Publisher Summary Polarization mode dispersion (PMD) is a linear effect that can be compensated in principle. In an ideal circularly symmetric fiber, the two orthogonally polarized modes have the same group delay. However, in reality, fibers exhibit a certain amount of birefringence because of imperfections in the manufacturing process or mechanical stress on the fiber after manufacture. It is noted that fluctuations in the polarization mode and fiber birefringence produced by the environment lead to dispersion that varies statistically with time and frequency. PMD causes different delays for different polarizations and when the difference in the delays approaches a significant fraction of the bit period, it leads to pulse distortion and system penalties. Environmental changes— including temperature and stress—cause the fiber PMD to vary stochastically in time. PMD, illustrating the basic concepts, the measurement techniques, the PMD measurement, the PMD statistics for first- and higher orders, the PMD simulation and emulation, the system impairments, and the mitigation methods has been summarized in the chapter. Both the optical and the electrical PMD compensations are considered.

Polarization-Mode Dispersion

Polarization mode dispersion (PMD), especially in "old" fibers, is considered harmful for installation and upgrading of trunk lines. An optical PMD equalizer should have several or many differential group delay (DGD) sections with polarization transformers in between which can endlessly transform any input polarization into a principal state of the following DGD section. The sections must practically have fixed DGDs unless there is only one section. The small-signal baseband transfer function for PMD, higher order PMD, and the necessary number of sections as well as their control by the output signals of an electrical filter bank in the receiver are also discussed in this context. Several PMD equalizers have been realized and successfully tested in transmission systems with bit rates of 10, 20, and 40 Gb/s. The systems operated stably with well-opened eye diagrams for DGDs ranging between 0 and 1.7 bit durations. Best performance is obtained from a distributed PMD equalizer with one piece of polarization-maintaining fiber twisted by 64 stepper motors. The principle can also be realized in LiNbO/sub 3/.

Polarization mode dispersion compensation at 10, 20, and 40 Gb/s with various optical equalizers

We quantify the benefits of using different techniques for compensation of polarization mode dispersion (PMD) in fiber-optic communication systems by means of numerical simulations. This is done both with respect to PMD-induced pulse broadening and in terms of system outage probability for different data formats [nonreturn-to-zero (NRZ) and return-to-zero (RZ)]. Attention is focused on simple and relevant single- and double-stage post-transmission compensators with a few degrees of freedom (DOF). It is generally believed that a PMD compensator with a polarization controller and a variable delay line can only compensate the PMD to the first order. We show, from analytical results, the counterintuitive fact that this scheme can also partially compensate for higher order PMD. We also investigate the benefit of using a polarizer as compensation element where the optical average power can be used as a feedback signal.

A comparison between different PMD compensation techniques

This paper presents a new method for the measurement of distributed polarization mode dispersion (PMD) in optical fibers. This method uses a polarization optical time-domain reflectometer (P-OTDR), and is based on a measurement of the degree of polarization of the backscattered light as a function of distance in the fiber. Both the average and the statistics of the degree of polarization are used to estimate the two relevant parameters for measuring PMD, namely, the beat length and the coupling length. At present, our P-OTDR gives qualitative results only. However, it enables to distinguish between high and low PMD sections in a long fiber link. This should already have practical applications, in particular for the characterization of installed fibers.

Distributed PMD measurement with a polarization-OTDR in optical fibers

We review recent research related to polarization mode dispersion (PMD) in fiber-optic transmission systems operating at 40 Gb/s and above. We investigate the benefits of using different passive techniques for mitigating the effects of PMD, including more advantageous data formats compared to the conventional nonreturn-to-zero (NRZ) format, soliton transmission, and forward error correction (FEC). We also compare a number of active pre- and posttransmission compensators.

Polarization-mode dispersion in high-speed fiber-optic transmission systems

We demonstrate a novel, adaptive optical polarization mode dispersion equalizer in a 10-Gb/s transmission experiment. The equalizer comprises a fast electrooptic polarization transformer an adjustable differential polarization delay line, and a simple electric distortion analyzer for automatic feedback control.

http://ieeexplore.ieee.org/iel4/5897/15800/00732706.pdf

D.L. Wilson

Papers

Automatic compensation of first order polarization mode dispersion in a 10 Gb/s transmission system