TUTORIAL Chirped-Pulse Amplification Ultrahigh peak power production from compact short-pulse laser systems Introduction of chirped-pulse ampli- It turns out that the hint to a solution THE AUTHOR fication (CPA) enabled the latest revolu- of this problem can be found as early as tion in production of high peak powers the time of the demonstration of the first from lasers through amplification of very laser, but the idea has been initially pro- IGOR JOVANOVIC posed to overcome a different issue – the short (femtosecond) laser pulses to pulse Igor Jovanovic is an power limitations of radars [1]. In 1985 it energies previously available only from Associate Professor of was realized by the group at the University long-pulse lasers. CPA has rapidly bridged Nuclear Engineering at of Rochester led by Gérard Mourou that the gap from its initial modest demon- Penn State University. this technique, termed chirped-pulse am- strations to multi-terawatt and petawatt- He received his undergraduate degree plification (CPA) [2], can also be applied in scale systems in research facilities and from the University of Zagreb in 1997 the optical domain, with revolutionary con- universities, as well as numerous lower- and his Ph.D. from the University of sequences for laser science and technology California, Berkeley in 2001. He is one of power scientific and industrial applica- and its applications. The idea of CPA is in- the pioneers of the technique of optical tions. deed simple and beautiful: given the limita- parametric chirped-pulse amplification. tions encountered by ultrashort laser pulses After receiving his Ph.D. he joined the With the advent of a mode-locked laser, it while propagating through the laser ampli- Lawrence Livermore National Labora- was quickly realized that the future of scal- fier, let us manipulate the ultrashort pulse tory as a staff physicist, and in 2007 he ing lasers to even higher peak powers will in a controllable and reversible fashion such became an Assistant Professor of Nuclear require solutions beyond the most obvious that the laser amplifier never encounters a Engineering at Purdue University. Since direct amplification of an ultrashort laser short, high power pulse, and only the laser 2010 he directs the Intense Laser Labo- pulse in a laser amplifier. Femtosecond la- system components compatible with such ratory at Penn State University and leads ser pulses pack very high peak powers and high peak powers can be exposed to it. It a research group studying high-power associated electric fields even in modest- is well known that the intensity limits as- laser technology, laser-matter interac- energy pulses, resulting in their proneness sociated with optical damage for standard tions, and advanced radiation detection to inducing beam distortions and optical reflective and diffractive optical compo- science and technology. damage in the optical components through nents are orders of magnitude greater than which they propagate, such as self-focusing those associated with nonlinear effects and ●● and its temporal counterpart, self-phase damage in transmission through the laser modulation. An important condition for ef- amplifier. If a convenient way exists to con- Igor Jovanovic ficient extraction of stored energy from the trollably stretch, amplify, and subsequently Department of Mechanical and Nuclear Engineering laser amplifier is the operation at fluences recompress the ultrashort laser pulse close The Pennsylvania State University (pulse energies per unit area) close to the to its original pulse duration, it is possible to 233 Reber Building characteristic saturation fluence for the giv- circumvent the damage limitations of laser University Park, PA 16802, USA en laser amplifier material, a demand easily amplifiers and scale the short-pulse lasers to Phone: +1 814 867 4329 Fax: +1 814 863 6382 met in nanosecond amplifiers without the extreme peak powers. E-mail: [email protected] risk of inducing optical damage. Operation Website: www.mne.psu.edu/IJ at those same fluences with pulse durations Components of a laser CPA system up to a million times shorter is not possible without inducing significant self-focusing Reversible manipulation of the temporal by mode-locked lasers. The stretcher oper- and self-phase modulation, resulting in cat- characteristics of ultrashort laser pulses is ates by introducing large, well-character- astrophic optical damage. One method for possible if a conjugate set of optical de- ized dispersion, i.e. time delay of different overcoming such energy density (beam in- vices termed stretcher (or expander) and spectral components of the ultrashort pulse tensity) limitations would be the increase of compressor is used (see Figure 1). An inher- to produce a long, chirped optical pulse. the laser amplifier aperture commensurate ent property of ultrashort pulses that can The compressor operates on the same prin- with the increase of peak power, but the be used for their controlled stretching and ciple, with dispersion that closely matches required apertures would be impractically recompression is their broad spectral band- that produced by the stretcher, but oppo- large, laser systems would be highly ineffi- width. Short pulse duration can be achieved site in sign. Thus the ultrashort pulse ini- cient as they would not operate near satura- only if coherent light consists of a range of tially generated in a mode-locked laser first tion fluence, and would suffer from prob- wavelengths which are carefully and orderly undergoes stretching in the pulse stretcher lems such as transverse parasitic lasing. synthesized – such as the pulses produced (to ps-ns durations), amplification to high 30 Optik & Photonik December 2010 No. 4 © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim TUTORIAL energies in the laser amplifier, followed by compression close to its original pulse dura- tion in the pulse compressor. Several important stretcher and com- pressor implementations have emerged in the optics community, and they all have in common the utilization of dispersive prop- erties of materials or specially constructed devices. Relatively large (~104), high- fidelity stretching ratios can be achieved in optical arrangements based on diffrac- tion gratings, which dominate the world of ultrahigh peak power CPA systems. Dif- fraction gratings are presently available in apertures on the order of 1 m2, allowing direct recompression of pulses to powers exceeding 1 PW [3]. Moderate stretching and compression ratios can be achieved with prism-based devices, optical fibers and Bragg gratings implemented either in bulk material or fibers. Finally, relatively small stretching and compression ratios FIG. 1: Conceptual schematic of a chirped-pulse amplification-based laser system. can be achieved by using the natural dis- persion of bulk material or specially de- signed reflective dielectric stacks (chirped pulse shaping devices that can selectively National Ignition Facility at Lawrence Liver- mirrors). All of those methods are in use delay or attenuate individual spectral com- more National Laboratory in California. Of today, with the choice among them usu- ponents of a laser pulse and utilize spatial course, CPA pulses carry much less energy ally determined by the requirements for light modulators or acousto-optic pro- and thus their advantages will be promi- the final pulse energy and pulse duration grammable dispersive filters. nent only in applications in which high at the CPA system output. The pulse recompressed at the output peak powers and focal intensities are de- The CPA process can result in both am- of the CPA system exhibit peak powers that sired. plitude and phase distortions, preventing usually greatly exceed that producible in the optimal recompression of the laser long-pulse lasers. For example, a standard pulse and achieving a high peak power. Applications of CPA modern tabletop CPA laser system, such as Stretching and recompression over many the one shown in Figure 2, found in uni- Numerous applications have emerged for orders of magnitude in pulse duration is versity laboratories and small research in- CPA in the past two decades, and they a process that requires high accuracies in stitutes exceeds the 400 TW peak power could be broadly classified into scientific, the design and manufacturing of optical available from the largest nanosecond laser industrial, medical, energy, and military/ components and in the construction of the system ever built – the recently completed security. Scientific applications were iden- stretcher and compressor. Additional spati- otemporal distortions are frequently intro- duced in stretchers and compressors. For this reason, and to minimize system size, stretchers and compressors are usually de- signed to provide a minimum stretching ratio compatible with achieving the re- quired pulse energy at the onset of signifi- cant pulse distortions in the laser amplifier. Such distortions lead to phase and spectral modification of the ultrashort pulse and can cause difficulties with pulse recompres- sion. Additional distortions can occur in the amplification process due to the amplifier material nonuniformities, nonlinearities, and thermal distortions. Significant effort has been expanded in the short-pulse laser community to achieve effective dispersion compensation, with much success. At the same time, the possibility of flexible disper- sion control has emerged and has enabled numerous practical applications. Active dispersion control in CPA systems can be FIG. 2: High-energy