Structural Design And Key Property Analysis Of Novel Photonic Crystal Fibers

Photonic crystal fibers (PCFs), one of the most important applications of photonic crystal (PC) technology, have attracted significant attention all over the world. Compared with traditional fibers, PCFs posses many unique properties, such as light guidance in air, endlessly single-mode transmission, controllable dispersion, and high nonlinearity and birefringence, which are expected to bring a huge impact to future optical communications. Worldwide efforts have been taken to the research and development (R&D) of PCFs in "novel structures, new applications".This dissertation focuses on the analysis and design of novel index-guiding PCFs aiming at some potential applications in future optical communications.It first presents an overall survey of the current state of the art in PCFs.In chapter 2, a versatile simulation platform based a full-vector finite element method (FEM) for the analysis of PCFs is established. In order to avoid spurious solutions, a hybrid edge/nodal element is applied and, to investigate the behavior of not only bound modes but leaky modes in optical waveguides, an anisotropic perfectly matched layer (PML) is employed as boundary condition at the edges of the computational window. The validity and usefulness of the FEM is verified when a rib anti-resonant reflecting optical waveguide (ARROW) and the hexagonal-lattice PCFs are taken into consideration. Perfect magnetic conductor (PMC) and perfect electric conductor (PEC) boundaries are discussed in detail in the analysis of a step-index fiber.In the third chapter, an analytical model for Rayleigh scattering in fibers is presented based on the full-vector finite element method. By using this model, the Rayleigh scattering losses in F-doped and GeO₂-doped High-index-core Bragg fibers, as well as PCFs with air holes arranged in hexagonal lattice are numerically investigated. Chapter 4 presents a novel design for realizing flattened dispersion in PCFs, using a square-lattice PCF with a central air-hole defect in the core region. The influences of the central air-hole defect on the mode field and dispersion are discussed in detail. Based on the mutual cancellation between the waveguide and the material dispersions, a nearly-zero dispersion-flattened PCF with dispersion within 0.3±0.3 ps/(km·nm) and confinement loss less than 0.1dB/km at wavelengths ranging from 1130nm to 1710nm is numerical demonstrated. Influence of varying PCF parameters on the dispersion properties of the dispersion-flattened PCF is analyzed. Owing to its ultra-fattened dispersion features, as well as low confinement losses and small effective mode area, the proposed PCF may be used for some nonlinear optical applications.Described in chapter 5 is a novel systematic scheme to achieve both high birefringence and low confinement loss in PCFs with finite number of air holes (i.e. 4 rings) in the cladding region, based on the fact that the modal birefringence of PCFs is dominated by the inner-ring air holes in PCFs. The relationships between fiber parameters and birefringence in the proposed PCFs are investigated. Numerical results show that fibers with modal birefringence in the order of 10^-3 and confinement loss less than 0.1dB/km can be easily realized in PCFs with only four rings of air holes in the cladding region.In the last chapter we propose a novel design for achieving wide-band single-polarization single-mode (SPSM) operation in photonic crystal fiber, using a rectangular-lattice PCF with two lines of three central air holes enlarged. The proposed PCF composed entirely of silica material is modeled by a full-vector finite element method with anisotropic perfectly matched layers. The polarization-dependent cutoff properties and confinement losses of the proposed structure are numerically analyzed as functions of PCF parameters and wavelengths. By adjusting the size of the central enlarged air holes, the position of the regime of single polarization can be tuned freely as required. The wide-band SPSM operation feature, low confinement losses and the small effective mode area are the main properties of the proposed PCF structure. Using this structure an ultra-wide-band SPSM-PCF with confinement loss less than 0.1dB/km within wavelength range from 1.20 to 1.66μm and effective mode area about 5.9μm² at 1.55μm is successfully designed. The proposed fiber is a nonlinear one that might be suitable for some nonlinear optical applications, or it can be used as polarizing elements in optical devices with wide SPSM operating bandwidths requirement such as an all fiber polarizer within the whole telecommunication window.