In fiber optics, why would RZ-DPSK be more suited for long-haul transmissions over NRZ-DPSK? Is the tolerance to non-linearity due to the power intensity or something else?
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RZ-DPSK is derived from NRZ-DPSK via a pulse carver, so you can see that a carrier will be more available than from the constant envelope of the NRZ. In NRZ the carrier arises from the phase shift chirps and non-linearities in that process.
Looking at the spectrum of the RZ vs. NRZ you will see a higher carrier to sideband ratio and a flatter power spectrum. All good things for recovering the signal on the other end.
At higher bit rates and long haul links RZ is more prone to dispersion as compared to NRZ.