4. Optical parametric chirped pulse amplification

where Δk = 0 represents the condition for quasi-monochromatic phase matching; Δk

In this case, two more equations must be added to the three-equation system (2) [22]

∂<sup>2</sup>ki ∂ω<sup>2</sup> i

Particularly in the case of high-energy laser pulse amplification, only a couple of existing highenergy lasers are suitable for OPA pumping. For this reason, usually the pump laser wavelength λ<sup>p</sup> represents the free-chosen parameter of the OPA process. For a certain nonlinear crystal, the other five parameters, including signal central wavelength, are deduced from the

UBBs of more than 100 nm, able to support amplification of sub-10-fs laser pulses, can be obtained in nonlinear crystals [22], like potassium dideuterium phosphate (DKDP) and BBO. Ultra-broad gain bandwidths for BBO and DKDP crystals, pumped by green nanosecond lasers, in NOPA configuration are shown in Figure 4. Gain bandwidths were calculated assuming plane interacting waves, uniform pump intensity distribution, no input idler beam, and negligible pump beam intensity depletion. For both NOPA processes, I consid-

currently used nonlinear crystals in case of about one-nanosecond pump pulse duration (e.g., the data sheets of the manufacturing company Altechna) [23]. Different lengths were considered for DKDP and BBO crystals, corresponding to similar gain values in the para-

The UBB phase-matching of DKDP crystals is centered around λS0 = 900 nm central wavelength, whereas the UBB of BBO crystals is centered in the range of 800 nm wavelength,

αDKDP = 0.92�, λS0 = 900 nm; UBBDKDP ≈ 135 nm. (b) 10-mm-long BBO crystal, λP(BBO) = 0.532 μm, θBBO = 23.8�, αBBO = 2.4�,

practically overlapped to the gain bandwidth of Ti:sapphire laser media.

� sin <sup>2</sup><sup>β</sup> v2 gski

An ultra-broad bandwidth (UBB) of phase-matching can be obtained for Δk

∂<sup>2</sup>ks ∂ω<sup>2</sup> s

νgs ¼ vgi cos β

cos β þ

where vgs and vgi are group velocities of signal wave and idler wave, respectively.

is the condition for optical parametric broad gain bandwidth.

50 High Power Laser Systems

five-equation system comprising Eqs. (2) and (6).

ered a flat pump intensity IP of 1 GW/cm<sup>2</sup>

metric amplification process.

Figure 4. NOPA gain spectra. IP = 1 GW/cm<sup>2</sup>

λS0 = 0.825 μm; UBBBBO ≈ 150 nm.

(0) = Δk

(1) = Δk

(0) = Δk

<sup>¼</sup> <sup>0</sup> (6)

, which can be accepted without damage risk of

. (a) 80-mm-long DKDP crystal; λP(DKDP) = 0.527 μm, θDKDP = 37.0�,

(1) = 0

(2) = 0.

OPCPA was proposed as an alternative solution for the amplification of large bandwidth stretched laser pulses [24] (Figure 5). Drawbacks of the Ti:sapphire CPA, particularly those related to the amplified spectral band narrowing, intensity contrast decrease, and thermal loading, can be overcome in OPCPA laser systems. Signal pulses generated by a broad bandwidth femtosecond oscillator are temporally stretched and synchronized to the pump pulses. Signal and pump pulses have similar durations, usually in the range of picoseconds or nanoseconds. The pump laser wavelength is chosen among the available high-energy green nanosecond lasers, such as frequency-doubled Nd:YAG (532 nm), Nd:glass (527 nm), Yb:YAG (515 nm) lasers. After OPCPA in one or more amplifier stages with nonlinear crystals, enhanced signal pulses can be temporally recompressed to get higher power femtosecond laser pulses.

Unlike CPA, OPCPA is free from gain narrowing and redshifting effects. Because the host crystal is transparent to the interacting beams, thermal loading is practically absent in the parametric amplification process.

On the other hand, in the case of OPCPA, the spectrum of the amplified laser pulse is sensitive to the angle between signal and pump laser beams. The parametric amplification of each signal spectral component depends on the local instantaneous pump radiation intensity. In order to keep a stable amplified signal spectrum from pulse to pulse, high temporal and spatial stability of the pump beams, as well as very stable experimental setup, are required.

Unlike CPA amplifiers, due to angular constraints between pump and signal wave vectors, imposed by the unique phase-matching geometry, in OPCPA experimental setups usually a single pump laser beam can be used (Figure 6). To amplify broadband chirped laser pulses, laser systems based on noncollinear OPCPA (NOPCPA) configuration, imposed by the conditions of UBB parametric amplification in nonlinear crystals, were developed [3, 25–29].

For high-energy final amplifiers of multi-PW laser systems, as much as 10<sup>2</sup> –103 J pump energy, within ~1 ns pulse duration, is required. It is a real challenge to build a single-beam laser able to deliver the pump pulses for these high-energy OPCPA stages.

Figure 5. Principle of optical parametric chirped pulse amplification.

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

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

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

53

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

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

Considering the currently available technical solutions, hybrid amplification represents a good

A couple of PW-class hybrid femtosecond laser systems are currently worldwide operated,

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

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

contrast, part of the ASE pedestal of these pulses is removed by a saturable absorber.

in the laser FE can be directly sent to the Ti:sapphire high-energy amplifiers.

the PW-class hybrid amplification femtosecond laser systems.

5.1. PW-class hybrid femtosecond laser systems

while other 10-PW laser facilities are under development.

choice for the development of petawatt-class femtosecond laser systems.

OPCPA stage.

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