# Ultrafast Mid-IR Laser Pulses Generation via Chirp Manipulated Optical Parametric Amplification
## 1 Introduction
Considering the group velocity mismatch between the three interacting pulses in OPA, we have proposed and experimentally investigated a collinear dual-crystal OPA (DOPA) scheme that compensates the group delay (GD) during amplification [93; 94]. A piece of BaF\({}_{2}\) crystal with an inverted group velocity relationship to the BBO crystal is employed in DOPA to compensate the temporal walk-off between the pulses and thereby improve the bandwidth and efficiency of the system. As a result, tunable spectra that support few-cycle pulse durations at non-degenerate regions are obtained. A novel frequency-domain OPA (FOPA) was proposed in [95]. Rather than amplifying the pulses in the temporal domain, FOPA employs frequency-domain amplification. Different ranges of the seed pulse are dispersed spatially, and the parametric process takes place in several pieces of nonlinear crystals that amplifies different spectral regions individually [96]. The broadband amplification results in a few-cycle Mid-IR pulse with millijoule level energy. As an alternative to directly generating few-cycle pulses in broadband OPA, the employment of spectral broadening in a hollow core fiber (HCF) accompanied with a narrowband OPA is also a promising approach. The self-phase modulation (SPM) of an ultrashort pulse in HCF filled with gas can efficiently broaden the pulse spectrum, and after the dispersion compensation, the broadened spectrum can be recompressed to few-cycle durations. The HCF compression has manifested its validity in a number of previous research studies [97; 98; 99; 100]. It is significant to point out that, restrained by the damage threshold of the employed components such as the nonlinear crystal, the periodically poled crystal, or the hollow core fiber, the energy scalability in the above systems are usually quite limited. The state-of-the-art Ti:sapphire laser system is capable of producing femtosecond laser pulses with the energy of hundreds of millijoules, which is difficult to be directly utilized in conventional OPA due to the above limitations. Therefore, the novel OPA schemes that might achieve better performance via manipulating the chirps of both pump and seed pulses are proposed. After temporally stretching the input pump pulse, an ultrahigh pulse energy can be fully utilized while the peak intensity of the incident beam remains lower than the damage threshold of the nonlinear crystal, hence evidently improving the energy scalability of the system. On the other hand, the possibility of individually manipulating pump and seed chirps enables one to match the central frequencies of input pulses in different slices, which is also beneficial for the system gain bandwidth. Stimulated by the recently proposed multi-plate spectral broadening scheme [101; 102], an intense supercontinuum with ultrabroad bandwidth can be used to pump the OPA system that further exploits the potential of chirp manipulated OPA scheme. In this paper, we focus on the chirp manipulated OPA scheme and review the developments utilizing this method, the structure of the paper is arranged as follows. In Section 2, the theoretical investigations on the high-energy OPA scheme are reported. In Section 3, the chirp manipulated OPA pumped by a spectrally broadened laser is analyzed and discussed. In Section 4, the latest experimental results on broadband pumped chirp manipulated OPA are presented. In Section 5, the prospects of the chirp manipulated OPA are discussed and the conclusions are drawn.