Previous papers have treated power transfer between HE11, TE01, and TM01 modes propagating on identical cylindrical fibers. Here we extend the theory to include power transfer between modes of any order propagating on uniform circular fibers of different radii and dielectric constant. A simple analytical expression for the coupling coefficient is derived. The error in using the decoupled two-mode form of the coupled-mode equations is determined. Examples are given to illustrate the extension of the two-fiber results to arrays of fibers with different properties. All results are presented in a dimensionless form applicable to circularly cylindrical fibers of arbitrary physical parameters.
Peter D. McIntyre and Allan W. Snyder, "Power transfer between optical fibers," J. Opt. Soc. Am. 63, 1518-1527 (1973)