The impact of multipath interference (MPI) in all-Raman multispan dispersion-compensated links operating at 42.67 Gb/s was studied. D+/D- and D+/D-/D+ compensation schemes were considered using different fiber sets. Optical signal-to-noise ratio (OSNR) penalties due to MPI were calculated for several backward-pumped system configurations, imposing system target OSNR levels suitable for intensity modulation with direct detection (IMDD) and differential phase-shift keying (DPSK). The analysis in this paper confirmed that the D+/D-/D+ OSNR penalty is typically much less than that of the D+/D- scheme. The authors then estimated the increase due to MPI in the number of spans required to satisfy a target OSNR for a given total link length, taking into account Kerr nonlinearities. It turned out that such an increase can be very significant (up to 15%–20%) with the D+/D- scheme and lower but nonnegligible (5%–10%) with the D+/D-/D+ schemes. The analysis confirms that, to substantially curtail the span increase, both forward and backward pumping should be adopted, as recent experimental results have shown. Finally, at the lower OSNR levels required by DPSK with respect to IMDD, the impact of MPI was shown to be smaller across all configurations.
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