Third Order Susceptibility
The third order susceptibility (χ(3)) is related to nonlinear effects such as the Kerr effect (where index changes with optical intensity), self phase modulation, SPM, (where the light modifies the glass index and its own propagation as it travels), and cross-phase modulation, XPM, (interference in propagation by light generated at another frequency). Many of these effects have desirable applications, and some fibers are designed specifically to enhance them, while conversely in many other applications any deviation from linear behavior is undesirable and quite the reverse is true. It would be fair to say that nonlinear fiber optic effects can be used to make interesting devices, but may not be useful in the networks linking those devices.
Related Terms: Kerr Coefficient