A measure of the extent to which the index of refraction changes with applied high electric fields; such as, several parts per ten thousand for applied fields of the order of twenty volts per centimeter.

Since the phase shift of a light wave is a function of the index or refraction of the medium in which it is propagating, the change in index can be used to phase modulate the light wave by shifting its phase at a particular point along the guide, by changing the propagation time to the point.