Nondestructive Structure Measurement of Optical-Fiber Preforms with Photoelastic Effect

The photoelastic effect is used to measure nondestructively the structure parameters of optical-fiber preforms. An He–Ne laser beam is introduced perpendicular to the preform axis, the transmitted beam is optically retarded by residual stresses in the preform, and the optical retardation is measured. A procedure for determining the core diameter and the refractive-index profile from the measured optical retardation is presented. It is applied to a step-index preform and a graded-index preform fabricated by the M.C.V.D. (Modified Chemical Vapour-phase Deposition) technique.