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Fiber optic cables, types and characteristics.

Fiber optic cable (aka fiber optic) is a fundamentally different type of cable compared to other types of electrical or copper cables. Information on it is transmitted not by an electrical signal, but by a light signal. Its main element is transparent fiberglass, through which light travels over huge distances (up to tens of kilometers) with insignificant attenuation.

The structure of a fiber optic cable is very simple and similar to the structure of a coaxial electrical cable, except that instead of a central copper wire, thin (about 1-10 microns in diameter) fiberglass is used, and instead of internal insulation, a glass or plastic sheath does not allow light to go outside the glass fiber. In this case, we are dealing with the regime of the so-called total internal reflection of light from the interface of two substances with different refractive indices (the refractive index of the glass shell is much lower than that of the central fiber). The metal braid of the cable is usually absent, since shielding from external electromagnetic interference is not required here, but sometimes it is still used for mechanical protection from the environment (such a cable is sometimes called armored, it can combine several fiber-optic cables under one sheath).

Fiber-optic cable has exceptional characteristics in terms of noise immunity and secrecy of transmitted information. In principle, no external electromagnetic interference is capable of distorting the light signal, and this signal itself, in principle, does not generate external electromagnetic radiation. It is almost impossible to connect to this type of cable for unauthorized eavesdropping on the network, as it requires breaking the integrity of the cable. Theoretically, the possible bandwidth of such a cable reaches 1012 Hz, which is incomparably higher than that of any electrical cables.

The cost of fiber optic cable has been steadily decreasing and is now roughly equal to the cost of thin coaxial cable. However, in this case, it is necessary to use special optical receivers and transmitters that convert light signals into electrical ones and vice versa, which sometimes significantly increases the cost of the network as a whole.

Typical signal attenuation in fiber optic cables at frequencies used in local area networks is about 5 dB / km, which is roughly the same as electrical cables at low frequencies. But in the case of a fiber-optic cable, with an increase in the frequency of the transmitted signal, the attenuation increases very slightly, and at high frequencies (especially above 200 MHz) its advantages over an electric cable are undeniable, it simply has no competitors.

However, fiber optic cable also has some disadvantages. The most important of them is the high complexity of installation (when installing connectors, micron accuracy is required, the attenuation in the connector greatly depends on the accuracy of the cleavage of the fiberglass and the degree of its polishing). To install the connectors, welding or gluing is used using a special gel that has the same refractive index of light as fiberglass.

In any case, this requires highly qualified personnel and special tools. Therefore, most often, fiber optic cable is sold in the form of pre-cut pieces of different lengths, on both ends of which the connectors of the required type are already installed.

Although fiber-optic cables allow for signal branching (for this, special splitters for 2-8 channels are produced), as a rule, they are used for transmission. After all, any branching inevitably greatly weakens the light signal, and if there are many branches, then the light may simply not reach the end of the network.

Fiber optic cable is less durable than electrical cable and less flexible (typical bending radius is about 10-20 cm). It is also sensitive to ionizing radiation, due to which the transparency of the glass fiber decreases, that is, the signal attenuation increases. It is also sensitive to sudden changes in temperature, as a result of which the fiberglass can crack.

Currently, optical cables are produced from radiation-resistant glass (they are, of course, more expensive).

Fiber optic cables are also sensitive to mechanical stress (shock, ultrasound) – the so-called microphone effect. To reduce it, soft sound-absorbing shells are used.

Use fiber-optic cable only in networks with a star and ring topology. In this case, there are no problems of matching and grounding. The cable provides perfect galvanic isolation of network computers.

In the future, this type of cable is likely to supersede electrical cables of all types, or at least strongly suppress them. The reserves of copper on the planet are depleting, and there are more than enough raw materials for the production of glass.

There are two different types of fiber optic cables:

Multimode, or multimode, cable, cheaper but of lower quality; Single mode cable, which is more expensive but has better performance.

The main differences between these types are associated with different modes of transmission of light rays in the cable.

In a single-mode cable, virtually all beams travel the same path, so they all reach the receiver at the same time, and the waveform is virtually unaltered. A single-mode cable has a center fiber diameter of about 1.3 µm and only transmits light at the same wavelength (1.3 µm). The dispersion and signal loss are very insignificant in this case, which makes it possible to transmit signals over a much greater distance than in the case of using a multimode cable.

For single-mode cable, laser transceivers are used that use light exclusively with the required wavelength. Such transceivers are still relatively expensive and not very durable. However, in the future, single-mode cable should become the main one due to its excellent characteristics.

In a multimode cable, the light paths have a noticeable spread, as a result of which the waveform at the receiving end of the cable is distorted. The central fiber has a diameter of 62.5 microns, and the diameter of the outer cladding is 125 microns (this is sometimes referred to as 62.5 / 125). A conventional (non-laser) LED is used for transmission, which reduces the cost and increases the lifespan of the transceivers compared to single-mode cable. The wavelength of light in a multimode cable is 0.85 µm.

The permissible cable length is 2-5 km. Currently, multimode cable is the main type of fiber optic cable, as it is cheaper and more affordable. The propagation delay of a signal in a fiber optic cable is not very different from that in an electrical cable. Typical latency for the most common cables is around 4-5 ns / m.

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