Professor Graham Reed: The calm before the storm
Silicon photonics luminaries series
Interview 3: Professor Graham Reed
Despite a half-century track record driving technology, electronics is increasingly calling upon optics for help. “It seems to me that this is a marriage that is really going to define the future,” says Graham Reed, professor of silicon photonics at the University of Southampton’s Optoelectronics Research Centre.
The optics alongside the electronics does not have to be silicon photonics, he says, but silicon as a photonics technology is attractive for several reasons.
“What makes silicon photonics interesting is its promise to enable low-cost manufacturing, an important requirement for emerging consumer applications,” says Reed. And being silicon-based, it is much more compatible than other photonics technologies. “It probably means silicon photonics is going to win out,” he says.
From Surrey to Southampton
Reed has been active in silicon photonics for over 25 years. As an academic at the University of Surrey, his first Ph.D. student was Andrew Rickman, who went on to found Bookham Technology and is now CEO of Rockley Photonics.
Rickman undertook the study of basic optical waveguide structures using silicon. “The first data we got, the waveguide losses were very high, 20 to 30dB per centimetre,” says Reed. “Within a year, we got the losses down to below 1dB per centimetre; that makes it viable.”
The research then broadened to include silicon modulators, a research topic Reed continues to this day.
Everything about silicon photonics is about low cost
The optical modulator is silicon photonics biggest achievement to date, argues Reed. “We were working on modulators in 1991 that worked at 20 megahertz,” he says. “Intel’s Mario Paniccia ribbed me when they got [a modulator] to 1 gigahertz.”
The Surrey group was not focussing on telecom when they started. “I never believed in the early 1990s that these things were going to go as fast as they became,” says Reed. Partly that was because the early work used much larger waveguides and to increase speed, the dimensions need to shrink.
In 2012, Reed and a dozen colleagues moved from the University of Surrey to the University of Southampton. Several factors led to the move. The University of Southampton was interested in the team, given its reputation and the rising importance of silicon photonics, while Reed was keen to make use of the university’s new on-site fabrication plant, which he describes as the best university fab in the UK and probably Europe.
“We were increasing frustrated with the fab facilities around the world,” says Reed. The team used multi-project wafers where companies and institutions have their circuits made on a shared wafer. However, such multi-project wafers have a lower run priority.
“Foundries do a good job but they often take much longer to deliver [the designs] than they aim,” says Reed. Worst case, it can take over three years to receive the chip design back. Given a project cycle typically lasts three years, this is a non-starter, he says: “Having a fab that you have a lot of control over is a big attraction”.
Research focus
Reed’s group is regularly approached by companies from all over the world. But it wasn't always like that. In the 1990s, getting funding to research silicon photonics was a challenge, he says.
The companies now contacting Reed’s group are either in the field and have a difficulty, or they want to enter the marketplace. “They want particular work done or a particular device worked upon,” he says.
Intel is one company that worked with Reed when they started their silicon photonics programme some dozen years ago.
Reed’s group’s research covers the development of individual optical components as well as systems. Much of the work is focussed on telecom and datacom, given that is where silicon photonics is most established, but the group is also conducting work using silicon photonics for longer wavelengths - 2 to 18 microns - known as the mid infra-red region.
Mid infra-red is an emerging field, says Reed: “People have seen the success of existing silicon photonics and are applying it to longer wavelengths.”
Such wavelengths are suited for sensing applications. “A lot of nasties - chemicals you’d want to sense - have characteristic absorption lines in this longer wavelength range,” he says.
Things also become easier at the longer wavelengths because the dimensions of the silicon features are more relaxed. However, additional materials are required that are transparent at these longer wavelengths, and these platforms all need developing. “Longer wavelengths equate to bigger waveguides; what gets more difficult are the sources and the detectors,” says Reed.
A third research activity his group is tackling is ongoing silicon photonics challenges such as wafer-scale testing, passive alignment, lowering power consumption and thermal stability issues.
Optical device work
Reed cites a low-channel-count multiplexer as an example of its research work on basic optical devices with the goal of helping commercialise silicon photonics.
“One of the issues in silicon photonics is to make things reliable and high yield,” says Reed. “One way to look at that is you need simplicity.”
The group has developed an angled multi-mode interference (MMI) multiplexer suited for 4 or 8 channel designs.
“It is so simple,” says Reed. The multiplexer is made in a single etch step and is based on large multi-mode waveguides that are more resilient to fabrication errors and layer thickness variations. The design is also more thermally stable than single-mode waveguides.
Another area is ring resonators - useful devices that can be used for a variety of tasks including modulation but which are sensitive to layer thickness variations as well as thermal stability issues. “If anyone is going to adopt ring resonators they need to find a way to make them athermal,” says Reed. “And they need a way to tune or trim to operate them to the resonance they need.”
Systems work
The group’s systems work addresses some of the same issues as the large systems vendors. However, the group is careful in the topics it chooses given their more modest university resources. “We are looking at more complex modulation systems but probably not for long haul communications,” says Reed.
Another research activity is looking at alternative ways to combine components. Using silicon photonics for integration in the mid infra-red range may give a new lease of life to the lab-on-a-chip concept. “People have talked about it for a long while but it hasn't really happened,” says Reed. “If you can do these things in a reliable and low-cost manner, maybe disposable chips are viable again.”
Silicon photonics challenges
Two current manufacturing challenges Reed highlights are the issues of passive alignment and wafer-scale testing.
Coupling the laser to a fibre or the silicon chip’s waveguide using passive alignment remains an ongoing challenge. “Everything about silicon photonics is about low cost,” says Reed. At present to attach a laser, it is typically turned on and aligned to the chip’s waveguide. This requires manual intervention and is time-consuming.
“The ideal scenario is to put a fibre down and it couples to the waveguide or laser and somehow you have aligned it,” he says. The challenge is the discrepancy in dimensions between the 10-micron fibre core and the waveguide, which is typically between 0.35- and 0.5-microns wide. Work is on-going to use mode converters or grating couplers such that the resulting optical loss is low enough to make passive alignment viable.
All these events are consistent with this field of technology pointing to mass markets
Wafer-scale testing remains another challenge. Grating couplers are one way designs can be tested while still on the silicon wafer. But these typically only allow the whole circuit to be tested - either it works or not - but you can’t test individual components. “If you are going to mimic the successes of electronics, you need to test more comprehensibly than that,” says Reed.
His group has developed an erasable grating that can be placed either side of a critical component to test it. These gratings can then be removed from the final circuit by using local laser annealing.
Reed expects the industry to overcome all these manufacturing challenges: “But it still means somebody has to have the brilliant idea”.
He is also somewhat surprised that there are not more silicon photonics products on the market, especially considering the huge investment in the technology made by some of the larger companies over the last decade.
He describes what is happening now as silicon photonics’ quiet period. Partly it is due to the vendors working to commercialise their technologies, partly it is the systems vendors that are developing next-generation products are evaluating the various technologies. “Until somebody jumps and that market takes off - and somebody will jump,” he says. “Then there will be ferocious activity.”
Opportunities
Reed is measured when assessing the future opportunities for the technology.
“It is not something that we strategise about - it is not what we do - but we get insights from time to time because of the people we work with and what they want,” he says. “The crucial thing is what facilitates the mass market because silicon photonics is really trying to bring photonics to the mass market.”
Reed does believe silicon photonics is disruptive: “If you look at the origins of what a disruptive technology is, it is a technology that works in one field but then it performs so well, it crosses the boundary into other areas”.
Silicon photonics was initially regarded as a short-reach technology but once the performance of its modulators started to drastically increase, the technology crossed the boundary into long-haul research, he notes. “That is the definition of a disruptive technology,” he says.
He also believes the technology has passed its tipping point. As evidence, he points to the investment made by the large companies and says it is inevitable that they will launch products: “So in that sense, the tipping point has already been and gone”.
In addition, he highlights the American Institute for Manufacturing Integrated Photonics (AIM Photonics) venture, the $610 million public and private funded initiative set up in 2015 to advance silicon photonics-based manufacturing.
“All these events are consistent with this field of technology pointing to mass markets,” says Reed. “If this was going to be indium phosphide that did that, why did not all that activity happen years ago?”
Start-up Sicoya targets chip-to-chip interfaces
“The trend we are seeing is the optics moving very close to the processor,” says Sven Otte, Sicoya’s CEO.
Sicoya was founded last year and raised €3.5 million ($3.9 million) towards the end of 2015. Many of the company’s dozen staff previously worked at the Technical University of Berlin. Sicoya expects to grow the company’s staff to 20 by the year end.
Otte says a general goal shared by silicon photonics developers is to combine the optics with the processor but that the industry is not there yet. “Both are different chip technologies and they are not necessarily compatible,” he says. “Instead we want the ASPIC very close to the processor or even co-packaged in a system-in-package design.”
Vertical-cavity surface-emitting lasers (VCSELs) are used for embedded optics placed alongside chips. VCSELs are inexpensive to make, says Otte, but they need to be packaged with driver chips. A VCSEL also needs to be efficiently coupled to the fibre which also requires separate lenses. ”These are hand-made transceivers with someone using a microscope to assemble,” says Otte. “But this is not scalable if you are talking about hundreds of thousands or millions of parts.”
He cites the huge numbers of Intel processors used in servers. “If you want to put an optical transceiver next to each of those processors, imagine doing that with manual assembly,” says Otte. “It just does not work; not if you want to hit the price points.”
In contrast, using silicon photonics requires two separate chips. The photonics is made using an older CMOS process with 130nm or 90nm feature sizes due to the relatively large dimensions of the optical functions, while a more advanced CMOS process is used to implement the electronics - the control loops, high-speed drivers and the amplifiers - associated with the optical transceiver. If an advanced CMOS process is used to implement both the electronics and optics on the one chip, the photonics dominates the chip area.
“If you use a sophisticated CMOS process then you pay all the money for the electronics but you are really using it for the optics,” says Otte. “This is why recently the two have been split: a sophisticated CMOS process for the electronics and a legacy, older process for the optics.”
Sicoya is adopting a single-chip approach, using a 130nm silicon germanium BiCMOS process technology for the electronics and photonics, due to its tiny silicon photonics modulator. “Really it is an electronics chip with a little bit of optics,” says Otte.
You can’t make a data centre ten times larger, and data centres can’t become ten times more expensive. You need to do something new.
Modulation
The start-up does not use a traditional Mach-Zehnder modulator or the much smaller ring-resonator modulator. The basic concept of the ring resonator is that by varying the refractive index of the ring waveguide, it can build up a large intensity of light, starving light in an adjacent coupled waveguide. This blocking and passing of light is what is needed for modulation.
The size of the ring resonator is a big plus but its operation is highly temperature dependent. “One of its issues is temperature control,” says Otte. “Each degree change impacts the resonant frequency [of the modulator].” Moreover, the smaller the ring-resonator design, the more sensitive it becomes. “You may shrink the device but then you need to add a lot more [controlling] circuitry,” he says.
Stefan Meister, Sicoya’s CTO, explains that there needs to be a diode with a ring resonator to change the refractive index to perform the modulation. The diode must be efficient otherwise, the resonance region is narrow and hence more sensitive to temperature change.
Sicoya has developed its own modulator which it refers to as a node-matched diode modulator. The modulator uses a photonic crystal; a device with a periodic structure which blocks certain frequencies of light. Sicoya’s modulator acts like a Fabry-Perot resonator and uses an inverse spectrum approach. “It has a really efficient diode inside so that the Q factor of the resonator can be really low,” says Meister. “So the issue of temperature is much more relaxed.” The Q factor refers to the narrowness of the resonance region.
Operating based on the inverse spectrum also results in Sicoya’s modulator having a much lower loss, says Meister.
Sicoya is working with the German foundry IHP to develop its technology and claims its modulator has been demonstrated operating at 25 gigabit and at 50 gigabit. But the start-up is not yet ready to detail its ASPIC designs nor when it expects to launch its first product.
5G wireless
However the CEO believes such technology will be needed with the advent of 5G wireless. The 10x increase in broadband bandwidth that the 5G cellular standard promises coupled with the continual growth of mobile subscribers globally will hugely impact data centres.
“You can’t make a data centre ten times larger, and data centres can’t become ten times more expensive, says Otte. “You need to do something new.”
This is where Sicoya believes its ASPICs can play a role.
“You can forward or process ten times the data and you are not paying more for it,” says Otte. “The transceiver chip is not really more expensive than the driver chip.”
Ranovus readies its interfaces for deployment
- Products will be deployed in the first half of 2015
- Ranovus has raised US $24 million in a second funding round
- The start-up is a co-founder of the OpenOptics MSA; Oracle is now also an MSA member.
Ranovus says its interconnect products will be deployed in the first half of 2015. The start-up, which is developing WDM-based interfaces for use in and between data centres, has raised US $24 million in a second stage funding round. The company first raised $11 million in September 2013.
Saeid Aramideh"There is a lot of excitement around technologies being developed for the data centre," says Saeid Aramideh, a Ranovus co-founder and chief marketing and sales officer. He highlights such technologies as switch ICs, software-defined networking (SDN), and components that deliver cost savings and power-consumption reductions. "Definitely, there is a lot of money available if you have the right team and value proposition," says Aramideh. "Not just in Silicon Valley is there interest, but in Canada and the EU."
The optical start-up's core technology is a quantum dot multi-wavelength laser which it is combining with silicon photonics and electronics to create WDM-based optical engines. With the laser, a single gain block provides several channels while Ranovus is using a ring resonator implemented in silicon photonics for modulation. The company is also designing the electronics that accompanies the optics.
Aramideh says the use of silicon photonics is a key part of the design. "How do you enable cost-effective WDM?" he says."It is not possible without silicon photonics." The right cost points for key components such as the modulator can be achieved using the technology. "It would be ten times the cost if you didn't do it with silicon photonics," he says.
The firm has been working with several large internet content providers to turn its core technology into products. "We have partnered with leading data centre operators to make sure we develop the right products for what these folks are looking for," says Aramideh.
In the last year, the start-up has been developing variants of its laser technology - in terms of line width and output power - for the products it is planning. "A lot goes into getting a laser qualified," says Aramideh. The company has also opened a site in Nuremberg alongside its headquarters in Ottawa and its Silicon Valley office. The latest capital will be used to ready the company's technology for manufacturing and recruit more R&D staff, particularly at its Nuremberg site.
Ranovus is a founding member, along with Mellanox, of the 100 Gigabit OpenOptics multi-source agreement. Oracle, Vertilas and Ghiasi Quantum have since joined the MSA. The 4x25 Gig OpenOptics MSA has a reach of 2km-plus and will be implemented using a QSFP28 optical module. OpenOptics differs from the other mid-reach interfaces - the CWDM4, PSM4 and the CLR4 - in that it uses lasers at 1550nm and is dense wavelength-division multiplexed (DWDM) based.
It is never good that an industry is fragmented
That there are as many as four competing mid-reach optical module developments, is that not a concern? "It is never good that an industry is fragmented," says Aramideh. He also dismisses a concern that the other MSAs have established large optical module manufacturers as members whereas OpenOptics does not.
"We ran a module company [in the past - CoreOptics]; we have delivered module solutions to various OEMs that are running is some of the largest networks deployed today," says Aramideh. "Mellanox [the other MSA co-founder] is also a very capable solution provider."
Ranovus plans to use contract manufacturers in Asia Pacific to make its products, the same contract manufacturers the leading optical module makers use.
Table 1: The OpenOptics MSA
End markets
"I don't think as a business, anyone can ignore the big players upgrading data centres," says Aramideh. "The likes of Google, Facebook, Amazon, Apple and others that are switching from a three-tier architecture to a leaf and spine need longer-reach connectivity and much higher capacity." The capacity requirements are much beyond 10 Gig and 40 Gig, and even 100 Gig, he says.
Ranovus segments the adopters of interconnect into two: the mass market and the technology adopters. "Mass adoption today is all MSA-based," says Aramideh. "The -LR4 and -SR10, and the same thing is happening at 100 Gig with the QSFP28." The challenge for the optical module companies is who has the lowest cost.
Then there are the industry leaders such as the large internet content providers that want innovative products that address their needs now. "They are less concerned about multi-source standard-based solutions if you can show them you can deliver a product they need at the right cost," says Aramideh.
Ranovus will offer an optical engine as well as the QSFP28 optical module. "The notion of the integration of an optical engine with switch ICs and other piece parts in the data centre are more of an urgent need," he says.
Using WDM technology, the company has a scalable roadmap that includes 8x25 Gig and 16x25 Gig (400 Gig) designs. Also, by adding higher-order modulation, the technology will scale to 1.6 Terabit (16x100 Gig), says Aramideh.
I don't see a roadmap for coherent to become cost-effective to address the smaller distances
Ranovus is also working on interfaces to link data centres.
"These are distances much shorter than metro/ regional networks," says Aramideh, with the bulk of the requirements being for links of 15 to 40km. For such relatively short distances, coherent detection technology has a high-power consumption and is expensive. "I don't see a roadmap for coherent to become cost-effective to address the smaller distances," says Aramideh.
Instead, the company believes that a direct-detection interconnect that supports 15 to 40km and which has a spectral efficiency that can scale to 9.6 Terabit is the right way to go. If that can be achieved, then switching from coherent to direct detection becomes a no-brainer, he says. "For inter-data-centres, we are really offering an alternative to coherent."
The start-up says its technology will be in product deployment with lead customers in the first half of 2015.