Refractive-index profiling of single-mode optical fibers and preforms

The application of the focusing method to measure the refractive-index profiles of single-mode optical fibers and preforms is described. In this automatic and nondestructive technique, the fiber or preform is immersed in index-matching oil, and collimated light is passed transversely through it. The intensity distribution of the transmitted light is detected with a video camera and processed with the aid of a computer-controlled video analysis system that provides the profile within a few minutes. The profiles of boron-doped and germanium-doped fibers and preforms with maximum Deltan varying from 0.0006 to 0.012 were measured with a repeatability of better than 1% and with a resolution better than 1 microm. The profiles of the preforms are in good agreement with those of the fibers and also with those of slab samples taken from the tips of the same preforms.