## 3 High-Energy Few-Optical Cycle Parametric Sources in the IR Spectral Region
### Optical Parametric Chirped Pulse Amplification
#### 3.2.2 Few-Cycle OPCPA Sources in the Mid-IR  
Because of the lack of non-linear media exhibiting transparency in the mid-IR spectral region, parametric amplification of this part of the spectrum has been difficult to achieve. In this sense, a significant advantage offered by periodically-poled crystals is the flexibility of operation in a spectral window, which commonly extends up to the mid-IR range. Titanium compounds, as KTP (Potassium Titanyle Phosphate), KTA (Potassium Titanyle Arsenate) and RTP (Rubidium Titanyle Phosphate), are also suitable for the mid-IR range. Recently, with the introduction of periodically poled crystal technology and with the development of growth techniques for single crystal synthesis, OPCPAs in the mid-IR have been realized.  
In [43; 44], high repetition rate (100 kHz) microjoule-level hundred-fs pulses at 3 \(\upmu\)m have been produced by means of OPCPAs systems based on periodically-poled crystals.  
The extension to a multi-m) few-cycle pulse regime was achieved by [PERSON] and coworkers in 2011 [45]. The scheme of their KTP/KTA-based source is reported in Figure 7. The front end of the system is made up of Yb:KGW oscillator, which is used for seeding a 1-mJ Yb:CaF\({}_{2}\) femtosecond amplifier at 1030 nm and a 250-mJ Nd:YAG picosecond amplifier at 1064 nm.  
The Yb:CaF\({}_{2}\) amplifier is used for pumping a white-light-seeded KTP-based OPA at the repetition rate of 0.5 kHz. As an output from the OPA, a signal is produced at 1.46 \(\upmu\)m with an energy of 65 \(\upmu\)j; the phase-stable idler beam at 3.9 \(\upmu\)m is discarded. The signal pulses are then stretched into a Grating-pr  
Figure 7: **(a)** Scheme of the OPCPA set-up for pulses amplification at 3.9 \(\upmu\)m; **(b)** measured (red) and retrieved spectral intensity (black) and phase (dashed); **(c)** temporal intensity profile corresponding to a pulse duration of 83 fs. Adapted with permission from [45], Copyright OSA Publishing, 2011.  
The Nd:YAG amplifier works at 20 Hz and is used for pumping two collinear OPCPA stages where 10 mm-long KTA crystals are used as non-linear media. A Type-II phase matching configuration is adopted.  
The first OPCPA stage is pumped by 50 mJ, and the 1.46 um seed is amplified to 0.5 mJ.  
The second OPCPA stage is pumped by up to 175-mJ pulses. This leads to the generation of uncompressed signal pulses with an energy of 22 mJ and uncompressed idler pulses at 3.9 um, with an energy of 13 mJ. After compression, the 3.9 um idler beam has an energy of 8 mJ. The retrieved idler pulse duration is 83 fs, corresponding to less than seven optical cycles of the fundamental frequency. In Figure 7, the idler spectrum and temporal profile after the second OPCPA stage are shown.  
Due to the high energy involved in the second OPCPA stage, the KTA crystal has uncoated Brewster cut faces so as to prevent optical damage in the coating. Brewster interfaces lead to angular dispersion in the 1.46-um signal beam entering the crystal. This angular dispersion is transferred to the idler. Thus, the idler beam suffers for a slight divergence, which is however comparable with the corresponding diffraction limit divergence. Moreover, the idler pulses are not CEP stable because they result from the interaction between pump and signal pulses, which are not locked in phase. The system can be modified for providing CEP stable mid-IR pulses by injecting into the OPCPA setup the idler beam coming out from the OPA front end. Indeed, the OPA idler is phase-locked and can be used for seeding the two OPCPA stages by remaining unaffected in phase. The main disadvantage of having the OPA idler as a seed for the OPCPA system consists of the necessity to use suitable mid-IR reflectors throughout the OPCPA setup.