6. Conclusions

After stretching to ~1 ns duration, laser pulses will be amplified up to the level of few 100 J in the all Ti:sapphire second CPA system (Figure 10). Ti:sapphire amplifiers AMP 1 are pumped by frequency-doubled Nd:YAG lasers at 10 Hz repetition rate. More than 4 J energy of the amplified chirped pulses can be obtained. By temporal compression of these pulses, 100 TW beams are generated in each arm of the HPLS. The next amplifier, AMP 2, is pumped by Nd: YAG lasers at 1 Hz repetition rate to get ~36 J pulse energy required for the generation of 1-PW temporally compressed laser pulses. In the last amplification stages, AMP 3.1 and AMP 3.2, Ti: sapphire crystals are pumped by 100-J energy frequency-doubled Nd:glass lasers (Atlas 100,

To attain the 10-PW peak power with as low as possible pulse energy, a large spectral bandwidth of laser pulses must be preserved throughout all amplification process. To compensate for redshifting and gain narrowing effects in the high-energy Ti:sapphire amplifiers, the output spectrum will be managed using reflective filters for spectrum shaping at the input of AMP 1, AMP 2, and AMP 3 amplifiers, similar to the solution proposed for the 10-PW Apollon laser in a previously published work [7]. Final spectral bandwidth as broad as 60 nm with the central wavelength of ~815 nm is expected, compared to ~35 nm bandwidth at ~845 nm central wavelength, calculated without spectrum control. The improved spectral bandwidth theoretically allows the generation of recompressed pulses as short as 15 fs. To secure the 10-PW peak power of the HPLS in case of 22- to 23 fs duration of the temporally recompressed pulses, more than 300 J energy of amplified chirped pulses could be obtained by full energy pumping of the last amplifier stage, AMP 3.2. Considering

To reach as high as 1023 W/cm<sup>2</sup> focused beam intensity, the 10-PW laser beam must be tightly focused in a few micrometers spot. Wavefront distortions, produced mainly by the thermal loading of Ti:sapphire amplifiers, give rise to focal intensity profile aberrations. They gather way by the enlargement of the focal spot size and the reducing of the energy content in the main spot. The associated Strehl ratio, which characterizes the peak intensity related to the ideal flat wavefront case, can decrease to values below 0.2 [10, 33]. Wavefront control and correction using adaptive optics are essential requirements for the laser beam focusing in an optimal and reproducible way. After high-energy amplifiers, deformable mirrors will be

Laser beam parameter Estimated value

Laser pulse peak power ≥ 10 PW Estimated pulse duration 15–25 fs Estimated pulse energy 150–250 J Repetition rate 1 pulse/min Intensity contrast ≥ 10<sup>12</sup>

, Ti:sapphire crystals with clear aperture diameter in the

Thales Optronique Company) at 1 pulse/min repetition rate [32].

range of 160–200 mm are necessary for the last amplifier stages.

Main specifications of the ELI-NP HPLS are summarized in the Table 1.

a safe laser fluence of 1–1.5 J/cm<sup>2</sup>

58 High Power Laser Systems

Table 1. Main specifications of ELI-NP HPLS.

Hybrid high-power femtosecond laser systems combine the advantages of ultra-broad bandwidth OPCPA in nonlinear crystals with the CPA technique in large size Ti:sapphire crystals. Ultra-broad gain bandwidths in the range of 150 nm can be obtained by noncollinear optical parametric chirped pulse amplification in nonlinear crystals, like BBO and DKDP, pumped by green lasers. A key feature of the hybrid amplification lasers consists in the adaptation of the phase-matching bandwidth of OPCPA nonlinear crystals to the gain bandwidth of the laseramplifying media, such as Ti:sapphire crystals and Nd:doped glasses. The ultra-broad phasematching bandwidth of BBO crystals and the gain bandwidth of Ti:sapphire laser crystals are spectrally overlapped. Many magnitude orders of amplification in hybrid femtosecond laser systems are obtained by OPCPA. Gain narrowing effect and ASE intensity pedestal are significantly attenuated compared to all Ti:sapphire amplifiers. It becomes easier to get high intensity contrast, large spectral bandwidth, and high-energy femtosecond laser pulses. Highpower laser pulses in the range of 10-fs pulse width can be generated by hybrid femtosecond laser amplifiers based on OPCPA in BBO crystals and CPA in Ti:sapphire crystals.

A 2 10 PW hybrid amplification femtosecond laser system is currently under construction at ELI-NP research facility. The Front-End is based on optically synchronized picosecond pulses OPCPA in BBO crystals. Picosecond stretched pulses of ~10 mJ energy, with spectral bandwidth broader than 70 nm, were obtained at the output of ELI-NP laser Front-End. After highenergy chirped pulse amplification in large aperture Ti:sapphire crystals and temporal recompression, 10-PW pulses of about 20 fs duration, with more than 1012 picosecond ASE intensity contrast, are expected at the output of ELI-NP high-power laser system.
