Coherent detection

high l op he n ing f the pe anage ailable that t the re hile t lated with different De Brujin sequences. The receivernoise loading thusallows, for theusual largeBERval-ues (of the order of 103) of DM lines with forward error correction (FEC), a reasonably fast BER estimation by direct Monte Carlo (MC) error counting. If lower BERs are of interest, one can use sophisti-cated pseudo-analytical BER evaluation techniques, such as the nary phase-shift keying (DPSK and DQPSK, respectively) with incoherent interferometric reception, and polarization-division-multiplexing coherent QPSK (PDM-QPSK). The schemes for the standard optical receivers that follow the channel-selection optical filter for the above formats are summarized in Fig. 1. Mach–Zehn-der delay interferometers with one symbol delay T and balanced photo-detection are used for DPSK and DQPSK [6,7]. The digital-signal-processing (DSP)-based coherent receiver for PDM-QPSK [7–9] uses a polarization beam splitter to split the polarization