10 mJ femtosecond Ti: Sapphire regenerative amplifier with large mode size

Abstract

With advent of chirped-pulse amplification, the peak power of femtosecond laser pulse was reached to petawatt (PW) or hundreds of terawatt (TW). Many progresses of high-field physics and ultrafast dynamics in matter are achieved using TW or PW laser. Pre-amplifier is an exponential growth amplifier which is also a bridge between oscillator and power amplifier. The best choice of pre-amplifier is amplification in regenerative cavity, due to its high stability and beam quality. The quality of pre-amplified laser pulse is significant to efficiency and beam quality of the successive power amplifier. High energy pre-amplifier with high beam quality will reduce the requirement of pump laser in final power amplifier. But typical regenerative amplifier only support low output energy of few millijoule. Higher energy from only one regenerative amplifier is crucial to whole laser system. High energy regenerative amplifier can be achieved by increasing the size of TEM00 in cavity. A new femtosecond Ti:sapphire regenerative amplifier with large mode size was demonstrated in this letter. The regenerative cavity is designed as stable linear resonator in which end mirrors are planar, the diameter of beam waist in Ti:sapphire crystal is larger than 2 mm, which can support high energy pulse amplified in cavity. By matching the focal spot of pump laser with the size of mode and optimization of cavity, the output laser energy up to 17.4 mJ was achieved under the pump energy of 60 mJ at repetition rate of 10 Hz, which corresponds to the efficiency of 29%. The amplified laser pulse from regenerative amplifier was compressed in a grating-pair compressor. By carefully alignment of incident angle and distance between the two gratings of compressor, the shortest pulses duration of 40.6 fs and energy of 13.9 mJ are obtained, which is a little bit longer than Fourier-transform limit based on spectrum of laser. The dispersion in the CPA laser system was also analyzed, after optimization of compressor, there are still high order dispersions uncompensated, which results in the duration of compressed pulses longer than Fourier-transform limit. Based on this large mode size regenerative amplifier, peak power of 1.9 TW laser pulses which compressed pulse energy of 81.4 mJ in 43 fs were also further realized by following only one stage of multipass amplifier. The beam quality (M2) was measured to be 1.6 and 1.5 in X and Y directions respectively, and the energy stability is 2.15% (rms). The results show that this large mode size regenerative amplifier is an ideal choice of pre-amplifier in TW laser system.