Spatially Resolved Characterization of the Coherence Area in the Incoherent Emission Regime of a Broad-Area Vertical-Cavity Surface-Emitting Laser

We present direct measurements of the spatial coherence area of a pulsed broad-area vertical-cavity surface-emitting laser using a reversing wavefront interferometer. With this technique, we can assess the size and uniformity of the coherence area across the laser aperture, being of importance for projection applications. We show that the output beam can be considered quasi-homogeneous and that the measured coherence area corresponds well with the coherence area deduced from the far-field emission profile. We demonstrate that the coherence area is limited in size by the radial temperature gradient in the device and discuss the origin of coherence variations.

I. Introduction

Vertical-Cavity surface-emitting lasers (VCSELs) find their use nowadays in a number of applications such as optical data communication and sensing. If required, these lasers can provide a single-mode circular beam at low cost. To obtain such a high-quality beam, it is necessary to restrict the transverse size of the cavity to a few micrometers. The small transverse size will limit the available output power to a few milliwatts typically [1]. This low output power level makes small-aperture VCSELs unsuitable for a number of applications such as illumination, projection, and printing. Then, the output power can easily be increased by raising the transverse size of the cavity [2]. This comes, however, at the expense of having a much reduced beam-quality: because of the large Fresnel number of such broad-area VCSELs (BA-VCSEL), multiple competing transverse modes will be emitted [3], [4].