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Optical Communication

March 13, 2023

Long before software feature creep, there was the start of the incessant creep up the radio frequency spectrum. AM radio broadcasting, at a center frequency of 1 MHz, was the first useful consumer application of radio frequencies. One MHz, which is just a hundred times higher than audio frequencies, paved the way for FM radio at a center frequency of 100 MHz, UHF television with a top frequency of 887 MHz, and the first cellphones, also at 800 MHz.

Presently, 5G cellphone frequencies are commonly at 1.7-4.7 GHz, with provision for higher frequencies of 24-47 GHz. A frequency of 47 GHz is five orders of magnitude higher than that of the lowest frequency AM radio station. Radio communications congestion is not totally mitigated by this migration to higher and higher frequencies. As an example, many consumer electronic devices use Wi-Fi for network connection. When there are multiple transmitters on a channels, collisions between competing signals result in lower speeds.

Snapshot of local lowband Wi-Fi spectrum

This is a snapshot of the local lowband WiFi spectrum at my house. These are just the devices that transmit an SSID. There are active IOT devices not present here. The four most powerful devices are in my house, and the others are neighbors' devices, including one printer. (Data collected using the Linux Wi-Fi scanner, LinSSID, and graphed using a combination of Gnumeric and Inkscape.Click for larger image.)

We are rushing towards a limit on radio frequency communication. The topmost radio frequency band on the electromagnetic spectrum is the Extremely high frequency band (EHF) that ends at 300 GHz. Above that, we have light in the form of the far infrared (FIR). Light can be used for line-of-sight communications; and, surprisingly, this was first done in 1880 by Alexander Graham Bell (1847-1922).

Bell and his assistant, Charles Sumner Tainter (1854-1940), invented the photophone, a device that enabled telephone communication on a beam of focused sunlight (see image). In this device, sound waves incident on a flexible mirror changed its shape from convex, which scatters light, to concave, which focuses light. A distant receiver converts the modulated light back to sound. This was first accomplished by the photoacoustic effect, and later by a selenium photodetector wired into an electric telephone receiver.

A photophone transmitter

Etching of a photophone transmitter. Solar radiation from the top of the image is focused onto the flexible mirror at the left.

(Wikimedia Commons image from the Biblioteca de la Universidad de Sevilla. Click for larger image.)

Today, optical communication is facilitated by the availability of light-emitting diodes (LEDs) and semiconductor photodetectors. One early use of these was in short-range data communication by infrared light in the 850-900 nanometer range. This is still used in many devices, such as the ubiquitous remote control. The Infrared Data Association (IrDA) established standards for such communication starting in 1994, although many remote controls now use more reliable radio frequencies.

The migration of room lighting to LEDs has given an opportunity to piggy-back free-space optical communication functionality inexpensively into room light fixtures. Such technology would reduce Wi-Fi congestion in office buildings, even if implemented only for download data, since there's generally more than a 10:1 ratio between the quantity of downloaded and uploaded data. The current snapshot of this ratio for my Linux desktop computer is 15.89. The infrastructure for visible light communication is everywhere. Outside our homes, there is LED lighting in streetlights and automobiles. Any computing device with a camera, such as tablet computers and smartphones, are already capable of being configured as optical receivers.

A Chinese research team has published a recent open access review article about high-speed visible light communication based on micro-LEDs[1-2] The high data rate of visible light communication will help to enable the next-generation 6G networks. Importantly, light communication can be done without a license, and it's both immune to electromagnetic interference and it does not produce high levels of electromagnetic interference.[2] This makes it a good communications medium for locations sensitive to electromagnetic interference, such as nuclear power plants, airports, hospitals, and underground mines.[2] As addressed in the review article, micro light-emitting diodes (μLEDs) are an excellent choice for for high-speed visible light communication, since they have a high modulation bandwidth.[1] The μLEDs will function also as high-speed photodetectors.[2]

Powering a desktop or tablet computer to receive, and possibly send, optical data is not a problem. Powering wearable electronic devices is more of a problem, since bulky battery packs are undesirable. Fortunately, wearable devices usually need just a small amount of power, since they lack large displays. Visible light communication devices do leak low intensity radio frequency signals during transmission, and a research team with members from the University of Massachusetts Amherst (Amherst, Massachusetts) and the Delft University of Technology (Delft, The Netherlands) have invented a low-cost method to harvest this energy by using the human body as an antenna.[3-5] Their research paper received the Best Paper Award from the Association for Computing Machinery’s 2022 Conference on Embedded Networked Sensor Systems.[4]

In their experiments, they found that surrounding objects act as antennas that enhance the radio frequency reception of an inductance coil used as an antenna.[3] A variety of objects act as antennas, but the human body gave the best response, up to ten times more than an isolated coil.[3-4] They decided that the best configuration was a coil of copper wire worn as a bracelet on the upper forearm, a device they call a Bracelet+, although ring, belt, anklet and necklace coils had good response.[4]

Power gain of RF reception for coils arranged on the human body.

power gain of radio frequency reception for inductance coils arranged on the human body. (Created using Inkscape. Human body outline from Wikimedia Commons. Data from fig. 13 of ref. 3.[3] Click for larger image.)

Experiment showed that microwatts of power could be harvested, which is enough to power simple body sensors such as blood glucose monitors, since these have a low sampling rate.[3-4] The authors note that their energy harvester costs less than fifty cents.[4]


  1. Tingwei Lu, Xiangshu Lin, Wenan Guo, Chang-Ching Tu, Shibiao Liu, Chun-Jung Lin, Zhong Chen, Hao-Chung Kuo and Tingzhu Wu, "High-speed visible light communication based on micro-LED: A technology with wide applications in next generation communication," Opto-Electronic Science, vol 1, no. 12 (December 29, 2022), article no. 220020, doi: 10.29026/oes.2022.220020. This is an open access article with a PDF file here.
  2. High-speed visible light communication based on micro-LED: A technology with wide applications in next generation communication, Press Release from Compuscript Ltd, January 6, 2023.
  3. Minhao Cui, Qing Wang, and Jie Xiong, "Next-generation Wireless Technology May Leverage the Human Body for Energy, Association for Computing Machinery, SenSys '22, November 6–9, 2022, Boston, Massachusetts. A PDF file can be found here.
  4. Next-generation Wireless Technology May Leverage the Human Body for Energy, University of Massachusetts Amherst Press Release, January 4, 2023.
  5. My son-in-law introduced me to the idea of holding an automobile remote alongside your face to get greater range. Some experiments to confirm this would be interesting.

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