  One salient characteristic of Quantum Cascade Laser (QCL) is its very shor
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 in sensing and
measurement.
