Full stabilization of a mid-infrared quantum cascade laser frequency comb with amplitude-modulated radiofrequency injection

Full stabilization of a mid-infrared quantum cascade laser (QCL) frequency comb referenced to a single-frequency QCL is achieved using external radio frequency (RF) injection as the sole actuator. The comb repetition frequency is locked by the frequency of the injected RF signal and the comb offset frequency is controlled by modulating the amplitude of the injected RF signal. This enables simultaneous stabilization of the two fundamental parameters that define a frequency comb. We achieve over an order of magnitude improvement in the stability of both the repetition and offset frequencies. This chip-scale semiconductor frequency comb, together with the externally controlled frequency stability demonstrated in this work, paves the way for compact mid-infrared photonic systems for various spectroscopic applications.