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40G/100G Coherent Detection Systems: Do They Deliver the Bang for the Buck?

The telecom industry seemingly has a firm grasp on 40G/100G coherent detection systems, with deployments extensively deployed in many regions across the globe. Coherent systems are heralded as an impressive solution to compensate for Polarization Mode Dispersion (PMD), thanks to advanced algorithms and advanced modulation formats.

But do they actually deliver the results?

EXFO partnered with General Photonics, an expert in lab measurements, to examine why PMD compensation might fail or be ineffective as well as the best options to mitigate the ripple effects.

It is important to remember that coherent detection systems perform four main polarization-related functions: 1) continuous state of polarization (SOP tracking), 2) polarization demultiplexing, 3) PMD compensation (PMDC), and 4) polarization-dependent loss (PDL) mitigation (PDLM). These functions are accomplished using high-speed DSP circuits and algorithms in the transceivers. Several factors can affect the performance of the first two functions and lead to degradation in the performance of PMD compensation and lead to increase bit error rate (BER), including:

  • Fast SOP changes
  • Abrupt SOP changes
  • Loss of SOP orthogonality
  • Fast PMD changes
  • Sudden PMD changes
  • Large PMD values
  • The presence of PDL

It makes sense that a fiber with lower PMD is less likely to exceed the system’s PMD threshold, but how can the other six risks be reduced? In short, PMDC failures can be reduced by measuring the PMD of each fiber and by not using the fibers with high PMD, because all seven risk factors for PMD compensation failure increase when the fiber PMD is high. EXFO offers a wide range of rugged, portable PMD testers, including the FTB-5700, the only single-ended dispersion tester on the market that can measure chromatic dispersion and PMD from a single location, eliminating the need for two technicians at each end of the link. EXFO also offers the industry’s only distributed PMD analyzer as well as the FTB-5500B PMD Analyzer designed to measure the PMD of amplified links. By systematically using these tools to measure the PMD of fibers during commissioning, operators can significantly reduce bit-error rates on 40G/100G networks.

To learn more about the findings from EXFO and General Photonic’s research on PMD, read the whitepaper “Why Coherent Detection Systems May Fail at Compensating for Polarization Mode Dispersion.”

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