Contents I. Introduction
To increase the bandwidth and save power consumption in data centers and high-performance computing systems, Co-Packaged Optics (CPO) has been expected to realize the next-generation server architecture. Several research institutes have been demonstrating silicon photonics-based transceivers, and they will be attractive for mass production with silicon wafer processes and wafer scale packaging. However, VCSEL-based transceivers are still very attractive in terms of saving power consumption. In fact, the ARPA-E sponsored MOTION project demonstrated a power density of 4 pJ/bit in phase 1 and the project has been targeting a lower power density of 2 pJ/bit in phase 2 [1]. VCSEL-based optical interconnects have been mostly employed for short reach applications of <100-m multi-mode fibers (MMFs) with a modulation speed of≥25 Gb/s per channel. A single-mode high-speed VCSEL is very attractive for the coverage of data center interconnects, which requires a longer distance of ≥2 km. To realize wide bandwidth and low power CPO solutions, the NICT B5G research project (#001) [2], BRIGHTEN, has launched since 2021 where a key component is a 1060-nm single-mode VCSEL-based 25-Gbaud ×16-channel optical transceiver with a very high-density optical interface using multi-core fibers (MCFs). According to the CPO collaboration JDF document [3], a land grid array (LGA) interface using a pitch size of 0.6 mm is specified and the footprint of transceiver is as wide as 20.7 mm ×20.1 mm. To decrease the mechanical size of transceiver and daughter board, it is much preferable to use a high-density electrical interface. Hence, we adopt the world narrowest pitch size of 0.3 mm for the electrical pluggable LGA interface. We also build a testing station for an ultra-compact 25-Gbaud × 16-channel transceiver whose footprint is as small as 15.9 mm × 7.7 mm [4]. In order to characterize the electrical interface and testing station, we firstly fabricated an 850-nm multi-mode VCSEL-based 56-Gb/s P AM4 × 8-channel optical transceiver employing commercially available electronics and photonics devices. The number of channels is 8 at the maximum due to the footprint. This transceiver uses a half of high-speed electrical contacts. Such a very small transceiver is also attractive as an optical engine to be built in a 400-Gb/s QSFP-DD pluggable transceiver [5]. In this paper, we describe the structural design and transmission characteristics of the 56-Gb/s PAM4 × 8-channel optical transceiver.
