Freely controllable single-optical-frequency comb for highly sensitive cavity ring-down spectroscopy.

Direct comb spectroscopy is a powerful tool for obtaining highly accurate spectroscopic information. However, because the number of comb modes is extremely large, the optical energy is distributed over many modes, resulting in ultrasmall optical power per comb mode and limiting the sensitivity of spectroscopic measurements. Here we demonstrate a freely controllable optical frequency comb based on the spectral peak phenomenon. Comb modes overlapping the CH₄ absorption spectrum are transformed into background-suppressed spectral peaks using a CH₄ gas cell and a nonlinear loop mirror. The generated comb is coherently amplified using a fiber Raman amplifier. A single comb mode is then extracted using a newly developed spectral filter with an ultrahigh resolution of 165 MHz. The maximum optical power of a single comb is estimated to be more than 10 mW. Using the selected comb mode, the ring-down decay signal from a high-finesse optical cavity is measured. The 2ν₃ band of CH₄ is accurately measured by comb-mode-resolved cavity ring-down spectroscopy with a sensitivity of 4.2 × 10