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?
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.