Old Research theme

Non-linear spectroscopy using an optical cavity as an absorption cell

Recent technical progress of dielectric coating has allowed us to construct a high-finesse optical cavity. We have applied it to non-linear spectroscopy of vibrationally excited states of molecules.

Demands for optical communications have driven development of high-performance diode lasers, which are also useful for spectroscopy. However, the output power is as small as a few mW, which is not large enough for non-linear spectroscopy of molecular overtone bands. To enhance the effective laser power, we have used the optical cavity as an absorption cell, and observed saturated absorption lines of the 2ν3 band of methane.1,2 　

Narrow spectral lines of atoms and molecules are usually used as precise frequency references. The figure below schematically shows an instrument for absolute frequency measurement of the 2ν3 band transitions of methane. We have measured the 12-THz difference frequency between the1.67-μm methane lines and the 1.54-μm acetylene line whose absolute frequency was previously measured. We have used an optical frequency comb generator to measure such large frequency differences.

Figure 1 shows schematic diagram of frequency difference measurements. Frequencies of two diode lasers are respectively stabilized at different saturated absorption lines, and the frequency difference between the lines is measured as a beat note with an accuracy of 40 kHz.3

List of publications

  1. K. Suzumura and H. Sasada, "Observation of saturation spectrum of the 2ν3 band of methane," Japanese Journal of Applied Physics, 34(12A), L1620-L1621 (1995).
  2. H. Sasada, K. Suzumura, and C. Ishibashi, "Coriolis-dependent Stark effect of the 2ν3 band of methane observed by saturated absorption spectroscopy," Journal of Chemical Physics, 105(20), 9027-9034(1996).
  3. K. Suzumura, C. Ishibashi, and H. Sasada, "Precise frequency-difference measurement between the 1.66-μm transitions of methane," Optics Letters, 22(17), 1356-1358 (1997).
  4. C. Ishibashi and H. Sasada, "Highly sensitive cavity-enhanced sub-Doppler spectroscopy of a molecular overtone band with a 1.66-μm tunable diode laser," Japanese Journal of Applied Physics, 38(2A), 920-922 (1999).
  5. C. Ishibashi, M. Kourogi, K. Imai, B. Widiyatmoko, A. Onae, and H. Sasada, "Absolute frequency measurement of the saturated absorption lines of methane in the 1.66-μm region," Optics Communications, 161, 223-226 (1999).
  6. C. Ishibashi and H. Sasada,"Near-infrared laser spectrometer with sub-Doppler resolution, high-sensitivity, and wide tunability: A case study in the 1.65-μm region of CH3I spectrum," Journal of Molecular Spectroscopy, 2000, 147-149 (2000).
  7. C. Ishibashi, R. Saneto and H. Sasada, "Infrared radio-frequency double-resonance spectroscopy of molecular vibrational-overtone bands using a Fabry-Perot cavity-absorption cell," Journal of the Optical Society of America B, 18, 1019-1030 (2001).