## 3 High-Energy Few-Optical Cycle Parametric Sources in the IR Spectral Region
### Available Sources Based on the OPA Design
#### 3.1.1 Millijoule-Level IR Parametric Sources  
A parametric amplifier in the near-IR spectral region was demonstrated in 2006 by [PERSON] et al. [23], providing 200-\(\upmu\)J, 15-fs CEP-stable pulses at 1.5 \(\upmu\)m. Using a similar design, the system was subsequently scaled up to the mJ level [28; 29], making it suitable for experiments in the strong-field regime. A detailed description of the mJ-energy source is reported below. This source consists of broadband near-IR OPA, centered around 1.5 \(\upmu\)m, providing millijoule-level pulses, which are passively stabilized by means of a DFG seeding process. A schematic view of its architecture is shown in Figure 2a. A Ti:sapphire (Ti:Sa) laser system, providing 60-fs pulses at \(\lambda\) = 800 nm with an energy of 10 mJ and a repetition rate of 10 Hz, is used for driving the parametric process. Its energy is split into three portions, one for generating an IR seed and two for pumping the two amplification stages. A phase-stable seed is generated through DFG among different spectral components of a spectrally-broadened laser pulse. Specifically, a small fraction of the driving laser beam (1 mJ) is focused through an iris in a krypton-filled gas cell. Here, spectral broadening occurs by filamentation [30]. The out-coming broadband pulses have an energy around 0.3 mJ and are compressed by means of chirped mirrors down to a duration of around 10 fs. These pulses are then focused into a 200 \(\upmu\)m-thick BBO crystal, where the DFG process takes place. As a result, a nJ-level CEP-stable IR seed is generated, which is subsequently boosted in energy by optical parametric amplification. Parametric amplification takes place in two stages. The first OPA stage consists of a 2-mm BBO crystal pumped by 2-mJ pulses, while the second one consists of a 3-mm BBO crystal pumped by 7-mJ pulses. In the second amplification stage, a small prism was inserted in the pump pathway in order to tilt the pulse front of 2\({}^{\circ}\) so as to match it with the pulse front of the seed beam [31]; as a result, the energy transfer from the pump to the seed is maximized. A slightly non-collinear Type-II phase-matching configuration is exploited in both stages for achieving few-cycle near-IR pulses. A spectral characterization of the system is reported in Figure 2b, showing the DFG spectrum (solid curve), as well as the IR spectra after the first (dashed curve) and the second (dashed\(-\)dotted curve) parametric amplification stages. The energy of the amplified DFG pulse after the second stage of amplification is 1.2 mJ, and its duration is around 17 fs, corresponding to 3.5 optical cycles (Figure 2c). As far as the energy stability, the measured peak-to-peak root-mean-squared (rms) fluctuation is better than 4%. Moreover the CEP stability of the out-coming IR pulses was characterized by means of an _f-2f_ optical system. As a result, an rms fluctuation of the CEP of 220 mrad was measured (Figure 3), which proved the passive stabilization mechanism to provide excellent performances in shot-to-shot stability. It is worth mentioning that, thanks to the low dispersion of BBO in the spectral region of interest, the IR pulses provided by this system are nearly transform-limited and do not require any further compression. Figure 2: (a) Scheme of the Optical Parametric Amplification (OPA) setup for the generation of high-energy Carrier-Envelope Phase (CEP)-stable near-IR pulses: BS, Beam Splitter; Difference Frequency Generation (DFG), 400 \(\upmu\)m-thick BBO crystal; POL, Polarizer; filter, neutral density filter for controlling intensity of the pump beam; (b) Spectra of the DFG pulses (solid curve) and of the pulses after the first (dashed curve) and second (dashed–dotted curve) OPA stage in a Type-II phase-matching configuration; (c) Temporal intensity profile corresponding to a pulse duration of 17 fs. The inset shows the spectrum of the amplified IR pulses (solid curve) and the retrieved spectral phase (dashed curve). Adapted with permission from [28], Copyright OSA Publishing, 2007.