Optimization of polarization-stable single- and multi-mode surface grating VCSELs towards high fabrication tolerance and superior performance

Abstract

Monolithically integrated surface gratings have proven to control the polarization of vertical-cavity surfaceemitting lasers (VCSELs) very reliably and effectively. To overcome the drawbacks of these devices with respect to threshold current or differential quantum efficiency we will present in this paper monolithically integrated surface gratings with a modified longitudinal position relative to the standing wave inside the laser cavity. It turns out that an optimum is reached for an additional quarter-wave layer on top of the upper Bragg mirror. In that way the diffraction from the grating and therefore its influence on the other laser properties like threshold, differential quantum efficiency or maximum output power can be reduced significantly, while the polarization control is still maintained. At the same time the requirements on the fabrication accuracy are very relaxed. 118 out of 120 fabricated devices with grating periods between 0.5 and 1.2 µm and grating depths between 35 and 105 nm exhibit a stable polarization orthogonal to the grating grooves. If one limits the lateral extension of the grating to a diameter smaller than the oxide aperture of the laser, the single-mode output power can be increased simultaneously.