A high-output, high efficiency femtosecond laser based on a single-stage multipass amplifier

A kilohertz laser producing 4 mJ pulses with 25 fs duration and 26% conversion efficiency was developed using a single-stage multi-pass amplifier. The laser signal spot size inside the Ti:sapphire gain medium was changed in a unique way to obtain high gain and high efficiency without damaging the crystal. To the best of our knowledge, this is the highest pulse energy ever obtained from kHz lasers with a single-stage multi-pass amplifier