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Refractive index profiles of waveguides are deduced from near-field intensity distributions of guided modes. To improve the accuracy of refractive index profiles, some measurements are taken with the center peak of the mode intensity profile blocked, allowing the increase of the optical power at the waveguide boundaries with a corresponding increase in the signal-to-noise ratio. Additional improvements reported here are related to the subsequent signal processing, which is performed mainly in the spatial frequency domain. Smooth and reliable profiles of channel waveguides are obtained, the method being very general and not presuming the functional dependence of n(x, y).