Spatially Filtered Feedback for Mode Control in Vertical-Cavity Surface-Emitting Lasers

Abstract

Transverse mode control of a vertical-cavity surface-emitting laser (VCSEL) is achieved by spatially filtered optical feedback. The spatial filter consisted of a pinhole, either 42 ?m or 81 ?m, in the 5.8 × magnified VCSEL image plane of a confocal lens arrangement. The pinhole was translated to map out the voltage drop across the VCSEL from spatially-selective feedback. For low injection currents, spatial filtered feedback enabled single fundamental mode operation due to higher-order mode suppression by appropriately locating the pinhole. For higher injection currents with five laser modes, spatial selection of feedback enabled control of the relative mode intensities and wavelengths. The maximum increase in mode intensity achieved was 1.8 × for the 4 ?m pinhole and 2.4 × for the 81 ?m pinhole. The maximum frequency shift achieved was - 2.7 GHz and - 7.1 GHz with the 42 ? and the 81 ?m pinholes. The spatially selective optical feedback produced long-lived changes to the VCSEL even after the feedback was blocked-results for two separate trials where the recovery time was 38 minutes are presented (although results varied for different bias and feedback conditions).