Abstract
Optical frequency combs (OFCs) have greatly simplified optical frequency measurements with a 10‒11
relative uncertainty level. Sub-Doppler resolution spectroscopy, which resolves overlapped Dopplerbroadened
spectral lines, gives the absolute frequencies of resolved transitions using OFCs. It has also
been used as a spectroscopic source with a broad tunable range, a high accuracy of the frequency
determination, and a short data acquisition time. For example, dual-comb spectroscopy has been applied
to various fields such as the linear and nonlinear spectroscopy of atoms and molecules. We developed a
comb-referenced sub-Doppler resolution infrared spectrometer and recently applied it to Stark
spectroscopy of methane. Methane molecules are usually considered nonpolar, but small permanent
electric dipole moments are induced by the vibrational and rotational motions. We recorded 20
transitions of the ν 3 band and accurately measured the Stark shifts. The electric dipole moments were
precisely determined by considering the vibration-rotation interactions.
