Photoacoustic and Fiber-Optic Interferometer Spectroscopic Method for Simultaneous Detection of Multiple Trace Gases

Dissolved gases in transformer oil are reliable indicators of operating conditions and fault types. Additionally, ambient water vapor can seriously affect the accuracy of photoacoustic dissolved gas analysis systems. Therefore, there is an urgent need for the development of the simultaneous detection of dissolved gases and water. A high-sensitivity, multiple-gas sensing system was developed by combining a differential photoacoustic cell and a water vapor fiber-optic sensor. The acoustic properties of the designed differential photoacoustic cell were analyzed through simulation and experimental validation for the differential and longitudinal modes, and the frequency difference between excitation and nonexcitation optical paths in the longitudinal mode was leveraged, achieving an amplitude response comparable to that of the differential mode. C₂H₂, CH₄, and CO measurements were performed at three resonance frequencies using two DFB and a QCL source. To monitor H₂O concentration and evaluate its effect on photoacoustic detection, a fiber-optic Fabry–Perot interferometer was developed using self-assembled microspheres with high specific surface area, single-mode optical fibers, and concentric tapered capillary tubes. Water vapor adsorption on the microspheres altered the refractive index, and cavity-length demodulation was employed to analyze the interference spectra to obtain the water vapor concentration. The water optical sensor showed high sensitivity of ∼112 pm/% for H₂O detection. Experimental results demonstrated that the dual-mode multicomponent gas sensor can achieve detection limits of 1.15, 241.07, and 367.32 ppb for CO, C₂H₂, and CH₄, respectively, with corresponding normalized equivalent noise absorption coefficients of 1.53 × 10^–8 cm^–1·W·Hz^–1/2, 4.56 × 10^–9 cm^–1·W· Hz^–1/2, and 3.75 × 10^–9 cm^–1·W·Hz^–1/2.