Abstract: This paper describes the optical fiber sensor based on modulated fiber grating model filter according to the principles of gas molecule's absorption spectra. This sensor system is important in real-time monitoring of the fault gases dissolved in transformer oil to ensure that transformer runs safely. Firstly this paper theoretically analyzes the optical quality of each component, including the wide band LED source, the modulated reflection fiber grating and gas molecules' absorption line, then gives their mathematical models. Some parameters in relation to these models are presented. In order to find out how the intensity of modulated light signal changes after the light passes the gas cell, a set of equations are given according to these models. These equations show that the variation of harmonic light intensity is determined by some factors such as modulation range and the wavelength difference between bias level of modulated fiber grating and peak of absorption line. A measurement system is built up, then detecting sensitivity is analyzed. Effects of temperature and noises are also analyzed. Some methods are presented to decrease their influences. !7
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