Colliding pulse mode locking of vertical-externalcavity surface-emitting laser

High-power semiconductor disk lasers producing short pulses at high repetition rates are attractive for numerous applications. The peak power achievable is often limited by the stability of the mode-locked regime as harmonic mode locking or nonstationary pulsed operation emerges at high pump powers. In this Letter, we present a new passive and robust mode-locking scheme for a vertical-external-cavity surfaceemitting laser (VECSEL). We placed the semiconductor gain medium and the semiconductor saturable absorber mirror (SESAM) strategically in a ring cavity to provide stable colliding pulse operation. With this cavity geometry, the two counterpropagating pulses synchronize on the SESAM, saturating the absorber together, which minimizes the energy lost and creates a transient carrier grating due to the interference of the two beams. The interaction of the two counterpropagating pulses is shown to extend the range of the mode-locking regime and to enable higher output power when compared to the conventional VECSEL cavity geometry. In this configuration, we demonstrate a pulse duration of 195 fs with an average power of 225 mW per output beam at a repetition rate of 2.2 GHz, giving a peak power of 460 W per beam, establishing a new (to our knowledge) state of the art in term of pulse duration and peak power combination. The remarkable robustness of the mode-locking regime is discussed and a rigorous pulse characterization presented.