We demonstrate the breakdown of the usual WKB expressions for tunneling ray power transmission coefficients on multimode optical wavequides with power law refractive index dependence, when the profile approaches the step index limit. A qualitative analysis provides a solution valid in this transition region, which also quantifies the domain of applicability of the WKB analysis. The latter is much more restricted than the usual condition on the slope of the profile predicts, and is due to the abrupt change in slope at the core-cladding interface.
© 1978 Optical Society of America
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