This article is part of the TechXchange: Silicon Photonics.
Co-packaged optics (CPO) technology developed by Broadcom reached a new level with Bailly, a 51.2-Tb/s optical switch that unites the company’s latest Tomahawk 5 switch chip with eight optical engines in one package.
On display in a demo at OFC last month, the Bailly chips are based on a new architecture that co-packages optical interconnects, which have long been plugged into the front of a switch, with a traditional switch ASIC.
To handle the surge in demand for AI and other workloads in the data center, cloud giants are increasingly using the power of photons to string together tens of thousands of servers and GPUs and other accelerators plugged into them across sprawling installations.
But the new switch uses silicon photonics to create optical interfaces in chiplet form. The chiplets are then directly attached to the switch ASIC in a package, providing major power and cost savings.
Broadcom said Bailly brings a major leap in performance over the “Humboldt” switch that it first debuted last year based on its 25.6-Tb/s Tomahawk 4, supporting 128 lanes of 100 Gb/s of optical connectivity.
Humboldt, which the company is building into Tencent’s data centers, contains four 3.2-Tb/s optical engines based on its silicon-photonics chiplets-in-package (SCIP) technology. Flexibility is the focus with the hybrid Humboldt: It uses optical I/O to supply a total of 12.8 Tb/s of connectivity out through the front of the switch, while the electrical I/O is used to connect to 32 400-Gb/s ports to supply the remaining bandwidth.
Broadcom is upping the ante with Bailly, doubling the optical engines co-packaged with the switch silicon to eight and the bandwidth of each of the interfaces to 6.4 Tb/s, resulting in an all-optical 51.2-Tb/s switch.
VP and GM of Optical Systems Near Margalit said every silicon photonics engine is based on optical I/O (PIC) and electrical I/O (EIC) chiplets. These building blocks are glued together in a wafer-scale package in a way that matches the pitch of the switch CPU’s SerDes.
Broadcom said the Bailly switch marks the first time it has manufactured a full CMOS electrical I/O chiplet, including a very-low-power TIA. It also packs a monolithically integrated optical MUX/DEMUX on the PIC.
Spotlight on Power Savings
The co-packaged optics at the heart of Bailly is designed to reduce excess power in the switch itself, which can create heat that limits how many and how closely the ports can be packed onto the front of the switch.
Modern data centers use optical transceivers that are plugged into the front of the switch. They turn data into light and then shoot it out around the system via optical fibers unfurling from the switch.
These connectivity modules are located a relatively long distance from the ASIC and the SerDes inside it. As a result, they have high-end digital signal processors (DSPs) that keep the signal from being scrambled at high speeds as it travels from the switch silicon over the electrical I/O on the board to the optics on the other end.
But as cloud and other technology firms step up to higher bandwidths in their data centers, electrical I/Os that connect the SerDes in the switch to the optics on the front of the box are becoming a key bottleneck.
By reducing the distance that usually separates the switch from the optics to a matter of millimeters, Broadcom said Bailly cuts down on the large amounts of power required to transfer data via electrical I/O.
The ability to drive the optics directly from the switch silicon brings a wide range of advantages. Broadcom said the all-optical Bailly switch expends only 5.5 W for every 400 Gb/s of high-bandwidth connectivity, a savings of more than 50% versus existing switches that must pair high-end DSPs and other expensive silicon. According to the company, pluggable modules on the market burn through from 14 to 16 W of power.
Bailly’s power savings are about 20% better than Humboldt (which expends 6.4 W of power for the same data rate), which helps reduce the cost of data transmission compared to existing switches, said Margalit.
Power efficiency is a high priority when it comes to building cloud data centers due to the high cost of electricity and the large carbon footprint of such massive operations.
Co-packaged optics also create more space to pack more optical ports in the switch, boosting bandwidth density. Alternatively, you also can space out the ports on the switch farther apart to improve air cooling.
The Bailly “can deliver double the bandwidth of a 25.6T standard solution without any increase in system power consumption,” said Margalit, who headed Intel’s silicon photonics unit before he joined Broadcom.
While the Tomahawk 5 is already shipping, the Bailly switch is set to enter mass production later this year.
This article is part of the TechXchange: Silicon Photonics.
