Analysis of Operating Regimes of Terahertz Quantum Cascade Laser Frequency Combs

In recent years, quantum cascade lasers (QCLs) have shown tremendous potential for the generation of frequency combs in the mid-infrared and terahertz portions of the electromagnetic spectrum. The research community has experienced success both in the theoretical understanding and experimental realization of QCL devices, capable of generating stable and broadband frequency combs. Specifically, it has been pointed out that four wave mixing (FWM) is the main comb formation process and group velocity dispersion (GVD) is the main comb-degradation mechanism. As a consequence, special dispersion compensation techniques have been employed, in order to suppress the latter and simultaneously enhance the former processes. Here, we perform a detailed computational analysis of FWM, GVD, and spatial hole burning (SHB), all known to play a role in QCLs, and show that SHB has a considerable impact on whether the device will operate as a comb or not. We therefore conclude that for a successful implementation of a quantum cascade laser frequency comb, one would need to address this effect as well.