Dynamics of the Angular Emission Spectrum of Large-Area VCSELs

High power VCSELs can be realized by scaling up the active area of bottom-emitting devices. This results in a large Fresnel number of the laser cavity. The laser beam cannot be described with Gauss modes in a simple way anymore, but is best described in terms of tilted plane waves, called Fourier modes. The beam quality and mode spectra depending on the applied current and the temperature of the VCSEL are investigated. Two-dimensional measurements of the near and the far field are combined with power and spectral measurement to characterize the VCSEL. Polarization and Fourier filtering are used to examine the spatially-dependent emission in detail. A rich dynamic in the angular emission profile for large-area VCSELs is observed and can be explained by considering the residual reflections from the AR-coated substrate-air interface and thermal effects. The presented theoretical model simulates the dynamics of the angular emission. The calculated angular and spectral profiles match the experimental observations very well over the whole parameter range. The influence of the active area is studied for diameters of the oxide aperture from 20 up to 300 mu m. For smaller diameters diffraction effects become more dominant, the Fresnel number is reduced and the emission spectrum gets closer to the Gauss mode description.