Abstract

Self-starting Kerr-lens mode locking (KLM) without any additional starting mechanism has been previously demonstrated^1,2 for pulses longer than 50 fs, where the KLM effect was optimized for the much longer pulses during the mode locking build-up phase. This may lead to saturation of the effective Kerr-lens saturable absorber and may ultimately limit further pulse shortening. In contrast, we have obtained self-starting sub-20-fs pulses from a KLM Ti:sapphire laser by using an antiresonant Fabry–Perot saturable absorber (A-FPSA)^3,4 The A-FPSA acts as a strong mode-locking-starting mechanism and supports the generation of KLM pulses over a wider range of laser parameters, such as intracavity losses and cavity stability. The efficient starting mechanism of the A-FPSA allows for an independent design of the self-starting mechanism and the fast saturable-absorber action due to KLM in the final stage of pulse shortening. Additionally, we demonstrate pulses of 40–80 fs, independent of the stability regime of the cavity, which shows that these pulses are generated by the A-FPSA, not by KLM. In this case, the A-FPSA is responsible for both self-starting and femtosecond-pulse formation.

© 1995 Optical Society of America