Novel gas-phase sensing scheme using fiber-coupled off-axis integrated cavity output spectroscopy (FC-OA-ICOS) and cavity-reflected wavelength modulation spectroscopy (CR-WMS).

By feeding back the reflected light from the first cavity mirror to a single-/multi-pass gas cell via a multi-mode fiber, we demonstrated a novel gas-phase analytical scheme for methane (CH) detection by combing fiber-coupled off-axis integrated cavity output spectroscopy (FC-OA-ICOS) and cavity-reflected wavelength modulation spectroscopy (CR-WMS). This scheme has an electrical module and two optical sensing modules which are connected through both single- and multi-mode optical fibers. Long-distance gas sensing application was conducted for verifying the analytical ability of the demonstrated technique exploiting the two fiber-coupled optical modules. A detection limit of 3 parts-per-million in volume (ppmv) for an 84 s averaging time and a precision of 56 ppmv for a 150 s averaging time were achieved using FC-OA-ICOS and CR-WMS, respectively. Two different CH measurement ranges were achieved in the sensor system with a wide dynamic range from ~15 ppmv to ~12% for CH detection. Field monitoring of CH leakage was performed for environmental analysis under a static and mobile mode using the wireless-controlled vehicle-mounted gas sensor. The proposed gas sensing scheme with fiber-coupled dual optical modules demonstrates a good potential for long-distance field CH measurements, especially for those in hazardous environment where in-situ human observation is impossible.