Optical EngineeringApplication of Fresnel diffraction to nondestructive measurement of the refractive index of optical fibers
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A nondestructive technique, based on Fresnel diffraction from phase objects, is proposed for specifying the refractive indices of optical fibers without requiring index matching liquid. The method involves measurement of normalized intensity distribution on the Fresnel diffraction pattern of a monochromatic plane wave diffracted from a fiber, and evaluation of the fiber core refractive index from the intensity distribution by the least-square method. It is also shown that for the case of an unknown fiber core radius, by fitting a suitable theoretical intensity distribution on the corresponding experimental intensity distribution, the core radius and refractive index can be obtained with reasonable accuracy. Simulation and experimental studies show that the technique is reliable and quite sensitive to refractive index change.