The ‘birth’ of optical wireless communications by Bell
“I have heard articulate speech produced by sunlight. I have heard
a ray of the sun laugh and cough and sing!”
Alexander Graham Bell, February 1879.
These were the words of Alexander Graham Bell that expressed his idea of the use of light for communications. The photophone, invented by Bell in 1880, is considered to be the precursor of optical wireless communications (OWC). Today, the broad research field of OWC can be separated to the two branches of free space optics (FSO) and visible light communications (VLC). Free space optical systems mostly comprise lasers for the outdoor transmission of information to photodetectors at distances up to many kilometres. The technology of VLC includes the use of light-emitting diodes (LEDs) as optical sources and is mainly applied to indoor environments such as offices, conference venues, hotels and hospitals.
The inception of wireless power transfer by Tesla
The ‘magic’ concept of wireless power transfer (WPT) or wireless energy transfer (WET) was inspired and demonstrated by Nikola Tesla in the late 19th century. During 1891–1904 Tesla was performing WPT experiments by inductive or capacitive coupling. In particular, he was using spark-excited radio frequency (RF) resonant transformers, now called Tesla coils, to generate high alternating current (AC) voltages and transmit power wirelessly at short distances. Nowadays, Tesla’s pioneering work has been the focus of research interest especially in the RF or microwave spectrum. The key components for energy harvesting (EH) and data communication of RF signals are rectifying antennas, known as rectennas. Rectennas are diode-based circuits able to convert RF power to direct current (DC) electrical power.
“Within a few years a simple and inexpensive device, readily carried about, will enable one to receive on land or sea the principal news, to hear a speech, a lecture, a song or play of a musical instrument, conveyed from any other region of the globe.”
Nikola Tesla, January 1905.
The simultaneous application of optical wireless communications and power transfer
Could the next step of OWC be the simultaneous application of WPT, and what receivers would be suitable for EH and communication? The answer was given by researchers at the Li-Fi Research and Development Centre in the University of Edinburgh. In , it was demonstrated practically for the first time that solar panels are capable of simultaneous EH in the order of some tens of milliwatt and high speed data communication of up to 12 megabits per second and are tuneable for both functionalities. The optical source used was a high brightness white LED. The wide availability and low cost of commercial LEDs able to transmit amounts of optical power in the order of a few watt offers a great potential for joint illumination and VLC. However, do LEDs represent the best choice for WET especially in applications where power needs to be delivered at long distances?
Research on optical wireless power transmission from LEDs and lasers
The wide radiation pattern of a LED source, similar to a sphere, is a major bottleneck for efficient light collimation and therefore power transmission. Light collimation is of critical importance, as it can effectively reduce the geometrical losses of an optical wireless link. In , an experimental study demonstrated that efficient power transmission from a LED to a solar panel at 5 m can be achieved by a respective increase in the dimensions of collimation optics such as lens, reflector, parabolic mirror. Also, the link efficiency, that is, the ratio of electrical power harvested from the load of a panel over the input DC electrical power to the optical source, was very low due to significant geometrical losses. For this reason, laser diodes (LDs) were selected for the implementation of an optical link for WPT in the follow-up study  due to their high directivity. In particular, the use of 5 LDs with collimation lenses resulted in an increased link efficiency of 7.5 times compared to the use of the powerful LED in . The levels of harvested power were in the order of some tens of milliwatt.
Optical and radio frequency wireless power transfer
Finally, what is the main advantage of optical wireless power transfer (OWPT) compared with RF-based WPT? In practical outdoor applications, solar panels can harvest optical power not only from ‘dedicated’ sources of LEDs or lasers but also from sun. Radio frequency-based EH does not have the ability to use power from a natural source and an electromagnetic source simultaneously, and therefore the wide use of solar panels everywhere offers great potential for autonomous communication devices operating under the principle of OWPT. Z. Wang, D. Tsonev, S. Videv, and H. Haas, “On the Design of a Solar-Panel Receiver for Optical Wireless Communications with Simultaneous Energy Harvesting,” IEEE J. Sel. Areas Commun., vol. 33, no. 8, pp. 1612-1623, Aug. 2015.  J. Fakidis, M. Ijaz, S. Kucera, H. Claussen, and H. Haas, “On the Design of an Optical Wireless Link for Small Cell Backhaul Communication and Energy Harvesting,” in Proc. IEEE 25th Annu. Int. Symp. Pers. Indoor and Mobile Radio Commun. (PIMRC), Washington, DC USA, Sep. 2-5 2014, pp. 58-62.  J. Fakidis, S. Kucera, H. Claussen, and H. Haas, “On the Design of a Free Space Optical Link for Small Cell Backhaul Communication and Power Supply,” in Proc. IEEE Int. Conf. Commun. Workshops (ICCW), London, UK, Jun. 8 2015, pp. 1408-1413.
Li-Fi PhD Research Student