Toward a Feedback-Insensitive Semiconductor Laser

We present a route toward design and fabrication of a conceptually new type of optical feedback insensitive integrated semiconductor ring laser with a weak optical isolator included in the ring. Although the approach has not resulted in the clear advantages that were aimed at, this detailed account of the process we believe is valuable to readers active in the field. After a summary of optical-feedback sensitivity in semiconductor lasers, we review various attempts toward reducing this sensitivity and discuss the feasibility of the concept of weak intra-cavity isolation. The design, model and theory of the novel integrated ring laser device are presented. The observed 5 dB of isolation and 3 dB increase of tolerance for external optical feedback demonstrate the feasibility of the intra-cavity isolation concept. However, the observed output performance of the single-mode laser, both with and without activated intra-cavity isolation, suggests the presence of a spurious intra-cavity reflection, strongly coupling the directional modes. Due to this coupling, the sensitivity to external optical feedback is comparable to that of a Fabry-Pérot type laser. Therefore, an alternate route is indicated toward a less complex laser cavity with potentially less intra-cavity reflection and better tolerance for external optical feedback.