ORIGINAL RESEARCH article
Front. Commun. Netw.
Sec. Optical Communications and Networks
Volume 6 - 2025 | doi: 10.3389/frcmn.2025.1642672
A DSP-Free Carrier Phase Recovery System using 16-Offset-QAM Laser Forwarded Links for 400Gb/s and Beyond
Provisionally accepted
- University of Washington, Seattle, United States
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Optical interconnects are becoming a major bottleneck in scaling up future GPU racks and network switches within data centers. Although 200Gb/s optical transceivers using PAM-4 modulation have been demonstrated, achieving higher data rates and energy efficiencies requires high-order coherent modulations like 16-QAM. Current coherent links rely on energy-intensive digital signal processing (DSP) for channel impairment compensation and carrier phase recovery (CPR), which consumes approximately 50pJ/b, 10 times higher than future intra-data center requirements. For shorter links, simpler or DSP-free CPR methods can significantly reduce power and complexity. While Costas loops enable CPR for QPSK, they face challenges in scaling to higher-order modulations (e.g., 16/64-QAM) due to varying symbol amplitudes. In this work, we propose an optical coherent link architecture using laser forwarding and a novel DSP-free CPR system using offset-QAM modulation. The proposed analog CPR feedback loop is highly scalable, capable of supporting arbitrary offset-QAM modulations (e.g., 4, 16, 64) without requiring architectural modifications. This scalability is achieved through its phase error detection mechanism, which operates independently of the data rate and modulation type. We validated this method using GlobalFoundry's monolithic 45nm silicon photonics PDK models, with circuit-and system-level implementation at 100GBaud in the O-band. We will investigate the feedback loop dynamics, circuit-level implementations, and phase-noise performance of the proposed CPR loop. Our method can be adopted to realize low-power QAM optical interconnects for future coherent-lite pluggable transceivers as well as co-packaged optics (CPO) applications.
Keywords: Intra-data center interconnect, Coherent communications, Optical transmitter, optical receiver, Analog Signal Processing, 16-QAM, offset-QAM, carrier phase recovery
Received: 06 Jun 2025; Accepted: 01 Sep 2025.
Copyright: © 2025 Rezaei, Sturm, Zeng and Moazeni. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
* Correspondence: Marziyeh Rezaei, University of Washington, Seattle, United States
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