Buried Subwavelength Gratings for Polarization Pinning of Bottom-Emitting GaAs VCSELs

We demonstrate bottom-emitting GaAs vertical-cavity surface-emitting lasers (VCSELs) incorporating a buried subwavelength grating in the top, highly reflective distributed Bragg reflector (DBR), with oxide aperture diameters ranging from 2 to 6. The buried grating successfully pins the polarization for aperture diameters up to 4 μm, and the 3 μm aperture VCSELs achieve single-mode, single-polarization operation, delivering 3.9 mW of output power with a side-mode suppression ratio exceeding 30 dB and an orthogonal polarization suppression ratio (OPSR) of 20 dB. Simulations and measurements indicate that the buried surface-etched grating can only weakly influence the lasing mode in the bottom-emitting configuration, thereby preventing polarization pinning of higher-order modes in larger-aperture VCSELs. The surface-etched gratings introduce minor degradation in threshold current, slope efficiency, and beam profile.