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.