Crosstalk degradation caused by optical amplification in a multichannel FSK heterodyne system

The crosstalk degradation caused by an optical amplifier in a four-channel FSK (frequency-shift-keyed) heterodyne communication system is measured. A bit error rate (BER) floor of 3*10/sup -4/ is observed when the channels are spaced by 200 MHz, FSK modulation at 45 Mb/s, and when the optical input signal is large enough such that the gain is compressed by 2 dB relative to its small-signal value. The receiver is substantially improved by reducing the optical power amplifier input. However, the sensitivity increases only to a maximum value beyond which it degrades as the optical power of the demodulated channel becomes small relative to the noise of the optical amplifier. The combined effect of the crosstalk and the amplifier noise yields an optimum sensitivity of 250 photons/b for BER=10/sup -9/. This result is 5 dB poorer than the sensitivity obtained in the absence of an optical amplifier.<>