Optical low coherence reflectometery for morphological characterization of static and dynamic systems

Thesis (Ph. D.)--University of Washington, 2000In this dissertation I describe and evaluate Optical Low Coherence Reflectometery (OLCR) for morphological characterization of static and dynamic scattering systems.OLCR is a white-light interferometric technique, developed in the mid-1980s to measure the amplitude and the signal loss due to discrete reflections within sophisticated optical assemblies. These reflections occur as the light passes from one optical medium to another. Based on this principle, OLCR was implemented for thickness and refractive index measurements of single- and multiple-layer polymer films. It also became the backbone of a new medical imaging technique, called Optical Coherence Tomography.The OLCR instrument used in this research is a commercially available Hewlett-Packard 8504A High Precision Reflectometer. It uses 9 micron single mode fibers as both the test probe (fixed mirror equivalent) and reference (moving mirror equivalent) legs of a fiber optic Michelson interferometer system. The optical design is based on low coherence (broad bandwidth) light that results in a nominal peak width of 10 microns for a discrete reflection of the 1.3 micron source beam.Unlike in the case of transparent materials, in nonuniform and scattering matrices light undergoes numerous scattering events. During each of these events, wave packets of light may change their direction of propagation as well as their phase properties. OLCR is designed to measure the extent of this phase (i.e., coherence) loss, which has been attributed, according to existing light scattering theories, to the size, concentration, uniformity, diffusion, and dielectric properties of the scatterers in the system, as well as to the wavelength and the coherence length of the light incident on the matrix. Information about the phase properties of scattered light and the related morphology of the matrix is convoluted in the OLCR scattering signal. The experiments presented herein work in concert with developing light scattering theories to extract all this information from the scattering signal, and thus characterize highly scattering systems.OLCR's optical fiber probe design and 180 degree backscattering geometry make this technique very suitable for nondestructive, on-line analysis that provides real-time, accurate information about chemical systems and industrial processes