Tri-frequency cavity ring-down spectroscopy for fast ppb-level CO2 measurement

Abstract

We report on the development of fast sensitive carbon dioxide (CO₂) measurement using tri-frequency cavity ring-down spectroscopy (Tri-CRDS), in which three laser frequencies (the carrier and its two sidebands generated by an EOM) with known intervals successively couple into a high-finesse (∼114200) optical cavity for generating different cavity-ring down times. Combined with absorption line shape analysis, the Tri-CRDS enables calibration-free CO₂ measurement without knowing the exact mirror reflectivity. Tight locking of the carrier laser frequency onto the CO₂ absorption line contributes to a long-term stable operation with a variation of < 10 MHz. In the near-infrared demonstration, we achieve a minimum detection limit (MDL) of 230 parts per billion (ppb) at a response time of 0.1 s. An Allan-Werle plot suggests an improved MDL of 4.3 ppb, corresponding to a noise-equivalent absorption coefficient (NEA) of 7.6 × 10⁻¹² cm⁻¹, for an optimum integration time of 111.5 s. This approach provides an attractive spectroscopic tool that may be utilized for precise trace-gas sensing in atmospheric greenhouse effect research.