The peak power performance of ultrafast fiber lasers scales with fiber
mode area, but large fibers host multiple modes that are difficult to
control. We demonstrate a technique for single-mode operation of
highly multimode fiber based on regenerative amplification. This
results in a short-pulse fiber source with, to our knowledge, an
unprecedented combination of features: high gain (>55dB) with negligible amplified
spontaneous emission, high pulse energy (>50µJ), good beam quality (M2≤1.3), and transform-limited
(300 fs) pulses from a single amplification stage. We discuss
peak intensity scaling to much higher levels and other opportunities
for short-pulse generation in regenerative fiber amplifiers.