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In optical communication systems employing phase-shift keying formats such as DQPSK, the demodulator functions as a digital phase decoder. When the two fiber output ports are photodetected and combined with a balanced receiver pair, the demodulator converts the phase transitions into an electrical digital signal of 1’s and 0’s. For DQPSK system, it is necessary to decode two signals propagating 90 degrees out of phase, each one at a baud rate, R. Thus, the DQPSK format requires two interferometers: one to decode the in-phase signal (I-signal) and another one to decode the quadrature signal (Q-signal).
Figure below shows the optical schematic for the DQPSK demodulator.
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