Multiplexed fiber-optic photoacoustic sensors for simultaneous detection of multi-point gases

A multiplexing scheme of fiber-optic photoacoustic (PA) gas sensors is demonstrated for simultaneous detection of multi-point gases. By sharing the PA demodulation device, the average cost of single point measurement can be significantly reduced. The passive fiber-optic PA gas sensing probe is integrated by a PA tube and a FabryPerot (F-P) interferometric (FPI) cantilever. The laser excited the PA signals is simultaneously incident into two sensing probes. The superimposed F-P interference spectrum containing multi-point gas concentration information is received by a high-speed spectrometer. For synchronous measurement of multi-point gases, a whitelight interferometry (WLI) based frequency division multiplexing (FDM) spectral demodulation method is exploited. A multi-point acetylene gas remote monitoring system is established by two PA gas sensing probes with 3 km fiber cable. The experimental results show that the sensing probes have achieved the detection limit of sub-ppm. The crosstalk between the two sensors is characterized by the gas concentrations at the two points, indicating a crosstalk of about - 35 dB. This multiplexing scheme of fiber-optic PA sensing probes has the merits of remote monitoring, low crosstalk, high sensitivity, intrinsic safety and low cost. It can be applied for coal spontaneous combustion monitoring, dissolved gas analysis and gas micro-leakage monitoring.