Widely tunable harmonic frequency comb in a quantum cascade laser

Self-starting harmonic frequency combs in quantum cascade lasers exhibit skipping of several tens of longitudinal modes of the cavity, producing widely spaced frequency combs which may be used for a number of applications, such as the generation of high-spectral-purity microwave and terahertz tones. Under pure electrical injection, the spacing of such combs is fixed by fundamental laser parameters and can hardly be controlled. Here, we demonstrate that harmonic frequency combs in quantum cascade lasers can be induced by optical injection of an external seed provided by a tunable source. This scheme enables wide tunability of the harmonic comb spacing, allowing the skipping between 44 and 171 longitudinal modes in a single device. Published by AIP Publishing.