ALEXANDER GRAHAM BELL was certain his greatest invention would change the world. He was almost right. The telephone was indeed revolutionary, letting people talk to each other across great distances as if they were in the same room. Unfortunately, Bell thought his greatest invention was not the telephone, but the photophone. That was a complete flop.
Perhaps it was just ahead of its time. Because the basic idea behind it – using pulses of light to bounce information through free space – is once again set to change the way we communicate. Radio waves have been the medium of choice for sending signals wirelessly for the best part of a century. But we’re rapidly reaching a crunch in how much data we can send. That, with advances in LEDs and lasers, mean light is starting to beat radio hands down, at least for some applications.
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Using light you could download the equivalent of a DVD box set in the blink of an eye. The ability to send digital signals could also be built into street lights and lighting in homes and offices, giving us internet coverage almost anywhere. And since our walls aren’t generally transparent, light could be ideal for keeping communications secure.
Those championing visible light communication, or Li-Fi, envision a bright future. “Everywhere I see a light I see Li-Fi,” says at the University of Edinburgh, UK, one of the technology’s main advocates. For Haas, light offers an alternative to radio that will let our devices exchange massive data streams without interruption. It opens up new possibilities too. For example, Li-Fi built into traffic lights and car brake lights would allow smart cars to receive traffic updates and exchange dash cam views.
Crunch time
But there are big questions. Though demonstration versions of the technology work well, the cost of components remains prohibitively high for most applications. Even when prices fall, as they inevitably will, does it follow that the telecoms industry will switch its allegiance from radio?
Bell’s photo phone may have been ahead of its time
Mobile communications using low-frequency electromagnetic waves – radio waves – is a spectacularly successful technology. You might say too successful. If you’re at the airport and can’t make a call or connect to the internet, a spike in newly arrived passengers has probably used up the capacity of the nearby cell towers that provide mobile signals. With more and more people sharing photos and streaming video – not to mention the internet of things, from cars to kettles, starting to talk to each other in the background – the problem is getting worse. “We’re throttling the capacity of our devices,” says at Boston University.
There are a bunch of ways to solve this problem. A lucrative one, for governments at least, is to reallocate slices of the radio spectrum, pinching bits from sectors like the military and auctioning them to the telecoms industry. In January 2014 ended with a handful of companies coughing up a record-breaking $45 billion to the US Federal Communication Commission.
Alternatively, multiple messages can be encoded in the same bit of radio spectrum. But implementing that requires ever more sensitive and expensive-to-develop devices like 4G phones. In any case, we are already close to the theoretical maximum number of things that can be overlaid in a single signal. Yet another option is to pack the landscape with cellular towers, so that fewer users have to share any one antenna.
For short-range communication, we can shimmy a little way along the electromagnetic spectrum to slightly higher frequencies, in the microwave region (see “Visibly better”). Wi-Fi is the current king of this arena. But even though it only has a modest range of 50 to 100 metres, you still hit interference problems when there are multiple networks close together, or many users at once.
Shift along to still higher frequencies, and you get to infrared and visible light – a part of the spectrum that is uncongested and unregulated. “Radio and light are the same stuff,” says Texas-based technologist Kevin Ashton, whose work at the Massachusetts Institute of Technology in the late 1990s drove the widespread adoption of radio-frequency identification (RFID), now used in contactless payment systems, for example.
“We’re not saying Li-Fi is a replacement for radio,” says Haas. “But this will provide relief elsewhere.” Haas, who coined the term, has been working on the technology at his lab in the Alexander Graham Bell building at the University of Edinburgh – a fitting place for the revival of an old technology (see “Speech by sunlight“). He foresees that five or 10 years from now, our devices will not only hook up to cellular towers and Wi-Fi networks, but to light bulbs. He isn’t alone in thinking this way. “We’re already spending all this money to light a room and send photons everywhere, so why not embed data in it?” says Little.
Li-Fi works by turning an LED bulb off and on to create a stream of bits for data transmission. The principle is just the same as sending a Morse code signal by flashing a torch – except that it happens billions of times a second. The faster the flicker, the more data you can send, and at the speeds in question there should be no danger of the flashing triggering epilepsy in susceptible people.
Although a pioneer of the technology, Haas didn’t invent it. He credits Japanese researchers at Keio University and IBM for laying the groundwork in the 1990s. Haas became involved in 2000, when he was working for Siemens. It was around then that three things happened: the practice of sending and receiving photos and video with our phones started to take off, the prospect of a spectrum crunch raised its head, and white LEDs came on the market.
Haas started playing around with those lights. His breakthrough was figuring out how to modulate the output of a single LED so it could carry multiple data streams at once. In 2006, he demonstrated this using an LED bulb fitted to an off-the-shelf desk lamp. He used it to transmit some cartoons, he recalls.
Since then, Haas has regularly given talks to the public about the technology’s potential, and in 2012 he founded a spin-out company, now called , to develop products for the consumer market.
But while PureLifi does have a few niche customers, the tech has yet to make a splash with the public. Ashton for one remains to be convinced that Li-Fi will have the kind of impact Haas and others expect it to. “A new technology with no installed base has to be very, very special to overcome all the advantages of incumbency,” he says. “There may well be certain conditions under which visible light is a far better solution than any alternative, but it’s not yet clear what they are.”
Forging an entirely new kind of connectivity for cellphones and other wireless devices is certainly a tall order. But Haas is undaunted, given the potential pay-off. In a lab test last year, at the University of Oxford and colleagues demonstrated an infrared data link able to carry 224 gigabits a second – fast enough to download several hours’ worth of high-quality video in an instant. In principle, visible light should give similar results. The researchers think that the technology could theoretically transmit at 3 terabits – trillions of bits – a second.
Using standard LEDs, Haas has built relatively cheap devices that transmit at rates similar to existing home Wi-Fi. For applications such as streaming video to multiple users, however, their performance can be significantly better because, rather than sharing a Wi-Fi router, each user can connect to their own light source. A dedicated streaming box could then talk to each light via the electrical cabling, sending video streams to every user simultaneously at rates equal to what a single user hogging a Wi-Fi router would enjoy.
“That capability has been shown, and now it’s about making applications,” says Haas. One of these is a Li-Fi demo Haas is building for the San Francisco basketball team Golden State Warriors, who are planning a new stadium for 2018. By harnessing the thousands of light bulbs in a stadium, you would get far better performance than Wi-Fi, he says: everyone could connect to the lights to watch instant video playbacks on their phone, for example.
Indoor location-tracking is another promising application. GPS does not work well indoors, and its error margin can be too big for practical purposes in any case. But the lights above your head can “see” exactly where you are. If you are in a supermarket, say, they could talk to your phone and send coupons to it for products on the shelf that you happen to be passing. Since little data needs to be transmitted, such systems can be set up relatively cheaply. Philips is testing one at a Carrefour department store in France.
If you don’t like that idea, Li-Fi could also boost privacy. Wi-Fi leaks through walls, letting people easily hijack or eavesdrop on signals. But light cannot escape a windowless room, making Li-Fi a straightforward way to keep wireless communication secure. Some companies are already using it for this reason, says Little. The technology would suit places like hospitals or airplanes, where privacy is key and radio interference can be a problem. Haas adds that certain intelligence services are also interested.
Traffic signals
The killer app, however, may be smart cars and roads. Almost every car-maker now has a division trying to beef up vehicle-to-vehicle communication, which will allow cars to share info about busy routes or dangerous road conditions. With Li-Fi, brake lights could notify the computer in the car behind that they are braking, for example. And LED traffic lights could broadcast details of congestion or roadworks ahead.
Since Li-Fi relies on the user being able to see the light source directly, it is less prone than radio to interference in crowded areas. “It’s likely that cars are in a congested area, and that’s where you get a problem with interfering radio signals,” says Little.
In the meantime, the first mass market Li-Fi products are already on the market: toys and games. Hasbro sells an electronic Scrabble-style game, in which the tiles have small LED lights that let them talk to each other to determine if they are in a sequence that spells a word, or not. Disney has a small team working out ways to use LED lights in their parks to send videos and games to the phones of people waiting in lines to keep them entertained. They are also developing table lamps that talk to toy cars with headlights that in turn talk to your computer.
These few examples aside, it will be a year or two at least before the costs of LEDs and Li-Fi chips come down enough to make mass adoption viable. Even Haas doesn’t yet have a Li-Fi network, or even a Li-Fi enabled desk lamp, at home. But he’s thinking about it. “The kids would love it,” he says.
(Images: Getty Images, Mary Evans)
Calling long-distance
For fast off-planet conversations, sometimes only light will do. In 2013, NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) demonstrated this by sending a message back to Earth by laser. Using near-infrared rather than visible light, in part to avoid blinding anyone, it was able to transmit data at 600 megabits a second – six times faster than NASA’s Lunar Reconnaissance Orbiter, for example, which relies on radio waves. By the time LADEE’s laser signal arrives from the moon, it only amounts to a few photons per bit of information – but that is enough for NASA to pick up using relatively cheap 40 centimetre optical telescopes.
For NASA, laser light communication has been a long time in the making. NASA cancelled projects in this area in the 1970s, 1980s and 1990s, says Don Cornwell, mission manager for LADEE. But advances in telecoms tech brought fibre-optic cables and better lasers, ones that NASA could repurpose if they proved robust. “We bought them off the shelf and saw which ones survived a shake and bake,” says Cornwell.
Radio waves tend to fan out with distance, whereas lasers keep a needle-like focus, making them the better option the further you go, says Cornwell. Plus lasers operate outside of the spectrum crunch (see main story), something even NASA has to contend with when using radio frequencies.
Cornwell says firms have been calling NASA for advice on how to make laser communications work – including Facebook, which wants to use laser-equipped drones to beam the internet out of the sky to remote sites in Africa or Asia. “A lot of companies are looking into this Sky-Fi,” says Cornwell.
Speech by sunlight
Having invented the telephone, Alexander Graham Bell moved on to something even better – or so he thought. In 1880, Bell showed off his photophone for the first time. It sent his voice through the air to a colleague 200 metres away who was holding a telephone connected to a big dish – the first wireless phone call.
The photophone worked by turning speech into vibrations in a diaphragm, then bouncing light off it. A receiving diaphragm caught the reflected beam, which contained subtle variations encoding the vibrations, and converted it back into the sound of the speaker’s voice. Bell thought the photophone could replace expensive telephone cables. But since the electric light had only just been invented and domestic lighting was still some years away, he had to rely on sunlight to make the system work.
“I have heard articulate speech by sunlight! I have heard a ray of the sun laugh and cough and sing!” he wrote at the time. “I have been able to hear a shadow and I have even perceived by ear the passage of a cloud across the sun’s disk.” But clouds were also the photophone’s downfall. It couldn’t be used when the sun was obscured.
Still, Bell was so proud of the photophone that he wanted to name his second daughter after it. Luckily for her, Bell’s wife thought otherwise and they named her Marian instead.
This article appeared in print under the headline “Light fantastic”
