On the joint effect of receiver impairments on direct-detection DQPSK systems

The effect of five different receiver (RX) impairments on the performance of amplified spontaneous emission (ASE)-noise-limited differential quadrature phase-shift keying (DQPSK) was analyzed jointly for the first time. This paper assessed the impact of detuning, finite extinction ratio, and delay error of the asymmetric Mach-Zehnder interferometer (AMZ), and of the delay and amplitude imbalance of the balanced photodetector (BPD). Three different RX and transmitter (TX) filtering set-ups and both return-to-zero (RZ) and nonreturn-to-zero (NRZ) codings were assumed to find out whether these different configurations showed a substantially different sensitivity to RX impairments. A very accurate bit error rate (BER) estimation technique based on Karhunen-Loeve (KL) series was also employed. It was found that DQPSK is surprisingly resilient to most impairments, either taken alone or in combination, across all three filtering set-ups and RZ and NRZ formats. The one critical exception is represented by the maximum tolerable detuning of the interferometers, which is about one sixth that of binary DPSK and maybe as low as 400 MHz at 40 Gb/s. The cumulative impact of all impairments, except detuning, can be expected to be less than 1 dB in most cases, whereas a detuning of 2% of the bit rate causes 3 dB of penalty by itself, independently of the electrical and optical filters used and of either RZ or NRZ coding. In practical systems, the overall penalty due to all effects combined may easily lead to a back-to-back sensitivity performance worse than intensity/modulation direct/detection (IM/DD).