Stronger Focusing Avoids SESAM Damage

Posted on 2008-07-02. Permanent link: http://www.rp-photonics.com/spotlight_2008_07_02.html

Ref.: encyclopedia articles on semiconductor saturable absorber mirrors, passive mode locking

The following curious situation can sometimes be observed when trying passive mode locking of a solid-state laser with a semiconductor saturable absorber mirror (SESAM): the SESAM works fine when operated with a relatively strongly focussed laser beam, but is quickly destroyed when the resonator design is modified such that the mode area on the SESAM becomes larger. This is really against all intuition: weaker focusing should lead to lower optical intensities, and thus reduce rather than increase the risk of damage!

The reason behind this peculiarity is that for too weak focusing on the SESAM, the mode-locking process can exhibit Q-switching instabilities. Here, the pulse energy undergoes strong fluctuations, and some of the pulses are much more intense than the average. Therefore, the peak intensity on the SESAM becomes even higher, despite the larger mode area.