Holographic measurement of refractive-index profile in the transition region of an optical fiber preform

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Abstract
The refractive-index profile at the preform-fiber transition region was measured using holographic shearing interferometry. It involved the measurements of both the horizontal and vertical deflection functions. Analysis is also given on the ray trajectories in the transition region. It is shown that the vertical deflection of the rays has a significant influence on the measurement of the horizontal deflection function which is needed for reconstruction of the index profile. It is found from measurement results that there is a variation of the index profile as the preform is transformed into fiber. This is especially pronounced at the cladding region where boron is doped.