Optical-fiber Internet cables – types and characteristics

Fiber optic internet network cable is a fundamentally different type of cable than other types of electrical or copper cables. Through it, information is transmitted not by an electrical signal, but by light. Its main element is transparent fiber glass, through which light passes over a long distance (up to tens of kilometers).

The structure of the optical fiber cable is very simple and similar to the structure of the coaxial electrical cable, only instead of a central copper wire, a thin (about 1-10 microns in diameter) glass fiber is used instead of internal insulation. , a glass or plastic shell that does not allow light to go beyond the glass fiber, in this case we are dealing with the so-called total internal reflection mode of light, from the boundary of two substances with different refractive indices (the index of refraction of the glass shell is much lower compared to the central fiber). Metal tapes of the cable are usually absent, since protection against 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 armor, it can combine several optical fiber cables).

Special characteristics of optical fiber cable

From the point of view of immunity to noise and secrecy of transmitted information, external electromagnetic interference cannot in principle disturb the light signal, and this signal itself does not generate external electromagnetic radiation. Connecting to this type of cable is practically impossible for unauthorized network eavesdropping, as this requires breaking the integrity of the cable. The theoretically possible bandwidth of such a cable reaches 1012 Hz, which is incomparably higher than any electrical cable. The cost of fiber optic cable has been steadily decreasing and is now roughly equivalent 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 signals and vice versa, which sometimes significantly increases the cost of the network as a whole.

Typical signal attenuation at fiber optic cable frequencies

Signal attenuation used in local area networks is about 5 dB/km, which is about the same as power cables at low frequencies. But in the case of optical fiber cable, as the frequency of the transmitted signal increases, the attenuation increases very slightly, and at high frequencies (especially above 200 MHz), its advantage over electric cable is undeniable.

However, fiber optic cable also has some disadvantages. The most important of them is the high complexity of the installation (micron accuracy is required when installing the connectors; the attenuation in the connector strongly depends on the accuracy of the fiberglass split and the quality of its polishing). To install connectors, welding or glue is used using a special gel that has the same light refractive index as fiberglass. In any case, it requires highly qualified personnel and special tools. Therefore, most often, fiber optic cable is sold in the form of pre-cut parts of different lengths, both ends of which are already installed with the desired type of connectors. Together with our team (Innocom) , we will be able to provide the services you want, which will help you in the IT infrastructure niche, you will get the quality service you deserve.

Although fiber optic cables allow signal splitting (special splitters for 2-8 channels are available for this), as a rule, they are used for transmission. After all, any branching necessarily significantly weakens the light signal, and if there is more resistance, 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 allowable bend radius is about 10-20 cm). It is also sensitive to ionizing radiation, due to which the transparency of the fiberglass decreases, that is, the signal attenuation increases. It is also sensitive to sudden changes in temperature, which can cause the fiberglass to crack. Currently, optical cables made of radiation glass are being produced (of course, they are more expensive).

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

Fiber optic cable is used

Only in star and ring topology networks. In this case, there is no problem of matching and grounding. The cable provides ideal galvanic isolation of networked computers. In the future, this type of cable is likely to replace all types of electrical cables, or in any case significantly replace them. The planet’s copper reserves are depleted and there is more than enough raw material for glass production.

There are two different types of fiber optic cables:

• Multimode, or multimode, cable, cheaper, but of lower quality;
• Single-mode cable, more expensive but with better performance.

The main differences between these types are related to the different modes of light rays passing through the cable.

single mode cable

Virtually all rays follow the same path, so they reach the receiver at the same time and the waveform is virtually undisturbed. Single-mode cable has a central fiber diameter of about 1.3 µm and transmits light only at the same wavelength (1.3 µm). Dispersion and signal loss are very small, which allows you to transmit signals over much longer distances than when using a multimode cable. For single-mode cable, laser transmitters are used, using light of only the required wavelength. Such transmitters are still relatively expensive and not very durable. However, in the future, single-mode cable should become mainstream due to its excellent characteristics.

Multimode cable

The paths of the light rays have a noticeable spread, as a result of which the shape of the signal at the receiving end of the cable is disturbed. The core fiber has a diameter of 62.5 microns and an outer sheath diameter of 125 microns (sometimes called 62.5/125). Conventional (non-laser) LEDs are used for transmission, which reduces cost and increases the life of transmitters compared to single-mode cable. Multimode cable has a wavelength of 0.85 µm. The permissible length of the cable reaches 2-5 km. Currently, multimode cable is the main type of fiber optic cable because it is cheaper and more readily available. The propagation delay of a fiber optic cable is not much different from that of electrical cables. The typical tension of the most common cables is about 4-5 N/m.

David Sarishvili

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