Keeping Chromatic Dispersion in Line at 40 Gbit/s

In addition to increased speed and bandwidth, the 40 Gbit/s technology comes with its own set of issues. Chromatic dispersion, which is caused by individual wavelengths traveling at different speeds in the fiber, is controllable at 10 Gbit/s. Since tolerance is fairly high, non-optimized compensation techniques can be used; namely, dispersion-compensating fiber (DCF), which acts as a broadband compensator. Although this seldom compensates for all the wavelengths with the same efficiency, at 10 Gbit/s transmission, broadband compensation is good enough (unless there are several spans in a row with several DCFs, each with their residual dispersion adding up). Residual dispersion can easily reach 300 ps/nm without causing any real issues.

In 40 Gbit/s transmission, on the other hand, the limited tolerance for chromatic dispersion leads to rather restrictive requirements when it comes to measuring link dispersion as well as applying compensator tolerances. A way to circumvent this is to use adaptive compensation. When using a non-return-to-zero (NRZ) modulation, the chromatic dispersion at 40 Gbit/s is 16 times more stringent than at 10 Gbit/s. The maximum residual dispersion of the 40 Gbit/s application codes is defined at 32 ps/nm. This tolerance has to be shared between the possible variation, during the lifetime of the link, and measurement precision and the tolerance of the dispersion compensator.

Based on a nominal chromatic dispersion of 17 ps/nm km at 1550 nm (i.e., 1360 ps/nm for 80 km), this results in a total possible tolerance of about 2.3% for both dispersion-compensator and link-dispersion tolerance. This imposes severe constraints on the component specifications as well as on the precision of the measurement equipment.

This brings us to accurate per-wavelength compensators. Since broadband compensators may have residual (positive or negative) CD at a certain wavelength, with the low tolerances required by 40 Gbit/s NRZ, these compensators are just not accurate enough across the DWDM band.

Often, these broadband compensators are optimized at 1550 nm. The following figure shows the danger of imperfect slope compensation with respect to one perfectly compensated channel:

Obviously, such a compensator is more than enough for 10 Gbit/s applications, but fails for 40 Gbit/s transmission. The figures below illustrate channel 1, 3 and 4, respectively:

In the next edition, we will discuss a different modulation format that helps overcome these issues.