Optimized integrated surface grating design for polarization-stable VCSELs

Monolithically integrated surface gratings have proven to control the polarization of single-mode and even multimode vertical-cavity surface-emitting lasers (VCSELs) very effectively. Unfortunately, up until now, the grating parameters have had to be known and realized very accurately for proper performance. The simulations and experimental results presented in this paper show in very good agreement that by changing the thickness of the cap layer of the VCSEL structure, the dependence of the polarization control on the grating parameters can be strongly reduced. With this modification, for multimode devices, we have achieved a stable polarization of all modes orthogonal to the grating grooves independent of the investigated grating period, the grating depth, and the orientation of the grating itself. The orthogonal polarization suppression ratio is, on average, 17.1 dB and exceeds 12 dB for 117 out of 120 highly multimode VCSELs. At the same time, the optimized layer design significantly reduces the diffraction in the far field, which occurs for grating periods larger than the emission wavelength of the laser