Study On Two Mode Excitation Mechanism Of Cascaded Gratings

Sensor based on fiber Bragg grating shows many advantages, such as corrosion resistance, anti-electromagnetic interference, large measuring range, good insulation and easy to reuse. And the cascaded fiber grating is showing better application prospects than single fiber grating has become the research focus of many scholars in recent years. In this paper, based on an in-depth study for the coupling mechanism of FBG and LPFG, we not only present the method to simulate the cascaded structure of FBG and LPFG but also present the method to measure concentration and temperature simultaneously in concatenated FBG and LPFG. The specific works are as follows:Firstly, this paper introduces some basic situation of fiber grating including fiber grating development, classification and application of the sensor. At the same time, analyses the effective refractive index in fiber grating, mode field distribution and coupling characteristics. The common theoretical analysis method contains the coupling mode theory and transfer matrix method is discussed, which lays the theoretical foundation and mathematical support for the modeling and Simulation of LPFG-FBG cascaded gratings.Secondly, the parameters of the fiber grating are discussed in detail, including the effect of the period, the length of the grating and the radius of the cladding, and the refractive index sensing properties of the long period fiber grating are discussed. The above discussion has played a leading role in the study of the parameter optimization and the refractive index sensing characteristics of LPFG-FBG cascaded gratings.Thirdly, based on the theory of transfer matrix method, the LPFG-FBG cascaded grating is modeled and simulated. Analyzing the impact of parameters of the FBG, LPFG and spacing fiber to reflection spectrum of concatenated FBG and LPFG based on the transfer matrix theory. And the refractive index sensing properties of cascaded gratings are simulated.Finally, the reflective spectrum for concatenated FBG and LPFG with the simulation method of transfer matrix theory is good consistent with practical value. Based on cascaded fiber gratings to build the experimental platform of some experiments were carried out to measure the temperature and refractive index. The experimental results show that LPFG-FBG cascaded grating with temperature compensation.