PRECISE FREQUENCY-DIFFERENCE MEASUREMENT BETWEEN THE 1.66-MU-M TRANSITIONS OF METHANE

We determined 66 frequency differences (FD's) between rovibrational li nes of methane at the 1.66-mu m region. Following the technique develo ped by Nakagawa et al. [Opt. Lett. 20, 410 (1995)], we measured the FD 's as the optical beat frequency between two external-cavity diode las ers locked at 1-MHz-wide saturated absorption dips of the methane line s. Even though the methane lines often overlap in Doppler-limited reso lution, the spectrometer that we use resolves them and determines thei r 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. (C) 1997 Optical Society of America.