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||Fibre optic communications
This technical report provides guidance for uniform measurements of the non-linear coefficient of single-mode fibres (see IEC 60793-2-50) in the 1 550 nm region. The non-linear coefficient is related to the following non-linear optical distortion effects as a combined parameter: - self-phase modulation (SPM); - cross-phase modulation (XPM); - four-wave mixing (FWM). Other fibre attributes, such as chromatic dispersion and polarisation mode 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 fibre, measurement of this power (absolute) and measurement of the output spectrum (which is modified by non- linear effects). Both methods use calculations that combine these measured results with those derived from other measurements such as attenuation (see IEC 60793-1-40) and chromatic dispersion (see IEC 60793-1-42). Both methods have limitations on the length of fibre that can be measured - in relationship with the chromatic dispersion at the wavelength being measured. Method A 1) requires injecting the light of two wavelengths into the fibre. The light of both wavelengths is constant at various power levels. At higher power, the lights beat due to the non-linear effect and produce an output spectrum that is spread. The relationship of the power level to a particular metric of spectrum spreading is used to calculate the non-linear coefficient. Method B ,  requires injecting pulsed light at a single wavelength. The pulses should be of duration substantially less than 1 ns and the input peak power of these pulses should be measured and related to the non-linear spreading of the output spectrum. Measurements of the non-linear coefficient are used to characterise specific single-mode fibre designs for the purpose of system design relative to power levels and distortion or noise effects derived from the non-linear optical behaviour.