Mode coupling in glass optical fibers and liquid-core optical fibers by three methods

•We test two functions for calculation of coupling characteristics in optical fibers.•The two functions are compared to the solutions of the power flow equation.•The SI glass optical fibers and liquid core optical fiber are investigated.•The two functions, while simpler to apply, are less accurate.

We test Slemon and Wells's function and recently reported Hurand et al.'s (Appl. Opt., 50, 492–499, 2011) function for calculation of coupling characteristics in step-index optical fibers against experimental measurements and against calculations by a related method that is based on the power flow equation. Compared are the coupling length Lc (which is the fiber length where the equilibrium mode distribution is achieved) and length zs (where steady-state distribution is achieved) in three step index glass optical fibers as well as a liquid core optical fiber. The two functions, while simpler to apply being just algebraic formulas, are less accurate over a wide range of numerical apertures. It is also shown that fibers with same coupling coefficient can have much different coupling characteristics.