What is and how optical fiber works.

High-speed Internet, digital television, mobile communications are possible thanks to thin glass threads stretching along the seabed between continents. If not for the fiber optic, you would hardly have read these lines.

The fundamental foundations of this technology were described in the middle of the 19th century. Then they tried to use water as a signal conductor - unsuccessfully. Materials suitable for the implementation of a bold idea were developed only after more than a hundred years.

Light conductor

In a normal wire, the signal is transmitted over a copper conductor. Information is carried by a stream of electrons - an electric current. Data is transmitted encrypted in binary code. If the impulse passes - it means one, does not pass - zero.

In a fiber-optic communication line, the coding principle is the same, but information is carried by photons or light waves, more precisely, both at the same time. Scientists have argued about the nature of light for so long that they eventually have combined incompatible theories. But you don't need to understand quantum wave duality to understand how light is used to transmit information in telecommunication networks.

It is enough to understand how to make light flow through wires for kilometers.The first thing that comes to mind is mirrors. Make a metal tube and coat the inside with a smooth layer such as silver.

Light entering from one side will bounce off the walls until it reaches the exit on the other side. Not a bad idea, but it won't work.First, making such a tube of the required length is an extremely difficult and therefore expensive task.

Secondly, the reflectance of silver is 99%, that is, the light entering the tube will lose energy and after 100 reflections it will completely go out.It is much better to do without mirrors. The foundations of geometric optics, laid down in the 19th century, will tell you how to do this.

The basic idea is easy to demonstrate with an example of an aquarium. A light beam from a source under water passes through the boundary between water and air - two media with different optical properties - and partly changes the direction of movement, and partly is reflected from the boundary of the two media as from a mirror.

If the angle of incidence of the beam is reduced, at a certain moment the light will cease to come out of the water at all and will be reflected completely, by 100%. The border of two environments works better than any mirror.

As it turned out, water is not needed to create such a border. Any two materials that transmit light differently - with different refractive indices - will do. Even a 1% difference is sufficient to create a fiber.

Glass wires

In lamps and toys, light guides are made of plastics, but more expensive and more transparent materials are needed to obtain a suitable optical fiber for communication.

Scientists have adapted quartz glass for this purpose. The core of an optical fiber preform is most often made from pure silicon dioxide. The outer layer is also made of quartz, but with an admixture of boron or germanium to reduce the refractive index.

Previously, to get such a blank, they simply inserted two glass tubes into each other, but today they often do differently. Hollow tubes of pure quartz are filled with a mixture of gases with a high germanium content and slowly heated until the germanium is deposited in a uniform layer on the inner surface.

After a sufficiently thick layer of germanium oxide has grown on the quartz glass, the pipe is heated until softened and pulled out until the cavity inside collapses.

So a rod with a diameter of 1 to 10 centimeters and a length of about 1 meter is obtained, already containing quartz in the core with the addition of germanium, having an increased refractive index and a pure quartz shell around it.

Such a workpiece is delivered to the top of a tower up to several tens of meters high. There, the lower part of the workpiece is again heated to one and a half thousand degrees - almost to the melting point, and the thinnest thread is pulled out of it. On the way down, the glass cools down and is dipped into a bath of polymer, which forms a protective layer on the quartz surface. This method produces up to 100 km of fiberglass from one workpiece. At the base of the tower, the cooled fiber is wound on a reel.

Yes, it is precisely wound: oddly enough, quartz fiber bends easily.The resulting fibers are collected in bundles of several pieces and sealed in polyethylene. Cables are then woven from these bundles.

Each cable can contain from two to three and up to several hundred optical fibers. Outside, for strength, they are braided with a polymer thread and receive another protective shell made of polyethylene.

Advantages and disadvantages of fiber

All these difficulties are justified because light is the fastest in the universe.

Thanks to this property of light, optical fiber has an unsurpassed information capacity. Twisted pair, like a telephone line, or coaxial cable, a conductor with a shield, transmits 100 megabits per second.

The most common for computer networks, an eight-core cable of 4 twisted pairs transmits up to 1000 megabits per second. Fiber optic on one core - three times more, up to 3000 megabits per second, and with the help of various experimental tricks, this threshold can be overcome.

In addition, fiber is much lighter than copper. At 9 microns thick - thinner than a human hair - a 100 km long quartz thread weighs about 15 g.Almost all modern data transmission lines are laid from fiber optic cables. They connect continents, countries and data centers.

In large cities, "optics" is also used when connecting apartment buildings to the global network, but the fiber is laid between the provider and the house, and ordinary twisted pair cables are distributed among the apartments.

With such a connection scheme, the maximum speed of access to the network for the subscriber still does not exceed 100 Mbit / s. For comparison, by passing an optical cable directly into an apartment, you can get a 1 Gbps channel, and yet the consumer rarely encounters a fiber-optic Internet.

It's not just that fiber is expensive to manufacture. Laying the cable is just the beginning. Signals passing through the communication line accumulate errors with distance and eventually completely fade. For a twisted pair, this happens after 1 km, for a coaxial cable, after about 5 km. After that, the signal has to be restored and amplified - regenerated.

In optical fiber, the regeneration distance is several times longer, but no matter how clean the quartz glass is, impurities, for example, millionths of a percent of water, remain in it.

The length of the fiber can be hundreds of thousands of kilometers, but after 100–200 km the attenuation of the optical signal still manifests itself.

Therefore, on the fiber-optic communication lines, intermediate amplifiers are installed, which restore the amplitude of the optical signal, and regenerators, which remove interference. Such equipment is much more expensive than amplifiers on traditional communication lines and requires qualified maintenance.

But most importantly, at the moment, gigabit communication channels are in little demand by ordinary people. Perhaps with the advent of smart homes, wearable computers, and the spread of ultra-high-definition video streaming, the demand for them will increase, but so far the speed provided by twisted pair cable is enough for the average consumer.

Without even coming into direct contact with this technology, each of us takes advantage of its benefits. Connection stability, low latency in signal transmission to the most remote servers and high speed of receiving a response from them, the ability to withdraw money from any ATM and make a call to any country in the world - all this is the merit of fiber, and it has no competitors in the project either.

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