Research On Fiber Optic Sensor Based On The Spherical Structure And Fiber Bragg Grating

As a new type of non-electrical sensor,fiber optic sensor has unique advantages over traditional electric sensors,including compact structure,light weight,immunity to electromagnetic interference,high temperature resistance and corrosion resistance.Since the advent of fiber optic sensors,it has been favored by scientific researchers,and widely concerned in many fields such as industrial manufacturing,agricultural production,meteorological monitoring,biopharmaceuticals,aerospace and defense military.Based on the previous research of optical fiber sensors,a novel all-fiber sensor is designed and fabricated by combining single mode fiber(Single mode fiber,SMF)spherical-shape structure with fiber Bragg grating(fiber Bragg grating,FBG).The main research contents include:1.The development background and significance of fiber optic sensors are expounded.The research status,development trends and main classifications are introduced.Afterwards,the research progress of fiber Bragg grating sensor and interference sensor are introduced respectively.On the basis of this,the application of spherical structure in fiber sensor is described separately.2.The mode coupling and inter-mode interference theory of fiber are analyzed in detail.The sensing principle of fiber Bragg grating is also expounded.By analyzing the basic principle of fiber Bragg grating,the sensing characteristics of different parameters are studied,and the temperature sensitivity of 11.9 pm/°C and the strain sensitivity of 1.3 pm/?? are obtained.3.Based on the spherical structure and the sensing characteristics of the fiber Bragg grating,a temperature-independent refractive index(Refractive index,RI)sensor is designed and verified experimentally.The sensing structure is fabricated by splicing a fiber Bragg grating with a single-mode fiber spherical structure,and the spherical structure is used to excite cladding modes,which have different effective refractive indices.By measuring the difference in wavelength shift between the reflected Bragg cladding mode and the core mode,a maximum sensitivity of 2.87 nm/RIU is achieved in the RI range of 1.357 RIU-1.458 RIU.When the temperature is raised from 25 °C to 225 °C,there is no significant change in the relative wavelength shift between the cladding mode and the core mode,indicating that this fiber optic sensor can be allowed for temperature-independent RI measurements.4.Based on the spherical structure and the sensing characteristics of fiber Bragg grating,a micro-displacement sensor with temperature compensation is designed and verified by experiments.The sensing structure is fabricated by cascading spherical structure connected series Bragg gratings.The first spherical structure is used to excite the cladding mode,and the second spherical structure can achieve coupling interference between the cladding mode and the core mode.Based on the inherent temperature characteristics of the fiber Bragg grating,it is only used to measure the temperature,which will reduce the influence of cross sensitivity.Sensing measurements can be achieved by observing the wavelength shift of the interference dip and the Bragg transmission dip when the micro-displacement and temperature changes.The micro-displacement and temperature sensitivity of the interference dip are-89.892 nm/mm and 0.088 nm/°C,respectively,and the temperature sensitivity of FBG transmission dip is 0.012 nm/°C.The temperature-compensated micro-displacement measurement can be obtained by calculating the sensing matrix.