Laser-Based Technique for Wireless Charging of Mobile Devices

Despite the fact that mobile devices (for example, smartphones, and tablets) enable people to work, communicate, and access information wirelessly, their batteries have to be charged by plugging them into an electric outlet. However, for the first time, researchers from the University of Washington have devised a technique for wireless and safe charging of a smartphone by using a laser.

The researchers have described in a paper published online in the Proceedings of the Association for Computing Machinery on Interactive, Mobile, Wearable and Ubiquitous Technologies in December 2017 that a narrow, invisible laser beam emitted from a laser emitter can supply charge to a smartphone located across a room—and can prospectively charge a smartphone as fast as a standard USB cable.

To achieve this, the researchers installed a thin power cell at the back of a smartphone. The power cell charges the smartphone by tapping the power from the laser. Moreover, the researchers developed tailor-made safety aspects, such as a metal, flat-plate heatsink on the smartphone to disperse the excess heat from the laser, and a reflector-based mechanism to turn the laser off if a person attempts to move along the path of the charging beam.

Safety was our focus in designing this system. 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.

Shyam Gollakota, Co-Author & Associate Professor - UW’s Paul G. Allen School of Computer Science & Engineering

Gollakota and Arka Majumdar, another co-author who is a UW assistant professor of physics and electrical engineering, headed the researchers who developed this wireless charging system and its safety aspects.

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. These features give our wireless charging system the robust safety standards needed to apply it to a variety of commercial and home settings.

Arka Majumdar, Researcher - UW Molecular Engineering & Sciences Institute

The charging beam is produced by a laser emitter configured by the researchers to generate a focused beam in the near-infrared spectrum. The safety system that turns the charging beam off focuses on harmless, low-power laser “guard beams,” emitted by another laser source co-positioned along with the charging laser-beam and physically “surrounds” the charging beam. The guard beams back are reflected back to photodiodes on the laser emitter by tailor-made, 3D printed “retroreflectors” positioned around the power cell on the smartphone.

Although the guard beams do not deliver any charge to the phone on their own, their reflection from the smartphone back to the emitter enables them to function similar to a “sensor” is a person moves in the guard beam path. The team developed the laser emitter such that it turns off the charging beam if an object (for example, body parts of a person) comes across the guard beams. The obstruction of the guard beams can be swiftly sensed to observe the most rapid motions of the human body, depending on many years of physiological investigations.

“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,” stated 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.”

It is anticipated that the next-generation nano-scale optical devices will have the potential to work at Gigahertz frequency, thereby decreasing the response time of the shutter to nanoseconds, stated Majumdar.

The smartphone is charged by the beam through a power cell located at the back of the phone. A steady power of 2W can be delivered by a narrow beam to an area of 15 square inches from a distance of nearly 4.3 m, that is, 14 feet. However, the emitter can be redesigned to increase the radius of the charging beam to an area of nearly 100 cm² from a distance of 12 m, or about 40 feet. This expansion indicates that the emitter can be designed to cover a wider charging surface (for example, tabletop or counter) and charge a smartphone positioned anywhere over the surface.

The scientists developed the smartphone to indicate its position by sending out high-frequency acoustic “chirps.” Although these chirps cannot be heard by human ears, they are adequately sensitive to be sensed by small microphones on the laser emitter.

“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,” stated Vikram Iyer, co-lead author of the study, who is a UW doctoral student in electrical engineering.

Upon detecting the smartphone to be on the desired charging surface, the emitter switches the laser on to initiate charging of the battery.

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

In order to make sure that the smartphone is not overheated by the charging beam, the researchers also positioned thin aluminum strips at the back of the smartphone, surrounding the power cell. These strips function like a heatsink, scattering excess heat from the charging beam and enabling the smartphone to be charged by the laser for many hours. They also tapped a small quantity of the heat to assist in charging the smartphone. They achieved this by positioned an almost flat thermoelectric generator on top of the heatsink strips.

The team is hopeful that their resilient heat-dissipation and safety aspects could allow laser-based, wireless charging of other devices, for example, tablets, cameras, and even desktop computers. Upon achieving this, the pre-bedtime chore of plugging in the tablet, smartphone, or laptop might, in future, be substituted by a less-complex task—positioning it on a table.

Rajalakshmi Nandakumar, a UW doctoral student in the Allen School, is the co-author of the study. The National Science Foundation, the Alfred P. Sloan Foundation, and Google Faculty Research Awards funded the study.