This paper proposes to include the construction and analysis of the difference spectrum of the modes in the usual procedure of determining the parameters of planar waveguides from the results of measurements of the spectrum of the waveguide modes. It is shown that analyzing the difference spectrum of the modes makes it possible to efficiently detect erroneous results of the measurements and to select the most suitable technique for calculating the refractive-index distribution of the test sample. Using the results of the analysis increases the reliability of reconstructing the profile of a planar lightguide and of the parameters of the kinetics of the process of forming a gradient layer determined from this analysis. This is especially useful when creating waveguides based on new materials.
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