Low frequency pulsation in the VCSEL coupled to a long external cavity

The authors report an experimental study of the instability behaviour of a VCSEL coupled to a very long external cavity (L_ex =50 cm). The coated window of the TO packaged device used in this work was removed to allow a high level of optical feedback to be applied from an external high quality mirror. A significant change in the threshold current was introduced by the external cavity, and a kink in the L-I characteristic was observed above the threshold current region of the solitary laser. Within the kink region of the L-I curve, low frequency pulsation (LFP) was observed which is different from the feedback-induced low frequency fluctuations (LFFs) noticed in conventional edge-emitting lasers. LFFs are usually attributed to the competition between the external cavity longitudinal modes, whereas UP observed in the present study can be attributed to competition between the orthogonal polarisation modes