Lasers could offer truly wireless smartphone charging

University of Washington scientists have created a system to charge smartphones and other devices wirelessly using lasers.

The system uses a narrow, invisible beam from a laser emitter, which can deliver charge to a smartphone sitting across a room. The system can also potentially charge a smartphone as quickly as a standard USB cable.

To accomplish this, the team mounted a thin power cell to the back of a smartphone, which charges the smartphone using power from the laser.

In addition, the team custom-designed safety features - including a metal, flat-plate heatsink on the smartphone to dissipate excess heat from the laser, as well as a reflector-based mechanism to shut off the laser if a person moves across the charging beam’s path.

Although wireless charging already exists in many high-end smartphones (mostly using the QI standard), it has limited functionality in that a device needs to be placed directly onto the charging surface. 

This laser system allows charging at much greater distances.

“Safety was our focus in designing this system,” said Shyam Gollakota, an associate professor who worked on the project.

“We have designed, constructed and tested this laser-based charging system with a rapid-response safety mechanism, which ensures that the laser emitter will terminate the charging beam before a person comes into the path of the laser.”

The charging beam is generated by a laser emitter that the team configured to produce a focused beam in the near-infrared spectrum.

The safety system that shuts off the charging beam centers on low-power, harmless laser ‘guard beams’, which are emitted by another laser source co-located with the charging laser-beam and physically ‘surround’ the charging beam.

Custom 3D printed retroreflectors placed around the power cell on the smartphone reflect the guard beams back to photodiodes on the laser emitter. The guard beams deliver no charge to the phone themselves, but their reflection from the smartphone back to the emitter allows them to serve as a sensor for when a person will move in the path of the guard beam.

The researchers designed the laser emitter to terminate the charging beam when any object - such as part of a person’s body - comes into contact with one of the guard beams.

“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,” said Gollakota. “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.”

The next generation of nano-scale optical devices are expected to operate with Gigahertz frequency, which could reduce the shutter’s response time to nanoseconds.

The beam charges the smartphone via a power cell mounted on the back of the phone. A narrow beam can deliver a steady 2W of power to a 15 square-inch area from a distance of up to 4.3 metres.

The emitter can be modified to expand the charging beam’s radius to an area of up to 100 square centimetres from a distance of 12 meters.

This extension means that the emitter could be aimed at a wider charging surface, such as a counter or tabletop, and charge a smartphone placed anywhere on that surface.

The researchers programmed the smartphone to signal its location by emitting high-frequency acoustic “chirps.” These are inaudible to our ears, but sensitive enough for small microphones on the laser emitter to pick up.

“This acoustic localisation system ensures that the emitter can detect when a user has set the smartphone on the charging surface, which can be an ordinary location like a table across the room,” said co-lead author on the project Vikram Iyer.

When the emitter detects the smartphone on the desired charging surface, it switches on the laser to begin charging the battery.

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

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