BPM Simulation And Experimental Study Of Multimode-interference Based Fiber–optic Temperature Sensors

Many kinds of multimode interference (MMI) based fiber-optic sensors are widely used in the area of fiber sensing because of their features such as flexible design, high sensitivity, quick response, fitting for many hard conditions. This paper makes a comprehensive study about application of multimode interference based fiber-optic sensors for temperature measurement. The numerical simulation model built by Wide-Angle Beam Propagation Method (WA-BPM), is used for calculating distribution of light field in sensors, and guides the further research of wavelength demodulation technology. New fiber-optic sensor of MMF-SMF-MMF (MSM) structure is suggested, and the experimental result is accord with the theoretical prediction.The main work and creative points of this thesis are as follows:(1) Model for calculation light distribution in multimode-interference based fiber-optic temperature sensors is built by WA-BPM. The analytical expression of light distribution is so complex that the old theoretical model can only obtain approximate conclusions. The difference formula of WA-BPM is deduced, and the software of simulating the light distribution in sensors is compiled by Matlab. Transmission spectrum can be obtained expediently by calculating the numerical conclusion of the light distribution in sensors, which guides the further research of wavelength demodulation technology.(2) Multimode interference based fiber-optic temperature sensor of MSM structure is presented. Different from fiber-optic temperature sensor of SMF-MMF-SMF (SMS) structure, interference of core and cladding modes become the foundation of this new sensor. Its sensitivity is 6 times more than the old one because of the different thermal optic coefficients between core and cladding modes. The cladding modes are more sensitive to the surrounding environment. This new sensor can be used in many other measurements, such as refractive index, strength of liquid.(3) Numerical simulation results of multimode interference based fiber-optic temperature sensors of SMS and MSM structure are obtained by the software.(4) The experimental result is accord with the numerical calculating result and theoretical model.