Investigation On The High Sensitivity Segmented Structure Of Evanescent Wave Optical Fiber Sensor

Among many types of optical sensors, fiber optic sensors have received considerable attention due to their advantages in small size, high sensitivity, and immunity to electromagnetic interference, and remote operation capability. In this paper, a segmented structure optic fiber sensor is proposed and demonstrated; it is based on evanescent wave absorption theory.The waveguide modes of the intrinsic straight and segmented structure are simulated and analyzed by using beam propagation method(BPM), which shows the high-order modes are excited repeatedly at the first transition of the each segmented region. The sensors are designed and measured base on the simulation results, and the sensing characteristics of segmented structure evanescent wave are analyzed from three aspects: theory, simulation and experiment.The main work of this thesis is as follows:1. The evanescent wave sensor is discussed theoretically, such as finite difference beam propagation method(FD-BPM). Moreover, the formula of the algorithm expressions is derivable from the modes theory of optical fiber.According to the expression, the process steps of the algorithms and simulation diagrams are presented, which lay a theoretical foundation for subsequent sensor simulation analysis.2. The basic theoretical knowledge of evanescent wave theory of optical fiber sensor is introduced. Through theoretical derivation, the formula of sensitivity of segmented structure evanescent wave optical fiber sensor is achieved. According to the formula, the sensitivity of sensor has relationship between the structural parameters of the fiber in the sensing region and the external environment parameters. The simulation results are investigated and the effect of the number of segments and the core diameters on the sensitivity of the sensors is investigated; the effect of the different structures and the concentration of solution on the sensitivity of the sensors are researched,which can provide a reference for the design of subsequent experimental sensors.3. The sensors are made by using the chemical corrosion metho. The sensitivity of the sensors is tested by using different concentrations methylene blue solution. The experimental results show that with the same the length of sensing region, and the core diameter decreases and the number of segments increases, the sensitivity of the sensor will be improved; with the fixed core diameters, the sensing region length of segmented structure sensors is 5cm, which is shorter than that of the conventional single straight sensor 6cm. However, the sensitivity of segmented structure sensors is higher than that of conventional single straight sensor. The segmented structure evanescent wave sensor can effectively stimulate the fiber low-order modes to the high-order modes, the sensitivity will be higher. The result is consistent with theoretical models and simulation analysis. The proposed sensor not only has a high sensitivity, but it is robust due to the larger core diameters and shorter length of the segmented region, which is suitable for materials spectrum measurements.