The evanescent wave theory (ewt) is extended to treat modal propagation along a general graded-index optical-fiber structure surrounded by a uniform cladding. A step or a valley may be present at the core–cladding interface. The fiber core may be specified in a more general form than was the case for untruncated profiles. The reformulation of the original ewt method uses the uniform corrective method, developed by Arnold for WKB calculations, iteratively to specify nonintegral radial mode numbers for the clad index profile. The integral radial mode numbers of the unclad profile are used as first approximations. Our method gives corrected values for propagation constants and group delays of guided modes and complex propagation constants of leaky modes. Numerical results show good agreement with a numerical solution for a parabolic-core fiber. For other profiles they specify accurate asymptotic upper and lower limits for the cladding influence. For leaky modes of a truncated near-parabolic profile we show agreement with results of a simple WKB formulation. A valley, present at the core–cladding interface, causes a reduction in the maximum intermodal group-delay difference from 3.1 to 1.9 nsec/km because of a strong attenuation of high-order (leaky) modes.
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