Feedback insensitive external-cavity semiconductor ring laser utilizing a weak intracavity isolator

External optical feedback (EOF), caused by the return loss of connected circuits, is well known to have a strong impact on the performance of semiconductor lasers [1]. Feedback effects are usually prevented by placing an optical isolator at the output of the laser. A strong isolation value of approximately 60dB is typically required for tunable lasers. It has proven difficult to implement sufficiently strong optical isolators in a form suited to photonic integration [2]. For this reason we have previously proposed a ring laser with weak intracavity isolation to enable a feedback insensitive integrated tunable laser [3]. In such a laser the isolator is used only to enforce unidirectional operation of the device and the returned signal couples in the lossy counter-propagating direction. Using our rate equation analysis it was predicted that 10dB intracavity isolation completely suppresses the effects of EOF over the operating range of interest in terms of output power, linewidth and relative intensity noise. In such a cavity, the weak isolation can now be implemented with known integration schemes [2].