The article describes a numerical method based on Fourier transform for studying propagating optical waves in dielectric planar waveguides. The inverse problem to the known direct one in waveguide investigation is proposed. namely a search of light wavelengths according to taken values of propagation constants. For each constant a set of wavelengths is obtained. https://www.parisnaturalfoodes.shop/product-category/ramen-lemongrss-ginger/