Investigation On Optical Microfiber Coupler Based Sensing Technology And Its Applications

As the most basic microfiber device,the microfiber coupler(MFC)has advantages of strong evanescent field,high sensitivity,high tenacity,low loss,etc.It is widely used in the research and development of the new sensing technology.This paper starts from the transmission characteristic and coupled-mode theory of microfiber,introduces the coupling phenomenon of light field between waveguides,and establishes the theoretical model of MFC.In the experiment,we combined the structure of MFC with principles such as thermo-optic/photoelastic effect,vernier effect,and dispersion turning point(DTP).From the general structure of MFC and the structure of double MFCs to the composite structure composed of other optical fiber structures,our purpose is to study the new sensing technology characterized by high sensitivity and quick response.On this basis,we proposed and designed the sensing solution based on MFC and completed the building of the related sensing system and measurement.This thesis explains the preparation process of sensors,experiment process,and analysis and discussion of experiment results in detail.The main contents of this thesis are as follows.(1)This thesis introduces the historical background of microfiber devices;explains the transmission characteristic of microfiber,coupled mode theory between microfiber,the theory of composite waveguide mode field,and the calculation process of coupling coefficient in detail;introduces the basic principle of MFC and its application in the field of optical fiber sensing;summarizes the preparation method of MFC;and states the building and use method of the biconical taper system of the Fused Biconical Taper.(2)Based on the characteristics of strong evanescent field and high tenacity of MFC,in the process of design and preparation of sensors,by preparation processes such as packaging,coating,and assembly,the MFC was combined with optical materials with thermo-optic/photoelastic effect and humidity sensing property.After changes of the external environment were transformed into the changes of the material properties,they were captured by the evanescent field of MFC and the optical signal modulation in the MFC was completed.Based on this principle,in the experiment,we proposed the design solutions of three kinds of sensors:1)The MFC was encapsulated into the polymethyl methacrylate(PMMA)capillary tube filled by index-matching fluid to prepare the high-sensitivity temperature sensor;2)PMMA microfiber with the humidity sensing property and silica microfiber were wound together to form the hybrid MFC and the surface of the MFC was covered by the propylene glycol humidity sensing film to prepare the humidity sensor;3)The principle of DTP was used and propylene glycol film was combined to prepare the ammonia sensor.(3)Based on the general structure of MFC,two MFCs were connected in parallel to form a composite structure based on two MFCs.At the same time,the vernier effect with the effect of sensitivity amplification was used to design and prepare the reflective index sensor with ultra-high sensitivity.In this thesis,coupled mode theory was used to construct the theoretical model of the sensor and its principle of sensing was analyzed.Through the experiment,the sensing property of the sensor was studied.Compared with the refractive index sensor with the general structure of MFC,its sensitivity was improved dozens of times.Based on the design solution of the refractive index sensor,the principle of variation rate conversion point(VRCP)of phase difference was used to realize the vernier effect with the bidirectional shift property and secondary amplification of the sensitivity.In the experiment,by combining with the gas-sensitive film technology,the ethanol gas sensor was designed and prepared and the ultra-high sensitivity of the structure of double MFCs was inherited,which also had an extremely high response speed.(4)As a carrier for recording information,intensity and phase are the common modulation objects in optical fiber sensing.We connected the MFC to the probe prepared by single-mode optical fiber in parallel and proposed the two-dimensional micro-displacement sensor based on the composite structure.When the probe moves in the probe plane,intensity modulation and phase modulation are completed at the same time.By analyzing the changes brought by two modulation methods in the output spectrum,we can realize the accurate two-dimensional micro-displacement measurement.This thesis simulates the theoretical model of the sensor structure,verifies the feasibility of the design solution of the sensor,and studies its reliance on temperature.