Jitter Margin Analysis of 56-Gb/s $\text{PAM}4\times 8-\text{Channel}$ VCSEL-Based Optical Transceiver for Co-Packaged Optics

We evaluate an optical link employing an ultra-compact 850-nm MM VCSEL-based 56-Gb/s $\mathbf{PAM}\mathbf{4} \times \mathbf{8}-\mathbf{channel}$ optical transceiver and three different kinds of MMFs (i.e., OM3, OM4, and OM5 fibers). We particularly analyze jitter margin characteristics for serial and 8-channel parallel signal transmissions. TDECQ values are also measured for optical eye diagrams transmitted through MMFs of several different conditions. The measured TDECQ values are as small as <1.9 dB within the specified distances of MMFs based on IEEE802.3cm. Jitter margin characteristics at the KP4-FEC threshold (i.e., BER of $\mathbf{2}.\mathbf{4}\times \mathbf{10}^{-\mathbf{4}})$ are narrower as increasing the distance of OM4, but a jitter margin of $\geq \mathbf{0}.\mathbf{35}$ U.I. can be kept within 110 m. We also measure no jitter margin degradation between serial and 8-channel parallel signal transmissions.

I. Introduction

To expand bandwidths and save power consumption in data centers and high-performance computing systems, Co-Packaged Optics (CPO) has been expected to the next-generation server architecture. Especially, VCSEL-based transceivers are very attractive in terms of saving power consumption with the aid of its low current drive characteristics. So far, the ARPA-E MOTION project has been demonstrating a power density of 4 pJ/bit in phase 1 and targeting a lower power density of 2 pJ/bit in phase 2 [1]. An 850-nm multi-mode (MM) VCSEL exhibited a modulation capability of a 56 Gb/s NRZ signal, but the multi-mode fiber (MMF) link distance was limited within 30 m. The NICT B5G BRIGHTEN project [2] was launched in 2021 where a 1060-nm InGaAs/GaAs single-mode (SM) VCSEL-based ultra-compact 25-Gb/s optical transceiver has been developed with a very high-density optical interface using multi-core fibers (MCFs). A single-mode high-speed VCSEL is very attractive for coverage of data center interconnects (DCIs), which require a longer distance of We reported a very high dense electrical pluggable interface employing a 0.3-mm pitch LGA to shrink a footprint of the optical transceiver as small as 7.7 [3]. We also built the first testing station (Gen. 1 testing station) for the optical transceiver. To characterize the electrical pluggable interface and testing station, we fabricated an 850-nm MM VCSEL-based 56-Gb/s optical transceiver employing commercially available electronics and photonics devices [4]. The footprint and electrical interface are compatible with the 16-channel optical transceiver. We reported the characteristics of individual optical links employing the 8-channel optical transceiver operated with a bit stream of 56 Gb/s PAM4 2¹³–1 pseudo-random binary sequence quaternary (PRBSQ) without aggressors in back-to-back. We obtained a bit error rate (BER) below the KP4 forward error correction (FEC) threshold (i.e., ) for each optical link.