Fiber optic cable is a fundamentally different type of cable compared to other types of electric or copper cables. Information on it is transmitted not by an electric signal, but by light. Its main element is transparent glass fiber, through which light passes over great distances (up to tens of kilometers) with slight attenuation.
The structure of the fiber optic cable is very simple and similar to the structure of a coaxial electric cable, only instead of a central copper wire here thin (with a diameter of about 1-10 μm) fiberglass is used, and instead of internal insulation, a glass or plastic sheath that does not allow light to go beyond the fiberglass. In this case, we are dealing with the regime of the so-called total internal reflection of light from the boundary of two substances with different refractive indices (the refractive index of a glass shell is much lower than that of a 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 (this cable is sometimes called armored cable, it can combine several fiber-optic cables under one sheath).
Fiber optic cable has exceptional characteristics for noise immunity and the privacy of transmitted information. No external electromagnetic interference, in principle, can distort the light signal, and this signal itself does not fundamentally generate external electromagnetic radiation. It is almost impossible to connect to this type of cable for unauthorized listening to the network, since this requires a violation of the integrity of the cable. Theoretically, the possible bandwidth of such a cable reaches 100 Gbps, which is incomparably higher than that of any electric cables. The cost of fiber optic cable is constantly decreasing and is now approximately equal to the cost of a thin coaxial cable. However, in this case, it is necessary to use special optical receivers and transmitters that convert light signals into electrical signals and vice versa, which sometimes significantly increases the cost of the network as a whole.
Ethernet cables also allow signal branching (for this purpose 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.
Ethernet cabling is less durable than electric, and less flexible (the typical value of the allowable 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 fiberglass can crack. At present, optical cables are made of radiation-resistant glass (they cost, of course, more expensive).
Cables for ethernet are also sensitive to mechanical stress (shock, ultrasound) - the so-called microphone effect. To reduce it, use soft sound-absorbing shells.
Cable for ethernet used only in networks with topology “star” and “ring”. In this case, there are no problems of coordination and grounding. The cable provides perfect galvanic isolation of network computers. In the future, this type of cable is likely to supersede all types of electrical cables or, in any case, greatly displace them. Copper reserves on the planet are depleted, and the raw materials for glass production are more than enough.
There are two different types of fiber optic cables:
- A multimode cable, cheaper, but less quality;
- Single-mode cable, more expensive but better.
The main differences between these types are associated with different modes of transmission of light rays in the cable.
In a single-mode cable, almost all beams travel the same path, as a result of which they all reach the receiver at the same time, and the waveform is practically not distorted. A single-mode cable has a central fiber diameter of about 1.3 microns and transmits light with only the same wavelength (1.3 microns). The dispersion and signal loss are very insignificant, which allows transmitting signals at a much greater distance than in the case of 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, a single-mode cable should become the main one due to its excellent characteristics.
In a multimode cable, the paths of light rays have a noticeable scatter, 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 sheath 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 life of the transceivers in comparison with a single-mode cable. The wavelength of light in a multimode cable is 0.85 μm. Permissible cable length reaches 2-5 km. Currently, multimode cable is the main type of fiber optic cable, as it is cheaper and more affordable. The propagation delay in a fiber optic cable is not much different from the delay in electric cables. A typical delay for the most common cables is around 4-5 ns / m.
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