Polarization Mode Dispersion - Compensating For PMD

Compensating For PMD

A PMD compensation system is a device which uses a polarization controller to compensate for PMD in fibers. Essentially, one splits the output of the fiber into two principal polarizations (usually those with dτ dω = 0, i.e. no first-order variation of time-delay with frequency), and applies a differential delay to bring them back into synch. Because the PMD effects are random and time-dependent, this requires an active device that responds to feedback over time. Such systems are therefore expensive and complex; combined with the fact that PMD is not yet the limiting factor in the lower data rates still in common use, this means that PMD-compensation systems have seen limited deployment in largescale telecommunications systems.

Another alternative would be to use a polarization maintaining fiber (PM fiber), a fiber whose symmetry is so strongly broken (e.g. a highly elliptical core) that an input polarization along a principal axis is maintained all the way to the output. Since the second polarization is never excited, PMD does not occur. Such fibers currently have practical problems, however, such as higher losses than ordinary optical fiber and higher cost. An extension of this idea is a single-polarization fiber in which only a single polarization state is allowed to propagate along the fiber (the other polarization is not guided and escapes).