24 January 2011 Avoiding fiber nonlinearities by choice of modulation format
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Abstract
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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Thomas F. Detwiler, Steven M. Searcy, Andrew J. Stark, Bert E. Basch, Stephen E. Ralph, "Avoiding fiber nonlinearities by choice of modulation format", Proc. SPIE 7960, Coherent Optical Communication: Components, Subsystems, and Systems, 79600I (24 January 2011); doi: 10.1117/12.875823; https://doi.org/10.1117/12.875823
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