Comb-calibrated sub-Doppler spectroscopy with an external-cavity quantum cascade laser at 7.7 μm.

We study the frequency noise and the referencing to a near-infrared frequency comb of a widely tunable external-cavity quantum-cascade-laser that shows a relatively narrow free-running emission linewidth of 1.7 MHz. The frequency locking of the laser to the comb further narrows its linewidth to 690 kHz and enables sub-Doppler spectroscopy on an N ₂ O transition of the ν ₁ band near 7.7 μm with sub-MHz resolution and absolute frequency calibration. The combined uncertainty on the measured transition center is estimated to be less than 50 kHz.