Applied To The Problem Of Planar Waveguide Devices And Liquid Crystal Devices In Optical Communication

Lightwave is employed in information technologies, implementing e.g. signal transmission, displaying, storing and sensing, etc. Optical devices are key elements in these technologies. Two types of optical devices, planar lightwave circuits (PLCs) and liquid crystal (LC) devices used in optical communications, are considered in this thesis. Novel structures, new calculation methods and new design methods are proposed in developing these two type devices.In developing the planar lightwave circuits, the coupling loss between a waveguide with a high refractive index difference and a single mode fiber is considered. A novel spot-size converter based on a Y-branch structure is proposed and numerical simulation results indicate that it can reduce the coupling loss effectively with low polarization dependent loss and good fabrication tolerance. A novel low-loss Y-branch structure is introduced and the corresponding designs are presented. The numerical and experimental results of this Y-branch structure indicate that the proposed Y-branch structure, without extra fabrication technologies, has a lower excess loss as compared with the conventional ones. Optimal designs of the structural parameters for the multimode interference coupler are carried out. Low loss and high uniformity of the multimode interference couplers are achieved when the waveguide is weakly guiding. Two simple, accurate and fast methods are proposed for the design of directional couplers with the couplings in the input and output regions considered. The simulation and experiments indicate that these two methods are simple, fast and accurate. A new optimal design of interleaves based on cascaded Mach-Zehnder interferometers is also presented. As compared with the two existing methods (digital signal processing methods and Fourier filters), the present method is simple and intuitive. The wavelength dependence of the directional couplers is modeled and with this model optimal designs of planar wavelength circuits based on Mach-Zehnder interferometers and their cascaded forms are presented.Liquid crystals are widely used in optical information processing technologies.Optical devices based on liquid crystals, such as variable optical attenuators (VOAs), switches and tunable filters, have been introduced into the area of optical communications recently. In order to simulate the electro-optical performance of a liquid crystal cell an iterative finite-difference method is proposed in the present thesis to calculate the distribution of LC directors under an applied voltage. This new method is very stable and simple, and can give accurate results quickly as compared with the existing methods. When the parameters of a LC cell are unknown, a new model with three characteristic parameters is introduced to predict the electro-optical performance of the LC cell. The new model can give a more accurate prediction to the electro-optical performance for LC cells of various thicknesses as compared with the existing two-parameter model. A new 2x2 extended Jones matrix is introduced to calculate the lightwave transmission of a liquid crystal cell. Electro-optical curves for LC cells have been analyzed and it has been shown mat a large attenuation range and a shallow attenuation slope can be achieved simultaneously for a variable optical attenuator based on a parallel-aligned LC cell with an appropriate surface anchoring strength. As compared with the existing liquid crystal-based VOA structure (using two cascaded LC cells with a particular material), the present structure is simple and has no special requirement for the LC materials. A new design method for a broadband linear polarization converter (LPC) is introduced and the performance of the LPC is improved after an optimization.