Harmonic comb generation in mid-infrared quantum cascade laser frequency combs by external cavity optical feedback

We demonstrate the generation of harmonic comb states in mid-infrared quantum cascade laser frequency combs by optical feedback injection from an external cavity. A Vernier-like method is presented to control the order of generated harmonic states up to the 6th order. In addition to increased repetition rates, such external cavity induced harmonic states also offer increased optical bandwidth and higher comb coherence. This scheme is easy to implement in spectroscopic systems based on quantum cascade laser frequency combs that can take advantage of these improved parameters.