Carrier-dependent nonlinear lens in a VECSEL gain medium

Vertical-external-cavity surface-emitting lasers (VECSELs) modelocked using saturable absorbers have become an established means of generating few hundred femtosecond optical pulses at Watt-level output powers and gigahertz repetition rates [1,2]. More recently there have been reports of self-modelocked VECSELs which do not use saturable absorbers, instead relying on nonlinear lensing in the laser gain sample as the necessary intensity dependent effect [3]. Design of these lasers is challenging as calculating the value of the nonlinear refractive index of a VECSEL gain sample is made complex by the fact that several physical effects contribute to it, including changes in carrier density, carrier thermalization, the Kerr effect and others. Given this complexity, calculation of accurate values for use in laser design is a difficult problem