Comb-assisted subkilohertz linewidth quantum cascade laser for high-precision mid-infrared spectroscopy

We report on the linewidth narrowing of a room-temperature mid-infrared quantum cascade laser by phase-locking to a difference-frequency-generated radiation referenced to an optical frequency comb synthesizer. A locking bandwidth of 250 kHz, with a residual rms phase-noise of 0.56 rad, has been achieved. The laser linewidth is narrowed by more than 2 orders of magnitude below 1 kHz, and its frequency is stabilized with an absolute traceability of $2×10−12$⁠. This source has allowed the measurement of the absolute frequency of a CO₂ molecular transition with an uncertainty of about 1 kHz.