Semiconductor ring laser frequency combs with active directional couplers

TL;DR

This paper presents electrically pumped semiconductor ring lasers with integrated active couplers that generate high-power frequency combs and allow for coherent control and advanced studies of resonant electromagnetic effects.

Contribution

It introduces a novel integrated ring quantum cascade laser design with active directional couplers, enabling high power, self-starting frequency combs and external signal injection for coherent control.

Findings

Output power exceeds ten milliwatts at room temperature.

Ability to control mode indices, quality factors, and coupling coefficients.

Demonstration of parametric amplification and four-wave mixing in the device.

Abstract

Rapid development of Fabry-Perot quantum cascade laser frequency combs has converted them from laboratory devices to key components of next-generation fast molecular spectrometers. Recently, free-running ring quantum cascade lasers allowed generation of new frequency comb states induced by phase turbulence. In absence of efficient light outcoupling, ring quantum cascade lasers are not suited for applications as they are limited in their power output to microwatt levels. Here we demonstrate electrically pumped ring quantum cascade lasers with integrated active directional couplers. These devices generate self-starting frequency combs and have output power above ten milliwatts at room temperature. We study the transmission of the ring-waveguide resonator system below the lasing threshold, which reveals the ability to individually control the mode indices in the coupled resonators, their quality factors, and the coupling coefficient. When the ring resonator is pumped above the lasing threshold, the intracavity unidirectional single-mode field parametrically amplifies an externally injected signal tuned into one of the ring resonances, generating an idler sideband via four-wave mixing. The ability to inject external optical signals into integrated laser cavities brings into reach coherent control of frequency comb states in ring semiconductor lasers. Furthermore, tunable coupled active resonators pumped below the lasing threshold enable a versatile platform for the studies of resonant electromagnetic effects, ranging from strong coupling to parity-time symmetry breaking.