DQPSK: When Is a Narrow Filter Receiver Good Enough?

In this paper, we investigate experimentally and via simulation the pros and cons of a narrow filter receiver for differential quadrature phase-shift keying based on a single optical filter and eschewing the conventional asymmetrical Mach-Zehnder interferometer structure. We quantify the performance differences between the two receivers, allowing system designers and operators to determine when the less complex narrow filter receiver might be the appropriate choice. We numerically optimize the 3-dB bandwidth and center frequency of the narrow filter and show it is more robust to carrier frequency detuning than the conventional solution. We show that the narrow filter receiver is more tolerant to chromatic dispersion (CD) than the conventional one, and equally tolerant to first-order polarization-mode dispersion. We show the impact of the 3-dB bandwidth on the receiver performance when CD accumulates. Finally, we show via experiments and simulations that the 3 dB advantage of the conventional receiver vanishes when the nonlinear impairments are fiber nonlinearities; comparing the two receivers at the optimum launch power for a 25 times 80 km system, the difference in optical SNR margin is reduced to ~1.6 dB. Experiments are conducted at 42 Gb/s using a commercially available narrow filter for reception.