We present channel capacity calculations for a multispan dense wavelength division multiplexed (DWDM) system that employs an ideal coherent optical receiver. Both dispersive and dispersion-free single-mode nonlinear optical fibers are considered. Degradation due to interference among Kerr nonlinear noise and optical amplifier noise accumulated along many spans is included in our model calculations. We will show that in the low-power quasi--linear regime, a multispan system can be approximated by an"equivalent"single-span system. The"equivalent"Kerr coefficient for most dispersive fibers is shown to increase with the square root of the number of spans, in contrast to the linear scaling dependence for a dispersion-free fiber. For a conventional fiber with β = -20 ps2/km, our calculated capacity of 10 (2 × 80 km), 8 (8 × 80 km), and 6 (32 × 80 km) bps/Hz indicates that today's technologies with 0.4 bps/Hz have only realized 5% of the theoretical total capacity. We have shown an increased (or decreased) capacity by about 1 bps/Hz per ten-fold increase (decrease) in dispersion.
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