Research On Refractive Index Sensors Of Thin-clad FBG And SNS Fiber Structures

Refractive index is one of the fundamental properties of a substance and indirectly reflects many of its physical and chemical properties.Accurate measurement of refractive index is essential in many fields such as biology,chemistry,food and the environment.Optical fiber refractive index sensors are currently a research hotspot in the field of refractive index sensing due to some of inherent excellent properties.In this paper,different types of optical fiber refractive index sensors are investigated,and thin-cladding Fiber Bragg Grating(FBG)and Single mode-No core-Single mode(SNS)fiber refractive index sensors are deeply studied.The main contents are as follows:Firstly,a review of optical fiber refractive index sensors is presented.The principles,structures and sensing performance of three types of optical fiber refractive index sensors are described in detail,taking into account representative research results from home and abroad.The differences and connections between the different types of refractive index sensors are also discussed and analyzed.Next,a thin-cladding FBG refractive index sensor is proposed and investigated.Starting from the three-layer model of optical fibers,the Bragg wavelength of thin-cladding FBG is theoretically analyzed as the diameter of the fiber decreases and under different ambient refractive indices.In the experiments,different thin-cladding FBG refractive index sensors were designed and fabricated,and the rationality and correctness of the theoretical analysis was verified experimentally by measuring the refractive index samples,and the deviations arising from the theory and experiment are also discussed.The proposed thin-cladding FBG refractive index sensor has the outstanding advantages of wavelength modulation,reflective structure,wide and narrow reflection band,and ease of multiplexing.It also has better mechanical strength and stability compared to FBG etched to the core.Finally,the refractive index sensor of the SNS fiber structure was studied.The multimode interference and refractive index sensing principles of the structure are studied first,and then the SNS structure is numerically simulated based on the finite difference beam propagation method.Using RSoft software,the influence of the length of no core fiber,the diameter of no core fiber and the environmental refractive index on the transmission field distribution and output transmission spectrum of the SNS fiber structure is studied,which provides a theoretical basis for the design of the structure in the subsequent experiments.In the refractive index sensing experiments,the fabrication of the sensor is described in detail,and the refractive index response of the interferometric trough in the transmission spectrum is tested and the refractive index sensitivity and resolution are in good agreement with the simulation results.