Performance limitations of a WDM coherent optical transmission system due to Raman amplifier induced crosstalk

Analysis is presented for a wavelength division multiplexing (WDM) transmission system with optical on-off-keying (OOK) modulation and heterodyne detection taking into account the effect of Raman amplifier induced crosstalk and beat noise components arising out of beating of pump power, signal power and crosstalk during the photodetection process. The results are numerically evaluated at a bit rate of 10 Gbps for several WDM channel, interchannel spacing, fiber length and local oscillator power in the receiver. The results are presented in terms of signal to crosstalk plus beat noise ratio and bit error rate (BER). It is found that BER deteriorates due to crosstalk induced by Raman amplifier at higher number of WDM channel and higher transmission distance. The allowable transmission distance for a given BER of 10^-9 and number of WDM channel are also evaluated. For example, the allowable transmission distance is approximately 92 Km for the number of channel is 4 whereas it reduces to almost 7 Km when the number of channel is increased to 16, at a BER of 10^-9 with pump power of 10 mW and interchannel spacing 12.5 GHz.