Study On Long-distance Optical Fiber Refractive Index Sensing Technique

Long-distance monitoring is useful and important for some applications such as chemical industry, marine environments assessment and pipeline monitoring,and refractive index(RI) is an inherent characteristic of a substance that is inextricably bound to the physical and chemical properties. So an accurate and long-distance measurement of RI has an extremely vital significance in some applications. The main focus of this paper is the research of long-distance RI sensing technique. This thesis sections as following:1. A Fresnel reflection-based optical fiber sensor system is proposed for long-distance RI measurement using the optical time domain reflectometry technique as an interrogation method. The surrounding RI from a long distance away can be measured easily by using this sensor system, which operates based on testing the Fresnel reflection intensity from the fiber-sample interface. This system is a simple configuration, which is easy to handle. Experimental results showed that the range of this measurement could reach about 100.8 km, and the RI sensitivities were from 38.71 d B/RIU to 304.89 d B/RIU in the RI range from 1.3486 to 1.4525.Moreover, to ensure its stability and accuracy, we bent the fiber link to artificially apply a loss in the link. It proved that this sensor had a good application future in long-distance RI sensing.2. A Fresnel reflection-based optical fiber sensor is proposed for a quasi-distributed RI detection. Fiber ends that are branched from a long fiber link by couplers work as sensing heads, and different sensing heads are separated from each other by fiber optic delay lines. Ambient RIs that are located at different positions in the long fiber link can be obtained simultaneously. One sensor which has two sensing heads is used to show the RI sensitivities are from 38.785 d B/RIU to305.430 d B/RIU, and the sensing range can reach 16 km. In addition, we add experimental verifications that including a three-point and a four-point RI measurement to further prove its ability to quasi-distributed measurement.3. A reflective long period grating-based sensor with Sagnac fiber loop mirror(SFLM) for simultaneous measurement of RI and temperature is proposed anddemonstrated. By cascading the SFLM to the end of a long period fiber grating(LPFG), the LPFG works as a reflection operation, which is convenient in some long-distance applications. Further, the SFLM and the LPFG have different sensitivities to RI and temperature. As a result, the RI and temperature measurement can be simultaneously achieved by monitoring the wavelength shifts of the LPFG and the SFLM’s dips in the reflection spectrum. Experimental results show that the temperature sensitivity can reach 1.533nm/ ℃, and the RI sensitivity is from16.864nm/RIU to 113.142nm/RIU when the RI range is from 1.333 to 1.430. The application for 40 km long distance RI and temperature measurement shows that the sensor has a potential application in a long-distance and dual-parameter sensing.