Preparation And Sensing Characteristics Of Few Mode Fiber Gratings

Fiber optic sensor has many advantages,such as immune to electromagnetic interference,high adaptable and small volume.In recent years,fiber optic sensor has been widely used in the aviation industry,bridge road monitoring,machinery and equipment and many other areas.The few mode optical fiber not only has the characteristics of the single mode fiber,but also has the characteristics of low nonlinearity of multimode fiber.Therefore,the few mold fiber has been more and more attention in the field of new sensing.In this paper,a combined sensor is proposed by combinded few mode fiber interferometer and Fiber Bragg Grating.In this paper,the sensing characteristics of temperature,strain and refractive index of the combined sensor are studied deeply.The paper mainly includes the following aspects:Firstly,the basic knowledge of the optical fiber is analyzed.The eigen equation and effective refractive index of the fiber mode are described numerically in detail.In addition,the common analytical method of fiber grating is discussed,i.e.,the coupling mode theory and transfer matrix method.This lays the theoretical foundation for the follow-up experiment and data processing.Secondly,the combined sensor production process is described.The influence factors on the interference spectrum of the few mode fiber interferometer are simulated by MATLAB.This provides the guidance for the actual production of the interferometer.Then,the forming gratings mechanism and characteristics of Fiber Bragg Grating fabricated by ultraviolet exposure are studied.The combined sensor is successfully prepared.The experimental results of the transmission spectrum agree well with the simulation results.Then Single Mode Fiber are manually welded at the ends of Fiber Bragg Grating.The combined sensor is successfully prepared.Thirdly,the sensing characteristics of the combined sensor are studied.The sensing mechanism of temperature,strain and refractive index are studied deeply.At the same time,a large number of theoretical simulations are carried,which analysises the characteristics of the combined sensor.The experimental results can verify the theoretical analysis well.Finally,the forming grating mechanism of Long Period Fiber Grating was studied.Few Mode-Fiber Long Period Fiber Grating is made by CO₂ laser.In order to make the wavelength range of the Long Period Fiber Grating transmission peak better controlled,the effects on Long Period Fiber Grating transmission peak of grating period,gate length,cladding thickness and other factors are deeply analyzed.The refractive index sensing principle of Long Period Fiber Grating is studied.Then,the refractive index characteristics of Few Mode Fiber Long Period Fiber Grating are simulated and analyzed numerically.