Resonant measurements of nonlinear lensing in a VECSEL gain sample

Abstract

In recent years there have been several reports describing self-modelocking (SML) in vertical-external-cavity surfaceemitting lasers (VECSELs). Some of these reports have suggested that the behaviour that has been observed results from nonlinear lensing in the VECSEL gain sample in a manner analogous to Kerr lens modelocking in solid state lasers. However to date none of the groups that have reported SML in VECSELs have performed measurements to ascertain whether nonlinear lensing occurs in the gain sample. Measurements of nonlinear lenses in VECSEL gain samples are therefore of value not only in order to understand the behaviour observed in the reports of SML, but also as a potentially crucial design tool for any mode-locked VECSEL, regardless of the modelocking method used. In a previous publication we described measurements which demonstrated that a defocusing nonlinear lens was present in an unpumped VECSEL gain sample, and that the inverse focal length of the lens increased with pump power, to the point of becoming a focusing lens for sufficiently high pump powers. Unfortunately, by necessity this measurement was performed using a probe laser which was not resonant with the quantum wells in the sample, meaning that the values measured may well be different from those experienced under operating conditions in a VECSEL. This paper describes a more complete characterisation of VECSEL gain sample nonlinear lensing with a probe laser whose wavelength is resonant with the gain sample quantum wells.