Multimode and single mode fiber-optic cables

One of the basic criteria of optical equipment classification is the type of the fiber optics with which it works. It is connected with the type of transmission of light in the fiber core - single-mode or multimode.

Optical fibers are composed of three essential parts, performing specific functions:

Transmission of light in optical fibers is based on the phenomenon of total internal reflection. The core, usually made of doped glass (e.g. GeO₂ + SiO₂), is the center through which light travels along, while the fiber 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 fiber from mechanical damage.

The main difference between single mode and multimode fiber optic cable is the way of light transmission in the fiber core. A multimode fiber 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 fiber core which transmits only one mode of light with a specific wavelength. In the case of a single mode fiber, the light wave propagates almost parallel to the axis of the fiber. Data rates in single mode optical fibers 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 multimode fibers. There are also dispersion-shifted fibers and nonzero dispersion-shifted fibers, for which the waveguide dispersion is practically eliminated in the third transmission window (1550 nm).

Multimode and single-mode fiber cores significantly differ in diameter. Single mode fiber core is usually between 8 - 10 micrometers (typically 9 um), while the diameter of a multimode fiber 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 fibers such as arc fusion splicing instruments or mechanical splicers are suitable for use with both types of fiber optic cables. The installer should carefully match the appropriate active devices, optical cables, and accessories.

The great advantage of single-mode fiber optic cables is the possibility of transmitting signals (without regeneration) up to 120 kilometers. In the case of multimode fibers, 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 multimode equipment - from active devices such as media and video converters to various accessories such as connectors, adapters, attenuators, and patchcords.