Abstract

Because of differential mode attenuation, the transmission losses of multimode graded-index fibers depend on the mode distribution propagating in the fiber, and accurate loss data which can be extrapolated to arbitrary fiber lengths are only achievable with an equilibrium mode distribution (EMD). The EMD establishes itself as a result of the selective loss of the various mode groups and the replenishing of their power via coupling from other modes. Since the distance within which this occurs may amount to more than 10 km's in recent low-mode-mixing fibers, techniques have been proposed to approximate this distribution by using re- stricted-numerical-aperature and spot-size excitation^1,2,3, mode filters⁴, mode scramblers^5,6,7, and long reference or dummy fibers⁸. However, the use of most of these techniques, per se, does not guarantee the creation of an EMD. For example, a low-mode-mixing fiber excited with lower-order modes does not establish the EMD even for a many-km fiber length. A necessary condition for the existence of the EMD is the constancy of the far-field radiation pattern (FFP) along the fiber as has first been utilized for the length-independent loss measurements of plastic-clad silica fibers⁹.