Acacia targets access networks with coherent QSFP-DD
- Acacia Communications has announced a 100-gigabit coherent QSFP-DD pluggable module.
- The module is the first of several for aggregation in the access network.
The second article addressing what next for coherent
Part 2: 100-gigabit coherent QSFP-DD
Acacia Communications has revisited 100-gigabit coherent but this time for access rather than metro networks.
Acacia’s metro 100-gigabit coherent pluggable product, a CFP, was launched in 2014. The pluggable has a reach from 80km to 1,200km and consumes 24-26W.
The latest coherent module is the first QSFP-DD to support a speed lower than the 400-gigabit 400ZR and ZR+ applications that have spurred the coherent pluggable market.
The launching of a 100-gigabit coherent QSFP-DD reflects a growing need to aggregate 10 Gigabit Ethernet (GbE) links at the network edge as 5G and fibre are deployed.
“The 10GbE links in all the different types of access networks highlight a need for a cost-effective way to do this aggregation,” says Tom Williams, vice president of marketing at Acacia.
Why coherent?
The deployment of 5G, business services, 10-gigabit passive optical networking (PON) and distributed access architecture (DAA) are driving greater traffic at the network edge.
Direct-detection optics is the main approach used for aggregation but Acacia argues coherent is now a contender.
Until now, Acacia has only been able to offer coherent metro products for access. The company believes a 100-gigabit coherent module is timely given the network edge traffic growth coupled with the QSFP-DD form factor being suited for the latest aggregation and switch platforms. Such platforms are not the high-capacity switches used in data centres yet port density still matters.
“We think we can trigger a tipping point and drive coherent adoption for these applications,” says Williams.
Using coherent brings robustness long associated with optical transport networks. “You just plug both ends in and it works,” he says.
In access, the quality of fibre in the network varies. With coherent, there is no need for an engineer to do detailed characterisations of the link thereby benefiting operational costs.
Adopting coherent technology for access also provides a way to scale. “You may only need 100 gigabits today but there is a clear path to 200 and 400 gigabit and the use of DWDM [dense wavelength-division multiplexing],” says Williams.
100-gigabit QDFP-DD
Acacia’s 100-gigabit QSFP-DD uses a temperature-controlled fixed laser and has a reach of 120km. The 120km span may rarely be needed in practice – up to 80km will meet most applications – but the extra margin will accommodate any vagaries in links.
The module uses Acacia’s 7nm CMOS low-power Greylock coherent digital signal processor (DSP). The Greylock is Acacia’s third-generation low power DSP chip that is used for its 400ZR and ZR+ modules.
The 100-gigabit QSFP-DD shares the same packaging as the 400ZR and ZR+ modules. The DSP, silicon-photonics photonic integrated circuit (PIC), modulator driver and trans-impedance amplifier (TIA) are all assembled into one package using chip-stacking techniques, what Acacia calls an opto-electronic multi-chip module (OEMCM).
“Everything other than the laser is in a single package,” says Williams. “The more we make optics look like electronics and the fewer interconnect points we have, the higher the reliability will be.”
The packaging approach brings size and optical performance benefits. The optics and DSP must be tightly coupled to ensure signal integrity as the symbol rates go up for 400-gigabit and soon 800-gigabit data rates. But this is less of an issue at 100-gigabit given the symbol rate is 32-gigabaud only.
Opportunities
The 100-gigabit QSFP-DD is now sampling and undergoing qualification. Acacia has yet to announce its general availability.
The company is planning other coherent modules for access including a tunable laser-based QSFP-DD as well as designs that meet various environmental requirements.
“We view coherent as moving into the access market and that will require solutions that address the entire market,” says Williams. That said, Acacia admits uncertainty remains as to how widely coherent will be adopted.
“The market has to play out and there are other competitive solutions,” says Williams. “We believe coherent will be the right solution but how that plays out near- to mid-term is uncertain.”
Books in 2019 - Final Part
Gazettabyte asks industry figures each year to cite the memorable books they have read. These include fiction, non-fiction and work-related titles.
In the second and final part, the recommendations during 2019 of Analysys Mason’s Dana Cooperson and Tom Williams from Acacia Communications are included.
Dana Cooperson, Research Director, Analysys Mason
I’ll cheat somewhat and go back several years when picking favourite books and then I’ll focus on titles read in 2019.
I’ve spent a lot of time over the past five years thinking about, helping my kids apply for, and paying for university education, so education-related books have been a focus.
My first recommendation is Excellent Sheep: The Miseducation of the American Elite and the Way to a Meaningful Life, by William Deresiewicz, an ex-professor and admissions counsellor at Yale.
I recommend it for its insight into the college admissions process, the business of US higher education, and how far some parents, prospective students, and colleges stray from what should be the goal: a good education. The recent “Varsity Blues” admissions scandal is a case in point.
The book, read after my first daughter’s run through the admissions obstacle course, validated my cynicism, but also left me and my younger daughter, who read it, empowered for our second attempt.
Three other education-related books offer different accounts of disadvantaged yet determined individuals who overcome challenging circumstances to become well-educated. And how friends and relatives can work to undermine those who strive for more. They also recount how difficult navigating the system can be for the disadvantaged and the crucial role of mentors.
Educated: A Memoir, by Tara Westover and Hillbilly Elegy: A Memoir of a Family and Culture in Crisis, by J. D. Vance, are well-known. These memoirs are insightful about the ‘anti-elite’, anti-education subcultures in the US (in Appalachia and survivalist Idaho, respectively).
Less well-known is A Hope in the Unseen: an American Odyssey from the Inner City to the Ivy League, by journalist Ron Suskind; by far my favourite of the three.
It traces the path of Cedric Jennings, a bright and determined African American boy from a poor, dangerous section of Washington, D.C., in the 1990s as he faces setback after setback in his quest for an education and a better life. It is a wonderfully written and deep book.
Other books gave me engrossing peeks into other eras, cultures, and species.
My 2019 reading started with Homegoing, by Yaa Gyasi, a story of two 18th century Ghanaian half-sisters, one of whom ends up enslaved in Mississippi. This epic novel spans eight generations of the sisters’ families and sheds light on the dark corners of the international slave trade and its legacy.
The central character of Eleanor Oliphant is Completely Fine, by Gail Honeyman, is smart, funny, and cringingly, endearingly quirky. The novel, set in present-day Scotland, has elements of a mystery as we slowly learn the roots of Eleanor’s trauma and just how twisted her psyche has become in her effort to outrun childhood tragedy.
I ended 2019 with A Gentleman in Moscow: A Novel, by Amor Towles. This novel’s plot spans Russian/ Soviet history from the Bolshevik revolution to the Cold War, and yet it unfolds almost entirely in a hotel.
Count Alexander Rostov, the titular protagonist, is an aristocrat whom the Bolsheviks deem a “former person” and sentenced to house arrest in Moscow’s Metropol Hotel. The Count, abetted by various friends and dogged by his chief antagonist, creates a life well-lived despite being a prisoner of the state. Here’s looking at you, Count!
Lastly, the book you didn’t know you needed to read about the species you didn’t know was so fascinating: The Soul of an Octopus: A Surprising Exploration into the Wonder of Consciousness, by journalist Sy Montgomery.
I’m never going to eat octopus again but that is a small price to pay for such an illuminating exposé on the physiology, lifecycle, and intelligence of the octopus; their personalities; and what we can learn about consciousness from a species alien to us.
Tom Williams, Vice President of marketing at Acacia Communications.
It may be a depressing story but the book that most impacted me in 2019 is entitled: What Made Maddy Run, by Kate Fagan.
It is a tragic story about a freshman, Madison Hollaran, at the University of Pennsylvania, who struggled with the pressures of freshman year as a scholarship athlete at an Ivy League school and committed suicide in her second semester.
Maddy seemed to have a perfect life as a star high-school athlete in soccer and track. She had a strong network of high-school friends and a supportive family, but she found herself lost at Penn and couldn’t find her way back to peace in her life.
Her family and close friends knew she was struggling but I don’t think anyone ever imagines events taking such a turn.
Maddy’s family provided the author with full access to her phone, computer and accounts. Stories from family and friends are interspersed with email and text discussions to provide a real sense of the pain she was struggling to communicate. Stitching these different strands together and the benefit of hindsight provide a fuller perspective.
As she approached her final act of desperation, several interactions presented themselves to offer her a different path out of the valley that she found herself in, but somehow she couldn’t recognise these opportunities. She had lost hope.
The book explores the pressures of freshman year, especially at an Ivy League school where students face a level of academic competitiveness never experienced before. Everyone there was at the top of their class in high-school.
In addition, athletes often feel the burden of living up to expectations to “earn” their scholarship. Their sport can become a responsibility or burden and no longer a source of enjoyment.
The book also explores how social media posts can disguise what someone like Maddie is feeling, making it even harder to recognise when a concerning situation has become a crisis.
As a parent of teenage daughters, I felt for her parents who knew she was struggling but didn’t know how to help. As parents, we want to fix our children’s problems, but as they approach adulthood, it is more difficult to have all the answers.
You know from the start how the book will end, but the chapter where she takes her life is as powerful as anything I’ve read. I can’t imagine how difficult it was for her family to provide the access to enable this book to be written, but I respect their strength and I hope it helps others in similar situations.
The book made a lasting impression on me.
Roy Rubenstein, Editor of Gazettabyte
I read some terrific titles in 2019 but none came close to the book What Dementia Teaches Us about Love, by Nicci Gerrard. (In the US, the title is The Last Ocean: A Journey Through Memory and Forgetting)
Gerrard is a journalist and novelist. She is also a co-founder of a campaign in the UK, named after her father, John, to allow carers to accompany dementia patients in hospitals. This follows her experience with her father who was left alone for days without visitors due to a virus outbreak.
Gerrard describes how, “… away from the home he loved, stripped of familiar routines and surrounded by strangers and machines, he swiftly lost his bearings and his fragile hold on himself. There is a great chasm between care and ‘care’, and my father fell into it.”
The book explores the disease – the gradual fragmentation of a person as they lose memory, language, recognition of their surroundings and, inevitability, their health.
But the book is more than that: it is a treatise on what it is to be human. What makes you, you? The grounding of memory and what it means to start forgetting. What is home? And the conflicting demands of caring: preserving the self while being endlessly drawn to caring for a loved one that is slowly losing and being lost.
The book is part memoir and part study. It is also sprinkled with moving human-interest stories. It may be hard to read at times but the book is uplifting.
Gerrard has written an original work on a topic that is not short of literature. Her writing also causes you to pause and reflect on what you’ve read.
For example, she starts the book with a story of how her father, after a decade of dementia, joins the family on a holiday in Sweden and visits a lake.
“My father, old and frail, swam out a few yards and then he started to sing. It is a song I’d never heard before, and never heard since …
“His self – bashed about by the years, picked apart by his dementia – was, in this moment of kindness, beyond language, consciousness and fear, lost and contained in the multiplicity of things and at home in the vast wonder of life.”
ECOC 2019 industry reflections II
Gazettabyte requested the thoughts of industry figures after attending the ECOC show, held in Dublin. In particular, what developments and trends they noted, what they learned and what, if anything, surprised them. Input from II-VI, Ciena, Fujitsu Optical Components and Acacia Communications. The second and final part.
State of play for 400 Gigabit Ethernet (GbE). Form factors ‘right-sized’ for faceplate densities
Sanjai Parthasarathi, chief marketing officer at II-VI
One new theme at ECOC is the demand for lower-cost 100-gigabit coherent transceivers for deployment in optical access for wireless access and fibre-deep cable TV. Such demand would significantly expand the market.
It was noteworthy at the show how 5G has become a significant factor influencing the wireless access market, with the potential for wide deployment of dense wavelength-division multiplexing (DWDM) technology with wavelength switching and tuning functions, not only in traditional network architectures but interesting new ones too.
This could drive significant demand for low-cost wavelength-selective switch (WSS) modules, tunable transceivers and 100-gigabit coherent transceivers, which is exciting.
As for surprises at the show, ECOC validated the view that developments in digital signal processor (DSP) technology for transceivers have accelerated to the point of having caught up with the state-of-the-art in photolithography, previously the province of DSPs for consumer electronics, high-performance computing and processors.
DSPs, for next-generation transceivers, are increasingly leveraging 7nm CMOS.
Patricia Bower, senior manager of product marketing at Ciena
A key talking point at ECOC was the state of play for 400 Gigabit Ethernet (GbE). Form factors ‘right-sized’ for faceplate densities – QSFP-DD, for example – and developments in short-range optical signalling supporting 100 gigabit-per-lambda are enablers for this next-generation client rate.
Market projections for 400GbE indicate a faster ramp for 400GbE than for 100GbE in previous years and that 400GbE client-side modules will ship in 2020 with broad, market-wide volumes ramping in 2021.
In parallel, 400-gigabit DWDM is projected to grow very strongly. Starting in early 2020, deployments of 800 gigabit-capacity DWDM systems will enable the industry to efficiently transport 400GbE anywhere in the network, including transoceanic propagation.
Following this, 400ZR will enable 400 gigabits-per-second over short point-to-point, single-span data centre interconnect links using coherent technology in the same compact QSFP-DD mechanical forms which will go hand-in-hand with the volume uptake of 400GbE.
Co-packaged optics
Discussions continued around approaches to package optics and electronics in switch-fabric ICs.
The consensus was that the approach will be mainstream in future 51.2 terabits-per-second (Tbps) switch chips, a couple of iterations from where we are today.
I learned more about the progress supporting wafer-scale manufacturability of co-packaged switch cores and optical input/ outputs, including on-chip laser integration.
Consideration of the relative trade-offs among power dissipation, cost, thermal management, and reliability compared to off-chip lasers are key. Electrical signalling also remains key in this approach. Even moving data off a chip package optically, electrical intra-chip signaling to the switching core is still needed for what effectively is a multi-chip module or modular system-on-chip.
Companies with key design skills in electrical and optical components will be best placed to address such designs.
I wasn’t surprised but pleased to see the progress by the industry for 400ZR demonstrated at the OIF booth. Various companies showed IC-TROSA electro-optic samples which is a contributing element for a 400ZR solution.
Mechanical mock-ups of the intended module packages (QSFP-DD and OSFP) were also shown as well as a mock-up of a switch-router platform to highlight 400ZR integration.
This level of progress is in line with the expected ramp-up of 400ZR in 2021.
Yukiharu Fuse, chief marketing officer, vice president/ general manager, business strategy division, Fujitsu Optical Components Limited
Several items were of interest at ECOC, but two I’d highlight are 400-gigabit coherent pluggable optics and XR Optics.
Vendors demonstrated the progress being made in the development of 400-gigabit coherent pluggable transceivers.
The key is their success is the development of a low-power coherent digital signal processor (DSP) that fits within a QSFP-DD or OSFP module, and this now seems feasible.
With this innovation, data centre operators will be able to install these modules in the slots used for client Ethernet, allowing the operators to support data centre interconnect without the need for transport gear.
The OIF-standardised 400ZR implementation will support linking data centres up to 120km apart using interoperable pluggable modules. The data centre operators also want longer reaches that ZR offers even if the power consumption of the transceiver inevitably goes up.
To address this, NEL and Acacia together with Lumentum and Fujitsu Optical Components introduced OpenZR+ to support longer distance links for data centre interconnect and other applications.
This will act as a potential de-facto standard with multi-source transceivers to support distances beyond ZR.
Such a development will be a big step for the data center operators, enabling wider coverage without the need for transport equipment.
XR Optics
Infinera introduced at ECOC a new concept of point-to-multi-point communications for access and aggregation network, dubbed XR Optics. Using Nyquist subcarriers, XR Optics can distribute up to 16 points according to the bandwidth requirements.
This concept may create a new market for coherent optics that until now has focussed on high-capacity, point-to-point applications.
Infinera introduced at ECOC a technology not a product. It will be interesting to see how the technology evolves into products and the support it gets with the goal of creating a multi-source supply chain.
I’m curious about the concept, though, with the key being how to achieve low-cost coherent optics needed for access and aggregation networks. I will watch this development with interest.
Tom Williams, vice president of marketing, Acacia Communications
We are seeing a trend toward increasing use of silicon photonics in client and transport optics. There are multiple approaches in the industry to address the challenges of power, size and cost, but silicon photonics has become established as an important technology for a variety of applications.
We were also happy to see the positive feedback for the OpenZR+ solution that we, in collaboration with several other companies, defined at the show.
I’ve participated in the 400ZR effort and the CableLabs project to define a coherent interface in access networks, so I was interested to learn more about the Infinera XR optics proposal. I’m still trying to understand the details, but it’s always interesting to see a different approach to solving a technical challenge.
As for unexpected developments at the show, I was surprised how difficult it can be to get a taxi in Dublin when Ariana Grande is in town!
Acacia heralds the era of terabit-plus optical channels
Acacia Communications has unveiled the AC1200-SC2 that delivers 1.2 terabits over a single optical channel.
The SC2 (single chip, single channel) is an upgrade of Acacia’s high-end AC1200 module. The AC1200 too is a 1.2-terabit module but uses two optical channels, each transmitting a 600-gigabit wavelength. The SC2 sends 1.2 terabits using two sub-carriers that fit within a single 150GHz-wide channel.
Each line is a data rate. Shown is the scope of how the baud rate and the modulation scheme can be varied and its impact on channel width, reach and data rate. Source: ADVA.
“In the SC2, we take care of everything so the user configures a single channel that is easier to manage in their network,” says Tom Williams, vice president of marketing at Acacia.
1.2-terabit channel
Acacia unveiled the AC1200 at the ECOC show in 2017. With its introduction, Acacia gained an advantage over its system-vendor rivals in bringing a 1.2-terabit coherent module to market using 600-gigabit wavelengths. The module supports up to 64-ary quadrature amplitude modulation (64-QAM) and a symbol rate of 69 gigabaud (GBd).
Systems vendors such as Ciena, with its WaveLogic 5, and Infinera, with its Infinite Coherent Engine 6 (ICE6), responded with their next-generation coherent designs that use symbol rates approaching 100GBd and support an 800-gigabit wavelength.
Sell-side research analysts interpreted the coherent developments as Acacia having a window of opportunity to exploit the AC1200 until the systems vendors’ coherent designs come to market in the coming year. The analysts also noted how 400 Gigabit Ethernet client signals better fit in an 800-gigabit wavelength compared to a 600-gigabit wavelength.
Then, in July, Acacia’s status as a merchant coherent technology supplier changed with the announcement that Cisco Systems is to acquire the company for $2.6 billion. Now, Acacia has detailed the SC2 as its acquisition awaits completion.
AC1200-SC2
The SC2 uses the same form factor and electrical connector as the AC1200 module, simplifying the upgrading of system designs using the AC1200. However, the SC2 module uses a single fibre pair for its optical output whereas the AC1200 uses two pairs, one for each channel.
The SC2 module shares the same Pico coherent digital signal processor (DSP) and baud rates as the AC1200. The Pico DSP uses fractional quadrature amplitude modulation (QAM) and an adjustable baud rate.
Fractional QAM allows the tuning of the transmitted data rate by using a mix of adjacent modulation formats. For example, 8-QAM and 16-QAM are alternated, and the percentage of time each is used determining the resulting data rate. In turn, the baud rate can be increased to widen the signal’s spectrum, if the optical channel permits, such that using a lower modulation scheme may become possible, improving the reach (see diagram above).
The AC1200 uses 50GHz- and 75GHz-wide channels while the SC2 uses 50-150GHz channels. For 600-gigabit and 1.2-terabit transmissions, the widest channels are used: 75GHz for the AC1200, and 150GHz for the SC2. “But as you go down in data rate, you can address the transmission in multiple ways,” says Williams. “You can run a higher modulation scheme in a narrow channel or, with a wider channel, run a lower modulation scheme to go further.”
The result optical performance means that the SC2 can be used for multiple applications: from short-span data centre interconnect where the full 1.2-terabit capacity is sent using 64-QAM, to metro-regional and long-haul distances using 800-gigabit and 16-QAM, all the way to ultra-long-haul terrestrial and subsea links with 400-gigabitand quadrature phase-shift keying (QPSK) modulation.
The AC1200 and the SC2 have comparable optical performance in terms of spectral efficiency and reach. This is unsurprising given how both modules use the same Pico DSP, baud rates and modulation schemes.
The AC1200 uses two 75GHz channels, each carrying 600 gigabits, to send 1.2 terabits, while the SC2 uses two sub-carriers in a 150GHz channel. However, the SC2 has a slight advantage since no guard band is needed between the two channels as is required with the AC1200 (unless the AC1200 is sending a two-channel ‘superchannel’ whereby no dead zone is needed between the channels).
Acacia is not detailing how it generates the optical sub-carriers besides saying the change stems from the interface between the Pico DSP and its silicon photonics-based photonic integrated circuit (PIC). The company will also not say if the SC2 uses a new PIC design.
Operational benefits
The fact that the SC2 and AC1200 deliver the same reach and capacity may explain why Acacia downplays the argument that the company has again leapfrogged its rivals with the advent of a module that sends 1.2 terabits over a single channel.
Instead, Acacia stresses the system and operational benefits resulting from doubling the data transmitted per channel.
“The SC2 module allows the entire capacity to be managed as a single channel,” says Williams. “The original [AC1200] module is well-suited to brownfield networks operating with 50GHz or 75GHz spacing, while the SC2 offers advantages in greenfield network architectures that can use channel plans up to 150GHz.”
Using a higher-capacity channel requires fewer optical components and reconfigurable optical add/ drop multiplexer (ROADM) ports thereby reducing networking costs, says Williams.
Using 150GHz-wide channels also aligns with an emerging consensus among network operators regarding wavelength roadmaps. “Network operators want to operate on some standardised grid based on regular multiples [50GHz, 75GHz] because it avoids fragmentation,” says Williams.
Availability
Acacia is already providing the SC2 module to certain customers that are undertaking validation testing. The firm is ready to ramp production based on particular customer demand.
Acacia will also be demonstrating its latest module at this week’s ECOC show being held in Dublin.
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.”
Acacia eyes pluggables as it demos its AC1200 module
The emerging market opportunity for pluggable coherent modules is causing companies to change their strategies.
Ciena is developing and plans to sell its own coherent modules. And now Acacia Communications, the coherent technology specialist, says it is considering changing its near-term coherent digital signal processor (DSP) roadmap to focus on coherent pluggables for data centre interconnect and metro applications.
Source: Gazettabyte
DSP roadmap
Acacia’s coherent DSP roadmap in recent years has alternated between an ASIC for low-power, shorter-reach applications followed by a DSP to address more demanding, long-haul applications.
In 2014, Acacia announced its Sky 100-gigabit DSP for metro applications that was followed in 2015 by its Denali dual-core DSP that powers its 400-gigabit AC-400 5x7-inch module. Then, in 2016, Acacia unveiled its low-power Meru, used within its pluggable CFP2-DCO modules. The high-end 1.2-terabit dual-core Pico DSP used for Acacia’s board-mounted AC1200 coherent module was unveiled in 2017.
“The 400ZR is our next focus,” says Tom Williams, senior director of marketing at Acacia.
The 400ZR standard, promoted by the large internet content providers, is being developed to link switches in separate data centres up to 80km apart. Acacia’s subsequent coherent DSP that follows the 400ZR may also target pluggable applications such as 400-gigabit CFP2-DCO modules that will span metro and metro-regional distances.
“There is a trend to pluggable, not just the 400ZR but the CFP2-DCO [400-gigabit] for metro,” says Williams. “We are still evaluating whether that causes a shift in our overall cadence and DSP development.”
AC1200 trials
Meanwhile, Acacia has announced the results of two transatlantic trials involving its AC1200 module whose production is now ramping.
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“There is a trend to pluggable, not just the 400ZR but the CFP2-DCO [400-gigabit] for metro”
In the first trial, Acacia, working with ADVA, transmitted a 300-gigabit signal over a 6,800km submarine cable. The 300-gigabit wavelength occupied a 70GHz channel and used ADVA’s Teraflex technology, part of ADVA’s FSP 3000 CloudConnect platform. Teraflex is a one-rack-unit (1RU) stackable chassis that supports three hot-pluggable 1.2-terabit sleds, each sled incorporating an Acacia AC1200 module.
In a separate trial, the AC1200 was used to send a 400-gigabit signal over 6,600km using the Marea submarine cable. Marea is a joint project between Microsoft, Facebook and Telxius that links the US and Spain. The cable is designed for performance and uses an open line system, says Williams: “It is not tailored to a particular company’s [transport] solution”.
The AC1200 module - 40 percent smaller than the 5x7-inch AC400 module - uses Acacia’s patented Fractional QAM (quadrature amplitude modulation) technology. The technology uses probabilistic constellation shaping that allows for non-integer constellations. “Instead of 3 or 4 bits-per-symbol, you can have 3.56 bits-per-symbol,” says Williams.
Acacia’s Fractional QAM also uses an adaptive baud rate. For the trial, the 400-gigabit wavelength was sent using the maximum baud rate of just under 70 gigabaud. Using the baud rate to the full allows a lower constellation to be used for the 400-gigabit wavelength thereby achieving the best optical signal-to-noise ratio (OSNR) and hence reach.
In a second demonstration using the Marea cable, Acacia demonstrated a smaller-width channel in order to maximise the overall capacity sent down the fibre. Here, a lower baud rate/ higher constellation combination was used to achieve a spectral efficiency of 6.41 bits-per-second-per-Hertz (b/s/Hz). “If you built out all the channels [on the fibre], you achieve of the order of 27 terabits,” says Williams.
Pluggable coherent
The 400ZR will be implemented using the same OSFP and QSFP-DD pluggable modules used for 400-gigabit client-side interfaces. This is why an advanced 7nm CMOS process is needed to implement the 400ZR DSP so that its power consumption will be sufficiently low to meet the modules’ power envelopes when integrated with Acacia’s silicon-photonics optics.
There is also industry talk of a ZR+, a pluggable module with a reach exceeding80km. “At ECOC, there was more talk about the ZR+,” says Williams. “We will see if it becomes standardised or just additional proprietary performance.”
Another development is the 400-gigabit CFP2-DCO. At present, the CFP2-DCO delivers up to 200-gigabitwavelengths but the standard, as defined by the Optical Internetworking Forum (OIF), also supports 400 gigabits.
Williams says that there a greater urgency to develop the 400ZR than the 400-gigabit CFP2-DCO. “People would like to ramp the ZR pretty close to the timing of the 400-gigabit client-side interfaces,” says Williams. And that is likely to be from mid-2019.
In contrast, the 400-gigabit CFP2-DCO pluggable while wanted by carriers for metro applications, is not locked to any other infrastructure build-out, says Williams.
Coherent optics players target the network edge for growth
Part 1: Coherent developments
The market for optical links for reaches between 10km and 120km is emerging as a fierce battleground between proponents of coherent and direct-detection technologies.
Interest in higher data rates such as 400 gigabits is pushing coherent-based optical transmission from its traditional long-distance berth to shorter-reach applications. “That tends to be where the growth for coherent has come from as it has migrated from long-haul to metro,” says Tom Williams, senior director of marketing at Acacia Communications, a coherent technology supplier.
Source: Acacia Communications, Gazettabyte
Williams points to the Optical Internetworking Forum’s (OIF) ongoing work to develop a 400-gigabit link for data centre interconnect. Dubbed 400ZR, the project is specifying an interoperable coherent interface that will support dense wavelength-division multiplexing (DWDM) links for distances of at least 80km.
Meanwhile, the IEEE standards group defining 400 Gigabit Ethernet has issued a Call-For-Interest to determine whether to form a Study Group to look at 400-Gigabit applications beyond the currently defined 10km 400GBASE-LR8 interface.
“Coherent moving to higher-volume, shorter-reach solutions shows it is not just a Cadillac product,” says Williams. Higher-volume markets will also be needed to fund coherent chip designs using advanced CMOS process nodes. “Seven nanometer [CMOS] becomes a very expensive prospect,” says Williams. “The traditional business case is not going to be there without finding higher volumes.”
Coherent moving to higher-volume, shorter-reach solutions shows it is not just a Cadillac product
Pico DSP
Acacia detailed its next-generation high-end coherent digital signal processor (DSP) at the OFC show held in Los Angeles in March.
Tom WilliamsDubbed Pico, the DSP will support transmission speeds of up to 1.2 terabits-per-second using two carriers, each carrying 600 gigabits of data implemented using 64-ary quadrature amplitude modulation (64QAM) and a 64 gigabaud symbol rate. The 16nm CMOS dual-core DSP also features an internal crossbar switch to support a range of 100-gigabit and 400-gigabit client interfaces.
ADVA Optical Networking is using the Pico for its Teraflex data centre interconnect product. The Teraflex design supports 3.6 terabits of line-side capacity in a single rack unit (1RU). Each 1RU houses three “sleds”, each supporting two wavelengths operating at up to 600 gigabits-per-second (Gbps).
But ADVA Optical Networking also detailed at OFC its work with leading direct-detection technology proponents, Inphi and Ranovus. For the data centre interconnect market, there is interest in coherent and direct-detection technologies, says ADVA.
Detailing the Pico coherent DSP before it is launched as a product is a new development for Acacia. “We knew there would be speculation about ADVA’s Teraflex technology and we preferred to be up front about it,” says Williams.
The 16nm Pico chip was also linked to an Acacia post-deadline paper at OFC detailing the company’s progress in packaging its silicon photonics chips using ball grid array (BGA) technology. Williams stresses that process issues remain before its photonic integrated circuit (PIC) products will use BGA packaging, an approach that will simplify and reduce manufacturing costs.
“You are no longer running the board with all the electronics through a surface mount line and then have technicians manually solder on the optics,” says Williams. Moreover, BGA packaging will lead to greater signal integrity, an important consideration as the data rates between the coherent DSP and the PIC increase.
It is an endorsement of our model but I do not think it is the same as ours. You still have to have someone providing the DSP and someone else doing the optics
Coherent competition
Ciena's recent announcement that it is sharing its WaveLogic Ai coherent DSP technology with optical module vendors Lumentum, Oclaro and NeoPhotonics is seen as a response to Acacia’s success as a merchant supplier of coherent modules and coherent DSP technologies.
Williams says Acacia’s strategy remains the same when asked about the impact of the partnership between Ciena and the optical module makers: to continue being first to market with differentiated products.
One factor that has helped Acacia compete with merchant suppliers of coherent DSPs - NEL and ClariPhy, now acquired by Inphi - is that it also designs the silicon photonics-based optics used in its modules. This allows a trade-off between the DSP and the optics to benefit the overall system design.
A challenge facing the three optical module makers working with Ciena is that each one will have to go off and optimise their design, says Williams. “It is an endorsement of our model but I do not think it is the same as ours,” he says. “You still have to have someone providing the DSP and someone else doing the optics.”
Coherent roadmap
Acacia has managed to launch a new coherent DSP product every year since 2011 (see diagram, above). In 2015 it launched its Denali DSP, the first to operate at line rates greater than 100Gbps.
Last year it announced the Meru, a low-power DSP for its CFP2-DCO module. The CFP2-DCO operates at 100Gbps using polarisation multiplexing, quadrature phase-shift keying, (PM-QPSK) and two 200Gbps modes: one using 16-ary quadrature amplitude modulation (PM-16QAM) and a longer reach variant, implemented using a higher baud rate and 8-ary quadrature amplitude modulation (PM-8QAM). The CFP2-DCO is already starting to be designed into platforms.
Since 2014, Acacia has launched a low-power DSP design every even year and a high-end DSP every odd year, with the Pico being the latest example.
Acacia has not said when the Pico coherent DSP will be generally available but ADVA Optical Networking has said it expects to launch the Teraflex in early 2018.
Acacia looks to co-package its coherent PIC and DSP-ASIC
- Acacia Communications is working to co-package its coherent DSP and its silicon photonics transceiver chip.
- The company is also developing a digital coherent optics module that will support 400 gigabit.
Acacia Communications is working to co-package its coherent DSP and its silicon photonics transceiver chip. The line-side optical transceiver company is working on a digital coherent optics module that will support 400 gigabits.
Acacia announced last November that it was sampling the industry’s first CFP2 Digital Coherent Optics (CFP2-DCO) that supports 100- and 200-gigabit line rates. The CFP2-DCO integrates the DSP and its silicon photonics chip within a CFP2 module, which is half the size of a CFP module, with each chip packaged separately.
The CFP2-DCO adds to the company’s CFP2-ACO design that was announced a year ago. In the CFP2-ACO, the CFP2 module contains just the optics with the DSP-ASIC chip on the same line card connected to the module via a special high-speed interface connector.
Now, Acacia is working to co-package the two chips, which will not only improve the performance of its CFP2-DCO but also enable new, higher-performance optical modules such as a 400-gigabit DCO. The Optical Internetworking Forum announced a new implementation agreement last December for an interoperable 400-gigabit ZR (80km) coherent interface.
Both [the DSP and silicon photonics chip] are based on CMOS processes. The next step for Acacia is to bring them into a single package.
Portfolio upgrades
Acacia has also upgraded its existing portfolio of coherent transceivers. The company has integrated the enhanced silicon photonics coherent transceiver in its AC100-CFP and its AC-400 5x7-inch modules.
The silicon-photonics transceiver achieves a more efficient coupling of light in and out of the chip and uses an improved modulator driver design that reduces the overall power consumption. The design also supports flexible grid, enabling channel sizes of 37.5GHz in addition to fixed-grid 50GHz channels.
The resulting AC100-CFP module has a greater reach of 2,500km and a lower power consumption than the first generation design announced in 2014. The enhanced PIC has also been integrated within the AC-400. The AC-400, announced in 2015, integrates two silicon photonics chips to support line rates of 200, 300 and 400 gigabits.
CFP2-DCO
Acacia is using the coherent transceiver photonic integrated circuit (PIC), first used in its CFP2-ACO, alongside a new coherent DSP to integrate the optics and DSP within the compact CFP2.
“The third-generation PIC is a mini PIC; in a gold box that is about the size of a dime, which is a third of the size of our original PIC,” says Benny Mikkelsen, founder and CTO of Acacia.
One design challenge with its latest DSP was retaining the reach of the original DSP used in the AC100-CFP while lowering its power consumption. Having an inherently low-power coherent DSP design in the first place is one important factor. Mikkelsen says this is achieved based on several factors such as the DSP algorithms chosen and how they are implemented in hardware, the clock frequencies used within the chip, how the internal busses are implemented, and the choice of bits-per-symbol used for the processing.
The resulting DSP’s power consumption can be further reduced by using an advanced CMOS process. Acacia uses a 16nm CMOS process for its latest DSP.
Other challenges to enable a CFP2-DCO module include reducing the power consumption of the optics and reducing the packaging size. “The modulator driver is the piece part that consumes the most power on the optics side,” says Mikkelsen.
Acacia's CFP2-DCO supports polarisation multiplexing, quadrature phase-shift keying (PM-QPSK) for 100 gigabits, and two modulation schemes: polarisation multiplexing, 8-ary quadrature amplitude multiplexing (PM-8QAM) and 16-ary QAM - for 200-gigabit line rates. In contrast, its -ACO supports just PM-QPSK and PM-16QAM.
At 100 gigabits, the DSP consumes about half the power of the Sky DSP used in the original AC100. Using PM-8QAM for 200 gigabits means the new DSP and optics support a higher baud rate - some 45 gigabaud compared to the traditional 32-35 gigabaud used for 100 and 200-gigabit transmission. However, while this increases the power consumption, the benefit of 8QAM is a 200-gigabit reach beyond 1,000km.
Mikkelsen stresses that a key reason the company can achieve a CFP2-DCO design is having both technologies in-house: “You can co-optimise the DSP and the silicon photonics”.
We think, at least in the near term, that the OSPF module seems to be a good form factor to work on
ACO versus DCO
Since Acacia now offers both the CFP2-ACO and CFP2-DCO modules, it is less concerned about how the relative demand for the two modules develops. “We don’t care too much which one is going to have the majority of the market,” says Mikkelsen. That said, Acacia believes that the CFP2-DCO market will become the larger of the two.
When the CFP2-ACO was first considered several years ago, the systems vendors and optical module makers shared a common interest. Systems vendors wanted to use their custom coherent DSP-ASICs while the -ACO module allowed component makers that didn't have the resources to develop their own DSP to address the market with their optics. It was also necessary to separate the DSP and the optics if the smaller CFP2 form factor was to be used.
But bringing CFP2-ACOs to volume production has proved more difficult than first envisaged. The CFP2-DCO is far easier to use, says Mikkelsen. The module can be plugged straight into equipment whereas the CFP2-ACO must be calibrated by a skilled optical engineer when a wavelength is first turned up.
Future work
Acacia is now looking at new module form factors and new packaging technologies. “Both [the DSP and silicon photonics chip] are based on CMOS processes,” says Mikkelsen. “The next step for Acacia is to bring them into a single package.”
In addition to the smaller size, a single package promises a slightly lower power consumption as well as manufacturing cost advantages. “We also expect to see higher performance once the DSP and optics are sitting next to each other which we believe will improve signal integrity between the two,” says Mikkelsen.
Acacia is not waiting for any industry challenges to be overcome for a single-package design to be achieved. The company points out that its silicon photonics chip is not temperature sensitive, aiding its co-packaging with the DSP.
Acacia is working on a 400-gigabit DCO design and is looking at several potential module types. The company is a member of the OSFP module MSA as well as the Consortium of On-Board Optics (COBO) which has started a coherent working group. “We think, at least in the near term, that the OSPF module seems to be a good form factor to work on,” says Mikkelsen.
Enabling coherent optics down to 2km short-reach links
Interview 5: Chris Doerr
Chris Doerr admits he was a relative latecomer to silicon photonics. But after making his first silicon photonics chip, he was hooked. Nearly a decade later and Doerr is associate vice president of integrated photonics at Acacia Communications. The company uses silicon photonics for its long-distance optical coherent transceivers.
Chris Doerr in the lab
Acacia Communications made headlines in May after completing an initial public offering (IPO), raising approximately $105 million for the company. Technology company IPOs have become a rarity and are not always successful. On its first day of trading, Acacia’s shares opened at $29 per share and closed just under $31.
Although investors may not have understood the subtleties of silicon photonics or coherent DSP-ASICs for that matter, they noted that Acacia has been profitable since 2013. But as becomes clear in talking to Doerr, silicon photonics plays an important role in the company’s coherent transceiver design, and its full potential for coherent has still to be realised.
Bell Labs
Doerr was at Bell Labs for 17 years before joining Acacia in 2011. He spent the majority of his time at Bell Labs making first indium phosphide-based optical devices and then also planar lightwave circuits. One of his bosses at Bell Labs was Y.K. Chen. Chen had arranged a silicon photonics foundry run and asked Doerr if he wanted to submit a design.
What hooked Doerr was silicon photonics’ high yields. He could assume every device was good, whereas when making complex indium phosphide designs, he would have to test maybe five or six devices before finding a working one. And because the yields were high, he could focus more on the design aspects. “Then you could start to make very complex designs - devices with many elements - with confidence,” he says.
Another benefit was that the performance of the silicon photonic circuit matched closely its simulation results. “Indium phosphide is so complex,” he says. “You have to worry about the composition effects and the etching is not that precise.” With silicon, in contrast, the dimensions and the refractive index are known with precision. “You can simulate and design very precisely, which made it [the whole process] richer,” says Doerr.
Silicon photonics is a disruptive technology because of its ability to integrate so many things together and still be high yield and get the raw performance
After that first wafer run, Doerr continued to design both planar lightwave circuits and indium phosphide components at Bell Labs. But soon it was solely silicon photonics ICs.
Doerr views Acacia’s volume production of an integrated coherent transceiver - the transmit and receive optics on the one chip - with a performance that matches discrete optical designs, as one of silicon photonics’ most notable achievements to date.
With a discrete component coherent design, you can use the best of each material, he explains, whereas with an integrated design, compromises are inevitable. “You can’t optimise the layer structure; each component has to share the wafer structure,” he says. Yet with silicon photonics, the design space is so powerful and high-yielding, that these compromises are readily overcome.
Doerr also describes a key moment when he realised the potential of silicon photonics for volume manufacturing.
He was reading an academic paper on grating couplers, a structure used to couple fibres to waveguides. “You can only make that in silicon photonics because you need a high vertical [refractive] index contrast,” he says. Technically, a grating coupler can also be made in indium phosphide but the material has to be cut from under the waveguide; this leaves the waveguide suspended in air.
When he first heard of grating couplers he assumed the coupling efficiency would be of the order of a few percent whereas in practice it is closer to 85 percent. “That is when I realised it is a very powerful concept,” he says.
Integration is key
Doerr pauses before giving measured answers to questions about silicon photonics. Nor does his enthusiasm for silicon photonics blinker him to the challenges it faces. However, his optimism regarding the technology’s future is clear.
“Silicon photonics is a disruptive technology because of its ability to integrate so many things together and still be high yield and get the raw performance,” he says. In the industry, silicon photonics has proven itself for such applications as metro telecommunications but it faces significant competition from established technologies such as indium phosphide. It will require more channels to be integrated for the full potential of silicon photonics as a disruption technology to emerge, says Doerr.
Silicon photonics also has an advantage on indium phosphide in that it can be integrated with electronic ICs using 2.5D and 3D packaging, saving cost, footprint, and power. “If you are in the same material system then such system-in-package is easier,” he says. Also, silicon photonic integrated circuits do not require temperature control, unlike indium phosphide modulators, which saves power.
Areas of focus
One silicon photonics issue is the need for an external laser. For coherent transceivers, it is better to separate the laser from the high-speed optics due to the fact that the coherent DSP-ASIC and the photonic chips are hot and the laser requires temperature control.
For applications such as very short reach links, silicon photonics needs a laser source and while there are many options to integrate the laser to the chip, a clear winning approach has yet to emerge. “Until a really low cost solution is found, it precludes silicon from competing with really low-cost solutions like VCSELs for very short reach applications,” he says.
Silicon photonic chip volumes are still many orders of magnitude fewer than those of electronic ICs. But Acacia says foundries already have silicon photonics lines running, and as these foundries ramp volumes, costs, production times, and node-sizes will continually improve.
Opportunities
The adoption of silicon photonics will increase significantly as more and more functions are integrated onto devices. For coherent designs, Doerr can foresee silicon photonics further reducing the size, cost and power consumption, making them competitive with other optical transceiver technologies for distances as short as 2km.
“You can use high-order formats such as 256-QAM and achieve very high spectral efficiency,” says Doerr. Using such a modulation scheme would require fewer overall lasers to achieve significant transport capacities, improving the cost-per-bit performance for applications such as data centre interconnect. “Fibre is expensive so the more you can squeeze down a fibre, the better,” he says.
Doerr also highlights other opportunities for silicon photonics, beyond communications. Medical applications is one such area. He cites a post-deadline paper at OFC 2016 from Acacia on optical coherent tomography which has similarities with the coherent technology used in telecom.
Longer term, he sees silicon photonics enabling optical input/ output (I/O) between chips. As further evolutionary improvements are achieved, he can see lasers being used externally to the chip to power such I/O. “That could become very high volume,” he says.
However, he expects 3D stacking of chips to take hold first. “That is the easier way,” he says.
Acacia unveils 400 Gigabit coherent transceiver
- The AC-400 5x7 inch MSA transceiver is a dual-carrier design
- Modulation formats supported include PM-QPSK, PM-8-QAM and PM-16-QAM
- Acacia’s DSP-ASIC is a 1.3 billion transistor dual-core chip
Acacia Communications has unveiled the industry's first flexible rate transceiver in a 5x7-inch MSA form factor that is capable of up to 400 Gigabit transmission rates. The company made the announcement at the OFC show held in Los Angeles.
Dubbed the AC-400, the transceiver supports 200, 300 and 400 Gigabit rates and includes two silicon photonics chips, each implementing single-carrier optical transmission, and a coherent DSP-ASIC. Acacia designs its own silicon photonics and DSP-ASIC ICs.
"The ASIC continues to drive performance while the optics continues to drive cost leadership," says Raj Shanmugaraj, Acacia's president and CEO.
The AC-400 uses several modulation formats that offer various capacity-reach options. The dual-carrier transceiver supports 200 Gig using polarisation multiplexing, quadrature phase-shift keying (PM-QPSK) and 400 Gig using 16-quadrature amplitude modulation (PM-16-QAM). The 16-QAM option is used primarily for data centre interconnect for distances up to a few hundred kilometers, says Benny Mikkelsen, co-founder and CTO of Acacia: "16-QAM provides the lowest cost-per-bit but goes shorter distances than QPSK."
Acacia has also implemented a third, intermediate format - PM-8-QAM - that improves reach compared to 16-QAM but encodes three bits per symbol (a total of 300 Gig) instead of 16-QAM's four bits (400 Gig). "8-QAM is a great compromise between 16-QAM and QPSK," says Mikkelsen. "It supports regional and even long-haul distances but with 50 percent higher capacity than QPSK." Acacia says one of its customer will use PM-8-QAM for a 10,000 km submarine cable application.
Source: Gazettabyte
Other AC-400 transceiver features include OTN framing and forward error correction. The OTN framing can carry 100 Gigabit Ethernet and OTU4 signals as well as the newer OTUc1 format that allows client signals to be synchronised such that a 400 Gigabit flow from a router port can be carried, for example. The FEC options include a 15 percent overhead code for metro and a 25 percent overhead code for submarine applications.
The 28 nm CMOS DSP-ASIC features two cores to process the dual-carrier signals. According to Acacia, its customers claim the DSP-ASIC has a power consumption less than half that of its competitors. The ASIC used for Acacia’s AC-100 CFP pluggable transceiver announced a year ago consumes 12-15W and is the basis of its latest DSP design, suggesting an overall power consumption of 25 to 30+ Watts. Acacia has not provided power consumption figures and points out that since the device implements multiple modes, the power consumption varies.
The AC-400 uses two silicon photonics chips, one for each carrier. The design, Acacia's second generation photonic integrated circuit (PIC), has a reduced insertion loss such that it can now achieve submarine transmission reaches. "Its performance is on a par with lithium niobate [modulators]," says Mikkelsen.
It has been surprising to us, and probably even more surprising to our customers, how well silicon photonics is performing
The PIC’s basic optical building blocks - the modulators and the photo-detectors - have not been changed from the first-generation design. What has been improved is how light enters and exits the PIC, thereby reducing the coupling loss. The latest PIC has the same pin-out and fits in the same gold box as the first-generation design. "It has been surprising to us, and probably even more surprising to our customers, how well silicon photonics is performing," says Mikkelsen.
Acacia has not tried to integrate the two wavelength circuits on one PIC. "At this point we don't see a lot of cost savings doing that," says Mikkelsen. "Will we do that at some point in future? I don't know." Since there needs to be an ASIC associated with each channel, there is little benefit in having a highly integrated PIC followed by several discrete DSP-ASICs, one per channel.
The start-up now offers several optical module products. Its original 5x7 inch AC-100 MSA for long-haul applications is used by over 10 customers, while it has two 5x7 inch modules for submarine operating at 40 Gig and 100 Gig are used by two of the largest submarine network operators. Its more recent AC-100 CFP has been adopted by over 15 customers. These include most of the tier 1 carriers, says Acacia, and some content service providers. The AC-100 CFP has also been demonstrated working with Fujitsu Optical Components's CFP that uses NTT Electronics's DSP-ASIC. Acacia expects to ship 15,000 AC-100 coherent CFPs this year.
Each of the company's module products uses a custom DSP-ASIC such that Acacia has designed five coherent modems in as many years. "This is how we believe we out-compete the competition," says Shanmugaraj.
Meanwhile, Acacia’s coherent AC-400 MSA module is now sampling and will be generally available in the second quarter.