Artificial intelligence (AI) and machine learning have become an integral part of the businesses of the webscale players.
The mega data centre players apply machine learning to the treasure trove of data collected from users to improve services and target advertising.
They can also use their data centres to offer cloud-based AI services.
Training neural networks with data sets is so intensive that it is driving new processor and networking requirements.
It is also impacting optics. Optical interfaces will need to become faster to cope with the amount of data, and that means interfaces with more parallel channels.
Anticipating these trends, a group of companies has formed the Continuous-Wave Wavelength Division Multiplexing (CW-WDM) multi-source agreement (MSA).
The CW-WDM MSA will specify lasers sources and the wavelength grids they use. The lasers will operate in the O-band (1260nm-1360nm) used for datacom optics.
The MSA is defining eight, 16 and 32 channels and will build on work done by the ITU-T and the IEEE.
This is good news for the laser manufacturers, says Chris Cole, Chair of CW-WDM MSA (pictured), given they have already shipped millions of lasers for datacom.
“In general, lasers are typically the hardest thing,” he says.
Wavelength count
The majority of datacom pluggable modules deployed today use either one or four optical channels. “When I started in optics 20 years ago it was all about single wavelengths,” says Cole.
Four channels were first used successfully for 40-gigabit interfaces. “That is when we introduced coarse wavelength-division multiplexing (CWDM),” says Cole.
Four wavelengths are the standard approach for 100, 200 and 400-gigabit optical modules. Spreading data across four channels simplifies the design of the electrical and optical interfaces.
“But we are ready to move on because the ability to increase parallel channels - be it parallel fibres or wavelengths - is much greater than the ability to push speed,” says Cole. “If all we do is rely on a four-wavelength paradigm and we keep pushing speed, we will run into a brick wall.”
Integration
Adopting more parallel channels will have two consequences on the optics, says Cole.
One is that photonic integration will become the only practical way to build multi-channel designs. Eight-channel designs are possible using discrete components but it won’t be cost-competitive for designs of 16 or more channels.
“It has to be photonic integration because as you get to eight and later, 16 and 32 wavelengths, it is not supportable in a small size with conventional approaches,” says Cole.
The MSA favours silicon photonics integration but indium phosphide or polymer integration platforms could be used.
The MSA will also cause wavelengths to be packed far more closely than the 20nm used for CWDM. Techniques now exist that enable tighter wavelength spacings without needing dedicated cooling.
One approach is separating the laser from the silicon chip - a switch chip or processor - that generates a lot of heat. Here, light from the source is fed to the optics over a fibre such that temperature control is more straightforward because the laser and chip are separated.
Cole also highlights the athermal silicon photonics of Juniper Networks that controls wavelength drift on the grid without requiring a thermo-electric cooler. Juniper gained the technology with its Aurrion acquisition in 2016.
Specification work
“Using the O-band has a lot of advantages,” says Cole. “That is where all the datacom optics are.”
The optical loss in the O-band may be double that of the C-band but this is not an issue for datacom’s short spans.
The MSA is to define a technology roadmap rather than a specific product, says Cole. First-generation products will use eight wavelengths followed by 16- and then 32-wavelength designs. Sixty-four and even 128 channel counts will be specified once the technology is established.
“Initially we did [specify 64 and 128 channels] but we took it out,” says Cole. “We’ll know a lot more if we are successful over three generations; we’ll figure out what we need to do when we get to that point.”
The MSA is proposing two bands, one 18nm wide (1291nm-1309nm) and the other 36nm wide (1282nm-1318nm). Eight, 16 and 32 wavelengths are assigned across both bands.
“It’s smack in the middle of the CWDM4 grid which is the largest shipping laser grid ever, and it is smack on top of the LWDM4 grid [used by -LR4 modules] which is the next highest grid to ship in volume,” says Cole.
The MSA will also specify continuous-wave laser parameters such as the output power, spectral width, variation in power between the wavelengths, and allowable wavelength shift.
Members
Cole started work on the CW-WDM MSA in collaboration with Ayar Labs while he was still at II-IV. Now at Luminous Computing, Cole, along with MSA editor Matt Sysak of Ayar Labs, and associate editor Dave Lewis of Lumentum, are preparing the first MSA draft and have solicited comments from members as to what to include in the specifications.
The MSA has 11 promoter members: Arista, Ayar Labs, CST Global, imec, Intel, Lumentum, Luminous Computing, MACOM, Quintessent, Sumitomo Electric, and II-VI.
The MSA has created a new observer member status to get input from companies that otherwise would be put off joining an MSA due to the associated legal requirements.
“So we have an observer category that if someone is serious and they want to see a subset of the material the MSA is working on and provide feedback, we welcome that,” says Cole.
The observer members are AMF, Axalume, Broadcom, Coherent Solutions, Furukawa Electric, GlobalFoundries, Keysight Technologies, NeoPhotonics, NVIDIA, Samtec, Scintil Photonics, and Tektronix.
“This MSA is meant to be inclusive, and it is meant to foster innovation and foster as broad an industry contribution as possible,” concludes Cole.
Further information
The CW-WDM MSA has several documents and technical papers on its website. The first document is the CW-WDM MSA grid proposal while the rest are technical papers addressing developments and applications driving the need for high-channel-count optical interfaces.