What is the 4 Wave Mixing problem in WDM? Can any one explain with example?

When three light waves with different lengths are simultaneously transmitted in a fiber, the interaction of quartz media will create a new light wave, whose frequency is the frequency combination of the three waves. This phenomenon is called four-wave mixing (FWM).

FWM causes optical power (the energy of a photon) to transfer from one channel to another channel and has the following impact on multi-channel systems:

• As the photon energy on the original wavelengths is impaired because of the transfer, the system BER and OSNR performance deteriorates.
• If the newly generated light wave is the same or overlapped with one of the original light waves, severe crosstalk results.
  

Great explanation! I have one more question regarding this phenomenon. I which scenarios can this problem occur?

Yes I agree with sevillaf. It would be better if anyone can give any practical exampl in DWDM domain

Wonderful explanation, a new term for me though.