Study On The Fiber Sensing System Based On Photonic Crystal Fiber

Owing to their excellent properties and unique optical characteristics over the conventional optical fibers, photonic crystal fibers (PCF) have attracted considerable attention in the scientific and industrial community since1996. In past decade, PCF have achieved great results in the areas of nonlinear optics, optoelectronics and optical communications. With the appearing of new structure of PCF and the remarkable progressing in the commercialization and fabrication of PCF, functional optical devices based on PCF have become global research focus, especially in the field of PCF sensing.The research work is supported by National Natural Science Foundation of China under Grant "New optical fiber parametric oscillator based on highly nonlinear optical fiber"(No.61007029) and two Nonprofit Applied Research Projects of Zhejiang Province "Hydraulic Sensing Research based on Novel Optical Fiber "(No.2011C21038) and " Development of Monitoring and Controlling Milk Component System"(No.2011C22051). In this thesis, we investigate a Mach-Zehnder (M-Z) interferometer type sensor based on a dual-core photonic crystal fiber (DC-PCF). Based on the insight study on method of fabricating, the sensing principle and optical properties, DC-PCF novel sensor has been applied to the hydraulic pressure sensing, strain sensing and the distributed strain sensing experimental research. Main content of the research work includes:1. Manufacture of the dual-core photonic crystal fiber sensor. At first, our teacher proposed a DC-PCF with two fiber cores separated with one air hole in the cross-section for pressure/temperature sensing applications. Then, the dual-core photonic crystal fiber was fabricated by ourselves in the Hong Kong Polytechnic University. A novel Mach-Zehnder interferometer type sensor was proposed by splicing a dual-core photonic crystal fiber with two single-mode fibers (SMFs) with suitable fusion splicing conditions. Owning to the mode index difference between the two fiber cores of DC-PCF, a transmission loss of8～10dB and extinction ratio of6～8dB MZI pattern was achieved and its transmission spectrum is sensitive to the micro-stress and hydraulic pressure applied on the DC-PCF sensor.2. Stress and hydraulic pressure sensor based on a dual-core photonic crystal fiber. Based on Mach-Zehnder interferometer principle, we applied the DC-PCF to the hydraulic and stress sensing experiment.The sensor based on a20-mm DC-PCF has a strain sensitivity of1.4pm/με with the measurement range form0to1500με and a pressure sensitivity of-41pm/MPa in a hydraulic pressure range from0to20MPa. In order to study the influence of DC-PCF length on the sensitivity of the interferometer, we also fabricate a40-mm DC-PCF interferometer for strain examination and get a sensitivity of1.9pm/με for strain sensing. It can be seen that the M-Z interferometer type sensor with a longer DC-PCF tends to have a high sensitivity.3. Design the system of distributed sensing based on dual-core photonic crystal fiber.We achieved the distributed sensing by using arrayed waveguide grating (AWG) with the function of wavelength division multiplexing. According to the characteristics of AWG center wavelength of each channel can shift with the chip’s temperature changes (10～11pm/℃), and the interference spectrum drift about0.6nm under the0～15MPa hydraulic pressure. Thus, the dual-core photonic crystal fiber sensor could be connected to the corresponding channel of AWG. Through scaning the AWG with a temperature ranging from30℃to90℃by computer, we obtain the interference peak value of each sensor in different hydraulic pressure. Finally we got the corresponding hydraulic pressure value by computer and achieved the distributed hydraulic sensing.