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

Abstract

We developed a high-precision spectroscopic system at 8.6 μm based on direct heterodyne detection and phase-locking of a room-temperature quantum-cascade-laser against an harmonic, 250-MHz mid-IR frequency comb obtained by difference-frequency generation. The $\sim 30\mathrm{dB}$ signal-to-noise ratio of the detected beat-note together with the achieved closed-loop locking bandwidth of $\sim 500\mathrm{kHz}$ allows for a residual integrated phase noise of 0.78 rad (1 Hz–5 MHz), for an ultimate resolution of $\sim 21\mathrm{kHz}$, 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 ${\mathrm{N}}_2\mathrm{O}$, with a relative precision of $3\times {10}^{-10}$.

© 2015 Optical Society of America

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