Coherent frequency combs produced by self frequency modulation in quantum cascade lasers

One salient characteristic of Quantum Cascade Laser (QCL) is its very short tau similar to 1 ps gain recovery time that so far thwarted the attempts to achieve self-mode locking of the device into a train of single pulses. We show theoretically that four wave mixing, combined with the short gain recovery time causes QCL to operate in the self-frequency-modulated regime characterized by a constant power in time domain and stable coherent comb in the frequency domain. Coherent frequency comb may enable many potential applications of QCL's in sensing and measurement. (C) 2014 AIP Publishing LLC.