Bidirectional Kerr-lens mode-locked femtosecond ring laser

We report the first bidirectional operation of a Kerr-lens mode-locked femtosecond Ti:sapphire laser. Kerr-lensing was used both to mode-lock the laser and to limit the pulse energy through passive negative feedback. e-switched mode-locked operation was also observed in the same cavity at a period of 4 to 7 mu s. The unidirectionality/bidirectionality of that operation could be controlled with an intracavity semiconductor configured as an energy limiter. A simple ABCD matrix method correctly predicts the locations of the apertures to achieve the positive feedback required for continuous wave mode-locking and the negative feedback for continuous wave bidirectional operation. The numerical model also estimates the observed beat frequency of 80-150 kHz. (C) 1997 Published by Elsevier Science B.V.