The optical system is then
in the strongly coupled regime and has established an equilibrium
mode distribution (EMD). We consider a fiber of sufficient length
L > Lc consisting of n segments, each of length l. This intrinsic
length characterizes the coupling strength of the fiber and can be
imagined as the mean free path of the energy packets, that is,
the average length an energy packet travels in a mode before it is
coupled to another guided mode. The coupling length for some
POFs (NA) has been found to be about 2 m [13].