Notes On Suitable Optical Fiber Cables - Beckhoff EK15 Series Documentation

Commissioning/application notes
Fig. 45: Internal and external port assignment EK1521, EK1521-0010
6.3.2

Notes on suitable optical fiber cables

General information on optical fiber types
Optical fiber are available as multimode and single mode types with different step and graded indices.
Step and graded index
Optical fiber cables consist of 2 concentric materials, the core and cladding, plus a protective (colored)
jacket. The core and the cladding have a different index of refraction, causing the light waves (modes; a
mode is a natural wave in the optical fiber) to be reflected back into the core at the boundary. Due to the step
change in the index of refraction this type of fiber is referred to as step index. A gradual/parabolic transition
between the index of refraction in the core and the coating (referred to as graded index) can be achieved by
mixing the materials. In a graded index fiber the modes are gradually diffracted back to the core, leading to
propagation-time compensation and significantly higher quality of the light pulse at the outlet compared with
a multimode step index fiber, where the different light modes have different signal run times (mode
dispersion) with associated front distortion.
Single mode
Single-mode fibers have a very thin core (9 µm) and therefore conduct only a single mode of the light used,
with high signal quality and virtually without mode dispersion. They are only available as step index fibers.
Due to the high signal quality they are suitable for large transmission bandwidths > 10 GHz*km and
distances > 50 km. The refractive index profile of single-mode fibers is dimensioned such that the multipath
propagation (intermodal dispersion), which is a problem with multi-mode fibers, is omitted – the signal light
propagates in a single-mode fiber only in a single guided fiber mode, hence the designation 'single-mode'.
This makes considerably larger transmission distances and/or bandwidths possible, and the limiting effect
that arises next is the color distortion of the transmitted mode.
EK1122, EK15xx Version: 3.4 55