GaAs wafer for passive mode locking and compression of energetic Q-switched pulses

We show experimentally that by use of an appropriately coated GaAs wafer as output coupler, either self-started passive mode locking or pulse width compression of Q-switched pulses of diode-pumped solid-state (DPSS) lasers can be achieved. Mode-locked pulses with duration in the picosecond time scale have been routinely generated in various Nd-doped DPSS lasers. Pulse width compression of the Q-switched pulses by a factor of more than three has been obtained. We have experimentally investigated the physical origin of the observed phenomena. It was suggested that they could be caused by the intensity-dependent cavity losses caused by the free-carrier-induced nonlinear refractive index grating in the wafer.