Nokia buys Elenion for its expertise and partnerships
Nokia will become the latest systems vendor to bolster its silicon photonics expertise with the acquisition of Elenion Technologies.
The deal for Elenion, a privately-held company, is expected to be completed this quarter, subject to regulatory approval. No fee has been disclosed.
“If you look at the vertically-integrated [systems] vendors, they captured the lion’s share of the optical coherent marketplace,” says Kyle Hollasch, director of optical networking product marketing at Nokia. “But the coherent marketplace is shifting to pluggables and it is shifting to more integration; we can’t afford to be left behind.”
Elenion Technologies
Elenion started in mid-2014, with a focus of using silicon as a platform for photonics. “We consider ourselves more of a semiconductor company than an optics company,” says Larry Schwerin, CEO of Elenion.
Elenion makes photonic engines and chipsets and is not an optical module company. “We then use the embedded ecosystem to offer up solutions,” says Schwerin. “That is how we approach the marketplace.”
The company has developed a process design kit (PDK) for photonics and has built a library of circuits that it uses for its designs and custom solutions for customers.
A PDK is a semiconductor industry concept that allows circuit designers to develop complex integrated circuits without worrying about the underlying transistor physics. Adhering to the PDK ensures the circuit design is manufacturable at a chip fabrication plant (fab).
But developing a PDK for optics is tricky. How the PDK is designed and developed must be carefully thought through, as has the manufacturing process, says Elenion.
“We got started on a process and developed a library,” says Larry Schwerin, CEO of Elenion. “And we modelled ourselves on the hyperscale innovation cycle, priding ourselves that we could get down to less than three years for new products to come out.”
The “embedded ecosystem” Elenion refers to involves close relationships with companies such as Jabil to benefit from semiconductor assembly test and packaging techniques. Other partnerships include Molex and webscale player, Alibaba.
Elenion initially focussed on coherent optics, providing its CSTAR coherent device that supports 100- and 200-gigabit transmissions to Jabil for a CFP2-DCO pluggable module. Other customers also use the design, mostly for CFP2-DCO modules.
The company has now developed a third-generation coherent design, dubbed CSTAR ZR, for 400ZR optics. The optical engine can operate up to 600 gigabits-per-second (Gbps), says Elenion.
Elenion’s work with the cloud arm of Alibaba covers 400-gigabit DR4 client-side optics as well as an 800-gigabit design.
Alibaba Cloud has said the joint technology development with Elenion and Hisense Broadband covers all the production stages: the design, packaging and testing of the silicon photonics chip followed by the design, packaging, assembly and testing of the resulting optical module.
Bringing optics in-house
With the acquisition of Elenion, Nokia becomes the latest systems vendors to buy a silicon photonics specialist.
Cisco Systems acquired Lightwire in 2012 that enabled it to launch the CPAK, a 100-gigabit optical module, a year ahead of its rivals. Cisco went on another silicon photonics shopping spree more recently with the acquisition of Luxtera in 2019, and it is the process of acquiring leading merchant coherent player, Acacia Communications.
In 2013 Huawei bought the Belgium silicon photonics start-up, Caliopa, while Mellanox Technologies acquired silicon photonics firm, Kotura, although subsequently, it disbanded its silicon photonics arm.
Ciena bought the silicon-photonics arm of Teraxion in 2016 and, in the same year, Juniper bought silicon photonics start-up, Aurrion Technologies.
Markets
Nokia highlights several markets – 5G, cloud and data centres – where optics is undergoing rapid change and where the system vendor’s designs will benefit from Elenion’s expertise.
“5G is a pretty obvious one; a significant portion of our optical business over the last two years has been mobile front-haul,” says Nokia’s Hollasch. “And that is only going to become more significant with 5G.”
Front-haul is optics-dependent and requires new pluggable form factors supporting lower data rates such as 25Gbps and 100Gbps. “This is the new frontier for coherent,” says Hollasch.
Nokia is not looking to be an optical module provider, at least for now. “That one we are treading cautiously,” says Hollasch. “We, ourselves, are quite a massive customer [of optics] which gives us some built-in scale straight away but our go-to-market [strategy] is still to be determined.”
Not being a module provider, adds Schwerin, means that Nokia doesn’t have to come out with modules to capitalise on what Elenion has been doing.
Nokia says both silicon photonics and indium phosphide will play a role for its coherent optical designs. Nokia also has its own coherent digital signal processors (DSPs).
“There is an increasingly widening application space for silicon photonics,” says Hollasch. “Initially, silicon photonics was looked at for the data centre and then strictly for metro [networks]; I don’t think that is the case anymore.”
Why sell?
Schwerin says the company was pragmatic when it came to being sold. Elenion wasn’t looking to be acquired and the idea of a deal came from Nokia. But once the dialogue started, the deal took shape.
“The industry is in a tumultuous state and from a standpoint of scenario planning, there are multiple dynamics afoot,” says Schwerin.
As the company has grown and started working with larger players including webscales, their requirements have become more demanding.
“As you get more into bigs, they require big,” says Schwerin. “They want supply assurance, and network indemnification clauses come into play.” The need to innovate is also constant and that means continual investment.
“When you weigh it all up, this deal makes sense,” he says.
Schwerin laughs when asked what he plans to do next: “I know what my wife wants me to do.
“I will be going with this organisation for a short while at least,” he says. “You have to make sure things go well in the absorption process involving big companies and little companies.”
Acacia bets on silicon as coherent enters its next phase
Gazettabyte interviewed Acacia Communications’ president and CEO, Murugesan ‘Raj’ Shanmugaraj, as the coherent technology company celebrates its 10th anniversary.
Raj Shanmugaraj
Acacia Communications has come a long way since Raj Shanmugaraj (pictured) first joined the company as CEO in early 2010. “It was just a few conference rooms and we didn't have enough chairs,” he says.
The company has since become a major optical coherent player with revenues of $340 million in 2018; revenues that would have been higher but for the four-month trade ban imposed by the US on Chinese equipment maker ZTE, an Acacia customer.
And as the market for coherent technology continues to grow, Acacia and other players are preparing for new opportunities.
“We are still in the early stages of the disruption," says Shanmugaraj. “You will see higher performance [coherent systems] in some parts of the network but there is going to be growth as coherent moves closer to the network edge.”
Here, lower power, flexibility and more integrated coherent solutions will be needed as the technology moves inside the data centre and closer to the network edge with the advent of 5G, higher-speed access and the Internet of Things (IoT).
Competitive landscape
Shanmugaraj prefers to focus on Acacia’s own strengths and products when asked about the growing competition in the coherent marketplace. However, recent developments present challenges for the company.
Systems vendors such as Huawei and Ciena are becoming more vertically integrated, developing not only their own coherent digital signal processor (DSP) ASICs but also optics. Ciena has also made its WaveLogic Ai DSP available to optical module makers Lumentum and NeoPhotonics and will sell its own optical modules using its latest WaveLogic 5 coherent silicon.
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“You will see higher performance [coherent systems] in some parts of the network but there is going to be growth as coherent moves closer to the network edge ”
New coherent digital signal processor (DSP) players are also expected to enter the marketplace alongside established competitors, NEL and Inphi. The entrance of new players developing coherent DSPs is motivated by the unit volumes promised by 400ZR, the emerging 80km data centre interconnect interface standard.
“We are proponents of the fact that the merchant market will continue to grow, driven by interoperability and standardisation,” says Shanmugaraj. Such growth will lead to multiple markets where coherent technology will play. “There are going to be a few winners, not just one or two,” he says.
Acacia’s revenues were hit in 2018 following the US Department of Commerce’s enforced trade ban imposed on ZTE. However, the company recorded a strong fourth quarter posting revenues of $107 million, up almost a quarter on the revenues a year earlier. This followed strong ZTE orders after the ban was revoked.
Shanmugaraj says diversification has always been a priority for the company, independent of the trade issues between the US and China. The company has also been working to diversify its Chinese customer base. “So we are well positioned as these trade issues get resolved,” he says.
Origins
Acacia was established in mid-2009 by a core team from Mintera, a sub-system supplier that provided 40-gigabit DPSK line cards to network equipment suppliers. But Mintera folded and was eventually sold to Oclaro in July 2010.
Before joining Acacia, Shanmugaraj was at systems vendor Alcatel-Lucent where he learned two lessons.
One is that the long-term success of a company is based on technology leadership. “You want to be driven by technology or you fall behind your competitors,” he says. The second lesson was that the largest systems companies build products internally before an ecosystem becomes established, after which they buy from merchant suppliers.
This matched the vision of Acacia’s founders that sought to exploit their optical expertise gained at Mintera to become a leading merchant supplier of coherent transmission technology.
Stealth years
Acacia remained in secrecy for nearly half its existence, only revealing its technology and products in 2014 with the launch of the AC-100 CFP coherent pluggable module. The AC-100 is aimed at metro networks delivering a transmission reach of 80km to 1,200km. However, Acacia had already been selling 5x7-inch modules for 100-gigabit long-haul and ultra-long-haul applications as well as a 40-gigabit ultra-long-haul module.
“In the early years, there were just a few companies working on coherent,” says Shanmugaraj. “We had to be careful in terms of what products we were developing and what customers we were going after.”
Shanmugaraj says Acacia secured multi-million dollar commitments from customers even before it had a product. “It was the expertise of the founding team as well as the product concepts they were proposing that got them the commitments,” he says.
The backing enabled the company to manage with only $53 million of venture funding prior to its successful initial public offering in 2016.
“This was a pretty significant feat,” says Shanmugaraj. “Hardware start-ups, whether semiconductor or systems companies, use significantly more cash; these are expensive technologies to get off the ground.”
Shanmugaraj describes the early years as intense, with staff working between 60 and 70 hours a week.The then start-up had to be prudent with funding, not growing too quickly yet having sufficient resources to meet orders from systems customers that had their own orders to fulfil.
Coherent technologies
Acacia’s founders chose silicon for its coherent solutions, to replace ‘exotic materials’ such as indium phosphide and lithium niobate used in traditional optical transmission systems.
The company backed silicon photonics for the coherent optics, an industry trailblazing decision. To this aim, Acacia recruited Chris Doerr, the renowned optical integration specialist and Bell Labs Fellow.
The company also decided to develop its own coherent DSPs. By developing the optics and the DSP, Acacia could use a co-design approach when designing the hardware, trading off the performance of the optics and the signal processing to achieve an optimal design.
Shanmugaraj explains that the company chose a silicon-based approach to exploit the huge investment made by the semiconductor industry in chips and their packaging. Basing the components on silicon would not only simplify high-speed networks, he says, but it would also lower their power consumption and enable products to be made more quickly and cheaply.
“The beauty of silicon photonics is that it can be placed right next to a heat source, in this case, the high-power coherent DSP ASIC that generates a lot of heat,” says Shanmugaraj. “This allows for smaller form-factor designs.” In contrast, indium phosphide-based optics need to be temperature controlled when placed next to a hot chip, he says.
“Five or six years ago, people were challenging whether silicon photonics was even going to work at 100 and 200 gigabits,” says Shanmugaraj. Acacia has now used silicon photonics in all its products, including its latest high-end 1.2 terabits AC1200 coherent module.
Shanmugaraj sees Acacia's portfolio of coherent products as the company's biggest achievement: "You see start-ups that come out with one product that is a bestseller but we have continued to innovate and today we have a broad portfolio."
AC1200
The AC1200 module supports two optical wavelengths, each capable of supporting 100 to 600-gigabit transmissions in increments of 50 gigabits.
The AC1200 can be used for data centre interconnect links through to long distance submarine links. Acacia recently demonstrated the AC1200 transmitting a 400-gigabit signal over a 6,600km submarine cable.
“We are seeing strong interest in our AC1200 from network operators and expect our equipment customers to begin deployments this quarter,” says Shanmugaraj.
There are several reasons why network operators are choosing to deploy the AC1200, he says: “High capacity is important in data centre interconnect edge applications where we expect hyperscale operators may use the AC1200 in its full 1.2-terabit mode, but these applications are also sensitive to cost, power and density.”
The AC1200 also provides higher capacity in a smaller footprint than the 5x7-inch form factors currently available, he says, while for longer-reach applications, the AC1200 offers a combination of performance and flexibility that is setting the pace for the competition.
The data centre interconnect market represents a good opportunity for coherent interconnect suppliers because the operators drive and deploy technology at pace, says Shanmugaraj. Hyperscalers are continually looking to add more capacity in the same size and power constraints that exist today. Accordingly, this has been a priority development area for Acacia.
To increase capacity, companies have boosted the symbol rate from 32 gigabaud to 64 gigabaud while systems vendors Ciena and Infinera have recently detailed upcoming systems that support 800-gigabit wavelengths that use a symbol rate approaching 100 gigabaud.
The AC1200, which is due in systems in the coming quarter, demonstrates silicon photonics based modulation operating at up to 70 gigabaud while first indium-phosphide 800-gigabit per wavelength systems are due by the year-end.
“We don’t really see silicon photonics lagging behind indium phosphide,” says Shanmugaraj. “We think there is a path to even higher baud rates with silicon photonics, and 128 gigabaud is the next logical step up because it would double the data rate without needing to increase the modulation order.”
Higher modulation orders are also possible but the benefits must be weighed against increased complexity, he says.
400-gigabit coherent pluggables
Shanmugaraj says that the 400ZR pluggable module standard continues the trend to reduce the size and power consumption of optical transport systems in the data centre.
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“You want to be driven by technology or you fall behind your competitors”
The current generation of data centre interconnect platforms, ranging from a 1 rack unit pizza box to a several rack-unit-sized chassis, were developed to be more compact than conventional optical transport platforms.
Now, with the advent of 400ZR that fits into a client-side QSFP-DD or OSFP module, data centre operators will be able to do away with such platforms for distances up to 80km by plugging the modules into the switch or router platforms and connecting them to open line systems.
“Costs come down because it [coherent] is getting down to the client-side form factors and that gives the hyperscalers more faceplate density,” says Shanmugaraj. “The hyperscalers also gain multi-vendor interoperability [with 400ZR] which is important as they want standardisation.”
Shanmugaraj admits that with the advent of 400ZR will bring greater competition. But he points out that the 400ZR is a complicated product to built that will challenge companies. Those players that have both the optics and a low-power DSP will have an advantage. “As long as it opens up the market wider, it is good for Acacia as it is in our control how we can win in the market,” says Shanmugaraj.
The industry expectation is that the 400ZR will start to be deployed in the second half of 2020.
There is also industry talk about 400ZR+, an interface that will be able to go beyond 80km that will require more advanced dispersion compensation and forward error correction schemes.
Shanmugaraj says it will be the same DSP ASIC that will support both the 400ZR and 400ZR+. However, a 400ZR+ interface will consume more power and so will likely require a larger module form factor than the ZR.
Meanwhile, the 400-gigabit CFP2-DCO pluggable for metro networks is built along the same lines as the 400ZR, says Shanmugaraj.
“Here you have applications like the Open ROADM MSA where network operators are trying to drive the same interoperability and not be stuck with one vendor,” he says. “This is driving the 400-gigabit evolution in the metro network for some of the largest telcos.”
There is also the open networking packet-optical opportunity, white-box platforms such as the Voyager and Cassini being developed by the Telecom Infra Project (TIP). Shanmugaraj says such white boxes rely on software solutions that are a work-in-progress and that much work is still to be done.
“The first generation showed that there is more work required to standardise the software and how that can be used by the hyperscalers,” he says. “It is an opportunity but we view it as more of a longer-term one.”
Emerging opportunities
The markets that are growing today are the metro, long haul, sub-sea and data centre interconnect, says Shanmugaraj.
The coherent applications that are emerging will result in products within the data centre as well as for 5G, access, the Internet of Things (IoT) and even autonomous vehicles.
Ultimately, what will lead to coherent being adopted within the data centre is the speed of the interfaces. “As you go to higher speeds, direct detection technology gets constrained [due to dispersion and other impairments],” says Shanmugaraj.
But for this to happen certain conditions will need to be met: the speed of interfaces on switches will need to increase, not just to 400 gigabits but 800 gigabits and greater.
“Looking to higher data rates beyond 400 gigabits, it gets more challenging for direct detect to achieve the necessary link budgets cost-effectively,” says Shanmugaraj. “It may be necessary to move from four-lane solutions to eight lanes in order to support the desired reaches. At the same time, we are working to make coherent more cost-effective for these applications.”
The other two conditions are the challenge of what form factors the coherent technology be squeezed into, andcost. Coherent optics is more expensive but its cost is driven by such factors as volumes, the level of automation that can be used to make the module, and the yield.
“There could be inflextion points where coherent becomes cost-competitive for some applications in the data centre,” says Shanmugaraj.
Companies will continue to innovate in both direct detect and coherent technologies and the market will determine the transition points. “But we do believe that coherent can be adopted inside data centres in the future,” he says.
In turn, metro and long-haul networks are already being upgraded in anticipation of 5G and the access requirements. “4G networks have a lot of 1-gigabit and 10-gigabit links but 5G has an order of magnitude higher throughput requirement,” says Shanmugaraj.
That means more capacity is needed for backhaul and that will lead to a proliferation of low-cost 100-gigabit coherent. A similar story is unfolding in access with the likes of the cable operators moving fibre closer to the network edge. This too will need low-cost 100-gigabit coherent interfaces.
IoT is a longer term opportunity and will be dependent on dense deployments of devices before the traffic will require sufficient aggregation to justify coherent.
“I don’t know if your refrigerator will have a coherent interface,” concludes Shanmugaraj. “But as you aggregated these [devices] into aggregation points, that becomes a driver for coherent at the edge.”
Data centre interconnect drives coherent
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NeoPhotonics announced at OFC a high-speed modulator and intradyne coherent receiver (ICR) that support an 800-gigabit wavelength
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It also announced limited availability of its nano integrable tunable laser assembly (nano-ITLA) and demonstrated its pico-ITLA, an even more compact silicon photonics-based laser assembly
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The company also showcased a CFP2-DCO pluggable
NeoPhotonics unveiled several coherent optical transmission technologies at the OFC conference and exhibition held in San Diego last month.
“There are two [industry] thrusts going on right now: 400ZR and data centre interconnect pizza boxes going to even higher gigabits per wavelength,” says Ferris Lipscomb, vice president of marketing at NeoPhotonics.
Ferris Lipscomb
The 400ZR is an interoperable 400-gigabit coherent interface developed by the Optical Internetworking Forum (OIF).
Optical module makers are developing 400ZR solutions that fit within the client-side QSFP-DD and OSFP pluggable form factors, first samples of which are expected by year-end.
800-gigabit lambdas
Ciena and Infinera announced in the run-up to OFC their latest coherent systems - the WaveLogic 5 and ICE6, respectively - that will support 800-gigabit wavelengths. NeoPhotonics announced a micro intradyne coherent receiver (micro-ICR) and modulator components that are capable of supporting such 800-gigabit line-rate transmissions.
NeoPhotonics says its micro-ICR and coherent driver modulator are class 50 devices that support symbol rates of 85 to 90 gigabaud required for such a state-of-the-art line rate.
The OIF classification defines categories for devices based on their analogue bandwidth performance. “With class 20, the 3dB bandwidth of the receiver and the modulator is 20GHz,” says Lipscomb. “With tricks of the trade, you can make the symbol rate much higher than the 3dB bandwidth such that class 20 supports 32 gigabaud.” Thirty-two gigabaud is used for 100-gigabit and 200-gigabit coherent transmissions.
Class 50 refers to the highest component performance category where devices have an analogue bandwidth of 50GHz. This equates to a baud rate close to 100 gigabaud, fast enough to achieve data transmission rates exceeding a terabit. “But you have to allow for the overhead the forward-error correction takes, such that the usable data rate is less than the total,” says Lipscomb (see table).
Source: Gazettabyte, NeoPhotonics
Silicon photonics-based COSA
NeoPhotonics also announced a 64-gigabaud silicon photonics-based coherent optical subassembly (COSA). The COSA combines the receiver and modulator in a single package that is small enough to fit within a QSFP-DD or OSFP pluggable for applications such as 400ZR.
Last year, the company announced a similar COSA implemented in indium phosphide. In general, it is easier to do higher speed devices in indium phosphide, says Lipscomb, but while the performance in silicon photonics is not quite as good, it can be made good enough.
“It [silicon photonics] is now stretching certainly into the Class 40 [that supports 600-gigabit wavelengths] and there are indications, in certain circumstances, that you might be able to do it in the Class 50.”
Lipscomb says NeoPhotonics views silicon photonics as one more material that complements its indium phosphide, planar lightwave circuit and gallium arsenide technologies. “Our whole approach is that we use the material platform that is best for a certain application,” says Lipscomb.
In general, coherent products for telecom applications take time to ramp in volumes. “With the advent of data centre interconnect, the volume growth is much greater than it ever has been in the past,” says Lipscomb.
NeoPhotonics’ interested in silicon photonics is due to the manufacturing benefits it brings that help to scale volumes to meet the hyperscalers’ requirements. “Whereas indium phosphide has very good performance, the infrastructure is still limited and you can’t duplicate it overnight,” says Lipscomb. “That is what silicon photonics does, it gives you scale.”
NeoPhotonics also announced the limited availability of its nano integrable tunable laser assembly (nano-ITLA). “This is a version of our external cavity ITLA that has the narrowest line width in the industry,” says Lipscomb.
The nano-ITLA can be used as the source for Class 50, 800-gigabit systems and current Class 40 600 gigabit-per-wavelength systems. It is also small enough to fit within the QDFP-DD and OSFP client-side modules for 400ZR designs. “It is a new compact laser that can be used with all those speeds,” says Lipscomb.
NeoPhotonics also showed a silicon-photonics based pico-ITLA that is even smaller than the nano-ITLA.“The [nano-ITLA’s] optical cavity is now made using silicon photonics so that makes it a silicon photonics laser,” says Lipscomb.
Instead of having to assemble piece parts using silicon photonics, it can be made as one piece. “It means you can integrate that into the same chip you put your modulator and receiver on,” says Lipscomb. “So you can now put all three in a single COSA, what is called the IC-TROSA.” The IC-TROSA refers to an integrated coherent transmit-receive optical subassembly, defined by the OIF, that fits within the QSFP-DD and OSFP.
Despite the data centre interconnect market with its larger volumes and much faster product uptakes, indium phosphide will still be used in many places that require higher optical performance. “But for bulk high-volume applications, there are lots of advantages to silicon photonics,” says Lipscomb.
400ZR and 400ZR+
A key theme at this year’s OFC was the 80km 400ZR. Also of industry interest is the 400ZR+, not an OIF specification but an interface that extends the coherent range to metro distances.
Lipscomb says that the initial market for the 400ZR+ will be smaller than the 400ZR, while the ZR+’s optical performance will depend on how much power is left after the optics is squeezed into a QSFP-DD or OSFP module.
“The next generation of DSP will be required to have a power consumption low enough to do more than ZR distances,” he says. “The further you go, the more work the DSP has to do to eliminate the fibre impairments and therefore the more power it will consume.”
Will not the ZR+ curtail the market opportunity for the 400-gigabit CFP2-DCO that is also aimed at the metro?
“It’s a matter of timing,” says Lipscomb. “The advantage of the 400-gigabit CFP2-DCO is that you can almost do it now, whereas the ZR+ won’t be in volume till the end of 2020 or early 2021.”
Meanwhile, NeoPhotonics demonstrated at the show a CFP2-DCO capable of 100-gigabit and 200-gigabit transmissions.
NeoPhotonics has not detailed the merchant DSP it is using for its CFP2-DCO except to say that it working with ‘multiple ones’. This suggests it is using the merchant coherent DSPs from NEL and Inphi.
Pilot Photonics makes a one terabit coherent comb source
Pilot Photonics has produced a four-wavelength laser chip for one-terabit coherent transmissions.
It is one of several applications the Irish start-up is pursuing using its optical comb source that produces multiple tunable outputs, the equivalent of a laser array.
The company is using its laser technology and photonic integration expertise to address Next Generation Passive Optical Network 2 (NG-PON2), coherent long-haul transmission, and non-telecom applications such as Light Detection and Ranging (LiDAR) and sensing.
Frank Smyth (right)
“We have a number of chips reaching maturity and we are transitioning from an R&D-focussed company to early commercial activity,” says Frank Smyth, CEO of Pilot Photonics.
Start-up
Pilot Photonics was founded in 2011 and developed a lab instrumentation product. But its limited market resulted in the company changing tack, adding photonic integration expertise to its optical comb source intellectual property.
The company secured two grants that furthered its photonic integration know-how. One - Big Pipes - was a European Commission Seventh Framework Programme (FP7) project addressing optical transport and data centre applications using combs. The second, an Irish government grant, helped the start-up to commercialise its comb technology.
But this was also a challenging period for the company which could only employ two full-time staff. “I wasn't even full time for a few years,” says Smyth, who worked evenings and weekends. “We went into a lean period out of necessity.”
But building a photonic integration capability gained the company a market presence and led to it raising nearly €1million in funding.
Pilot Photonics now has 11 staff and two products being evaluated by customers. One is a directly-modulated laser for NG-PON2 while the second is a fibre-sensing product. The coherent four-channel source chip will soon be its third evaluation product.
The company is also working on a further funding round of several million Euro that it hopes to close by the year-end.
Optical comb source
There are several ways to implement an optical comb source. These include solid-state and fibre-based comb sources commonly used for scientific instrumentation but they are unsuited for high-volume applications, says Smyth.
Pilot Photonics’ approach, dubbed gain switching, is suited to high-volume applications and involves the direct modulation of a laser chip. “A close competitor of our technology is mode-locked laser diodes,” he says. This is the technology used by Ranovus for its module designs.
The start-up claims its technology has distinct advantages. “Our approach gives you better optical properties such as a narrow line-width," he says. The source also offers tunable wavelength spacing, in contrast to most optical combs that use a fixed-cavity design. Pilot Photonics says it can tune the spacing of the sources with sub-kilohertz precision.
The advantage of the comb source for coherent transmission is that a single chip can replace four or eight distinct lasers, saving packaging, size and cost
Pilot Photonics’ comb sources exploit injection locking between two lasers. Injection locking refers to an effect when two closely matched oscillating systems - in this case, lasers - interact to become synchronised.
The start-up’s comb source comprises a short-cavity ‘slave’ laser and a long-cavity ‘master’ one. The slave laser is modulated with a sine wave, turning the laser on briefly each cycle, to create a train of optical light pulses.
Linking the two lasers, injection locking occurs which increases the coherence between the output pulses. As Smyth explains, this reduces the jitter of the slave laser’s output in that the laser is turned on and off at the same exact points each cycle. This turns the slave’s output, when viewed on a spectrum analyser, into equally-spaced narrow line-width light sources.
The dimensions of the master laser’s cavity set the sources’ line widths while their spacing is dictated by the modulating sine wave. The master laser also determines the central wavelength of the comb sources while the sine wave’s frequency sets the spacings either side. “The master laser gives you a locked centre point and then the tones emanating from the centre can be tuned quite precisely,” says Smyth.
Pilot Photonics’ core intellectual property is making the indium-phosphide optical comb source using its patented gain-switching approach.
Photonic integration
The start-up has built a library of indium-phosphide optical functions in addition to the lasers used for the comb source. The functions include semiconductor optical amplifiers, waveguides, optical couplers, splitters and an active optical filter.
The splitters are used to place the comb source output on waveguides while an active optical filter on each selects the wanted source.
“This [active optical filter] is what we use to separate out individual comb lines so we can do fancy things with them,” says Smyth. For example, modulating the source with data, or beating two sources together for frequency multiplication to create sources in the millimetre wave or sub-terahertz ranges.
Pilot Photonics’ optical circuits are built in an indium-phosphide foundry where the comb source fabrication in done without using regrowth stages. This equates to fewer mask stages to process the indium-phosphide wafer. “There is no regrowth of material back over etched areas,” says Smyth. Fewer steps equates to a less-costly manufacturing process and improved yields.
The start-up sees NG-PON2, the 10-gigabit four-wavelength PON standard, as the largest and closest market opportunity for the company. Coherent optical transport is another telecom market the company is pursuing.
“The next closest opportunity is optical fibre sensing,” says Smyth, pointing out that there are several optical fibre sensoring techniques that can be made using their laser as a pulse source.
The company is also developing LiDAR technology and is involved with the European Space Agency to develop a light source for high-frequency metrology applications including atomic clocks and gravity meters.
“It is a very broad range of applications that we can apply the technology to,” says Smyth.
NG-PON2
Pilot Photonics is not using its source technology as a comb for an NG-PON2 optical line terminal (OLT) but rather as a directly modulated laser for the customer premises equipment’ optical network unit (ONU).
“What we have done is develop a wavelength-tunable directly-modulated laser for NG-PON2,” says Smyth. The benefit of its design is that the laser chip meets the stringent specification of the ONU by being tunable, meeting a reach of 40km and enabling sub-$100 designs.
The start-up is engaged with several potential NG-PON2 customers including manufacturers, systems vendors and module makers, and has delivered an evaluation board with the chip to its lead customer.
Two or three network equipment manufacturers are eager to evaluate the chip
Coherent source
The advantage of the comb source for coherent transmission is that a single chip can replace four or eight distinct lasers, saving packaging, size and cost.
Smyth estimates that a four-channel comb source is a third of the cost of a design using four single-mode lasers. The power consumption is also less; only one thermo-electric cooler is required instead of four.
Pilot Photonics says that it has demonstrated its four-channel comb-source transmitting over hundreds of kilometres.
The comb source can be used to send 400-gigabit (100 gigabit/wavelength) and 1-terabit (250 gigabit/wavelength) super-channels. “We’ve done two terabits using 16-QAM on most of the channels and QPSK on the outer ones,” says Smyth.
There are also other system performance benefits using a comb source. There is no need for guard bands to separate between the tones. “You are packing them as tight as can be allowed, the ultimate in spectral efficiency,” he says.
Smyth also points out that non-linear compensation techniques can be used because the frequency spacings are known precisely. Using non-linear compensation methods benefits reach; the laser source can be launched at higher power and the non-linear effects that result can be compensated for.
Pilot Photonics has shown its sources spaced as close as 6.25GHz to 87.5GHz apart. The start-up also says the tones do not need to be evenly spaced.
The start-up now has its four-channel comb-source chip on an evaluation board that it is about to deliver to interested systems vendors and large-scale data centre operators.
“Two or three network equipment manufacturers are eager to evaluate the chip,” says Smyth. “They are less forthcoming as to what they are applying it to.”
Ciena to sell its own coherent modules
The systems vendor is expanding its offerings to include WaveLogic modem chips and coherent optical modules.
Ciena is developing its own coherent modules to sell to the telecom and datacom markets.
The system vendor has set up the Optical Microsystems Division business unit to promote its WaveLogic coherent modem technology to the marketplace. Until now it has licensed its WaveLogic Ai digital signal processor (DSP) to module makers Lumentum, NeoPhotonics and Oclaro. But now it is planning to sell its own coherent modules.
In a job advert for a head of sales channel development, Ciena says the Optical Microsystems Division's goal is ‘to develop and productize electro-optic components and modules for sale to global systems integrator customers to be incorporated in their products for sale to telecom and data network customers’.
And at the recent European Conference on Optical Communication (ECOC) held in Rome, a network equipment manufacturer said it was approached by Ciena enquiring if it was interested in buying coherent modules from the company.
Ciena would not comment when asked if it will sell its own coherent modules. Instead, the company pointed to statements it made during its fourth quarter 2017 earnings call that outlined the creation of the Optical Microsystems Division with the stated goal of generating $50 million annual revenues by year-end 2020.
[At ECOC], a network equipment manufacturer said it was approached by Ciena enquiring if it was interested in buying coherent modules from the company
Optical Microsystems Division
Until Ciena announced in early 2017 the licensing of its 400-gigabit WaveLogic Ai to Lumentum, NeoPhotonics and Oclaro, systems vendors kept their coherent DSPs in-house. And with good reason. These are the chips that power their leading optical platforms and enable product differentiation.
Ciena’s announcement at the time showed a willingness to pursue a different business model. By licensing its DSP to optical module makers, Ciena could break into important new markets such as China even though the move would benefit its competitors using its advanced DSP for their platforms.
But the market has changed since Ciena made the announcement and now the company is deciding how best to proceed, says Mike Genovese, managing director and senior equity research analyst at MKM Partners.
“At the time of the announcement it seemed there was a big opportunity selling the [coherent] modem into Chinese OEMs,” says Genovese. “But that seems less likely now because Chinese OEMs want to assemble their own modules out of components they buy and make.”
The result is that the opportunity has shifted to data centre interconnect. “But there are decisions that need to be made,” says Genovese. “For example, does Ciena want to make its modem product a [pluggable] 400ZR solution?”
It is a view shared by Sterling Perrin, principal analyst, optical networking and transport at Heavy Reading.
“It [the licensing of its DSP] was originally built around breaking into the China market. That strategy now looks must riskier than it did originally, so I’m certain they are looking at every alternative,” says Perrin. ”The main goal is to get the most return-on-investment on the money they put into building a WaveLogic generation, and using that money to fund the next generation of DSP investment.”
At the time of the announcement it seemed there was a big opportunity selling the [coherent] modem into Chinese OEMs. But that seems less likely now.
Pluggables are going to become an important opportunity for coherent technology, says Andrew Schmitt, founder and directing analyst at Cignal AI. Schmitt says the next stage of coherent’s development - what he calls the fourth generation of coherent - will be pluggable from the start and more standards-based than any wavelength-division multiplexing (WDM) pluggable that has preceded it.
“It will address a large portion of the overall market - not just cloud operator data centreinterconnect,” says Schmitt. “Equipment vendors will need to adjust their strategies as many standalone optical hardware applications will be displaced by pluggable coherent.”
Ciena also has all the required technologies. As well as its WaveLogic modem technology, it has high-speed optical component expertise that it gained with the 2016 acquisition of Teraxion’s photonics division. The Teraxion group had indium phosphide and silicon photonics technologies.
All change
The agreement between Ciena and the three optical module makers also included an option where future WaveLogic DSPs would be made available to the three for applications such as 400-gigabit pluggables.
NeoPhotonics says that Ciena’s general strategy of bringing its WaveLogic Ai technology to a larger market and application space has not changed.
Equipment vendors will need to adjust their strategies as many standalone optical hardware applications will be displaced by pluggable coherent
Is Ciena going straight to market with future WaveLogic-based modules?
“How the modules are marketed may follow different models in the future; there is always an evolution in business models as the market shifts,” says Ferris Lipscomb, NeoPhotonics’ vice president of marketing. “Our intention is to continue to be a partner and bring value to the Ciena Microsystems business wherever possible.”
Lumentum would not comment on what the status was regarding using future coherent DSPs, nor would it say whether Ciena is to sell its own modules. Lumentum did say that it has a close relationship with Ciena and that it continues to support partnership opportunities.
But the possibility of Ciena selling modules to the marketplace is not ruled out by Ciena’s third optical module partner, Oclaro.
Yves LeMaitre, chief strategy officer at Oclaro says that Ciena’s recent announcements could point to a new strategic direction. “At this point, it is unclear how they are going to do this,” he says. Oclaro also does not know yet if it will gain access to new WaveLogic designs.
LeMaitre views the options with Ciena’s coherent technology as part of a broader debate as to how systems vendors should adapt their business models in an environment of change brought about by software-defined networks and open design frameworks.
The fact that internet content providers purchase optics directly, as do certain service providers, creates a dilemma for the systems vendors. “How are they going to go to the market to address this?” says LeMaitre. “Are they going to rely on a partnership with module makers or are they going to address the market on their own?”
Oclaro says it remains ‘very interested’ in working with Ciena if it is willing to give the module maker access to future DSP designs for pluggables.
400ZR will signal coherent’s entry into the datacom world
- 400ZR will have a reach of 80km and a target power consumption of 15W
- The coherent interface will be available as a pluggable module that will link data centre switches across sites
- Huawei expects first modules to be available in the first half of 2020
- At OFC, Huawei announced its own 250km 400-gigabit single-wavelength coherent solution that is already being shipped to customers
Coherent optics will finally cross over into datacom with the advent of the 400ZR interface. So claims Maxim Kuschnerov, senior R&D manager at Huawei.
Maxim Kuschnerov400ZR is an interoperable 400-gigabit single-wavelength coherent interface being developed by the Optical Internetworking Forum (OIF).
The 400ZR will be available as a pluggable module and as on-board optics using the COBO specification. The IEEE is also considering a proposal to adopt the 400ZR specification, initially for the data-centre interconnect market. “Once coherent moves from the OIF to the IEEE, its impact in the marketplace will be multiplied,” says Kuschnerov.
But developing a 400ZR pluggable represents a significant challenge for the industry. “Such interoperable coherent 16-QAM modules won’t happen easily,” says Kuschnerov. “Just look at the efforts of the industry to have PAM-4 interoperability, it is a tremendous step up from on-off keying.”
Despite the challenges, 400ZR products are expected by the first half of 2020.
400ZR use cases
The web-scale players want to use the 400ZR coherent interface to link multiple smaller buildings, up to 80km apart, across a metropolitan area to create one large virtual data centre. This is a more practical solution than trying to find a large enough location that is affordable and can be fed sufficient power.
Once coherent moves from the OIF to the IEEE, its impact in the marketplace will be multiplied
Given how servers, switches and pluggables in the data centre are interoperable, the attraction of the 400ZR is obvious, says Kuschnerov: “It would be a major bottleneck if you didn't have [coherent interface] interoperability at this scale.”
Moreover, the advent of the 400ZR interface will signal the start of coherent in datacom. Higher-capacity interfaces are doubling every two years or so due to the webscale players, says Kuschnerov, and with the advent of 800-gigabit and 1.6-terabit interfaces, coherent will be used for ever-shorter distances, from 80km to 40km and even 10km.
At 10km, volumes will be an order of magnitude greater than similar-reach dense wavelength-division multiplexing (DWDM) interfaces for telecom. “Datacom is a totally different experience, and it won’t work if you don’t have a stable supply base,” he says. “We see the ZR as the first step combining coherent technology and the datacom mindset.”
Data centre players will plug 400ZR modules into their switch-router platforms, avoiding the need to interface the switch-router to a modular, scalable DWDM platform used to link data centres.
The 400ZR will also find use in telecom. One use case is backhauling residential traffic over a cable operator’s single spans that tend to be lossy. Here, ZR can be used at 200 gigabits - using 64 gigabaud signalling and QPSK modulation - to extend the reach over the high-loss spans. Similarly, the 400ZR can also be used for 5G mobile backhaul, aggregating multiple 25-gigabit streams.
Another application is for enterprise connectivity over distances greater than 10km. Here, the 400ZR will compete with direct-detect 40km ER4 interfaces.
Having several use cases, not just data-centre interconnect, is vital for the success of the 400ZR. “Extending ZR to access and metro-regional provides the required diversity needed to have more confidence in the business case,” says Kuschnerov.
The 400ZR will support 400 gigabits over a single wavelength with a reach of 80km, while the target power consumption is 15W.
The industry is still undecided as to which pluggable form factor to use for 400ZR. The two candidates are the QSFP-DD and the OSFP. The QSFP-DD provides backward compatibility with the QSFP+ and QSFP28, while the OSFP is a fresh design that is also larger. This simplifies the power management at the expense of module density; 32 OSFPs can fit on a 1-rack-unit faceplate compared to 36 QSFP-DD modules.
The choice of form factor reflects a broader industry debate concerning 400-gigabit interfaces. But 400ZR is a more challenging design than 400-gigabit client-side interfaces in terms of trying to cram optics and the coherent DSP within the two modules while meeting their power envelopes.
The OSFP is specified to support 15W while simulation results published at OFC 2018 suggest that the QSFP-DD will meet the 15W target. Meanwhile, the 15W power consumption will not be an issue for COBO on-board optics, given that the module sits on the line card and differs from pluggables in not being confined within a cage.
Kuschnerov says that even if it proves that only the OSFP of the two pluggables supports 400ZR, the interface will still be a success given that a pluggable module will exist that delivers the required face-plate density.
400G coherent
Huawei announced at OFC 2018 its own single-wavelength 400-gigabit coherent technology for use with its OptiX OSN 9800 optical and packet OTN platform, and it is already being supplied to customers.
The 400-gigabit design supports a variety of baud rates and modulation schemes. For a fixed-grid network, 34 gigabaud signalling enables 100 gigabits using QPSK, and 200 gigabits using 16-QAM, while at 45 gigabaud 200 gigabits using 8-QAM is possible. For flexible-grid networks, 64 gigabaud is used for 200-gigabit transmission using QPSK and 400 gigabits using 16-QAM.
Huawei uses an algorithm called channel-matched shaping to improve optical performance in terms of data transmission and reach. This algorithm includes such techniques as pre-emphasis, faster-than-Nyquist, and Nyquist shaping. According to Kuschnerov, the goal is to squeeze as much capacity out of a network’s physical channel so that advanced coding techniques such as probabilistic constellation shaping can be used to the full. For Huawei’s first 400-gigabit wavelength solution, constellation shaping is not used but this will be added in its upcoming coherent designs.
Huawei has already demonstrated the transmission of 400 gigabits over 250km of fibre. “Current generation 400G-per-lambdas does not enable long-haul or regional transmission so the focus is on shorter reach metro or data-centre-interconnect environments,” says Kuschnerov.
When longer reaches are needed, Huawei can offer two line cards, each supporting 200 gigabits, or a single line card hosting two 200-gigabit modules. The 200-gigabits-per-wavelength is achieved using 64 gigabaud and QPSK modulation, resulting in a 2,500km reach.
Up till now, such long-haul distances have been served using 100-gigabitwavelengths. Now, says Kuschnerov, 200 gigabit at 64 gigabaud is becoming the new norm in many newly built networks while the 34 gigabaud 200 gigabit is being favoured in existing networks based on a 50GHz grid.
Lumentum jolts the industry with Oclaro acquisition
Lumentum announced on Monday its plan to acquire Oclaro in a deal worth $1.8 billion.
The prospect of consolidation among optical component players has long been mooted yet the announcement provided the first big news jolt at the OFC show, being held in San Diego this week.
Alan Lowe“Combined, we will be an industry leader in telecom transmission and transport as well as 3D sensing,” said Alan Lowe, president and CEO of Lumentum, on an analyst call discussing the deal.
Lumentum says their joint revenues totalled $1.7 billion with a 39% gross margin over the last year. And $60 million in synergies are forecast in the second year after the deal closes, which is expected to happen later this year.
The $1.8 billion acquisition will comprise 56 percent cash and 44 percent Lumentum stock. Lumentum will also raise $550 million to help finance the deal.
“This is a big deal as it consolidates the telecom part of the component market,” says Daryl Inniss, business development manager at OFS Fitel and former market research analyst.
Background
Lowe said that ever since Lumentum became a standalone company three years ago, the firm concentrated on addressing the increase in optical communications demand that started in late 2015 and then last year on ramping the production of its 3D sensing components. “Execution on major M&As had to wait,” he said.
The company investigated potential acquisitions and evaluated several key technologies including silicon photonics and indium phosphide. This led to it alighting on Oclaro with its indium phosphide and photonic integrated circuit (PIC) expertise.
Lowe also highlighted Oclaro’s strategy of the last five years of first trimming its business lines and then successfully executing on delivering optical transmission products.
Oclaro’s CEO, Greg Dougherty, CEO of Oclaro, described how his company has focussed on delivering differentiated photonic chip products to various growing end markets. “This is a very good combination for both companies and for the industry,” said Dougherty.
There is no overabundance in [optical] chip designers worldwide and together we have the strongest chip designer team in the world
Business plans
Lumentum’s business includes telecom transport components, modules and sub-systems. Its products include reconfigurable optical add/drop multiplexers (ROADMs), pump lasers, optical amplifiers and submarine products. In the second half of 2017, Lumentum’s telecom revenue mix was split three quarters telecom transport with transmission products accounted for the remaining quarter. Other Lumentum businesses include industrial lasers and 3D sensing.
In contrast, Oclaro’s focus in solely transmission components and modules, with the revenue mix in its most recent quarter being 53 percent telecom line side and 47 percent datacom client-side products.
The combined R&D resources of the merged company will allow it to do a much better job at supporting datacom products using the new QSFP-DD and OSFP form factors. “Right now I’m guessing that Alan is spread thin and I know the Oclaro datacom team has been spread thin,” says Dougherty.
The acquisition will also pool the two companies’ fabrication facilities.
Lumentum has already moved its lithium niobate manufacturing to its main gallium arsenide and indium phosphide fab in San Jose, California. San Jose also hosts a separate planar lightwave circuit fab.
Oclaro, which is headquartered in San Jose, has three photonic chip fabrication sites: an indium phosphide laser fab for datacom in Japan that makes directly modulated lasers (DMLs) and electro-absorption modulated lasers (EMLs), an indium phosphide fab in the UK that manufactures coherent optical components and sub-assemblies, and a lithium niobate fab in Italy.
The acquisition will also bolster the company’s chip design resources. “There is no overabundance in [optical] chip designers worldwide and together we have the strongest chip designer team in the world,” says Dougherty.
Lumentum plans to assign some of the chip designers to tackle a burgeoning pipeline of 3D sensing product designs.
In 2017 Lumentum reported three customers that accounted for nearly half of its revenues, while Oclaro had four customers, each accounted for 10 percent or more of its sales, in 4Q 2017. Oclaro selected customers include the webscale players, Amazon, Google and Microsoft, as well as leading systems vendors such as Ciena, Cisco, Coriant, Huawei, Juniper, Nokia and ZTE.
Both Oclaro and Lumentum, along with Neophotonics, signed an agreement with Ciena a year ago to use its WaveLogic Ai DSP in their coherent module designs.
Lumentum plans to provide more deal details closer to its closure. Meanwhile, the two CEOs will continue to run their companies with Oclaro’s Dougherty remaining at least during the transition period.
Further information:
For the link to the acquisition presentation, click here.
Coherent gets a boost with probabilistic shaping
Nokia has detailed its next-generation PSE-3 digital signal processor (DSP) family for coherent optical transmission.
The PSE-3s is the industry’s first announced coherent DSP that supports probabilistic constellation shaping, claims Nokia.
Probabilistic shaping is the latest in a series of techniques adopted to improve coherent optical transmission performance. These techniques include higher-order modulation, soft-decision forward error correction (SD-FEC), multi-dimensional coding, Nyquist filtering and higher baud rates.
Kyle Hollasch
“There is an element here that the last big gains have now been had,” says Kyle Hollasch, director of product marketing for optical networks at Nokia.
Probabilistic shaping is a signal-processing technique that squeezes the last bit of capacity out of a fibre’s spectrum, approaching what is known as the non-linear Shannon Limit.
“We are not saying we absolutely hit the Shannon Limit but we are extremely close: tenths of a decibel whereas most modern systems are a couple of decibels away from the theoretical maximum,” says Hollasch.
Satisfying requirements
Optical transport equipment vendors are continually challenged to meet the requirements of the telcos and the webscale players.
One issue is meeting the continual growth in IP traffic: telcos are experiencing 25 percent yearly traffic growth whereas for the webscale players it is 60 percent. Vendors must also ensure that their equipment keeps reducing the cost of transport when measured as the cost-per-bit.
Operators also want to automate their networks. Technologies such as flexible-grid, reconfigurable optical add/drop multiplexers (ROADMs), higher-order modulation and higher baud rates all add flexibility to the optical layer but at the expense of complexity.
There is an element here that the last big gains have now been had
“It is easy to say software-defined networking will hide all that complexity,” says Hollasch. “But hardware has an important role: to keep delivering capacity gains but also make the network simpler.”
Satisfying these demands is what Nokia set out to achieve when designing the PSE-3s.
Capacity and cost
Like the current PSE-2 coherent DSPs that Nokia launched in 2016, two chips make up the PSE-3 family: the super coherent PSE-3s and the low-power compact PSE-3c.
The PSE-3s is a 1.2-terabit chip that can drive two sets of optics, each capable of transmitting 100 to 600 gigabit wavelengths. This compares to the 500-gigabit PSE-2s that can drive two wavelengths, each up to 250Gbps.
The low-power PSE-3c also can transmit more traffic, 100 and 200-gigabit wavelengths, twice the capacity of the 100-gigabit PSE-2c.
Nokia has used a software model of two operators’ networks, one an North America and another in Germany, to assess the PSE-3s.
The PSE-3s’ probabilistic shaping delivers 70% more capacity while using a third fewer line cards when compared with existing commercial systems based on 100Gbps for long haul and 200Gbps for the metro. When the PSE-3s is compared with existing Nokia PSE-2s-based platforms on the same networks, a 25 percent capacity gain is achieved using a quarter fewer line cards.
Hollasch says that the capacity gain is 1.7x and not greater because 100-gigabit coherent technology used for long haul is already spectrally efficient. “But it is less so for shorter distances and you do get more capacity gains in the metro,” says Hollasch.
Probabilistic shaping
The 16nm CMOS PSE-3s supports a symbol rate of up to 67Gbaud. This compares to the 28nm CMOS PSE-2s that uses two symbol rates: 33Gbaud and 45Gbaud.
The PSE-3s’ higher baud rate results in a dense wavelength-division multiplexing (DWDM) channel width of 75GHz. Traditional fixed-grid channels are 50GHz wide. With 75GHz-wide channels, 64 lightpaths can fit within the C-band.
The PSE-3s uses one modulation format only: probabilistic shaping 64-ary quadrature amplitude modulation (PS-64QAM). This compares with the PSE-2s that supports six modulations ranging from binary phase-shift keying (BPSK) for the longest spans to 64-QAM for a 400-gigabit wavelength.
Using probabilistic shaping, one modulation format supports data rates from 200 to 600Gbps. For 100Gbps, the PSE-3s uses a lower baud rate in order to fit existing 50GHz-wide channels.
In current optical networks, all the constellation points of the various modulation formats are used with equal probability. BPSK has two constellation points while 64-QAM has 64. Probabilistic shaping does not give equal weighting to all the constellation points. Instead, it favours those with lower energy, represented by those points closer to the origin in a constellation graph. The only time all the constellation points are used is at the maximum data rate - 600Gbps for the PSE-3s.
Using the inner, lower energy constellation points more frequently than the outer points reduces the overall average energy and this improves the signal-to-noise ratio. That is because the symbol error rate at the receiver is dominated by the distance between neighbouring points on the constellation. Reducing the average energy still keeps the distance between the points the same, but since a constant signal power level is used for DWDM transmission, applying gain increases the distance between the constellation points.
“We separate these points further in space - the Euclidean distance between them,” says Hollasch. “That is where the shaping gain comes from.”
Changing the probabilistic shaping in response to feedback from the chip, from the network, we think that is a powerful innovation
Using probabilistic shaping delivers a maximum 1.53dB of improvement in a linear transmission channel. In practice, Nokia says it achieves 1dB. “One dB does not sound a lot but I call it the ultimate dB, the last dB in addition to all the other techniques,” he says.
By using few and fewer of the constellation points, or favouring those points closer to the origin, reduces the data that can be transported. This is how the data rate is reduced from the maximum 600Gbps to 200Gbps.
To implement probabilistic shaping, Nokia has developed an IP block for the chip called the distribution matcher. The matcher maps the input data stream as rates as high as 1.2 terabits-per-second onto the constellation points in a non-uniform way.
Theoretically, probabilistic shaping allows any chosen data rate to be used. But what dictates the actual data rate gradations is the granularity of the client signals. The Optical Internetworking Forum’s Flex Ethernet (FlexE) standard defines 25-gigabit increments and that will be the size of the line-side data rate increments.
Embracing a single modulation format and a 75GHz channel results in network operation benefits, says Hollasch: “It stops you having to worry and manage a complicated spectrum across a broad network.” And it also offers the prospect of network optimisation. “Changing the probabilistic shaping in response to feedback from the chip, from the network, we think that is a powerful innovation,” says Hollasch.
The reach performance of the PSE-3s using 62Gbaud and PS-64QAM. The reach performance of the PSE-2s is shown (where relevant) for comparison purposes.
Product plans
The first Nokia product to use the PSE-3 chips is the 1830 Photonic Service Interconnect-Modular, a 1 rack-unit compact modular platform favoured by the webscale players.
Nokia has designed two module-types or ‘sleds’ for the 1830 PSI-M pizza box. The first is a 400-gigabit sled that uses two sets of optics and two PSE-3c chips along with four 100-gigabit client-side interfaces. Four such 400-gigabit sleds fit within the platform to deliver a total of 1.6 terabits of line-side capacity.
In contrast, two double-width sleds fit within the platform using the PSE-3s. Each sled has one PSE-3 chip and two sets of optics, each capable of up to a 600-gigabit wavelength, and a dozen 100-gigabit interfaces. Here the line-side capacity is 2.4 terabits.
Nokia says the 400-gigabit sleds will be available in the first half of this year whereas the 1.2 terabit sleds will start shipping at the year-end or early 2019. The first samples of the PSE-3s are expected in the second half of 2018. Nokia will then migrate the PSE-3s to the rest of its optical transport platform portfolio.
So has coherent largely run its course?
“In terms of a major innovation in signal processing, probabilistic shaping is completing the coherent picture,” says Hollasch. There will be future coherent DSP chips based on more advanced process nodes than 16nm with symbol rates approaching 100GBaud. Higher data rates per wavelength will result but at the expense of a wider channel width. But once probabilistic shaping is deployed, further spectral efficiencies will be limited.
Acacia announces a 1.2 terabit coherent module
Channel capacity and link margin can be maximised by using the fractional QAM scheme. Source: Acacia.
The company is facing increasing market competition. Ciena has teamed up with Lumentum, NeoPhotonics, and Oclaro, sharing its high-end coherent DSP expertise with the three optical module makers. Meanwhile, Inphi has started sampling its 16nm CMOS M200, a 100- and 200-gigabit coherent DSP suitable for CFP2-ACO, CFP-DCO, and CFP2-DCO module designs.
The AC1200 is Acacia’s response, extending its high-end module offering beyond a terabit to compete with the in-house system vendors and preserve its performance lead against the optical module makers.
Enhanced coherent techniques
The AC1200 has an architecture similar to the company’s AC400 5x7-inch 400-gigabit module announced in 2015. Like the earlier module, the AC1200 features a dual-core coherent DSP and two silicon photonics transceiver chips. But the AC1200 uses a much more sophisticated DSP - the 16nm CMOS Pico device announced earlier this year - capable of supporting such techniques as variable baud rate, advanced modulation and coding schemes so that the bits per symbol can be fine-tuned, and enhanced soft-decision forward error correction (SD-FEC). The AC400 uses the 1.3 billion transistor Denali dual-core DSP while the Pico DSP has more than 2.5 billion transistors.
The result is a two-wavelength module design, each wavelength supporting from 100-600 gigabits in 50-gigabit increments.
Acacia is able to triple the module’s capacity to 1.2 terabits by incorporating a variable baud rate up to at least 69 gigabaud (Gbaud). This doubles the capacity per wavelength compared to the AC400 module. The company also uses more modulation formats including 64-ary quadrature amplitude modulation (64-QAM), boosting capacity a further 1.5x compared to the AC400’s 16-QAM.
Acacia has not detailed the module’s dimensions but says it is a custom design some 40 percent smaller in area than a 5x7-inch module. Nor will it disclose the connector type and electrical interface used to enable the 1.2-terabit throughput. However, the AC1200 will likely support 50 gigabit-per-second (Gbps) 4-level pulse-amplitude modulation (PAM-4) electrical signals as it will interface to 400-gigabit client-side modules such as the QSFP-DD.
The AC1200’s tunable baud rate range is around 35Gbaud to 69Gbaud. “The clock design and the optics could truly be continuous and it [the baud rate] pairs with a matrix of modulation formats to define a certain resolution,” says Tom Williams, senior director of marketing at Acacia Communications. Whereas several of the system vendors’ current in-house coherent DSPs use two baud rates such as 33 and 45Gbaud, or 35 and 56Gbaud, Acacia says it uses many more rates than just two or three.
The result is that at the extremes, the module can deliver from 100 gigabits (a single wavelength at some 34Gbaud and quadrature phase-shift keying - QPSK) to 1.2 terabits (using two wavelengths, each 64-QAM at around 69Gbaud).
The module also employs what Acacia refers to as very fine resolution QAM constellations. The scheme enables the number of bits per symbol to be set to any value and not be limited to integer bits. Acacia is not saying how it is implementing this but says the end result is similar to probabilistic shaping. “Instead of 2 or 3 bits-per-symbol, you can be at 2.5 or 2.7 bits-per-symbol,” says Williams. The performance benefits include maximising the link margin and the capacity transmitted over a given link. (See diagram, top.)
The SD-FEC has also been strengthened to achieve a higher coding gain while still being a relatively low-power implementation.
Using a higher baud rate allows a lower order modulation scheme to be used. This can more than double the reach. Source: Acacia
The company says it is restricted in detailing the AC1200’s exact performance. “Because we are a merchant supplier selling into system vendors that do the link implementations, we have to be careful about the reach expectations we set,” says Williams. But the combination of fractional QAM, a tunable baud rate, and improved FEC means a longer reach for a given capacity. And the capacity can be tuned in 50-gigabit increments.
Platforms and status
ADVA Optical Networking is one vendor that has said it is using Acacia’s 1.2-terabit design for its Teraflex product, the latest addition to its CloudConnect family of data centre interconnect products.
Is ADVA Optical Networking using the AC1200? “Our TeraFlex data centre interconnect product uses a coherent engine specifically developed to meet the performance expectations that our customers demand,” says ADVA's spokesperson.
Teraflex is a one-rack-unit (1RU) stackable chassis that supports three hot-pluggable 1.2-terabit ‘sleds’. Each sled’s front panel supports various client-side interface module options: 12 x 100-gigabit QSFP28s, 3 x 400-gigabit QSFP-DDs and lower speed 10-gigabit and 40-gigabit modules using ADVA Optical Networking’s MicroMux technology.
Samples of the AC1200 module will be available in the first half of 2018, says Acacia. General availability will likely follow a quarter or two later.
Has coherent optical transmission run its course?
Feature: Coherent's future
Three optical systems vendors share their thoughts about coherent technology and the scope for further improvement as they look two generations ahead to symbol rates approaching 100 gigabaud
Optical transmission using coherent detection has made huge strides in the last decade. The latest coherent technology with transmitter-based digital signal processing delivers 25x the capacity-reach of 10-gigabit wavelengths using direct-detection, according to Infinera.
Since early 2016, the optical systems vendors Infinera, Ciena and Nokia have all announced new coherent digital signal processor (DSP) designs. Each new generation of coherent DSP improves the capacity that can be transmitted over an optical link. But given the effectiveness of the latest coherent systems, has most of the benefits already been achieved?
Source: Infinera
“It is getting harder and harder,” admits Kim Roberts, vice president, WaveLogic science at Ciena. “Unlike 10 years ago, there are no factors of 10 available for improvement.”
Non-linear Shannon limit
It is the non-linear Shannon limit that defines how much information can be sent across a fibre, a function of the optical signal-to-noise ratio.
Kim Roberts of CienaThe limit is based on the work of famed mathematician and information theorist, Claude Shannon. Shannon's work was based on a linear communication channel with added Gaussian noise. Optical transport over a fibre is a more complex channel but the same Shannon bound applies, although assumptions for the non-linearities in the fibre must be made.
Roberts stresses that despite much work, the industry still hasn't figured out just what the upper limit is over a fibre for a given optical signal-to-noise ratio.
It is getting harder and harder. Unlike 10 years ago, there are no factors of 10 available for improvement.
"There are papers that show that with this method and this method, you can do this much," says Roberts. "And there are other papers that show that as the power goes up, there is no theoretical limit until you melt the fibre."
These are theoretical things, he says, but the key is that the headroom available remains unknown. What is known is that the theoretical limit remains well ahead of practical systems. Accordingly, systems performance can be improved using a combination of techniques and protocols coupled with advances in electro-optics.
Design goals
A key goal when designing a new optical transmission system is to increase the data sent for a given cost i.e. decrease the cost-per-bit. This is an ongoing requirement as the service providers contend with ever growing network traffic.
Another challenge facing engineers is meeting the demanding power, density and thermal constraints of their next-generation optical transport system designs.
One way to reduce the cost-per-bit is to up the symbol rate to increase the data sent over a wavelength. Traditional 100-gigabit and 200-gigabit dense wavelength-division multiplexing (DWDM) systems use 32-35 gigabaud (GBaud). The latest coherent DSPs already support more than one baud rate: Nokia’s PSE-2s coherent DSP supports 33Gbaud or 45Gbaud while Ciena’s WaveLogic Ai chipset supports 35Gbaud or 56Gbaud.
Having a choice of baud rates coupled with the various modulation scheme options means the same number of bits can be sent over a range of optical reaches. The more complex the modulation scheme, the closer the points are in a constellation and the harder it is to correctly detect the data at the receiver in the presence of noise. Accordingly, using the combination of a simpler modulation scheme and a higher baud rate allows the same data to be sent further.
Capacity-reach is what matters: how much capacity you can extract for a given reach
Nokia's 1.4-billion transistor PSE-2s supports two 200 gigabit-per-second (Gbps) formats: polarisation-multiplexing, 16-ary quadrature amplitude modulation (PM-16QAM) at 33Gbaud, or using PM-8QAM at 45Gbaud. The 200-gigabit wavelength has an optical reach of some 800km using 16-QAM at 33Gbaud but this rises to 1,600km when PM-8QAM at 45Gbaud is used. Alternatively, using 45Gbaud and PM-16QAM, more data can be sent: 250 gigabits-per-wavelength over 800km.
Nokia's Randy EisenachCoherent systems designers are not stopping there. “The next higher baud rate the industry is targeting is 61-68 Gbaud,” says Randy Eisenach, senior product marketing manager, optical networks at Nokia.
Operating at the higher gigabaud range - Infinera talks of 65-70Gbaud - a single transmitter-receiver pair sends twice the amount of data of traditional 32-35Gbaud systems using the same modulation format. But the higher-baud rates require the electro-optics to operate twice as fast. The analogue-to-digital and digital-to-analogue converters of the coherent DSP must sample at twice the baud rate - at least 130 billion samples-per-second. A 65-70Gbaud rate also requires silicon implemented using a more advanced and expensive CMOS process mode - 16nm instead of 28nm. In turn, the optical modulator and drivers need to work well at these higher rates.
“The optical networking industry is well on its way to solving these engineering and component issues in the next year or so,” says Eisenach.
The capacity-per-wavelength also goes up with baud rate. For shorter reach links, 400-600 gigabits-per-wavelength are possible at 65-70Gbaud and, according to Pravin Mahajan, Infinera’s director of product and corporate marketing, power consumption in terms of watts-per-gigabit will improve by some 2.5x.
Pravin Mahajan of InfineraAnd the system vendors are not stopping there: the next baud rate hike after 65-70Gbaud will be in the region of 80-100 Gbaud. The coherent DSPs that will support such data rates will need to be implemented using 7nm CMOS process (see table).
“Capacity-reach is what matters: how much capacity you can extract for a given reach,” says Mahajan. “These successive generations [of faster baud rates] all keep moving that curve upwards.”
DSP features
In addition to the particular baud rates chosen by the vendors for their DSP designs, each includes unique features.
Instead of modulating the data onto a single carrier, Infinera’s FlexCoherent DSP uses multiple Nyquist sub-carriers spread across a channel. The number of subs-carriers varies depending on the link. The benefit of the approach, says Infinera, is that it allows a lowering of the baud rate used which increases the tolerance to non-linear channel impairments experienced during optical transmission.
The FlexCoherent DSP also supports enhanced soft-decision forward-error correction (SD-FEC) including the processing of two channels that need not be contiguous. This is possible as the FlexCoherent DSP is dual-channel which particularly benefits long-haul and subsea applications, claims Infinera. By pairing two channels, the FEC codes can be shared. Pairing a strong channel with a weak one and sharing the codes allows some of the strength of the strong signal to be used to bolster the weaker one, extending its reach or even allowing a more advanced modulation scheme to be used.
Infinera has just announced that by using Nyquist sub-carriers and the FEC gain sharing technologies, its customer, Seaborn Networks, is able delivering 11.8 terabits of capacity over a 10,600km submarine link.
Nokia’s PSE-2s DSP has sufficient processing performance to support two coherent channels. Each channel can implement a different modulation format if desired, or the two can be tightly coupled to form a super-channel. Using 45Gbaud and PM-16QAM, two 250-gigabit channels can be implemented to enable a 500-gigabit muxponder card. The PSE-2s can also implement 400-gigabit wavelength but that is the only format where only one channel can be supported by the PSE-2s.
Ciena’s WaveLogic Ai, meanwhile, uses advanced coding schemes such that it no longer mentions particular modulation schemes but rather a range of line rates in 50-gigabit increments.
Coding schemes with names such as set-partition QPSK, matrix-enhanced PM-BPSK, and 8D-2QAM, have already started to appear in the vendors’ coherent DSPs.
“Vendors use a lot of different terms essentially for the same thing: applying some type of coding to symbols to improve performance,” says Eisenach.
There are two main coding approaches: constellation shaping, also known as probabilistic shaping, and multi-dimensional coding. Combining the two - probabilistic shaping and multi-dimensional coding - promises enhanced performance in the presence of linear and non-linear transmission impairments. These are now detailed.
Probabilistic shaping
The four constellation points of QPSK modulation are equidistant from the origin. With more advanced modulation schemes such as 16-QAM, the constellation points differ in their distance from the origin and hence have different energies. Points in the corners of the constellation, furthest from the origin, have the most energy since a point’s power is the square of the distance from the origin.
Here the origin is at the centre of the square 64-QAM constellation. With probabilistic shaping, more of the points closer to the origin are chosen with the resulting data rate going down. Source: Nokia
Probabilistic shaping uses the inner constellation points more than the outer points, thereby reducing the overall average energy and this improves the signal-to-noise ratio. To understand why, Ciena points out that the symbol error rate at the receiver is dominated by the distance between neighbouring points of the constellation. Reduced the average energy still keeps the distance between the points the same, but when gain is applied to restore the signal’s power levels, the effect is to increase the distance between points. “It means we have better separation between the points, we’ve expanded everything,” says Roberts.
Using probabilistic shaping delivers a maximum 1.53dB of improvement in a linear transmission channel. “That is the theoretical limit,” says Roberts. “In a non-linear world, we get a greater benefit from shaping beyond just shaping the noise.”
Probabilistic shaping also has another benefit: it allows the number of bits sent per symbol to be defined.
Using standard modulation schemes such as 64-QAM with no constellation shaping, 6 bits-per-symbol are sent. Using shaping and being selective in what points are used, fewer bits are sent and they don’t need to be integer values. “I can send 5.7, 5.6, 5.3, even 5.14 bits-per symbol,” says Roberts. “Until I get to 5 bits, and then I have a choice: do I use more shaping or do I start with 32-QAM, which is 5 bits-per-symbol.”
Technology A shows today's coherent DSPs: operating at 30-35Gbaud and delivering 100, 150 and 200Gbps capacities per wavelength. Technology B is Ciena's WaveLogic A. Operating at 56Gbaud, it delivers up to 400Gbps per wavelength in 50Gbps. Technology C will continue this trend. Operating around 70Gbaud, up to 600Gbps per wavelength will be possible in even finer speed increments of 25Gbps. Is this Ciena's next WaveLogic? Source: Ciena
This is very useful as it allows fine control of the data sent such that operators can squeeze just enough data to suit the margins available on a particular fibre link. “You don't have to choose between 100-gigabit and 200-gigabit wavelengths,” says Roberts. "You can use smaller jumps and that sometimes means sending more capacity.”
Three things are needed to fine-tune a link in this way. One is a coherent DSP that can deliver such variable increments on a wavelength using probabilistic shaping. Also needed is a flexible client signalling scheme such as the OIF’s Flexible Ethernet (FlexE) protocol, a protocol mechanism to vary the Ethernet payload for transmission. Lastly, intelligent networking software is required to determine what is happening in the network and the margins available to assess how much data can be squeezed down a link.
Ciena says it has not implemented probabilistic shaping in its latest WaveLogic Ai coherent DSP. But given the Ai will be a family of devices, the technique will feature in upcoming coherent DSPs.
Nokia published a paper at the OFC event held earlier this year showing the use of probabilistic shaping over a transatlantic link. Using probabilistic-shaped 64-QAM (PS-64QAM), a spectral efficiency of 7.46b/s/Hz was achieved over the 5,523km link. This equates to 32 terabits of capacity over the fibre, more than 2.5x the 12 terabits of the existing DWDM system that uses 100Gbps PM-QPSK.
Advanced coding
Multi-dimensional coding is another technique used to improve optical transmission. A 16-QAM constellation is a two-dimensional (2D) representation in one polarisation, says Roberts. But if both polarisations of light are considered as one signal then it becomes a 4D, 256-point (16x16) symbol. This can be further extended by including the symbols in adjacent time slots. This forms an 8D representation.
Non-linear compensation has been an interesting research topic. Nokia continues to investigate the topic and implementation methods but the benefits appear small for most real-world applications
The main two benefits of multi-dimensional coding are better noise performance and significantly better performance in the presence of non-linear impairments.
Nokia’s PSE-2s uses coding for its set-partition QPSK (SP-QPSK). Standard PM-QPSK uses amplitude and phase modulation, resulting in a 4-point constellation. With SP-QPSK, only three of the four constellation points are used for each symbol. A third fewer constellation points means less data is transported but the benefit of SP-QPSK is extended reach due to the greater Euclidean distance between the symbol points created by carefully mapping the sequence of symbols. This results in 2.5dB of extra gain compared to PM-QPSK, for a reach beyond 5,000km.
Using the PSE-2’s 45Gbaud symbol rate, the fewer constellation points of SP-QPSK can be compensated for to achieve the same overall 100Gbps capacity as PM-QPSK at 33Gbaud.
Infinera’s FlexCoherent uses what it calls matrix-enhanced PM-BPSK, a form of averaging that adds 1dB of gain. “Any innovation that adds gain to a link, the margin that you give to operators, is always welcome,” says Mahajan.
Ciena’s WaveLogic 3 Extreme coherent DSP supports the multi-dimension coding scheme 8D-2QAM to improve reach or capacity of long-reach spans.
Such techniques mean vendors have a wealth of available choices available. It is also why Ciena has stopped referring to modulation schemes and talks about its WaveLogic Ai at 35Gbaud supporting 100-250Gbps data rates in 50-gigabit increments while at 56Gbaud, the WaveLogic Ai delivers 100-400Gbps optical channels in 50-gigabit steps.
Probabilistic shaping and multi-dimensional coding are distinct techniques but combining the two means the shaping can be done across dimensions.
Design engineers thus have various techniques to keep improving performance and there are other directions too.
Forward-error correction is about 2dB from the theoretical limit and with improved design Ciena’s Roberts expects 1dB can be reclaimed.
In turn, signal processing techniques could be applied at the transmitter to compensate for expected non-linear effects. “Non-linear compensation has been an interesting research topic,” says Eisenach. “Nokia continues to investigate the topic and implementation methods but the benefits appear small for most real-world applications.”
So is there much scope for further overall improvement?
“There is still a lot more juice left," says Mahajan.
“It [coherent transmission improvement] is getting harder and harder,” adds Roberts. “It is taking more mathematics and more and more CMOS gates, but Moore’s law is providing lots of CMOS gates.”
This is an updated and extended version of an article that first appeared in Optical Connections magazine earlier this year.