Highly Sensitive Optical Fiber Photoacoustic Sensor for In Situ Detection of Dissolved Gas in Oil

A highly sensitive optical fiber photoacoustic (PA) sensor was proposed and demonstrated for in situ detection of dissolved gas in oil. Two optical fibers were used for the transmission of pump light and probe light, respectively. The miniature detection module integrated by optical fiber sensor and oil–gas separation membrane made it passive and anti-electromagnetic interference. The dissolved gas in oil passed through the organic membrane into the micro-chamber of the PA sensor. The PA signal generated by the gas absorption of laser energy was detected by a Fabry–Perot interferometric cantilever. To reduce the equilibrium time of oil–gas separation and improve the PA response, structural parameters of the sensor were optimized. The experimental results showed that the temperature had an influence on the response time and PA response. The response time was ~1.8 h at 50 °C. To realize temperature compensation, the temperature was measured simultaneously by averaging the measured Fabry–Perot cavity length. In addition, the detection limit of dissolved acetylene (C ₂ H ₂ ) gas reached $0.5~\mu \text{L}$ /L. Since oil pumping was not required and the oil did not contact with the ambient air, the oil was not polluted and consumed to achieve in situ measurement.