Nonlinear Fiber Optics - 4 ed. Agrawal

1.3. Fiber Nonlinearities 17
1.3.3 Importance of Nonlinear Effects
Most measurements of the nonlinear-index coefficient n 2 in silica fibers yield a value
in the range 2.2–3.4 × 10 −20 m 2 /W (see Appendix B), depending on the core composition
and whether the input polarization is preserved inside the fiber or not [95]. This
value is small compared to most other nonlinear media by at least two orders of magnitude.
Similarly, the measurements of Raman- and Brillouin-gain coefficients in silica
fibers show that their values are smaller by two orders of magnitude or more compared
with other common nonlinear media [47]. In spite of the intrinsically small values
of the nonlinear coefficients in fused silica, the nonlinear effects in optical fibers can
be observed at relatively low power levels. This is possible because of two important
characteristics of single-mode fibers—a small spot size (mode diameter