Longitudinal scaling of VECSEL output power maintaining narrow linewidth

Abstract

Output power of VECSELs had been scaled by lateral scaling to tens of watts and beyond. Longitudinal scaling, employing multiple VECSEL devices in a single resonator, has the potential to scale up the power as well. However, some of the devices need to be placed at a fold of the resonator and inherently suffers from the spectral instability. The standing wave pattern created at a fold of a standing wave cavity exhibits that of 4-wave interference, and the resulting pattern shows high contrast modulation in the plane of the quantum well. The phase of that modulation depends on the phase relationship between the forward and backward beams, which differs for different longitudinal modes. This results in a situation similar to the special hole burning effect in solid-state lasers in which case the standing wave pattern is in the longitudinal direction. Because of the resonant periodic gain structure, VECSELs do not suffer from spatial hole burning if the device is placed at the end of the standing-wave cavity and single-frequency operation can be obtained relatively simply. This no longer holds when the VECSEL device is placed at the fold of a standing wave cavity. Twisted-mode configuration addresses this and allows narrow-linewidth or single-frequency operation of multi-device VECSELs. By having forward and backward modes in oppositely rotating circular polarization, the standing wave pattern does not show modulation in the planes of quantum wells, recovering the advantage of resonant periodic gain.