John Bowers: We are still at the dawn of photonics
After 38 years at the University of California, Santa Barbara (UCSB), Professor John Bowers (pictured) is stepping away from teaching and administrative roles to focus on research.
He welcomes the time it will free for biking and golf. He will also be able to linger, not rush, when travelling. On a recent trip to Saudi Arabia, what would have centered around a day-event became a week-long visit.
Bowers’ career includes significant contributions to laser integration and silicon photonics, mentoring some 85 PhD students, and helping found six start-ups, two of which he was the CEO.
Early Influences
Bowers’ interest in science took root while at high school. He built oscilloscopes and power supplies using Heathkits, then popular educational assemblies for electronics enthusiasts. He was also inspired by his physics and chemistry teachers, subjects he majored in at the University of Minnesota.
A challenging experience led him to focus solely on physics: “I took organic chemistry and hated it,” says Bowers. “I went, ‘Okay, let’s stick to inorganic materials.’”
Bowers became drawn to high-energy physics and worked in a group conducting experiments at Fermilab and Argonne National Laboratories. Late-night shifts – 10 PM to 6 AM – offered hands-on learning, but a turning point came when his mentor was denied tenure. “My white knight fell off his horse,” he says.
He switched to applied physics at Stanford, where he explored gallium arsenide and silicon acoustic devices, working under the supervision of the late Gordon Kino, a leading figure in applied physics and electrical engineering.
Bowers then switched to fibre optics, working in a group that was an early leader in single-mode optical fibre. “It was a period when fibre optics was just taking off,” says Bowers. “In 1978, they did the first 50-megabit transmission system, and OFC [the premier optical fibre conference] was just starting.”
Bell Labs and fibre optics
After gaining his doctorate, Bowers joined Bell Labs, where his work focused on the devices—high-speed lasers and photodetectors—used for fibre transmission. He was part of a team that scaled fibre-optic systems from 2 to 16 gigabits per second. However, the 1984 AT&T breakup signalled funding challenges, with Bell Labs losing two-thirds of its financial support.
Seeking a more stable environment, Bowers joined UCSB in 1987. He was attracted by its expertise in semiconductors and lasers, including the presence of the late Herbert Kroemer, who went on to win the 2000 Nobel Prize in Physics. Kroemer developed the double heterostructure laser and played a big part in enticing Bowers to join. Bowers was tasked with continuing the laser work, something he has done for the last 40 years.
“Coming to Santa Barbara was brilliant, in retrospect,” says Bowers, citing its strong collaborative culture and a then newly formed materials department.
Integrated lasers
At UCSB, Bowers worked on integrated circuits using indium phosphide, including tunable lasers and 3D stacking of photonic devices.
At the same time, the field of silicon photonics was starting after Richard Soref wrote a seminal paper proposing silicon as an optical material for photonic integrated circuits (PIC).
“We all knew that silicon was a terrible light emitter because it is an indirect band-gap material,” says Bowers. “So when people started talking about silicon photonics, I kept thinking: ‘Well, that is fine, but you need a light source, and if you don’t have a light source, it’ll never become important.’”
Bowers tackled integrating lasers onto silicon to address the critical need for an on-chip light source. He partnered with Intel’s Mario Paniccia and his team, which had made tremendous progress developing a silicon Raman lasers with higher powers and narrower linewidths.
“It was very exciting, but you still needed a pump laser; a Raman laser is just a wavelength converter from one wavelength to another,” says Bowers. “So I focused on the pump laser end, and the collaboration benefitted us both.”
Intel commercialised the resulting integrated laser design and sold millions of silicon-photonics-based pluggable transceivers.
“Our original vision was verified: the idea that if you have CMOS processing, the yields will be better, the performance will be better, the cost will be lower, and it scales a lot better,” says Bowers. “All that has proven to be true.
Is Bowers surprised that integrated laser designs are not more widespread?
All the big silicon photonics companies, including foundry TSMC, will incorporate lasers into their products, he says, just as Intel has done and Infinera before that.
Infinera, an indium phosphide photonic integrated circuit (PIC) company now acquired by Nokia, claimed that integration would improve the reliability and lower the cost, says Bowers: “Infinera did prove that with indium phosphide and Intel did the same thing for silicon.”
The indium phosphide transceiver has a typical failure rate of 10 FIT (failures per ten billion hours), and if there are 10 laser devices, the FIT rises to 100, he says. By contrast, Intel’s design has a FIT of 0.1, and so with 10, the FIT becomes on the order of 1.
Silicon lasers are more reliable because there’s no III-V material exposed anywhere. Silicon or silicon dioxide facets eliminate the standard degradation mechanisms in III-V materials. This enables non-hermetic packaging, reducing costs and enabling rapid scaling.
According to Bowers, Intel scaled to a million transceivers in one year. Such rapid scaling to high volumes is important for many applications, and that is where silicon photonics has an advantage.
“Different things motivate different people. For me, it’s not about money, it’s more about your impact, particularly on students and research fields. To the extent that I’ve contributed to silicon photonics becoming important and dynamic, that is something I’m proud of.”
-Professor John Bowers
Optical device trends
Bowers notes how the rise of AI has surprised everyone, not just in terms of the number of accelerator chips needed but their input-output (I/O) requirements.
Copper has been the main transmission medium since the beginning of semiconductor chips, but that is now being displaced by optics – silicon photonics in particular – for the communications needs of very high bandwidth chips. He also cites companies like Broadcom and Nvidia shipping co-packaged optics (CPO) for their switching chips and platforms.
“Optics is the only economic way to proceed, you have to work on 3D stacking of chips coupled with modern packaging techniques,” he says, adding that the need for high yield and high reliability has been driving the work on III-V lasers on silicon.
One current research focus for Bowers is quantum dot lasers, which reduce the line width and minimise reflection sensitivity by 40dB. This eliminates the need for costly isolators in datacom transceivers.
Quantum dot devices also show exceptional durability, with lifetimes for epitaxial lasers on silicon a million times longer than quantum well devices on silicon and 10 times less sensitivity to radiation damage, as shown in a recent Sandia National Labs study for space applications.
Another area of interest is modulators for silicon photonics. Bowers says his group is working on sending data at 400 gigabits-per-wavelength using ‘slow light’ modulators. These optical devices modulate the intensity or phase, of light. Slowing down the light improves its interaction in the material, improving efficiency and reducing device size and capacitance. He sees such modulators is an important innovation.
“Those innovations will keep happening; we’re not limited in terms of speed by the modulator,” says Bowers, who also notes the progress in thin-film lithium niobate modulators, which he sees as benefiting silicon photonics, “We have written papers suggesting most of the devices may be III-V,” says Bowers, and the same applies to materials such as thin-film lithium niobate.
“I believe that as photonic systems become more complex, with more lasers and amplifiers, then everyone will be forced to integrate,” says Bowers.
Other applications
Beyond datacom, Bowers sees silicon photonics enabling LIDAR, medical sensors, and optical clocks. His work on low-noise lasers, coupled to silicon nitride waveguides, reduces phase noise by 60dB, enhancing sensor sensitivity. “If you can reduce the frequency noise by 60dB, then that makes it either 60dB more efficient, or you need 60dB less power,” he says.
Applications include frequency-based sensors for gas detection, rotation sensing, and navigation, where resonance frequency shifts detect environmental changes.
Other emerging applications include optical clocks for precise timing in navigation, replacing quartz oscillators. “You can now make very quiet clocks, and at some point we can integrate all the elements,” Bowers says, envisioning chip-scale solutions.
Mentorship and entrepreneurial contributions
Bowers’ impact extends to mentorship, guiding so many PhD students who have gone on to achieve great success.
“It’s very gratifying to see that progression from an incoming student who doesn’t know what an oscilloscope is to someone who’s running a group of 500 people,” he says.
Alan Liu, former student and now CEO of the quantum dot photonics start-up Quintessent, talks about how Bowers calls on his students to ‘change the world’.
Liu says it is not just about pushing the frontiers of science but also about having a tangible impact on society through technology and entrepreneurship.”
Bowers co-founded UCSB’s Technology Management Department and taught entrepreneurship for 30 years. Drawing on mentors like Milton Chang, he focused on common start-up pitfalls: “Most companies fail for the same set of reasons.”
His own CEO start-up experience informed his teaching, highlighting interdisciplinary skills and team dynamics.
Mario Paniccia, CEO of Anello Photonics, who collaborated with Bowers as part of the Intel integrated laser work, highlights Bowers’ entrepreneurial skills.
“John is one of the few professors who are not only brilliant and technically a world expert – in John’s case, in III-V materials – but also business savvy and entrepreneurial,” says Paniccia. “He is not afraid to take risks and can pick and hire the best.”
Photonics’ future roadmap
Bowers compares photonics’ trajectory to electronics in the 1970s, when competing CMOS technologies standardised, shifting designers’ focus from device development to complex circuits. “Just like in the 1970s, there were 10 competing transistor technologies; the same consolidation will happen in photonics,” he says.
Standardised photonic components will be integrated into process design kits (PDKs), redirecting research toward systems like sensors and optical clocks.
“We’re not at the end, we’re at the beginning of photonics,” emphasises Bowers.
Reflections
Looking back, would he have done anything differently?
A prolonged pause follows: “I’ve been very happy with the choices I have made,” says Bowers, grateful for his time at UCSB and his role in advancing silicon photonics.
Meanwhile, Bowers’ appetite for photonics remains unwavering: “The need for photonic communication, getting down to the chip level, is just going to keep exploding,” he says.
Richard Soref: The new frontiers of silicon photonics
Interview 4: Professor Richard Soref
John Bowers acknowledges him with ‘kicking off’ silicon photonics some 30 years ago, while Andrew Rickman refers to him as the ‘founding father of silicon photonics’. An interview with Richard Soref.
It was fibre-optic communications that started Professor Richard Soref on the path to silicon photonics.
“In 1985, the only photonic chip that could interface to fibre was the III-V semiconductor chip,” says Soref. He wondered if an elemental chip such as silicon could be used, and whether it might even do a better job. He had read in a textbook that silicon is relatively transparent at the 1.30-micron and 1.55-micron wavelengths used for telecom and it inspired him to look at silicon as a material for optical waveguides.
Soref's interest in silicon was a combination of the potential of using the chip industry’s advanced manufacturing infrastructure for electro-optical integration and his own interest in materials. “I’m a science guy and I have curiosity and fascination with what the world of materials offers,” he says. “If I have an avenue like that, I like to explore where the physics takes us.”
In 1985 Soref constructed and did experiments on waveguides based on un-doped silicon resting upon a doped silicon substrate. It turned out not to be the best choice for a waveguide and in 1986 Soref proposed using a silicon-on-insulator waveguide instead, what has become the mainstream approach for the silicon photonics industry.
Silicon-on-insulator had a far greater refractive index contrast between the waveguide core and its cladding and is far less lossy. And while Soref didn’t build such structures, “it stimulated others to develop that major, major waveguide, so I’m proud of that”.
The original waveguide idea was not a wasted one, though. Soref and then research assistant, Brian Bennett, used the undoped-on-doped silicon waveguide structure to study and quantify free-carrier electro-modulation effects. These effects underpin the workings of the bulk of current silicon photonic modulators. Soref says their published academic paper has since been cited over 1,800 times.
Soref is approaching his 80th birthday and is a research professor at the University of Massachusetts in Boston. He has spent over 50 years researching photonics, silicon photonics and the broader topic of mid-infrared wavelengths and Group IV photonics, as well as spending five years researching liquid crystals for displays and electro-optical switching. For 27 years he was employed at the Air Force Research Laboratory. He has also worked at the Sperry Research Center and the MIT Lincoln Laboratory.
Applications go beyond telecom and optical interconnect, and perhaps the most important application is sensing
Group IV photonics
Soref’s research interests are broad as part of his fundamental interest in material science. In more recent years he has focused on Group IV photonics but not exclusively so.
The term silicon-photonics is firmly entrenched in the global community, he says, a phrase that includes on-chip germanium photo-detectors and even, with heterogeneous integration, III-V materials. Group IV photonics is a superset of silicon photonics and includes silicon-germanium-tin materials (SiGeSn) and well as silicon carbide. Such materials will likely be used in the monolithic silicon chip of the future, he says.
He has published papers on alloys such as silicon germanium carbon and silicon germanium tin. “I was estimating what these never-before-seen materials would do; you could create new alloys and how would those alloys behave,” says Soref.
Silicon germanium tin offers the possibility of a direct bandgap light emitter. “It is a richer material science space, with independent control of the bandgap and the lattice parameter,” says Soref.
Adding tin to the alloy lengthens the wavelength of operation, typically in the 1.5-micron to 5-micron range, the near infra-red and part of the mid infra-red part of the spectrum. “Applications go beyond telecom and optical interconnect, and perhaps the most important application is sensing,” says Soref.
The applications in this wavelength range include system-on-a-chip, lab-on-a-chip, sensor-on-a-chip and sensor-fusion-on-a-chip for such applications as chemical, biological, medical and environmental sensing. Such sensor chips could be in your smartphone and play an important role in the emerging Internet of Things (IoT). “Sensing could be a very important economic foundation for Group IV photonics,” says Soref.
And Soref does not stop there. He is writing a paper on Group III nitrides for ultra violet and visible-light integrated photonics: “I think silicon and Group IV are limited to the near-, mid- and longwave infra red”.
Challenges
Soref points to the work being done in developing commercial high-volume manufacturing: the use of 300mm silicon wafers, developing process libraries and perfecting devices for volume manufacturing. He welcomes AIM Photonics, the US public-private venture investing $610 million in photonics and manufacturing.
But he argues that there should also be an intellectual space for growth, “a wider space which is not so practical but which will become practical”. He cites the emerging areas of sensing and microwave photonics. “That is the frontier,” says Soref. “And the foundry work should not prevent that intellectual exploration.”
An important application area for microwave photonics is wireless, from 5GHz to 90GHz. Soref envisages a photonic integrated circuit (PIC), or an opto-electronic IC (OEIC) that features electronics and optics on-chip, that communicates with other entities often via fibre but also wirelessly.
“That means RF (radio frequency) or microwave, and for microwave that requires a transmitter and receiver on the chip,” says Soref. Such a device would find use in the IoT and future smartphones.
Microwave designs in the past used an assemblage of discrete components that makes a system on a board. These new microwave PICs or OEICs could perform many of the classical functions such as spectral analysis, optical control of a phased array microwave antenna, microwave signal processing, and optical analogue to digital conversion (ADC) and optical digital to analogue conversion (DAC).
This is analogous to the convergence of computing and photonics, says Soref. In computing, the signal goes from the electrical domain to the optical and back, while for microwave photonics it will be conversions between the microwave and photonic domains on the chip.
There are also quantum-photonic applications: quantum computing, quantum cryptography and quantum metrology where photonic devices could play a role.
Opportunities
These are the three emerging opportunities areas Soref foresees for Group IV photonics emerging in the next decade: sensors, microwave photonics and the quantum and computing worlds in addition to the existing markets of telecom and optical interconnect.
Soref is not sure that silicon photonics has yet reached its tipping point. “To make silicon photonics and Group IV photonics ubiquitous and pervasive, it takes a lot of investment and a lot of commercial results,” he says. “We have not yet arrived at that stage of economic foundation.”
New optical devices
Soref also highlights how continual advances in CMOS feature size, from 45nm down to 7nm, promise new photonic components that could become commonplace.
Soref cites the example of a silicon-on-isolator nanobeam. The nanobeam is a strip waveguide with air holes, in effect a one-dimensional photonic crystal lattice in a waveguide.
The nanobeam structure is of interest as it performs the same role as the micro-ring resonator, a useful optical building block used in such applications as modulation.
“The photonic crystal structure requires extreme control of dimensions to reduce unwanted scattering, so it needs very fine lithography,” says Soref. People have argued such structures are impractical due to the unrealistic dimensional control needed.
“But foundries have shown you can get a very high-quality photonic crystal in a silicon fab,” he explains. “This foundry advantage would enable new components that might have seemed too difficult or marginal on paper.”
Significant progress in silicon photonics may have been achieved since his first work in 1985, but as Soref highlights, it is still early when assessing the full significance of the technology.
Tackling system design on a data centre scale
Interview 1: Andrew Rickman
Silicon photonics has been a recurring theme in the career of Andrew Rickman. First, as a researcher looking at the feasibility of silicon-based optical waveguides, then as founder of Bookham Technologies, and after that as a board member of silicon photonics start-up, Kotura.
Andrew Rickman
Now as CEO of start-up Rockley Photonics, his company is using silicon photonics alongside its custom ASIC and software to tackle a core problem in the data centre: how to connect more and more servers in a cost effective and scaleable way.
Origins
As a child, Rickman attended the Royal Institution Christmas Lectures given by Eric Laithwaite, a popular scientist who was also a professor of electrical engineering at Imperial College. As an undergraduate at Imperial, Rickman was reacquainted with Professor Laithwaite who kindled his interest in gyroscopes.
“I stumbled across a device called a fibre-optic gyroscope,” says Rickman. “Within that I could see people starting to use lithium niobate photonic circuits.” It was investigating the gyroscope design and how clever it was that made Rickman wonder whether the optical circuits of such a device could be made using silicon rather than exotic materials like lithium niobate.
“That is where the idea triggered, to look at the possibility of being able to make optical circuits in silicon,” he says.
If you try and force a photon into a space shorter than its wavelength, it behaves very badly
In the 1980s, few people had thought about silicon in such a context. That may seem strange today, he says, but silicon was not a promising candidate material. “It is not a direct band-gap material - it was not offering up the light source, and it did not have a big electro-optic effect like lithium niobate which was good for modulators,” he says. “And no one had demonstrated a low-loss single-mode waveguide.”
Rickman worked as a researcher at the University of Surrey’s physics department with such colleagues as Graham Reed to investigate whether the trillions of dollars invested in the manufacturing of silicon could also be used to benefit photonic circuits and in particular whether silicon could be used to make waveguides. “The fundamental thing one needed was a viable waveguide,” he says.
Rickman even wrote a paper with Richard Soref who was collaborating with the University of Surrey at the time. “Everyone would agree that Richard Soref is the founding father of the idea - the proposal of having a useful waveguide in silicon - which is the starting point,” says Rickman. It was the work at the University of Surrey, sponsored by Bookham which Rickman had by then founded, that demonstrated low-loss waveguides in silicon.
Fabrication challenges
Rickman argues that not having a background in CMOS processes has been a benefit. “I wasn’t dyed-in-the-wool-committed to CMOS-type electronics processing,” he says. “I looked upon silicon technology as a set of machine-shop processes for making things.”
Looking at CMOS processing completely afresh and designing circuits optimised for photonics yielded Bookham a great number of high-performance products, he says. In contrast, the industry’s thrust has been very much a semiconductor CMOS-focused one. “People became interested in photonics because they just naturally thought it was going to be important in silicon, to perpetuate Moore’s law,” says Rickman.
You can use the structures and much of the CMOS processes to make optical waveguides, he says, but the problem is you create small structures - sub-micron - that guide light poorly. “If you try and force a photon into a space shorter than its wavelength, it behaves very badly,” he says. “In microelectronics, an electron has got a wavelength that is one hundred times smaller that the features it is using.”
The results include light being sensitive to interface roughness and to the manufacturing tolerances - the width, hight and composition of the waveguide. “At least an order of magnitude more difficult to control that the best processes that exist,” says Rickman.
“Our [Rockley’s] waveguides are one thousand times more relaxed to produce than the competitors’ smaller ones,” he says. “From a process point of view, we don’t need the latest CMOS node, we are more a MEMS process.”
If you take control of enough of the system problem, and you are not dictated to in terms of what MSA or what standard that component must fit into, and you are not competing in this brutal transceiver market, then that is when you can optimise the utilisation of silicon photonics
Rickman stresses that small waveguides do have merits - they go round tighter bends, and their smaller-dimensioned junctions make for higher-speed components. But using very large features solves the ‘fibre connectivity problem’, and Rockley has come up with its own solutions to achieve higher-speed devices and dense designs.
“Bookham was very strong in passive optics and micro-engineered features,” says Rickman. “We have taken that experience and designed a process that has all the advantages of a smaller process - speed and compactness - as well as all the benefits of a larger technology: the multiplexing and demultiplexing for doing dense WDM, and we can make a chip that already has a connector on it.”
Playing to silicon photonics’ strengths
Rickman believes that silicon photonics is a significant technological development: “It is a paradigm shift; it is not a linear improvement”. But what is key is how silicon photonics is applied and the problem it is addressing.
To make an optical component for an interface standard or a transceiver MSA using silicon photonics, or to use it as an add-on to semiconductors - a ’band-aid” – to prolong Moore’s law, is to undersell its full potential. Instead, he recommends using silicon photonics as one element - albeit an important one - in an array of technologies to tackle system-scale issues.
“If you take control of enough of the system problem, and you are not dictated to in terms of what MSA or what standard that component must fit into, and you are not competing in this brutal transceiver market, then that is when you can optimise the utilisation of silicon photonics,” says Rickman. “And that is what we are doing.” In other words, taking control of the environment that the silicon sits in.
It [silicon photonics] is a paradigm shift; it is not a linear improvement
Rockley’s team has been structured with the view to tackle the system-scale problem of interconnecting servers in the data centre. Its team comprises computer scientists, CMOS designers - digital and analogue - and silicon photonics experts.
Knowing what can be done with the technologies and organising them allows the problems caused by the ‘exhaustion of Moore’s law’ and the input/output (I/O) issues that result to be overcome. “Not how you apply one technology to make up for the problems in another technology,” says Rickman.
The ending of Moore’s law
Moore’s law continues to deliver a doubling of transistors every two years but the associated scaling benefits like the halving of power consumed per transistor no longer apply. As a result, while Moore’s law continues to grow gate count that drives greater computation, the overall power consumption is no longer constant.
Rickman also points out that the I/O - the number of connections on and off a chip - are not doubling with transistor count. “I/O may be going from 25 gigabit to 50 gigabit using PAM–4 but there are many challenges and the technology has yet to be demonstrated,” he says.
The challenge facing the industry is that increasing the I/O rate inevitably increases power consumption. “As power consumption goes up, it also equates to cost,” says Rickman. Clearly that is unwelcome and adds cost, he says, but that is not the only issue. As power goes up, you cannot fully benefit from the doubling transistor counts, so things cannot be packed more densely.
“You are running into to the end of Moore’s law and you don’t get the benefit of reducing space and cost because you’ve got to bolt on all these other things as it is very difficult to get all these signals off-chip,” he says.
This is where tackling the system as a whole comes in. You can look at microelectronics in isolation and use silicon photonics for chip-to-chip communications across a printed circuit board to reduce the electrical losses through the copper traces. “A good thing to do,” stresses Rickman. Or you can address, as Rockley aims to do, Moore’s law and the I/O limitations within a complete system the size of the data centre that links hundred of thousands of computers. “Not the same way you’d solve an individual problem in an individual device,” says Rickman.
Rockley Photonics
Rockley Photonics has already demonstrated all the basic elements of its design. “That has gone very well,” says Rickman.
The start-up has stated its switch design uses silicon photonics for optical switching and that the company is developing an accompanying controller ASIC. It has also developed a switching protocol to run on the hardware. Rockley’s silicon photonics design performs multiplexing and demultiplexing, suggesting that dense WDM is being used as well as optical switching.
Rockley is a fabless semiconductor company and will not be building systems. Partly, it is because it is addressing the data centre and the market has evolved in a different way to telecoms. For the data centre, there are established switch vendors and white-box manufacturers. As such, Rockley will provide its chipset-based reference design, its architecture IP and the software stack for its customers. “Then, working with the customer contract manufacturer, we will implement the line cards and the fabric cards in the format that the particular customer wants,” says Rickman.
The resulting system is designed as a drop-in replacement for the large-scale data centre players’ switches they haver already deployed, yet will be cheaper, more compact and consume less power, says Rockley.
“They [the data centre operators] can scale the way they do at the moment, or they can scale with our topology,” says Rickman.
The start-up expects to finally unveil its technology by the year end.
Photonics and optics: interchangeable yet different
Many terms in telecom are used interchangeably. Terms gain credibility with use but over time things evolve. For example, people understand what is meant by the term carrier [of traffic] or operator [of a network] and even the term incumbent [operator] even though markets are now competitive and 'telephony' is no longer state-run.
"For me, optics is the equivalent of electrical, and photonics is the equivalent of electronics - LSI, VLSI chips and the like" - Mehdi Asghari
Operators - ex-incumbents or otherwise - also do more that oversee the network and now provide complex services. But of course they differ from service providers such as the over-the-top players [third-party providers delivering services over an operator's infrastructure, rather than any theatrical behaviour] or internet content providers.
Google is an internet content provider but with its gigabit broadband service it is rolling out in the US, it is also an operator/ carrier/ communications service provider. And Google may soon become a mobile virtual network operator.
So having multiple terms can be helpful, adding variety especially when writing, but the trouble is it is also confusing.
Recent discussions including interviewing silicon photonics pioneer, Richard Soref, raised the question whether the terms photonics and optics are the same. I decided to ask several industry experts, starting with The Optical Society (OSA).
Tom Hausken, the OSA's senior engineering & applications advisor, says that after many years of thought he concludes the following:
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People have different definitions for them [optics and photonics] that range all over the map.
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I find it confusing and unhelpful to distinguish them.
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The National Academies's report is on record saying there is no difference as far as that study is concerned.
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That works for me.
Michael Duncan, the OSA's senior science advisor, puts the difference down to one of cultural usage. "Photonics leans more towards the fibre optics, integrated optics, waveguide optics, and the systems they are used in - mostly for communication - while optics is everything else, especially the propagation and modification of coherent and incoherent light," says Duncan. "But I could easily go with Tom's third bullet point."
"Photonics does include the quantum nature, and sort of by convention, the term optics is seen to mean classical" - Richard Soref
Duncan also cites Wikipedia, with its discussion of classical optics that embraces the wave nature of light, and modern optics that also includes light's particle nature. And this distinction is at the core of the difference, without leading to an industry consensus.
"Photonics does include the quantum nature, and sort of by convention, the term optics is seen to mean classical," says Richard Soref. He points out that the website Arxiv.org categorises optics as the subset of physics, while the OSA Newsletter is called Optics & Photonics News, covering all bases.
"Photonics is the larger category, and I might have been a bit off base when throwing around the optics term," says Soref. If only everyone was as off base as Professor Soref.
"We need to remember that there is no canonical definition of these terms, and there is no recognised authority that would write or maintain such a definition," says Geoff Bennett, director, solutions and technology at Infinera. For Bennett, this is a common issue, not confined to the terms optics and photonics: "We see this all the time in the telecoms industry, and in every other industry that combines rapid innovation with aggressive marketing."
That said, he also says that optics refers to classical optics, in which light is treated as a wave, whereas photonics is where light meets active semiconductors and so the quantum nature of light tends to dominate. Examples of the latter would be photonic integrated circuits (PICs). "These contain active lasers components, semiconductor optical amplifiers and photo-detectors " says Bennett. "All of these rely on quantum effects to do their job."
"We need to remember that there is no canonical definition of these terms, and there is no recognised authority that would write or maintain such a definition" - Geoff Bennett
Bennett says that the person who invented the term semiconductor optical amplifier (SOA) was not aware of the definition because the optical amplifier works on quantum principles, the same way a laser does. "So really it should be a semiconductor photonic amplifier," he says.
"At Infinera, we seem for the most part to have abided to the definitions in terminology that we use, but I can’t say that this was a conscious decision," says Bennett. "I am sure that if our marketing department thought that photonic sounded better than optical in a given situation they would have used it."
Mehdi Asghari, vice president, silicon photonics research & development at Mellanox, says optics refers to the classical use and application of light, with light as a ray. He describes optics as having a system-level approach to it.
"We create a system of lenses to make a microscope or telescope to make an optical instrument using classical optics models or we use optical components to create an optical communication system," he says. This classical or system-level perspective makes it optics or optical, a term he prefers. "We are not concerned with the nature - particle versus wave - of light, rather its classical behaviour, be it in an instrument or a system," he says.
But once things are viewed closer, at the device level, especially devices comparable in size of photons, then a system-level approach no longer works and is replaced with a quantum approach. "Here we look at photons and the quantum behaviour they exhibit," says Asghari.
In a waveguide, be it silicon photonics (integrated devices based on silicon), a planar lightwave circuit (glass-based integrated devices), or a PIC based on III-V or active devices, the size of the structure or device used is often comparable or even smaller than the size of the photons it is manipulating, he says: "This is where we very much feel the quantum nature of light, and this is where light becomes photons - photonics - and not optics."
ADVA Optical Networking's senior principal engineer, Klaus Grobe, held a discussion with the company's physicists, and both, independently, had the same opinion.
"Both [photonics and optics] are not strictly defined," he says. "Optics clearly also includes classic school-book ray optics and the like. Photonics already deals with photons, the wave-particle dualism, and hence, at least indirectly, with quantum mechanics, and possibly also quantum electro-dynamics (QED)."
Since in fibre-optics for transport, ray-propagation models no longer can be used, and also since they rely on the quantum-mechanical behaviour, for example of diode receivers, fibre-optics are better filed under photonics, says Grobe: "But they are not called fibre-photonics".
So, the industry view seems to be that the two terms are interchangeable but optics implies the classical nature of light while photonics suggests light as particles. Which term includes both seems to be down to opinion. Some believe optics covers both, others believe photonics is the more encompassing term.
Mellanox's Asghari once famously compared photons and electrons to cats and dogs. Electrons are like dogs: they behave, stick by you and are loyal; they do exactly as you tell them, he said, whereas cats are their own animals and do what they like. Just like photons. So what is his take?
He believes optics is more general than photonics. He uses the analogy of electrical versus electronics to make his point. An electronics system or chip is still an electrical device but it often refers to the integrated chip, while an electrical system is often seen as global and larger, made up of classical devices.
"For me, optics is the equivalent of electrical, and photonics is the equivalent of electronics - LSI, VLSI chips and the like," says Asghari. "One is a subset or specialised version of the other due to the need to get specific on the quantum nature of light and the challenges associated with integration."
"Optics refers to all types of cats, be it the tiger or the lion or the domestic pet. Photonics refers to the so called domestic cat that has domesticated and slaved us to look after it" - Mehdi Asghari
To back up his point, Ashgari says take a look at older books and publications that use the term optics. The term photonics started to be used once integration and size reduction became important, just as how electrical devices got replaced with electronic devices.
Indeed, this rings true in the semiconductor industry: microelectronics has now become nano-electronics as CMOS feature sizes have moved from microns to nanometer dimensions.
And this is why optical fibre or the semiconductor optical amplifier are used because these terms were invented and used when the industry was primarily engaged with the use of light at a system level and away from the quantum limits and challenges of integration.
"In short, photonics is used when we acknowledge that light is made of photons with all the fun and challenges that photons bring to us and optics is when we deal with light at a system level or a classical approach is sufficient," says Asghari.
Happily, cats and dogs feature here too.
"Optics refers to all types of cats, be it the tiger or the lion or the domestic pet," says Asghari. "Photonics refers to the so called domestic cat that has domesticated and slaved us to look after it."
Last word to Infinera's Bennett: "I suppose the moral is: be aware of the different meanings, but don’t let it bug you when people misuse them."
Q&A with photonics pioneer, Richard Soref - Part 1
Richard Soref has spent over 50 years researching photonics, contributing groundbreaking work in the areas of liquid crystals, silicon photonics and the broader topic of mid-infrared wavelengths and Group IV photonics. For 27 years he was employed at the Air Force Research Laboratory. He has also worked at the Sperry Research Center, the MIT Lincoln Laboratory, and is now a research professor at the University of Massachusetts in Boston.
In part 1 of a two-part interview with Gazettabyte, he details his research interests, explains what is meant by Group IV photonics, and discusses why photonics has not matched the semiconductor industry in terms of integration, and how that could change.
Optics is a seemingly small subset of physics but really optics is a huge field with a deep, variegated nature waiting to be discovered
Richard Soref
Q: Having gained your Ph.D. in 1963, you have spent your career in what you call the science of light. What is it about photonics that you find so captivating?
RS: I’ve been drawn to its diversity and classical beauty. Photonics used to be called optics until it was re-labelled by the OSA. Optics is a seemingly small subset of physics but really optics is a huge field with a deep, variegated nature waiting to be discovered. To make progress, you need multiple disciplines, and I’ve always been captivated by the materials science aspect that opens the door to new physics and new devices.
Can you outline your career and how you ended up as a research professor at the University of Massachusetts at Boston?
RS: A general outline is that I chose employers who would encourage exploration of new avenues, would give me freedom to fly or fall – and both are built into research. Basic research is where my talents and passion align. And it helps to be obsessive.
In the early years, I worked mostly alone. Then the pleasures of collaboration became important, and for decades I have been fortunate to have outstanding research partners who did heavy lifting of things like quantum mechanics and electromagnetic modelling.
At Lincoln Lab, I continued the nonlinear optical studies that I began during my Stanford Ph.D. work. Sperry Research was an excellent environment until it fell victim to the corporate research shutdown contagion. Ironically or prophetically, impurity-doped silicon infrared sensors were an early focus at Sperry.
Lithium niobate sparked my 40-year interest in electro-optics: Pockels [effect], Kerr [electro-optic effect], Franz-Keldysh [effect] and more. My extensive work on liquid crystals gained a lot of traction, and at an Information Display show I met scientists from South Korea who told me that my early papers helped them with their commercial flat-panel TV products. It was fulfilling to hear that.
Apart from some governmental distractions, the Air Force Research Laboratory (AFRL) years were a happy time and I welcomed the support of the Air Force Office of Scientific Research (AFOSR); the AFOSR is a national treasure.
Fibre-optic telecom emerged in 1985 when I was at AFRL. The fibres needed semiconductor assistance, I felt. That’s when the new silicon insights came to me. I’m glad that I was able in 1989 to identify silicon-on-insulator (SOI) as the primary waveguided-network platform, and I’m gratified that brilliant and innovative research groups around the world entered this field early on. They strived successfully to perfect this technology. To do my part, I tried to surround the problem in a 1985-2015 series of papers, among them my 1991 silicon-germanium-tin (SiGeSn) direct-gap prediction and my 1992 opto-electronic integrated circuit (OEIC) proposal. My most-cited work is a 1987 paper on free-carrier electro-optics.
Summarising, I had two visions at AFRL: Group IV photonics and long-wave integrated optoelectronics, where long wave denotes anything from two to 200 microns, although the mid-infrared wavelengths are key because of their room temperature possibilities. Perhaps there is a third vision: the multi-technology 3D chip on which seven technologies including bio-chemical could be combined.
Sadly those creative years drew to a close when the Massachusetts laboratory was shut down by the Air Force and the party moved to Wright Patterson AFB in Ohio. At that point, I joined the University of Massachusetts in Boston to stay near family and to keep the technical flame alive in research. I’m still collaborating with wonderful people, most of them young.
Can you provide rigour regarding some definitions? Starting with silicon photonics, silicon mid-infrared optics and Group IV photonics, can you define each and do you have a preference for a particular term?
RS: The silicon-photonics term is strongly entrenched in the global community. The phrase includes on-chip germanium photo-detectors and presumably germanium lasers. Nevertheless, I think this term is a bit narrow or misleading about the silicon-germanium-tin materials (SiGeSn) that likely will be used in the monolithic silicon chip of the future.
I am in the minority when I say that I prefer the wider term Group IV photonics (GFP) which takes into account those three-part alloys as well as diamond and graphene. This GFP term was coined in 2003 in my office at Hanscom when Greg Sun and I were dreaming constructively about a new, dedicated IEEE conference, the international meeting I co-founded in 2004.
In the coming years, the purely photonic integrated circuit, the PIC chip, will evolve, after money is spent, into the opto-electronic chip, and the transistors will be CMOS or BiCMOS or heterojunction bipolar
What about the OEIC, how does it differ from silicon photonics? And lastly, nano-photonics, how does it compare to silicon photonics?
RS: The opto-electronic integrated circuit describes the synergistic marriage of nano-photonics and nano-electronics on the same silicon chip. Others have called this an electronic-photonic integrated circuit or EPIC. In essence, the OEIC is a transistorised photonic chip containing electronic intelligence, signal processing, computation, electrical control of active photonic devices, and perhaps RF wireless transceiving capability, which I strongly advocate.
In the coming years, the purely photonic integrated circuit, the PIC chip, will evolve, after money is spent, into the opto-electronic chip, and the transistors will be CMOS or BiCMOS or heterojunction bipolar. These possibilities illustrate the diversity of GFP.
As for nano-photonics, it is a subset of silicon photonics populated by wave-guided components whose smallest cross-section dimension is 15 to 30 percent of the free-space wavelength. Photonics, like electronics, started as micro and shrank to nano. The term nano means nanometer-scale and applies also to quantum-dot diameter, quantum-well layer thickness and photonic-crystal air hole diameter.
In over half a century, electronics has undergone an extraordinary transformation from simple integrated circuits to profoundly complex ones. Yet while integrated optics was spoken of as early as 1969 in the Bell Labs paper by Stuart Miller, integration has been far more modest. Why?
RS: The main roadblock has been the lack of compelling applications for medium scale and large scale photonic integration. Perhaps this was a lack of vision or a lack of market to drive the integration research.
Another inhibiting factor is the large expense, the cost-per-run of making a photonic integrated circuit, although the OPSIS user foundry [before it closed] and other user facilities have mitigated entry costs to some extent. Additional factors are the area-footprint and volume of the photonic building blocks. The photonic device size is generally larger, or much larger, than the size of the modern individual transistor.
Is this about to change?
RS: To some degree, yes. People are packing photonic components together in a circuit but there are limits on how closely this can be done. These constraints lead me to wonder whether photonic integration will follow the same historical path as micro- and nano-electronics, the same developmental story. Will there be a Moore’s law for photonics with PIC packing density doubling every 18 months? The billion-photonic circuit is not on the cards, so I doubt that the law will hold.
The diffraction limit of optics and the single-mode criteria set lower limits on photonic size, although plasmonic devices go below those dimensional limits and are compatible with photonics.
I see glimmers today of where LSI can make a difference. A near-term use is a 128x128 array of electro-optical phase shifters for optical beam steering. More speculatively, we have electro-optical logic arrays, spectrometers on-chip, optical neural networks, dense wavelength-division multiplexers and demultiplexers, quantum processors, and optical computers using dense nano-LED arrays.
The government has deeper pockets than industry for sustained R&D efforts
What are the major challenges today making optical devices using a CMOS fabrication process?
RS: A partially-met challenge is to actualise a stable and reliable process in a 130nm or 65nm CMOS node for manufacturing the active and passive photonic parts of the on-chip network. We need process procedures for principal components which are recipes defined with a new design library. Whether to standardise photonics is an open question. When and where to place transistor circuits on-chip is a challenge. Putting transistors on a separate chip is a near-term alternative.
It takes art as well as science to determine the opto-electronic layering and to decide whether the available processing temperatures necessitate fabrication at the front end or back end of the overall process.
I believe that a manufacturing initiative is an essential next step for GFP to convince friends and skeptics alike of the long-term commercial and military value of GFP offered in new generations of energy-efficient ultra-performance chipsets. The government has deeper pockets than industry for sustained R&D efforts, so I believe that the Department of Defense can be the force driving GFP expansion into higher realms at very low costs per chip.
That’s why I welcome the new 5-year Integrated Photonics Institute project as part of the National Network for Manufacturing Innovation, funded by the RAMI bill [Revitalize American Manufacturing and Innovation Act]. It is a bill that would train the workforce, while public-private partnerships will transform research into products and will deploy infrastructure that supports US-enterprise competitiveness.
For the second part, click here