【作者】 汪静丽；

【导师】 姚建铨；

【作者基本信息】 天津大学 ， 光电子技术， 2011， 博士

【摘要】 太赫兹(THz)波的传输研究是其能够广泛应用的关键技术。一方面,研究THz波在大气中的传输特性,将对其应用到空间通信、大气科学以及遥感等领域产生重要影响;另一方面,研究具有高双折射的THz波导,将会使其在高比特率通信系统、高性能光器件、滤波器及传感器等中得到广泛应用,具有实际意义。普通光纤存在偏振态漂移、模间干扰和交叉敏感等若干问题,使光纤传感器的应用受到了限制。光子晶体光纤(PCF)作为一种新型光纤,由于其结构设计的灵活性以及性能的优越性,应用于传感后有望克服上述缺点,正逐渐成为传感领域的又一个研究方向。本文围绕THz波传输和基于填充混合液体的PCF温度传感研究展开,主要内容和创新点归纳如下:1.分析了THz波大气传输的研究概况,包括大气传输的基本原理,辐射传输方程的推导以及对各类THz波大气传输模型的比较,并指出尚待解决的问题,最后对THz辐射大气传输研究给出了若干建议和展望。2.设计了一种新型的高双折射混合格子THz PCF,对其芯区亚波长尺寸的空气孔采用三角形和长方形格子的混合排列,通过打破结构对称性实现了高的模式双折射。仿真结果表明:合理选择光纤参数后,该THz PCF在很宽的频率范围内,都具有较高的双折射(达到10-2)和低的限制损耗,且通过改变某些光纤参数,可以灵活地控制光纤的模式双折射或限制损耗特性。3.设计了一种具有大空气芯的聚合物椭圆环波导,它可通过在某一方向挤压普通圆形环波导得到。与之前报道的具有复杂横截面的空气芯THz波导不同的是,我们所提出的聚合物椭圆形环波导的结构非常简单,易于制造。数值结果表明:椭圆环波导存在高的模式双折射,在很宽的THz频率范围内都达到了10-2数量级,且由于大部分模式功率集中于空气芯内,使得由聚合物材料吸收引起的波导损耗大大降低。4.设计了一种基于类领结单元的新型聚合物THz光纤,其中芯区内引入的类领结单元能够增加结构的非对称性,从而获得高的模式双折射。仿真结果表明,这类光纤在很宽的频率范围内双折射值都达到了10-2数量级,特别是在三角形格子的THz光纤中,其双折射最大值达到≈5.11×10-2 (f=1.2THz)。且由于部分模式功率分布于空气孔内,由聚合物材料吸收所引起的波导损耗大大降低。5.设计了两种单偏振单模(SPSM)THz PCF。第一种是正方形格子排列的THz PCF,主要利用了折射率匹配耦合原理,使得波导中基模的某一偏振态模和包层缺陷模发生耦合,从而导致该芯区模的大部分功率泄露至包层缺陷中而无法在光纤内传输,实现了SPSM运行。第二种是基于菱形空气孔的THz PCF,通过在芯区附近引入缺陷,使得基模的某一偏振态模无法在波导内传输,从而实现了SPSM特性。仿真结果表明:光纤中引入缺陷的位置、个数及其尺寸均会对SPSM运行的区域和宽度产生影响。6.分析填充不同混合液后,两种PCFs(LMA-5和LMA-8)中的温度传感特性。理论计算表明选择合适的光纤,填充恰当的混合液后,随着温度的变化,光纤的有效折射率、模场分布和限制损耗变化明显,从而使得基于此类PCF的传感器对温度变化很灵敏。随后我们设计了相关的实验方案,进行了实验验证,结果表明实验数据与理论计算值的变化趋势一致。更多还原

【Abstract】 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.更多还原