Yb:YAG single-crystal fiber amplifiers for picosecond lasers using the divided pulse amplification technique

International audienceA two-stage master-oscillator power-amplifier (MOPA) system based on Yb:YAG single-crystal-fiber (SCF) technology and designed for high peak power is studied to significantly increase the pulse energy of a low-power picosecond laser. The first SCF amplifier has been designed for high gain. Using a gain medium optimized in terms of doping concentration and length, an optical gain of 32 dB has been demonstrated. The second amplifier stage designed for high energy using the divided pulse technique allows us to generate a recombined output pulse energy of 2 mJ at 12.5 kHz with a pulse duration of 6 ps corresponding to a peak power of 320 MW. Average powers ranging from 25 to 55 W with repetition rates varying from 12.5 to 500 kHz have been demonstrated. The requirements of industrial and scientific applications have driven a lot of research and development work on high-power ultrafast diode-pumped solid-state laser systems. In the last few years, among technologies such as fiber, slab, or thin disk, the single-crystal fiber (SCF) has demonstrated strong potential for the development of laser systems with simple and compact geometries. In the continuous wave regime, 250 W was generated from a SCF-based oscillator [1], showing the strong potential of this concept for high power extraction. More recently, SCF amplifiers for short pulses with high-gain and high-nonlinearity thresholds have been demonstrated. In the femtosecond regime, high-repetition-rate systems based on SCF allow high average power, 160 W in linear polarization [2], and 85 W in cylindrical polarization [3]. In the high-energy regime, a common way to push the limit of low-repetition-rate systems using SCF, bulk crystals, or fibers is to exploit the large spectral width of femtosecond pulses to decrease the nonlinear effects by stretching the input pulses and recompressing the amplified pulses. This way, a SCF amplifier has generated 400 femtosecond pulses with an energy higher than 1 mJ [4]. Recent experimental efforts to improve the performance of high-energy ultrafast sources have focused on coherent combining. The idea is to create several independently amplified pulses, in the space or time domain, in order to decrease the peak intensity in the gain medium. By combining the chirped pulse amplification (CPA) technique and the spatial distribution of the amplification process onto two SCF channels, compressed pulses with durations of 695 fs and energies of 3 mJ were recently obtained [5]. The divided pulse amplification (DPA) concept is an alternative passive coherent combining technique [6,7,8] which has been successfully implemented in femtosecond-fiber-CPA systems to generate pulse energies of 1 mJ [9]. It consists of the generation of pulse replicas with birefringent crystals or delay lines and recombines them after amplification. It can be easily implemented passively in double-pass amplifiers using a 90° polarization rotation between the two passes. In the picosecond regime, the narrow spectral width does not allow easy and efficient exploitation of the CPA technique and DPA is therefore an attractive alternative. Recent studies have demonstrated the possibility of amplifying picosecond pulses in SCF without relying on CPA for pulse energies beyond 700 μJ and peak powers of 28 MW [10]. Implementing DPA on SCF amplifiers opens the way to high-energy direct picosecond pulse amplification with peak powers over 100 MW. In this Letter, we present a high-energy picosecond laser system based on a low-power picosecond mode-locked oscilla-tor amplified in a two-stage SCF amplifier. In the picosecond regime, due to the low spectral width, in our case below 1 nm, CPA techniques are less convenient to implement compared to the femtosecond regime [4]. For simplicity, the strategy employed here to push further the limitations in terms of peak power and energy is only based on DPA. The combination of a high-gain amplifier stage and a high-energy amplifier stage 1628 Vol. 41, No. 7 / April 1 2016 / Optics Letters Letter 0146-9592/16/071628-04 Journa