Coherence radius and mode size of a broad-area vertical-cavity surface-emitting laser in the incoherent emission regime

We present experiments measuring the complex degree of coherence of a Broad-Area Vertical-Cavity Surface-Emitting Laser (BA-VCSEL) when it is driven into a regime of spatially incoherent emission. This high-power, spatially incoherent emission regime is quite uncommon for semiconductor lasers but; call be useful in e.g. illumination and projection systems, as the low degree, of spatial coherence may help to reduce speckle. The near-field coherence properties are measured for different positions in the VCSEL's aperture using a 180 degrees reversing-wavefront Michelson interferometer. We give evidence that the coherence area is much smaller than the VCSEL's aperture and that, the intensity fluctuations across the coherence area are small, therefore allowing the BA-VCSEL to be considered a quasi-homogeneous source. We explain the reason for the relatively small coherence radius of about 1.4 mu m based on the mode size in a planar cavity together with the thermal gradient within the VCSEL aperture.