Robustness to laser frequency offset in direct-detection DPSK and DQPSK systems

We assess, both by measurement and simulation, the receiver performance degradation caused by frequency offsets between the optical source and the delay interferometer for nonreturn-to-zero (NRZ) and return-to-zero (RZ) differential-phase-shift-keying (DPSK) and differential-quadrature-phase-shift-keying (DQPSK) systems. We rind that NRZ- and RZ-DQPSK systems are about six times more sensitive to frequency offsets than NRZ- and RZ-DPSK systems operating at the same bit rate. We explain the reason for this behavior by means of eye aperture considerations.