A Dynamical Theory of the Electromagnetic Field

This specification describes a novel apparatus and method for testing optical wave guides to determine the optical attenuation and to locate faults in the optical fiber. The light for test purposes is launched into the fiber by one or more light beams that surround the wave guides and that are focussed to converge from around the circumference and at the axis of the wave guide; the convergence being at low angles of incidence, so that some of the light enters the wave guide. The remaining light launched into the wave guide in opposite directions at a location, nearer to one end than to the other, is measured at opposite ends of the wave guide. The ratio of the light at the opposite ends of the wave guide and the difference in the length of travel of the light from the region of entry to each end is used to compute attenuation. Faults are found by internal reflections of the light beam in the wave guide.