Absolute frequency measurement of molecular transitions by a direct link to a comb generated around 3-µm

A 3-¼m continuous-wave difference-frequency source is directly referenced to a mid-infrared optical frequency comb synthesizer by measuring their beat-note signal by a fast HgCdTe detector. Absolute frequency metrology of molecular vibration spectra is demonstrated by locking the 3-¼m coherent radiation to the nearest comb tooth and tuning the comb mode spacing across the Doppler-broadened absorption profile of a CH4 ro-vibrational transition.