Mode-dependent attenuation in optical fibers

A theory with which one can calculate modal distributions within a multimode fiber excited by a source of arbitrary spatial coherence, but within the mode-continuum limit, is presented. The theory is specialized to the case of incoherent source excitation, which is arbitrarily limited in both spatial and angular extent. The theory is then applied to the problem of determining the significance of various experimental techniques designed to measure differential mode attenuation. It is found that standard measurement techniques measure a quantity that differs from the actual differential mode attenuation. It is also illustrated that it can be difficult to calculate accurately the differential mode-attenuation coefficient from measurement data.