Evanescent Field Fiber Sensor And Hydrogen Sensing

As the important element of the Internet of Things/Industry 4.0（Made in China 2025）,Sensor is one of the fundamental areas of the electric devices.Optic fiber sensors have been widely used to monitor a series of physical,chemical and biological parameters of the environment.In order to reduce the size of the fiber sensor and improve the sensing sensitivity as much as possible,the evanescent field fiber sensor has received more and more attention from researchers in recent years.This dissertation mainly focuses on the evanescent field fiber sensors based on femtosecond laser micromachining technology,a new type of optic fiber hydrogen sensor is designed,and some innovative research results have been achieved.The main contents are as follows:1.The dissertation reviews the fabrication and sensing application of the evanescent field fiber sensors reported in recent years,introducing the development of the micro-/nano-fiber sensor and the in-fiber waveguide fiber sensor,and introducing the development of the optic fiber hydrogen sensor.Introducing the interaction mechanism between the femtosecond laser and the transparent materials,the principle of evanescent field and sensing mechanism,and the fiber Bragg grating principle.2.The oxyhydrogen flame microfiber taper system was built.The system consists of a precision displacement platform,hydrogen gas path,fiber 3D fixture,and control software.The LabVIEW program of microfiber taper system is designed and written.The microfiber obtained by the system has low transmission loss and good surface flatness.The system can theoretically produce microfibers up to 1 meter in length.3.The femtosecond laser micro processing system and the femtosecond laser point-by-point method are used to work on the Bragg grating.The laser processing parameters and grating structure parameters of the point-by-point Bragg grating are explored and optimized.4.The microfiber Bragg grating is successfully fabricated in microfiber with a diameter of 10 microns.The spectrum of the structure in the air was tested,and several resonance peaks were observed.The temperature sensing performance and refractive index sensing performance of the structure were successfully characterized.The results show that the structure has good linearity sensitivity of¹0 pm/°C similar to that of ordinary single-mode fiber at room temperature;a higher refractive index sensitivity （¹60nm/RIU）at 1.42 in the range of 1.31-1.43.5.We demonstrated a new type of optic fiber hydrogen sensor based on the surface waveguide structures coating the palladium films.The preparation process of the device is illustrated.According to the sensing test,the hydrogen sensitivity of the fiber hydrogen sensor can reach-3.41 dB/%-H₂ between 1%to 5%,which has better sensing performance advantages compared with the traditional evanescent field hydrogen sensor.