Enabling Technology in High-Baud-Rate Coherent Optical Communication Systems

High-baud-rate coherent optical system is essential to support the ever-increasing demand for high-speed applications. Owing to the recent progress in advanced modulation formats, over 1 Tb/s single-carrier data transmission has been achieved in the laboratory, and its commercial application is envisioned in the near future. This paper presented the trend of increasing baud rate and utilizing high-order quadrature-amplitude modulation (QAM), and several enabling technologies in the coherent optical communication systems. We first discussed recent progress of high-order QAM system and digital signal processing technology. Furthermore, we compared the transmission performance of three different ultrahigh-order QAM formats. The paper then reviewed the commonly used methods of achieving over 100 GBaud optically modulated signals. Besides, five attractive modulators and their corresponding modulation structures are illustrated. Key performance parameters including electrode length, 3-dB bandwidth, half-wave voltage, extinction ratio and optical loss are also compared. Finally, the trade-off between the baud rate and QAM orders in implementing high-speed systems are investigated in simulations. The results show that for the coming 800 GbE or 1.6 TbE, PDM-64-QAM might be an idea choice by considering the trade-off between the link reach and required system bandwidth. By adopting the latest probabilistic shaping technology, higher-order QAM signals, such as PS PDM-256-QAM, could be favorable for long reach applications while using extra system bandwidth.