# Ultrafast Mid-IR Laser Pulses Generation via Chirp Manipulated Optical Parametric Amplification
## 2 High-Energy Dual-Chirped-OPA (DC-OPA)
Although Ti:sapphire laser systems with >100 TW peak power and 10 Hz repetition rate are already available, they cannot be applied in an OPA as they stand owing to the damage threshold of the BBO crystals. Moreover, the concept of OPCPA does not work if we employ a high-energy Ti:sapphire laser. If we only stretch the seed pulse in OPCPA, the intensity of the pump pulse from the high-energy Ti:sapphire laser is still far beyond the crystal damage threshold. If a Ti:sapphire laser with sufficient energy can be applied to pump an OPA while preventing damage to BBO crystals, we will conveniently obtain a high IR energy. To this end, a dual-chirped OPA (DC-OPA), which simultaneously obtains high peak power and ultrashort pulse duration in the IR region, is proposed in [107] and shown in Figure 1. The DC-OPA is seeded with a chirped, broadband seed pulse and pumped by a stretched, broadband pump pulse. In our envisaged DC-OPA system, automatically synchronized pump and seed pulses are obtained because they come from a common source. The pump pulse can be spatially separated into two pulses on a beam splitter, one strong and the other one weaker. The strong one will be used as a pump pulse, and the weaker one will be used to generate the seed pulse via white-light generation, e.g., in a sapphire plate. Figure 2 shows the dependence of conversion efficiency and bandwidth for output signal on the GDD values of seed pulses. The pump pulse at 800 nm has a Gaussian temporal profile with a duration of 35 fs that is stretched to 792 fs by adding a GDD of \(-\)10,000 fs\({}^{2}\). The added GDD values of 35-fs seed pulse at 1400 nm are varied by a step of 2000 fs\({}^{2}\). Under a pump intensity of 100 GW/cm\({}^{2}\) and a designed energy gain of 10\({}^{3}\), it is found that the optimized seed GDD ranges from 3000 fs\({}^{2}\) to 5000 fs\({}^{2}\) for signal pulse (Figure 2a) and 3500 fs\({}^{2}\) to 5500 fs\({}^{2}\) for idler pulse (Figure 2b). Relatively high total conversion efficiency (over 40%) with a broad bandwidth supporting a sub-40-fs signal and idler pulses can be obtained, when we choose the proper GDD values (Pump: \(-\)10,000 fs\({}^{2}\); Seed: \(+\)4000 fs\({}^{2}\)) for avoiding gain by narrowing and maintaining good temporal overlap. Before recompression of the signal and idler pulses, it is essential to know their spectral properties. We chose a set of GDD values (pump: \(-\)10,000 fs\({}^{2}\), seed: 4000 fs\({}^{2}\)) within the optimized range to treat this issue. It is shown in Figure 3a that an amplified signal pulse at 1.4 \(\upmu\)m exhibits a smooth distribution with a bandwidth of 79 nm, which is just slightly narrower than the injected spectra of the seed pulse. Furthermore, it maintains a good beam quality for the signal pulse, as shown by the inset plot. After phase compensation, the pulse duration of the obtained signal is 40 fs (Figure 3b), which is the inset plot for the spatial profile of the obtained signal. At present, the 10 TW-class (e.g., 40 fs, 0.4 J) Ti:sapphire laser system is commercially available. From the conversion efficiency of DC-OPA, if we apply all the laser energy (0.4 J) for DC-OPA with an optimized GDD condition, we can expect  
Figure 1: Scheme of dual-chirped optical parametric amplification (DC-OPA). Figure 2: Signal pulse conversion efficiency (blue circle) and bandwidth (red triangle) from the DC-OPA with respect to the seed GDD (0-10,000 fs\({}^{2}\)). (**a**) Type-I and (**b**) Type-II phase matching. The conversion efficiency and bandwidth are calculated at the gain saturation point in the BBO crystal. to generate more than 100 mJ signal energy before compression. Although our estimation does not include an optical loss by compressor, we will obtain more than enough TW IR laser power. Using the concept of DC-OPA, a total output energy of 1.4 \(\upmu\)m (signal) and 1.9 \(\upmu\)m (idler) of 210 mJ has recently been experimentally realized [108], pumped by a 700-m) Ti:sapphire laser source.