Equilibrium Mode Distribution (EMD) is a steady state in multimode fibers where the relative power density
between the different modes is balanced. The steady state is reached when the modes in the optical fiber
have propagated over a certain distance, the so-called Equilibrium Length. After the Equilibrium Length, the EMD state is reached. In practice, the Equilibrium Length varies from a few meters to more than one kilometer.
After the Equilibrium Length, the numerical aperture at the output of the optical fiber is independent of the numerical aperture of the optical source.