Design of Lock-In Amplifier in the Optical Fiber Methane Harmonic Detection

A new safety system of optical fiber detection for methane measurement is described based on the near-infrared absorption mechanism. An optical fiber methane gas detector is designed for methane gas concentration below 5% v/v. The detector bases on tunable diode laser absorption spectroscopy, scans the methane gas absorption lines locate at 1653.722 nm, uses the orthogonal lock-in amplifier technology to extract weak first harmonic signal and second harmonic signal, eliminates light intensity disturbance by using the ratio of the second harmonic peak and the first harmonic average value, realizes the gas concentration measurement. This paper mainly analyzes the principle and key technology of orthogonal lock-in amplifier in the optical fiber methane gas detector. Experimental results show that the response time is 1s, the measurement accuracy up to 500ppm.