Methods for crosstalk measurement and reduction in dense WDM systems

We propose a scheme for the monitoring and reduction of crosstalk arising from 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 level of crosstalk from a given channel can be determined by measuring the power of the corresponding tone. Crosstalk from other channels can be cancelled in a linear fashion by weighting and summing the photocurrents of the desired channel and several adjacent interfering channels. Alternatively, in nonlinear crosstalk cancellation, decisions are made on the interfering signals, and these decision are weighted and summed with the photocurrent of the desired channel. For example, assuming an optical filter having a Gaussian passband, the channel density can be increased from 20 to 30%, depending on the number of adjacent channels detected. The signal-to-interference ratio can be increased by 10-20 dB and the system can achieve a BER<10/sup -9/ under conditions where, without interference cancellation, the signal-to-interference ratio would be less then 10 dB.