Research On Fiber Optic Sensor Based On Multimode Interference

The optical fiber sensors based on multimode interference (MMI) havebeen an active area of research due to their simple structure, low-cost, ease ofprocessing, anti-electromagnetic interference, corrosion resistance and highsensitivity. In this paper, the principle of the proposed sensors based on multimodeinterference (MMI) is presented in detail firstly. Then the numerical analysis of lightdistribution of single-mode-multimode-single-mode (SMS) fiber structure isdiscussed by using beam propagation method (BPM). A detailed simulation study ofthe SMS fiber structure with different structural and physical parameters has beencarried out. Finally, the proposed SMS fiber structure for measuring refractive index,relative humidity and liquid level is proposed and experimentally demonstrated andthe SMS fiber structure based on multimode interference could be a great potentialfor sensing.The main work of this thesis is as follows:1. The multimode interference in the single-mode-multimode-single-mode fiberstructure is discussed theoretically, such as modes coupling and evanescent wavetheory. Moreover, the formula of a resonant wavelength of output spectrum isderivable from the theory of multimode interference, which provides the theoreticalfoundation to the further research of optical fiber structure based on multimodeinterference.2. The beam propagation method was employed for modeling the propagationof light along the SMS fiber structure. A serial analysis on the propagation of lightalong the SMS fiber structure with different wavelength of the input light, length ofthe multimode fiber core, diameter of the multimode fiber core and externalrefractive index surrounding the multimode fiber core is taken on for a deepunderstanding on the SMS fiber structure. The result provides practical value for theSMS fiber structure using as fiber sensor.3. The SMS fiber structure based on multimode interference for measuring refractive index of liquids, relative humidity (RH) and liquid level are experimentallydemonstrated. Theoretical analysis and simulation of the SMS fiber structure havedemonstrated that the longer of the multimode fiber length or the smaller of themultimode fiber core diameter, the higher of the refractive index sensitivity. Theexperiment results show a good agreement with the simulation. Furthermore, theproposed sensors can be demodulated by optical power in special wavelength range,so it could be great potential for low-cost and high sensitivity optical fiber sensorsdeveloping.This article analyzes the characters of optical fiber sensors based on multimodeinterference (MMI) on three aspects such as theory, simulation and experiment. Theinnovations of this paper are as follows:1. The possibility of SMS fiber structure for sensing applications is predicted bynumerical simulation BPM.2. Multimode interference theory is applied into the fiber-optic refractometer,relative humidity (RH) sensor and liquid level sensor. Moreover, influence ofdifferent length of the multimode fiber core and diameter of the multimode fiber coreare studied in detail. It is proved that the longer of the multimode fiber length or thesmaller of the multimode fiber core diameter, the higher of the refractive indexsensitivity.This research enriches the theory of optical waveguides and lays a foundationfor industry developing of the optical fiber sensors based on multimode interference.