Polarization pairing kills distortion

Polarization pairing kills distortion
Polarization-mode dispersion (PMD) is a significant barrier to achieving single-channel data rates at 10 Gbits/second and beyond in optical communication systems. PMD results in pulse broadening and distortion, and leads to system performance degradation. A considerable effort has been devoted in recent years to mitigate the effects of PMD, based on optical, electrical and optoelectrical PMD compensators. Among the PMD compensation techniques, electrical-domain (post-detection) approaches are particularly attractive because of their potential for compact and cost-effective implementation in the chip sets at the receiver.

Electronic equalizers using simple feed-forward and decision feedback structures have been proposed for mitigating intersymbol interference (ISI) in optical communications, and have been recently implemented and tested at 10 Gbits/s using integrated silicon germanium technology as analog equalizers for PMD mitigation. However, they do not deliver the performance gains typically expected, and the optimization of filter coefficients adaptively, even with the simple least mean squares (LMS) algorithm, is still a challenging task at the high data rates at which optical systems operate.

Since all high-data-rate systems use direct detection, the polarization phase information is lost during detection. Polarization diversity can provide additional advantages for PMD mitigation by making more efficient use of the available information.

We are describing a novel polarization diversity receiver that uses simple fixed optics and electronics to obtain significantly better performance than is possible with the use of an equalizer (with no diversity) that requires optimization of the filter taps by some means. There is little additional improvement by incorporating electronic equalization into the diversity receiver structure, since the diversity structure eliminates the major part of the ISI due to PMD.

In the system, an incoming signal is equally split into three pairs of orthogonal polarizations that are detected by independent photodetectors. The advantage of using pairs of orthogonal polarizations is to allow the detection of the total signal power in such a way that the amplitude margin can be increased while maintaining the original noise distribution in the system. This differs from a structure that uses single polarizations and hence has higher sensitivity to noise.

In the first branch of the proposed diversity receiver, a linear polarization beam splitter (LPBS) is used to split the signal between vertical and horizontal polarizations. In the second branch, a quarter-wave plate converts the signal from circular to linear polarization before the polarization beam splitter (PBS), that is rotated by 45 degrees with respect to the first PBS, which splits the signal into right- and left-circular polarizations. In the third branch, a PBS splits the signal into two diagonal polarizations. The combiner/equalizer block combines the pairs of signals synchronized by an electrical delay.

Discrete samples are obtained with a clock recovery subsystem and a decision circuit. Finally, the decision module selects the branch that has the maximum amplitude margin.

The detection of three pairs of orthogonal polarizations allows the receiver to obtain at least one pair of orthogonal polarizations of the signal with a reduced PMD distortion. Equalization then leads to an additional penalty reduction when added to the diversity receiver with three orthogonal polarizations. An equalizer requires optimization of the filter coefficients-either user-tuned or adaptively computed. On the other hand, a diversity receiver with the simple combiner does not require any coefficient optimization and can be realized with the current technology at 10-Gbit/s speeds and beyond.

It does require multiple photodetectors for each wavelength channel. Tests showed that the diversity receiver achieves a larger reduction in power penalty than can be achieved using an electronic equalizer. Also shown is that combining an equalizer with the diversity receiver offers little additional improvement.

Other contributors to this article include T. Adal and I.T. Lima Jr., researchers, Department of Computer Sciences and Electrical Engineering at Maryland University.

The complete paper is being presented at the Optical Fiber Communications Conference.

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