The first laser was introduced in 1960, and at the same time, communication systems based on its use began to develop. After almost 10 years, optical fiber was invented - the foundation of the Internet in its current state. Cables based on it have almost the same structure: in the center, there is a fiber for transmitting light, around it there is a sheath-damper that prevents the loss of radiation. Above it is insulation for added protection.
The core and shell are made of quartz glass. The difference between the two is that the refractive index of the damper is lower. To implement the necessary tasks, a minimal difference is enough, which sometimes amounts to hundredths and thousandths.
“Mode” is the path of the refraction of a light signal in the nucleus. The diameter of the shells of all types of fiber optics is 125 micrometers. The diameter of the central part of a single-mode fiber is 9 μm, of a multi-mode fiber is 50 (sometimes 62.5).
A single-mode fiber has a stepped refractive index profile: it is the simplest in terms of production. A multimode mode cannot work with such a profile at large distances, otherwise the signals begin to creep into them - they use a gradient metric. The peculiarity of this profile is that the transition in the refractive indices between the core and the shell occurs smoothly (and not abruptly from one indicator to another).
The main characteristic of the fiber is the broadband coefficient. The characteristics of multimode fibers are divided according to this parameter into 4 classes from OM1 to OM4. If OM1 is only suitable for expanding an existing system, then OM4 is also suitable for building new networks. It provides speeds up to 10 Gbit / second at a distance of up to 550 meters. Single-mode fibers are divided into OS1 and OS2: the second are suitable for use in any situation, there are practically no restrictions.
Optical cable selection rules
Application performance, as well as the distance at which it should work, have a major impact on the choice of fiber optic cable:
- for speeds of more than 10 Gbit / second, regardless of distance - single-mode fiber (also for 10-gigabit applications and distances over 550 meters);
- for 10 Gb / s applications and distances up to 550 meters - OM4;
- 10 Gb / s, up to 300 meters - OM3;
- 1 Gbit / s, up to 600-1100 meters - OM4;
- 1 Gbit / s, 600-900 meters - OM3;
- 1 Gbit / s, up to 550 meters - OM2.
Multimode cables cost superior to single-mode cables because the price depends on the core diameter. This does not mean that using a single-mode cable is more profitable: it requires more expensive equipment. To understand which option is best in a particular case, you need to rely on the cost of the system as a whole, and not on the separate price of the cable and network devices.
Single-mode cables are used in the following areas:
- marine and intercontinental cable lines;
- long-distance landlines;
- provider lines;
- cable television systems;
- GPON systems;
- cable systems in information processing centers;
- cable systems from 550 meters.
Multimode fiber is used in:
- cable systems inside buildings;
- horizontal segments of cable networks;
- information processing centers (as a supplement).
Multimode fibers are suitable in situations where the network structure allows it, primarily distance. The low cost of equipment helps to cover the cost of an expensive cable.
Optical Cable Testing
Testing cable networks involves the use of equipment, at one end of which there are radiation sources, and at the other meters. The devices take into account the presence of a small amount of losses: this is stipulated in the relevant standards, among which are TIA / EIA and ISO / IEC. If verification of long lines is required, refractometers are used.
Fiber optic cable
Network segments will last a long time if handled carefully. The use of special napkins and other cleaning products will significantly extend the life of the product.
Damage to cables sometimes occurs during repairs, such as digging trenches. You can find the location of the fault using the diagnostic tool. The OTDR will help determine the distance to the point at which the signal is lost.
High-quality network diagnostics require the use of reliable, expensive equipment. Budget devices will help to find a break, poor welding or bending, but with the help of them you can not get a detailed report. More advanced equipment for testing optical cables with single-mode and multimode fiber has the following features:
- detailed cable network diagnostics;
- event table compilation;
- report generation with all network characteristics.
Accuracy of the report is important for obtaining a network passport. Some welded joints are made so high quality that a reflectometer is not able to determine insignificant losses on them. However, they are still there, and the presence of welding and the smallest losses should be reflected in the report. Of great importance is the quality of the software used in the hardware. It is able to set the event, and then measure the loss at a specific site in manual mode.
Most professional equipment involves the possibility of expanding functionality. Additional options are opened using the following elements:
- video microscope for checking the ends;
- radiation source;
- optical phone.
Equipment from manufacturers of Fluke Networks, Greenlee Communication, and a number of other popular brands is in special demand. Qualitative testing of the network helps identify its weaknesses. Proper operation of the equipment and the absence of signal loss is the key to a long and uninterrupted operation of the cable system.