Charging with a Laser

The wireless 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. (Source: M. Stone, U. Washington)

Although mobile devices such as tablets and smart­phones let us communicate, work and access infor­mation wire­lessly, their batteries must still be charged by plugging them in to an outlet. But engi­neers at the Univer­sity of Washing­ton have for the first time developed a method to safely charge a smart­phone wire­lessly using a laser. A narrow, invisible beam from a laser emitter can deliver charge to a smartphone sitting across a room and can 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 tries to move in the charging beam’s path.

“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 Majum­dar, who is also a researcher in the UW Mole­cular Engi­neering & Sciences Institute. “These features give our wireless charging system the robust safety standards needed to apply it to a variety of commer­cial and home settings.”

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 3-D 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 comes into contact with one of the guard beams. The blocking of the guard beams can be sensed quickly enough to detect the fastest motions of the human body, based on decades of physio­logical studies.

“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, added Majumdar.

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 15 square-inch area from a distance of up to 4.3 meters, or about 14 feet. But 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 12 meters, or nearly 40 feet. 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 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,” said Vikram Iyer, a UW doctoral student in elec­trical engi­neering. 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.” To ensure that the charging beam does not overheat the smart­phone, the team also placed thin aluminum strips on the back of the smart­phone around the power cell. These strips act as a heatsink, dissipating excess heat from the charging beam and allowing the laser to charge the smart­phone for hours. They even harvested a small amount of this heat to help charge the smart­phone by mounting a nearly-flat thermo­electric generator above the heatsink strips.

The researchers believe that their robust safety and heat-dissi­pation features could enable wireless, laser-based charging of other devices, such as cameras, tablets and even desktop computers. If so, the pre-bedtime task of plugging in your smart­phone, tablet or laptop may someday be replaced with a simpler ritual: placing it on a table. (Source: U. Washington)

Reference: V. Iyer et al.: Charging a Smartphone Across a Room Using Lasers, Proc. of the ACM on Interactive, Mobile, Wearable and Ubiquitous Tech. 1, 143 (2017); DOI: 10.1145/3161163

Link: Molecular Engineering & Sciences Inst., University of Washington, Seattle, USA

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