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Building circuits of light
Move over, electrons. Ton Backx and his team are putting photons front and centre as they lay the groundwork for the coming era of photonics.
Photonics has long been heralded as the next big thing in computing, a technology for the 21st century in which photons replace electrons to offer light-speed super- energy-efficient data processing and myriad new devices that benefit society. Yet for the most part the dream of light-based chips has yet to make it out of the lab.
► Ton Backx – serial entrepreneur and CEO of both the Institute for Photonic Integration and Photon Delta at Eindhoven University of Technology – believes we are now on the cusp of ushering in this new photonic era. And he is putting in place the fundamental research, technology and infrastructure needed to do so.
TECHNOLOGIST. What is photonics?
TON BACKX What you are most likely familiar with are micro-electronics and microelectronic circuits, out of which your computer systems are built – basically any product that you buy these days makes use of these microelec-tronic circuits. Photonics is similar to microelectronics. In photonics devices, instead of using electrons as the information carriers, you use photons, which are packages of light energy.
T. Why do we need an alternative to electronics?
TB. The problem we are facing is that the handling and processing of data, which we do in data centres, is still done by microelectronics, and microelectronics is not very energy efficient. In 2016, 5% of all electrical power generated was used for processing data in data centres and telecommunications centres – and that’s growing by a factor of about 1.7 per year. Also, with the Internet of Things, Netflix, Facebook, YouTube and other Internet activities, data centres are growing capacity by a factor of 2 per year.
Microelectronics is a limiting factor if we want to grow traffic at the same rate it has been growing for the past 20 years. And this means that we need to do something different very rapidly.
T. How is photonics better?
TB. Looking at the energy efficiency of the most advanced microelectronics chip, processing a bit of data consumes roughly a picojoule. When you do this for photon-based technologies, the energy budget is less than a femtojoule per bit – so a factor of a thousand less energy demand. The other advantage is that when you look at the frequencies you can cover with photons, they are a factor of about 1,000–10,000 higher than the spectrum you can cover with electrons. So you can go to much higher frequencies and you can do that far more energy efficiently.
T. What can you do with a photonic circuit that you can’t with microelectronics?
TB. You can build a single-circuit LIDAR [like radar, but using light waves]. LIDAR is extremely important for autonomous cars in the automotive industry. When you can provide the full functionality in a single circuit and develop that at a price level that complies with what the automotive industry needs, then basically you are in business.
You can build an optical coherence tomography (OCT) sensor system. OCT is the optical equivalent of ultrasound, but it can do 3D visualization of tissue at the cell level and even look at the metabolism within the cells. This technology is already used today but consists of a cabinet stuffed with all sorts of equipment. What we can do with photonic integrated circuit technology is reduce a cabinet of electronics to a single photonic circuit – and with higher accuracy and higher resolution.
Another application is sensor systems for aeroplanes; you build a single photonic integrated circuit and attach it to a bundle of optical fibres to build a nervous system, like ours in the human body, that can continuously monitor load on the heavily loaded parts of the aeroplane. Development is already underway, and prototypes have already been implemented in Airbus and Boeing aircraft.
T. How do you build integrated circuits out of light?
TB. When you look at a microelectronics circuit, you have active components which are the transistors, and you have passive components; resistors and capacitors. With those three building blocks and the electrical wires connecting them, you can build any type of functionality.
When you look at photonic integration technology, you also have basic building blocks, which are an optical amplifier, a polariser and a phase shifter. With those three components, together with a light guidewave, which is the equivalent of an electric wire, you can engineer any functionality to build any type of processing you can do with microelectronics. The only difference is that the information carrier is going to be a photon instead of an electron.
T. What is holding up photonic circuit deployment?
TB. If you look at photonic integrated circuits today, the complexity is comparable to the complexity the microelectronics industry was integrating into their circuits in the mid-1970s. That means we can integrate about 3,000 components onto a single circuit. It is not a very high integration density when you can integrate billions of components into a single microelectronics circuit. So, one of the major challenges we are facing is increasing photonic integration density.
T. Where will the first applications of photonic integrated circuits be?
TB. The leading market will be data and telecommunications. We have to think of communication speeds of 400 Gb per second across optical interconnects in 2019 and rapidly growing to 1–2 Tb per second in the coming years, which can only be realised with photonic integrated circuit technology.
The same applies when 4G is replaced by 5G. We need to replace current base stations with something much smaller and far less energy demanding that can handle much more traffic – which means we have to use optical technology. I expect that photonic integrated circuits will be the next-generation technology that will be used to solve many of the challenges the world faces.
T. What kind of challenges?
TB. If you look at how we want to move towards preventative healthcare, we need to be able to detect illnesses, like cancer for example, at a very early stage and at low cost. The sensor systems that can do that – detect cancer, monitor your blood – are photonic integrated technology-based. When you look at smart cities, one of the key things is the control of air pollution, and the sensors you need for that are the photonic integrated circuits that can provide that functionality at low cost. There will be many, many applications we can’t dream of today.
T. Where does silicon fit into this?
TB. Silicon is a very attractive material for photonic applications simply due to the fact that we have an infrastructure for processing it, with very extensive factories all over the world that can mass-produce silicon-based circuits. But from an application point of view, silicon is not attractive at all because it has the highest losses – the losses are at least a factor of 10 higher than in indium phosphide – and you can’t produce active components: you will always need another material connecting to these silicon-based photonic circuits to get light into the circuit.
The only problem with indium phosphide is that, as a semiconductor material, it is not as far developed as silicon is, so it requires further investment in basic infrastructure to provide indium phosphide materials and circuits based upon them at scale levels comparable to what you can do with silicon today.
T. What is your team at TU/e doing to drive forward photonic chip technology?
TB. We are working on fundamental research into how we structure our materials to physical properties in terms of photonic applications, and being able to synthesise these materials to build devices with them.
We are also formulating similar types of standards to those in the microelectronics industry. This involves fundamental research on standard building blocks – standard optical amplifier, phase shifter, polariser – from which standard functional blocks can be made, and ultimately the circuits with the functionalities we are looking for. It also means creating the software libraries which enable people to design photonic integrated circuits.
Ton Backx began his professional association with TU/e in the 1970s. He received his MSc in Electrical Engineering in 1976 and PhD in 1987, stemming from mobile-base control process optimisation research intended for all kinds of industrial applications he conducted for Philips. Backx then started and successfully ran a number of advanced control companies, alongside a part-time professorship at TU/e from 1992, before being invited to become Dean of the Faculty of Electrical Engineering in 2006.
Ten years later, Backx stepped down from his position as Dean to take on a new role as CEO of TU/e’s new Institute for Photonic Integration and Photon Delta, organising all photonic integration-related activities at and around the university. He is also a member of the governing board of the EuroTech Universities Alliance.
Electrons vs. Photons
In copper wires, data is transmitted through electron motion. Because of the friction with copper atoms, energy is lost. Thus, electronic circuits make sense only for the transmission of small quantities of data over short distances.
Photons travel through fibre-optic cables made of glass or polymers. There is no friction inside the cable and therefore no loss of energy. As there is no electronic interference, several signals can be sent simultaneously.
Why is it difficult to put photons on chips?
The problem is integration density. Billions of components can be put on microelectronic chips but only few thousands on photonic chips. Increasing the density of photonic single circuits will be one of the big challenges in coming years.
The terms that make photonics
A description of nature in which energy and other quantities are defined as discrete packages with the characteristics of both particles and waves.
An elementary particle of light which exhibits wave-particle duality, meaning it has the properties of both waves and particles. Our modern understanding of the photon is attributed to Albert Einstein, who used it to explain experimental observations that did not fit the classical wave model of light. The photon always moves at the speed of light in a vacuum.
Light Amplification by Stimulated Emission of Radiation. A device that emits light through a process of optical amplification, based on the stimulated emission of electromagnetic radiation. Lasers have an enormous range of applications, from fibre optics in communications, to laser printers and modern surgical procedures.
Cables used as a means of transmitting light between two locations. In communications they permit transmission over longer distances, and at higher bandwidths, than wire cables. Optical fibres are manufactured by drawing glass (silica) or plastic to a diameter similar to that of a human hair.
By Benjamin Skuse
Displays of the future
From LCD televisions to the latest force-sensitive touchscreen technology, electronics and photonics are pushing the envelope ever outward.
Liquid crystal display
Liquid crystal displays (LCDs) appeared on the market in the 1970s. Within 20 years, they became the leading technology for watches and calculators as well as for laptop screens, desktop monitors and televisions. LCDs compromised the long-standing success of cathode-ray tubes and large-screen flat plasma display panels.
How it works
In LCDs, each pixel – the switchable unit in a display – consists of a layer of liquid crystal (LC) material trapped between two polarising glass plates. Transparent electrodes on the glass plates apply an electric voltage that switches the LC on and off. In many LCDs, an additional backlight illuminates the LC material, for example blue light-emitting diodes (LEDs) combined with broad spectrum phosphors provide a white light source. Traditional colour LCDs use red, green and blue filters.
Strengths and weaknesses
The image quality of LCDs has improved over the years, and photonics occasionally lends an additional hand: for example, quantum dots – nanometre-sized semiconductor particles – can be incorporated into backlight units to obtain more vibrant colours without the need for bespoke filters. Nevertheless, LCDs have issues with contrast, depending on the light conditions, and a limited viewing angle.
Organic light emitting diodes
Around 2012 a different kind of display started to overtake LCDs. The new technology, building on basic research that dated to the 1960s, was based on organic light-emitting diodes (OLEDs), in which organic semiconductors replace the materials found in conventional LEDs.
How it works
In OLED displays, thin films of organic material are placed between two electrodes. A voltage is applied, causing the organic semiconductor layers to emit light at specific wavelengths. Red, green and blue OLEDs can be arranged as pixel arrays to produce full-colour images. The emission wavelengths of the OLEDs can be tuned by varying the applied voltage.
Strengths and weaknesses
OLEDs offer several promising features for flat display panels. They don’t require backlighting, which reduces the thickness and overall weight of the device. OLED displays produce vivid colours, achieve better contrast and guarantee wide viewing angles. The versatile fabrication process of OLED displays has also allowed companies to explore previously uncharted territory, such as flexible and transparent displays. The latest smartphones by Samsung and Apple are based on OLEDs. On the downside, this technology is more expensive than LCD, and the materials involved can degrade, for example due to exposure to oxygen and water.
In the 1960s, a group of researchers at the University of Illinois developed an optical touch screen based on scanning infrared technology for the computer learning system they were developing, PLATO (Programmed Logic for Automatic Teaching Operations). Decades later, devices such as Apple’s first iPhone began to incorporate touch functionality. Now it is increasingly sought after in all types of displays.
How it works
The idea behind scanning infrared technology is relatively simple: a grid of infrared light beams (often generated by LEDs) covers the screen. Phototransistors translate an interruption in the light as a touch input. This technology is durable and can be scaled to large areas, but it requires a large number of light sources and detectors. Most present-day existing touchscreens in smartphones and tablets rely on electrical rather than optical technologies. For example, in projected capacitive touchscreens, the input given by the user’s finger tapping on the surface of the display changes ever so slightly the capacitance between conductive electrodes.
Strengths and weaknesses
Everyone agrees that touchscreens are a success, but the ideal solution still hasn’t been found. For example, sensitivity to the force of the applied touch would be desirable. Touchscreens can also become challenging under certain conditions, such as if the surface is wet.
In 2011, researchers at the Technical University of Denmark (DTU) presented an optical touchscreen based on waveguide sensing. The DTU team’s idea has now been taken up by WaveTouch, a joint venture between OPDI Technologies, a Danish optics innovation company, and O-Net Communications Ltd in Hong Kong. The optical nature of this touchscreen technology makes it – in principle – more robust to ambient conditions traditionally disruptive to electronics-based touch-sensitive platforms. WaveTouch will be targeting niche markets where their features may have an advantage against available technologies – for example, for outdoor portable navigation.
How it works
A single laser light source is placed in one corner of the screen, producing the light beams that constitute the touch-sensitive grid. The beams propagate in a planar waveguide (in this case a transparent plastic plate) through total internal reflection (TIR). Here, a touch input given by a finger or stylus on the sensitive surface breaks the TIR, allowing some light to escape the waveguide. TIR in the context of touchscreens is not an entirely new concept, but the specific realisation devised at DTU is novel.
Strengths and weaknesses
Because WaveTouch technology requires only one light source and a small detector array, the number of components is drastically reduced. The touchscreen is also sensitive to the force of the applied touch, which opens the door to an even broader range of functionalities.
By Gaia Donati
Harnessing optics to handle the data crunch
Photonics may hold the answer to coping with huge data traffic. But a big challenge remains: converting electronic data into light on silicon chips.
The world is on the brink of a massive traffic jam – not in vehicles, but in data. In many countries, traffic is increasing by around 40% annually, according to Beyond Fast, a report conducted by the Eindhoven University of Technology and Dutch consultancy firm Dialogic. That rate is only set to rise, with global internet traffic projected to increase nearly threefold over the next five years.
Data centres bear the brunt of the strain. “Seventy-five per cent of data exchange is within and between data centres – much larger than the exchange between centres and users,” says Bert Jan Offrein, photonics manager at IBM Research.
Speeding things up
Data centres are scaling at a staggering rate, and on numerous fronts, to keep up with the rise in cloud computing demand. Every new generation of processor chip contains more transistors than the last, server boards contain increasing numbers of processors, and data centres get bigger to house more server racks. The largest data centre on Earth – the Citadel Campus in Tahoe Reno, Nevada – now covers nearly 700,000 square metres, the equivalent of 61 football pitches.
The downside of storing data racks in such vast configurations is that even communicating rack-to-rack requires data to be pushed over distances and at bandwidths that current fibre-optics channels weren’t designed to support. Moving beyond a rate of ~100 Terabits per second per fibre core creates severe distortions in data signals and can lead to a phenomenon called fibre fusing, in which the fibre core melts.
Lars-Ulrik Aaen Andersen and his fellow researchers at the Technical University of Denmark (DTU) are leading the way in developing next-generation fibre optics that can handle this traffic problem. They have broken the existing transmission barrier by building optical multiplexer systems based on what’s known as a high-count, single-mode, multi-core fibre. When combined with amplifiers, the system is capable of ultra-high capacity optical transmission of 1 petabit – 1015 bits, the equivalent of around 223,000 DVDs – per second over 1,000 km.
“The exciting thing about breaking this capacity crunch is that it’s not speculative theorising; we’ve been able to physically demonstrate it,” says Andersen. The system could potentially lead to a tenfold reduction in the average data centre’s cost, energy and space per bit.
Moving modulation on-chip
In order to move data through a fibre optic cable, the electronic signal on the chip must be converted to light. This is traditionally performed in-cable, meaning lasers, detectors and electronic devices that help modulate the pulses of light all sit within the cable housing. Transmission performance improves when this electro-optical conversion happens as close to the chip as possible. So research labs around the world have been looking at ways of bringing the optics ever closer to the processor. The ultimate goal is to perform the conversion on the chip itself. That would allow electrical and optical pathways to run side-by-side at the nanometre scale. Converting electronic data into light on silicon chips is complicated, says Jonathan Finley, a researcher at the Technical University of Munich’s Walter Schottky Institut. “In computing, semiconductor physics is based on 60 years of working almost exclusively with silicon, and silicon doesn’t emit light.” Enter the new field of materials science: silicon photonics.
Finley and his colleagues have developed tiny nanowire lasers 1,000 times thinner than a human hair that can generate and route light on the chip. “We’ve grown filamentary whiskers of gallium arsenide on top of silicon waveguides,” Finley explains. “The material isn’t new; it was used in the first laser ever demonstrated. However, we discovered that when you make these crystals very thin – in this case 300 nm in diameter – they perform incredibly efficiently.”
To put this performance in perspective, a typical search engine enquiry uses around 1 nanojoule of electricity per bit. The lab record using these new structures currently stands at a femtojoule per bit: “That’s a million times more energy efficient per bit of information,” Finley says. “When scaled up, the effect on IT’s global energy consumption would be drastic.”
In the last four years, a number of other labs have also demonstrated optical generation on silicon chips, including Andersen’s lab at DTU. “Some of our work with on-chip laser communication is really important,” he notes. “I don’t think we’re far away from a breakthrough that will make it into a commercial product.”
Finley is equally optimistic. “I’d estimate commercial rollout of on-chip lasers sooner than many people predict: around six to eight years,” he says.
Progress with polymers
In this new era of silicon photonics, polymers may make it possible to integrate photonics on both compound semiconductors and silicon platforms. Under the guidance of British photonics pioneer Michael Lebby, US-based Lightwave Logic Inc. has developed novel ridge waveguide modulators using organic polymers. The material is more temperature stable than previous designs, and provides a platform with vast potential for scaling performance and energy efficiency. And because the polymers can withstand extreme heat, they can be sprayed onto the silicon during standard chip production processes.
The modulators already have bandwidth capable of transmitting data rates greater than 50 gigabits per second (Gbps) and could be used in various 4 x 50 configurations to transmit 400 Gbps, a data rate many data centres are moving towards. The team is now working on advanced polymer structures that will enable transmitters to operate up to 800 Gbps. “Polymer photonic integrated circuits will address the challenges facing the ‘heavy data’ industry over the next decade,” Lebby says. “With high temperature stability, reliability, high performance, low power, and simple fabrication techniques, polymers are ideally suited as a vehicle to create polymer engines not just for data applications, but for healthcare, consumer, and automotive industries as well, as the drive to battery-based hand-held products grows.”
A new era for photonic computing
With photonics moving deeper and deeper into computer systems, could the future of computing be completely photonic? Researchers are already imagining a future in which computing eschews electrons altogether in favour of light. “The biggest misconception around optical computing is that it would work like the computers we have today,” Finley says. “Electronic transistors perform particular mathematical operations when you input 1s and 0s. In optical computing, the transistor might differentiate something, or integrate it, or change its phase: the outputs could be far more nuanced.”
The energy efficiency of all-photonic systems is particularly interesting to engineers working on neuromorphic computing, which mimics the brain’s architecture. “Electronic computers are relatively slow, and the faster we make them the more power they consume,” says University of Exeter Professor C. David Wright. He recently helped demonstrate a fully integrated all-photonic synapse that resembled its biological counterpart.
“Conventional computers are also pretty ‘dumb’, with none of the in-built learning and parallel processing capabilities of the human brain,” he says. “We tackle both of these issues – by developing not only new brain-like computer architectures, but also by working in the optical domain to leverage the huge speed and power advantages of the upcoming silicon photonics revolution.”
By Ben McCluskey