Microstructure Fibers Based On Twisted Structure And Analysis Of Their Mode Characteristics

Optical fiber plays an important role in many fields such as data communication,health care,processing and manufacturing,and is closely related to human daily life.The demand for the function of optical fiber is more and more extensive.Microstructure optical fiber is an attractive optical fiber because its structure can be designed flexibly.As a result,it has obtained many advantages that ordinary optical fibers do not have,and has rapidly developed a variety of functional devices with its unique advantages.The application of twist on flexible photonic crystal fibers provides more possibilities for it.Twisted photonic crystal fibers have good application prospects in fiber sensing,supercontinuum,polarization conversion and filtering,orbital angular momentum excitation and transmission,optical tweezers,etc.In this thesis,the index-guiding photonic crystal fiber is twisted to realize the fiber that can support the transmission of multiple orbital angular momentum modes at the same time.The influence of the structural parameters of the designed helical photonic crystal fiber on the transmission characteristics of the fiber is theoretically analyzed.The dual-core all-solid photonic crystal fiber is twisted to realize the circular polarization beam splitter The designed twisted photonic crystal fiber structure and circular polarization beam splitter are simulated,and the results are analyzed.The main works of this thesis are as follows:1.The twisted index-guiding photonic crystal fiber structure is used to effectively separate different modes.The twisted fiber can be used for the parallel transmission of multiple orbital angular momentum（OAM）modes with low confinement loss.The two degenerate modes before twisting will split into two non-degenerate vector modes in the twisted fiber.Adjust the degree of separation of different modes by adjusting the twist rate,and at the same time,convert a part of the momentum into the azimuth direction to form the orbital angular momentum.The effects of twist rate,core diameter,air hole diameter-spacing ratio and other parameters on the number of modes and loss are discussed.Optimized fiber parameters are determined.The twist rate is 4 rad/mm,the cladding diameter is 125μm,the diameter and spacing of the air holes are 1.1μm and 2.9μm,respectively.It can support up to 19 modes in the range of 1.20～1.28μm at the same time under this parameter.15 are modes that carry orbital angular momentum among them.In the wavelength range of 1.52～1.56μm,up to 16transmission modes can be supported,13 of which carry orbital angular momentum.Although there are several modes in these two ranges that do not carry OAM,they are circular polarization modes that do not couple to each other in the twisted fiber.The fiber can be used in mode division multiplexing technology to form multiple transmissions.The confinement loss of the modes is kept below 10^-4 d B/cm.2.The twisted all-solid photonic bandgap fiber is modeled,and the influence of the twist rate on the light-guiding wavelength range of the bandgap photonic crystal fiber is analyzed.In a light-guiding band,the polarization modes in the twisted dual-core all-solid fiber are simulated.The transmission,coupling and separation of the left-handed and right-handed circular polarization modes in the helically twisted fiber are discussed using the modes coupling theory.The circularly polarized modes in the fiber before and after twisting are corresponded.The effects of structural parameters of the fiber and twist rate on the effective refractive index and coupling length of the circularly polarized mode are discussed.By optimizing the parameters,the shortest length that can maximize the separation of the two polarization states is reduced by an order of magnitude compared with that before optimization.