## 3 High-Energy Few-Optical Cycle Parametric Sources in the IR Spectral Region
### Available Sources Based on the OPA Design
#### 3.1.2 Sub-Two-Cycle IR Sources Based on Narrowband Parametric Amplification System  
In the OPA system described above, a very powerful Ti:Sa laser source is used for achieving broadband IR pulse with an energy on the millijoule-level. On the other hand, high-power lasers suffer for strict limitations on the repetition rate. In fact, in order to avoid damage and thermal degradation effects inside the laser active media, a low repetition rate was used. A different scheme is now presented for producing few-cycle IR pulses that is based on a narrow-band parametric amplification stage followed by a spectral broadening stage. In this scheme, higher repetition-rate laser sources are used, even if some price is paid in terms of energy. The pulses originating from a narrow-band parametric amplifier are spectrally broadened, by exploiting a non-linear process occurring in noble gas atoms, namely the Self-Phase Modulation (SPM) process. The possibility to combine a narrow-band OPA and a compression stage based on SPM strongly relies on the capability to produce IR femtosecond pulses with an energy above the threshold required for efficiently driving non-linear processes in noble gases. In this sense, a first implementation was proposed by [PERSON] et al. in 2007 [32]. In their setup, a multi-cycle CEP-stabilized IR OPA pumped by a 1-kHz Ti:Sa laser system is used, producing pulses at 2 \(\upmu\)m with an energy around 500 \(\upmu\)J and a duration above 50 fs. SPM in a plasma filament is then used for broadening the pulses spectrum, thus approaching the few-optical-cycles regime. CEP stability is preserved during filamentation, and the residual rms CEP drift of the output pulses is smaller than 0.1 rad. This scheme thus allows for the generation of CEP-stable 270 \(\upmu\)J pulses, with a measured FWHM of 17 fs. An alternative approach for generating even shorter pulses in the sub-two-cycle regime was reported in 2011 by [PERSON] et al. [33; 34] based on spectral broadening in a Hollow Core Fiber (HCF). In Figure 4, their experimental setup is shown. It consists of a high energy OPA combined with a pulse broadening and compression stage. The OPA is pumped by a non-CEP-stabilized 1-kHz Ti:Sa laser delivering 3.8 mJ, 35-fs pulses at a wavelength of 800 nm. The laser power is split into four branches, one used for the generation of a seed and the remaining ones used for pumping three amplification stages. A broadband seed is produced by focusing the radiation into a sapphire plate where white-light generation takes place. As already mentioned, the white-light generation process preserves the CEP fluctuations of the original laser pulses. Thus, a non-CEP stable seed is generated in the near-IR spectral range, which undergoes parametric amplification into a Type-II BBO crystal (BBO 1). Here, the signal is amplified, and an idler is generated at 1.8 \(\upmu\)m. Since both pump and seed share the same CEP fluctuations, the idler is CEP-stable, its phase resulting from the difference between pump and seed phases. In this first parametric stage, a collinear configuration is used, in order to avoid the idler from being affected by spatial chirp. The 1.8 \(\upmu\)m-idler is selected  
Figure 3: A sequence of interferograms acquired with an _f-2f_ interferometer over 28 s is reported. In the right panel, the CEP fluctuations are shown corresponding to a rms value of 220 mrad. Adapted with permission from [29], Copyright OSA Publishing, 2008.  
mirror and amplified in the two subsequent parametric amplification stages (BBO 2, BBO 3), which are implemented using a slightly non-collinear Type-II phase-matching configurations. After BBO 3, near-IR pulses at 1.8 \(\upmu\)m with an energy of 650 \(\upmu\)J and an almost transform-limited duration of 35 fs are available. In order to reduce pulse duration down to the few-cycles scale, these pulses are focused into an argon-filled HCF using a 1-m focusing optic. Here, SPM takes place, leading to a significant spectral broadening.