1. Introduction

High-power femtosecond laser systems were developed using chirped pulse amplification (CPA) technique [1]. PW-class Ti:sapphire laser systems have been demonstrated worldwide in the last years [2–6].

To attain a high peak pulse power, we need high pulse energy in a short pulse duration. In a laser amplifier system, the maximum acceptable laser fluence is restricted by the damage risks. The amplified laser pulse energy is limited by the size of currently existing optical components,

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© The Author(s). Licensee InTech. 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 eproduction in any medium, provided the original work is properly cited.

like Ti:sapphire crystals and diffraction gratings from optical compressors. To attain multi-PW peak pulse power in CPA systems based on available optical components, it is necessary to deliver the output laser energy in 10-fs range duration laser pulses. The recompressed amplified pulse duration is inversely proportional to the spectral bandwidth, which contains the phaselocked spectral components, which means flat-phase spectral bandwidth. The spectral band narrowing and the increase of the recompressed amplified pulse duration are the result of gain narrowing and redshifting in Ti:sapphire crystals [7]. A broad spectral bandwidth of amplified laser pulses throughout CPA laser systems can be preserved by special techniques, like optical cross-polarized wave (XPW) generation [8] and spectral filters for spectrum management [7]. Flat spectral phase over a large bandwidth can be obtained by the correction of high-order phase distortions using acousto-optic programmable dispersion filters (AOPDFs) [9].

borate (BBO) crystals pumped by green lasers and the gain bandwidth of Ti:sapphire crystals are practically overlapped. In this case, a large spectral gain bandwidth can be preserved over the whole hybrid amplification chain. High-power, high-intensity contrast recompressed femtosecond pulses can be obtained. OPCPA in BBO crystals up to mJ energy level in the Front-End, followed by CPA in large-aperture Ti:sapphire crystals up to 10/100 Joules, represents a

High-Power, High-Intensity Contrast Hybrid Femtosecond Laser Systems

http://dx.doi.org/10.5772/intechopen.70708

45

A couple of worldwide developed hybrid amplification high-power femtosecond laser systems are described. The hybrid amplification configuration has been considered as an appropriate solution for the 2 10 PW femtosecond laser system of the Extreme Light Infrastructure:

2. Chirped pulse amplification in broad spectral bandwidth laser media

gratings, up to few-hundred picoseconds or about one nanosecond pulse duration.

The principle of CPA in broad gain bandwidth laser media (e.g., Ti:sapphire crystals) is presented in Figure 1. Femtosecond laser pulses generated by a large spectral bandwidth oscillator are temporally stretched with dispersive optical elements, in most cases diffraction

The Ti:sapphire laser is a four-level system as depicted by a simplified energy level diagram in Figure 1. Ti:sapphire crystals are optically pumped by nanosecond green lasers. By absorption of pump laser photons, the Ti atoms are raised from the ground energy level E1 to the spectral band E4. The excited atoms are rapidly transferred by nonradiative transitions from the absorption band E4 to the upper laser energy level E3. The spontaneous fluorescence lifetime of Ti atoms on the upper laser level is about 3 μs. The Ti atoms are accumulated on the upper laser level giving rise to a population inversion between E2 and E3 laser levels. Under these conditions, an input laser pulse with photon energy quanta corresponding to the energy difference between the E3 upper level and the E2 lower level is amplified by stimulated laser transitions between E3 and E2 levels. The generated laser radiation is coherently added to the

Figure 1. Chirped pulse amplification (CPA) in Ti:sapphire laser crystals. Bν, amplified pulse frequency bandwidth; τp,

suitable solution for PW-class femtosecond laser systems.

Nuclear Physics (ELI-NP) facility.

input radiation.

temporally compressed pulse duration.

For many research applications, very high laser intensity in the focused beam is required. The capability to tightly focus the laser beam in a very small spot is one of the most important features of the high-power CPA laser systems. Tight focusing significantly depends on the quality of the amplified pulse beam wavefront. Thermal loading of Ti:sapphire crystals is one of the main reasons of wavefront distortions in CPA systems. The focused beam intensity related to the ideal case of an undisturbed flat wavefront, having the same intensity profile as the real beam, is given by the Strehl ratio (SR) [10]. By focusing 300-TW femtosecond laser beams in few-μm diameter spots, 2 1022 W/cm2 peak intensity has been obtained [11]. More than 10<sup>23</sup> W/cm<sup>2</sup> peak power is expected by tightly focusing 10-PW femtosecond laser pulses.

If a laser intensity of about 10<sup>11</sup> W/cm2 is attained on the target before the main laser pulse, the generated pre-plasma could disturb the experiment. High-intensity contrast becomes a crucial laser beam parameter for accessing high-field physics in various experimental targets. Some techniques for improving the intensity contrast of laser emission, such as saturable absorbers [12, 13] and XPW [14–16], were used inside high-power femtosecond CPA laser systems. Plasma mirrors, based on self-induced plasma shuttering, were proposed for improving intensity contrast after the temporal compression of amplified chirped laser pulses [17, 18]. Reaching an intensity contrast in the range of 1012 represents a challenging task for a multi-PW all Ti:sapphire CPA laser.

Optical parametric chirped pulse amplification (OPCPA) in nonlinear crystals provides large amplification spectral bandwidth and improves the intensity contrast of the amplified pulses outside the temporal window of the parametric amplification process [4, 19]. In case of OPCPA with high-energy laser pulses, an important technical problem consists in the generation of a single pump beam with simultaneously difficult-to-accomplish specifications: hundreds of Joules laser pulse energy, about one nanosecond pulse duration, spatial and temporal smooth and nearly flat intensity profile, very good stability from pulse to pulse, high repetition rate.

Hybrid femtosecond lasers combine OPCPA in nonlinear crystals at low-medium energy with CPA in large size Ti:sapphire crystals at high energy. A key feature of high-power 10-fs laser systems consists in the adaptation of the parametric amplification phase matching bandwidth of nonlinear crystals to the spectral gain bandwidth of laser amplifying Ti:sapphire crystals. The ultra-broad phase-matching spectral bandwidth near 800 nm wavelength of beta-barium borate (BBO) crystals pumped by green lasers and the gain bandwidth of Ti:sapphire crystals are practically overlapped. In this case, a large spectral gain bandwidth can be preserved over the whole hybrid amplification chain. High-power, high-intensity contrast recompressed femtosecond pulses can be obtained. OPCPA in BBO crystals up to mJ energy level in the Front-End, followed by CPA in large-aperture Ti:sapphire crystals up to 10/100 Joules, represents a suitable solution for PW-class femtosecond laser systems.

like Ti:sapphire crystals and diffraction gratings from optical compressors. To attain multi-PW peak pulse power in CPA systems based on available optical components, it is necessary to deliver the output laser energy in 10-fs range duration laser pulses. The recompressed amplified pulse duration is inversely proportional to the spectral bandwidth, which contains the phaselocked spectral components, which means flat-phase spectral bandwidth. The spectral band narrowing and the increase of the recompressed amplified pulse duration are the result of gain narrowing and redshifting in Ti:sapphire crystals [7]. A broad spectral bandwidth of amplified laser pulses throughout CPA laser systems can be preserved by special techniques, like optical cross-polarized wave (XPW) generation [8] and spectral filters for spectrum management [7]. Flat spectral phase over a large bandwidth can be obtained by the correction of high-order phase

For many research applications, very high laser intensity in the focused beam is required. The capability to tightly focus the laser beam in a very small spot is one of the most important features of the high-power CPA laser systems. Tight focusing significantly depends on the quality of the amplified pulse beam wavefront. Thermal loading of Ti:sapphire crystals is one of the main reasons of wavefront distortions in CPA systems. The focused beam intensity related to the ideal case of an undisturbed flat wavefront, having the same intensity profile as the real beam, is given by the Strehl ratio (SR) [10]. By focusing 300-TW femtosecond laser beams in few-μm diameter spots, 2 1022 W/cm2 peak intensity has been obtained [11]. More than 10<sup>23</sup> W/cm<sup>2</sup> peak power is expected by tightly focusing 10-PW femtosecond laser pulses. If a laser intensity of about 10<sup>11</sup> W/cm2 is attained on the target before the main laser pulse, the generated pre-plasma could disturb the experiment. High-intensity contrast becomes a crucial laser beam parameter for accessing high-field physics in various experimental targets. Some techniques for improving the intensity contrast of laser emission, such as saturable absorbers [12, 13] and XPW [14–16], were used inside high-power femtosecond CPA laser systems. Plasma mirrors, based on self-induced plasma shuttering, were proposed for improving intensity contrast after the temporal compression of amplified chirped laser pulses [17, 18]. Reaching an intensity contrast in the range of 1012 represents a challenging task for a multi-

Optical parametric chirped pulse amplification (OPCPA) in nonlinear crystals provides large amplification spectral bandwidth and improves the intensity contrast of the amplified pulses outside the temporal window of the parametric amplification process [4, 19]. In case of OPCPA with high-energy laser pulses, an important technical problem consists in the generation of a single pump beam with simultaneously difficult-to-accomplish specifications: hundreds of Joules laser pulse energy, about one nanosecond pulse duration, spatial and temporal smooth and nearly flat intensity profile, very good stability from pulse to pulse,

Hybrid femtosecond lasers combine OPCPA in nonlinear crystals at low-medium energy with CPA in large size Ti:sapphire crystals at high energy. A key feature of high-power 10-fs laser systems consists in the adaptation of the parametric amplification phase matching bandwidth of nonlinear crystals to the spectral gain bandwidth of laser amplifying Ti:sapphire crystals. The ultra-broad phase-matching spectral bandwidth near 800 nm wavelength of beta-barium

distortions using acousto-optic programmable dispersion filters (AOPDFs) [9].

PW all Ti:sapphire CPA laser.

44 High Power Laser Systems

high repetition rate.

A couple of worldwide developed hybrid amplification high-power femtosecond laser systems are described. The hybrid amplification configuration has been considered as an appropriate solution for the 2 10 PW femtosecond laser system of the Extreme Light Infrastructure: Nuclear Physics (ELI-NP) facility.
