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Multi-mode and single mode fibre-optic cables

One of the basic criteria of optical equipment classification is the type of the fibre optics with which it works. It is connected with the type of transmission of light in the fibre core - single-mode or multi-mode.
Optical fibres are composed of three essential parts, performing specific functions:

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The structure of a fibre
1. core
2. cladding
3. buffer coating


Transmission of light in optical fibres is based on the phenomenon of total internal reflection. The core, usually made of doped glass (e.g. GeO2 + SiO2), is the centre through which light travels along, while the fibre cladding is made of pure glass (SiO2). Such combination of materials is dictated by their refraction indexes. To achieve total internal reflection, the refraction index of the cladding (pure glass) must be lower than the refractive index of the core (doped glass). The buffer coating surrounding the cladding is a cover layer usually made from a thermoplastic material and special gels, which protects the fibre from mechanical damage.


The main difference between single mode and multi-mode fibre optic cable is the way of light transmission in the fibre core. A multi-mode fibre core transmits many modes (to simplify - beams of light with the same wavelength). The propagation of multiple modes causes modal dispersion, which translates into a significant reduction in the range or speed of signal transmission. Simply, the signal is spread in time because the propagation velocity of the optical signal is not the same for all the modes due to their different path lengths between the transmitter and receiver, resulting from different angles of reflection of light beams from the boundaries of the core.


The phenomenon of modal dispersion is practically eliminated in a single-mode fibre core which transmits only one mode of light with a specific wavelength. In the case of a single mode fibre, the light wave propagates almost parallel to the axis of the fibre. Data rates in single mode optical fibres are limited by polarization mode dispersion and chromatic dispersion. Chromatic dispersion is a combination of material dispersion and waveguide dispersion. The phenomena lead to signal degradation due to varying delay in arrival time between different components of the signal, however they do not affect the signal quality so significantly as in the case of multi-mode fibres. There are also dispersion-shifted fibres and nonzero dispersion-shifted fibres, for which the waveguide dispersion is practically eliminated in the third transmission window (1550 nm).

 

Propagation of light in:
1 - multi-mode optical fibre,
2 - single-mode optical fibre.


Multi-mode and single-mode fibre cores significantly differ in diameter. Single mode fibre core is usually between 8 - 10 micrometers (typically 9 um), while the diameter of a multi-mode fibre core is 62.5 or 50 micrometers. In both cases the typical diameter of the cladding is 125 micrometers.


There is no visible difference between the cabling - the installer has to pay attention to the marking of the cables and cooperating equipment. In most cases, devices for connecting optical fibres such as arc fusion splicing instruments or mechanical splicers are suitable for use with both types of fibre optic cables. The installer should carefully match the appropriate active devices, optical cables, and accessories.


The great advantage of single-mode fibre optic cables is the possibility of transmitting signals (without regeneration) up to 120 kilometers. In the case of multi-mode fibres, the maximum transmission range is about 2 km. Of course, the actual transmission distance is determined by the applied optical devices and their capabilities. DIPOL offers a range of single-mode and multi-mode equipment - from active devices such as media and video converters to various accessories such as connectors, adapters, attenuators, and patchcords.