Characteristics Investigation And Fabrication Of Bent Fiber Interferometric Sensors

Fiber optic sensing technology is rapidly developing with optical fibers, optoelectronics and fiber optic communication technologies; currently it is gradually becoming one of the important symbols of the national informationization level. Because of their distinctive advantages over traditional sensors, including small size, immunity to electromagnetic interference, multiplexing capability and easy to build sensor network, fiber optic sensors have attracted more and more attentions.In this thesis, according to the analysis of the current development status of fiber optic sensor, aiming at simplifying the production process and improving the performances (including detection precision, sensitivity and stability) of fiber sensor, the research procedure of this thesis is based on the research of bending fiber-based interferometric fiber optic sensor, we exploited and optimized various fiber sensor structures, and obtained temperature-insensitive dual channel RI sensor structure, bending fiber dual channel RI sensor and tapered fiber-based Fabry-Perot (FP) interferometer with controllable temperature sensitivity, and implemented the research and application of a variety of novel interferometric fiber optic sensor. The main contents of this paper are as follows:1. We designed a bent-fiber-interference-based sensor structure by incorporation fiber bending loss with Mach-Zehnder interference. Through the analysis of the working principle of the sensor structure, we concluded that the structure can be used as environment refractive index (RI) sensor, and experimentally verified its sensing performance. By controlling the different bending angle, different sensor structures can be obtained, and achieved a maximum RI sensitivity as -204nm/RIU. Finally, we investigated sensing characteristics of this sensor with different bending angles.2. According to the deficiencies and limitations of present sensors, we designed a wavelength modulation bent-fiber-based sensor. Both principle and fabrication method aredetailedly introduced, we investigated the RI and temperature sensing characteristics of this sensor. Also, we proposed a dual-channel fiber sensor, and verify the feasibility by experimental research, the experiment result indicated that the dual channel measurement without crosstalk can be realized. In addition, to further analysis of the temperature response, we investigated the temperature sensing properties, which demonstrated that this sensor is more appropriate for RI measurement at lower temperature and can realize temperature-insensitive RI sensing. Simultaneously, the temperature sensitivities of these two channels are similar, it is beneficial torealize the temperature self-compensation.3. In order to further optimize the perfonnance of the bent-fiber-based sensors, we proposed and demonstrated a novel sensor configurationby employing long period grating and bent-fiber structure for simultaneous measurement of temperature and RI. We adopted relative wavelength demodulation method to achieve the temperature compensation, and obtained temperature-insensitive measurement results. However, the bandwidth of the fiber interferometer spectrum is wide, which reduced the detection accuracy and resolution, we also presented a novel RI sensor based ona simple fiber ring laser incorporating a bent fiber filter. After investigated the laser-output performance of fiber ring laser, we verified the feasibility of the fiber ring laser-based sensor system for RI measurement.4. We proposed an optimization design of fiber FP interferometer for eliminating the temperature effect and enhancing other measurement accuracy. We successfully solved the problem of the temperature cross-sensitivity of FP interferometer by introducing a free reflection end-faceinto the FP cavity. Theoretically, we defined an equivalent thermal expansion coefficient of the FP interferometer; the temperature effect can be eliminated by optimizing the design parameters. We adjusted the temperature sensitivity by changing the design parameters. Finally, the strain test is implemented to demonstrate the good mechanical strength of the proposed FP interferometers.