Steady state Modal Distribution in Multimode fiber, achieved some distance from the source, where the relative power in the Modes becomes stable with increasing distance.

That 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.

Note 1: In practice, the equilibrium length may vary from a fraction of a Kilometer to more than a kilometer.

Note 2: After the equilibrium length has been traversed, the Numerical Aperture of the fiber’s output is independent of the numerical aperture of the optical source, i.e ., beam, that drives the fiber. This is because of Mode Coupling and stripping, primarily by small perturbations in the fiber’s geometry which result from the manufacturing and cabling processes.

Note 3: In the ray-optics analogy, the equilibrium mode distribution may be loosely thought of as a condition in which the “outermost rays” in the fiber Core are stripped off by such phenomena as microbends, and only the “innermost rays” continue to propagate. In a typical 50-m core multimode Graded-Index fiber, Light propagating under equilibrium conditions occupies essentially the middle seven-tenths of the core and has a numerical aperture approximately seven-tenths that of the full numerical aperture of the fiber. This is why in-line optical attenuators based on the principle of Gap Loss may be ineffective or induce a lower-than-rated Loss IF they are inserted near the optical receiver. To be fully effective, gap-loss attenuators should be inserted near the optical transmitter, where the core is fully filled. [After FAA] Synonym S equilibrium mode power distribution, steady-state condition.