Broadband all-order polarization mode dispersion compensation using liquid-crystal modulator arrays

Polarization mode dispersion (PMD) is a potential limiting factor in long-haul high-speed optical communications, especially in systems beyond 10 Gb/s. Although considerable effort has been devoted to compensation methods for PMD, most of the research is restricted to a small bandwidth, within the limits of the first- and second-order PMD approximations. For the first-order approximation to be valid, the distortions induced by PMD must remain less than a few tenths of the pulse duration or the bit period for return-to-zero (RZ) or non-return-to-zero (NRZ) systems, respectively. In this work, as far as the authors know, the application of ultrafast optical pulse-shaping techniques for experimental broadband all-order PMD compensation is demonstrated for the first time. PMD is treated as arbitrary variations of state of polarization (SOP) and phase versus wavelength, in an all-order sense. Two fiber-pigtailed pulse shapers are implemented in a serial manner to compensate the polarization and phase spectra independently. The first step corrects the wavelength-dependent polarization states to a fixed wavelength-independent state. This reduces the PMD compensation problem to a generalized chromatic dispersion compensation problem. Consequently, equalization of the spectral phase in the second step restores the clean broadband pulse signal. In the experiments, compensation of subpicosecond pulses (14 nm bandwidth around 1550 nm) that are anomalously spread to more than 2 ps due to PMD is demonstrated. These results are potentially relevant for future ultra-high-speed time-division-multiplexed (TDM) systems. Research challenges that must be addressed to bring this approach to practical application are also briefly enumerated, and the potential for scaling to a range compatible with wavelength-division-multiplexed (WDM) systems is discussed.