A family of near-field spot excitation or detection techniques have recently been developed to measure the refractive index profile of optical fibers. The present paper discusses the effect of finite spot size of the illuminating beam on the accuracy of the reconstructed profile. A wave theoretical analysis is performed, and a series of numerical examples are given. Experiments of the spot excitation near-field scanning method are performed for a nearly parabolic index fiber, resulting in a satisfactory agreement with the numerical analysis. Furthermore, a quantity is introduced which is a function of the refractive index, wavelength, and spot size of the illuminating beam. This quantity serves to determine the suitable spot size for measuring the optical fibers having different parameters.
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