LIMITATIONS IN THE PERTURBATION ANALYSIS OF BENT FINITE-CLAD FIBERS AND WAVE-GUIDES

The transverse shift in the field distribution and the correction to the propagation constant of the fundamental and symmetric cladding modes on bent finite-clad single-mode fibres and slab waveguides are evaluated from perturbation theory for effective index values extending below the cladding index. Analytical results are derived in both geometries for the step-profile that are valid within the overall limitations of the theory. However, it is found that, for the fibre geometry only, the method breaks down at certain discrete wavelengths because of degeneracies that occur between the HE(12) (LP(02)) and TE(01) (LP(11)) mode propagation constants.