Interference immunity of fiber-optic communication lines with spectral multiplexing of channels

The performance of fiber-optic communication lines with spectral multiplexing of channels is analyzed for interference immunity and its improvement. The major sources of interference and insertion losses are crossovers, which generate crosstalk, and demultiplexers in the receiver. The signal intensity at the output of the j-th demultiplexer for the i-th spectral channel is calculated first, assuming independent signal sources. The the ratio of peak signal power to average interference power is calculated, considering quantum noise, thermal and shot noise, mode beats, and background illumination. A demultiplexer is designed on this basis, but optimization of the channels is an indeterminate problem of which the solution depends on the optimality criterion and on the given values of the other interference indicators not selected as optimality criterion. The interference immunity of such communication lines can be improved by post-detector compensation of crosstalk, which requires only a multipole network with appropriate j-th input to i-th output transmission coefficients, and by considering that the interference immunity will decrease as the useful signal bandwidth increases.