Abstract: In optical planar and/or channel waveguides, determination of two parameters such as: the depth and the waveguide index with the use of the m-line spectroscopy (prism coupling method) involves measuring of at least two coupling angles of the guiding modes. Most of integrated optical devices operate in single-mode regime but such device developed for 2nd and 3rd transmission windows are multimodal within the VIS range thus the conventional m- line study with the use of the He-Ne laser yields in evaluation of the waveguide parameters under examination. However, the conventional m-line procedure of the determination of the waveguide parameters fails for single-mode structures developed for the application within the VIS-range. A method presented in the paper involves an additional two-beam interferometric examination of the slab with a waveguiding structure. The interference of the beams reflected from the upper and lower surfaces of the slab results in a fringe pattern. Within the region of the waveguide boundary one observes the fringe shift related to the difference of optical paths of the beams in- and outside of the waveguide. The microscopic study of that fringe pattern results in an additional relation of the waveguide depth and its index. A substitution of the waveguide depth by the last relation results in a modified modal equation for the waveguide index that can be solved by conventional numerical methods.!8
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