COHERENT CROSSTALK IN MULTICHANNEL FSK DD LIGHTWAVE SYSTEMS DUE TO 4-WAVE-MIXING IN SEMICONDUCTOR OPTICAL AMPLIFIERS/

Coherent crosstalk in multichannel lightwave transmission systems due to four-wave mixing in semiconductor optical amplifiers is analyzed. F or direct detection of evenly spaced frequency channels, coherent beat ing between signal and intermodulation products occurs when the channe l 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 s pectral hole burning and dynamic carrier heating gain mechanisms must be included in addition to carrier modulation and gain saturation to a ccurately predict both incoherent and coherent crosstalk for channel s pacing greater than 5 GHz. Degradation of the eye pattern due to coher ent 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 cha nnels 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 limitat ions alone.