Network Readiness for ROADM

The ROADM mainly replaced O-E-O routers. Since ROADMs dont have the pulse-reshaping capability of the router, chromatic dispersion (CD) and polarization mode dispersion (PMD) are not nulled at the ROADM site, and the all-optical link is now longer, as seen in the two figures below:

Figure 3: Before the ROADM, optical links were short

Figure 4: With ROADM in the network, the all-optical link can be increased multifold, requiring CD and PMD characterization

Recommended Tests

Both the EXFO CD (FTB-5800) and PMD (FTB-5500B) analyzers are well-suited to this advanced optical testing application as they can easily test through multiple amplifiers with great speed and accuracy.  

More on Chromatic Dispersion Issues

For chromatic dispersion, since wavelengths can come and go to and from different networks, compensating at the receiver end proves extremely complex, if not impossible. This is why ROADMs typically have integrated dispersion-compensating fibers (DCF) (after the line amplifiers and before the switch itself). This ensures that all wavelengths, regardless of origin and destination, are relatively well compensated for.  

Longer links with several cascaded ROADMs may require an additional tunable compensation at the receiving site to make up for the leftover dispersion caused by the inaccuracy of the DCF and by the contribution of the ROADM itself. End-to-end testing may be required. It is important to note that for the only effective way to perform end-to-end testing is to go through the EDFA (since they contribute to the total CD), but bypass the ROADM since testing for CD through the ROADM is not possible.  

More on PMD Issues

PMD causes a different challenge. PMD in fiber increases as the square root of the sum of the squares because PMD in fiber is stochastic; i.e., random. The PMD in ROADMs, however, is deterministic and accumulates linearly. As an example of the impact this can have, consider a system whose PMD tolerance is 10 ps (typical for 10 Gbit/s SONET/SDH networks), a ROADM exhibiting 1ps of PMD (typical), and a tolerance for the fiber (including EDFA) down to 9 ps. A network with cascaded ROADMs will see the fiber PMD tolerance decrease proportionally. Again, this stresses the importance of testing every single link that will be part of the ROADM network and why end-to-end testing may be required. It is important to note that for the only effective way to perform end-to-end testing is to go through the EDFA (since they contribute to the total CD), but bypass the ROADM since testing for CD through the ROADM is not possible.