Polarization-induced pulse spreading in birefringent optical fibers with zero differential group delay

The polarization properties of concatenations of trunks of birefringent fibers and elements with polarization-dependent losses are analyzed. We show both theoretically and experimentally that the concatenation can have zero differential group delay over a whole range of wavelengths but that a pulse propagating down the concatenation can still experience significant pulse spreading. In this example the two main methods used for characterizing polarization mode dispersion in optical fiber systems, i.e., Jones matrix eigenanalysis and the interferometric method, give different results. This counterintuitive example underlines the need for a careful assessment of the basic concepts related to polarization effects in the presence of polarization-dependent losses.