Simultaneous optimisation of filter bandwidths and interferometer free spectral range for DPSK and DQPSK modulation

The authors investigate the optimal bandwidths for transmitter and receiver filters for differential phase-shift keying (DPSK) and differential quadrature phase-shift keying (DQPSK) optical modulation in dense wavelength-division multiplexing (WDM) scenarios. Single-channel setups were investigated as reference. Simultaneously, the optimal free spectral range (FSR) of the delay interferometer (DI) needed at the receiver for both modulation formats was searched. Transmitter (Tx) and receiver (Rx) optimal setups were focused and the propagation effects were not considered. It is demonstrated that the optimal FSR, in general, is larger than the symbol rate for both DPSK and DQPSK modulation formats.