Transition to spatially incoherent emission of a broad-area VCSEL: evolution of beam profiles, spectra and coherence properties

Broad-area vertical-cavity surface-emitting laser (BA-VCSELs) can be made to emit spatially incoherent light by driving them with strong current pulses, as we demonstrated in [1]. This state of incoherent emission is of fundamental importance as it is not readily observable in semiconductor lasers. It can be useful in, for example, projection systems where a low degree of spatial coherence might help to reduce the speckle-contrast. How the gradual disappearance of the modal structure takes place and how the coherence properties change during the transition is an important question that has not yet been studied in detail. To confirm the assumption of non-modal emission and to identify the key processes involved in the emanation of this state, we have experimentally investigated how the BA-VCSEL's emission properties evolve during the transition from modal to non-modal emission and back again. In this study we focus on the evolution of the near-field and far-field profile, the emitted spectrum and the near-field spatial coherence.