Amplified feedback and spontaneous emission injection in quantum cascade ring laser systems

Ring lasers exhibit rich operational regimes such as unidirectional, bidirectional, or bistable operation. The two-mode dynamics of the counter-propagating modes - clockwise (CW) and counterclockwise (CCW) and their selection have gained attention in the mid-infrared; however, the underlying switching mechanism has been largely unexplored. Previously, we experimentally demonstrated robust and deterministic mode selection in a ring quantum cascade laser (QCL) with an active outcoupling waveguide. Here, we apply the Lang-Kobayashi framework to our system to model the effects of spontaneous emission in the waveguide arms as well as amplified optical feedback from the facets on the mechanics of mode switching. We find that coherent feedback from facet reflections agrees upwards of 93% with the experimental behavior, indicating that amplified feedback is the dominant mechanism driving the mode selection dynamics.