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We report a strong reduction of spatial coherence of the emission of large aperture vertical-cavity surface-emitting lasers when they are driven by microsecond electrical pulses. We give evidence that this is due to a breakdown of the modal emission of these lasers. The spatial decoherence manifests itself in the formation of a Gaussian far field intensity distribution. The coherence radius we extract is 1.4 micrometer under these operating conditions, irrespective of the Fresnel number of the vertical-cavity surface-emitting laser. Finally, the spatial coherence properties can be varied by changing the pulse duration or pulse amplitude.

We demonstrate the control of the spatiotemporal emission dynamics of a broad-area semiconductor laser in an external optical feedback configuration formed by a spatially filtering mirror. The emission properties are studied with a single-shot streak camera with temporal resolution of approximately 7 ps and spatial resolution of approximately 5 microm. Our results show a significant reduction of the spatial filamentation and, furthermore, suppression of the spatiotemporal instabilities, which are both intrinsic emission properties of standard high-power broad-area lasers. Associated with the control of the emission dynamics, strong improvement of the beam quality, which is essential for numerous high-power applications, is possible.

Broad-area vertical-cavity surface-emitting lasers (BA-VCSELs) can exhibit a state of spatially incoherent emission, as we recently reported in [M. Peeters et al., Opt. Express, 13, 9337 (2005)]. Here, we experimentally study the evolution of a BA-VCSEL under pulsed operation from well-defined modal emission with a multitude of transverse cavity modes to such spatially incoherent emission. The transition is studied using a high-speed intensified CCD camera and differential image analysis with which single-shot measurements of the imaged nearfield, farfield, spatial coherence, and spectral emission properties are acquired. This combination of experimental characterization tools allows for a detailed description of the BA-VCSEL's emission behavior, which is necessary for an in-depth understanding of the processes involved. We find the interplay between the thermal chirp and the build-up of a spatially distributed thermal lens to be decisive for the break-up of the global cavity modes.

Abstract-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.