Correlation of nonlinear noises from different spans in 100 Gb/s multi-span fiber optic lines

• Nonlinear noises in different spans of multi-span line are correlated.
• Interaction of nonlinear noises can be described by correlation function.
• Correlation model enables calculation of nonlinearity for all types of lines.

The accumulation of nonlinear noise in multi-span communication lines was investigated by numerical modeling and experimentally for different values of accumulated dispersion at input of each span. One coherent 100G channel was investigated (wavelength 1549.32 nm, DWDM channel C35). It has been established that interaction of nonlinear noises from two different spans can be described by a correlation function that depends only on values of input dispersion for these spans. It confirms the idea that the nonlinear noise in a fiber optic line is formed mainly due to signal to signal interaction, and the influence of the signal to noise interaction can be neglected. The shape of correlation function was investigated by numerical modeling and experimentally, and its simple approximation was offered. The nonlinear noise in any multi-span line with arbitrary dispersion plan can be calculated based on the investigated correlation function for two spans. It was shown that in a compensated line the offered theoretical model based on correlation function corresponds well to simpler theoretical model based on superlinear dependence of total noise on number of spans.