PDM-16QAM transmission of 135 GBaud enabled by 120 GSa/s DAC and Tomlinson-Harashima pre-coding

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 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 withoutTHpre-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 140GBaud PDM-16QAM signal can be predicted to be approximately 100 km, even with the imperfect timing recovery operation. (C) 2021 Optical Society of America

Automated summary

Experimental results confirm that TH pre-coding can enhance the speed and capacity of PDM-16QAM signals in optical coherent systems, showing significant improvement in performance under optical back-to-back conditions. The ratio of symbol rate to DAC sampling rate achieved in this study is the highest reported for DAC-based optical coherent systems to date, demonstrating promising potential for long-distance transmission.