Fiber optic cable (also fiber-optic) — this is a fundamentally different type of cable compared to other types of electrical or copper cables. Information thereon is not transmitted electric signal, and light. The main element of it — is a transparent glass through which light travels great distances (up to tens of kilometers) with little attenuation.
The structure of the fiber optic cable is very simple and similar to that of a coaxial electrical cable, only instead of a central copper wire is used thin (diameter of the order of 1-10 microns) glass fibers, and instead of internal insulation — glass or plastic shell, which does not allow light to go beyond the glass.
Fiber optic cable offers exceptional performance for noise immunity and privacy of information transmitted. No external electromagnetic interference, in principle, can not distort the light signal, and the signal of this principle does not give rise to electromagnetic radiation. Connect to this type of cable for eavesdropping network is almost impossible, because it requires the integrity of the cable.
However, fiber optic cable has some disadvantages. The most important of them — the high complexity of installation (installing the connectors with a micron precision, the accuracy of the cleaved fiber and the degree of polishing is highly dependent attenuation in the socket). To install the connector used welding or bonding with a special gel having the same refractive index as glass. In any case, this requires highly skilled personnel and special tools. Therefore, most fiber optic cable is sold in the form of pre-cut pieces of different lengths on both ends of which are fitted desired type connectors.
There are two different types of fiber optic cables:
1. Multi-mode or multi-mode cable, cheaper, but lower quality;
2. Single-mode cable 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 almost all single-cable beams pass the same path as a result they reach the receiver simultaneously, and the waveform is not substantially distorted. Singlemode cable having a central fiber diameter of about 1.3 microns, and transmits only light with the same wavelength (1.3 microns). Dispersion and signal loss while not very significant ¬ that can transmit signals at a much greater distance than in the case of multimode cable. For single-mode laser transceivers cable applied using only light with a desired wavelength. These transceivers are still relatively expensive and not very durable. However, in the long term single-mode cable must be a major thanks to its excellent performance.
In multimode cable trajectories of light rays have noticeable spread, resulting waveform at the receiving end of the cable is distorted. The central fiber has a diameter of 62.5 mm and the diameter of the outer casing — 125 microns (sometimes referred to as 62,5 / 125). For transmission using an ordinary (non-laser) light-emitting diode, which reduces costs and increases the lifetime transceiver over singlemode cable. The wavelength of light in the multimode cable is 0.85 microns. Permissible cable length up to 2-5 km. Currently multimode cable — the main type of fiber optic cable, as it is cheaper and more affordable. Propagation delay of a fiber optic cable is not much different from a delay in electrical cables. The typical amount of delay for the most common cable is about 5.4 ns / m.