System performance of coherent transmission over cascaded in-line fiber amplifiers

The limitations of cascaded in-line amplifier systems using coherent modulation-demodulation schemes are examined by evaluating the product of the data rate and the transmission distance. The linear amplified spontaneous emission (ASE) accumulation is shown to make the maximum value of the data rate-distance product increase proportionally with the ratio of the amplifier output signal power to the noise figure. It is also shown that the Kerr-nonlinearity-induced phase noise limits the product of the data rate and the third power of the distance, the maximum value of which is inversely proportional to both the amplifier output signal power and the noise figure. The fiber dispersion is known to limit the product of the distance and the square of the data rate by causing waveform distortion. By taking these three relations into account, it is concluded that coherent signal transmission has a maximum in-line amplifier system length of 10³-10⁴ km in the gigabit-per-second range. Among these three factors, the nonlinearity-induced phase noise has the greatest impact