Refractive-index profiling of preform rods by a photoelastic method: application to VAD single-mode fiber preforms

An automatic and nondestructive method for measuring the refractive-index profile of optical fiber preforms using the photoelastic effect is presented. The method is applied to single-mode fiber preforms fabricated by the VAD technique. A collimated beam of linearly polarized light is passed transversely through the preform. The transmitted light is detected with a video camera equiped with a large diameter lens, a quarter-wave plate, and an analyzer. The refractive-index profile is computed from the light intensity distribution. Excellent agreement is obtained with core diameters and profiles determined from interference microscopic observation on preform slab samples.