5. High-power laser systems based on hybrid amplification

A schematic configuration of a high-power hybrid femtosecond laser system is shown in Figure 7. In the low-energy amplification section, usually called the Front-End (FE) of the laser system, femtosecond pulses generated by a laser oscillator are temporally stretched up to 10 ps – 100 ps. Stretched laser pulses are mainly amplified by OPCPA from the nJ energy level up to 10 mJ – 100 mJ energy range, corresponding to seven to eight orders of magnitude amplification. By high-energy CPA in broad gain bandwidth laser media, chirped laser pulses, stretched in the range of one nanosecond pulse duration, are amplified by three to four orders of magnitude up to 10 J – 100 J and then temporally compressed back to the femtosecond range.

OPCPA is considered an appropriate technique in the low energy amplification FE, where large enough nonlinear crystals and good-quality beam ps-ns pump lasers are available. Output FE laser pulses with large spectral bandwidths, recompressible with high intensity contrast, are further amplified in high-energy Ti:sapphire amplifier stages.

In high-power laser systems (HPLS) based exclusively on parametric amplification, the technical bottlenecks move toward very high-energy pump lasers. Pump energy in the kJ range, few nanosecond pulse duration, flat intensity profile, and stable temporal and spatial beam profiles are required for pumping final OPCPA stages.

In femtosecond laser systems, the maximum amplified pulse energy is restricted by the size of available amplifying media. The clear aperture of largest available Ti:sapphire crystals for CPA

Figure 7. Basic configuration of a hybrid chirped pulse amplification laser system.

is smaller than the aperture of some available OPCPA nonlinear crystals, like DKDP, for example. Nevertheless, there is an advantage of Ti:sapphire CPA: for optical pumping of large-aperture Ti:sapphire crystals, several green pump lasers can be used, with output pulse energy of 50–100 J, less restrictive requirements concerning pulse duration, and much higher repetition rate compared to a single-beam kJ pump laser necessary for pumping a high energy OPCPA stage.

Because most of the amplification in hybrid lasers is realized by OPCPA, gain narrowing and ASE effects are attenuated compared to all Ti:sapphire amplifiers. It becomes easier to get highintensity contrast, high-energy laser pulses, recompressible down to femtosecond pulse duration.

In a hybrid femtosecond pulse amplification system, based on both OPCPA and CPA, a key feature is the matching of the ultra-broad gain bandwidth of the nonlinear crystal to the amplification spectral band of Ti:sapphire laser crystals. In this case, stretched pulses amplified in the laser FE can be directly sent to the Ti:sapphire high-energy amplifiers.

The ultra-broad gain band of DKDP crystals is centered near 900 nm. OPCPA based on DKDP crystals can be used in hybrid femtosecond laser amplifiers. In this case, seed laser pulses must have a broad bandwidth adapted to the ultra-broad phase-matching spectral band of DKDP crystals. In DKDP-OPCPA laser systems equipped with Ti:sapphire broadband femtosecond oscillators, complicated experimental setups were realized to generate broadband laser pulses with the central wavelength shifted near 900 nm [25, 26]. BBO crystals, pumped by frequencydoubled Nd lasers, have a "lucky" ultra-broad phase-matching bandwidth in the range of 800 nm, practically overlapped to the gain bandwidth of Ti:sapphire laser crystals. Due to more than 100 nm phase-matching bandwidth, BBO crystals pumped by green lasers can support the amplification of stretched laser pulses recompressible at sub-10 fs pulse duration [22]. The available few centimeters clear aperture BBO crystals are large enough for OPCPA up to 100 mJ signal pulse energy. For this reason, BBO crystals are frequently used in the FEs of the PW-class hybrid amplification femtosecond laser systems.

Considering the currently available technical solutions, hybrid amplification represents a good choice for the development of petawatt-class femtosecond laser systems.

## 5.1. PW-class hybrid femtosecond laser systems

5. High-power laser systems based on hybrid amplification

Figure 6. Schematic description of broadband noncollinear OPCPA in nonlinear crystals.

52 High Power Laser Systems

contrast, are further amplified in high-energy Ti:sapphire amplifier stages.

Figure 7. Basic configuration of a hybrid chirped pulse amplification laser system.

are required for pumping final OPCPA stages.

A schematic configuration of a high-power hybrid femtosecond laser system is shown in Figure 7. In the low-energy amplification section, usually called the Front-End (FE) of the laser system, femtosecond pulses generated by a laser oscillator are temporally stretched up to 10 ps – 100 ps. Stretched laser pulses are mainly amplified by OPCPA from the nJ energy level up to 10 mJ – 100 mJ energy range, corresponding to seven to eight orders of magnitude amplification. By high-energy CPA in broad gain bandwidth laser media, chirped laser pulses, stretched in the range of one nanosecond pulse duration, are amplified by three to four orders of magnitude up to 10 J – 100 J and then temporally compressed back to the femtosecond range. OPCPA is considered an appropriate technique in the low energy amplification FE, where large enough nonlinear crystals and good-quality beam ps-ns pump lasers are available. Output FE laser pulses with large spectral bandwidths, recompressible with high intensity

In high-power laser systems (HPLS) based exclusively on parametric amplification, the technical bottlenecks move toward very high-energy pump lasers. Pump energy in the kJ range, few nanosecond pulse duration, flat intensity profile, and stable temporal and spatial beam profiles

In femtosecond laser systems, the maximum amplified pulse energy is restricted by the size of available amplifying media. The clear aperture of largest available Ti:sapphire crystals for CPA A couple of PW-class hybrid femtosecond laser systems are currently worldwide operated, while other 10-PW laser facilities are under development.

A high spatiotemporal quality PW-class laser system has been developed at Advanced Photon Research Center, Japan Atomic Energy Agency [3]. This laser system is based on a double CPA configuration (Figure 8). In the first CPA section, femtosecond laser pulses generated by a Ti: sapphire oscillator are stretched, pre-amplified in Ti:sapphire amplifiers, and temporally recompressed to get mJ-energy output pulses with sub-30 fs duration. To improve the intensity contrast, part of the ASE pedestal of these pulses is removed by a saturable absorber.

In the second CPA section, the intensity-filtered pulses, stretched up to ~1 ns pulse duration, are amplified by OPCPA. The conventional regenerative amplifier used in all Ti:sapphire

Figure 8. Schematic drawing of a hybrid PW-class laser amplifier based on low-energy OPCPA in BBO crystals and Ti: sapphire high-energy amplification [3].

pump pulses are electronically synchronized. Up to 250 J energy, stretched laser pulses are amplified in two flash-lamp pumped Nd:glass amplifiers, with shifted peak gain wavelengths of the Nd-doped glasses. The gain spectral bandwidth of mixed Nd:glass amplifiers is about 15 nm near 1 μm central wavelength. First amplifier stage consists in a 64-mm-diameter silicate rod amplifier. In the second amplifier stage, the laser pulse passes four times through two pairs of 315 mm aperture phosphate disk amplifiers. Finally 186 J, 168 fs compressed pulses, with

Figure 9. Schematic drawing of a PW laser system based on OPCPA in BBO and YCOB crystals and high-energy

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

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

55

A high-contrast 1.16 PW laser system was developed at Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, by combining low-energy femtosecond optical parametric amplification with high-energy Ti:sapphire amplification [4]. Sub-10 fs pulses, with 4 nJ pulse energy, generated by a Ti:sapphire oscillator are split into two beams. About 70% energy pulse was used as the seed for a Ti:sapphire CPA to obtain 5 mJ, 50 fs pulses, which are frequency-doubled by BBO crystals to generate second harmonic pulses for pumping the twostage broad bandwidth NOPA. The other 30% of the femtosecond oscillator pulse energy is used as the broadband seed, at 800 nm central wavelength, for the first NOPA. The optical synchronization of the signal and the pumping femtosecond pulses was accurately controlled by a Herriot telescope delay line. This way, large bandwidth femtosecond pulses are amplified to ~26 μJ energy without any temporal stretching in the two-stage NOPA with BBO crystals. The parametric amplification process which involves the signal and pump pulses occurs on few 10 fs timescale. The background noise beyond this time range cannot be amplified, and the amplified signal pulse contrast is improved by a factor equal to the parametric gain. In the second CPA stage, the clean signal pulses were stretched to about 600 ps. The medium energy Ti:sapphire amplifiers are pumped by 532 nm wavelength nanosecond Nd:YAG lasers running at 1 Hz repetition rate. The last high-energy amplifier consists in an 80-mm-diameter, 40-mm-thickness Ti:sapphire disk, pumped by a nanosecond Nd:glass laser, 120 J at 527 nm wavelength, 1 pulse/20 minutes repetition rate. After temporal compression, more than 32 J pulse energy at ~28 fs pulse duration, corresponding to a peak power up to 1.16 PW, with

In the frame of the French project Apollon, an optically synchronized OPCPA with picosecond pulses was proposed for the Front-End configuration of a 10 PW laser system [28]. A similar

estimated nanosecond range contrast better than 1012 were obtained.

amplification in mixed glasses at 1 μm spectral range [27].

enhanced intensity contrast ratio of 1010, has been obtained.

lasers is replaced by a two-stage OPCPA with BBO crystals, pumped by a frequency-doubled Nd:YAG nanosecond laser. Pump and seed pulses are electronically synchronized with a timing jitter of 0.5 ns. To avoid the parametric fluorescence, the OPCPA stages are operated with relatively high-energy seed pulses in a low gain mode. Seed pulses of ~2.5 μJ are amplified up to ~5 mJ, keeping each BBO stage to less than 100 amplification factor. Hereby, the intensity contrast of amplified pulses in the nanosecond time range is significantly improved, practically with the parametric amplification factor.

Laser pulses are amplified up to 3 J energy level in two Ti:sapphire stages pumped by 10 Hz repetition rate green (532 nm wavelength) Nd:YAG lasers. Final Ti:sapphire amplifier is pumped by a single-shot green (527 nm) nanosecond Nd:glass laser with ~60 J pulse energy. Pump laser beams with smooth homogenized spatial intensity profile are delivered to a largeaperture 80 mm diameter Ti:sapphire laser crystal. A near-homogenous flat-top intensity profile of the amplified pulse beam was obtained. After temporal compression, 20 J/38 fs pulses with more than 0.5 PW peak power and more than 1010 intensity contrast in subnanosecond temporal range were generated.

1.1 PW laser based on hybrid optical parametric chirped pulse amplification and mixed Nd: glass amplifiers (Figure 9) has been demonstrated at Texas Center of High Intensity Laser Science, Austin, USA [27].

Nano-Joule-energy seed pulses with 16 nm FWHM spectrum centered at 1058 nm are generated by a tunable Ti:sapphire laser oscillator. Hundred-femtosecond oscillator pulses are stretched to more than 1 ns pulse duration. Stretched pulses are amplified by approximate nine orders of magnitude in three OPCPA stages to attain ~1 J pulse energy: two stages with pairs of BBO crystals and the last one with a pair of yttrium calcium oxoborate (YCOB) crystals. OPCPA crystals are pumped by frequency-doubled Nd:YAG lasers at 532 nm. The parametric amplification takes place in a near-degeneracy type of interaction, where pump wavelength is about two times shorter than the signal wavelength, which assures a broad enough gain bandwidth to amplify ~30 nm broadband signal pulses. Nanosecond seed and

Figure 9. Schematic drawing of a PW laser system based on OPCPA in BBO and YCOB crystals and high-energy amplification in mixed glasses at 1 μm spectral range [27].

pump pulses are electronically synchronized. Up to 250 J energy, stretched laser pulses are amplified in two flash-lamp pumped Nd:glass amplifiers, with shifted peak gain wavelengths of the Nd-doped glasses. The gain spectral bandwidth of mixed Nd:glass amplifiers is about 15 nm near 1 μm central wavelength. First amplifier stage consists in a 64-mm-diameter silicate rod amplifier. In the second amplifier stage, the laser pulse passes four times through two pairs of 315 mm aperture phosphate disk amplifiers. Finally 186 J, 168 fs compressed pulses, with estimated nanosecond range contrast better than 1012 were obtained.

lasers is replaced by a two-stage OPCPA with BBO crystals, pumped by a frequency-doubled Nd:YAG nanosecond laser. Pump and seed pulses are electronically synchronized with a timing jitter of 0.5 ns. To avoid the parametric fluorescence, the OPCPA stages are operated with relatively high-energy seed pulses in a low gain mode. Seed pulses of ~2.5 μJ are amplified up to ~5 mJ, keeping each BBO stage to less than 100 amplification factor. Hereby, the intensity contrast of amplified pulses in the nanosecond time range is significantly improved,

Figure 8. Schematic drawing of a hybrid PW-class laser amplifier based on low-energy OPCPA in BBO crystals and Ti:

Laser pulses are amplified up to 3 J energy level in two Ti:sapphire stages pumped by 10 Hz repetition rate green (532 nm wavelength) Nd:YAG lasers. Final Ti:sapphire amplifier is pumped by a single-shot green (527 nm) nanosecond Nd:glass laser with ~60 J pulse energy. Pump laser beams with smooth homogenized spatial intensity profile are delivered to a largeaperture 80 mm diameter Ti:sapphire laser crystal. A near-homogenous flat-top intensity profile of the amplified pulse beam was obtained. After temporal compression, 20 J/38 fs pulses with more than 0.5 PW peak power and more than 1010 intensity contrast in sub-

1.1 PW laser based on hybrid optical parametric chirped pulse amplification and mixed Nd: glass amplifiers (Figure 9) has been demonstrated at Texas Center of High Intensity Laser

Nano-Joule-energy seed pulses with 16 nm FWHM spectrum centered at 1058 nm are generated by a tunable Ti:sapphire laser oscillator. Hundred-femtosecond oscillator pulses are stretched to more than 1 ns pulse duration. Stretched pulses are amplified by approximate nine orders of magnitude in three OPCPA stages to attain ~1 J pulse energy: two stages with pairs of BBO crystals and the last one with a pair of yttrium calcium oxoborate (YCOB) crystals. OPCPA crystals are pumped by frequency-doubled Nd:YAG lasers at 532 nm. The parametric amplification takes place in a near-degeneracy type of interaction, where pump wavelength is about two times shorter than the signal wavelength, which assures a broad enough gain bandwidth to amplify ~30 nm broadband signal pulses. Nanosecond seed and

practically with the parametric amplification factor.

nanosecond temporal range were generated.

Science, Austin, USA [27].

sapphire high-energy amplification [3].

54 High Power Laser Systems

A high-contrast 1.16 PW laser system was developed at Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, by combining low-energy femtosecond optical parametric amplification with high-energy Ti:sapphire amplification [4]. Sub-10 fs pulses, with 4 nJ pulse energy, generated by a Ti:sapphire oscillator are split into two beams. About 70% energy pulse was used as the seed for a Ti:sapphire CPA to obtain 5 mJ, 50 fs pulses, which are frequency-doubled by BBO crystals to generate second harmonic pulses for pumping the twostage broad bandwidth NOPA. The other 30% of the femtosecond oscillator pulse energy is used as the broadband seed, at 800 nm central wavelength, for the first NOPA. The optical synchronization of the signal and the pumping femtosecond pulses was accurately controlled by a Herriot telescope delay line. This way, large bandwidth femtosecond pulses are amplified to ~26 μJ energy without any temporal stretching in the two-stage NOPA with BBO crystals. The parametric amplification process which involves the signal and pump pulses occurs on few 10 fs timescale. The background noise beyond this time range cannot be amplified, and the amplified signal pulse contrast is improved by a factor equal to the parametric gain. In the second CPA stage, the clean signal pulses were stretched to about 600 ps. The medium energy Ti:sapphire amplifiers are pumped by 532 nm wavelength nanosecond Nd:YAG lasers running at 1 Hz repetition rate. The last high-energy amplifier consists in an 80-mm-diameter, 40-mm-thickness Ti:sapphire disk, pumped by a nanosecond Nd:glass laser, 120 J at 527 nm wavelength, 1 pulse/20 minutes repetition rate. After temporal compression, more than 32 J pulse energy at ~28 fs pulse duration, corresponding to a peak power up to 1.16 PW, with enhanced intensity contrast ratio of 1010, has been obtained.

In the frame of the French project Apollon, an optically synchronized OPCPA with picosecond pulses was proposed for the Front-End configuration of a 10 PW laser system [28]. A similar solution was considered for the 2 10 PW laser system of the ELI-NP laser facility from Bucharest-Magurele [29].
