Refractive Index Sensing Of Asymmetric Fiber Couple

In this thesis,a refractive index sensing system based on asymmetric fiber coupler has been presented.and the sensing analysis of different types of asymmetric fiber couplers,the measurement of refractive index sensing using asymmetric fiber couplers has been achieved.At the same time,the sensing characteristics of different types of asymmetric fiber couplers were compared and analyzed.The main research contents are as follows:(1)Structure and principle analysis of asymmetric optical fiber coupler.The structural characteristics of the asymmetric fiber coupler and the geometric optical transmission principle of different structural regions have been analyzed in detail.According to the relevant characteristics of symmetric fiber couplers and the coupling mode and mode interference theory,the wave optical transmission principle of asymmetric fiber couplers was deduced,and the optical energy coupling characteristics and light field mode distribution characteristics of different types of asymmetric fiber couplers were obtained.(2)Fabrication of asymmetric optical fiber coupler.The asymmetric optical fiber coupler is mainly fabricated by melt-drawing cone machine.Based on the fabrication parameters of symmetric optical fiber couplers,the parameters of different types of asymmetric optical fiber sensing structures composed of single-mode and 50/62.5/105multi-mode optical fibers have been optimized.And the coupler was encapsulated to eliminate the crosstalk caused by the remaining physical parameters in the subsequent refractive index sensing.(3)Analysis of refractive index sensing characteristics.By building the asymmetric optical fiber coupler refractive index sensing measurement system,the optimal interference spectrum was obtained through the comparison of the interference spectrum of different connection modes and different types of asymmetric optical fiber couplers.And the optimal interference spectrum was also used as sensing measurement.The refractive index sensing analysis of different types of asymmetric optical fiber couplers with effective coupling zones of 25 μm,18 μm and 11 μm was carried out respectively.Through the comparison of asymmetric optical fiber couplers with single-mode and 50multi-mode optical fiber,62.5 multi-mode optical fiber,105 multi-mode optical fiber,the highest sensitivity within a small range of 1.3330-1.3363 was 3275.174 nm/RIU,4357.869 nm/RIU and 4978.048 nm/RIU respectively.Since the asymmetric fiber coupler consisting of single mode and 105.0 multi-mode fiber has the maximum sensitivity response in the small range,its refractive index sensitivity response in the large range of 1.340-1.440 was further studied.and it can be concluded that the asymmetric fiber coupler can also obtain good refractive index sensitivity in the large range measurement range.