LEDs Are Just a Holding Place: Get Ready for Laser LiFi

On October 26, 1958, Pan American World Airways whisked 111 passengers on a Boeing 707 from New York to Paris. With that, the age of commercial jet travel was on. Piston-driven propeller planes would continue, but jet engines such as the one on the 707 flew faster and farther. They would rule the skies. Now the developers of laser LiFi hope that their own 1958 is coming soon.

Like the early developers of jet engines, they believe they have a technology that will usher in significant strides in velocity, distance, and several other key performance areas compared to the propellers they are trying to replace - in their case, LiFi transmitted via LEDs.

"Any motivation to want to use LiFi to begin with is the reason that one would want to ultimately go with laser LiFi," said Paul Rudy, co-founder of Santa Barbara, CA-based SLD Laser, where he is also senior vice president of business development. "Laser has substantially higher speed capability. You're talking about orders of magnitude faster than any LED."

Chao Shen, co-founder and technical lead of SaNoor Technologies, the spinout of Thuwal, Saudi Arabia's King Abdullah University of Science and Technology, picked up the thought.

"There are many advantages to using lasers," Shen said. "One can have 100 times higher speed and 100 times longer transmission distance when using laser LiFi in comparison with LED LiFi."

And none other than the man regarded by many as the "Father of LiFi," Harald Haas, sees the technology's future lying in laser light, despite having a commercial interest in LED LiFi. Haas cited several reasons why lasers will emerge, with speed among them.

"It can go an order of magnitude faster, I see a clear path to 100 gigabit per second in the next year or two, and we are looking at one terabit per second in the next five years," said Haas, a professor at the University of Edinburgh in Scotland, where he is chair of mobile communications.

By comparison, 100 Gb/s is 100 times faster than the 1Gb/s that Haas' commercial enterprise, pureLiFi, has experimentally demonstrated in public using LEDs, and probably 400 times or more faster than LED LiFi has achieved in any practical setting. Add a factor of 1,000 by the time laser speeds hit a terabit.

Shen, Rudy, and Haas all presented at SPIE Photonics West in February, where they joined a number of other laser and light communication experts in a 90-minute Light-Based Sensors and Communication panel session. They described the progress to date and the challenges that lie ahead on the road to making laser diodes a mainstay of wireless data communications.

Light vs radio

To appreciate the journey, first, a quick LiFi primer: LiFi, short for light fidelity, is an evolving wireless communications technology that transmits data through the air via a modulated light source. It uses light waves rather than the radio spectrum tapped by WiFi, cellular, Bluetooth and other more commonly known wireless systems.

LiFi comes with other benefits as well. Lightwaves do not cause electromagnetic interference the way radio waves do. That means they can potentially transmit in areas where other wireless technologies can cause problems, such as in hospitals, on a factory floor or in a plane. There's a security benefit too in that they are harder to intercept than radio waves because they require a direct line of sight to the light source - they do not travel through walls.

Although LiFi traces its commercial roots back to 2012 when Haas co-founded pureLiFi in Edinburgh to deliver LiFi through LED light sources, it has yet to take off in any big way.

One reason is that makers of smart phones, laptops and gadgets have yet to embed LiFi receivers in their devices, the way they do with WiFi. And they look unlikely to do so until at least 2021, as a standards battle drags on between backers of protocols from the Institute of Electrical and Electronics Engineers (IEEE) on the one hand, and of an approach endorsed by the International Telecommunication Union (ITU) on the other.

Until then, end users will have to attach USB sticks or other types of optically equipped "dongles" to their devices in order to communicate via LiFi.

Meanwhile, optical specialists like Shen, Rudy and Haas are working hard at advancing the state-of-the-art, trying to move it from LED chips to laser chips. "Why would anyone want to use LEDs [for LiFi] in the first place? The answer is LEDs are in light bulbs today, are safe and reliable and you can leverage the fact that the cost structures are already sort of consumer style," noted SLD's Rudy.

A high-speed laser LiFi transmitter developed by SaNoor Technologies allows the LiFi transmission data rate going beyond 10 Gb/s. Credit: SaNoor Technologies

Horses for courses

"LEDs are wonderful for lots of things, but LEDs are not high-speed devices," he continued. "What's happening is you're getting data rates that are pretty similar to WiFi. Maybe you'll get tens or hundreds of megabits per second. But you're not going to get 10 or 20 gigabits per second from a high lumen light bulb that lights up a room. LED LiFi is constrained by the device."

And as SaNoor's Chen explained, the device - the LED - uses a fundamentally different and slower light emitting process than does a laser chip. The LEDs' spontaneous emission technology is paced by a relatively sluggish electron link, while the stimulated emission of laser chips can be modulated at much higher frequencies, meaning much faster connection speeds can be delivered.

With that and other attributes working in lasers' favor, Haas at the University of Edinburgh is confident of breaking speed barriers. As part of a joint five-year project led by the University of Leeds and including Edinburgh and the University of Cambridge, Haas foresees demonstrating 100 Gb/s speed "within one-and-ahalf to two years."

The project, called Terabit Bidirectional Multi User Optical Wireless System for 6G (TOWS) does not intend to stop there. As its name implies, it is targeting a terabit a second, a threshold it thinks it can hit by March 2024, when not coincidentally its £8 million funding from the UK government's Engineering and Physical Sciences Research Council (EPSRC) expires.

The project also includes a long list of external academic and industrial collaborators who have pledged support from around the world.

One of those collaborators is Airbus, perhaps because laser LiFi enthusiasts believe that the technology could become a medium for plane-to-plane or plane-to-ground communication. (Air France recently trialed LiFi-delivered data service on a Paris-to-Toulouse flight aboard an Airbus A321). Other industry collaborators include Babcock International Group, Cisco, Microsoft, Deutsche Telekom and the BBC. From academia, King Abdullah University of Science and Technology - which will also be part of today's Light-Based Sensors and Communication panel - is connected to the project, as is China's Tsinghua University and the University of Science and Technology of China, as well as Britain's University of Oxford, University College London, and Bristol University, to name just a few.

In the distance

SLD, which already makes laser light sources that emit light at a distance for illumination purposes such as flashlights and car headlights, is adapting them for LiFi communication purposes as well, with some amount of speed and distance trade off. Rudy noted that an SLD flashlight is currently capable of throwing light across a kilometer; that sort of distance could probably handle data transmission speeds of around 10 Gb/s, he said.

Haas envisions distances even longer than a kilometer, a target that he called "only a threshold." While his Edinburgh team thus far has maxed out at about 80 meters at a Gbit per second speed, he is confident in stretching that distance. "We'll see how much we can get beyond a kilometer," he noted.

A lot of the work on laser LiFi will proceed in lockstep with work on laser lighting in general, as developers try to move laser chips more and more into general and specialty lighting applications.

But while laser light for general illumination might compromise on communication specifications, the speeds and feeds should still be superior to LED LiFi.

Laser LiFi will come in a variety of forms. Sometimes laser chips will be purpose built for LiFi communications, with no illumination in mind. Case in point: SaNoor's Chen noted that infrared laser LiFi could overcome the efficiency challenges that face high-speed laser LiFi, so SaNoor is working on infrared.

But it is also developing green and blue lasers, again not for illumination, but specifically for data transmission underwater, where infrared is ineffective. In fact, underwater is a major target application for SaNoor, to help marine exploration vehicles transmit data to ships, buoys and so forth. (SaNoor also makes lasers for illumination underwater and in other settings).

Collectively, Chen, Rudy, and Haas see a broad set of applications for laser LiFi, as it potentially helps on-road cars and trucks communicate with each other, supports vehicle-to-infrastructure communications, planeto- ground, plane-to-plane and so on.

It could also become a broadband transmission technology when outfitted on streetlights. Another potential application: Haas sees laser LiFi supporting individualized replays at sports stadiums that focus on particular players and might include augmented reality features.

It will come with plenty of challenges. While lasers in principle can modulate much faster than LEDs, engineering them to do so will take some doing, noted Haas. Equally, in its early stages, high-speed laser LiFi is energy inefficient. He quipped, "We have to make sure that we don't need a power plant next to the transmitter and receiver to make it operational. We are only at the start of the LiFi revolution. It is really important to unlock the wireless connectivity of the future."

Mark Halper is a freelance business, technology, and science journalist who covers everything from media moguls to subatomic particles. A version of this article was originally published in the 2020 Photonics West Show Daily.

Related SPIE content:

Harald Haas: LiFi: Transforming Fibre into Wireless

Harald Haas: Progress in LED technology removes barriers for LiFi communication

Advanced LiFi technology: Laser light

Can LiFi stave off the wireless spectrum crunch?

High-speed wireless networking using visible light