The team mounted a thin PV cell to the back of a smartphone, illuminated by an infrared laser. To make the system safe, the team designed safety features such as 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 tries to move in the charging beam's path.
The safety system that shuts off the charging beam centres 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 high power charging beam. Custom 3D printed reflectors 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 the reflection from the smartphone back to the emitter allows them to serve as a sensor, stopping the charging beam when any object crosses a guard beam.
"Safety was our focus in designing this system," said Shyam Gollakota, an associate professor in the UW's Paul G. Allen School of Computer Science & Engineering. "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."
"In addition to the safety mechanism that quickly terminates the charging beam, our platform includes a heatsink to dissipate excess heat generated by the charging beam," said Arka Majumdar, a UW assistant professor of physics and electrical engineering. "These features give our wireless charging system the robust safety standards needed to apply it to a variety of commercial and home settings."
"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