EQUILIBRIUM MODE DISTRIBUTION (EMD)

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 .