# Ultrafast Mid-IR Laser Pulses Generation via Chirp Manipulated Optical Parametric Amplification
## 2 High-Energy Dual-Chirped-OPA (DC-OPA)  
The power scalability of OPA is limited by both the pump laser energy and the size of the nonlinear crystals. In order to apply more pump energy, naturally, the concept of CPA is transferred to OPA, forming a new scheme called OPCPA. In an OPCPA, the seed pulse is first stretched to a duration that is comparable to a pump pulse with a markedly reduced peak intensity, and then a high-energy pump is used to amplify the seed. Finally a much higher amplified pulse energy than that from an OPA is obtained. In this scheme, to achieve a good temporal overlap between pump and seed pulses, one can employ a high-energy laser system with a relative long pulse duration (for example, a picosecond or nanosecond Nd:YAG system) as the pump source. Limited by the gain bandwidth of a collinear OPCPA geometry, a picosecond pump laser usually results in a longer than 100-fs amplified signal pulse [103], which is not sufficient for many applications. To obtain a relatively short IR pulse, a noncollinear OPCPA of 1.56-\(\upmu\)m pulses using KTA is later proposed and demonstrated [104], in which 100-fs pulses from an erbium fiber laser were stretched to 100 ps and the KTA crystals were synchronously pumped by high-power 100-ps pulses from a Nd:YLF regenerative amplifier at 1.053 \(\upmu\)m. This work opens the door for power scaling of OPCPA to simultaneously obtain a high peak power and an ultrashort pulse duration less than 100 fs. Following this pioneering work, [PERSON] et al. [100] have demonstrated a 4-stage KTP/KTA OPCPA, which produce a CEP-stable multi-mJ 1.5-\(\upmu\)m \(\sim\)70-fs pulse based on the fusion of a diode-pumped solid-state (DPSS) femtosecond Yb:KGW MOPA system and picosecond Nd:YAG solid-state technology. Even if the abovementioned works show the ability of generating IR output energy reaches the multi-mJ level in OPCPA, OPCPA in general faces important technical challenges [42], such as the requirement of a specific pump laser, the difficulty in synchronization with an external pump laser, and the unwanted generation of parasitic superfluorescence accompanying the primary pulse in broadband high-parametric-gain configurations [105]. Keep in mind that how much pump energy can be applied is actually determined by whether the peak pump intensity exceeds the damage threshold of a nonlinear medium. In an OPA, although periodically poled nonlinear crystals such as LiNbO\({}_{3}\) (PPLN) and stoichiometric LiTaO\({}_{3}\) (PPSLT) have high nonlinearity with high conversion efficiency, the available pump intensity in such OPA is quite low owing to the low damage threshold of the crystal and its AR coatings. Therefore, OPA with PPLN and PPSLT crystals might be suitable for generating ultrashort pulse durations with moderate pulse energy but promising for high repetition rates (>1 kHz). On the other hand, beta-BaB\({}_{2}\)O\({}_{4}\) (BBO) is one of the most outstanding nonlinear optical crystals for obtaining broadband IR pulses, which has unique properties: a wide transparency region (0.19-3.5 \(\upmu\)m), wide phase-matching range (0.41-3.5 \(\upmu\)m), large nonlinear coefficient, and high damage threshold. In the OPA scheme with BBO, an output energy up to 7 mJ with a pulse width of 40 fs [106] was achieved at a signal wavelength near 1.4 \(\upmu\)m using a terawatt Ti:sapphire laser system 0.8 \(\upmu\)m. In this work, the pump intensity is already close to the crystal damage threshold, a further increase in the output power of the OPA must enlarge the aperture of the BBO crystals. However, the power scalability of the OPA is limited by the available aperture size of the BBO crystals (typically ~20 \(\times\) 20 mm\({}^{2}\)). On the other hand, a BBO OPA pumped by 800-nm pulses can easily produce a few-cycle IR pulse with a multi-mJ output energy. Since the maximum output is limited by the damage threshold of the BBO crystals, the power scalability for the pump laser is inferior to that of the OPCPA.