 Cross-phase modulation

Cross-phase modulation occurs when one beam changes the wave phase of another through an intensity-dependent optical response. In an optical Kerr effect medium, a pump changes the refractive index sampled by a probe; propagation over length $L$ gives the probe an additional phase approximately $(2\pi /{\lambda }_0)L\Delta n$.

A delayed weak probe can therefore acquire a time-dependent phase from a strong pump optical pulse. Interfering the probe with an unaffected reference makes the phase change observable as an intensity change. Changing the pump intensity or blocking the pump tests the coupling. The coefficient depends on the beams' relative polarization and the material response; a phase change need not imply appreciable transfer of energy to the probe.

Cross-phase modulation (XPM) is a nonlinear optical phenomenon that occurs in optical fibers and other nonlinear media. It arises when two or more optical signals (or channels) co-propagate through a nonlinear medium, such as a fiber optic cable. The intensity of one signal can affect the phase of another signal due to the nonlinear dependence of the refractive index on the intensity of the light.