Coherence-multiplexed FMCW spectroscopy for long-distance multi-point gas sensing

We present a long-range, multi-point spectroscopic gas-sensing method that incorporates coherence multiplexing into optical frequency-modulated continuous-wave (FMCW) spectroscopy to significantly increase the sensing distance. In this method, the sensing region is segmented to suppress the accumulation of phase noise along the optical path, and the optical delay of the reference is accordingly adjusted segment by segment to achieve optimal match of optical coherence. As a proof-of-concept, multi-point acetylene gas sensing is experimentally demonstrated, achieving a minimum detectable concentration of 0.94 ppm and a sensing spatial resolution of 0.46 m over a distance of 11 km, extending the sensing range by a factor of 60. Provided a sufficient optical power budget is maintained, the system can potentially multiplex thousands of sensors. The proposed coherence-multiplexed FMCW spectroscopy relaxes requirements on laser coherence, detection bandwidth, and computing power, offering a low-cost and high-capacity route to distributed gas monitoring in large-scale, long-distance scenarios.