5.2. High-power laser system of the ELI-NP research facility

High-power laser system (HPLS) of the ELI-NP laser facility, developed by Thales Optronique, consists in a two-arm 10 PW peak power femtosecond laser amplifier. ELI-NP HPLS combines the advantages of a high amplification factor FE, based on OPCPA at low energy level, with the high energy amplification in large-size Ti:sapphire crystals pumped by high-energy frequency-doubled Nd:YAG and Nd:glass lasers [29]. For each arm, two additional output beams of 100 TW pulse peak power at 10 Hz repetition rate and 1 PW at 1 Hz are available. The schematic drawing of the ELI-NP 2 10 PW laser system is shown in Figure 10.

FE is based on OPCPA with optically synchronized seed and pump pulses of 20 to 25 ps duration (Figure 11). Seed and pump pulses for OPCPA are created by amplifying two output pulses generated by an ultra-broad bandwidth Ti:sapphire femtosecond oscillator (Venteon Company).

Pump pulse is obtained by amplifying a pJ-energy pulse, generated at the edge of the fs oscillator spectral bandwidth, with the central wavelength of 1064 nm and ~10 nm spectral bandwidth. In the first diode-pumped Ytterbium-doped fiber amplifier, pulse energy is increased to the nJ range. After spectral filtering in a Fiber Bragg Grating, the spectral bandwidth of the pulse is reduced to less than 0.1 nm bandwidth and its energy decreases in the range of 10 pJ. After amplification in the second Ytterbium-doped fiber amplifier, near-

Fourier-transform-limited pulses of ~1 nJ energy and ~25 ps pulse duration are amplified in bulk Nd:YAG amplifiers and frequency-doubled in a Lithium Triborate (LBO) crystal to get

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The broadband seed pulses of nJ pulse energy at ~800 nm central wavelength are temporally stretched to 100 ps in a diffraction grating stretcher. An acousto-optic programmable dispersion filter (AOPDF), Dazzler type, is used to compensate for high-order phase distortions [9]. Stretched pulses are amplified to the mJ level in a Ti:sapphire regenerative amplifier and recompressed in the range of few 10 fs pulse duration. Femtosecond pulses amplified in the first CPA are intensity filtered and spectrally broadened by XPW generation in two barium fluoride (BaF2) crystals. The femtosecond pulses with improved intensity contrast are stretched to ~20 ps pulse duration in a bulk glass stretcher. Broad bandwidth picosecond pulses are amplified in a two-stage NOPCPA with BBO crystals. To preserve a high intensity contrast during the parametric amplification process, the parametric fluorescence is hampered by keeping a relatively low parametric gain, less than 100 amplification factor for each NOPCPA stage. Outside the temporal window of the parametric process, the intensity contrast

By combining XPW and NOPCPA, the intensity contrast of recompressed pulses after FE amplification was improved by at least six orders of magnitude. More than 1012 ASE intensity contrast has been estimated in the few 10 ps range before the main femtosecond pulse. Spectral bandwidth narrowing and redshifting effects, specific to all Ti:sapphire amplification, were drastically reduced in the FE. Picosecond pulses with more than 70 nm FWHM spectral bandwidth were obtained at the FE output [30, 31]. Amplified pulses of ~10 mJ energy, split into two equal-energy beams, represent the seed pulses for A and B HPLS Ti:sapphire ampli-

more than 80 mJ pump pulse energy at 532 nm wavelength and 10 Hz repetition rate.

Figure 11. ELI-NP HPLS front-end based on optically synchronized OPCPA with BBO crystals.

is improved by a numerical factor equal to the parametric gain.

fication arms.

Figure 10. Schematic drawing of the 2 10 PW ELI-NP femtosecond laser system.

High-Power, High-Intensity Contrast Hybrid Femtosecond Laser Systems http://dx.doi.org/10.5772/intechopen.70708 57

Figure 11. ELI-NP HPLS front-end based on optically synchronized OPCPA with BBO crystals.

solution was considered for the 2 10 PW laser system of the ELI-NP laser facility from

High-power laser system (HPLS) of the ELI-NP laser facility, developed by Thales Optronique, consists in a two-arm 10 PW peak power femtosecond laser amplifier. ELI-NP HPLS combines the advantages of a high amplification factor FE, based on OPCPA at low energy level, with the high energy amplification in large-size Ti:sapphire crystals pumped by high-energy frequency-doubled Nd:YAG and Nd:glass lasers [29]. For each arm, two additional output beams of 100 TW pulse peak power at 10 Hz repetition rate and 1 PW at 1 Hz are available. The

FE is based on OPCPA with optically synchronized seed and pump pulses of 20 to 25 ps duration (Figure 11). Seed and pump pulses for OPCPA are created by amplifying two output pulses generated by an ultra-broad bandwidth Ti:sapphire femtosecond oscillator (Venteon

Pump pulse is obtained by amplifying a pJ-energy pulse, generated at the edge of the fs oscillator spectral bandwidth, with the central wavelength of 1064 nm and ~10 nm spectral bandwidth. In the first diode-pumped Ytterbium-doped fiber amplifier, pulse energy is increased to the nJ range. After spectral filtering in a Fiber Bragg Grating, the spectral bandwidth of the pulse is reduced to less than 0.1 nm bandwidth and its energy decreases in the range of 10 pJ. After amplification in the second Ytterbium-doped fiber amplifier, near-

schematic drawing of the ELI-NP 2 10 PW laser system is shown in Figure 10.

5.2. High-power laser system of the ELI-NP research facility

Figure 10. Schematic drawing of the 2 10 PW ELI-NP femtosecond laser system.

Bucharest-Magurele [29].

56 High Power Laser Systems

Company).

Fourier-transform-limited pulses of ~1 nJ energy and ~25 ps pulse duration are amplified in bulk Nd:YAG amplifiers and frequency-doubled in a Lithium Triborate (LBO) crystal to get more than 80 mJ pump pulse energy at 532 nm wavelength and 10 Hz repetition rate.

The broadband seed pulses of nJ pulse energy at ~800 nm central wavelength are temporally stretched to 100 ps in a diffraction grating stretcher. An acousto-optic programmable dispersion filter (AOPDF), Dazzler type, is used to compensate for high-order phase distortions [9]. Stretched pulses are amplified to the mJ level in a Ti:sapphire regenerative amplifier and recompressed in the range of few 10 fs pulse duration. Femtosecond pulses amplified in the first CPA are intensity filtered and spectrally broadened by XPW generation in two barium fluoride (BaF2) crystals. The femtosecond pulses with improved intensity contrast are stretched to ~20 ps pulse duration in a bulk glass stretcher. Broad bandwidth picosecond pulses are amplified in a two-stage NOPCPA with BBO crystals. To preserve a high intensity contrast during the parametric amplification process, the parametric fluorescence is hampered by keeping a relatively low parametric gain, less than 100 amplification factor for each NOPCPA stage. Outside the temporal window of the parametric process, the intensity contrast is improved by a numerical factor equal to the parametric gain.

By combining XPW and NOPCPA, the intensity contrast of recompressed pulses after FE amplification was improved by at least six orders of magnitude. More than 1012 ASE intensity contrast has been estimated in the few 10 ps range before the main femtosecond pulse. Spectral bandwidth narrowing and redshifting effects, specific to all Ti:sapphire amplification, were drastically reduced in the FE. Picosecond pulses with more than 70 nm FWHM spectral bandwidth were obtained at the FE output [30, 31]. Amplified pulses of ~10 mJ energy, split into two equal-energy beams, represent the seed pulses for A and B HPLS Ti:sapphire amplification arms.

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

installed in each HPLS amplification arm. Output beam wavefronts with more than 0.8 Strehl

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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.

intensity contrast, are expected at the output of ELI-NP high-power laser system.

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

This book chapter is supported by the Extreme Light Infrastructure: Nuclear Physics (ELI-NP) Phase II, a project cofinanced by the Romanian Government and the European Union through the European Regional Development Fund - the Competitiveness Operational Program (1/7

Institute for Nuclear Physics and Engineering, Extreme Light Infrastructure – Nuclear Physics,

ratio and near-diffraction-limited focal spots are expected.

6. Conclusions

Acknowledgements

July 2016, COP, ID 1334).

Address all correspondence to: razvan.dabu@eli-np.ro

Author details

Magurele, Romania

Razvan Dabu

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 a safe laser fluence of 1–1.5 J/cm<sup>2</sup> , Ti:sapphire crystals with clear aperture diameter in the 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.

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


Table 1. Main specifications of ELI-NP HPLS.

installed in each HPLS amplification arm. Output beam wavefronts with more than 0.8 Strehl ratio and near-diffraction-limited focal spots are expected.
