Ph.D. Thesis, University of Sydney, Australia, August 1994 Theory of Optical Fiber Modal Fields with Application to the Design of Evanescent Field Devices Kai Ming Lo Abstract: In this thesis, we present a model for calculating the modal properties of an optical fiber having an infinite cladding, and several non-overlapping embedded homogeneous cylinders of arbitrary position, radii and refractive index. This model is based on the Rayleigh method with which it is possible to obtain the propagation constants and the vector electromagnetic modes for multi cylinders of step refractive index. The Rayleigh method is then extended to cover cylinders of radially inhomogeneous refractive index.
D-shaped optical fiber evanescent field absorption sensors are initially studied semi-quantitatively using one dimensional planar waveguide model. This Rayleigh method is then employed to analyze optical fibers with side pit. Such fibers with a “large side pit” approach the D-shaped optical fiber geometry as the radius of the side pit approaches infinity. Side pit fibers with radially inhomogeneous cores are studied via the extended Rayleigh method. Evanescent fiends surrounding the exposed cores of a D-shaped fiber are compared with the experimental results using near-field scanning optical microscopy.
We use a four-layer planar waveguide and a coated water side pit fiber to find the sensitivity improvement for the coated over the uncoated guiding structures The coated water side pit fiber is modelled by a water side pit fiber with a thin high-index coating in the inner wall of the pit. A six-fold sensitivity improvement is predicted for a coated over the uncoated water side pit fiber.
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