Nonlinear refraction is a light intensity dependency of refractive index attributable to the third-order susceptibility, χ3, and is also called as the optical Kerr effect. The refractive index change due to the optical Kerr effect is expressed as a function of time, n(t), and is proportional to a light intensity, |E(t)|2.




n0 is a linear (small-signal) refractive index of medium. n2 is a nonlinear refractive index coefficient, n2=3.18×10-20m2/W for silica glass optical fiber.