## 3 High-Energy Few-Optical Cycle Parametric Sources in the IR Spectral Region
### Perspectives in OPAs and OPCPAs Development  
An attractive perspective for the future development of compact ultra-broadband tunable parametric OPA and OPCPA in the near-IR and mid-IR lies in the exploitation of fiber-doped laser sources. Fiber-doped lasers are robust high repetition rate turn-key systems, capable of providing pulses in the near-IR spectral window and characterized by high spatial quality and temporal duration down to the few-tens of fs [46]. The output of these fiber lasers may be used for directly seeding a cascade of OPAs, leading to a substantial reduction of the complexity in the experimental setup. By using a fiber-doped laser system in combination with a powerful ps-solid-state laser system, parametric amplification of the near-IR radiation can be achieved, even above the mJ level. So far, near-IR OPCPA source at the 10-mJ energy-level have been reported [47; 48] delivering pulses in the multi-cycle regime (\(\sim\)200 fs) at repetition rate between 10 Hz and 1 kHz. In recent years, the interest in developing few-cycle long-wavelength sources has fostered research on fiber-based system. That is because the near-IR beam provided by these systems can be used for generating a seed radiation in the mid-IR by DFG process. In [49; 50], a dual-output fiber oscillator delivering femtosecond pulses at both 1550 and 1050 nm is used as a front-end of an OPCPA (Figure 8). In particular, the 3-um seed pulses for the OPCPA chain are generated by performing DFG between the 1550- and 1050-nm pulses. As a result, a CEP stable mid-IR seed is obtained. The seed is subsequently amplified by means of energy exchange with a 100-kHz ps pump beam in a three-stage OPA. Therefore, few-cycle pulses at 3 um are produced with an energy that is however well below the mJ level (\(\sim\)5 uJ). Thus, by integrating fiber-based near-IR sources into OPCPAs, a significant scaling in the setup performances can potentially be attained. In addition, these sources provide the possibility for mid-IR seed light generation directly from a DFG process. This approach is suitable for CEP stability and allows one to overcome limitations discussed for [45], related to the use an idler beam as a mid-IR seed for OPCPAs. On the other hand, the IR pulse energy delivered by these sources is still very low with respect to conventional OPAs and OPCPAs based on solid-state lasers. New perspectives in parametric amplifiers' development have opened with the introduction of two different approaches, namely waveform synthesizers and Frequency-domain Optical Parametric Amplifiers (FOPAs). A promising route for the generation of extremely short optical pulses consists of synthesizing multiple sources. Synthesis is obtained by coherently combining the output of a number of different sources operating at different wavelength, so that a broader spectrum is constructed. The coherent combination of different near-IR OPCPAs, as well as of different fiber-based systems has been recently demonstrated [51; 52], leading to sub-single cycle waveforms with an energy in the order of some tens of \(\upmu\)J. Systems based on this technology are suitable for energy scaling and can provide a solution for overcoming fundamental limitations, like limited parametric amplification bandwidth. However, improved performances both in terms of scalability and pulse duration lead, in these cases, to an increased complexity in the setup. Indeed, synthesizing a sub-single cycle waveform requires both an accurate shaping of the pulses temporal profile and a full control on the CEPs and on the relative timing among the pulses. Acusto-optic programmable filters and balanced cross-correlators are used for properly shaping and timing the pulses with attosecond relative time stability. A completely different scheme for achieving parametric amplification of CEP-stable pulses is Frequency-domain Optical Parametric Amplification (FOPA) [53].