Mode-Locking of VECSELs by Crossed-Polarization Gain Modulation

We analyze in detail an approach for inducing passive mode-locking (ML) in vertical-external-cavity surface-emitting lasers (VECSELs) without using any saturable absorber but directly exploiting the lightwave polarization degree of freedom. Part of the VECSEL output, which is forced to be linearly polarized, is reinjected into the optical amplifier after a delay time and rotating its polarization state by 90^deg. In the active medium, both polarizations interact via cross-gain modulation. A comprehensive time-domain model including polarization and carrier spin dynamics is derived from fundamental principles. The model is used to evaluate the feasibility and robustness of the ML scheme. We obtain mode-locked pulses of ~ 10-ps duration at repetition rates up to 20 GHz. The reinjection delay time defines resonance conditions where the maximum power and shortest pulsewidth are obtained. Stable fundamental ML is surrounded by regimes of harmonic and incomplete ML. A transition leading from self-pulsation to ML is obtained for sufficiently long reinjection delay times.