Annular cavity VCSELs: enhanced beam quality and speckle-less illumination.

Vertical-cavity surface-emitting lasers (VCSELs) are valued for their high coupling efficiency, directionality, and low cost. Broad-area (BA)-VCSEL, in particular, offers the possibility of a customizable degree of spatial coherence and high output power for low-speckle imaging, illumination, communication, and augmented reality technologies. However, new designs of BA-VCSEL for scalable fabrication need to be developed. In this study, we present an annular cavity method for engineering optical characteristics and spatial coherence of VCSELs by altering the transverse shape of their cavities. Our experimental and simulation results reveal that annular VCSELs, with a doughnut shape, exhibit higher optical power density and lower threshold current due to more efficient utilization of gain medium compared to conventional BA-VCSELs. Notably, the annular VCSEL with a 20-µm radius central hole achieves a maximum optical power density of 1.09 kW/cm with a significant increase in external quantum efficiency (21.9%), outperforming the conventional BA-VCSEL (0.596 kW/cm and 16.1%). Spatial coherence analysis indicates that the designed annular VCSEL produces speckle patterns with lower intensity variations, enabling a higher number of mutually incoherent modes. The annular cavity design effectively achieves both low spatial coherence and Q-factor spoiling.