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fiber optics (FO)

The optical fiber (FO), Fiber Optics (FO), is standardized in DIN 47002 and VDE 0888 and states that it is a conductor in which modulated light is transmitted. The FO can be made of glass fiber or plastic and is characterized, among other things, by its extremely high transmission rate, which can be up to several billion bits per second (bit/s).

The transmission technology on optical fibres is based on intensity modulation, which takes the form of amplitude modulation, frequency modulation or phase mod ulation. An increase in transmission capacity is possible by modulating different wavelengths of light. Furthermore, optical waveguides are insensitive to electromagnetic interference, are largely tap-proof and, if they are made of glass, have extremely low attenuation values.

The structure of optical fibres

In terms of construction, an optical fiber consists of a cylindrical core glass, the surrounding cladding glass, and the coating. This standard concept is called Silica Clad Silica (SCS). Optical core glass and optical cladding glass are made of high purity fused silica with different refractive indices. In all SCS optical fibers, the core glass and cladding glass are mechanically inseparable.

Structure of optical

Structure of optical

fibers The structure is already determined during manufacture before the fiber is drawn. The outer coating, the primary coating, forms a mechanically resistant protective layer. It is surrounded by the secondary coating, which is not necessarily connected to the glass fiber. In addition to the aforementioned types of optical fibers, there are also hybrid optical fibers that consist of a glass core and a polymer cladding. Examples of this are Hard Clad Silica (HCS) and Plastic Clad Fiber (PCF). There are also the pure plastic optical fibers, Plastic Optical Fiber (POF). And there are optical fibers in which light is transmitted in air instead of the core glass. These optical fibers are called hollow-core fibers.

Optical fibres

with different modes

with different modes

In the core of an optical fibre with core glass, light components of different irradiation angles propagate. In order for a light beam to be guided in the core at all, it must be reflected at the interface between the core glass and the cladding glass. And this always towards the core glass. Such a reflection only occurs if the refractive index of the core glass is greater than that of the cladding glass and if the angle of incidence of the light onto the fiber end face is smaller than the maximum angle of acceptance. In this case, frequent reflection at the boundary layer between the core glass and the cladding glass results in light waves of different travel times, the modes. Light waves that travel a longer distance in the fiber are also referred to as high-mode light beams, while light waves that travel close to the fiber axis are referred to as low-mode beams.

Optical fibers with different modes

Structure and refraction profile of the single-mode fiber

Structure and refraction profile of the single-mode fiber

If several modes contribute to signal transmission in an optical fiber, they are also referred to as multimode fibers. Optical fibers in which only one mode is transmitted due to a very small core diameter (about 5 µm to 10 µm) are called single-mode fib ers.

Structure and refractive profile of the step index profile fiber

Structure and refractive profile of the step index profile fiber

In the case of multimode fibers, a further distinction can be made with regard to the type of refractive index
profile

profile

within the fiber. A distinction is made between step index and gradient fibers. In the case of step fibers, the core glass and cladding glass have a fixed refractive index, and the densities of the respective materials are constant.

Structure and refractive profile of the grad

Structure and refractive profile of the grad

ed fiber The graded fiber has a parabolic refractive index profile in the core. Beams travelling at different speeds in a graded fibre cover different distances in the glass fibre. Since the phase velocity of the beams is constant in such a fiber, the differently transported beams also have different travel times.

The effects of dispersion lead to a very strong change of the output signal in relation to the input signal, especially with long media lengths or high data rates. The gradient fiber was designed to circumvent the resulting differences in propagation time. This is a multimode fiber with a parabolic refractive index profile in the core. In these fibers, the beam path of the modes is no longer straight but almost sinusoidal. Common fibers, some of which are standardized (e.g. by ITU, German standard VDE 0888), have a core diameter of 50 µm, 62.5 µm, 85 µm with a cladding diameter of 125 µm.

Classification of optical fibres

Transmission rates of optical fibres depending on the distance

Transmission rates of optical fibres depending on the distance

In order that optical fibres can be used for Gigabit Ethernet, 10 Gig abit Ethernet and 100 Gigabit Ethernet, the quality of optical fibres is classified, similar to the class specifications of TP cables. The ISO/IEC qualifies gradient fibers into four OM classes and monomode fibers into the two OS classes. Within the classes, a distinction is made between light-emitting di ode and laser diode for the feed. For example, OM class 3 (OM3) has a bandwidth of 2,000 MHz x km at 850 nm wavelength, OM4 is suitable for 100 Gigabit Ethernet.

Optical fibers can be interconnected by detachable and fixed connections. A detachable connection of two fibers can be made using fiber optic connectors, and a non-detachable connection can be made using splices, which is characterized by the lowest attenuation values.

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Englisch: fiber optics - FO
Updated at: 06.02.2019
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