Investigations On Several Key Problems In The Preparation Of Chiral Fiber Gratings

Chiral fiber gratings (CFGs) are a new kind of fiber grating structure formed by spinning the linear birefringence fibers at the melting state at a very high speed. They own not only the wavelength selectivity as the conventional gratings, but also the circular polarization selectivity. In the aspect of maintaining circular polarization, CFGs possess unique advantage,which can be seen as a kind of natural all-fiber circular polarizers.As is well known, circular polarization technology has been widely used in optical fiber current sensor, coherent optical communication and fiber optic gyro system. The effective preparation of CFGs is a notable research direction for promoting the development of the circular polarization technology. Therefore, in this thesis, we aim at the effective preparation of the long period CFGs, and focus on solving the problems appeared in the process of grating preparation, both theoretical and practical. Our research results will provide theoretical guidance for the experiments of the preparation.First, we discuss the birefringence effect of stress-induced birefringence fiber which can be used for the preparation of CFGs. Based on this, we further study the the birefringence effect of birefringence fiber with multi-lobe stress region nonuniformly arranged along the azimuthal direction in the cladding of the cross section. We focus on the dependence of birefringence on the angular angles between adjacent stress regions on the number of stress region. The study provides a feasible plan to enlarge the birefringence effect of the stress-induced fiber.Second, according to the design scheme of preparation platform of long period CFGs, we discuss the characteristics of CFGs from two aspects as following:1) Considering the theoretical simulation basis which are used to design the experimental scheme, they are all obtained under the assumptions of continuous twist to fabricate chiral fiber grating. which are different from our experimental scheme in mechanical stepping. Based on coupled mode theory, we study the effects of the step of the stepping system on the spectral characteristics of long period CFGs, and suggest the selection range of the step in an effective fabrication;2) Due to the mechanical characteristics of the preparation platform, the existence of gradient twist rate at the two ends of the chiral fiber grating will affect the performance of the grating. Through modifying the original coupled-mode equations for uniform chiral fiber grating, we analyze the effect of the length with gradient twist rate on performance of the chiral fiber grating. Further, we identify the range of the gradient length in order to its existence has no effect on the performance of the grating.Finally, we study the mode coupling rules and polarization selectivities in long-period chiral fiber gratings based on eccentric elliptical-core fibers by using the full vector coupled-mode theory. We consider both low order and high order perturbation effects of the relative dielectric constant of CFGs in the process of analysis, and obtain the mode coupling selection rules and polarization selection properties of long period CFGs. Then, we demonstrate the plentiful circular polarization-selectivity properties of the CFGs based on eccentric elliptical-core fibers by numerical simulation. The results verify the mode coupling rules and polarization selectivities obtained by theoretical analysis, and provide the foundation for the wide application of CFGs.