Nubis’ bandwidth-packed tiny optical engine

Nubis Communications has revealed its ambitions to be an optical input-output (I/O) solutions provider. Its tiny 1.6-terabit optical engine measures 5mm x 7.5mm. The optical engine has a power consumption of below 4 picojoule/bit (pJ/b) and a bandwidth density of 0.5 terabits per millimetre.

“Future systems will be I/O with an ASIC dangling off it.” Nubis Communications has ended its period of secrecy to unveil an optical engine targeted at systems with demanding data input-output requirements.

The start-up claims its optical engine delivers unmatched bandwidth density measured in terabits per millimetre (T/mm) and power consumption performance metrics. “In the timeframe of founding the company [in 2020], it became obvious that the solution space [for our product] was machine learning-artificial intelligence,” says Dan Harding, the CEO of Nubis.

Company Background

Nubis has raised over $40 million, with the lead investor being Matrix Partners. Venture capital company Matrix Partners backed Acacia Communications, acquired by Cisco in 2021.

Other Nubis backers are Weili Dai, a co-founder of Marvell Technologies, and Belgium-based imec.xpand.

“We have raised enough money to get to production with our product,” says Harding, who joined Nubis in 2021 from Broadcom.

Peter Winzer is the CTO and founder of the company. Formerly at Nokia Bell Labs, Winzer was the 2018 winner of the Optica (then OSA) and IEEE Photonics Society’s John Tyndall Award for his work on coherent optical communications.

Nubis has 40 staff, mostly engineers.

“As a team, we are multidisciplinary,” says Winzer. The company’s expertise includes silicon photonics, analogue IC design including serialisers/ deserialisers (serdes), packaging – electrical and optical, and software including advanced simulation tools.

“It is all geared towards a systems solution,” says Winzer. “We are not just looking at the PIC [photonic integrated circuit] or the electronics; we have the system and the architecture in mind.”

The input-output challenge

Machine learning workloads continue to grow at a staggering pace, doubling more than twice each year. Not surprisingly, computing systems running such workloads are struggling to keep up.

Scaling such systems not only requires more processing – more graphics processing units (GPUs) – but also networking to connect clusters of GPUs.

What the compute vendors want is any-to-any connectivity between processors and between clusters. This is creating a tremendous input-output challenge in terms of bandwidth density while keeping the power consumption under control.

“Over half the power of that cluster can be taken up by traditional optics,” says Harding. “So it is clear that the industry wants new solutions.”

“Whatever cents-per-gigabit [figure] you use, if you multiply it by the I/O capacity, the number you’ll get is many times that of [the cost of] an ASIC,” adds Winzer. “We say that future systems will be I/O with an ASIC dangling off it.”

Target applications

Nubis’ optical engine can support retimed and direct-drive designs.

Direct drive refers to how the optical engine does away with a DSP, using the remote ASIC’s serdes instead. The benefit of direct drive is that the optical engine’s power consumption is lower since no local serdes are needed.

“There are two optical I/O approaches: socketed and chiplet-based, and we support both,” says Winzer.

Nubis is addressing first the socketed optical I/O opportunity as it is the nearer-term market.

“Optical I/O is really what we do, and at high speed,” says Winzer. “We are doing 100 gigabit and 200 gigabit [per channel]; we are not dabbling in low speed, wide and parallel.”

Nubis says its engine is also aimed at the pluggable module and active optical cable markets.

Design details

Nubis’ optical engine is a 16 x 112-gigabit design with a footprint of 5mm x 7.5mm.

“Because we have our electronics flip-chipped on top, that’s the entire footprint,” says Winzer. “We maintain that it is the highest density by far of any optical engine.”

Nubis says many parallel fibres can be interfaced to the optical engine despite its tiny size.

Supporting parallel fibres is essential for machine learning systems as the fibres are fanned out to enable any-to-any connectivity.

Nubis’ engine uses a 4 by DR4 fan-out architecture with 36 fibres arranged in a 3×12 array.

Surface coupling in a 2D array interfaces the 36 fibres to the PIC: 32 fibres are for data and four for the external laser light source.

There is only a physical limit to the number of fibres that can be connected if edge coupling is used, says Winzer. But surface coupling in a 2D array means the optical engine delivers 5-10x more density than its competitors.

The start-up also has designed the engine’s electronics: the optical modulator driver and the trans-impedance amplifier (TIA). The electronics use advanced equalisation to boost the electrical channel, given direct drive has demanding requirements, says Harding.

The XT1600 optical module

Nubis’ first product is the XT1600 optical module. Here, a substrate houses the company’s PIC and electronics onto which is packaged a lid containing the optical fibres.

Nubis has developed in-house the packaging and the fibre attach solution.

The substrate is 15x15mm, somewhat larger than the engine. Harding says this is deliberate to support products under development.

The 1.6 terabits – in fact, 16x112Gbps full duplex – module has a 2km reach. Its power consumption is below 4 pJ/b.

The fibres exit the module vertically and bend to the side. “[Going] vertical is good but the 2D is the much more important aspect here,” says Winzer.

A 2D approach is logical, says Nubis. An electrical ball grid array (BGA) all the bottom surface. It makes sense that the optics is similarly massively 2D, especially for designs where its a 100-gigabit electrical signal in and a 100-gigabit optical signal out.

Multiple rings of optical I/O engines can surround the ASIC because the fibres exit vertically. “Nobody else can do that because they are escaping from the [PIC] edge,” says Winzer.

Winzer highlights another benefit of the design.

The Universal Chiplet Interconnect Express (UCIe) specification calls for 2T/mm bandwidth escape density. An optical chiplet can only achieve this if wavelength-division multiplexing (WDM) is used due to the large fibre size. Nubis can achieve this density optically without having to use WDM because of 2D surface coupling.

Doing all-to-all at scale remains a big system challenge. “We’re just a part of that challenge,” says Harding. But for optical I/O to become pervasive in the data centre over the next five years, the optics must be significantly lower power, smaller, and efficient.

“If you crack that 2D nut, you can do many, many great things down the road,” says Winzer. “We’ve solved a huge technology problem that allows us to scale much better than anybody else.”

Status

Nubis has not named its foundry and contract manufacturing partners but says they are large, high-volume manufacturers.

Harding says there are now up to five credible silicon photonic foundries available.

“There was some early product definition which some foundries were better suited to support,“ says Harding. “And there was a robustness of the initial PDKs [process design kits] to get us an early product that was important to us.”

Choosing a contract manufacturer proved easier, given the maturity of the players.

Nubis’ first product has 16 optical channels each at 112 gigabit, but future designs will offer N by 224-gigabit channels.

Meanwhile, the XT1600 optical engine is available for sampling.