optical fibers in long line communication links fail often in tensile strength . analysis of tensile properties reveals a variation in tensile strength with fiber length , pointing to uncommon structural defects randomly occurring along the fiber length . failures occur from crack propagation at these defects . the defects can be revealed according to the invention by nondestructive monitoring during production via off - plane light scattering . a narrow light beam incident across the fiber diameter is refracted in a pattern characteristic of the fiber geometry . variations in that pattern are produced by structural variations in the fiber or by surface contamination . the nature of the pattern change can indicate the type of defect .