The nonlinear coefficient (nLc) is the ratio of the Kerr nonlinear refractive index, n₂, and the effective area, A_eff (see FOTP-132), as:

The nonlinear coefficient is related to the following nonlinear optical distortion effects as a combined parameter:

Self-phase modulation (SPM)

Cross-phase modulation (XPM)

Four-wave mixing (FWM)

Other fiber attributes, such as chromatic dispersion, also influence the transmission.

Two methods are given, with details specific to each in normative annexes. They are:

Method A Continuous-wave dual-frequency

Method B Pulsed single-frequency

Both methods require injecting very high powers (5 dBm or more) into the fiber, measurement of this power (absolute), and measurement of the output spectrum - which is modified by nonlinear effects. Both methods use calculations that combine these measured results with those derived from other measurements such as attenuation (FOTP-61 or FOTP-78) and chromatic dispersion (FOTP- 168, FOTP-169, or FOTP-175). Both methods have limitations on the length of fiber that can be measured - in a relationship with the chromatic dispersion at the wavelength being measured.