Regenerator tackles the problems of PMD

By TAMI FREEMAN, FibreSystems Europe -- Polarization-mode dispersion can cause serious headaches in 10- and 40-Gbit/sec optical systems, inducing signal errors and traffic outages in a manner that is both unpredictable and seemingly random in the appearances it makes across the network. Now, however, researchers at the Chalmers University of Technology in Sweden have come up with a new take on the problem, one that involves the use of optical regenerators for PMD compensation.

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By TAMI FREEMAN, FibreSystems Europe -- Polarization-mode dispersion (PMD) can cause serious headaches in 10- and 40-Gbit/sec optical systems, inducing signal errors and traffic outages in a manner that is both unpredictable and seemingly random in the appearances it makes across the network. Now, however, researchers at the Chalmers University of Technology in Sweden have come up with a new take on the problem, one that involves the use of optical regenerators for PMD compensation.

A big benefit of using regenerators for PMD compensation is their ability to correct other sources of signal impairments, such as chromatic dispersion, nonlinearities, and noise. "In future, regenerators might be a standard, low-cost component that can mitigate any transmission problem," explains researcher Magnus Karlsson.

Regenerators come in two forms: 2R (which reshape and reamplify the signal) and 3R (2R plus signal retiming). Previous research into the use of 2R regeneration for PMD compensation disclosed severe timing problems, in particular for return-to-zero (RZ) pulses. Adding clock recovery and retiming, however, greatly improves the regeneration of PMD-impaired data.

Karlsson and colleagues used a series of simulations to investigate a 3R regenerator's ability to reduce the outage probability in an optical transmission line. The simulated set-up comprised a transmission line with the 3R regenerator placed midway between the transmitter and receiver.

The researchers compared the performance of a conventional three-degrees-of-freedom PMD compensator (placed immediately before the receiver) with that of 3R regenerators with power margins of 2 and 5 dB. For non-RZ (NRZ) signals, the 5-dB regenerator exhibited a similar performance to the conventional compensator, whereas the 2-dB regenerator did not perform as well.

For RZ signals, the research group considered a fixed receiver (in which the threshold and sampling time are fixed) and an adaptive receiver (which samples at a time and decision level that minimizes the bit-error rate).

The conventional compensator exhibited a near-identical performance with both receiver types. The regenerators, on the other hand, exhibited significantly improved performance when used with the adaptive receiver, due to the timing-misalignment property of PMD-impaired data.

In this scenario, the 5-dB regenerator reduced the outage probability from around 2 x 10-1 to about 10-3 at an average differential group delay of 42% of the bitslot, while the conventional compensator only reduced the outages to around 10-2. The 2-dB regenerator performance was similar to that of the PMD compensator.

The researchers concluded that the 3R regenerator offers a similar PMD-mitigating performance to that of conventional PMD compensators. However, for systems limited solely by PMD, conventional compensators are probably a more cost-effective choice.

"Regenerators offer the advantage when you have other impairments as well as the PMD that need to be fixed," Karlsson explains. "Next, we aim to look into the problems and challenges of regeneration experimentally, mainly using all-optical configurations."

Tami Freeman is deputy editor, FibreSystems Europe in association with LIGHTWAVE Europe.

• This article originally appeared in FibreSystems Europe in association with LIGHTWAVE Europe, September 2004 p10.
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Replacing standard PMD compensators with regenerators could enable signal regeneration that is independent of the source of distortion.
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