Caustic Patterns Associated With Melt Zones in Solidified Glass Samples - Part I: Symmetric Cases

When a plane light wavefront propagates along a cylindrical glass rod in which a rapid monotonic decrease in cross section occurs, some light may be refracted from the glass and become externally visible. For a sample with homogeneous optical properties, the amount of emerging light and its intensity distribution are strongly influenced by the rate at which the cross section decreases. In a previous study of melt, or drawdown, zones of solidified samples taken from a laser-heated fiberdrawing system, the boundaries between the regions of emitted light and shadow were seen to be loci of intense illumination properly identified as "caustics." 1 These caustics were shown, by both experiment and analysis, to arise from various internal reflections and a refraction of the light from the surface. It was noted that the number of caustics increases as the rate of change of the cross section increases. No light is emitted from a very gradually tapered sample, while a great deal of light and numerous caustics are emitted from a sample with a very rapid taper. Also, as the rate of change of the cross section increases, the propagation 3209