We apply a multi-layer perceptron as the timing error detector for PPM signal clock recovery. Simulation results show that the proposed MLP-based CDR method can improve the accuracy of phase detection with large sampling frequency offset over 1200 ppm.

Direct detection (DD) is one of the most attractive solutions for short and medium reach optical fiber transmission in comprehensive consideration of digital signal processing (DSP), power consumption, and cost. Various single side band (SSB) DD techniques have been widely investigated to detect and process the complex signal. In this paper, we propose a novel approach which uses only high-speed digital input/outputs (IO) to generate both quadrature phase shift keying (QPSK) signal and virtual carrier signal. A transmission of 4 × 32 Gb/s QPSK signal over 1200-km standard single mode fiber (SSMF) in block-wise manner has been successfully demonstrated. Only a single polarization in-phase-andquadrature (IQ) modulator and one single photodiode (PD) are used for each channel. Compared with conventional DSP of coherent detection, the DSP in the proposed scheme is also simplified that only fixed frequency compensation is required without additional phase noise elimination. The overall system complexity has been greatly reduced.

We experimentally demonstrate an 8×10Gb/s PDM-QPSK real-time coherent communication system spaced with 12.5GHz over a free-space optical link. All the channels of the system have achieved error-free transmission performance.