| Chiral fiber gratings (CFGs) are a new kind of optical waveguide structures, and can be manufactured by spinning optical fibers or preform with non-circular symmetry cross section at melting state at high speed. CFGs have period modulation of refractive index along the longitudinal propagation direction as well as the azimuthal direction. As their properties are associated with the twist handedness, so is called "Chiral" fiber gratings. As CFGs have polarization selectivity as well as wavelength selectivity, they have been attracting much attention since proposed in2003. Now they are currently being commercialized in many applications such as circularly polarized polarizers, filters, sensing especially in high temperature and hash environment.In the thesis, the properties and applications of chiral long period fiber gratings (CLPGs) are systematacially studied in depth, including several aspects:Establish the unified coupled-mode analysis model for CLPGs with N-fold rotation symmetry cross section; and give the phase-and angular-matching conditions for mode couplings; to guide the design of CLPGs, study the mode birefringence of some special fibers; analyze the properties of the right-handed single-and double-helix CLPGs, especially on the mode-coupling selectivity and the polarization selectivity of the gratings; meanwhile, develop the applications in multi-parameter sensing and the conversion of the orbital angular momentum of fiber modes. Based on the above study, a number of innovative achievements are obtained. The main contents of the thesis are summarized as follows.According to the conventional coupled-mode theory, choosing the circularly polarized mode basis, the unified coupled-mode analysis model for CLPGs with N-fold rotation symmetry cross section is established, and the phase-and angular-matching conditions for mode couplings are obtained. To understand the effects of specialty optical fibers on properties of CLPGs, the mode birefringent properties of fibers with multi-lobe stress region and fibers with geometrical cross section deformation are analyzed in detail. It is obtained that the birefringence of fibers with2-lobe stress region is double that with single-lobe stress region, and the birefringence will disappear for fibers with N-lobe (N^3) stress region uniformly along the azimuthal direction in the fiber cladding. For fibers with geometrical cross section deformation, the condition of determining mode birefringence is obtained, and the fundamental modes will not have birefringence for fibers with N-fold (N≥3) rotation symmetry cross section, however, for higher angular order modes, the even and odd modes may have birefringence. In addition, for theoretically analyzing and numericallysimulating the properties of CLPGs, the analytical field expressions of the hybrid cladding modes and TE/TM modes as well as their dispersion relationship and normalized power are deduced.The mode coupling properties are analyzed in detail for right-handed single-and double-helix CLPGs. It is found that when right-and left-handed circularly polarized (RCP and LCP) core modes simultaneously couple with a pair of degenerate cladding modes in weakly guiding approximation, CLPGs are wavelength-selective but not polarization-selective, however, when only RCP (LCP) couples with a special cladding mode, CLPGs are polarization-selective as well as wavelength-selective. Taking elliptic-core fiber-and Panda fiber-based CLPGs for example, the properties of geometric deformation and stress-induced double-helix CLPGs are theoretically analyzed and numerically simulated in detail. Furthermore, the physical explanation of polarization selectivity in CLPGs is given from the point of view of the azimuthal rotation symmetry of the field of optical modes and the discrete rotation symmetry of the fiber cross section.Taking small core-offset fiber-and elliptic-core fiber-based CLPGs for example, their temperature and strain sensitivities are simulated. Two kinds of materials are considered: Silica grass and polymer materials. As polymer materials have much lower Young’s modulus, larger thermal expansion coefficient and larger thermo-optic coefficient than silica grass, polymer CFGs have advantages over both strain and temperature sensing. Based on the polarization selectivity of the mode coupling of the RCP and LCP core modes with a pair of degenerate cladding modes in weakly guiding approximation, a new scheme of simultaneously sensing temperature and stain have been proposed. And the multi-parameter sensing application of small core-offset fiber-based CLPGs is discussed. In addition, a new scheme of manufacturing CLPGs based on polarized laser-induced birefringent materials is proposed.The conversion of the orbital angular momentum (OAM) of fiber modes associated with mode couplings in CLPGs, as well as the interaction of spin angular momentum (SAM) and OAM, is studied. It is found that beams with OAM of±(?) and±3(?) can be generated by single-helix CLPGs, and beams with OAM of±N(?) and±(±N±2)(?) can be generated by N-helix (N≥2) CLPGs, in particular, with OAM of±2h and±4h for double-helix CLPGs. Based on right-handed elliptic-core CLPGs, the generators of OAM beams are designed, including a dual-OAM converter generating OAM beams with charge-2and4at dual wavelengths, and a broadband converter based on adiabatic coupling. Furthermore, Taking CLPGs as acoustic vortices, a physical picture of conversion of the angular momentum associated with mode couplings in CLPGs is given. |
PDF Full Text Request