Abstract
VCSELs have major applications in high-speed data- and tele-communications such as local/metro area networks, optical interconnects and free-space optical links. Even though VCSEL was invented more than three decades ago and rapidly entered into system applications, a method of mass producing monolithic, polarization stable VCSELs at industry scale is still on-demand. Unfortunately, due to the high symmetry of VCSEL along its optic (emission) axis, VCSELs do not have intrinsic polarization stability. As the injection current increases above threshold, polarization randomly switches between the two orthogonal directions and leads to relative intensity noise and degrades the bit error rates during signal transmission. Until now polarization mode in VCSELs has been stabilized by growing VCSELs on high-index and off-angled substrates; introducing single/multiple asymmetric shapes in active and current aperture layers; growing asymmetric quantum nanostructures in active layers; introducing stress externally on the top DBRs; fabricating 1D shallow surface, high-contrast sub-δ gratings and integrating with SP elements on the topmost layer of VCSEL; introducing photonic nanostructures into VCSEL cavities, and so on. Our current review not only focuses on successful demonstrations and patents in the past but also identifies suitable methods for mass production of polarization stable VCSELs for practical applications.
Keywords: Birefringence, in-plane anisotropy, polarization control, Semiconductor laser, Surface emitting laser, VCSEL, strained quantum well, Spin-Flip Model, Polarization Switching
6