Some enabling techniques for polarization mode dispersion compensation

Progress toward 40-Gb/s polarization-mode dispersion (PMD) compensation is presented in several areas. A single-waveplate polarization scrambler that generates Stokes´ parameters with just three harmonics has been realized. Together with an arrival-time detection scheme, it allows the detection of about 1 ps of PMD within 2.4 μs in a 40-Gb/s nonreturn-to-zero transmission setup. A scrambler that operates independent of its input polarization has also been realized. Both scramblers can be shared among a number of wavelength-division-multiplexed (WDM) channels for which PMD is to be detected. Furthermore, an inverse scattering technique has been implemented to determine differential group-delay profiles of an improved distributed PMD compensator in X-cut, Y-propagation LiNbO₃. In this context, the most common kind of higher order PMD is identified, which typically persists after compensation of first-order PMD. In a fiber link with polarization scrambling, it is shown to be measurable in a most efficient manner by comparing the rising and falling slopes of the detected signal using two oppositely poled one-way rectifiers placed at the output of a differentiator.