Chirped-Pulse Amplification Ultrahigh Peak Power Production from Compact Short-Pulse Laser Systems

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

energies in the laser amplifier, followed by compression close to its original pulse duration in the pulse compressor. Several important stretcher and compressor implementations have emerged in the optics community, and they all have in common the utilization of dispersive properties of materials or specially constructed devices. Relatively large (~104), high- fidelity stretching ratios can be achieved in optical arrangements based on diffraction 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 dispersion 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 required pulse energy at the onset of significant pulse distortions in the laser amplifier. Such distortions lead to phase and spectral modification of the ultrashort pulse and can cause difficulties with pulse recompression. 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 dispersion control has emerged and has enabled numerous practical applications. Active dispersion control in CPA systems can be FIG. 2: High-energy multipass cryogenically cooled Ti:sapphire amplifier. achieved effectively by using specialized (Credit: Amplitude Technologies)

laser facilities in Europe through the Ex- treme Light Infrastructure (ELI) project will require continuous and rapid advances in CPA technology. Important challenges and limitations for this technique include the compressible energy, pulse duration, prepulse contrast, and average power. For example, the ultimate limit in CPA pulse energy is presently determined by the size and damage threshold of the final compressor component (usually a diffraction grating), which is exposed to the high-energy recompressed pulse. FIG. 3: Conceptual schematic of an optical parametric chirped-pulse amplification system. Numerous advances have been made to increase the available grating sizes, optical tified first, and are to this day driving the pulses can interact with materials very dif- damage thresholds, and to coherently tile advances in CPA technology. Some of the ferently from long pulses, enabling high- gratings into larger apertures. The output most striking examples include precision removal of very small amounts of pulse duration is limited by the bandwidth • the ultrafast pump-probe experiments for materials without melting, a property very of the available laser gain media used, chemistry, biology, and material science, useful in materials processing. Such high- with Ti:sapphire, Yb:glass and Nd:glass be- • studies of the dynamic properties of ma- precision material removal is also very in- ing the most popular choices today. With terials, teresting in medical applications, where the advent of ultrahigh peak powers and • production and studies of hot dense mat- minimal damage to surrounding tissue can peak intensities it became apparent that ter resembling extreme processes in as- result when ultrashort pulses are used in the temporal fidelity of the pulse, i.e. the trophysics (high-energy density physics), surgical procedures. A particularly interest- ratio of the power contained in the main • generation of even shorter (attosecond) ing application of CPA is in the develop- pulse to the power contained in the light laser pulses for studying ultrafast proc- ment of future energy sources based on arriving at earlier times can be as critical as esses, and inertial confinement fusion. Experimental other parameters. Prepulse contrast ratios • pushing the “intensity frontier”, where evidence exists that ultrahigh intensity la- required in short pulse experiments today previously unexplored high-field physics sers based on CPA can significantly relax the frequently exceed 1010, the level attainable is studied at focused intensities that to- requirements on the laser drivers and tar- only recently in selected CPA systems. Fi- day already exceed 1022 W/cm2. gets in fusion facilities. In the military arena nally, industrial applications and especially the use of ultrashort pulses is being consid- laser-driven particle accelerators will require Industrial applications have been motivated ered for efficient production of long, con- higher repetition rates. This issue is present- primarily by the notion that ultrafast laser ductive plasma channels in the air (plasma ly being addressed by innovative laser de- filaments), and high-energy density physics signs such as fiber lasers, by development The University is being used to validate the performance of of new materials, and by advances in diode nuclear weapons. pumping technology. Pennsylvania State University One of the most exciting discoveries ap- University Park, PA, USA plicable to all of the areas discussed above Optical parametric chirped-pulse is that CPA lasers can serve to produce The Pennsylvania State University (popu- ultrashort, ultrabright beams of ionizing amplification (OPCPA) larly known as Penn State) is one of larg- radiation from relatively compact, inex- In the last decade we witnessed a rapid est public research universities in the pensive systems. Since its initial demonstra- development of a new implementation of United States, spanning 24 campuses tion, CPA has been used to produce X-rays, CPA which does not utilize direct laser am- and a virtual World Campus offering gamma-rays, terahertz radiation, as well as plification – termed the optical parametric distance education. The student popu- energetic electron, proton, and heavier ion chirped-pulse amplification (OPCPA) [4]. lation of Penn State is close to 90,000, beams. The recent rapid advances in the Such systems are similar to CPA systems, with half of the students located at the technique of laser wakefield acceleration of but utilize a laser to pump a nonlinear me- flagship campus at University Park. Penn electrons are very promising for replacing dium (optical nonlinear crystal such as be- State is ranked among the top 15 public expensive and large accelerator facilities. ta-barium borate, lithium triborate, and po- universities in the United States and has This is especially useful in frontier scientific tassium dihydrogen phosphate). Through a one of the largest federal and industry- research, medical therapy and diagnostics, nonlinear difference-frequency generation funded research programs, with a partic- and security. parametric process it is possible to efficient- ular strength in science and engineering. ly transfer energy from an energetic nano- Optics and laser research is conducted in second pump pulse to a stretched femto- CPA limitations the Colleges of Engineering and Science, second pulse (see Figure 3). It is important but also in the university-affiliated Ap- Development of CPA has already encoun- to note the difference in the motivation for plied Physics Laboratory and the Electro- tered several important challenges and limi- ultrashort pulse stretching here compared Optics Center. tations, which are the subject of vigorous to conventional CPA: OPCPA is an instanta- international research and development neous process in which comparable pulse www.psu.edu activities. For example, the unprecedented durations are needed for the stretched sig- development of intense, ultrahigh power nal and the pump – typically on the order

of nanoseconds in high-energy applica- er through reduced pulse durations has References tions. OPCPA systems exhibit many attrac- revolutionized the field of lasers. We are tive characteristics, such as high gain, very still witnessing the wave of innovations in [1] G. E. Cook: “Pulse Compression-Key to More Effi- broad spectral bandwidth that only weakly many areas of science and technology that cient Radar Transmission”, IEEE Proc. IRE 48, 310 depends on gain, spectral tunability, and resulted from this important insight. CPA is (1960) negligible thermal load. Those charac- presently an indispensible technique in sci- [2] D. Strickland, G. Mourou: “Compression of am- teristics led to the widespread use in the entific research, but also in industrial and plified chirped optical pulses”, Opt. Commun. preamplifier stage for high-energy short- medical applications. Furthermore, it is 56, 219 (1985) pulse lasers [5], and in schemes that seek to making inroads into even more areas tra- [3] M. D. Perry et al: “Petawatt laser pulses”, Opt. produce higher peak powers through even ditionally dominated by a different technol- Lett. 24, 160-162 (1999) shorter pulses than those available from CPA ogy, such as particle accelerators. There is [4] A. Dubietis, G. Jonusauskas, A. Piskarskas: “Power- systems [6]. Another attractive application no doubt that we will continue to experi- ful femtosecond pulse generation by chirped and has been the prepulse contrast enhance- ence an evolutionary development of CPA stretched pulse parametric amplification in BBO ment, where it has been demonstrated that technology, with occasional breakthroughs crystal”, Opt. Commun. 88, 437–440 (1992) the OPCPA performance greatly exceeds such as OPCPA that have a high probability [5] I. Jovanovic et al.: “Optical parametric chirped that of comparable CPA systems based on to support the de facto equivalent of the pulse amplifier as an alternative to Ti:sapphire direct laser amplification. Moore’s Law for laser power and the scope regenerative amplifiers”, Appl. Opt. 41, 2923 and importance of laser applications. (2002) [6] C. Hernandez-Gomez et al.: “The Vulcan 10 PW Conclusion Project”, J. of Physics: Conf. Series 244, 032006 The advent of CPA in the optical domain (2010) and its application to boost the peak pow-