Spatial decoherence of pulsed broad-area vertical-cavity surface-emitting lasers

Abstract

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.

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

“…We show that the coherence area is much smaller than the VCSEL aperture and that the intensity is uniform over the scale of the coherence area. This further substantiates the conjecture put forward in [5] that a BA-VCSEL can be operated as a quasi-homogeneous source. We also show that the spatially and intensity weighted average of the measured coherence radius corresponds well with the coherence radius deduced from the far-field divergence angle.…”

“…With this knowledge, we can now make the link between the complex degree of coherence discussed above and the measurements of the far-field profile discussed in [5]. For a similar device and driving conditions as described in this paper, we reported in [5] that the BA-VCSEL no longer shows a structured modal far-field.…”

“…In our measurements, we use an oxide confined BA-VCSEL with an aperture radius of 25 m, similar to the device used in [5]. The BA-VCSEL emits at a wavelength of 840 nm, with a maximum CW output power of 70 mW at an injected current of 80 mA.…”

“…In our experiments, we drive the VCSEL with a pulsed current of 240-mA amplitude, 1-s pulse length, and a duty cycle of 0.5%. These pulse conditions induce the nonmodal, incoherent emission regime [5].…”

“…Besides emission in multiple transverse modes, we have shown [5] that one can obtain a drastically different emission regime with BA-VCSELs by driving these devices with s-pulses. Using the proper pulse amplitude, the BA-VCSEL will no longer emit globally defined transverse modes but instead it will emit a nonmodal, spatially incoherent beam.…”