Hi Mani,
The amount of introduced noise depends on a number of details of the design (power levels, spacing of the signals, overlap in time). We have a great tutorial and overview of the XPM effect at the following link though that could probably explain it better than I could:
Effects of Cross Phase Modulation (XPM) and Four-Wave Mixing (FWM)
Let me know if there is anything else you are having trouble with!