MEMS tunable VCSEL with polarization selected by sub-wavelength pitch grating

A sub-wavelength grating has been added to an optically-pumped MEMS tunable VCSEL structure to control polarization. This "P_VCSEL" is a thermocompression-bonded sandwich of a 1/2-VCSEL with gain and semiconductor DBR attached to a silicon MEMS mirror on an electrostatically actuated membrane for wavelength control. The sub-wavelength pitch grating is etched into the surface of the 1/2-VCSEL. Optical layer stack simulations have accurately predicted the tuning behavior of the P_VCSELs but not the polarization behavior. The polarization selection does not occur through threshold gain asymmetries as originally believed, but rather through polarization-dependent loss in the grating. We believe the rough sidewalls of the grating teeth are responsible for the polarization-dependent loss. While the selected-against polarization loss is very high, the selected-for polarization loss is still higher than desirable. This reduces power output, especially at short wavelength. However, 91 nm tuning in the 1050 nm band was achieved, suitable for application in optical coherence tomography.