Energy-efficient ultracompact 106-Gb/s PAM4 × 8-channel VCSEL-based CPO transceiver

This paper describes an energy-efficient ultra-compact 106-Gb/s PAM4 × 8-channel VCSEL-based Co-Packaged Optics (CPO) transceiver for networks and AI/ML systems in data centers. This CPO transceiver employs a wide bandwidth coreless build-up organic substrate to an interposer and it contributes realizing a record narrow 0.3-mm pitch LGA electrical interface and then also enables an extremely small footprint of 7.7 mm × 15.9 mm. Differential transmission lines formed in the interposer exhibit 3-dB bandwidth of >46 GHz even when including the 350-μm gold wires between electrical pads of a 53-Gbaud linear-drive EIC and electrical pads of the interposer. Small-signal responses over 100-m OM4 links demonstrate electro-optic 3-dB bandwidth of >30.0 GHz, and electro-electro 6-dB bandwidth of 26.9 GHz, which are wider than the Nyquist frequency for 53-Gbaud operation. When operating the CPO transceiver by a bit stream of 64-Gb/s and 106-Gb/s PAM4 PRBSQ 2¹³-1 signals in a loopback optical link with a 100-m OM4 fiber, BER characteristics are lower than the required forward-error correction threshold for PCIe 6.0 and IEEE802.3db. The optical links exhibit low link energy of 4.5 pJ/bit for the 106-Gb/s PAM4 operation, which successfully meets the targeted value for energy-efficient interconnects in AI/ML systems.