Engineering the spectral bandwidth of quantum cascade laser frequency combs

TL;DR

This paper investigates methods to optimize and control the spectral bandwidth of quantum cascade laser frequency combs, combining numerical simulations and experiments to enhance their application in broadband spectroscopy.

Contribution

It introduces a comprehensive approach to maximize comb spectral width by analyzing nonlinear effects, dispersion, mirror losses, and injection locking techniques.

Findings

Optimal dispersion compensation enhances bandwidth.

High mirror losses promote comb sidemode proliferation.

Injection locking stabilizes and maximizes spectral width.

Abstract

Quantum cascade lasers (QCLs) facilitate compact optical frequency comb sources that operate in the mid-infrared and terahertz spectral regions, where many molecules have their fundamental absorption lines. Enhancing the optical bandwidth of these chip-sized lasers is of paramount importance to address their application in broadband high-precision spectroscopy. In this work, we provide a numerical and experimental investigation of the comb spectral width and show how it can be optimized to obtain its maximum value defined by the laser gain bandwidth. The interplay of nonoptimal values of the resonant Kerr nonlinearity and the cavity dispersion can lead to significant narrowing of the comb spectrum and reveals the best approach for dispersion compensation. The implementation of high mirror losses is shown to be favourable and results in proliferation of the comb sidemodes. Ultimately, injection locking of QCLs by modulating the laser bias around the roundtrip frequency provides a stable external knob to control the FM comb state and recover the maximum spectral width of the unlocked laser state.