Multi-point spectroscopic gas sensing based on coherent FMCW interferometry

We present an innovative spectroscopic method based on coherent optical frequency-modulated continuous-wave (FMCW) interferometry that can realize multi-point gas detection with high spatial resolution, high sensitivity, and high selectivity. This method takes full advantage of the intrinsic capability of spatial localization of the coherent FMCW, meanwhile efficiently decodes the spectral information from the reflected optical signals. Gas sensors are deployed by adopting bus topology, i.e., distributed along a single backbone fiber in the measurement arm of the FMCW interferometer. For validation, a multi-point acetylene gas sensing system with three sensing nodes is experimentally demonstrated. The transmission spectra of the three gas sensors are accurately extracted, and their corresponding gas concentrations are efficiently retrieved with a low crosstalk below -30 dB. The demonstrated system achieves a sensitivity of 55 ppm (noise equivalent absorbance of 0.004) over a distance of 52 m, with a sensing spatial resolution of 30 cm and a spectral resolution of 0.5 GHz. Our proposed method promotes a novel way for the development of multi-point spectroscopic gas sensing systems for challenging applications such as gas leakage detection and gas emission monitoring, where spatially resolved chemical analysis over a large area is required.