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Lasers charging smartphone
A smartphone is powered up using a laser-based charging system. The charging laser and guard lasers are normally invisible to the human eye, but red beams have been inserted in place of the guard beams for demonstration purposes. (University of Washington Photo / Mark Stone)

Engineers at the University of Washington have demonstrated that it’s possible to charge up your smartphone using laser beams.

Perhaps the deeper question is, why?

If laser charging can be conducted quickly and safely, that would mark a big step toward freeing up mobile devices ranging from phones and tablets to drones and laptops.

The beaming system is described in a paper published online in the Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies. It can deliver a steady 2 watts of power to a 15-square-inch area from a distance of 14 feet, or from up to 40 feet away with further modifications.

“The beam delivers charge as quickly as plugging in your smartphone to a USB port,” co-lead author Elyas Bayati, a UW doctoral student in electrical engineering, said today in a news release. “But instead of plugging your phone in, you simply place it on a table.”

As you might expect, it’s not really that simple.

Bayati and his colleagues had to answer three big questions: How do you get the laser to sense the mobile device’s location? How do you keep the invisible, near-infrared laser beam from hitting someone who gets in the way? And how do you keep the device that’s being charged from heating up to hazardous temperatures?

To address the first challenge, the researchers programmed their experimental smartphone to signal its location by emitting high-frequency audio chirps. The chirps are inaudible to human ears, like a dog whistle, but strong enough to be picked up by small microphones on the laser beam emitter.

When the emitter senses that the phone is sitting on the desired charging surface, it begins to switch on its laser system, aimed at a thin power cell that’s mounted on the back of the smartphone.

A high-powered charging beam emanates from the central port of the UW team’s prototype laser emitter. Four low-powered guard beams are emitted from ports surrounding the guard beam. Next to each guard beam port are clear photodiodes, which detect the guard beams when they’re reflected back to the emitter by retroreflectors on the phone. (UW Photo / Mark Stone)

But there’s not just one laser beam. The charging beam is surrounded by a set of harmless, low-power laser beams. In order for the charging beam to fire, the guard beams have to have an unimpeded shot at a corresponding set of tiny retroreflectors placed around the power cell.

If the retroreflectors don’t bounce the guard beams back to the emitter, the charging beam is terminated.

Co-author Shyam Gollakota, an associate professor in UW’s Paul G. Allen School of Computer Science and Engineering, said the guard beams and retroreflectors form the basis of a “rapid-response safety mechanism.”

“The guard beams are able to act faster than our quickest motions because those beams are reflected back to the emitter at the speed of light,” he explained. “As a result, when the guard beam is interrupted by the movement of a person, the emitter detects this within a fraction of a second and deploys a shutter to block the charging beam before the person can come in contact with it.”

Heatsink assembly
The UW team’s prototype heatsink assembly, which can mounted on the back of a smartphone, consists of a photovoltaic cell (silver square, top) attached to a thermoelectric generator (in white). The generator is mounted on top of an aluminum heatsink. The entire assembly is only 8mm (0.3 inches) thick and 40mm (1.6 inches) wide. (UW Photo / Mark Stone)

The researchers addressed the overheating challenge by putting thin aluminum strips on the back of the smartphone, surrounding the power cell. The strips act as a heatsink, dissipating excess heat from the charging beam. It’s even possible to convert some of that heat into electricity by mounting a thermoelectric generator above the strips.

“These features give our wireless charging system the robust safety standards needed to apply it to a variety of commercial and home settings,” said co-author Arka Majumdar, a UW professor of physics and electrical engineering.

If and when the technology is ready to take to market, the researchers might not have to go far for advice. Gollakota is the co-founder of Jeeva Wireless, a startup that’s developing a system for ultra-low-power wireless connectivity. Last year, Jeeva raised $1.2 million in a seed funding round.

In addition to Gollakota, Majumdar and Bayati, the authors of “Charging a Smartphone Across a Room Using Lasers” include co-lead author Vikram Iyer and Rajalakshmi Nandakumar.

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