When guiding is due to a continuously varying, rather than discontinuous, index of refraction,n(z), andn(z)peaks at some point inside the propagation medium, there may result "buried modes," i.e., modes whose exponential decay takes place inside the guiding medium rather than, as is usual, in an adjacent medium. The conditions for obtaining these modes are discussed in the Wentzel-Kramer-Brillouin (WKB) approximation and the mode equations given for both these modes and the usual ones for an illustrative case.
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E. M. Conwell (1975) studied this question.
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