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).
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.
“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.”
Oclaro showcases its pluggable CFP2-DCO at ECOC
Multi-sourcing CFP2-DCO modules, coherent digital signal processor (DSP) partnerships, new laser opportunities and the latest on Lumentum’s acquisition of Oclaro. A conversation with Oclaro’s chief strategy officer, Yves LeMaitre.
Oclaro demonstrated its CFP2 Digital Coherent Optics (CFP2-DCO) pluggable module working with Acacia Communications’ own CFP2-DCO at the recent European Conference on Optical Communication (ECOC), held in Rome.
Yves LeMaitreOclaro announced earlier this year that it would use Acacia’s Meru coherent DSP for a CFP2-DCO product.
The company also announced at ECOC the availability of a portfolio of single-mode lasers that operate over an extended temperature range.
“We see two new laser opportunities for us,” says LeMaitre. “The upgrade of the access networks and, concurrently, the deployment of 5G.”
Coherent pluggables
The CFP2-DCO is a dense wavelength-division multiplexing (DWDM) module that supports 100-gigabit and 200-gigabit data rates. With the CFP2-DCO design, the coherent DSP is integrated within the module, unlike the CFP2 Analog Coherent Optics (CFP2-ACO) where the DSP chip resides on the line card.
“A concern of the market is that there has been essentially only one source of CFP2-DCO for the last few years and it was Acacia,” says LeMaitre. “Now there will be a broader supply for people who want coherent pluggables.”
Oclaro has been selling a CFP2-ACO but the company could not address those systems vendors that do not have their own DSP yet want to use coherent pluggables. “Now we can leverage our optics and combine it with Acacia’s DSP and bring another source of the CFP2-DCO,” says LeMaitre.
Acacia’s Meru is a low-power DSP that supports 200 gigabit-per-second (Gbps) wavelengths using either 8-ary quadrature amplitude modulation (8-QAM) or 16-QAM. Using 8-QAM enhances the optical reach at 200 gigabit. Oclaro’s CFP2-DCO uses its indium phosphide-based optics whereas Acacia’s module uses the company’s silicon photonics technology.
Oclaro sees the deal with Acacia as a first step, given the coming generation of 400-gigabit coherent modules including the 400ZR.
Production of Oclaro’s CFP2-DCO will commence in early 2019.
WaveLogic Ai DSP
Oclaro, along with module makers Lumentum and NeoPhotonics, signed an agreement in 2017 with Ciena to use the equipment maker’s 400-gigabit WaveLogic Ai coherent DSP. Oclaro is now shipping the 400-gigabit optical module that uses the Ciena DSP.
“The market for these types of large 400-gigabit form-factor modules in fairly limited as it is already addressed by many of the network equipment manufacturers,” says LeMaitre. “It [the module] is targeted at a few customers and a few opportunities.”
When the agreement with the three module makers was announced, there was talk of Ciena developing coherent DSPs for emerging applications such as 400-gigabit pluggables. However, Ciena has since decided to bring its own coherent modules to the marketplace and Oclaro does not yet know if it will get access to Ciena’s future coherent DSPs.
“We remain very interested in working with Ciena if they give us access to a DSP that could fit into pluggable coherent solutions but we have no agreement on that,” says LeMaitre.
There is an expectation in terms of dollar-per-bit that 400-gigabit modules are not yet meeting
Access and 5G wireless
At ECOC, Oclaro announced the availability of extended-temperature 10-gigabit and 25-gigabit lasers for access network and 5G deployments. The company also detailed its electro-absorption modulated laser (EML) supporting single-wavelength 100-gigabit transmissions for the data centre.
LeMaitre says the latest laser opportunities stem from the expansion and speed upgrades of the access infrastructure as well as upcoming 5G deployments. “This is resulting in a new lease of life for single-mode lasers because of the faster speeds and increased distances,” he says. These distances range from 10-40km and even 80km.
The environmental conditions required for these applications means the lasers must operate over industrial temperature (I-Temp) ranges, from -40 to 85oC and even higher.
Oclaro’s 25-gigabit directly-modulated laser (DML) for 5G fronthaul and mid-haul applications operates at up to 95oC. This means the laser does not need a thermo-electric cooler, simplifying the module design and reducing its power consumption. The laser has also been operated at 50 gigabit-per-second (Gbps) using 4-level pulse-amplitude modulation (PAM-4).
LeMaitre says the architectures for 5G will vary depending on the density of deployments and the primary application such as broadband or the Internet of Things.
Oclaro also announced an extended temperature range DML for 10-gigabit passive optical networks such as XGS-PON and 10GE-PON. The laser, which operates at the 1270nm wavelength, is used at the optical network unit (ONU) at the premises. Oclaro is also developing new 10-gigabit EMLs for the downstream link, from the PON optical line terminal (OLT) to the ONU. Transmission distances for such PONs can be 20km.
The company recently expanded laser production at its Japan and UK facilities, while the 10- and 25-gigabit lasers are now being mass-produced.
400 Gigabit Ethernet
Oclaro was one of five companies that took part in a 100-gigabit single-wavelength interoperability demonstration organised by the Ethernet Alliance at the show. The other four were Applied Optoelectronics, InnoLight Technology, Source Photonics, and Sumitomo Electric Industries.
The company showed its EML operating at 50 gigabaud with PAM-4 in the 100-Gigabit QSFP28 module. The 50Gbaud EML can operate uncooled such that no thermo-electric cooler is needed.
Oclaro says it will soon start sampling a 400-gigabit QSFP-DD FR4 module. The 2km four-channel FR4 developed by the 100-Gigabit Single Lambda MSA will use four 50Gbaud lasers. Volume production of the FR4 module is expected from the second quarter of 2019.
LeMaitre says 400-gigabit modules for the data centre face two key challenges.
One is meeting the power consumption of the new form factor modules such as the QSFP-DD. The optics for a four-wavelength design consumes 3-4W while the accompanying PAM-4 digital signal processor can consume 7-8W. “A transceiver burning 10-12W might be an issue for large-scale deployments,” says LeMaitre. “There is a power issue here that needs to be fixed.”
The second challenge for 400-gigabit client-side is cost. The price of 100-gigabit modules has now come down considerably. “There is an expectation in terms of dollar-per-bit that 400-gigabit modules are not yet meeting,” says LeMaitre. If the DSPs have yet to meet the power needs while the cost of the new modules is not in line with the dollar-per-bit performance of 100-gigabit modules, then 400-gigabit modules will be delayed, he says.
Acquisition
Lumentum’s acquisition of Oclaro, announced in March, continues to progress.
LeMaitre says two of the main three hurdles have now been overcome: anti-trust clearance in the U.S. and gaining shareholder approval. What remains is achieving Chinese clearance via the State Authority for Market Regulation.
“Until the merger deal is closed, we have to continue to operate as two separate companies,” says LeMaitre. But that doesn't prevent the two firms planning for the day when the deal is completed. Issues being worked through include the new organisation, the geographic locations of the companies’ groups, and how the two firms will work together to build a combined financial model.
The deal is expected to close before the year-end.
Aurrion mixes datacom and telecom lasers on a wafer
"There is an inevitability of the co-mingling of electronics and optics and we are just at the beginning"
Eric Hall, Aurrion
Aurrion's long-term vision for its heterogeneous integration approach to silicon photonics is to tackle all stages of a communication link: the high-bandwidth transmitter, switch and receiver. Heterogeneous integration refers to the introduction of III-V material - used for lasers, modulators and receivers - onto the silicon wafer where it is processed alongside the silicon using masks and lithography.
In a post-deadline paper given at OFC/NFOEC 2013, the fabless start-up detailed the making of various transmitters on a silicon wafer. These include tunable lasers for telecom that cover the C- and L-bands, and uncooled laser arrays for datacom.
The lasers are narrow-linewidth tunable devices for long-haul coherent applications. According to Aurrion, achieving a narrow-linewidth laser typically requires an external cavity whose size makes it difficult to produce a compact design when integrated with the modulator.
Having a tunable laser integrated with the modulator on the same silicon photonics platform will enable compact 100 Gigabit coherent pluggable modules. "The 100 Gig equivalent of the tunable XFP or SFP+," says Eric Hall, vice president of business development at Aurrion.
Hall admits that traditional indium-phosphide laser manufacturers will likely integrate tunable lasers with the modulator to produce compact narrow-linewidth designs. "There will be other approaches but it is exciting that we can now make this laser and modulator on this platform," says Hall. "And it becomes very exciting when you make these on the same wafer as high-volume datacom components."
Aurrion's vision of a coherent transmitter and a 16-laser array made on the same wafer. Source: Aurrion
The wafer's datacom devices include a 4-channel laser array for 100GBASE-LR4 10km reach applications and a 400 Gigabit transmitter design comprising 2x8 wavelength division multiplexing (WDM) arrays for a 16x25Gbps design, each laser spaced 200GHz apart. These could be for 10km or 40km applications depending on the modulator used. "These arrays are for uncooled applications," says Hall. "The idea is these don't have to be coarse WDM but tighter-spaced WDM that hold their wavelength across 20-80oC."
Coarse WDM-based laser arrays do not require a thermo-electric cooler (TEC) but the larger spacing of the wavelengths makes it harder to design beyond 100 Gigabit, says Hall: "Being able to pack in a bunch of wavelengths yet not need a TEC opens up a lot of applications."
Such lasers coupled with different modulators could also benefit 100 Gigabit shorter-reach interfaces currently being discussed in the IEEE, including the possibility of multi-level modulation schemes, says the company.
Aurrion says it is seeing the trend of photonics moving closer to the electronics due to emerging applications.
"Electronics never really noticed photonics because it was so far away and suddenly photonics has encroached into its personal space," says Hall. "There is an inevitability of the co-mingling of electronics and optics and we are just at the beginning."