CPFSK is the most widely used modulation scheme in coherent optical tr
ansmission. Theoretical analyses so far predicted a much more stringen
t linewidth requirement compared to ASK and FSK systems with noncohere
nt IF-detection. In this paper we present a unified linewidth analysis
of ASK, FSK, CPFSK, and DPSK receivers, with leads to the conclusion
that the linewidth performance of these different schemes is to a larg
e extent identical. The inclusion of post-detection filtering in the a
nalysis is critical for obtaining these results, which are supported b
y practical system measurements on a 1 Gbit/s phase diversity CPFSK re
ceiver.