Comparative study of dispersion characteristics of uniaxial crystalline optical fiber under the extreme cases of helix pitch angles

Abstract

Some special cases of dispersion characteristics of a uniaxial crystalline optical fiber with a helical winding on the core-cladding boundary are investigated theoretically. In the present case, the optical fiber is doubly unconventional: (i) in the choice of uniaxial crystalline optical fiber and (ii) in the choice of sheath helix between the core and cladding. Field components, boundary conditions, and eigenvalue equations for HE and EH modes are obtained. We consider two special cases, i.e. ψ = 0° and ψ = 90° to obtain simpler eigenvalue equations which contain Bessel and modified Bessel functions and their derivatives. The nature of the dispersion curve remains unaffected with the anisotropy and only cutoff frequency is lowered for the positive uniaxial crystals. The helix pitch angle does not affect the modal cutoffs but it effectively controls the number of guided modes. This property has promising importance in long-distance communication where only a few modes are desired to be guided in order to minimize the inter-modal dispersion.