Research On Optical Fiber Sensing Technology Based On The Spectrum Absorption

In recent years, the petroleum, coal, metallurgy and chemical industry have developed rapidly.Along with the development of industry, many kinds of poisonous gases are produced. At the sametime, there are many cases happened because of poisonous gases. Some kinds of gases are very easyto explode, and they indeed can harm human being’s health and public property. It is very importantto inspect the poisonous and explosive gases.Compared with the conventional gas sensors, optical fiber gas sensors have many advantages,such as excellent flexibility, small volume, light weight, high sensitivity and anti-electromagnetisminterference. They have obtained wide attention in recent years.Based on principle of absorption spectrum and Beer-Lambert law, this dissertation analyzes themathematical model of gas sensor based on differential absorption and harmonic detection based onnarrow-band optical source. A new type of gas sensor system based on differential absorption theoryis designed.Main researches performed in this dissertation include:1、A new optical fiber gas sensor system based on a scanning light source is presented, and anovel optical gas sensor system is designed. By using broadband light source——ASE(AmplifiedSpontaneous Emission) and a tunable Fabry-Perot filter (FFP) driven by a saw-tooth wave, a scanninglight source is developed. This scanning light source can sweep over several gas absorption peaks atthe same time, which is necessary to realize a variety of gas detection.2、By using parallel resonant circuit, the linear distortion of the scanning light source’s drivingsaw-tooth wave is reduced. As the Fabry-Perot tunable filter is a capacitive device, it can cause thelinear distortion of the saw-tooth wave. By selecting the appropriate inductance and resistancecomponents to match the tunable Fabry-Perot filter, it can generate parallel resonance. In this way, itcan eliminate the linear distortion of the saw-tooth waveform. As a result, it can ensure the stability ofthe scanning light source.3、A new kind of digital average filtering method based on spectrum is put forward. By usingETALON and reference grating, it can provide reference wavelength, and realize the spectrumreproducing of the output signal. Therefore, the same N spectral signals can be superposed. Thismethod is able to overcome the uncertainty effects caused by the transmission wavelength of tunableFabry-Perot filter and filter’s driving voltage. Experimental results show that gas detection sensitivityand signal-to-noise ratio is remarkably improved. 4、Although different detection method can diminish the source jitter and other instability, it cannot eliminate other background noise generated by the system. In this paper, a modified least squaremethod is used to fit the envelope of gas absorption lines. By dividing the gas absorption lines and theenvelope fit line of absorption lines, it can reduce the drift of the background noise curve due to thechange of experimental environment. The modified fitting curve does not carry any information onabsorption peaks of gas, so it can eliminate the changes of fitting curve accounting for concentrationsof gas. As a result, the accuracy of measurement can be improved.5、Experiments including one and two gases respectively are performed. The analysis ofexperimental results shows that the outputs of these two experiments are similar and verify thatfiber-optic gas sensor system based on a scanning light source can be applied to dectect two or moretypes of gases.