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. 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.