Optical fiber preform diagnostics.

A sensitive, nondestructive, and noncontacting method is described for obtaining structural information on the preforms from which optical fibers are produced. The technique allows the determination of the core size and the core eccentricity from direct observation of light traversing the preform normal to its axis. Also observable are the core-cladding interface structure, individual deposition layer structure and variations, imperfections within the core and the cladding, and the presence of an axial refractive-index depression. Results and implications from a variety of multimode and single-mode preforms are presented along with a theoretical analysis of the observations.