CW-WDM MSA charts a parallel path for optics
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.
Chris Cole
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.
The making of integrated optics
A US initiative is bringing together leading companies with top academics and universities to create a manufacturing infrastructure for the widespread adoption of integrated photonics.
The US sees integrated photonics as a strategic technology and has set up the American Institute for Manufacturing Integrated Photonics - AIM Photonics - to advance the technology and make it available to a wider community of companies. AIM Photonics, with $610 million of public and private funding, is a five-year initiative ending in 2020. AIM’s long-term goal is to be self-sustaining.
Doug Coolbaugh
“Right now the infrastructure is focussed on electronics and CMOS but photonics is going to be the future,” says Doug Coolbaugh, chief operations officer at AIM Photonics. “There is no other way to do it [very high bandwidth] except using light for ultra fast communications.”
Technologies start at universities and in the labs of companies with large R&D budgets. IBM and Intel, for example, have been developing silicon photonics for over a decade and the technology is ready for deployment. However, the intellectual property developed remains with such companies.
“AIM is not only creating the manufacturing infrastructure for integrated photonics but also ideas and intellectual property that can be used by companies for new products,” says Coolbaugh.
All the elements are being addressed so that small to medium businesses and entrepreneurial ventures can use integrated photonics for their products; companies too small to develop the technology themselves. “That will accelerate the silicon photonics ecosystem and allow new products to come out much faster than it would normally take,” says Coolbaugh.
Manufacturing
Silicon photonics luminary, Lionel Kimerling, professor of materials science and engineering at MIT, and an active member of AIM Photonics, views its focus on manufacturing as an important development.
The discipline of manufacturing is something that the chip industry has mastered through designing process integration, selecting materials and all the qualification standards used to meet system requirements, he says, but is less developed in the photonics industry.
AIM is making available a chip fabrication plant to interested companies. SUNY Polytechnic Institute has been working with MIT for the last six years to develop a 300mm-wafer silicon photonics line at its Albany site. The fab offers a multi-project wafer service whereby several designs can be made on a single wafer, allowing costs to be shared among companies.
AIM is not only creating the manufacturing infrastructure for integrated photonics but also ideas and intellectual property that can be used by companies for new products
A design kit is also being developed featuring key building blocks needed to make an integrated photonics circuit. AIM is working with leading semiconductor industry design automation companies Cadence, Synopsys and Mentor Graphics to provide the software tool environment for designers to develop circuits. “This design environment is compatible with the silicon photonics process here in our fab,” says Coolbaugh.
A packaging and prototyping facility located in Rochester, New York is also being set up. “Photonics packaging is relatively new and certain aspects have not been developed that much,” says Coolbaugh.
Another issue is developing skilled engineers and technicians able to design and manufacture integrated photonics circuits. Whereas electronic chip designers typically have a first degree, photonics engineers tend to have a doctorate because of the deep understanding needed. “This is one of the things we find we are lacking significantly,” says Coolbaugh. “There are just not enough skilled people in the industry to fulfil these needs.”
Professor Kimerling says he is spending much of his time putting together educational material to help attract individuals to pursue a career in silicon photonics. Much of the technology is in place, he says, what is required is to make it accessible to people. “I don’t have 40 more years in the industry, but I could influence the next 40 years by creating these instructional materials and career paths, and getting roadmap consensus that can drive the industry,” says Kimerling.
AIM is also working with universities and companies to develop technology and intellectual property alongside the manufacturing centres. Four research areas have been chosen, covering datacom, analogue RF for telecom involving Infinera, sensors and phased arrays. These are areas where AIM sees products emerging in volume in the next five years.
Keren Bergman, whose work focusses on the intersection of photonics and computing systems, mentions how AIM Photonics has already benefited her research group through much closer interactions with companies in the area of datacom. “It has had a big impact on our work,” says Bergman, professor and director at the Lightwave Research Laboratory at Columbia University.
Each year AIM will review and add new research topics. “There are new ideas, new materials and new manufacturing processes that will be developed,” says Coolbaugh. He cites the use of silicon photonics to drive robots as an emerging application area.
Status
AIM expects the entire manufacturing infrastructure to be in place in the next couple of years.
“Right now it is only the photonics design part but we will also be putting in interposers for packaged designs," says Coolbaugh. Interposers are a key technology that allows the co-packaging of chip dice, an approach known as system-in-package or 2.5D packaging.
AIM expects to offer multi-project wafers with interposers and system-in-package by 2017, with the ability to add CMOS dice in 2018. AIM is also developing a test, assembly and packaging facility which it expects to be available by 2018. “Testing is a really critical component of this entire infrastructure,” says Coolbaugh.
The goal is to develop new ways of fast-testing photonics on wafers, while there will be the high-speed testing of circuits at Rochester. “What we design has got to work in the fab, the fab has got to test well and then what we package has to be consistent with what we deliver to the packaging house,” says Coolbaugh. “The entire flow has to integrate exactly.”
A start-up or small company wanting to make a product can already use the design kit - which continues to evolve - and benefit from AIM’s multi-project wafer service. Then there will be the Rochester packaging and prototyping site. Low volumes can be made at the Albany fab while AIM will pass higher-volume manufacturing requests to leading chip fabrication players such as GlobalFoundries.
Companies can take a concept, develop their own product and have their own business. “We provide the entire chain for the infrastructure,“ says Coolbaugh. ”Right now, this is only available to large companies.”
If all goes to plan, what impact will AIM have on integrated optics and silicon photonics in particular? “It will be a worldwide impact,” says Coolbaugh. “Just because we want to create the infrastructure in the US doesn’t mean we are limiting our customers to the US.”
Further information
For AIM Photonics presentations, click here
The text is based on an article that first appeared in Optical Connections magazine
Teraxion embraces silicon photonics for its products
Teraxion has become a silicon photonics player with the launch of its compact 40 and 100 Gigabit coherent receivers.
The Canadian optical component company has long been known for its fibre Bragg gratings and tunable dispersion compensation products. But for the last three years it has been developing expertise in silicon photonics and at the recent European Conference on Optical Communications (ECOC) exhibition it announced its first products based on the technology.
"You don't have this [fabless] model for indium phosphide or silica, while an ecosystem is developing around silicon photonics"
Martin Guy, Teraxion
"We are playing mainly in the telecom business, which accounts for 80% of our revenues," says Martin Guy, vice president, product management & technology at Teraxion. "It is clear that our customers are going to more integration and smaller form-factors so we need to follow our customers' requirements."
Teraxion assessed several technologies but chose silicon photonics and the fabless model it supports. "We are using all our optical expertise that we can apply to this material but use a process already developed for the CMOS industry, with the [silicon] wafer made externally," says Guy. "You don't have this [fabless] model for indium phosphide or silica, while an ecosystem is developing around silicon photonics."
The company uses hybrid integration for its coherent receiver products, with silicon implementing the passive optical functions to which the active components are coupled. Teraxion is using externally-supplied photo-detectors which are flip-chipped onto the silicon for its coherent receiver.
"We need to use the best material for the function for this high-end product," says Guy. "Our initial goal is not to have everything integrated in silicon."
Coherent receiver
A coherent receiver comprises two inputs - the received optical signal and the local oscillator - and four balanced receiver outputs. Also included in the design are two polarisation beam splitters and two 90-degree hybrid mixers.
Several companies have launched coherent receiver products. These include CyOpyics, Enablence, NEL, NeoPhotonics, Oclaro and u2t Photonics. Silicon photonics player Kotura has also developed the optical functions for a coherent receiver but has not launched a product.
One benefit of using silicon photonics, says Teraxion, is the compact optical designs it enables.
The Optical Internetworking Forum (OIF) has specified a form factor for the 100 Gigabit-per-second (Gbps) coherent receiver. Teraxion has developed a silicon photonics-based product that matches the OIF's form factor sized 40mmx32mm. This is for technology evaluation purposes rather than a commercial product. "If customers want to evaluate our technology, they need to have a compatible footprint with their design," explains Guy. This is available in prototype form and Teraxion has customers ready to evaluate the product.
Teraxion will come to market with a second 100 Gigabit coherent receiver design that is a third of the size of the OIF's form factor, measuring 23mmx18mm (0.32x the area of the OIF specification). The compact coherent receivers for 40 and 100Gbps will be available in sample form in the first quarter of 2013.
Teraxion's OIF-specification 100 Gig coherent receiver (left) for test purposes and its compact coherent receiver product. Source: Teraxion
"We match the OIF's performance with this design but there are also other key requirements from customers that are not necessarily in the OIF specification," says Guy.
The compact 100Gbps design is of interest to optical module and system vendors but there is no one view in terms of requirements or the desired line-side form-factor that follows the 5x7-inch MSA. Indeed there are some that are interested in developing a 100 Gigabit CFP module for metro applications, says Guy.
Roadmap
Teraxion's roadmap includes further integration of the coherent receiver's design. "We are using hybrid integration but eventually we will look at having the photo-detectors integrated within the material,” says Guy.
The small size of the coherent design means there is scope for additional functionality to be included. Teraxion says that customers are interested in integrating variable optical attenuators (VOAs). The local oscillator is another optical function that can be integrated within the coherent receiver.
In 2005 Teraxion acquired Dicos Technologies, a narrow line-width laser specialist. Teraxion's tunable narrow line-width laser product - a few kiloHertz wide - is available in the lab. "The purpose of this product is not to be deployed on the line card - right now," says Guy. "We believe this type of performance will be required for next-generation 100 Gig, 400 Gig, 1 Terabit coherent communication systems where you will need a very 'clean' local oscillator."
Teraxion is also working on developing a silicon-photonics-based modulator. The company has been exploring integrating Bragg gratings within silicon waveguides for which it has applied for patents. This is several years out, says Guy, but has the potential to enable high-speed modulators suited for short-reach datacom applications.