**1. Introduction**

Ultra-high peak power laser pulses are the only way today to reach extremely high concentration of the energy in small volume after focusing with intensity 10<sup>22</sup> W/cm<sup>2</sup> and above. This possibility makes the pulse laser systems as remarkable instruments for scientific research, if the high average power (high repetition rate) could be simultaneously reachable for industrial applications. The pulsed lasers require a much lower energy to get ultra-high powers comparing to continuous wave (CW)-regime. The situation became very promising when Q-switching [1] and mode-locking [2] regimes of laser operation were invented (**Figure 1**). This permitted to get a short enough laser pulse: nanosecond-level (10−9 s) at first and then consequently

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

Nevertheless, the next multiorder step in increasing laser powers was done more than 20 years after the Q-switch invention, when chirped pulse amplification (CPA) technology was suggested in the mid-1980s and was related with significant reduction of the pulse duration for increasing peak power [7, 8] (**Figure 1**). The main goal of the CPA method is the reduction of optical elements and amplifier crystals size. If one will try to amplify the short laser pulse directly, undesirable nonlinear distortions of the pulse in the gain media and other optical elements will be reached very soon. This leads to self-focusing of the laser beam, and thus a growing intensity, which damages the optical elements. Therefore, one has to increase the beam diameter and the apertures of the amplifiers and others optics to avoid these problems. Nevertheless, the attempt to keep the intensity under the self-focusing threshold requires enormous apertures of the optical elements (e.g., several meters in diameter for petawatt

**Figure 2.** (a) NIF laser and target area building; (b) laser bay 2, one of NIF's two laser bays (from https://www.theguardian.com/ environment/gallery/2009/may/28/national-ignition-facility-fusion-energy#img-7. http://archive.boston.com/bigpicture/2010/10/

New Generation of Ultra-High Peak and Average Power Laser Systems

http://dx.doi.org/10.5772/intechopen.70720

65

Progressive idea of the CPA was adopted for the optical diapason and it gave an alternative way to increase pulse energy. Instead of enlarging the transverse beam size, increasing the pulse duration was suggested for further amplification, which was then followed by the pulse compression. Historically, first time, this idea was applied in the microwave diapason for radar technology in the 1960s. Soon, the stretching and compression of the laser pulse was suggested using different methods, among which were the use of compression in dispersive media [9], multilayer film interferometers [10] and, perhaps the most productive and widely explored today, the stretching and compression with a pair of diffraction gratings [11]. The first time when the pulse, chirped in a dispersive media (1.4 km of optical single mode fiber), was amplified and further compressed by a pair of diffraction gratings was done in 1985 [7, 8]. However, the distortion of the chirped pulse, introduced by the higher orders dispersion of the optical fiber made difficult to compensate by the grating compressor, and thus the output pulse dura-

Most successful realization of this idea (Martínez-type stretcher setup containing a telescope between the gratings [12]) is presented in **Figure 3**, where the stretcher and compressor were both built on the base of diffraction gratings. The main clue can be explained very easily. Short pulse should possess a wide spectrum due to the Heisenberg uncertainty principle or other words, consist from the many harmonics. We can redirect different harmonics (parts) of this spectrum along different optical paths via diffraction, and thus get a different group

output power).

the\_national\_ignition\_facility.html).

tion was limited of a few picoseconds.

**Figure 1.** Intensity evolution since the first laser demonstration in 1960, with the different regimes of optics and electrodynamics. Black boxes (joules) indicate typical laser energies. Blue boxes (electron volts) indicate typical laser-plasma accelerated particle energies. (Courtesy G. Mourou.) http://www.spie.org/newsroom/4221-exploring-fundamental-physicsat-the-highest-intensity-laser-frontier?highlight=x2404&ArticleID=x88664&SSO=1 (QCD, quantum chromodynamics; QED, quantum electrodynamics; E, electric field; e, electron charge; λ<sup>c</sup> , compton wavelength; m<sup>0</sup> , electron mass; c, speed of light. Ep, proton energy; mp, proton mass. E<sup>e</sup> , electron energy; C<sup>3</sup> , cascaded conversion compression; ELI, extreme light infrastructure; ILE, institut de la lumiere extreme; CUOS, Center for Ultrafast Optical Science University of Michigan; HHG, high harmonic generation; CPA, chirped pulse amplification).

picosecond (10−12 s) and femtosecond (10−15 s) pulse durations using mode-locking oscillators. The lasers based on these technologies, and applying also master oscillator power amplifier (MOPA) configuration, achieved MW and GW-levels of power or 1011–1014 W/cm<sup>2</sup> after focusing. The ability to reach these high levels of power allowed to look forward to the application of these systems in other new and very promising areas, such as charged particles acceleration [3] and inertial confinement of the fusion nuclear reaction (ICF) [4]. The consequent evolution of these laser systems resulted in the tremendous National Ignition Facility (NIF) laser in the United States [5], and the Megajoule in Europe [6].

The prior, with its output energy in 192 laser beams about 4 MJ, is delivering to the target a power of up to 500 Terawatt. The laser occupies the area as large as three football fields (see **Figure 2a**) and consists of two laser bays, one of which is presented in **Figure 2b**.

So, a large area is required for these multichannel facilities due to damage threshold of the optical elements which is normally limited at 10 J/cm<sup>2</sup> for ns-scale of the pulse duration. Further increasing of the power is associated with unacceptable gigantism of the laser systems.

**Figure 2.** (a) NIF laser and target area building; (b) laser bay 2, one of NIF's two laser bays (from https://www.theguardian.com/ environment/gallery/2009/may/28/national-ignition-facility-fusion-energy#img-7. http://archive.boston.com/bigpicture/2010/10/ the\_national\_ignition\_facility.html).

Nevertheless, the next multiorder step in increasing laser powers was done more than 20 years after the Q-switch invention, when chirped pulse amplification (CPA) technology was suggested in the mid-1980s and was related with significant reduction of the pulse duration for increasing peak power [7, 8] (**Figure 1**). The main goal of the CPA method is the reduction of optical elements and amplifier crystals size. If one will try to amplify the short laser pulse directly, undesirable nonlinear distortions of the pulse in the gain media and other optical elements will be reached very soon. This leads to self-focusing of the laser beam, and thus a growing intensity, which damages the optical elements. Therefore, one has to increase the beam diameter and the apertures of the amplifiers and others optics to avoid these problems. Nevertheless, the attempt to keep the intensity under the self-focusing threshold requires enormous apertures of the optical elements (e.g., several meters in diameter for petawatt output power).

Progressive idea of the CPA was adopted for the optical diapason and it gave an alternative way to increase pulse energy. Instead of enlarging the transverse beam size, increasing the pulse duration was suggested for further amplification, which was then followed by the pulse compression. Historically, first time, this idea was applied in the microwave diapason for radar technology in the 1960s. Soon, the stretching and compression of the laser pulse was suggested using different methods, among which were the use of compression in dispersive media [9], multilayer film interferometers [10] and, perhaps the most productive and widely explored today, the stretching and compression with a pair of diffraction gratings [11]. The first time when the pulse, chirped in a dispersive media (1.4 km of optical single mode fiber), was amplified and further compressed by a pair of diffraction gratings was done in 1985 [7, 8]. However, the distortion of the chirped pulse, introduced by the higher orders dispersion of the optical fiber made difficult to compensate by the grating compressor, and thus the output pulse duration was limited of a few picoseconds.

picosecond (10−12 s) and femtosecond (10−15 s) pulse durations using mode-locking oscillators. The lasers based on these technologies, and applying also master oscillator power amplifier

infrastructure; ILE, institut de la lumiere extreme; CUOS, Center for Ultrafast Optical Science University of Michigan; HHG,

**Figure 1.** Intensity evolution since the first laser demonstration in 1960, with the different regimes of optics and electrodynamics. Black boxes (joules) indicate typical laser energies. Blue boxes (electron volts) indicate typical laser-plasma accelerated particle energies. (Courtesy G. Mourou.) http://www.spie.org/newsroom/4221-exploring-fundamental-physicsat-the-highest-intensity-laser-frontier?highlight=x2404&ArticleID=x88664&SSO=1 (QCD, quantum chromodynamics;

, electron energy; C<sup>3</sup>

ing. The ability to reach these high levels of power allowed to look forward to the application of these systems in other new and very promising areas, such as charged particles acceleration [3] and inertial confinement of the fusion nuclear reaction (ICF) [4]. The consequent evolution of these laser systems resulted in the tremendous National Ignition Facility (NIF) laser in the

The prior, with its output energy in 192 laser beams about 4 MJ, is delivering to the target a power of up to 500 Terawatt. The laser occupies the area as large as three football fields (see

So, a large area is required for these multichannel facilities due to damage threshold of the opti-

increasing of the power is associated with unacceptable gigantism of the laser systems.

after focus-

, electron mass; c, speed

for ns-scale of the pulse duration. Further

, compton wavelength; m<sup>0</sup>

, cascaded conversion compression; ELI, extreme light

(MOPA) configuration, achieved MW and GW-levels of power or 1011–1014 W/cm<sup>2</sup>

**Figure 2a**) and consists of two laser bays, one of which is presented in **Figure 2b**.

United States [5], and the Megajoule in Europe [6].

QED, quantum electrodynamics; E, electric field; e, electron charge; λ<sup>c</sup>

high harmonic generation; CPA, chirped pulse amplification).

of light. Ep, proton energy; mp, proton mass. E<sup>e</sup>

64 High Power Laser Systems

cal elements which is normally limited at 10 J/cm<sup>2</sup>

Most successful realization of this idea (Martínez-type stretcher setup containing a telescope between the gratings [12]) is presented in **Figure 3**, where the stretcher and compressor were both built on the base of diffraction gratings. The main clue can be explained very easily. Short pulse should possess a wide spectrum due to the Heisenberg uncertainty principle or other words, consist from the many harmonics. We can redirect different harmonics (parts) of this spectrum along different optical paths via diffraction, and thus get a different group

**1.1. Limitation on the output energy**

space and time with a ps accuracy; and so on [15].

As mentioned above, CPA laser systems have reached sub-petawatt output powers with the energy as low as several tens of Joules [16]. However, for that new generation of the ultra-high power lasers, the modest energy will no longer be enough, and the kJ-level has to be reached setting the next milestone at tens or even hundreds of petawatt [17, 18]. The modern ability to stretch pulse duration is restricted at the few orders by the existing grating technology, so further energy increase requires once again the enlargement of amplifier apertures due to the damage threshold. There are two candidates for the final amplifier of the ultra-high CPA systems: the optical parametrical amplifiers [15] and the laser amplifiers. Both of them possess their own advantages and shortcomings. OPA has several attractive properties as the gain bandwidths large enough to support light pulses as shorter as 10 fs; low amplifier heating due to "Optical cooling," which leads to the potential of a high-repetition-rate; large high-quality nonlinear amplifier crystals; high pulse contrast due to the absence of amplified spontaneous emission (ASE) outside the pump pulse duration. At the same time, OPA possesses low efficiency (below 20%); there are severe requirements on the pump beam quality, such as limitation on the spatial fluctuations in amplitude; short pump pulses (below 1 ns), in order to match with the stretched signal pulses for higher efficiency and their precise matching in

New Generation of Ultra-High Peak and Average Power Laser Systems

http://dx.doi.org/10.5772/intechopen.70720

67

The laser amplifiers are free of most of these restrictions, but possess problems of their own, which include severe losses due to th disk shape of the crystals and very high transverse gain and so the high transverse ASE (TASE), and possible parasitic generation (TPG). TASE as well as TPG leads to significant depletion of the inverted population, and thus the stored energy. The ASE from the pumped Ti:Sa crystal before and after the threshold of the parasitic generation is demonstrated in **Figure 4** [16]. The absence of ASE from the significant part of crystal demonstrates the reduction of the population inversion and so loses the stored energy. Because of this, enlarging the aperture of the amplifier fails to be an unlimited means of obtaining more output energy under the constraints imposed by the damage threshold limit. Therefore, in this case, the main limitation that arises on the path toward ultra-high output power and intensity is the restriction on the pumping and extraction energy imposed by

**Figure 4.** ASE from the pumped Ti:Sapphire crystal before and after threshold of parasitic generation [16].

**Figure 3.** Simplified layout of the CPA laser system (from http://www.llnl.gov/str/Petawatt.html).

delay for each, finally guiding them consequently to the same direction and expanding such a way to the pulse duration. If in the stretcher, harmonics with longer wavelengths were directed along the shorter optical paths (positive or normal dispersion), then the compressor has to accomplish the reverse (negative or anomalous dispersion). Since the diffraction gratings are used both in the stretcher and the compressor, the functions of stretching and compression will be close to each other and the higher-order dispersion can be compensated up to 4th order.

Simplified layout of the classical CPA laser system is presented in **Figure 3**. Here the short pulse from the mode-locked oscillator passing through the stretcher is expanded usually by 4–5 orders. Further, the pulse with low intensity is amplified in the chain of amplifiers by several orders of magnitude, and is then compressed back to the short duration.

Several different laser active medias were used for oscillators and amplifiers in the CPA systems, such as dye, Nd:YAG, Nd:Glass, and so on. The most preferable from them was Ti:Sapphire (Ti:Sa) crystal due to its very large bandwidth emission spectra (FWHM ~ 200 nm), very high thermal conductivity and mechanical hardness. This setup was able to achieve pulse durations as short as below 10 femtosecond, if the Ti:Sa oscillators [13] with a few fs mode-locked pulse and amplifiers are adopted. Besides the laser amplifiers [14], optical parametrical amplifier (OPA) has been presented in CPA schemas [15].

Exploiting this technology, researchers developed laser systems with 100 s terawatt level output power and 1019 W/cm<sup>2</sup> intensity (see **Figure 1**) but, as any other technology, CPA has some boundaries on the continued improvement of its parameters, the foremost of which is the limitation on the output energy.
