Fiber-optic Dispersion Investigator

– Led Investigation of Polarization Mode Dispersion Impact on 10 Gb/s Fiber-optic Transmission Performance –

Situation

In 1998, while working at Nortel in Tier-3 Technical Product Support for long-haul fiber-optic networks, a support call came in for a number of optical links suffering unexplained performance problems. Field personnel had done basic troubleshooting and neither Tier-1 nor Tier-2 support was able to find the problem source.

Actions

I led remote field teams and local design personnel through this six-week technical investigation. After ruling out product-related problems through troubleshooting, I:

  • Developed custom software tools to collect performance data from field equipment via modem(!)
  • Developed Microsoft Excel scripts/macros to parse, partially analyze, and plot the data
  • Analyzed error trends over time and comparing different problematic optical links
  • Cross-examined field performance problems with optical fiber manufacturers and types
  • Involved a physicist from the Optical Link Design group to help identify fiber characteristics that could be at fault
  • Instructed field personnel to use test equipment to measure polarization mode dispersion (PMD) in the problem links, confirming that PMD was the culprit
  • Reproduced and confirmed the phenomenon by testing on our laboratory system

Outcomes

The installed fibers turned out to have higher polarization mode dispersion (PMD) than the optical link design team had anticipated would be present in the field. PMD causes one polarization axis of laser light to travel slower down the fiber compared to the other (orthogonal) polarization axis. These two components travelling at right angles, but slightly different speeds, caused blurring of the receiver “eye” (i.e., poor distinction between ones and zeros), resulting in data performance errors.

High PMD can occur in optical fiber for:

  • Physical reasons, such as fiber manufactured with slightly oval cross-section or that was unevenly stretched when the fibers were laid
  • Optical reasons, such as manufactured with a non-uniform index of refraction in the radial direction (cross-section) and/or along the length of the fiber

Based on our research into the global market share of fiber manufacturers and fiber types, we estimated that an alarming 10% of world-wide installed fiber, at the time, suffered from PMD that was high enough to prevent error-free 10 Gb/s optical transmission on those fibers.

To wrap up the investigation, I co-authored an engineering bulletin to all Nortel long-haul fiber-optic customers world-wide. In the bulletin, I:

  • Highlighted the fiber manufacturers, types, and manufacturing periods that could be problematic for 10 Gb/s and higher transmission systems
  • Provided instructions for Nortel field personnel and customers to measure PMD for planned and deployed systems

I won an executive award for my integral role in helping to resolve this unexpected and high-visibility issue.