We experimentally verified that Tomlinson–Harashima (TH) pre-coding for an optical coherent system can enhance the speed of polarization division multiplexing (PDM) 16 quadrature amplitude modulation (QAM) signal to be higher than 135 GBaud with 120 GSa/s digital-to-analog converter (DAC) and electrical/optical/electrical bandwidth of the system narrower than 57 GHz at $- 30\;{\rm dB}$. The improvement of achievable capacity based on the received SNR in optical back-to-back conditions could be obviously observed by comparing the PDM-16QAM signal with and without TH pre-coding. The normalized generalized mutual information of the 135 GBaud PDM-16QAM signal for 120 km standard single mode fiber transmission was much higher than the threshold of error-free operation of a 5/6 code rate of digital video broadcasting satellite second generation low-density parity check forward error correction. To the best of our knowledge, the ratio of the symbol rate to the DAC sampling rate (135 GBaud over 120 GSa/s) is the highest ever reported for DAC-based optical coherent systems. The reachable distance of a 140 GBaud PDM-16QAM signal can be predicted to be approximately 100 km, even with the imperfect timing recovery operation.

中文翻译：

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通过 120 GSa/s DAC 和 Tomlinson-Harashima 预编码实现 135 GBaud 的 PDM-16QAM 传输

我们通过实验验证了用于光学相干系统的 Tomlinson-Harashima (TH) 预编码可以将偏振分复用 (PDM) 16 正交幅度调制 (QAM) 信号的速度提高到高于 135 GBaud 和 120 GSa/s 数字-到模拟转换器 (DAC) 和系统的电/光/电带宽小于 57 GHz，$- 30\;{\rm dB}$. 通过比较使用和不使用 TH 预编码的 PDM-16QAM 信号，可以明显地观察到基于光背靠背条件下接收 SNR 的可实现容量的提高。120km标准单模光纤传输135GBaud PDM-16QAM信号归一化广义互信息远高于数字视频广播卫星二代低密度奇偶校验5/6码率无差错运行阈值检查前向纠错。据我们所知，符号率与 DAC 采样率的比率（135 GBaud 超过 120 GSa/s）是有史以来报告的基于 DAC 的光学相干系统的最高值。140 GBaud PDM-16QAM 信号的可达距离可以预测为大约 100 公里，即使定时恢复操作不完善。