Direct phase-locking of a 8.6-mu m quantum cascade laser to a mid-IR optical frequency comb: application to precision spectroscopy of N2O

We developed a high-precision spectroscopic system at 8.6 mu m based on direct heterodyne detection and phaselocking of a room-temperature quantum-cascade-laser against an harmonic, 250-MHz mid-IR frequency comb obtained by difference-frequency generation. The similar to 30 dB signal-to-noise ratio of the detected beat-note together with the achieved closed-loop locking bandwidth of similar to 500 kHz allows for a residual integrated phase noise of 0.78 rad (1 Hz-5 MHz), for an ultimate resolution of similar to 21 kHz, limited by the measured linewidth of the mid-IR comb. The system was used to perform absolute measurement of line-center frequencies for the rotational components of the nu(2) vibrational band of N2O, with a relative precision of 3 x 10(-10). (C) 2015 Optical Society of America