Quantum Limited DPSK Receivers with Optical Mach-Zehnder Interferometer   Demodulation
  We present an analysis of quantum-limited DPSK receivers with optical Mach-Zehnder interferometer (MZI) demodulation. It is shown for the first time that the quantum limits for DPSK/MZI receivers with single-port and balanced detections exactly differ from 3-dB in receiver sensitivity, obtained by both Poisson and Gaussian noise statistics. 
Xiupu Zhang, Guodong Zhang