Why is RZ-DPSK more nonlinear tolerant than NRZ-DPSK?
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.