Research On Optical Fiber Refractive Index Sensor Based On Multimode Interference And Evanescent Field

Refractive index sensor has important potential applications in many fields such as biological medicine,chemical and national defense.Present chemical index sensors usually need long analysis cycle and large precise instruments.Optical fiber index sensor has many advantages such as fast response,high sensitivity and resistance to electromagnetic interference.In this paper,theory based on multimode interference and evanescent field is introduced in detail.A detailed simulation study of SMS(singlemode-multimode-singlemode fiber based on multimode fiber interference)fiber structure has been carried out with RSOFT.We discuss the potential applications based on SMS and segmented strutureoptic fiber.The main work of this thesis is as follows:1.Theory based on multimode interference and evanescent field is introduced in detail.The formula of resonant wavelength and transmission depth of evanescent of output spectrum is derivable from the theory of multimode interference,which provides the theoretical foundation to the further research of optical fiber structure.2.The beam propagation method was employed for modeling the propagation of light along the SMS fiber structure.A analysis has been done on the propagation of light along the SMS fiber structure with different wavelength of the input light.The different wavelength will also affect the cycle of self-imaging effect.3.A kind of refractive index sensor based on SMS has been designed.The experimental results coincide with theoretical results.The use of no-core fiber can avoid the uncertainty when getting rid of cladding layer.The sensor sensitivity can reach 460 nm/RIU.4.A kind of segmented structure optic fiber evanescent wave sensor is designed which has a refractive index sensitivity of 444 nm/RIU.The low temperature cross-sensitivity of this equipment is due to low thermo-optic coefficients.