Failure of the scalar approximation near cutoff frequency of a step-index fiber mode

The feasibility of the scalar approximation used in the theory of weakly-guiding optical fibers is considered near the cutoff frequency of the HE/sub 1n/(LP/sub 0n/) mode. The asymptotic behavior of polarisation corrections for transverse and longitudinal wavenumbers of a lossy fiber mode is investigated both analytically and numerically. These results as well as the transmittance calculated for an irregular fiber consisting of single-mode and bimodal segments demonstrate the failure of scalar approximation near the cutoff frequency.