The sensitivity of FM spectroscopy can be dramatically enhanced by location of the sample in a high-finesse cavity, for example, ∼5 orders of magnitude in this study. To avoid conversion of laser frequency noise into amplitude noise by the cavity, we choose the rf modulation frequency to match the cavity’s free spectral range. In this way small frequency fluctuations produce no additional noise, and a pure FM dispersion signal is recovered in transmission. We present a systematic study of the detection sensitivity, signal line shape and size, and slope at the central tuning. Experimentally, using a weakly absorbing gas such as C<sub>2</sub>H<sub>2</sub> or C<sub>2</sub>HD placed inside an external high-finesse resonator, we obtained an integrated absorption sensitivity of 5×10<sup>−13</sup> (1×10<sup>−14</sup>/cm) for the gas’s weak near-IR molecular overtone transitions. As an interesting application, a Nd:YAG laser was well stabilized on the P(5) line of the C<sub>2</sub>HD (ν<sub>2</sub>+3ν<sub>3</sub>) band by this technique. The high attainable sensitivity permitted selection of slow molecules with low power and gas pressure to give a linewidth 13 times below the room-temperature transit-time limit.
Long-Sheng Ma, Jun Ye, Pierre Dubé, and John L. Hall, "Ultrasensitive frequency-modulation spectroscopy enhanced by a high-finesse optical cavity: theory and application to overtone transitions of C2H2 and C2HD," J. Opt. Soc. Am. B 16, 2255-2268 (1999)