Crosstalk measurement and reduction in dense WDM systems using subcarrier tone channel identification and linear cancellation

Proposes a scheme for the monitoring and reduction of crosstalk arising front the limited stop-band rejection of optical bandpass filters in dense WDM systems. The optical carrier at each wavelength is modulated with a subcarrier tone unique to that wavelength. The crosstalk level can be determined by measuring the power of the tone. The crosstalk from other channels can be cancelled by weighing and summing the photocurrents of several adjacent channels. Assuming a Gaussian passband for example, the channel density can be increased by 30% to 50%, depending on the number of adjacent channels detected, and the system can achieve a BER <10^-9 even if the signal-to-interference ratio is less than 10 dB