Contents I. Introduction
Co-Packaged Optics (CPO) is attracting attention to expand bandwidth and save power consumption in data center networks. Silicon photonics-based transceivers for CPO have been demonstrated by several research institutes because they will be attractive for mass production with silicon wafer processes and wafer-scale packaging [1], [2]. However, vertical cavity surface emitting laser (VCSEL)-based transceivers are promising in saving power consumption with the aid of the low current drive characteristics. VCSEL-based optical links have been mostly employed for short-reach applications of <100-m multi-mode fibers (MMFs) with a modulation speed of Gbaud per channel. A single-mode high-speed VCSEL is very attractive for the coverage of data center interconnects, which requires a longer distance of . The NICT B5G BRIGHTEN project [3] has been launched since 2021 to realize wide bandwidth and low-power network switch servers using a 1060-nm coupled-cavity single-mode (SM) VCSEL-based optical transceiver. The optical transceiver employs an ultra-high-density optical interface using an SM multi-core fiber (MCF). This SM-MCF enables a remarkable reduction in the size of the optical transceiver. Furthermore, it is much preferable to use a high-density electrical interface to decrease the mechanical size of the optical transceiver and daughter board. Therefore, we designed and fabricated an electrical pluggable interface that uses the world's smallest 0.3-mm pitch land grid array (LGA). In our previous work, we built the first testing station (Gen. 1 testing station) to characterize a optical transceiver whose footprint is as small as [4]. We also reported the RF characteristics of the testing station with an 850-nm multi-mode (MM) VCSEL-based 28-Gbaud optical transceiver employing commercially available electronic and photonic devices [5]. We demonstrated a BER of 10−12 for both 28-Gbaud NRZ and PAM4 operations at a heatsink temperature of 25°C. However, a 3-dB bandwidth of this testing station was limited to 10.2 GHz at the worst channel due to the impedance-mismatched vias in a core layer. To purely evaluate the optical transceiver, the 3-dB bandwidth of a test station should be GHz (i.e., a Nyquist frequency of 25 Gbaud). In this work, we design and fabricate the next-generation testing station (Gen. 2 testing station) using a coreless building-up multi-layer substrate to avoid the impedance mismatch of the vias in the core layer. Furthermore, we placed electrical connectors more densely to shorten the length of electrical transmission lines for bandwidth expansion. We demonstrate the total jitter margin improvement of the optical link by employing the Gen. 2 station.
