Polarization maintaining fiber coupler for coherent DPSK systems.

We have characterized the performance of a polarization maintaining fiber (PMF) coupler at lambda=1.5microns and demonstrated its use as an optical mixer in a 400 Mbit/s DPSK heterodyne transmission experiment. The receiver sensitivity of -56.6 dBm (or 42 photons/bit) at a bit error rate level 1x10 sup (-9), or 3. 0 dB receiver penalty from the shot noise limit at infinite LO power, compares well with the best result using a non-PMF coupler. Control and stability of the polarization states in the PMF mixer indicates that polarization mismatch of the transmitted and local oscillator signals is not part of the power penalty.