A theoretical analysis of polarisation-bistable vertical-cavity surface-emitting lasers used as Doppler velocimeters

This work presents a theoretical analysis that can explain the increase in sensitivity of a vertical-cavity surface-emitting laser (VCSEL) used as a Doppler velocimeter. The authors use an intensity rate equation model, consisting of two photon density equations (one for each polarisation mode) and one charge carrier equation. Effects of self- and cross-saturation between the modes are included, giving rise to a small region of bistability around the polarisation switching point. The Doppler-shifted optical feedback is introduced in the equations following the standard Lang-Kobayashi approach. When the VCSEL is biased inside the bistable region and when the induced modulation amplitude exceeds a critical level proportional to the width of the bistable region, the laser switches periodically between the two polarisation modes. In this way the polarisation resolved output shows a much better signal-to-noise ratio as the switching is between the on and off state of the selected mode.