Polarization Mode Dispersion in Distributed Erbium-Doped Fibers

In the design of distributed erbium-doped fibers (DEDFs) for ultra-high bit-rate (>40 Gbit/s) soliton transmission, polarization mode dispersion (PMD) is an important design parameter. We have measured the PMD values of DEDFs from two different sources and found them to be an order of magnitude higher than those suitable for ≥40 Gbit/s soliton transmission. Theoretical modeling and microscopic inspection of fiber core ellipticity have been carried out to understand the high PMD values in these DEDFs. It is believed that core ellipticity and erbium doping process-induced stress are the main reasons for the high PMD values in these fibers. This highlights an important design and fabrication problem which must be solved before DEDFs can fulfil their promise as a channel for long-haul ultra-high bit-rate soliton transmission.