Amplified Feedback and Spontaneous Emission Injection in Quantum Cascade Ring Laser Systems

TL;DR

This paper investigates the mode switching mechanisms in quantum cascade ring lasers, highlighting the dominant role of amplified feedback and spontaneous emission effects through experimental validation and theoretical modeling.

Contribution

It introduces a model applying the Lang-Kobayashi framework to quantum cascade ring lasers, revealing the key role of amplified feedback in mode selection.

Findings

Coherent feedback from facet reflections matches experimental data over 93%

Amplified feedback is identified as the main driver of mode switching

Spontaneous emission effects significantly influence the laser dynamics

Abstract

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.