A Study Of Macrobending Loss Characteristics Of Single-mode Fibers With Metallic Coating

Fiber-optic sensing technology is a type of new sensing technology,which employs optical wave as signals carrier and optical fiber as medium to transmit and sense the measured external signals.As the transmission medium,the characteristics of optical fiber will affect the study and application of fiber-optic sensening.Fiber-optic macrobend sensors based on the macrobending loss characteristics of optical fiber have not been studied extensively and need further research.Metallized coating can not only form metalized optical fiber,but change the macrobending loss characteristics of optical fiber,which provides a basis for realizing metalized optical fiber macrobend sensing and the corresponding smart metal structure.The dissertation is focusing on the research of single-mode fibers and the related macrobending loss characteristics,including main factors,the methods to realize nickel coating on the surface of the fiber,two models to predict macrobending loss in optical fiber with complex refractive index coating and to analyze thermal stress and the asscosiated photoelastic effect for the metalized macrobending fiber,the performance of temperature sensing for macrobend fiber,and the encapsulation process of fiber with nickel coating by laser welding.The main research contents are as follows.(1)Three types of theoretical models for predicting macrobending loss in single-mode fibers with different structures were analyzed.The correction method based on bending stress induced photoelastic effect for the calculation of fiber macrobending loss was studied.The effects of coating,bend radius,wavelength,bend turns and MAC value on the characteristis of fiber macrobending loss were simulated and analyzed.The simulated results show that the bend sensitivity of single-mode fibers can be judged by MAC value.(2)After studying the technologies of electroless plating nickel and electroplating nickel,a nickel protective layer was applied on the surface of a single-mode fiber,forming a rigid structure of bending fiber.After a comparative analysis,laser welding method was used to encapsulate the metalized fiber into metal matrix.The thermal test of the encapsulated bending fiber verified the feasibility of laser welding.(3)A model to predict macrobending loss in single-mode fibers with complex refractive index coating based on the model by L.Faustini was developed.The effects of bend radius,refractive index of coating and wavelength on the fiber macrobending loss characteristics were simulated and analyzed based on the new model.The experiments for a fiber with different coatings show:(i)the new model can accurately predict the macrobending loss in a single-mode fiber with a real or complex refractive index coating;(ii)the strong bend loss oscillations of the fiber with complex refractive index coating as a function of bend radius or wavelength occur;(iii)large imaginary part of the refractive index of the coating can intensify the bend loss oscillation behaviors.(4)A model to analyze thermal stress and the asscosiated photoelastic effect for the metalized macrobend fiber was developed,which was used to study the effects of fiber structure on thermal stress and refractive index.The thermal stress dependent refractive index coefficiect Kn was used to correct the thermal induced change of refractive index.Theoretical and experimental study of the performance of temperature sensing for macrobend fibers show that the macrbending loss response to temperature for the fiber with infinite-cladding structure was nearly linear and the response for the fiber with infinite-coating structure often appeared oscillations.However,monotonous response and high temperature resolution can be obtained in a large temperature range.