| The research on transmission properties of terahertz (THz) wave is important for its applications. On the one hand, the atmospheric transmission properties of THz radiation are the basis of THz space applications, such as space based communications, atmosphere science, remote sensing and so on. On the other hand, THz waveguides with high birefringence have great potential in high-bit-rate communication systems, high quality optical devices, filtering, sensing, etc.Conventional fiber sensors are limited by the problems of polarization drift, mode interference, cross-sensitivity and so on. Sensors based on photonic crystal fibers (PCFs) with unique properties and design flexibility, are expected to overcome the difficulties and gradually becoming a new research focus.The main contents and key innovative points are as follows:1. The research status of THz wave atmospheric propagation properties is summarized, a detailed description on the basic theories and radiative transfer equations is given, and a comparison among various atmospheric propagation models is made. Finally, some suggestions and prospects are given for the research and development.2. We propose a novel highly birefringent THz PCF with subwavelength circular air hole pairs in the core which are arranged as hybrid crystal lattice structure. Its high mode birefringence is realized by reducing structure symmetry in core. Simulation results show that this kind of THz PCFs exhibit high birefringence on a level of 10-2 and low confinement loss over a wide THz frequency range. Moreover, the birefringence or confinement loss can be controlled flexibly by adjusting fiber parameters.3. We analyze a simple birefringent THz waveguide which is a polymer elliptical-tube with a cross section of elliptical ring structure. It can be achieved by stretching a normal circular-tube in one direction. Simulations show that this kind of waveguide exhibits high birefringence on a level of 10-2 over a wide THz frequency range. Moreover, as a majority of modal power is trapped in the air-core inside the polymer elliptical-tube, and the THz waveguide guiding loss caused by material absorption can be reduced effectively.4. A novel highly birefringent polymer THz fiber based on a near-tie unit is proposed. The introduction of near-tie units in the fiber core can enhance asymmetry to realize high mode birefringence. Simulation results show that the polymer THz fiber exhibits high birefringence on a level of 10-2 over a wide frequency range, and an extremely large birefringence (≈0.051065 at f=1.2THz) is obtained in a trianglar lattice THz fiber. Moreover, as a part of mode power is trapped in the air holes, the THz fiber guiding loss caused by polymer material absorption can be reduced effectively.5. Two single-polarization single-mode (SPSM) THz PCFs are investigated. The first is square lattice THz PCF. One of the polarized states of fundamental mode is eliminated by index-matching coupling with the modes of two cladding defects, and SPSM property is obtained. The Second is a THz PCF based on rhombic air-holes. By introducing defects into to the cladding of PCF, one of the polarized fundamental modes is forbidden to propagate in the waveguide and SPSM fiber is realized. Numerical results show that the position, number and size of defects will affect the location and width of SPSM region.6. We analyze the temperature sensing properties of two kind of PCFs (LMA-5 and LMA-8) based on different liquid mixture. Theoretical calculations show that the effective refractive index, distribution of mode field and the confinement loss become highly dependent on temperature when proper PCF and liquid mixture are chosen. In the experiments, the results agree well with the theoretical simulations.
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