  Wireless charging for mobile Internet of Things (IoT) devices such as
smartphones is extremely difficult. To reduce energy dissipation during
wireless transmission in mobile scenarios, laser or narrow radio beams with
sophisticated tracking control are typically required. However, reaching the
necessary tracking accuracy and reliability is really difficult. In this paper,
inspired by the features of optical resonators and retroreflectors, we develop
an experiment on a self-aligned resonant beam charging system for long-distance
mobile power transfer. It exploits light resonances inside a
double-retroreflector-based spatially separated laser resonator (SSLR), which
eliminates the requirement for any kind of tracking control. Focal telecentric
cat's eye retroreflectors are employed here. The SSLR was investigated by both
theoretical calculation and experiment. We also well assembled the transmitter
and the receiver and demonstrated its application in mobile smartphone
charging. The results show that above 5-W optical power (also obtained more
than 0.6-W electrical power) transferring with negligible diffraction loss to a
few-centimeter-size receiver is realized while the receiver moves arbitrarily
within 2-m vertical distance and 6{\deg} field of view from the transmitter.
The maximum horizontal moving range is up to 18cm. This wireless charging
system empowers a smartphone in mobile operation with unlimited battery life
without the need for a cable.
