An optical technique for measuring the core radius of high-Δ optical fibers is proposed in which a laser beam is scattered off the core of a fiber immersed in index-matching fluid. Using a fiber segment that was pulled to form a near-field optical tip, the technique is used to measure variations in the core radius of step-index fibers down to the scale of 1 nm. Standard techniques for measuring the index profile of optical fibers rely on scanning a focused spot of visible light across the cleaved fiber face. Their resolution is therefore limited approximately to the wavelength of visible light, or about 0.5 μm. For step-index fibers with large relative index difference, Δ, and core radii of only a few micrometers, this limited resolution results in a significant distortion of the measured profile. Much greater resolution could theoretically be obtained with scattering or interferometric techniques, ideally to a small fraction of the wavelength. Measurements at the nanometer scale could provide useful information on draw-induced changes in the profile and could conceivably test theories that attribute the abnormally high loss of high-Δ fibers to fluctuations in the core radius. This paper describes a high-resolution scattering technique suitable for high-Δ step-index fibers. The method consists of scattering a focused laser beam off the core of the fiber and analyzing the fringes of the scattering pattern away from the forward direction. It is similar to a conventional light scattering technique used to provide real-time feedback to control the fiber diameter during the draw, except that the fiber is immersed in index-matching fluid. This effectively removes the cladding, vastly simplifies the scattering pattern, and permits much more direct information about the core to be obtained. Analysis of the resulting scattering pattern permits nanometer-scale fluctuations of the core radius to be observed.
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