ON THE PERFORMANCE OF OPTICAL DIRECT-DETECTION DPSK IN THE PRESENCE OF RECEIVER IMPAIRMENTS

We analyze the impact of certain receiver impairments on directdetection DPSK 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 interferometer. Introduction: Optical direct-detection DPSK has recently been employed in a breakthrough experiment, transmitting 2.5 TbiVs over 4,000 km using 64x40 Gbitls (l). This result marks an important departure from the use of conventional IWDD formats. It proves that phase modulation, in the low-non-linearity scenario made possible by FECs and Raman amplification, can withstand ultra-long haul transmission and outperform IWDD, even at 40 GbiVs per channel. The practicality of DPSK outside of the laboratory environment, however, has yet to be ascertained. DPSK needs a rather complex receiver (RX) that requires an asymmetric Mach-Zehnder filter and a Balanced Photo- Detector (BPD), as shown in Fig.1. In principle, the Mach-Zehnder needs to be accurately phase-tuned so that the two output ports create perfect constructive and destructive interference in CW. The BPD is supposed to have a perfect responsivity matching between its two arms. These requirements are difficult to satisfy. Whether DPSK is going to be a practical option for the future generation of DWDM systems may eventually depend on its resilience on such imperfections. The purpose of this paper is to assess such resilience and find out the resulting tolerances. We also compare the extent of the associated penalties to the potential sensitivity gain of DPSK vs. IWDD. The Analysls: We assume the RX structure of Fig. 1, followed by an electrical post-detection filter. The TX is assumed ideal with NRZ coding. To compute the BER, we resorted to a well-known analytical approximated approach (21,(3). The method yields a fairly good estimate of the BER in the practical case of a Bessel post-detection filter of bandwidth 0.6-0.7'Rb and a generic bandpass optical RX filter of bandwidth N.Rb, with Rb the bit-rate. If there are no RX impairments, it is then possible to obtain a closed-form expression of the BER of DPSK, as it was done already in 131. In the presence of impairments. however, the analysis gets much more complex. We extended the method by resorting to results from the general theory of quadratic forms of random variables (4). It was no longer possible to derive an analytical BER, but we were able to obtain a closed-form expression of the characteristic function of the distribution of the decision signal. Then, the actual BER can be accurately obtained through numerical integration as shown in (6), (7).