Objective  Methane (CH₄) gas detection plays an important role in many fields, and rapid detection of CH₄ gas is of great significance for early warning of accidents. CH₄ gas detection methods include electrochemical method, tunable diode absorption spectroscopy (TDLAS), combustion catalytic method, non-dispersive infrared method (NDIR), photoacoustic spectroscopy (PAS), etc. Electrochemical measurement of CH₄ gas has high sensitivity properties, but it requires frequent calibration. Combustion catalysis has a fast time response, but produces error in low oxygen. With the rapid development of laser technology and acoustic detection technology, photoacoustic spectroscopy has attracted more and more attention for its fast response, zero background and high sensitivity in trace gas detection. Traditional photoacoustic spectroscopy technology using capacitive microphone as acoustic signal sensor, but the electrical characteristics of the capacitive microphone limits in electromagnetic interference (EMI), high temperature and explosive environmental applications. Fabry-Perot interferometric fiber microphone has the advantages of concentrated sensing area, strong environmental adaptability, easy miniaturization and high sensitivity. In recent years, the all-optical PAS technology based on the Fabry-Perot interferometer has attracted the attention of many researchers. A feasible experimental scheme is proposed for the rapid and safe detection of CH₄ gas leakage in industry by using fiber microphone based Fabry-Perot interferometric principle. Now it is enough to be used in the industrial leak detection of CH₄ gas, which is contributed to achieve methane gas leakage warning and protect workers safe.

Methods  Traditional photoacoustic spectroscopy techniques use condenser microphones as photoacoustic signal sensors, but the electrical characteristics of capacitive microphones limit their use in environments such as electromagnetic interference. In this paper, an all-optical photoacoustic spectroscopy device for CH₄ gas leakage detection is proposed. The technology is divided into photoacoustic signal generation module and photoacoustic signal detection module. The photoacoustic signal generation module is as follows. The excitation light source generated by the modulated 1653 nm DFB laser enters the photoacoustic cell to generate a photoacoustic signal, the photoacoustic signal is received by the optical fiber microphone, and the photoacoustic signal is transformed from the optical signal to the optical signal through the optical fiber microphone; The photoacoustic signal demodulation module is as follows. The light source generated by the 1 310 nm DFB laser enters the Fabry-Perot (F-P) cavity through the circulator to form an interference signal. Then the self-made optical fiber microphone is used for acquisition, and the interference optical signal is demodulated by the intensity demodulation method based on temperature feedback adjustment, and the advantage of this method is to achieve stable control of the interference signal Q point for a long time and quick response. The final signal is amplified by a lock-in amplifier, collected by the signal acquisition card (NI, USB-6003), and input into the computer to realize the detection of the generated photoacoustic signal.

Results and Discussions  The CH₄ gas in the laboratory is tested using the photoacoustic spectroscopy device, The experimental results show that the detection limit of using the optical fiber optic microphone is 7.47 ppm (1 ppm=1×10⁶). According to the Allan variance results, the detection limit of CH₄ gas is 0.23 ppm at an average time of 142 s. Compared with other photoacoustic spectroscopy technologies, the proposed photoacoustic sensing system has the advantages of good stability, fast response speed and simple optical path, and the whole experimental system is simple.

Conclusions  A cantilever fiber optic microphone is designed through the simulation and optimization of the cantilever beam. The Q-point stabilized intensity demodulation technology reduces the interference of the ambient temperature to the microphone, thereby ensuring the stability of the fiber microphone for long-term operation. In the experiment, the resonance frequency of the optical fiber microphone and the H-shaped photoacoustic cell was matched to realize the double resonance enhancement of the photoacoustic signal, and a set of high-sensitivity all-optical photoacoustic spectroscopy experimental device was built. The acoustic signal generation and detection of the experimental device are based on optical principle and optical fiber structure, which realizes the all-optical and high-sensitivity detection of CH₄ gas. The detection limit of the experimental setup for CH₄ gas was 7.47 ppm. According to the Allan variance results, the minimum detection limit of the all-optical photoacoustic spectroscopy device is 0.23 ppm under the condition of an average time of 142 s. The photoacoustic sensing system proposed in this experiment has the characteristics of good safety and simple structure. At present, the photoacoustic device can meet the detection level of CH₄ gas leakage in the industry.