Prediction of a Forbidden Zone in Light Interference Fringes of Glass Microbubbles

Abstract

Light interference microscopy has been used to characterize 50 to 100 μm o.d. glass microbubbles for use as laser fusion targets. In support of this programme, we have written a parallel-light, ray-trace code that follows the refracted paths of rays passing through the microbubbles. The code ultimately calculates the increase in optical path length as one moves from rays passing through the centre of the microbubble to those at the edge. The code also predicts four types of ray paths, of which only two are strong enough to produce measurable light in the interference microscope. Most interesting is the code's prediction of a forbidden zone in which no interference fringes appear. Attempts to verify this effect with laser target microbubbles gave inconclusive results because of improper dimensions and lack of sphericity in the microbubbles. Experiments with high-refractive-index, thick-walled glass cylinders might confirm the code's predictions.