Suppression of beat noise from optical amplifiers using coherent receivers

Coherent optical fiber communications have been studied intensively because of their high receiver sensitivity and high-frequency selectivity. With the advent of an erbium-doped fiber amplifier (EDFA), however, the first advantage seems to have become less attractive. Nevertheless, the combination of the EDFA and coherent techniques offers a number of attractive features. In this paper, we show both theoretically and experimentally that the excess beat noises (common-mode and image-band beat noises) from optical amplifiers can be suppressed by using coherent receivers such as a balanced receiver and a double-stage phase-diversity (DSPD) receiver. The noise figure (NF) of the excess-noise-suppressed coherent receivers with an optical preamplifier is shown to be 0 dB. Bit-error-rate (BER) formulas are also discussed