Measurement of residual internal stresses in optical fiber preforms

A non-destructive experimental procedure is presented which enables the determination of residual thermal stresses in optical fiber preforms. The procedure is based on integrated photoelasticity. We carry out the measurement of the optical retardation using the traditional Senarmont compensation method combined with a fringe shifting technique. The radial distribution of the axial stress is reconstructed using Abel transform. We have investigated two different refractive-index profiles, adopted in the realization of monomode and multimode optical fibers. It was shown that in both cases the results obtained experimentally and those analytically predicted by the Timoshenko elastic model were in good agreement. To obtain accurate experimental results, it was necessary to apply a correction procedure to take into account the fact that the trajectories of the light rays inside the preforms are not straight lines.