Bearing in mind that the Chromatic Dispersion (CD) tolerance of coherent systems can be up to 50.000ps/nm it is clear that CD measurements have lost part of their value. (We will be discussing the line side technology and the effect of chromatic and polarisation mode dispersion in an article very soon)

However there are several cases where it still makes good sense to perform a CD measurement. One such example are one-sided CD measurements based on an OTDR method to identify the type of installed fiber. Coherent system have an average 6dB lower receiver sensitivity and with this restriction some deployments are requiring now the use of Raman amplification. But to have a proper network design and some reasonable Raman amplication it is mandatory to know the expected gain of this special type of amplification.

In many cases a service provider may know only the cable manufacturer and the deployed fiber standard within this cable, such as G.655 compliant fibre, but not what the very exact fibre type is.

With an OTDR based one-sided CD measurement you can determine precisely the effective mode field diameter and with this the expected gain.

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