= 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.
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