As most of you reading this know, Chromatic Dispersion (the spreading of a signal because different wavelengths propagate at different speeds through fiber) is a primary concern for anyone deploying a 10 Gbps network. In this post, I will discuss the different methods of dispersion compensation available and how they might be implemented in various network scenarios.

Optical vs. Electrical Compensation

Today, the most popular method of dispersion compensation involves the deployment of spools of dispersion-compensating fiber (DCF). Typically, these spools are placed near the termination point of the fiber link to be compensated. They allow for the compensation of a large number of DWDM channels at a fixed value. This turns out to be a very inefficient method of compensation, and here’s why:

• DCF adds additional losses to your link budget which must be accounted for
• DCF is expensive
• These losses and costs are incurred without the benefit of extending the overall link

One alternative method is to include the DCF in the link itself. However, this is only practical for those who are laying new fiber and can engineer the cable such that the DCF is periodically interleaved with the non-DCF such that the overall link dispersion is controlled. See “Long-Haul Soliton WDM Transmission with Periodic Dispersion Compensation and Dispersion Slope Compensation” published here for an example of how this was accomplished to engineer an 8000 km optical link.

Tunable Dispersion Compensation

Using a variety of techniques, tunable dispersion compensators can be built that will compensate up to ± 1600 ps/nm of Chromatic Dispersion. These are optoelectronic devices that sit just in front of the photodetector on a line card and compensate the dispersion prior to signal recovery.

Fiber Bragg Gratings (FBG’s) are being employed as tunable compensators. By changing the properties of the FBG, certain wavelengths can be delayed (or accelerated) as desired. This accomplished by either heating the fiber or applying mechanical stress along its length. The result is a device that can predictably alter the latency of some, or all, of the wavelengths being transported by a specific fiber.

Civcom recently announced its Free-Path Manageable Dispersion Compensation Module (M-DCM), which is an etalon based tunable optical dispersion compensator. The M-DCM is a multi-channel device that can compensate up to +/- 1700 ps/nm of chromatic dispersion.

Another provider, TeraXion recently demonstrated its Fiber Bragg Grating (FBG)-based Tunable Dispersion Compensator, another multi-channel device that can tune over +/- 1600 ps/nm.

Electrical Dispersion Compensation (EDC)

Electrical dispersion compensation (EDC) has recently begun to emerge as a viable alternative to optical dispersion compensation (ODC). Several companies are touting EDC solutions that compensate for up to +/- 2800 ps/nm (+/- 2000 ps/nm is more believable) of chromatic dispersion. EDC is based on adaptive equalization algorithms and architectures that have existed for decades.

• Feed-forward equalizer (FFE)
• Decision Feedback Equalizer (DFE)
• Fixed Delay Tree Search (FDTS)
• Maximum Likelihood Sequence Estimation (MLSE)

Unfortunately, EDC does not scale well to bit rates higher than 10 Gbps (at least not today). The success of an EDC solution is highly dependent on such factors as bit rate, OSNR, and timing/jitter characteristics. None of these factors affect the ODC solutions available today. However, emerging optical modulation schemes like DuoBinary and DQPSK may make EDC a more viable solution. Only time will tell.