Abstract: This invention provides a DPSK demodulator and a DQPSK demodulator. Both of the demodulators are based on polarization delay-line interferometers. They can be integrated with photodetectors in fiber-optic communication systems. The demodulators consist of polarization beam shifter, polarization beam splitter and wave plates. Coupling of the demodulators with photodetectors can be through free space or fibers. Time delay generated in the interferometer can be controlled with a phase shifter, using either thermal, piezoelectric, mechanical or electrical means. Examples of phase shifter using a piezo bender and an actuator respectively are also disclosed.