Residual dispersion

The slope compensation of DCF is curved, whereas the dispersion characteristic for most fibers is closer to linear. This explains why it is difficult to achieve low residual dispersion, especially for LEAF. The figure below shows a typical example of the residual dispersion having LEAF compensated by DCF. The residual dispersion is normally highest at the blue side and can be as high as 12-14%. This is not only a problem at 40   Gb/s but also at 10   Gb/s for long distance systems. Proximion´s technology allows full dispersion compensation (less than 1% residual dispersion) over the whole operating range.

Figure 4. Typical residual dispersion of LEAF compensated by Dispersion Compensation Fiber (DCF)

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Positive dispersion

As a last example we will show the possibility to create a positive dispersion compensator. Some vendors, making systems for sub marine applications, want to use fiber with negative dispersion to achieve better transmission properties. They have therefore a need to compensate for this negative dispersion without adding to much loss and still with low residual dispersion. This can simply be achieved in DCM-FB by “just” reversing the fiber connected to the circulator. The light is then inserted from the blue-side of the grating instead of the red side as illustrated in figure 9. The advantages are the same as for the “normal” dispersion compensator.

Figure 5. Positive dispersion compensator – principle

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Integration of the solution

In contrast to traditional DCF solutions, the small form factor of DCM-FB allows for plug-in solutions, thus enabling full integration in the system. This has several advantages:

• The footprint of the whole system decreases which also reduces the operation costs.
• The system can be shipped with preinstalled DCMs which minimizes the risk of mistakes in the commissioning process.
• By simply adding an identification circuit on the board the DCM-FB can be an integrated part of the inventory system which is desirable from an O&M perspective.

Amplifier design

A typical double-stage amplifier node consists in a pre-amplifier, a booster and in-between, various functionalities as shown in figure 5.

Figure 6. In-line building blocks in a typical transmission node

Thanks to their low insertion loss, DCM-FB will allow for more functionality in each node, without exceeding the power budget available. Alternatively, the whole amplifier may be re-designed to take full advantage of the characteristics of the DCM.

Figures 6 and 7 show a comparison between the designs based on traditional DCF and on DCM. The lower middle-stage loss in the DCM case allows for lower power, cheaper pumps, or even fewer pumps to be used in the pre-amplifier and the booster stages. The noise level of the total solution with DCM is also lower.

Figure 7. Traditional mid-stage amplifier solution designed for high internal loss

Figure 8. Mid-stage amplifier solution tailored for DCM-FB

Note that the circulator in figure 7 also fulfills the function of the two isolators in figure 6, which also reduces the cost of the overall solution.

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High dispersion DCM

The features of the DCM-FB allow also for new system designs, which were not possible before. It is for example possible to design DCM for up to 5000 ps/nm with insertion losses similar to today’s 2000ps/nm DCF.
Figure 9. Link design with High Dispersion DCM

This design can be very effective for point-to-point links. The cost savings lie in less gear for the dispersion compensation as well as simpler and cheaper amplifiers.

Face lift upgrade

Figure 10. Simple “face lift” solution using an in-line DCM-FB

luggable transceivers like SFPs are now introduced in many systems. The advantages having these are obvious but still they are somewhat limited from a dispersion point of view, even if the transceiver vendors are doing their best to improve current performances. Even 2.5 Gb/s system can therefore be dispersion limited, also without optical amplifiers. A simple “face lift” solution is to place a DCM-FB directly after the multiplexer. Having amplifiers and/or having higher bit rates make the benefits even larger.

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Dispersion adjustment in legacy systems

The freedom to produce virtually any kind of dispersion in DCM-FB gives the opportunity to compensate for residual dispersion in DCF solutions. For example, in many legacy systems the residual dispersion limits the distance. This is usually solved with back-to-back transponders. The residual dispersion can easily be removed by tailored DCM-FBs which allows the system operator to remove the back-to-back transponders.