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Bit-Error Rates in High-density Wavelength Division Multiplexed Heterodyne Optical ASK Systems

Crosstalk in high-density wavelength, or frequency division multiplexing, is a significant problem in the implementation of coherent optical systems because of several factors. One of the major issues is the phase noise present in the transmitter and receiver lasers. In this paper, we examine the effects of interference from adjacent channels in heterodyne communication systems appearing at the output of the postdetection lowpass filter. Both ideal laser sources having zero phase noise and practical laser sources with non-zero phase noise are considered. We relate the interference to the bit-error rate (BER) obtainable at the output of the digital receiver. In particular, we obtain two expected bit-error rates depending on adjacent channel bit combinations and determine their behavior as a function of signal-to-noise ratio (SNR) and receiver threshold setting. It is observed that there are certain ranges of threshold settings that minimize the bit-error rates and that laser phase noise causes significant degradation in receiver BER performance. The effects of intermediate frequency (IF) filter parameters on the BER performance are also investigated.