The original problem for eigenwaves of weakly guiding
optical fibers formulated on the plane is reduced to a convenient for numerical solution
linear parametric eigenvalue problem posed in a disk. The study of the solvability of
this problem is based on the spectral theory of compact self-adjoint operators.
Properties of dispersion curves are investigated for the new formulation of the problem.
An efficient numerical method based on FEM approximations is developed. Error estimates
for approximate solutions are derived. The rate of convergence for the presented
algorithm is investigated numerically.