Study On Special Fibers Based On Mode Coupling And Higher Order Mode Photonic Bandgap Fibers

ABSTRACT:Special optical fibers and related devices have been widely used in such fields as fiber communication, fiber sensing, biomedicine, materials processing and military technology etc. This dissertation is directly supported by the National High Technology Research and Development Programs of China (863 program) "Special fibers for communications" and National Nature Science Fund "Research on fabrication and application of photonic crystal fibers for broadband dispersion and dispersion slope compensation". Special optical fibers based on the mode coupling effect have been studied theoretically and experimentally. Moreover, higher order mode photonic bandgap fibers have been explored theoretically. The main achievements of the dissertation are listed as follows:1. Based on the mode coupling effect of bent fiber, the mode effective index differences between the two supermodes are fitted linearly, and the analytical equation of low loss criterion for bend transitions is deduced. Based on this low loss criterion for bend transitions, two typical adiabatic bend transition paths, the optimum profile and the linear profile, are proposed. It is pointed that the length scale of the linear profile is an important parameter while designing an adiabatic bend transition path.2. An Archimedean spiral profile is designed as an adiabatic bend transition path. A grooved mandrel is fabricated and the geometry of the grooved curve follows the Archimedean spiral profile. Using a supercontinuum source and a free space link, the low loss criterion for bend transitions is demonstrated experimentally. It is shown that a fiber can be tightly bent with low loss if the bend transition is gradual enough to be adiabatic. It is also shown that the low loss criterion for bend transitions can be considered to be a numerical verification for further experimental and practical investigations.3. Based on the plane wave modeling and the finite element modeling, a quick method is proposed to explore the guided modes in photonic bandgap fibers. In this method, a simplified model with an equivalent air cladding is proposed to simplify the numerical analysis. Based on this method, a novel photonic bandgap fiber with pure silica core is designed to confine and guide higher order modes by photonic bandgap effects, exhibiting potentials for research on higher order modes. 4. Based on the mode coupling effect, a novel design of low confinement loss and high negative dispersion photonic crystal fiber, which comprises of a Ge-doped inner core and a pure silica ring core, with a high dispersion of-1120 ps/nm/km and a low confinement loss of 1.01×10-6 dB/km at the wavelength of 1550nm is proposed. The confinement loss is decreased by over four orders of magnitude through introducing the large holes in the most outer ring. The stack-and-draw fabrication method is improved and some dual-core photonic crystal fibers with different structural parameters for dispersion compensation are fabricated.5. Modified Split Step Fourier Method is firstly used to solve the nonlinear coupled mode equations. Through the numerical analysis of optical pulse propagation in twin core fiber, it is shown that twin core fiber can play a role of passive mode-locking device as saturable absorbers. To splice twin core fiber with standard single mode fiber, an asymmetric twin core fiber is designed. To excite and detect optical signals in the two cores, a method for fabricating a flat asymmetric twin core fiber is proposed. Some kinds of twin core fibers are fabricated to meet different needs by the stack-and-draw fabrication method.