<title>Design of a broadband optical waveguide for WDM systems: propagation solution by the finite element method</title>

Abstract

The enormous transmission capacity of the order of thousand of Gb/s offered by single mode fibers cannot be fully utilized with the conventional step index fiber. To increase the bit rate and the repeater spacing, some more advanced fiber designs have to be considered. However for these designs and the new ones to come to a numerical technique, which is fast and accurate, is required for calculating the dispersion. Most of the existing numerical approaches are good enough for the calculation of propagation constant, though some of them are not suited for arbitrary refractive index profile. However for the numerical calculation of dispersion, first and second derivatives of the propagation constant are required. So the propagation constant must be highly accurate so as to ensure accuracy of dispersion calculations. A powerful numerical technique, the quadratic finite element method (FEM) is used for analyzing the modal characteristic of single mode optical fiber with arbitrary refractive index profile. The simulated results when compared with earlier reported ones for step index profile, confirms the accuracy of the proposed numerical technique. It is shown that multiple cladded fiber is better suited for Wideband Systems. Dispersion flattened fibers have been proposed in the past with W-profile, but all such fiber designs have been observed to be bend sensitive in the long wavelength window, as one has to operate very close to the cutoff of the fundamental mode. The cutoff wavelength of the fundamental mode can be increased by introducing a second region of raised index into W-structure and thus, the light which penetrated (leaked) into the outer cladding earlier can now be retrapped. Computations show that by properly optimizing the profile parameters, a fiber can be designed where the dispersion can be kept confined with in +/- 1.0 ps/km-nm over a wide entire wavelength span from 1290 to 1550 nm. The wavelength span includes both the low loss transmission window near 1310 and 1550 nm. The results suggest an excellent wideband optical waveguide for future WDM systems.