**1. Introduction**

The new generation of femtosecond technology based on short-pulse-pumped optical parametric chirped-pulse amplification (OPCPA) [1] holds promise for scaling the peak power and average power of few-cycle pulses simultaneously. This progress would benefit a number of fields, notably attosecond science [2, 3] by allowing to scale attosecond pulse generation at higher photon energy and higher flux.

In OPCPA, the amplified bandwidth is not limited by the energy level structure of a laser medium or gain narrowing [4], as is the case in laser amplifiers [5]. Therefore, OPCPA appears to be the method of choice for the production of ultrashort pulses (down to the few-cycle regime) at high peak and average power. Employing short pulses (several-ps to sub-ps) to

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pump an OPCPA allows higher peak intensity in the nonlinear medium as the damage threshold intensity of materials scales with the inverse square root of the pulse duration [6]. The high pump intensity makes it possible to achieve the required gain in a shorter crystal, which leads to greater amplification bandwidth.

Further advantage with a short crystal is that the effect of transverse walk-off is reduced, the temporal contrast can be enhanced, and stretchers and compressors can be simpler. However, the crystal length does not decrease as rapidly as the pulse duration, so the temporal walk-off relative to the pulse duration increases for short pulses. A simple analytical analysis shows that the optimum pump-pulse duration to achieve a high conversion efficiency and a broad‐ band gain is around 1 ps [7].

Nevertheless, all these advantages of short-pulse-pumped OPCPA remain useless without an efficient, reliable, and powerful pump source. Such pump lasers are required to deliver highenergy near-1-ps pulses with near-diffraction-limited beam quality at repetition rates in the kHz to MHz range.

Heretofore, due to the lack of suitable pump lasers, the few-cycle OPCPA delivered either high-energy pulses at a low repetition rate [8, 9] or low-energy pulses at a high repetition rate [10]. Nowadays, Yb-doped lasers in the thin-disk, fiber or slab geometries [1, 11–15] are capable of delivering high-energy, high average power pulses with ps-pulse duration. Among these laser technologies, the recent advances in Yb:YAG thin-disk lasers have started to fulfill the criteria for suitable pump sources for OPCPA systems and hold promise to change the current state of the art of OPCPA systems to few-cycle pulses with higher energy and average power [1, 16].

This chapter is devoted to the recent progress in Yb:YAG-pumped, few-cycle OPCPA systems. In Section 1, a brief overview on the fundamentals of OPCPA is presented. In Sections 2 and 3, novel techniques for increasing the conversion efficiency are discussed. In Section 4, a technique for extension of the amplification bandwidth is discussed.

In a medium with second-order nonlinearity, a high-energy photon (called pump) can decay to two newly generated photons with lower frequencies (called seed and idler). In the presence of initial seed photons, the decay of pump photons is stimulated and consequently more photons at the seed frequency are generated. The seed photons after amplification are named signal and the process is called optical parametric amplification (OPA). The frequency of the generated signal and idler photons is defined by the conservation of energy. However, the amplification bandwidth can be increased by fulfilling conservation of momentum between pump, signal, and idler pulses, which can be tuned by the type, thickness, and temperature of the nonlinear medium and also the geometry of the three interacting beams.

To obtain a strong pump-to-signal and idler energy conversion, the spatial and temporal overlap between seed and pump pulses in the nonlinear medium should be maximized. The optimum temporal overlap between the pump and seed pulses can be ensured by temporal stretching of the seed pulses to the temporal window of pump pulses. This technique is the combination of chirped-pulse amplification (CPA) [17] and OPA, hence called optical para‐ metric chirped-pulse amplification.

In addition to the above-mentioned parameters, the conversion efficiency in OPA or OPCPA systems also depends on the thickness of the nonlinear medium, peak intensity of the pump pulses, and the initial seed energy. Conversion efficiency scales up by increasing the thickness of the nonlinear medium as long as the phase-matching condition (conservation of momen‐ tum) between the three interacting beams is satisfied. At higher pump peak intensity to induced nonlinear polarization in the nonlinear medium is stronger and therefore larger amplification is achieved. Moreover, by increasing the seed-to-pump energy ratio, the conversion efficiency at lower amplification gain can be achieved.

The optimization of the pump-to-signal conversion efficiency of multicycle OPCPA systems has been the subject of several studies [18–22]. In the next two sections, the feasibility and realizability of two techniques to increase the conversion efficiency of few-cycle OPCPA systems are discussed.
