For a general class of graded-index fiber profiles, modal propagation constants and group delays were previously evaluated asymptotically to O(k−9) by using the systematic calculation scheme of evanescent wave theory. Here, the numerical results obtained directly are improved by using a nonlinear transformation technique developed by Shanks. This technique extracts information from the diverging as well as converging part of an asymptotic series, whereas the direct method utilizes the converging part alone. We obtain accurate results for a profile far from parabolic shape as well as for one of near-parabolic shape. For the profile far from parabolic shape the results quantify the error in group delay for the WKB technique for the lowest-order mode to approximately 30 ps/km. For high-order modes the WKB results agree with the transformed results.
© 1980 Optical Society of America
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