Research On Gas Concentration Measuring Method Based On Harmonic Detection Technology

With the development of the optical fiber sensing technology, the study of optical fiber gas sensor based on near-infrared spectrum absorption has been developed rapidly both at home and abroad. However, direct absorption spectrum technology is vulnerably influenced by fluctuations of light source and optical path and the noise introduced by detection circuit. Thus research on harmonic detection technology is vitally significant. By modulating to the wavelength of light source, signals passed through the measured gas will be accordingly modulated and then contain harmonics signals. Using locked-in amplifier can detect these harmonic signals related with gas concentration. Taking the advantage of harmonic detection technology, the anti-jamming capability of the system will be greatly improved. Meanwhile, the sensitivity of detection system can also be greatly improved. Thus harmonic detection technology has a very important application value.In this paper, the method of gas concentration measurement based on harmonic detection technology is studied through theory and experiments. The main research contents including:(1) According to the principle of harmonic detection technology based on narrow band light source, the principle of second harmonic detection technology based on broadband light source is analyzed. Then the impact of modulation parameters on the harmonic signals is analyzed theoretically. Then optimized choice is carried. The principle of multiple modulation technique to improve the detection sensitivity is analyzed.(2) A cantilever structure is designed to achieve wavelength modulation. Through simulation analysis, chose the structure of equal strength cantilever. Its material is carbon fiber, having a length of 20cm, bottom width of 4cm and thickness of 0.15cm. Select a fiber grating whose center wavelength and bandwidth is respectively 1531.59nm and 0.18nm. An electromagnet is designed and produced. Eventually a wavelength modulation module is produced to achieve the modulation of central wavelength of the light source.(3) The impact of the modulation frequency and voltage modulation amplitude on the second harmonic signals are studied experimentally. The outputs of the detector and the impact of the random noise are anglicized with different voltage modulation frequency and voltage modulation amplitude. The voltage modulation frequency and the voltage modulation amplitude is respectively determined to 40Hz and 2V ultimately.(4) The gas detection system based on harmonic detection technology is designed and set up. The experiment to measure the acetylene gas concentration is carried out. By calculating the experimental data, the sensing characteristics are analyzed. Gas measurement resolution is up to 500ppm, the stability is 3.92%, the sensitivity of the system is 1?V/ppm.