This paper presents a technique for calculating the maximum of the transverse refractive-index distribution in an asymmetric planar waveguide from the measured mode spectrum. It is proposed to consider in addition a conventional symmetric waveguide, assuming that the branches of its profile are formed by the desired profile of the waveguide being analyzed and its mirror image relative to the sample surface. According to the properties of planar waveguides, the measured mode spectrum of an asymmetric waveguide in this case corresponds to the spectrum of the odd modes of such a symmetric waveguide. Extrapolating the mode spectra of both waveguides into the region of negative orders and shifting one spectrum along the mode-order axis by 0.25 makes it possible to determine the maximum refractive index in the waveguide at the point where the curves constructed in this way intersect. It is shown that the technique developed here can provide a lower error level than the traditional calculational methods.
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