We measured the modal indices of a planar waveguide made from GeSeSb glass sandwiched between and air by using the prism-coupling technique. Based on the measured indices of the TE modes, we determined the position of the turning point corresponding to each mode of the waveguide by using the inverse WKB method. Using the modified fitting criterion introduced previously [Appl. Opt. 33, 3227 (1994)], we accurately determined the spatial profile of the refractive index for such a waveguide. Such a graded-index profile is probably caused by compositional variation of the GeSeSb guiding layer.
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