A performance evaluation of WDM-Nyquist systems generated by optical flat comb source and based on POLMUX-QPSK, POLMUX-DQPSK, POLMUX-16QAM and POLMUX-64QAM

In this paper, we implement a WDM-Nyquist transmission system with 12.5 GHz channel spacing generated through an Optical Flat Comb Source (OFCS). Each channel could carry one of four different modulation format as Polarization Multiplexing-Quadrature Phase Shift Keying (POLMUX-QPSK), Polarization Multiplexing-Differential Quadrature Phase Shift Keying (POLMUX-DQPSK), Polarization Multiplexing-Quadrature Amplitude Modulation based on 16 (POLMUX-16QAM) and 64 (POLMUX-64QAM) with Return-to-Zero (RZ) pulse carving and 33% duty cycle. Numerical simulations of 1 Tbit/s superchannel have been carried out, in order to evaluate the performances of the different modulation format using appropriate metrics. We discuss their back-to-back receiver sensitivity and the required Optical-to-Noise Signal Ratio (OSNR) for 3.8•10 -3 and 10 -9 Bit Error Rate (BER). We find that POLMUXQPSK has the best receiver sensitivity and the lower OSNR penalty compared to the other modulation formats. With POLMUX-QPSK sensitivity as reference, we can observe a power penalty of 13.4 dB (for a BER equal to 3.8•10 -3 ) for POLMUX-64QAM. In addition, we can observe an OSNR penalty of 21 dB in OSNR for POLMUX-64QAM compared to POLMUX-QPSK modulation format for a BER equal to 3.8•10 -3 . However, it is important to mention that POLMUX- 64QAM presents higher Spectral Efficiency (SE). We study also the robustness of these four optical modulation formats for transmission of 1 Tbit/s in dispersion compensated WDM-Nyquist systems using two kinds of transmission fibers: SMF (Single Mode Fiber) and NZDSF (Non-Zero Dispersion Shifted Fiber). We find that POLMUX-QPSK is the suitable modulation format in dispersion compensated WDM-Nyquist systems using NZDSF fiber. We simulate the nonlinear effect tolerance by considering self-phase modulation (SPM), cross-phase modulation (XPM) and four-wave mixing (FWM) for the different modulation format. We observe that POLMUX-QPSK has the best robustness against the cited nonlinear fiber effects in NZDSF fiber.