Detection of atmospheric methane is based on a huge amount of fundamental data provided by high-resolution spectroscopy in the infrared region. Doppler-limited and sub-Doppler resolution spectroscopy of methane have been discussed in particular for the 3.3 and 1.67μm band transitions. To attain sub-Doppler resolution in the 1.67μm band transitions, we employed a Fabry Perot cavity as an absorption ce11. This enabled us to observe nonlinear saturation spectra with a spectral resolution of about 1 MHz even using the weak optical field from a laser diode, because the optical field strength in the cavity is greatly enhanced at the antinodes of the optical standing wave. We precisely measured small Stark shifts of seven tetrahedral components of the vibration rotation transitions, sixty-six difference frequencies between the components, and the absolute frequency of two transitions. To enhance the sensitivity, noise-immune cavity-enhanced optical- heterodyne molecular spectroscopy (NICE-OHMS) was also applied.
