The impact of receiver imperfections on the performance of optical direct-detection DPSK

The goal of this paper is to analyze the effects of key receiver impairments on the performance of differential phase-shift keying (DPSK), like the responsivity imbalance between the two arms of the receiver balanced photodetector and the detuning of the asymmetric Mach-Zehnder filter. We extend to DPSK a semianalytical technique, based on the Karhunen-Loeve series expansion, which can be used to obtain reliable performance results in the simulation of IM/DD optical systems. We show that the DPSK receiver imperfections may substantially decrease the sensitivity gain of DPSK over standard IM/DD systems. In particular, the performance turns out to be remarkably insensitive to the responsivity imbalance between the two arms of the receiver balanced photodetector but highly sensitive to the detuning of the asymmetric Mach-Zehnder filter. We also analyze the validity of the application of the Q parameter evaluation, which is extensively used in theory and simulation of intensity-modulation direct-detection (IM/DD) optical systems, to the study of the bit error rate performance of direct-detection DPSK optical systems, showing that its inaccuracy can be very large.