Linear versus Nonlinear Optics

Essentially, all conventional optics such as mirrors and lenses are linear because the light that enters is the same as the light that exits.

In the case of a nonlinear optical application, light goes in, but the light that comes out is modified by passage through the material and the interaction of other fields that may be applied. In electro-optics (Second-order nonlinear optics) light is modified by the influence of an electric field. In Third-order nonlinear optics (all optical processes) the modification is only caused by intense light fields.