Modal and coupling characteristics of low-order modes in thermally diffused expanded core fibers

In this paper, the Galerkin's numerical method is applied in order to calculate propagation constants, optical intensity distributions and far-field-patterns of low-order LP-modes in thermally diffused expanded core (TEC) fibers. The problem of efficient coupling between Gaussian laser beams and single-mode TEC-fibers is also considered. It is demonstrated that TEC fibers are good candidates for Laser source-to-fiber optical coupling scheme with long working distance. The numerical results are compared with the corresponding ones obtained by a one-parameter variational method utilizing Laguerre–Gauss functions. Galerkin's method is generally stable and more accurate when the mode is away from the cut-off region. On the other hand, the variational method provides an explicit form of the modal fields if the variational parameter is accurately determined.