## 3 High-Energy Few-Optical Cycle Parametric Sources in the IR Spectral Region
### Available Sources Based on the OPA Design
#### 3.1.3 High-Energy Tunable IR Parametric Source  
For OPA systems based on a design as the one described in the previous sections, the possibility for achieving energy scaling is fundamentally connected to the advances in the front-end laser technology. However, in modern Ti:Sa laser systems, the energy scalability competes with the repetition rate, making the development of high-energy high-repetition rate sources a significant technical issue. Tunability is a key feature of OPA systems. In order to obtain tunable ultrashort pulses, a broadband seed must be used. However, achieving tunability in an ultrashort pulse duration regime  
Figure 4: **(a)** Scheme of the OPA setup for the generation of sub-two-cycle CEP-stable near-IR pulses; **(b)** results of the IR pulse temporal characterization: measured (gray) and retrieved spectral intensity (black) and phase (dashed red); **(c)** temporal intensity profile corresponding to a pulse duration of 9.5 fs. FS, Fused Silica. Adapted with permission from [33], Copyright OSA Publishing, 2011.  
is a challenging task, with the spectral dispersion of the signal pulses being the main limitation. Since the spectral phase is extremely sensible to the experimental conditions upon which seed generation, as well as signal amplification occur, an accurate optimization of parameters as pump/seed pulses energy and duration, as well as a suitable seed line design are required. We present here a novel parametric source recently developed in our laboratories and capable of providing enhanced performances with respect to the OPA systems described in Section 3.1.1, in terms of repetition rate and tunability. An advanced Ti:Sa front-end laser source is used for pumping a two-stage parametric amplifier, delivering 1 kHz 1 mJ\(\sim\)25-fs pulses centered at a wavelength that is tunable between 1.3 \(\upmu\)m and 2 \(\upmu\)m. The OPA scheme is similar to that shown in Figure 2a. Ti:Sa front-end laser provides \(>\)15 mJ 25 fs 800-nm pulses at a repetition rate of 1 kHz. An \(\sim\)7-mJ portion of the pulse energy is used for driving the parametric amplifier. In particular, \(\sim\)330 \(\upmu\)J are sent into the seed generation line. Here, a Kr-filled HCF is used for spectral broadening and pulse compression below 10 fs. The output pulses have an ultra-broadband spectrum, from 550 nm to 1 \(\upmu\)m, nearly approaching an octave extension. DFG is thus performed into a 3 mm-thick BBO crystal. A DFG spectrum is obtained extending from 1.3 \(\upmu\)m to 2 \(\upmu\)m. CEP-stable DFG pulses are subsequently amplified by means of two amplification stages, where 3 mm-thick BBO crystals are used in a Type-II phase-matching configuration. The first amplification stage is pumped by \(\sim\)330 \(\upmu\)J pulses; in the second amplification stage, 6-mJ pump pulses are used. Along the two pump lines, glass windows of proper thickness have been placed for achieving a fine tuning of the pump pulses duration, thus resulting in an optimization of the amplified DFG signal energy and duration. At the output of the OPA system, we obtain ultrashort pulses, tunable according to the seed DFG spectrum. The energy and the duration of the signal pulses slightly depends on the wavelength. The energy varies from 600 \(\upmu\)J to 1 mJ, the peak value being measured at 1.5 \(\upmu\)m. As far as the duration, temporal characterization measurements at two different wavelengths are shown in Figure 5a-d. At 1.4 \(\upmu\)m (Figure 5a,b), a 16-fs pulse duration has been obtained, corresponding to 3.5 optical cycles of the fundamental frequency. At 1.9 \(\upmu\)m (Figure 5c,d), transform limited pulses have been obtained with a duration of 32 fs, corresponding to five optical cycles of the fundamental frequency. The CEP stability of the amplified pulses has been quantified by means of _f-2f_ measurements (see Figure 5e). As a result, an rms value of 200 mrad was measured. An active feedback loop stabilization system is currently being implemented in order to compensate for slow drifts.