Multifocal Resonator Configurations for Ker-Leus Mode-Locking

Kerr-lens mode-locking (KLM) has become one of the most popular mode-locking schemes employed in ultrafast solid-stale lasers. The incorporation of both gain and mode-locking mechanisms within the active crystal, as in a classical four-mirror X-folded cavity, however, puls contradictory requirements onto the beam parameters inside the active medium. To achieve strong KLM action, the resonator should be close to the instability region, the beam must be lightly focused, and the active medium displaced relative to the beam waist. These requirements have to be traded off in order to achieve good laser action Due to its unique combination of properties, Ti:sapphire remains the only laser medium which offers satisfactory performance in a classical KLM scheme. In order to use KLM with other active media which may possess smaller nonlinearity, gain or damage threshold, or with other pumping schemes, not supporting tight focussing, an alternative scheme is to separate gain and KLM action. The separation of mutually contradicting requirements provides sufficient degrees of freedom, which may be the only way to realize the KLM scheme [1], or at least allow to optimize the results [2]. Although this idea has already been realized experimentally yielding promising results, the lack of comprehensive multifocal resonator analysis did not permit wider usage so far.