II-VI’s VCSEL approach for co-packaged optics

Co-packaged optics was a central theme at this year’s OFC show, held in San Diego. But the solutions detailed were primarily using single-mode lasers and fibre.

Vipul Bhatt

The firm II-VI is beating a co-packaged optics path using vertical-cavity surface-emitting lasers (VCSELs) and multi-mode fibre while also pursuing single-mode, silicon photonics-based co-packaged optics.   

For multi-mode, VCSEL-based co-packaging, II-VI is working with IBM, a collaboration that started as part of a U.S. Advanced Research Projects Agency-Energy (ARPA-E) project to promote energy-saving technologies.

II-VI claims there are significant system benefits using VCSEL-based co-packaged optics. The benefits include lower power, cost and latency when compared with pluggable optics.

The two key design decisions that achieved power savings are the elimination of the retimer chip - also known as a direct-drive or linear interface - and the use of VCSELs.

The approach - what II-VI calls shortwave co-packaged optics - integrates the VCSELs, chip and optics in the same package.

The design is being promoted as first augmenting pluggables and then, as co-packaged optics become established, becoming the predominant solution for system interconnect.

For every 10,000 QSFP-DD pluggable optical modules used by a supercomputer that are replaced with VCSEL-based co-packaged optics, the yearly electricity bill will be reduced by up to half a million dollars, estimate II-VI and IBM.

VCSEL technology

VCSELs are used for active optical cables and short-reach pluggables for up to 70m or 100m links.

VCSEL-based modules consume fewer watts and are cheaper than single-mode pluggables.

Several factors account for the lower cost, says Vipul Bhatt,  vice president of marketing, datacom vertical at II-VI.

The VCSEL emits light vertically from its surface, simplifying the laser-fibre alignment, and multi-mode fibre already has a larger-sized core compared to single-mode fibre.

"Having that perpendicular emission from the laser chip makes manufacturing easier," says Bhatt. "And the device's small size allows you to get many more per wafer than you can with edge-emitter lasers, benefitting cost."

The tinier VCSEL also requires a smaller current density to work; the threshold current of a distributed feedback (DFB) laser used with single-mode fibre is 25-30mA, whereas it is 5-6mA for a VCSEL. “That saves power,” says Bhatt.

Fibre plant

Hyperscalers such as Google favour single-mode fibre for their data centres. Single-mode fibre supports longer reach transmissions, while Google sees its use as future-proofing its data centres for higher-speed transmissions.

Chinese firms Alibaba and Tencent use multi-mode fibre but also view single-mode fibre as desirable longer term.

Bhatt says he has been hearing arguments favouring single-mode fibre for years, yet VCSELs continue to advance in speed, from 25 to 50 to 100 gigabits per lane.

“VCSELs continue to lead in cost and power,” says Bhatt. ”And the 100-gigabit-per-lane optical link has a long life ahead of it, not just for networking but machine learning and high-performance computing.“

II-VI says single-mode fibre and silicon photonics modules are suited for the historical IEEE and ITU markets of enterprise and transport where customers have longer-reach applications.

VCSELs are best suited for shorter reaches such as replacing copper interconnects in the data centre.

Copper interconnect reaches are shrinking as interface speeds increase, while a cost-effective optical solution is needed to support short and intermediate spans up to 70 meters.

“As we look to displace copper, we're looking at 20 meters, 10 meters, or potentially down to three-meter links using active optical cables instead of copper,” says Bhatt. “This is where the power consumption and cost of VCSELs can be an acceptable premium to copper interconnects today, whereas a jump to silicon photonics may be cost-prohibitive.”

Silicon photonics-based optical modules have higher internal optical losses but they deliver reaches of 2km and 10km.

“If all you're doing is less than 100 meters, think of the incredible efficiency with which these few milliamps of current pumped into a VCSEL and the resulting light launched directly and efficiently into the fibre,” says Bhatt. “That's an impressive cost and power saving.”

Applications

The bulk of VCSEL sales for the data centre are active optical cables and short-reach optical transceivers.

“Remember, not every data centre is a hyperscale data centre,” says Bhatt. ”So it isn't true that multi-mode is only for the server to top-of-rack switch links. Hyperscale data centres also have small clusters for artificial intelligence and machine learning.”  

The 100m-reach of VCSELs-based optics means it can span all three switching tiers for many data centres.

The currently envisioned 400-gigabit VCSEL modules are 400GBASE-SR8 and the 8-by-50Gbps 400G-SR4.2. Both use 50-gigabit VCSELs: 25 gigabaud devices with 4-level pulse amplitude modulation (PAM-4).

The 400GBASE-SR8 module requires 16 fibres, while the 400G-SR4.2, with its two-wavelength bidirectional design, has eight fibres.

The advent of 100-gigabit VCSELs (50 gigabaud with PAM-4) enables 800G-SR8, 400G-SR4 and 100G-SR1 interfaces. II-VI first demonstrated a 100-gigabit VCSEL at ECOC 2019, while 100-gigabit VCSEL-based modules are becoming commercially available this year.

Terabit VCSEL MSA

The Terabit Bidirectional (BiDi) Multi-Source Agreement (MSA) created earlier this year is tasked with developing optical interfaces using 100-gigabit VCSELs.

The industry consortium will define 800 gigabits interface over parallel multi-mode fibre, the same four pairs of multi-mode fibre that support the 400-gigabit, 400G-BD4.2 interface. It will also define a 1.6 terabit optical interface.

The MSA work will extend the parallel fibre infrastructure from legacy 40 gigabits to 1.6 terabits as data centres embrace 25.6-terabit and soon 51.2-terabit switches.

Founding Terabit BiDi MSA members include II-VI, Alibaba, Arista Networks, Broadcom, Cisco, CommScope, Dell Technologies, HGGenuine, Lumentum, MACOM and Marvell Technology.

200-gigabit lasers and parallelism

The first 200-gigabit electro-absorption modulator lasers (EMLs) were demonstrated at OFC '22, while the next-generation 200-gigabits directly modulated lasers (DMLs) are still in the lab.

When will 200-gigabit VCSELs arrive?

Bhatt says that while 200-gigabit VCSELs were considered to be research-stage products, recent interest in the industry has spurred the VCSEL makers to accelerate the development timeline.

Bhatt repeats that VCSELs are best suited for optimised short-reach links.

“You have the luxury of making tradeoffs that longer-reach designs don't have,” he says. “For example, you can go parallel: instead of N-by-200-gig lanes, it may be possible to use twice as many 100-gig lanes.”

VCSEL parallelism for short-reach interconnects is just what II-VI and IBM are doing with shortwave co-packaged optics.

Shortwave co-packaged optics

Computer architectures are undergoing significant change with the emergence of accelerator ICs for CPU offloading.

II-VI cites such developments as Nvidia’s Bluefield data processing units (DPUs) and the OpenCAPI Consortium, which is developing interface technology so that any microprocessor can talk to accelerator and I/O devices.

“We're looking at how to provide a high-speed, low-latency fabric between compute resources for a cohesive fabric,” says Bhatt. The computational resources include processors and accelerators such as graphic processing units (GPUs) and field-programmable gate arrays (FPGAs).

II-VI claims that by using multi-mode optics, one can produce the lowest power consumption optical link feasible, tailored for very-short electrical link budgets.

The issue with pluggable modules is connecting them to the chip’s high-speed signals across the host printed circuit board (PCB).

“We're paying a premium to have that electrical signal reach through,” says Bhatt. “And where most of the power consumption and cost are is those expensive chips that compensate these high-speed signals over those trace lengths on the PCB.”

Using shortwave co-packaged optics, the ASIC can be surrounded by VCSEL-based interfaces, reducing the electrical link budget from some 30cm for pluggables to links only 2-3cm long.

“We can eliminate those very expensive 5nm or 7nm ICs, saving money and power,” says Bhatt.

The advantage of shortwave co-packaged optics is better performance (a lower error rate) and lower latency (between 70-100ns) which is significant when connecting to pools of accelerators or memory.

“We can reduce the power from 15W for a QSFP-DD module down to 5W for a link of twice the capacity,” says Bhatt, “We are talking an 80 per cent reduction in power dissipation. Another important point is that when power capacity is finite, every watt saved in interconnects is a watt available to add more servers. And servers bring revenue.”

This is where the 10,000-unit optical interfaces, $0.4-$0.5 million savings in yearly electricity costs comes from.

The power savings arise from the VCSEL’s low drive current, the use of the OIF’s ultra short-reach (USR) electrical interface and the IBM processor driving the VCSEL directly, what is called a linear analogue electrical interface.

In the first co-packaged optics implementation, IBM and II-VI use non-return-to-zero (NRZ) signalling.

The shortwave co-packaged optics has a reach of 20m which enables the potential elimination of top-of-rack switches, further saving costs. (See diagram.)

Source: II-VI

II-VI sees co-packaged optics as initially augmenting pluggables. With next-generation architectures using 1.6-terabit OSFP-XD pluggables, 20 to 40 per cent of those ports are for sub-20m links.

"We could have 20 to 40 per cent of the switch box populated with shortwave co-packaged optics to provide those links," says Bhatt.

The remaining ports could be direct-attached copper, longer-reach silicon-photonics modules, or VCSEL modules, providing the flexibility associated with pluggables.

"We think shortwave co-packaged optics augments pluggables by helping to reduce power and cost of next-generation architectures."

This is the secret sauce of every hyperscaler. They don't talk about what they're doing regarding machine learning and their high-performance systems, but that's where they strive to differentiate their architectures, he says.

Status

Work has now started on a second-generation shortwave design that will use PAM-4 signalling. “That is targeted as a proof-of-concept in the 2024 timeframe,” says Bhatt.

The second generation will enable a direct comparison in terms of power, speed and bandwidth with single-mode co-packaged optics designs.

Meanwhile, II-VI is marketing its first-phase NRZ-based design.

“Since it is an analogue front end, it's truly rate agnostic,” says Bhatt. “So we're pitching it as a low-latency, low-power bandwidth density solution for traditional 100-gigabit Ethernet.”

The design also can be used for next-generation PCI Express and CXL disaggregated designs. 

II-VI says there is potential to recycle hyperscaler data centre equipment by adding state-of-the-art network fabric to enable pools of legacy processors. “This technology delivers that,” says Bhatt.

But II-VI says the main focus is for accelerator fabrics: proprietary interfaces like NVlink, Fujitsu’s Tofu interconnect or HPE’s Cray’s Slingshot.

“At some point, memory pools or storage pools will also work their way into the hyperscalers’ data centres,” says Bhatt.