PROPAGATION CHARACTERISTICS OF NEAR SQUARE LAW OPTICAL FIBRES - A PERTURBATION ANALYSIS.

A first order perturbation analysis was made to obtain the propagation constants for an optical fiber characterized by a near square dielectric constant profile. Closed form expressions for the propagation constants and group velocities in such a medium were obtained and the results were compared with those obtained by the WKB approximation. Although both the methods give almost identical results for multimode fibers, the perturbation method appears to be more useful as it gives closed form analytical expressions for the propagation constants. The variation of pulse broadening with the displacement of an incident Gaussian beam from the fiber axis was also studied and the results were compared with available experimental data.