Everywhere you turn, optical engineering is at the heart of new technologies. No wonder 2015 has been named the Year of Light.
Food crops. Microchips. The Internet. Solar panels. CT scans. Holograms. A geek’s shopping list? No – just a handful of life’s essentials, but all of them based on the exploitation of a curious natural wonder: light. With properties spanning the familiar ones of reflection, refraction and diffraction, to sparking photosynthesis in plants to the action-at-a-distance spookiness of quantum entanglement, light is currently the gift that keeps on giving to scientists and engineers. Whether it is to explore in ever-greater detail what is going wrong inside our bodies, to building secure quantum cryptography or constructing an interplanetary optical Internet for future space farers, our thirst for squeezing ever more uses from light is undiminished. Yet, at the same time, 1.3 billion people in the developing world have no access to electric light – and their day screeches to a halt at sundown unless they want to risk fire and respiratory illness with naked flame light sources.
It need not be that way. “Innovative solutions are at hand, such as solar-powered LED lighting,” says Eric Rondolaat, CEO of Philips Lighting in Eindhoven. It’s to further develop such sustainable light-based ideas that the United Nations has dubbed 2015 the “International Year of Light and Light-Based Technologies”, in a bid to show how optical engineering can answer such problems.
“At many special events we’re considering past successes with light and identifying new horizons in light science,” says John Dudley, a physics professor from the University of Franche-Comté, in France, who runs the Year of Light for the UN. One new idea, he says, will be explored via a citizen science experiment in which smartphone light sensors will measure pollution.
So where else is light going? Here Technologist takes a look at some of light’s past successes and where it is headed next.
Communications// Internal reflections
Our hyper-connected world owes its almost instantaneous connectivity to bundles of optical fibres, hair-thin rods of glass that carry signals encoded in flashing laser beams under the oceans and around the world. Developed by a number of engineers through the mid-20th century, optical fibres have largely replaced the long distance wires that had been in use since the first viable electric telegraph was commercialised by Charles Wheatstone and William Cooke in London in 1837. The problem with copper wires? Thanks to electrical resistance, they lose too much of a signal’s power over distance. They also have a limited bandwidth and are susceptible to electromagnetic interference. An optical fibre, on the other hand, has very low light losses and extremely high bandwidth. And as optical fibres are made of non-conducting materials they cannot act as an antenna and so are immune to electromagnetic interference.
The secret of fibres is total internal reflection [Fig. 01]. In a medium of constant refractive index, light travels in a straight line. But if a light beam launched in the middle of a fibre towards one of its edges meets a material of different refractive index, it will bend slightly. The more the index changes nearer the wall, the more it will bend back towards the centre of the fibre. By chemically varying the refractive index of the glass across its width so that it varies from the centre to the edge of the fibre, light beams swerve back and forth until they reach a detector at the end of the fibre.
Progress in boosting fibre speeds has been profound. In 2011 Karlsruhe Institute of Technology in Germany streamed data at 26 terabits per second, breaking a 2009 Danish-set record of 1 tb/s. But in 2014 a research group at the Technical University of Denmark took the record back by sending data at 43 tb/s.
Despite this frequent smashing of transmission records, it is far from safe to assume we will always have the Internet capacity to carry data hungry streaming services (like 3D movies). The more laser beams of multiple wavelengths we launch into fibres, the more likely they are to interact and interfere, causing noise that might lose a signal’s ability to be boosted every 100 km or so by an optical amplifier. As yet, no one knows the capacity limit of the fibres the world has come to depend on, so a multinational group led by a team including Gerhard Kramer of the Technische Universität München, is urgently probing that right now. The late information theorist Claude Shannon worked out that it is possible to encode data so that it can be transmitted through any noisy channel at a maximum capacity – but whether clever coding will lead to the kind of transmission speeds we demand remains to be seen.
Wave and particle
Why is light so useful? Because its mixture of wavelike and particle-like properties lends it an amazing array of applications.
In the 18th century, Sir Isaac Newton realised that light was a wave because of the way a prism refracts light, creating a colourful spectrum. But the sharp-edged shadows that light can create made him suspect it was a particle, too. It was not until the early 20th century that Albert Einstein, in his Nobel-Prize-winning quantum theory, worked out that that light comprises particles – called photons – that travel in waves. He calculated that the energy in a photon was simply equal to its wave-vibration frequency by the multiplied Planck constant.
This means that in addition to the wave reflection effects that make optical fibres work, or the diffraction effect in holograms, LED designers can make an electron drop an energy level, kicking out a packet of energy in the form of a photon of known colour and energy. Conversely, in a solar panel, an incoming photon from the sun can knock low-energy electrons into a high–energy conduction state – generating a current – by transferring their energy to the electron.
Illustrating this wave-particle duality has been difficult, but earlier this year researchers at EPFL set up a standing light wave on a nanowire and blasted it with electrons. This, they say, allowed them to photograph the way the photons in the standing wave interacted with the particles.
Electronics // Speed and more speed
Light is impacting electronics in two major ways: first, in speeding up today’s computer circuits with on-chip “photonic” light-speed communications, and second, in allowing peculiar quantum effects to create secure encryption systems and “quantum computers” that, it is hoped, will operate at speeds today’s computer architects can only dream about.
The photonic idea kicked off in 1982 when David Fraser of Chelsea College at the University of London speculated that it might be possible to gain much more speed from computers by connecting their microprocessors and memory chips with extremely short optical fibres, rather than the printed copper tracks common on circuit boards. He has been proven correct. Nanoscale photonic circuits are now being readied for primetime in cloud-data-centre server chips and supercomputers thanks to work on how to optically connect such chips by research teams the world over, but particularly at the Eindhoven University of Technology and at IBM’s Lab in Yorktown Heights, New York.
Photons play an entirely different trick in a form of cryptography called quantum key distribution (QKD). In this, a cryptographic key that unlocks secret content is represented by the polarisation angles of a long series of photons sent through an optical fibre to a receiver. So a +45 degrees angle might represent a digital 1, and a -45 degrees angle a 0. However, if a hacker tries to intercept this data they are immediately foiled by quantum mechanics: observation of the polarisations, a quantum property, mysteriously destroys them. So the receiver knows immediately that the channel has been breached. Already on the market, QKD is constantly being refined in terms of speed and distance [Fig. 02].
Quantum computing, however, has yet to demonstrate its promise. Conventional digital computers fetch data from memory, process it and then store it. Quantum computers promise much, much faster processing because photons can exist in a state of quantum superposition – that is, entangled in two different states (polarisations) simultaneously. These quantum bits (qubits) should allow exponentially faster processing. This has yet to be demonstrated, but don’t bet against it.
Energy // Spread your wings, feel the force
A remarkable light-powered journey has been underway this year: an aircraft driven by electric motors powered only by sunlight flew more than halfway around the world. Called Solar Impulse 2, the solar-panel-covered carbon-fibre single-seater aircraft is funded by many of Switzerland’s major corporations and experts in solar-energy harvesting. EPFL has had a major role in the plane’s design as the project’s science advisor since 2003.
Solar Impulse 2 is just another example of humankind pushing a light-based technology to its limits [Fig. 03]. Even how to best charge the plane’s batteries is unclear, says project spokesman Marc Baumgartner. “It depends on the season, time, sun angle, region, temperature and many other factors.” However, the insulation on the batteries proved too good, causing them to suffer irreversible damage on the plane’s record-breaking trip from Japan to Hawaii. The team hopes to redesign the insulation and fit fresh batteries, continuing its round-the-world attempt in April 2016. In other words, solar energy is a moveable feast – the amount you can harvest depends entirely on ambient sunniness. It was ever thus. Invented in its current form by Gerald Pearson and colleagues at Bell Labs in New Jersey in 1954, the first silicon semiconductor solar cells managed to convert only 4 per cent of incident sunlight into electricity – one tenth of what is possible in labs today. But if you really want limitless light energy, you have to harvest it in earth orbit. Above the clouds, weather and pollution, and without having to worry about loss of power at night or in colder seasons, a solar harvesting station in geostationary earth orbit could intercept light power continuously. A space-based solar power station would use a 3-km long array of mirrors to focus light on football field sized arrays of solar panels, creating electricity. That power would then be used to generate microwaves, which are beamed down to earth for reconversion to electricity.
“Solar Impulse 2 is just another example of humankind pushing a light-based technology to its limits.”
The advantage is that all the technologies to do this exist. The downside? The eye-watering cost. But with climate change such a threat, the U.S. National Space Society puts it well: “The cost of space solar power development always needs to be compared to the cost of not developing space solar power.”
Medicine // Heal the selfie
Ever since 1895, when Wilhelm Röntgen first used light in what he dubbed the X-ray spectrum to peer at his wife’s hand bones, light has been revolutionising medicine. After Röntgen (and his Nobel Prize) came the laser, used for a variety of therapies, especially in dermatology and for fixing detached retinas, and that was followed by much improved medical imaging machines like the computer tomography (CT) X-ray scanner. But light is not resting on such laurels: its next trick is democratising medicine – via the machine of the moment, the smartphone [Fig. 04].
The light sources and light sensors on a phone make them a great vehicle for diagnostics – and soon a host of conditions will be diagnosable via phone. First out of the blocks is the London School of Hygiene and Tropical Medicine, which, along with a number of expert partners, has developed Peek, a multifunctional eye-testing system based on a smartphone. Peek runs eye tests on the phone screen, but crucially also uses the white-light LED lamp with a lens add-on to allow a medic to examine the retina. In remote places in sub-Saharan Africa and South America, with no healthcare infrastructure, the hope is that preventable diseases that cause blindness can be picked up early. Still another optical phone add on, called the MoleScope lens, allows people to take diagnostic-quality high-definition images of moles that they suspect may in fact be skin cancer.
Using an associated app, made by MetaOptima of Vancouver, Canada, they can then e-mail the HD images to experts who can advise if the user needs to see a dermatologist.
Mental health can also be assessed by phone. The camera can be used as a light sensor that checks how often someone goes out into broad daylight or when they turn the lights out. That might sound strange, but people suffering from depression or loneliness tend to stay indoors and sleep longer. So a team at Dartmouth College in New Hampshire has developed an app designed, in part, to detect depression and loneliness amongst its student population. In early tests it is working well.
One problem: there is no name yet for cell phone-based ailment analysis. Dare we suggest: “selfie diagnosis”?
Entertainment // Helmets on we’re going in
The only 3D images most of us see day-to-day are the still holograms used for authentication on credit cards. But true, wraparound 3D imagery will soon be flooding our minds if plans by Facebook and Microsoft come off. And it won’t be based on diffraction of laser-light as holograms are. Instead, a battery of new light-emission and display technologies will be harnessed. Last year, Facebook bought a start-up called Oculus VR, maker of a virtual reality headset called Oculus Rift. While VR is not new, well-thought-out consumer systems for using it are – and Oculus has taken the time to work out precisely how such a product should work. “We set out to finally deliver on the dream of virtual reality,” says CEO Brendan Iribe. The technology at the Rift’s heart has two organic light-emitting-diode (OLED) screens with no motion blur or visible, dotty pixels and providing a wide field of view [Fig. 05]. It hits the market in January 2016. However, Rift blocks out the real world, so moving around safely will be tough while you use it. To the rescue comes Microsoft, which is developing Hololens, designed to overlay 3D graphics on your real-world view – so you can move around safely and interact with virtual images. Hololens will let you see virtual computer screens, giant TVs and, say, your own photos writ large on your walls, in mid-air or on your fridge door – and interact with them using gestures.
While VR offers a pretty unnatural lighting experience, some researchers are coming to the rescue. At Ecole Polytechnique Fédérale de Lausanne, architect Marilyne Andersen is investigating how piping in ambient daylight – with glass, mirrors and optical fibres – deep inside buildings can improve the psychological wellbeing of the people living and working within it. It’s not easy, though. Andersen points out in a recent paper on this “daylighting” idea that risks include producing too much interior glare or causing overheating. She is right to be cautious: in 2013, a London skyscraper with an ambitious concave frontage began focussing light on a street below, melting car bodywork, bicycle seats, paintwork and carpets in shop doorways.
Lighting design, it seems, should not be taken lightly.
By Paul Marks @PaulMarks12