Leaky mode attenuation coefficients for graded index optical fibers are derived using the WKB approximation. For the case of a parabolic index profile, the attenuation coefficients for explicitly evaluated and used to calculate the leaky mode contribution to near field and attenuation measurements. Failure to observe the expected leaky mode contributions in several graded index fibers is interpreted as evidence for the presence of an additional loss mechanism.
Robert Olshansky, "Leaky modes in graded index optical fibers," Appl. Opt. 15, 2773-2777 (1976)