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

The transverse shift in the field distribution and the correction to t he propagation constant of the fundamental and symmetric cladding mode s on bent finite-clad single-mode fibres and slab waveguides are evalu ated from perturbation theory for effective index values extending bel ow the cladding index. Analytical results are derived in both geometri es 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 deg eneracies that occur between the HE(12) (LP(02)) and TE(01) (LP(11)) m ode propagation constants.