The future of optical I/O is more parallel links

Chris Cole has a lofty vantage point regarding how optical interfaces will likely evolve.

As well as being an adviser to the firm II-VI, Cole is Chair of the Continuous Wave-Wavelength Division Multiplexing (CW-WDM) multi-source agreement (MSA). 

Chris Cole

The CW-WDM MSA recently published its first specification document defining the wavelength grids for emerging applications that require eight, 16 or even 32 optical channels.

And if that wasn’t enough, Cole is also the Co-Chair of the OSFP MSA, which will standardise the OSFP-XD (XD standing for extra dense) 1.6-terabit pluggable form factor that will initially use 16, 100 gigabits-per-second (Gbps) electrical lanes. And when 200Gbps electrical input-output (I/O) technology is developed, OSFP-XD will become a 3.2-terabit module. 

Directly interfacing with 100Gbps ASIC serialiser/ deserialiser (serdes) lanes means the 1.6-terabit module can support 51.2-terabit single rack unit (1RU) Ethernet switches without needing 200Gbps ASIC serdes required by eight-lane modules like the OSFP.

“You might argue that it [the OSFP-XD] is just postponing what the CW-WDM MSA is doing,” says Cole. “But I’d argue the opposite: if you fundamentally want to solve problems, you have to go parallel.”

CW-WDM specification

The CW-WDM MSA is tasked with specifying laser sources and the wavelength grids for use by higher wavelength count optical interfaces.

The lasers will operate in a subset of the O-band (1280nm-1320nm) building on work already done by the ITU-T and IEEE standards bodies for datacom optics.

In just over a year since its launch, the MSA has published Revision 1.0 of its technical specification document that defines the eight, 16 and 32 channels. 

The importance of specifying the wavelengths is that lasers are the longest lead items, says Cole: “You have to qualify them, and it is expensive to develop more colors.”

In the last year, the MSA has confirmed there is indeed industry consensus regarding the wavelength grids chosen. The MSA has 11 promoter members that helped write the specification document and 35 observer status members.

“The aim was to get as many people on board as possible to make sure we are not doing something stupid,” says Cole.

As well as the wavelengths, the document addresses such issues as total power and wavelength accuracy.

Another issue raised is four-wavelength mixing. As the channel count increases, the wavelengths are spaced closer together. Four-wavelength mixing refers to an undesirable effect that impacts the link’s optical performance. It is a well-known effect in dense WDM transport systems where wavelengths are closely spaced but is less commonly encountered in datacom.

“The first standard is not a link budget specification, which would have included how much penalty you need to allocate, but we did flag the issue,” says Cole. “If we ever publish a link specification, it will include four-wavelength mixing penalty; it is one of those things that must be done correctly.”

Innovation

The MSA’s specification work is incomplete, and this is deliberate, says Cole.

“We are at the beginning of the technology, there are a lot of great ideas, but we are going to resist the temptation to write a complete standard,” he says.

Instead, the MSA will wait to see how the industry develops the technology and how the specification is used. Once there is greater clarity, more specification work will follow.

Matt Sysak

“It is a tricky balance,” says Cole. “If you don’t do enough, what is the value of it? But if you do too much, you inhibit innovation.”

“The key aspect of the MSA is to help drive compliance in an emerging market,” says Matt Sysak of Ayar Labs and editor of the MSA’s technical specification. “This is not yet standardised, so it is important to have a standard for any new technology, even if it is a loose one.”

The MSA wants to see what people build. “See which one of the grids gain traction,” says Sysak.

Ayar Labs’ SuperNova remote light source for co-packaged optics is one of the first products that is compliant with the CW-WDM MSA.

Sysak notes that at recent conferences co-packaged optics is a hot topic but what is evident is that it is more of a debate.

“The fact that the debate doesn’t seem to coagulate around particular specification definitions and industry standards is indicative of the fact that the entire industry is struggling here,” says Sysak.

This is why the CW-WDM MSA is important, to help promote economies of scale that will advance co-packaged optics.

Applications

Cole notes that, if anything, the industry has become more entrenched in the last year.

The Ethernet community is fixed on four-wavelength module designs. To be able to support such designs as module speeds increase, higher-order modulation schemes and more complex digital signal processors (DSPs) are needed.

“The problem right now is that all the money is going into signal processing: the analogue-to-digital converters and more powerful DSPs,” says Cole.

His belief is that parallelism is the right way to go, both in terms of more wavelengths and more fibers (physical channels).

“This won’t come from Ethernet but emerging applications like machine learning that are not tied to backward compatibility issues,” says Cole. “It is emerging applications that will drive innovation here.”

Cole adds that there is hyperscaler interest in optical channel parallelism. “There is absolutely a groundswell interest here,” says Cole. “This is not their main business right now, but they are looking at their long-term strategy.”

The likelihood is that laser companies will step in to develop the laser sources and then other companies will develop the communications gear.

“It will be driven by requirements of emerging applications,” says Cole. “This is where you will see the first deployments.”