Signal Transmission Devices Review. Prt.4 Fiber-optic cable (FOC)

This type of cables is the most effective transmission media. Comparably to other communication lines. FOC has some advantages:

vast signal transmitting distance, low leakage;

broad signal transmitting frequency band;

freedom from electromagnetic noise;

ground loop has no effect on it;

protected from external access;

high level of safe use in rooms with risk of fire and explosion based on its physical structure.

Fiber-optic communication lines allow to create multitasking nets and can be used for varied aims: as a system of data transferring in the cable television, in telephony, in guard systems.

FOC consists of a central load-bearing member, around which optical fiber located. It is sheathed and has load-bearing unit. Installing FOC can take place at different conditions, so that different constructive variants of FOC exist:

ropes are used for air lines;

armored cables are used for underlaying of cable;

moistureproof FOC – for cable laying in the water.

It is also necessary to meet technical requirements that are described in the FOC-manufacturer instruction during the installation and construction work, especially point about a cable bend radius and maximum cable stretching.

Optical fiber has a central light conductor with high refraction index. The conductor is enclosed from outside, and the shell has a low refraction index. It helps the light ray to reflect from the shell without getting out from it. According to the refraction index optical fiber can be divided in two main groups: multimode and monomode cables. Several rays of light pass through the inner conductor in one time. The rays reflect from the outer conductor at various angles. The monomode cable inner conductor has a small diameter, which is commensurable with the transmitted wave length. That is why the light rays passing along the led lamp longitudinal axis and have no reflection from outer conductor.

Every fiber-optic information transfer scheme consists of next elements:

signal source;

modulator-amplifier in conjunction with laser and photodiode emitter. LED radiates a wide spectrum light at a wide angle in several different directions . It is used in multimode cables. The laser emitter has a high power, a tight angle of light feeding and generates a one-length wave light;

fiber-optic cable as a signal transmission medium;

an photodiode at the receiving side and a booster;

The monomode cable has the balance of advantage over the multimode cable. Its service band is twice wider, higher transmission range and less of losses. Though the price of the facility is much more higher, so it’s recommended to use multimode cable at the FOCL sections, with the length less than 10 km. It is necessary to remember about the losses at the supply splits and optical splices during the cable tracks design calculations, because the every jointing brings a signal attenuation, the rate of which depends on professionalism of the staff. The fiber has a heterogeneous structure, which is also needed to be taken into account when the distance calculating.