American-Israeli development makes it possible to greatly increase the volume of optical fiber communications
In 1966, the Chinese engineer Charles Kao came up with a revolutionary idea - the use of thin glass tubes to transmit information through light. This idea - Kao said - will make it possible to control light and even bend it (which will of course also make it possible to use it to transmit information in a non-straight line), will make it possible to transfer a large amount of information and send it over long distances without suffering from the decay or decrease in signal strength, as happens to an electric signal in a copper cable . Soon the brilliant idea took off (this is not a typo, but a play on words), and in the 70s the first optical fibers were already produced in research laboratories. In the decades that followed, they already began to be used in communication networks, and today optical fibers are the backbone that routes most of the communication in our world. Telephone, computer communication, data traffic, cable TV - all of these go through intricate networks of optical fibers, which reach (almost) all corners of the globe. Each such fiber is about the thickness of a hair, and the large communication cables, such as submarine cables, contain a large number of fibers. Kao, by the way, received the Nobel Prize in Physics in four years, mainly for this invention.
The tube is filling up
"In the 80s, the optical fibers were like the national carrier - a pipe with a huge diameter through which all that flows is a thin trickle of water," says Prof. Moshe Tor, from the Faculty of Engineering at Tel Aviv University. "Today it seems that we are not far from reaching the maximum amount of information that we can transmit in these fibers. Experts are divided on whether this will happen in two, three, or five years. But it's not far. With the existing technologies we will not be able to catch up with the growing demand". The use of many optical fibers and an improvement in the technologies for transmitting information through them, have also led over the years to a significant decrease in communication prices. "If we can't transmit more information via fiber, this will mean that in order to double the amount of information, it will be necessary to double the infrastructure, which will of course require financial investment and will make communication more expensive."
do not mix
A new development by Tor and his colleagues in the USA, may postpone the end by several good years, thanks to harnessing the unique properties of light for a more efficient transfer of information. Today, each photon (particle of light) can transmit several hundreds of gigabits per second (that is, several hundred billion "bits" of information signals marked for convenience with "0" or "1"). It is possible to transmit simultaneously, in a single fiber, photons in about 160 different wavelengths, and each of them transmits such an amount of information. If you try to transmit more information, the photons start interfering with each other, and the transmission of information goes wrong.
The optical fibers are divided into two groups: single-mode fiber, in which the light moves in only one and only way along the fiber, and these are mainly used for long-range communication. The second type is a multi-mode fiber, where the light can move in several different ways: for example in a straight line, in a zig-zag from side to side, and so on. Such fibers are cheaper, but limited to transmitting information over short distances. The work of Prof. Tor and his colleagues, led by Siddharth Ramachandran (Ramachandran) from Boston University and Alan Willner (Willner) from the University of Southern California, offers a new approach to multimodal information transmission. "There are two approaches to transferring such information," Tor says. "One - sending the photons in different ways so that they will mix in their passage through the fiber, and then separating them using the methods of advanced electronics. The other - use of modes of oscillation that do not cause photons to mix with each other."
the curving light
Tor and his colleagues chose the second method, harnessing the circular angular momentum of the photons for this purpose. Light particles moving in this way will look like a spiral, or like a screw, what the researchers call "twisted light". The modes of movement of these photons differ from each other both in the direction of rotation (right or left), and in the degree of density of the rotations (screwing). "We were able to use such photons to transfer 400 gigabit of information per second, which is a lot," Tor says. "They do not mix with each other, and by using four modes at each wavelength, we were able to transmit 1.6 terabits per second." In an article in the journal Science, the researchers report that they were able to transmit light using this method to a distance of one kilometer - farther than the use of multi-cycle fibers.
Change is inevitable
The method developed by Tor and his colleagues is not only of interest to the normal communications market, and it may also be of great importance to communications and military data traffic. Therefore, the American Defense Research Agency, DARPA, which operates within the framework of the US Department of Defense, participates in funding the research.
Even though the new method makes use of fibers that already exist in the market, its activation still requires an upgrade of the infrastructure, and the activation of new technologies at the entry points of the light into and out of the fiber. This is also the reason why the researchers did not register a patent for the method - they revealed an existing physical principle, and the patents - Tur estimates - will be registered in the future for these accompanying technologies. The full potential of the method will only become clear in further research, when the researchers discover how many different modes can be transmitted simultaneously in one fiber (now, as mentioned, they managed to use four modes).
Wide use of this method will also require the replacement of the long-range fibers with fibers of a slightly different type. However, Tor estimates that in the coming years there will be no escape from this change. "If the market develops the way it is developing today, in about five years the technology will exhaust itself, and then they will have to turn to the method we developed or a similar method, or they will have to start laying more and more lines to meet the demand, at considerable expense and investing more energy in each transfer of information."
Related links:
Abstract of the article on the website of the journal Science
