Running two or more systems over one optical fibre is not arbitrary. It can happen that an existing system uses amplitude modulation and a new system will use much more modern modulation format, which is considered to be one of the phase or multilevel phase modulation formats. The old and the new channels can be interleaving or each would occupy its dedicated band. The result can be origination of crosstalk from amplitude modulation channels to frequency modulation channels. The crosstalk is known as Cross-phase modulation (XPM).
It is associated with much higher energy transmitted by amplitude modulation formats, which can be transferred to other channels of the wavelength multiplex.
Cross-phase modulation (XPM) is the change in the optical phase of optical radiation at a certain wavelength caused by the interaction with radiation at another wavelength in a nonlinear medium.
It can be achieved by: