Fiber optic glossary

Our fiber optic glossary gives you an overview of the most important fiber optic terms. Here you will find explanations of important terms and further information.

APC/UPC connection

The main difference lies in the fiber end face. In APC, it is inclined at 8° to the horizontal axis, while in UPC it is perpendicular to it. APC is more suitable for applications requiring high precision fiber optic signaling. Less sensitive digital systems can work equally well with UPC. Note: Typically APC connectors are green.

Bandwidth-length product

Calculated by multiplying the maximum bandwidth by the length of an optical fiber. In addition to attenuation, another important parameter for the quality of multimode fibers.

BERT (Bit Error Rate Test):

Comprehensive performance test. Calculated from the number of bits received with errors and the total number of bits transmitted by the other end during the same period. This provides a holistic picture of system performance.

Breakout cables

Pre-assembled fiber optic cables consisting of one or more optical fibers housed in a common cable jacket and equipped with fiber optic connectors at one end. They are used wherever a direct plug connection is required without patch panels, junction boxes or other components.

Mini breakout cable

Particularly suitable for laying and pulling in cable ducts and shafts, in underfloor areas, as a patch and adapter cable and as a connection cable to the workplace inside buildings.

Bundle core

A type of fiber optic cable, with several optical fibers, which are jointly surrounded stress-free by a filled or unfilled plastic tube.

Dead zone

Area behind a reflective event that can no longer be evaluated with an OTDR. Depends on the pulse width of the measuring pulse, the larger the longer the dead zone.


Modal dispersion is a distortion mechanism in multimode fibers because the modes tend to disperse when they are too far apart. Chromatic dispersion occurs because multiple wavelengths travel through the glass at different speeds. The transmitted signal is distorted and stretched.

Insertion Loss

The attenuation experienced by a signal as it flows through a component or transmission system. The value is defined as the ratio of outgoing and incoming signal power and is expressed in decibels.


Central component in a fiber optic connector, holds the fiber and guides it during the mating process. Often made of hard metal, ceramic, plastic or a combination of these.

Fiber optic connector types

The majority of connectors used today are plug-plug connections. The connectors used must have the lowest possible insertion loss and a high return loss. Likewise, they must be reproducible and maintainable over several hundred connection cycles. The most common connector types in communications technology are LC and SC, and in older installations also ST and E-2000 for long-distance transmissions.

Fresnel losses

Loss during the transition from one optical fiber to another, caused by a gap between the optical fibers and passing the glass-air interfaces twice. Partial back reflection of the light occurs there in each case, which consequently means a loss.

FTTX (Fiber To The X)

Generic term for broadband access technologies as a combination of optical fibers and copper twisted pairs. Stands for all fiber technologies in the „last mile“ such as Fiber to the Home (FTTH), Fiber to the Curb (FTTC), Fiber to the Desk (FTTD), etc.

Launch and tail cord

Both are needed to make an accurate OTDR measurement. The launch cord bridges the dead band of the instrument connector and helps to achieve a stable output condition. The tail cord is used to measure the last connector on the link. Pre- and post fibers should be min. 75m long for multimode, min. 150m long for singlemode (depending on the dead zones of the OTDR used or the measurement parameters set).


Discrete light waveforms propagating in the core glass of an optical fiber, provided that the coupling of the light into the fiber occurs below the acceptance angle. Whereas in a singlemode fiber only a single mode, the fundamental mode, is capable of propagation, in a multimode fiber there are many hundreds of modes, which differ, among other things, in field distribution and propagation speed. In the case of optical fibers, a distinction is made between single mode fibers, which know only one mode, and multimode fibers, which know many modes of lower and higher order.

Multimode fiber/multimode

Can transmit multiple light beams/modes simultaneously at typical wavelengths of 850 and 1310nm. Mainly used for short to medium distance lengths up to medium data rates.

OTDR (Optical Time Domain Reflectometer)

Optical time domain reflectometry is a method for professional measurement and testing of optical fibers. With the OTDR method, faults in fiber optic cables can be localized directly, and transmission-related parameters can also be measured and analyzed. These measurements can also form the basis for proving and documenting fault-free workmanship. Later, during operation, the OTDR is used for fault diagnosis, for example to localize fiber breaks.

Rayleigh attenuation

Light scattering that can be caused by material inhomogeneities in an optical fiber. return loss: Quality characteristic of fiber optic cables, defined as the ratio of injected energy to reflected energy. The value is given in decibels and should be as high as possible.

Refractive index

The factor by which the speed of light in optical media is less than in a vacuum.Coating: Optical fiber itself is made of high-purity fused silica. The core is surrounded by a glass cladding and covered with a plastic layer (primary coating). This makes the fiber flexible and protects it from breakage.

Singlemode fiber

A fiber in which only one mode can propagate. Used today in all areas of optical transmission with typical transmission wavelengths of 1310 and 1550nm. Mainly used for longer distances or very high data rates.

Splice (fusion/mechanical)

Mechanical or fusion splicing are the two widely used techniques for fiber optic connection. Each of the splicing methods has its requirements, advantages and disadvantages. Most often, fusion splicing is preferred.

Video microscope

Allows you to safely visually inspect fiber optic connectors (1.25mm and 2.5mm ferrules) for scratches and dirt that are difficult to inspect with standard microscopes and require time-consuming removal.