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Abstract— By eliminating self- and cross-phase modulation,
constant-intensity modulation techniques can improve the
spectral efficiency of WDM systems. In the linear regime, the
spectral efficiency with constant-intensity modulation is found to
be about 1.10 bit/s/Hz more than half the Shannon limit. In
WDM systems limited by cross-phase modulation, constant-
intensity modulation allows the launching of higher optical power
and yields increased spectral efficiency.
I. INTRODUCTION
ecently, Mitra and Stark [1] calculated the maximum
spectral efficiency of dense wavelength-division-
multiplexed (WDM) systems that are 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), in which
the intensity of each signal perturbs the fiber refractive index,
thereby modulating the phase of all the other signals. In
addition, as the signals propagate, fiber dispersion converts
XPM-induced phase modulation to intensity noise. With
constant-intensity modulation, such as phase or frequency
modulation [2]-[4] (or, to a certain degree, polarization
modulation [5]), both self-phase modulation (SPM) and XPM
cause only non-time-variant phase shifts, eliminating both
phase and intensity distortion. Under the assumptions in [1],
increasing launched power leads to a monotonic increase in
spectral efficiency, leading to a higher spectral efficiency
than that limited by XPM. However, laser intensity
fluctuations or imperfect phase/frequency modulation cause
intensity noise [3] and fiber dispersion converts phase
modulation to amplitude variation [4]. In this paper, we
calculate the maximum spectral efficiency that can be
achieved using constant-intensity modulation techniques,
assuming the use of low-noise lasers and nearly ideal
phase/frequency modulators, and assuming careful control of
fiber dispersion.
The work of K.-P. Ho was supported in part by an Earmarked Grant f