The Study Of Hydrogen Sensitive Detector Based On The Long Period Fiber Gratings

The initial studies on gas detector are conductance-type. Detection of gas concentrations is sensitive by detecting changes in conductivity of the films. But this "gas-conductance" type detectors are need to be heated when it is working. There is a great security risk when the "gas-conductance" type detector works in an explosive environment. Optical gas detector in the recent year have many advantages, such as high sensitivity, small size, light weight, good electrical insulation, is not toxic, anti-electromagnetic interference, corrosion resistance, etc. And it is very good to make up for the "gas-conductance" type detector's disadvantages. In particular, the coated long period fiber gratings detector combines the advantages of optical thin film sensor and long-period fiber gratings sensor, and it has become the main development direction of gas detector.Firstly, this article have studied the coupled mode theory of the long-period fiber gratings, the transmission rate, cross-coupling rate, bandwidth, cladding mode effective refractive index and other expressions are derived. Combination the relations of long-period fiber gratings resonant wavelength with the radius and the changes of outside cladding refractive index, Analysis the refractive of long period fiber gratings transmission spectrum caused by the refractive index of fiber parameters and by the environmental change and obtained the characteristics of long period fiber gratings refractive index-sensitive. Then studied the sensor characteristics of the long-period fiber gratings which have three packs and four layer structure model and the hydrogen sensitive optical mechanism of the Pd/WO3 film, design a long-period fiber gratings detectors which was sensitive to hydrogen and it's encapsulate structure. And then established a system with a long-period fiber gratings detectors to detect hydrogen based on the theory of the optical fiber sensor. The system used double optical path and double detectors, and effectively reduce the impact of temperature on the refractive index sensing, and proposed a method to calculate the concentration of hydrogen which use the calibration curve. Finally, analysis the loss of the system theoretical, and provides some ideas of solving the cross-interference of the temperature and stress.