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Overview

Robust 100Gb/s Transmission using Coherent Pol-Mux QPSK

Description

Polarization multiplexing of QPSK (Pol-Mux QPSK) is a good candidate for the implementation of next generation high-speed transmission systems. It helps to reduce the requirements on electrical and opto-electrical components because it requires a symbol-rate of only one fourth of the bit-rate. Digital signal processing (DSP) combined with coherent detection has the potential to mitigate the impact of linear transmission impairments like chromatic dispersion (CD) and polarization mode dispersion (PMD). This example illustrates the usage of DSP to mitigate the impact of phase noise*, CD and polarization-induced distortions.
* Phase and frequency mismatch between laser transmitter and local oscillator.

Typical Results

The simulation setup is displayed in Figure 1. It consists of a 100 Gb/s Pol-Mux QPSK transmitter, a transmission fiber, a polarization-diversity receiver (90° hybrid) and two DSP modules. The impact of residual CD (50 ps/nm) and polarization cross-talk** (Pol-Xtalk) is mitigated using a MIMO structure consisting of 5-tap FIR filters. The coefficients of the filters are optimized using the constant-modulus algorithm (CMA). The phase noise is corrected using a multi-symbol phase estimation (MSPE) technique based on the Viterbi & Viterbi algorithm. The impact of CD, Pol-Xtalk and phase noise on the constellation diagram is illustrated in Figure 2. The constellation diagrams before (blue) and after (green) equalization in presence of the combined effects are displayed in Figure 3.
** Misalignment between the states of signal polarization and the polarization beam splitter in front of the receiver.

Keywords

Coherent, Polarization Multiplexing (Pol-Mux), Quadrature Phase-Shift Keying (QPSK), Digital Signal Processing (DSP), Finite Impluse Response (FIR), Constant Modulus Algorithm (CMA), Multi-Symbol Phase Estimation (MSPE)