Behavior of the first higher-order modes of a circular core optical fiber whose core cross-section changes into an ellipse

The vector finite element method is used for investigating the behavior of a set of the first higher-order, four-fold degenerate modes (TE/sub 01/, TM/sub 01/, and two-fold degenerate HE/sub 21/ modes) of a circular core optical fiber whose core cross-section changes into an ellipse. It is found that the TE/sub 01/ mode becomes the first higher-order mode polarized along the minor axis, E/sub 21//sup y/ mode, in a dual-mode region. The remaining three modes become, respectively, the first higher-order mode polarized along the major axis, E/sub 21//sup x/ mode, and the second higher-order modes polarized along the major and minor axes, E/sub 12//sup x/ and E/sub 12//sup y/ modes, in the order of magnitude of the effective index. It is also found that the effective index of the E/sub 21//sup x/ mode is smaller than that of the E/sub 21//sup y/ mode in a small ellipticity region.<
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