Flexible endoscopes for confocal and multiphoton imaging have the potential to revolutionize the medical field by obviating the need for invasive biopsies; however, these high expectations can be achieved only by reducing endoscope size and by improving image resolution. In this dissertation, methods for enhancing the performance of current endoscopes are explored by studying the properties of multi-core fibers using numerical modeling and experimental analysis. Numerical simulation tools are based on the normal mode expansion of the fields, coupled mode theory, and the multipole method.
Image fibers (multi-core step-index fibers commonly used in fiber endoscopes) have small, closely spaced cores that are predicted through basic theoretical analysis to be strongly coupled. These image fibers are explained to successfully transmit images because of nonuniformities in their cross-section that reduce inter-core coupling. The wavelength, average core size, and degree of variation in core size determine the strength of coupling between adjacent cores, such that fibers with smaller cores at longer wavelengths require more nonuniformity in order for reliable image transmission. Guidelines are given for assessing the performance of image fibers in a particular system. In addition, due to the random nature of this effect, strong core coupling can be observed experimentally, demonstrating that the quality of images from current endoscopes is still compromised by crosstalk. Multi-core photonic crystal fibers (PCFs) are a potential alternative for use in flexible endoscopes. PCFs achieve tighter mode confinement than image fibers and are therefore predicted to allow higher core densities with less crosstalk and, ultimately, improved image contrast and resolution. The fabrication of these fibers, however, typically introduces nonuniformities into the photonic crystal cladding. Random nonuniformities in the air hole size and location are shown to reduce the coupling length and the coupling efficiency. When the air holes are large, variations in the lattice of less than 1% are sufficient to cause essentially independent core propagation. Nonuniformities are also shown to increase the core birefringence although the dispersion and loss of PCFs are rather robust to variations. Understanding the characteristics of core coupling is a first step towards improving the design of current endoscopes.