Imagine walking into an airport or grocery store and your smartphone
automatically starts charging. This could be a reality one day, thanks to a
new wireless laser charging system that overcomes some of the challenges
that have hindered previous attempts to develop safe and convenient
on-the-go charging systems.
"The ability to power devices wirelessly could eliminate the need to carry
around power cables for our phones or tablets," said research team leader
Jinyong Ha from Sejong University in South Korea. "It could also power
various sensors such as those in Internet of Things (IoT) devices and
sensors used for monitoring processes in manufacturing plants."
In Optics Express, the researchers describe their new system, which uses
infrared light to safely transfer high levels of power. Laboratory tests
showed that it could transfer 400 mW light power over distances of up to 30
meters. This power is sufficient for charging sensors, and with further
development, it could be increased to levels necessary to charge mobile
devices.
Several techniques have been studied for long-range wireless power transfer.
However, it has been difficult to safely send enough power over meter-level
distances. To overcome this challenge, the researchers optimized a method
called distributed laser charging, which has recently gained more attention
for this application because it provides safe high-power illumination with
less light loss.
"While most other approaches require the receiving device to be in a special
charging cradle or to be stationary, distributed laser charging enables
self-alignment without tracking processes as long as the transmitter and
receiver are in the line of sight of each other," said Ha. "It also
automatically shifts to a safe low power delivery mode if an object or a
person blocks the line of sight."
Going the distance
Distributed laser charging works somewhat like a traditional laser but
instead of the optical components of the laser cavity being integrated into
one device, they are separated into a transmitter and receiver. When the
transmitter and receiver are within a line of sight, a laser cavity is
formed between them over the air—or free space—which allows the system to
deliver light-based power. If an obstacle cuts the transmitter-receiver line
of sight, the system automatically switches to a power-safe mode, achieving
hazard-free power delivery in the air.
In the new system, the researchers used an erbium-doped fiber amplifier
optical power source with a central wavelength of 1550 nm. This wavelength
range is in the safest region of the spectrum and poses no danger to human
eyes or skin at the power used. Another key component was a wavelength
division multiplexing filter that created a narrowband beam with optical
power within the safety limits for free space propagation.
"In the receiver unit, we incorporated a spherical ball lens retroreflector
to facilitate 360-degree transmitter-receiver alignment, which maximized the
power transfer efficiency," said Ha. "We experimentally observed that the
system's overall performance depended on the refractive index of the ball
lens, with a 2.003 refractive index being the most effective."
Laboratory testing
To demonstrate the system, the researchers set up a 30-meter separation
between a transmitter and a receiver. The transmitter was made of the
erbium-doped fiber amplifier optical source, and the receiver unit included
a retroreflector, a photovoltaic cell that converts the optical signal to
electrical power and an LED that illuminates when power is being delivered.
This receiver, which is about 10 by 10 millimeters, could easily be
integrated into devices and sensors.
The experimental results showed that a single-channel wireless optical power
transfer system could provide an optical power of 400 mW with a channel
linewidth of 1 nm over a distance of 30 meters. The photovoltaic converted
this to an electrical power of 85 mW. The researchers also showed that the
system automatically shifted to a safe power transfer mode when the line of
sight was interrupted by a human hand. In this mode, the transmitter
produced an incredibly low intensity light that did not pose any risk to
people.
"Using the laser charging system to replace power cords in factories could
save on maintenance and replacement costs," said Ha. "This could be
particularly useful in harsh environments where electrical connections can
cause interference or pose a fire hazard."
Now that they have demonstrated the system, the researchers are working to
make it more practical. For example, the efficiency of the photovoltaic cell
could be increased to better convert light into electrical power. They also
plan to develop a way to use the system to charge multiple receivers
simultaneously.
Reference:
Nadeem Javed et al, Long-range wireless optical power transfer system using
an EDFA, Optics Express (2022).
DOI: 10.1364/OE.468766