Beam propagation study of on-wafer multiple-defect photonic crystal VCSEL arrays

Abstract

Vertical-emitting laser arrays operating in a coherently coupled regime offer the potential for high brightness and lowpower. Previous work focused on quantifying the beam propagation factor of on-wafer single-emitter photonic crystal vertical cavity surface emitting laser (PCSEL) devices for comparison with typical measured values of the spectral linewidth and side-mode suppression ratio. Expanding on this work, here we report on a novel method of characterizing the beam propagation factor for 2x1 multiple-defect coherently coupled PCSEL arrays. First, the on-wafer 2x1 PCSEL arrays were characterized to determine the range of injection currents that produced a coherently or incoherently coupled output using a 3-D power map. Both operating regions are explored here. After measuring the spectrum, the beam profiles were captured using a vertically mounted beam profiling system. Each individual laser in the 2x1 array was first operated and characterized independently. The device was then characterized operating in an incoherent and coherent coupled mode, respectively. The beam propagation factor, or M2, was calculated for each set of data using a weighted least-squares curve fit and in accordance with the ISO Standard 11146. As expected, the individual lasers making up the 2x1 array produced near-Gaussian beam profile with M2 close to 1. With both laser elements operating, regardless of the state of coherence, the output beam adopted an asymmetry and the M2 value increased predominately on the lateral axis. In this effort, a parametric study of the beam propagation factor of devices emitting near 850nm is presented.