Sub-Baudrate Sampling at DAC and ADC: Toward 200G per Lane IM/DD Systems

As optical transceivers evolve toward 200G per lane, the comparatively low bandwidth and sampling speed of the commercially available high-speed digital-to-analog converters (DACs) and analog-to-digital converters (ADCs) increasingly become the dominating obstacle toward success. On the other hand, DACs and ADCs are essential to make use of digital signal processing for impairment compensation and error correction; altogether driving systems' engineers to a hard design dilemma. With the purpose of alleviating this compromise, we propose a transceiver structure-encompassing both hardware and software-enabling the generation, transmission, and detection of strongly over-filtered data signals whose baudrate exceeds the sampling rates of (potentially) both the DAC and ADC. We call this approach sub-baudrate sampling, a technique which does not increase the processing complexity at the transmitter side and does not even require the information about the channel state for pre-distortion or pre-coding purposes. We experimentally assess the performance of sub-baudrate sampled signals in two blocks of experiments: first the proof of concept in back-to-back configuration, including up to 112-Gbaud ON-OFF keying at 0.785 samples per symbol in ~25-GHz aggregate bandwidth. Then the C-band transmission demonstration, showing up to 100-Gbaud quaternary pulse-amplitude modulation (0.92 samples per symbol) and 125-Gbaud ON-OFF keying (0.736 samples per symbol) over 1 and 80-km of single-mode fiber, respectively, with more than 3-dB receiver sensitivity margin at the 7% overhead hard-decision bit error rate limit.