Research On Refractive Index Sensing Based On Chaotic Correlation Fiber Loop Ring Down System

Refractive index is one of the fundamental properties of a material.The measurement of refractive index has important applications in chemical,biological,environmental monitoring and food safety.Optical fiber sensors have the characteristics of anti-electromagnetic interference,corrosion resistance,high temperature resistance and high sensitivity.Compared with measuring spectrum and intensity,fiber loop ring down system(FLRDs)have great advantages,such as insensitivity to light-source-intensity fluctuations,fast response,the ring down enhanced detection sensitivity and low cost.Sensitivity of the system can be achieved by reducing the length.In this paper,a new type of refractive index sensor is proposed by combining tapered fiber and chaotic correlation FLRDs.And a real-time monitoring for chaotic signal processing system is achieved based on the LabVIEW.This article mainly includes the following four parts from theory and experiment:(1)On the basis of elaborating the principle and development of optical fiber sensing,the advantages of chaotic correlation FLRDs was analyzed,and the research progress and application of chaotic laser were explored.(2)The melt-pull method was used to prepare the tapered fiber of the evanescent field sensor.Combining the wave optics theory with the principle of chaotic correlation FLRDs,the analytical relationship of chaotic correlation FLRDs refractive index sensing was deduced,and the correctness of the theory was proved by the experiment.(3)According to the refractive index sensing principle of chaotic correlation FLRDs,a chaotic signal processing system was built,and data exchange was completed through serial communication technology.The system realizes data collection,data processing,and achieves real-time monitoring.(4)Using the 1550nm chaotic fiber laser and fiber coupler,the TSMF chaotic correlation FLRDs refractive index sensing system was built,and the relationship between the ring down time of the peaks of autocorrelation coefficient and the refractive index was studied.The results show that(1??_R-1??_R0)has an excellent linear relationship with the refractive index n,and R² is as high as 0.997.The sensing demonstrates a good stability and repeatability.The influence of the change in fiber loop length on the sensing system was also investigated.The results show that the sensitivity of chaotic correlation FLRDs increases with the decrease of the loop length,and the sensitivity of the sensing system is as high as 0.08 ns^-1 RIU^-1when the ring length is 4.67m.The change of the loop length has little effect on the detection limit(DL)of the system.The DL of 10^-4 RIU were achieved.