Calculations have been made of mode properties of optical fibers with ring index profiles. Three idealized profiles with regions of depressed index on axis were chosen for study. The quantities of interest are the propagation constant for various modes, the group delay, and the near and far field mode patterns. Near and far field patterns are compared with Gaussian TEM laser modes. The results are particularly relevant to studies of nonlinear interactions in fiber waveguides.
R. H. Stolen, "Modes in fiber optical waveguides with ring index profiles," Appl. Opt. 14, 1533-1537 (1975)