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

Abstract

We determined 66 frequency differences (FD's) between rovibrational lines of methane at the 1.66-mum 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.

Comb coherence-transfer and cavity ring-down saturation spectroscopy around 1.65 μm: kHz-accurate frequencies of transitions in the 2ν3 band of 12CH4

“…65,66 To the best of our knowledge, the first measurements of sub-Doppler saturated absorption (Lamb dips) in the 2n 3 band of methane were reported in the 1990s by the group of Sasada of Keio University in Yokohama (Japan). [68][69][70] Optical frequency combs were not available at that time and precise frequency differences between 2n 3 transitions were obtained as the optical beat frequency between two external-cavity diode lasers locked at 1 MHz-wide saturated absorption dips of the methane lines. A number of frequency differences mostly between Q-branch transitions of the 2n 3 band were reported with a precision better than 40 kHz.…”

Highly Sensitive Cavity-Enhanced Sub-Doppler Spectroscopy of a Molecular Overtone Band with a 1.66 µm Tunable Diode Laser

“…Another frequency modulation is applied to NICE-OHMS by the EOM at 478.5 MHz, which is equal to the FSR of the FP cell, and the modulation index is 0.092, which is limited by the small gain of the rf-amplifier used. A sawtooth voltage is applied to the PZT to sweep the frequency of the cavity mode, while the laser frequency follows it through the fre-sitivity of the previous spectrometers [6][7][8][9][10][11][12][13] does not induce any noise in the FM signal. They observed overtone transitions of C 2 HD, C 2 H 2 , and CO 2 with a stable 1.064 µm Nd : YAG laser 16) and obtained a spectral resolution of 70 kHz.…”

“…7-10) Following them, Suzumura and co-workers observed the saturation spectrum of the 2ν 3 band of methane in the 1.66 µm region, 11) and conducted sub-Doppler resolution spectroscopy. 12,13) The technique [6][7][8][9][10] is also sensitive because of its large absorption length. However, even a small frequency difference between the laser and the cavity mode is converted into a large amplitude noise of the radiation intensity transmitted through the FP cell because the high-finesse cavity works as a sensitive frequency discriminator.…”