Equilibrium Modal Distribution (EMD)

EMD of light travelling in an optical waveguide or fiber, is the distribution of light that is no longer changing with fibre length or with input modal excitation. Achieved some distance from the source, where the relative power in the modes becomes stable with increasing distance.
The condition in a multimode fiber wherein after propagation has taken place for a certain distance, called the equilibrium length, the relative power distribution among modes becomes statistically constant and remains so for the course of further propagation down the fiber.