Tunable Frequency Comb Generation Using Quantum Cascade Lasers Subject to Optical Injection

We dissect the enabling capabilities of the tunable period-one (P1) limit cycles of optically injected quantum cascade laser (QCL) oscillators for the generation of optical frequency combs. As such, we theoretically investigate the P1 dynamics of a QCL using a single-mode rate equation model. We find that such a P1 limit cycle occupies a rather large and wide region of the optical frequency detuning and injection level ratio map. We have not recorded evidence of chaos in this injected laser system, in marked contrast with quantum well and quantum dot cases. Contrary to interband semiconductor lasers, the QCL's oscillation frequency is generally smaller than the detuning frequency, and is reduced with increasing injection strength, due to the strong injection pulling effect. When the optical injection is operated in the vicinity of the Hopf bifurcation, the P1 oscillation produces dense optical frequency combs, owing to both the frequency pulling effect and the four-wave mixing effect. The comb spacing is continuously tunable from subgigahertz up to a few gigahertz, via fine control of either the detuning frequency and/or the injection ratio. This novel approach of the frequency comb generation is of prime importance for high-resolution detection of narrow absorption lines of gas molecules.