100W class compact Yb:YAG single crystal fiber amplifier for femtosecond lasers without CPA

Single crystal fibers (SCF) represent an alternative technology for ultrashort pulse amplification to high average power in a simple architecture. SCF have an aspect ratio of a short rod fiber or a thin and long crystal and benefit several advantages from the both concepts. Relatively short interaction length and large signal beam diameter mitigate the nonlinear effects and allow direct amplification of femtosecond pulses avoiding the standard chirped pulse amplification (CPA) technique. In this paper, we demonstrate the amplification of femtosecond pulses up to 160 W of average power using a compact and simple two-stage SCF amplifier without CPA. High brightness pumping and a double-pass signal configuration of the first stage allowed us to reach the small signal gain value of 32.5 dB, i.e. almost 2000, which is the highest reported value. Additionally, we implemented for the first time the bidirectional pumping scheme in the second stage. With the total pump power of 300 W, we achieved the highest average power of femtosecond pulses and the highest extraction efficiency from the SCF, i.e. 160 W and 42 %, respectively. The pulse duration at the maximum output power was measured to be 800 fs assuming sech² temporal shape. The amplification details and spectral, temporal and spatial characterization of the output beam will be presented.