## 3 High-Energy Few-Optical Cycle Parametric Sources in the IR Spectral Region
### Optical Parametric Chirped Pulse Amplification
#### 3.2.1 Few-Cycle OPCPA Sources in the Near-IR  
The generation of near-IR millijoule-level few-cycle femtosecond pulses at 1.5 \(\upmu\)m was reported for the first time by [PERSON] and coworkers in 2009 [38]. Their OPCPA setup is based on Type-II KTA/KTP crystals pumped by picosecond Nd lasers (Figure 6). The front end of the OPCPA consists of a CEP-stable seeder that provides low-energy IR pulses to be boosted and a Nd:YAG source, providing high-energy pump pulses. The seeder (see [39] for details) is based on an OPA system driven by a femtosecond source, made up of a Yb:KGW oscillator plus a power amplifier. Specifically, the 130 \(\upmu\)J, 250 fs, 10-kHz pulses at 1.03 \(\upmu\)m coming out from this source are used to generate a broadband seed by means of white-light generation. The seed undergoes parametric amplification in two OPA stages. Here, idler pulses, CEP-stable, tunable from 1.4 to 1.6 \(\upmu\)m with an energy of 4 \(\upmu\)J and a duration sub-40 fs are generated. These idler pulses are used for seeding the OPCPA. A portion of the beam produced by the Yb:KGW oscillator and centered at 1.064 \(\upmu\)m is stretched and then delivered to the Nd:YAG system used for pumping. This system consists of a regenerative and a double-pass amplifiers working at a repetition rate of 20 Hz. Amplification of the 1.064 \(\upmu\)m beam takes place so that 100 mJ 60-fs pulses are generated. Synchronization between seed and pump pulses derives from the fact that the beam from the Yb:KGW oscillator is used as an input both in the seeder and in the Nd:YAG amplifier. In order to match pump and seed pulse duration, the seed undergoes temporal stretching inside a grating-based stretcher so that a pulse duration of 40 ps is obtained. An acusto-optic device is used for finely shaping temporal dispersion. Parametric amplification of the IR seed occurs into two stages, namely Stages 3 and 4. Idler pulses at \(\sim\)3.4 \(\upmu\)m are generated. Both in Stage 3 and in Stage 4, a Type-II phase-matching configuration is chosen. Ten millimeter-thick KTP crystals are used as non-linear media. That is because, unlike borate crystals, KTP is transparent for the idler wavelength in the mid-IR, thus preventing thermal load inside the amplification media and enabling efficient amplification of a signal in the spectral region around 1.5 \(\upmu\)m.  
In Stage 3, a 7-mJ pump beam is used, while in Stage 4, the pump energy can be tuned between 45 and 90 mJ. The dimension of the pump beams is changed for matching the dimension of the seed by means of proper telescopes. The high energy in the second pump beam line is delivered to Stage 4 by a vacuum relay imaging system. As a result of the two parametric amplification steps, CEP-stable pulses at around 1.5 \(\upmu\)m with an energy up to 12.5 mJ are generated. The spectral bandwidth of the IR pulses is 80 nm, corresponding to a transform limited duration of \(\sim\)65 fs. To approach the few-cycle regime, spectral broadening by filamentation in a gas cell is performed. In order to prevent additional non-linear effects from affecting the temporal structure of the IR pulse during propagation in the gas, a low input pulse energy is used, 2.2 mJ. With low energy, filamentation in a self-compression regime is achieved. As a result, the IR pulses are compressed down to a 19.8-fs duration, corresponding to four optical cycles of the fundamental frequency. The output energy is 1.5 mJ. By properly optimizing the experimental conditions (beam diameter, focusing length, gas type and pressure, cell length), higher energy can be used as an input for filamentation, and higher output energy (\(\sim\)5 mJ) can be obtained. However, the energy scalability of this system is low because of energy limitations imposed for attaining a suitable filamentation regime. Moreover, the extremely high power delivered by the pump source prevents high repetition rate operation regimes.