  This letter presents a comparison of exact probability density function with
the Gaussian noise approximation in optically pre-amplified DPSK receivers with
optical Mach-Zehnder interferometer demodulation (MZI) and balanced detection,
including the impact of phase noise. It is found that the Gaussian noise
approximation significantly over-estimates ASE-ASE beat noise in DPSK receivers
with balanced detection particularly when phase noise is negligible, compared
to IM/DD receivers, ASE- amplified spontaneous emission. However, the Gaussian
noise approximation is still applicable for DPSK receivers with balanced
detection and the measured 3-dB advantage is predicted by the Gaussian noise
distribution.
