Modal analysis of layer-structured optical fibers

Abstract

Numerical modeling techniques play a key role in the development of new fiber optics, e.g., for telecommunications or sensing applications. In particular, the refractive-index profiles of various specialty optical fibers are increasingly designed with numerical tools. In this thesis, analytical and numerical mode-analysis techniques are applied to layer-structured specialty fibers as well as to partially coherent optical fields. The work is divided into three parts. In the first part, hollow-core and annular-core optical fibers are considered. The accuracy of the weak-guidance approximation for the low-order modes of such fibers is first assessed. Then, the approximation is employed to investigate the use of hollow-core multimode fibers in evanescent-wave guiding of laser-cooled atoms. Two schemes for creating the required evanescent-wave field on the inner surface of the fiber are analyzed. These are based on the selective excitation of tubular fiber modes with Laguerre-Gaussian input beams and on the dithering of a Gaussian beam at the fiber input to average out the speckles due to multimode interference. The first part of the thesis is concluded with a full-vector analysis of the self-imaging properties of annular-core fibers. In the second part, microstructured optical fibers with coated inclusions are studied. The effect of high-refractive-index dielectric inclusions on the photonic bandgap of a photonic-crystal fiber is characterized. Also, a novel design for a fiber-integrated surface-plasmon-resonance sensor based on a three-hole microstructured fiber with metal inclusions is proposed. The sensor makes use of an aqueous analyte infiltrated into the fiber pores. In the third part, the coherent-mode analysis of partially coherent light is applied to a random, stationary optical field which rotates with respect to an observer. The spectral changes due to the rotation are exemplified for a field with Gaussian Schell-model space correlations.