Investigation and Optimization of a Line-Locked Quartz Enhanced Spectrophone for Rapid Carbon Dioxide Measurement

We have developed a rapid quartz enhanced spectrophone for carbon dioxide (CO₂) measurement, in which the laser wavelength was tightly locked to a CO₂ absorption line and a custom quartz tuning fork (QTF) operating at 12.5 kHz was employed. The intrinsic QTF oscillation-limited response time, as well as the optimal feedback interval, was experimentally investigated. By tightly locking the laser to the R(16) transition of CO₂, we obtained a stable laser operation with its center wavelength variation kept within 0.0002 cm^-1, merely three times the laser linewidth. The reported CO₂ sensor achieved a detection limit of 7 ppm, corresponding to a normalized noise equivalent absorption coefficient (NNEA) of 4.7 × 10^-9 W·cm^-1·Hz^-1/2, at a response time of 0.5 s. The detection limit can be further improved to 0.45 ppm at an integration time of 270 s, illustrating a good system stability. This spectrophone enables the realization of compact and fast-response gas sensors for many scenarios, where CO₂ concentration from sub-ppm to hundreds of thousands of ppm is expected.