Someday charging your smartphone may be as simple as just placing it on a table or countertop.

A team of engineers from the University of Washington has created a method to safely charge a smartphone wirelessly from across the room using a narrow, invisible beam from a laser emitter that could potentially charge the phone as fast as a standard USB cable.

The researchers mounted a thin power cell to the back of a smartphone, which charges the phone using power from the laser. They also designed safety features, including a metal, flat plate heat sink to dissipate excess heat from the laser and a reflector-based mechanism to shut off the laser if someone moves in the charging beam’s path.

“Safety was our focus in designing this system,” co-author Shyam Gollakota, PhD, an associate professor in the UW’s Paul G. Allen School of Computer Science & Engineering, said in a statement. “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.”

A laser emitter that is configured to produce a focused beam in the near-infrared spectrum generates the charging beam. The safety system centers on low power, harmless laser guard beams, which are emitted by another laser source co-located with the charging laser-beam and physically surrounding the charging beam.

The retroreflectors are custom 3D printed and placed around the power cell on the phone to reflect the guard beams back to photodiodes on the laser emitter.

The laser emitter terminates the charging beam when a human body meets 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,” Gollakota said. “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 narrow beam can deliver a steady 2W of power to a 15 square-inch area from a distance of up to about 14 feet. However, the emitter can be modified to expand the charging beam’s radius to an area of up to 100 square centimeters from a distance of nearly 40 feet, which enables the emitter to be aimed at a wider charging surface—including a counter or tabletop— and charge a phone placed anywhere on that surface.

During the study, the researchers programmed a smartphone to signal its location by emitting high-frequency acoustic chirps that are inaudible to human ears, but sensitive enough for small microphones on the laser emitter to pick up.

“This acoustic localization 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,” co-lead author Vikram Iyer, a UW doctoral student in electrical engineering, said in a statement.