A DSP-Free Carrier Phase Recovery System using 16-Offset-QAM Laser Forwarded Links for 400Gb/s and Beyond

TL;DR

This work tackles the power and complexity burden of carrier phase recovery in high-order coherent links by introducing a DSP-free CPR strategy based on offset-QAM and laser-forwarding. The approach detects phase error from the average I/Q differences and corrects it with a LO-phase shifter, enabling modulation-agnostic scalability to 4-, 16-, and higher-offset-QAM without architectural changes. Through circuit- and system-level simulations in GlobalFoundries’ 45 nm silicon-photonics PDK at 100 GBaud in the O-band, the authors analyze loop dynamics, laser phase noise effects, and practical tolerances, showing that 400 Gb/s via 16-offset-QAM can meet KP4 FEC under realistic linewidths and LO–Rx mismatches. The proposed method, combined with laser-forwarding, offers a path to low-power, coherent-lite transceivers for future data-center interconnects and co-packaged optics.

Abstract

Optical interconnects are becoming a major bottleneck in scaling up future GPU racks and network switches within data centers. Although 200 Gb/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 - 10x 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 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.

Figures (10)

• Figure 1: The constellation of (a) 4-QAM (QPSK) (b) 4-offset-QAM modulation with and without the phase error between the Rx and LO paths.
• Figure 2: (a) Block diagrams of the proposed offset-QAM coherent receiver using Method 1 (b) and Method 2 (c) for LO phase recovery.
• Figure 3: The error signal generated by Method 1 and Method 2.
• Figure 4: (a) The linear small-signal phase modeling of the CPR system (b) the simplified phase model after averaging.
• Figure 5: Magnitude and phase plots of the open-loop transfer function of the CPR system.