Precise frequency-difference measurement between the 166-µm transitions of methane

We determined 66 frequency differences (FD’s) between rovibrational lines of methane at the $1.66-\mathit{\mu }\mathrm{m}$ region. Following the technique developed by Nakagawa et al. [Opt. Lett. 20, 410 (1995)], we measured the FD’s as the optical beat frequency between two external-cavity diode lasers locked at 1-MHz-wide saturated absorption dips of the methane lines. Even though the methane lines often overlap in Doppler-limited resolution, the spectrometer that we use resolves them and determines their FD’s with better than 40-kHz precision. This fact demonstrates that the methane lines are promising candidates for frequency reference and that this technique has great potential for high-resolution molecular spectroscopy.