF 3 50 15 300

In conclusion, with chirped pulse amplification, the field of optical science is entering uncharted territory in physics. The intensities that can be generated arc such that during the interaction the electrons arc rclativistic, and x-ray radiation and formidable pressures due to pondcromotivc forces as high as gigabars can be produced. These ultrahigh-pcak-powcr lasers will undoubtedly have a profound impact on laser science.

-121 — REFERENCES

1) Strickland D. and MourouG.: Opt. Commun. 56,219(1985). 2) Maine P., Strickland D., Bado P., Pcssot M., and Mourou G.: IEEE J. Quantum Electron. OE-24. 398 (1988). 3) Maine P. and Mourou G.: Opl. Lett. £, 467 (1988). 4) Fcrray M„ Lomprc L. A., GobcrtO., L'Huillicr A., Mainfray G., Manus C, and Sanchez A.: Opl. Commun. 75, 278 (1990). 5) Perry M. D., Patterson F. G., and Weston J.: Opt. Lett. 15, 381 (1991); Patterson F. G. and Perry M. D.: J. Opl. Soc.Amcr.Bft 2384 (1991). 6) Pcssot M., Squicr J.,Bado P., Mourou G., and Hartcr D.: IEEE J. Quantum Electron. 25,61 (1989); Pcssot M., Squicr J., Mourou G., and Hartcr D.: Opt. Lett. J4, 797 (1989). 7) Kmctcc J., Macklin J. J..and Young J. F.: Opt. Lett. J6, 1001 (1991). 8) Sullivan A., Hamster H., Kaptcyn H. C, Gordon S., White W., Nathcl H., Blair R. J., and Falcone R.W.: Opt. Lett. J5,1406(1991). 9) Dilmirc T. and Perry M.D.: Opl. Lcit. 18,426 (1993). 10) Bcaud P., Richardson M., Micsak E., and Chai B. T.: Opt. Lett. J8,1550 (1993). 11) White W. E., Hunter J. R., Van Wocrkom L., Ditmirc T., and Perry M. D.: Opt. Lett. 17,1067 (1992). 12) Sautcrct C, Husson D., Thicll G., Sczncc S., Gary S.. Migus A., and Mourou G.: Opt. Lett. J6,238 (1991). 13) Rouycr C, Mazataud E., Allais I., Pierre A., Sczncc S., Saulcrci C., Mourou G., and Migus A.: Opt. Lett. 18, 214 (1993). 14) Yamakawa K., Shiraga H., and Kato Y.: Opt. Lett. 16, 1593 (1991). 15) Li K., L'Huillicr A., Fcrray M., Lomprc L. A., and Mainfray G.: Phys. Rev. A 29,5751 (1989). 16) The reader is referred to Martin J.-L., Migus A., Mourou G., and Zcwail A. H., cds.: "Ultrafasl Phenomena VIII," Springer-Vcrlag, Berlin (1993). 17) Zyssct B., Fujimoto J. G., Puliafito C. A., Birngrubcr R., and Dculsch T. F.: Laser in Surgery and Medicine 9, 193 (1989). 18) Ho P. P., Wang L., Liang X., Galland P., Kalpaxis L. L., and Alfano R. R.: Optics and Photonics News 4,23 (1993) and references therein; L. Wang el al.. Science 253.769 (1991) and references therein. 19) Lcith E., Chen C, Chen H„ Chen Y., Dilworth D., Lopez J., Rudd J., Sun P. C, Valdmanis J., and Vosslcr G.: J. Opt. Soc. Am. A 2,1148(1992). 20) Shcn Y. R.: "Principles of Nonlinear Optics," and references therein, Wiley Intcrscicncc, New York (1984); Kocchncr W.: "Solid-State Laser Engineering, Third Edition," Springcr-Vcrlag, New York (1990). 21) Born M. and Wolf E.: "Principles of Optics," Pcrgamon, Oxford (1970). 22) Spcncc D. E., Kcan P. N., and Sibbctt W.: Opt. Lett. 16, 42 (1991). 23) TapidJ.L.and MourouG.: Opt. Lett. 17,136(1992). 24) Trcacy E. B.: IEEE J. Quantum Electron. OE-5.454 (1969).

— 122 — 25) Martinez O. E.: IEEE J. Quantum Electron. OE-23.1385 (1987). 26) Pcssot M.: Opt. Commun. fi2,419 (1987). 27) Lcmoff B. E. and Barty C. P.: Opt. LcU. IS, 1651 (1993); Zhou J., Huang C.-P., Shi C, Mumanc M., and Kaptcyn H. C: Opt. Lett. 1&24 (1993).

— 123- FIGURES theoretical limit

pw

TW chirped pulse amplification ,_ GW 3CD • mode locking o Q. MW ^ ' Q switching

KW \ free-running

1960 1970 1980 1990

FIG. 1. Evolution of Laser Peak Power

CD c CD

10-12

0.001 0.01 0.1 1 10 100 103 104 IQ5 106 107 108 repetition rate (Hz) >

FIG. 2. Average Power of Femtosecond Lasers

-124- 1020

10i5 _

T3 s o

10'0

1960 1970 1980 1990

FIG. 3. History of Laser-Matter Interaction

1*• oscillator stretcher amplifier compressor

FIG. 4. Chirped Pulse Amplification

— 125 —