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Modern fiber optic cables contain multimode gradient fibers (code letter "G") or singlemode fibers (code letter "E"). In simplified terms, several different light beams (modes) travel simultaneously on different paths through the fiber in multimode fibers, and only one in singlemode fibers (these "light beams" symbolically represent the preferred direction of propagation of the main energy distribution of the electromagnetic wave "light").

The light is guided in the inner area of the fiber. The outer area ensures that light that has left the inner area cannot re-enter it, which would lead to signal distortion. The inner area is called the core in multimode fibers and the mode field in singlemode fibers. The outer area is called the cladding for both types of fiber.

As the core/mode field and cladding have different refractive indices, the light is reflected at the boundary between the two areas (total internal reflection). As a result, as much light as possible is guided in the core/mode field. Nowadays, fibers with a core diameter of 50 μm are used for multimode fibers, previously also with 62.5 μm.

The two fiber types must not be mixed on the same route, as otherwise high light losses occur, especially during the transition from 62.5 μm to 50 μm. The diameter of the mode field for single-mode fibers varies depending on the fiber manufacturer and wavelength and is 9 - 10 μm. The diameter of the cladding is 125 μm for all three fiber types.

Fiber optic cables do not necessarily have to be made of glass. They can also be made entirely or partially of plastic. Polymer optical fibers, also known as polymer optical fibers or POF for short, are made entirely of plastic. The English terms are "polymeric optical fiber" or "plastic optical fiber".

In contrast to glass fibers, polymer fibers cannot be connected with thermal splices as the plastic would melt due to the high temperature. Polymer fibers are connected with connectors or clamps. Precise, straight cuts are possible with sharp knives; grinding and polishing the fibers is not necessary.

Bend-insensitive optical fibers offer significant advantages in installations where space is limited. With full transmission bandwidth, bend-insensitive fiber types can also be installed in tight bends. However, not all of them are backwards compatible with conventional fibers. The ITU-T G.657 standard defines bend-insensitive single-mode fibers. The G.657.A series is compatible with the standard single-mode fibers according to ITU-T G.652. G.657B series fibers are usually not 100 percent compatible, but they offer even tighter bending radii than the A series fibers.

Depending on the manufacturer, bend insensitive multimode fibers (BIMMF) are backwards compatible with conventional OM3 or OM4 fibers. The fiber data sheet provides information on compatibility; in case of doubt, it is advisable to request confirmation of compatibility with other fibers.

More important than the bending radius of the fiber, however, is that of the cable. Expensive bend-insensitive fibers are useless if the cable containing them requires a large minimum bending radius.

Zero-waterpeak fibers, which offer a large usable wavelength range, are very important for WDM systems. WDM stands for Wavelength Division Multiplexing. Whereas in conventional transmission, light of only one wavelength travels in a single-mode fiber, in WDM systems several light beams of different wavelengths are transmitted in the same fiber.

Each channel is assigned its own wavelength. In order to ensure uniform transmission, the optical properties of the optical fiber must be as uniform as possible over the entire area used. Even if there are still very few WDM systems in LANs, care must be taken when selecting fibers to ensure that a future migration to WDM is possible through the use of zero-waterpeak fibers.

Pre-assembled cabling components with MPO connectors are being used more and more frequently in fiber optic networks - in data centers, server rooms, technical rooms of telecommunications providers and increasingly also in building cabling. The individual components are pre-assembled ready for connection and only need to be plugged together, which significantly reduces the installation effort. In addition, cabling with MPO connectors offers a simple migration path to 40 and 100 Gigabit Ethernet via multimode fibers, in which eight or twenty fibers are used simultaneously.

MPO connectors are protected against twisting by a key on the top of the connector. Depending on whether two connectors are plugged together with the keys on different sides (key up to key down, so-called type A) or on the same side (key up to key up, so-called type B), different fiber positions are connected to each other. Type C corresponds to type A, but fibers in a cable are swapped in pairs in order to connect sending and receiving during the transition to two-fiber networks.