Underwater wireless "Internet"

Many of us use Wi-Fi every day, but who has heard of his “relative,” Li-Fi? Today, the US Navy is funding the development of an underwater data transfer technology based on the principle of high-frequency light pulses. And the work published in Nature Nanotechnology may well help to embody this technology. The authors of the article produced artificial material that reacts unusually to light. Thanks to this property, it is possible to create high-speed light data transmitters.



The principle of operation of Li-Fi is similar to Morse code. LED flashes correspond to logical ones and zeros. The sequence of light pulses received by the optical receiver is converted into a digital signal. The flash rate is such that the human eye is not able to distinguish it, and the higher the speed, the greater the bandwidth of such a transmission channel.

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Mentioned artificial material can increase the frequency of LED flashes by one or two orders of magnitude. “You can use very cheap LEDs and increase speed 50 times,” says Zhaowei Liu, an optical engineer at the University of California, author of the article. It would also be possible to initially use faster LEDs and increase the frequency to a similar extent.



Li-Fi can be used for specific tasks, for example, to improve the quality of submarine communications, because radio waves propagate poorly in the water column, and acoustic transmission technologies do not provide high speed too. Also, Li-Fi can be useful in petrochemical enterprises and in aviation, where the use of Wi-Fi leads to interference in equipment.



There is another possible area of ​​application of promising technology. The US Federal Communications Commission warned that the range of available frequencies for wireless communications is becoming too "densely populated." Li-Fi can reduce the number of frequencies used today. The optical spectrum is approximately 10,000 times wider than the radio frequency spectrum, and can accommodate much more data channels. In addition, visible light does not interfere with radio waves, which allows you to simultaneously work Wi-Fi and Li-Fi. You can also switch from one to another, by analogy with hybrid cars. Also, Li-Fi can be integrated into the already existing infrastructure of external and internal lighting.







The highest data transfer rate through a single LED, achieved in the laboratory, is 3.5 gigabits per second over a distance of 5 cm. This is still less than the record of transmission speed through a radio channel at 237 GHz, which is 100 gigabits per second , but there is potential for further growth.



At longer distances and in field conditions, the transmission speed through Li-Fi is lower. For example, specialists from the Heinrich Hertz Institute in Germany reached a speed of 500 megabits / sec at a distance of 1 to 2 m, and 100 megabits / sec at a distance of more than 20 m.



Liu and his colleagues plan to increase the bandwidth by introducing “hyperbolic metamaterial” (“hyperbolic metamaterial”) into the LEDs. This substance consists of several alternating 10-nm layers of silicon and silver. “Sandwiches” with a thickness of 305 nm are placed on a glass substrate, with grooves cut out in each layer. Next, the stack of layers is covered with transparent plastic with the addition of rhodamine dye molecules. These molecules fluoresce when they absorb light. The researchers irradiated them with a laser and measured the brightness and frequency of flicker during fluorescence, noting multiple growth.







“This study demonstrates the great potential of the material invented,” says Zubin Jacob, an engineer from the University of Albert. Hyperbolic metamaterials have unusual properties due to the applied patterns, whose size is less than the length of light in the range from 400 to 700 nm. When light falls on such a material, a so-called plasmonic resonance occurs, in which electrons begin to oscillate at the same frequency as the material. This property is not found in nature. When plasmon resonance coincides with fluorescent radiation, the latter can increase, which leads to an increase in brightness and flicker rate.



German researchers claim that they can help colleagues from the United States to bring Li-Fi to the level of commercial use. The typical LEDs available today are optimized for lighting, not for data transmission, so their parameters can be changed within relatively small limits. So the new material can be an excellent solution in this matter.