Numerical simulation of optical Stark effect saturable absorbers in mode-locked femtosecond VECSELs using a modified two-level atom model.

The interaction of an optical pulse with a quantum well saturable absorber is simulated using a semi-classical two-level-atom model which has been modified to approximate spectral hole burning in the carrier distribution. Saturable absorption behaviour is examined in the limit where pulse duration approaches the carrier-carrier scattering time. For long pulses bleaching dominates the absorber response but as the pulse duration approaches the carrier-carrier scattering timescale an additional pulse shaping mechanism becomes active, allowing the absorber to continue to shorten pulses beyond the limit set by bleaching. Examination of the spectral and temporal absorption profiles suggests that intense pulses experience additional pulse shortening from the optical Stark effect.