On mode coupling and low-frequency fluctuations in external-cavity laser diodes

Abstract

The role which mode coupling plays in determining the oscillating mode of a semiconductor laser and ensuring its stability is illustrated for the case of an external-cavity laser diode. We show that the tendency of a laser to oscillate in the mode with lowest threshold gain is counteracted by self-stabilization due to mode coupling. For an external-cavity laser at low bias current and with moderate to strong optical feedback the occurrence of low-frequency fluctuations (LFF) can be seen as a consequence of a reduced self-stabilization effect. This picture supports our previously developed description of LFF as bistable switching followed by iterative shifts of the laser spectrum towards the mode with lowest threshold gain. The description is shown to agree with measurements of the time evolution of the laser spectrum during LFF.