Microstructured Fibers Based On Coupling Theory And Their Application

Due to the unique properties and flexible design, microstructured fibers have attracted intensive attention and become the focus of special fibers. As the improvement of fiber fabrication technology, there is a rapid development for the microstructured fibers and fiber-based components. Supported by National Nature Science Foundation Program, Innovation Foundation for Excellent Doctorial Candidates of Beijing Jiaotong University and The Project of Australia Science Council and oriented to the requirement of ultrabroadband optical fiber components and low-loss THz fibers, this dissertation focuses on investigating ultrabroadband polarization splitters, low-loss flexible THz fibers and its application in THz imaging system. The main achievements of the dissertation are listed as below:1. From the point of view of fabrication and application feasibility, an ultrabroadband splitter based on a dual-core microstructured fiber was proposed. Two fluorine-doped cores were introduced to reduce its sensitivity to wavelength. The simple structure with one elliptical air hole makes the fiber easy to fabricate. The polarization splitter has an ultrawide bandwidth of 400nm within the wavelength range from 1.23μm to1.63μm when the extinction ratio is better than -20dB. Its ultrawide bandwidth covers O, E, S, L and U communication bands. At the wavelength of 1.55μm, the ER is as low as -35dB. The distance between two cores is 17.3μm which is large enough to splice with single mode fibers and polarization maintaining fibers as input and output ports. The splicing loss is only 0.38dB when splicing with single mode fiber, which makes the polarization splitter compatible with fiber communication system.2. A modified structure with a microstructure core was proposed. The microstructured core consisting of an array of five small air holes was used to replace the elliptical air hole. This structure reduced the fabrication difficulty significantly. It can achieve an ultrawide bandwidth of 300nm. The mode field areas of splitter and single mode fibers match well, so the splicing loss is as low as 0.04dB when splicing with single mode fibers. A microstructured fiber with a microstructured core consisting three small holes was fabricated successfully by two-step stack and draw method, which demonstrated the fabrication feasibility.3. A low-loss tube-lattice hollow-core THz fiber based on Zeonex material was proposed. Taking advantage of tube lattice structure and Zeonex material, the flexible THz fiber can achieve low loss and wide bandwidth. By controlling the thickness of tube-wall in cladding, the transmission windows can be adjusted. The fiber with tube-wall thickness of 92μm has one transmission window from 0.64THz to 1.45THz. The lowest loss is lower than 0.1dB/m. The fiber with the tube-wall thickness of 378μm has three transmission windows whose losses are lower than 5dB/m. The fiber also shows good flexibility, and it can bend at the radius of 12.7cm.4. The false-color THz imaging system based on THz-TDS system was proposed. Three primary colors were defined by the integration of three frequency ranges. In THz images, different colors represent different materials and different saturation represents different material intensity. Comparing with gray-scale THz imaging system, false-color THz imaging system has an advantage on fast imaging and makes THz images contain more information. The resolution of the imaging system is about 0.4mm which is good enough to meet the requirement of rapid security check.5. It is the first time to realize the mobile THz imaging using our homemade low-loss flexible THz fiber. The THz fiber has the tube-wall thickness of 378μm and three transmission windows. Its three transmission windows are defined as the integration ranges of three primary colors according to the false color imaging. The imaging results can clearly indicate the background, the container and two kinds of white powders with different colors. This work breaks the space limit of THz imaging system and extends its application.