Avoiding fiber nonlinearities by choice of modulation format
2011
Nonlinear refraction in fiber optic links is a capacity limiting mechanism, whereby the phase of each propagating signal
is modulated by intensity variations of signals in nearby channels. The transition to coherent detection enables a wide
variety of modulation formats to be considered. Indeed, the choice of modulation format plays a primary role in
determining the degree of amplitude variation in the channel as well as the robustness to the phase noise impairment that
nonlinearities induce. On one hand, constant envelope formats (or nearly-constant) avoid fluctuations in the signal and
produce lower nonlinearity-based impairments. Alternatively, star-QAM modulation formats enhance the receiver's
robustness to phase noise. Using simulated and experimental results we demonstrate the effectiveness of each format in
avoiding fiber nonlinearity effects for both standard fiber (17ps/nm-km) and NZDF (5 ps/nm-km). We show sensitivity
of several formats to nonlinear phase modulation from adjacent channels. We show the interaction between dispersion
and constant envelope formats that guides the applications in which constant envelope formats, such as continuous phase
modulation (CPM) provide gain over non-constant formats, such as QPSK. Consideration is made to scaling to 100
Gb/s and beyond in practical implementations.
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