Cross Phase Modulation in Optical Fibers (Fiber Optics)

[Elbereth]

Hello everyone.
I have been practicing for my exam in fiber optics, and stumbled upon the following question:

We have a transmission system with 3 channels, composed of a single mode fiber and a dispersion compensation fiber. Specifications are given for each. We need to calculate the channel's frequency spacing in a way that XPM (cross phase modulation is less than 1).

My problem is I cannot figure out what is exactly meant by XPM being less than 1 and what formula relates the cross phase modulation to the channel spacing. I know that the strength of the cross phase modulation increases with the no. of channels and becomes stronger as channel spacing becomes smaller but I cannot figure out how to relate it using an equation.

Any help would be appreciated.

 

[sophiecentaur]

How are you defining cross phase modulation? It sounds like a non-linear effect ? Where would that occur in a fibre?