Measurement of fiber-cladding diameter uniformity by use of whispering-gallery modes: nanometer resolution in diameter variations along millimeter to centimeter lengths.

Variations in fiber-cladding diameter on a nanometer scale were measured along millimeter to centimeter lengths by use of whispering-gallery modes (WGMs) in the elastic scattering of the fiber. The fiber was side coupled with a wavelength-tunable Gaussian beam. The scattered light was imaged approximately 1:1 onto a multichannel photodiode array detector. Based on the WGM wavelength shifts along the fiber, the taper of the fiber cladding's diameter was measured on a nanometer/millimeter scale. The fiber's surface roughness amplitude (in nanometers) and granular size ( approximately 100 microm) along centimeter-length fibers could also be revealed by use of higher- Q resonances.

Perturbation model for computing optical fiber birefringence from a two-dimensional refractive-index profile.

A vector perturbation model for computing optical fiber birefringence for an arbitrary two-dimensional index profile is developed. The vector wave equation is solved to yield the unperturbed vector f ields for an azimuthally symmetric refractive index. These f ields are used as the basis for the degenerate perturbation analysis. Unlike with the scalar perturbation theory, only f irst-order perturbation analysis suff ices for the computation of birefringence. Computed birefringence for various perturbations are reported.

Energy-density distribution inside large nonabsorbing spheres by using Mie theory and geometrical optics.

Mie theory and geometrical-optics ray tracing are used to obtain the distribution of electric energy density inside a nonabsorbing micrometer-sized sphere illuminated by a polarized plane wave. The Mie solution shows the multiply reflected geometrical-optics rays inside a sphere having a diameter of ~ 150 free-space wavelengths (size parameter = circumference/wavelength = 500). The geometrical-optics result shows the major features of the Mie solution and provides a physical interpretation of the electromagnetic interactions that result in the observed energy-density distributions. Both solutions show internal on-axis energy-density maxima inside the shadow surface of the sphere. The region of greatest enhanced energy density is approximately one internal wavelength in diameter and approximately twenty internal wavelengths in length.

Numerical optimization of 3-D SAR distributions in cylindrical models for electromagnetic hyperthermia.

Numerically optimized SAR (specific absorption rate) distributions in a source free 3-D multilayered concentric cylindrical (MCC) model are presented. The fields were expanded in the modes of the MCC. Cost functions which specify mathematically the relative weight assigned to differences between an SAR distribution and a desired SAR distribution were defined. The coefficients of the modes, which minimize the cost function, were obtained using gradient search optimization methods. The optimized SAR distributions shown were computed using three different cost functions and two different radial locations for the center of the region where the desired SAR is largest. A five-layered model, including the outer water layer for cooling and improved matching with the source, was used. The frequency was 70 MHz. The current and charge distributions computed on a perfectly conducting cylindrical surface just outside the model are also shown. The surface current and charge distributions depend strongly on the relative importance of the cost for acute heat and systemic heat. A technique is developed for generating a new set of basis functions for reducing the number of unknowns to be optimized. We suggest that the approach shown could be useful in designing hyperthermia applicators.

Limits on focused power deposition for electromagnetic hyperthermia.

Computations of SAR distributions in homogeneous regions and in multilayered concentric cylindrical models of the torso are presented to illustrate how an increase in the localization of the fields and/or a decrease in attenuation of the fields in one dimension is accompanied by a decrease in the localization and/or an increase in the attenuation in the other dimensions. Sharp gradients that may exist in the EM intensity patterns at the surface of a body become less pronounced further into the body because the wave components having higher spatial frequencies are most rapidly attenuated.