Abstract
An automatic and nondestructive focusing method to determine the refractive index distributions of optical fiber preforms is presented. The 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 vidicon camera equipped with a low f-number lens focused just outside the core boundary. Recording and processing of the light distribution follow with the aid of a computer-controlled video-analysis system. The refractive index profile of the preform is computed from the intensity distribution, plotted, and fitted with a power-law curve within several minutes time. The repeatability of the index measurement is better than 1%, and excellent agreement is obtained with profiles determined interferometrically using slab samples from the tips of the same preforms.
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