CD Testing for 2.5 Gb/s CWDM Applications: Crucial Q&As
Francis Audet, Eng., Senior Product Manager

Several years ago, DWDM networks became the ultimate in high-speed transmission. Now that cost control is a top priority for many providers, CWDM networks are increasingly being deployed, which entails several differences; namely, where CD testing is concerned. While DWDM applications did not require CD testing at lower transmission speeds, these measurements are essential to adequate CWDM transmission .

How does CD in CWDM differ from CD in DWDM?

In DWDM systems, the established chromatic dispersion (CD) threshold level for 10 Gb/s transmission is approximately 1100 ps/nm. Since DWDM systems usually operate on G.652 fiber, which typically exhibits a CD level of <19 ps/nm.km in the C band, this translates to 60 to 80 km between regeneration or compensation sites.

Transmission at 2.5 Gb/s is four times slower than at 10 Gb/s, so the CD tolerance for 2.5 Gb/s is about 16 times higher (i.e., around 18 000 ps/nm). This is due to the fact that spacing between bits is four times larger, and modulation creates four times less chirp (signal broadening). Conclusion? DWDM links operating at 2.5 Gb/s have a higher tolerance and therefore do not necessarily require CD testing, whereas links operating at 10 Gb/s benefit greatly from CD testing, especially between 60 and 80 km, as not doing so could lead to unacceptable bit error rate.

For CWDM systems, on the other hand, ITU-T Recommendation G.695 established that the maximum CD tolerance for 2.5 Gb/s transmission is 1600 ps/nm—a much lower value than for DWDM systems. Since CWDM uses the L band, the maximum CD level at the highest CWDM channel (1617 nm) is approximately 21 ps/nm.km (on G.652 fiber), which leads to a limit of 76 km between sites.

Therefore, in CWDM systems, CD testing is extremely important, even at a low bit rate such as 2.5 Gb/s. DWDM guidelines do not apply to CWDM.

Why are DWDM and CWDM CD tolerance levels so different?

In order to save costs, which is the main purpose of the CWDM network, distributed (DFB) lasers in CWDM deployments are directly modulated, whereas DWDM networks use lasers with external modulation. The direct consequence of this is the occurrence of higher chirp in CWDM networks, meaning that the pulse starts broader. CWDM transmission can therefore accept a lot less CD, and reaches its threshold limits a lot quicker than DWDM transmission.

What about testing in the O Band?

For attenuation purposes, Recommendation G.695 states that CWDM systems can go up to 80 km for wavelengths of 1471 nm and higher; wavelengths in the O-E bands are limited to 40 km. With smaller distances and lower internal CD, testing in the O to E range is not necessary. Only S, C, and L bands are at risk, since CD and distances are higher.

What about 10 Gb/s CWDM transmission?

Recommendation G.695 has not yet defined a CD threshold for 10 Gb/s CWDM transmission, but it is safe to say that the value will be much lower than the one set for 2.5 Gb/s transmission. Transmission on G.652 fiber over 10 Gb/s CWDM networks will be a huge challenge, so for now, the medium of choice is the G.655 NZDSF fiber, whose maximum CD is 10 ps/nm.km on the highest channel. Still, accurate CD testing is mandatory, and it will surely be the key to success, even for short links.