Abstract
Recently, Mitra and Stark1 calculated the channel capacity of dense wavelength-division-multiplexed (WDM) systems, limited by both optical amplifier noise and fiber nonlinearities. Mitra and Stark argued that the capacity of WDM systems is limited most fundamentally by cross-phase modulation (XPM). As the signals propagate, fiber dispersion converts XPM-induced phase modulation to intensity noise. With constant-intensity modulation formats, such as phase or frequency modulation2–4 (or, to a certain degree, polarization modulation,5 both self-phase modulation (SPM) and XPM cause only constant phase shifts, eliminating both phase and intensity distortion. When constant-intensity modulation is used, under the assumptions in,1 increasing launched power leads to a monotonic increase in channel capacity, leading to a higher channel capacity than that limited by XPM. However, laser intensity fluctuations or imperfect phase/frequency modulation cause intensity noise3 and fiber dispersion converts phase modulation to amplitude variations.4 In this paper, we calculate the maximum channel capacity that can be achieved using constant-intensity signal, assuming the use of low-noise lasers and nearly ideal phase/frequency modulators, and assuming careful control of fiber dispersion.
© 2002 Optical Society of America
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