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Coherent crosstalk in multichannel lightwave transmission systems due to four-wave mixing in semiconductor optical amplifiers is analyzed. For direct detection of evenly spaced frequency channels, coherent beating between signal and inter-modulation products occurs when the channel bit rate is greater than the laser linewidth. Crosstalk and maximum input power limitations for an FSK/DD system are calculated for up to 100 channels and channel spacing up to 100 GHz. It is shown that the spectral hole burning and dynamic carrier heating gain mechanisms must be included in addition to carrier modulation and gain saturation to accurately predict both incoherent and coherent crosstalk for channel spacing greater than 5 GHz. Degradation of the eye pattern due to coherent crosstalk is shown to exceed the incoherent contributions by up to 33 dB even when laser phase statistics and bit pattern statistics are accounted for. The result of this crosstalk mechanism is a predicted decrease in the maximum input power per channel to -43 dBm for 100 channels at 100 GHz channel spacing and -52 dBm for 100 channels at 5-GHz channel spacing for an optical SNR of 23 dB, representing a decrease in input power per channel of 25 dBm over incoherent crosstalk limitations alone.
Date of Publication: Jan. 1996