We discuss technology options and challenges for scaling intra-datacenter interconnects beyond 1 Tb/s bandwidths, with focus on two possible approaches: pulse amplitude modulation (PAM)-based intensity modulation-direct detection (IM-DD) and baud-rate sampled coherent technology. In our studies, we compare the performance of various orders of PAM modulation (PAM4 to 8). In addition to these fixed PAM signaling options, a flexible PAM (FlexPAM) technique leveraging granularity in spectral efficiency (SE) is proposed to maximize link margin. For baud-rate sampled coherent technology, we propose a simplified digital signal processing (DSP) architecture to bring down power consumption of the coherent approach closer to that of IM-DD PAM. We also propose two new phase noise tolerant 2D coherent modulation formats to relax the laser linewidth requirement. In closing, a comparative study of fixed IM-DD PAM versus coherent polarization multiplexed-quadrature amplitude modulation (PM-QAM) is presented for a 1.6 Tb/s solution (200 Gb/s per dimension), with consideration of link loss/reach budget, power consumption, implementation complexity, as well as fan-out granularity.

中文翻译：

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超过 1Tb/s 的数据中心内部互连技术：IM-DD 还是 Coherent？

我们讨论了将数据中心内互连扩展到超过 1 Tb/s 带宽的技术选项和挑战，重点是两种可能的方法：基于脉冲幅度调制 (PAM) 的强度调制直接检测 (IM-DD) 和波特率采样相干技术。在我们的研究中，我们比较了不同阶 PAM 调制（PAM4 到 8）的性能。除了这些固定的 PAM 信令选项之外，还提出了一种利用频谱效率 (SE) 粒度的灵活 PAM (FlexPAM) 技术来最大化链路余量。对于波特率采样相干技术，我们提出了一种简化的数字信号处理 (DSP) 架构，以降低相干方法的功耗，使其更接近 IM-DD PAM 的功耗。我们还提出了两种新的抗相位噪声 2D 相干调制格式，以放宽激光线宽要求。最后，针对 1.6 Tb/s 解决方案（每维 200 Gb/s）提出了固定 IM-DD PAM 与相干偏振复用正交幅度调制 (PM-QAM) 的比较研究，并考虑了链路损耗/到达预算、功耗、实现复杂性以及扇出粒度。