Study On CH₄ Optical Fiber Sensing System Based On Spectrum Absorption

Gas-sensing technology is an important forefront topic in the research field of sensing technology now. Gas-sensing technology, which employs semiconductor laser and light emitted diode as light resource, has multitude advantages over traditional approaches, such as high sensitivity, multiple selection, wide dynamic range, good signal to noise ratio and quick response time. Especially optical fiber sensor is more safe and reliable, for electrical insulation has been realized by the separation of optical fiber sensor from detection and signal processing circuit. With the rapid development of optical fiber communication technology, low-cost and high quality near inferred laser and light emitted diode working at room temperature become widely available. The emit wavelength from these light resource is not only consistent with the three low-decay windows of fiber optics, but also with many wide-frequency or recombination absorption spectrum of environmental and industrial gases. Therefore optical fiber gas sensor can fulfill on-line measurement in high-temperature, poisonous and dangerous situation where human being and traditional instruments can hardly accomplish.In this paper, CH4 is used as a research example. Based on the theory and experi- mental approaches of inferred absorption all optical fiber sensing, mathematical model and system design for all optical fiber on-line gas measurement are presented. The detailed research contents are as following:(1) Based on molecule spectrum theory and experimental data, presented are fine parameters of CH4 combined frequency band v2+v3 and wide frequency band 2v3, including absorption spectrum central wavelength, line space, intensity distribution, line type and line width. These fine parameters have played solid foundation for measurement wavelength selection, and effectively eliminated cross sensitivity. (2) According to Lambert-beer law and related optical signal processing technology, analysis formula is presented to express the relationship between spectrum intensity and gas density after the two widely used semiconductor light resource LED and DFBLD functioned with inferred absorption spectrum line. (3) Since the gas inferred absorption coefficient is very small, when the light ab- sorption measurement is directly used, there are not only light route interference, but also no relationship between content and density. Then the signal is very difficult to amplify,and measurement sensitivity is strictly limited. Therefore we should try to avoid the signal measurement component, which has no relationship with density.(4) Difference absorption measurement and wavelength modulation harmonic mea- surement are two fundamental approaches for gas light absorption detection. For di- fferent light resource, system designs for harmonic detection have great differences, but all of them are based on derivative spectrum characteristics and even symmetric gas absorption line (or F-P chamber multiple vertical pattern symmetry). With ground wave as feedback signal, the central wavelength can be precisely locked on the location of gas absorption central position.(5) Presentedthe detailed system components including lock-in amplifier, weak signal measurement circuit and microcomputer controlled circuit.(6) Experimental research has been completed including LED spectrum characteri- stics, relationship between transmission light intensity and CH4 density, result analysis and calibration.