Single-mode nonlinear amplification of femtosecond pulses in a multimode-fiber regenerative amplifier

Nonlinear amplification in large-core fibers is a potentially attractive approach to high-power short-pulse generation, but transverse mode coupling and the consequent beam degradation are concerns. Robust single-mode operation of a multimode fiber amplifier can be achieved in a regenerative amplifier, but this has only been demonstrated in linear amplification. Here, we investigate single-mode nonlinear amplification in a multimode-fiber regenerative amplifier. Pronounced mode coupling is observed when the amplified pulse accumulates a large nonlinear phase; however, cavity-assisted mode selection can suppress mode coupling and improve the spatial beam quality. By optimizing the seed pulse duration, pulse energies up to 10 μJ with good beam quality are achieved and subsequently compressed to 98 fs duration. Factors that limit the performance of this amplifier are discussed.