What is step index fiber

Glass fiber / optical waveguide (LWL)

Optical fibers, or LWL for short, are thin plastic fibers that can transmit optical signals in the form of light or light signals over long distances. While electrical signals in copper lines travel as electrons from one end to the other, the photons (light particles) take on this task in optical waveguides (LWL).

With fiber optic cables, optical signals can be bridged over long distances without an amplifier. A high bandwidth is possible despite long distances. The bandwidth of a single fiber optic cable is around 60 THz. No commercial system provides that. But the capacity can be increased almost at will simply by adding more wavelengths as a carrier. No copper cable and no radio system can do that. That is why fiber optic cables are the transmission medium of the future.

Glass fiber or fiber optic cable

The glass fiber is an optical waveguide (LWL), the fibers of which consist of the basic material glass. It is often confused with the term fiber optic cable. Optical fiber is the generic term for all light-conducting cables, which also includes glass fibers. Optical fibers are available as glass, quartz or plastic fibers.
A fiber optic cable is often referred to as glass fiber even if the basic material is not glass.

Principle of a transmission system based on a fiber optic cable


Depending on the data form, an analog-digital conversion takes place first. As a rule, the data are available as electrical signals, which are then amplified by a driver stage. Before transmission, the electrical signals must be converted into optical signals. Special light-emitting diodes (LEDs) or laser diodes serve as light generators for this purpose. The light is fed directly into the fiber optic cable. At the other end of the fiber optic cable, the light pulses are converted back into electrical signals. A photo element, for example a photo transistor, generates electrical pulses from the light. A digital-to-analog conversion then takes place when the data has to be transferred to the receiver in analog form and amplified.

Communication networks with fiber optic cables

In order to achieve high speeds in communication networks, one usually relies on optical connections between the nodes. In the control centers and switching centers, the transmitted light signals are mostly converted into electrical signals, evaluated and further processed. They are then converted back into light signals for further transmission. This is where the disadvantages of optical transmission systems become apparent. For processing, optical signals usually first have to be converted into electrical signals.

Structure of a fiber optic cable

Optical fibers (LWL) made of plastic have a diameter of about 0.1 mm. They are extremely flexible but also sensitive. Therefore, a professional handling of fiber optic cables is required.
The fiber core (core glass) is the central area of ​​an optical waveguide, which is used to guide the light. The core consists of a material with a higher refractive index than the cladding on top. A reflection takes place on the walls inside the fiber optic cable, so that the light beam is guided around every corner with almost no loss. The effect of the reflection between the core glass and the cladding glass is called total reflection.
The cladding glass is the optically transparent material of an optical waveguide on which the reflection takes place. Cladding glass, also known as cladding, is a dielectric material with a lower refractive index than the core. The dielectric material is non-metallic and non-conductive. So it does not contain any metallic components.
The coating is the plastic coating that is applied to the surface of the cladding glass as mechanical protection.
The protective material that is extruded onto the coating is called buffering. It protects the cable from environmental influences. Buffering is also available as a tube that isolates the fiber from stress in the cable when the cable is moved.

Advantages of fiber optic cables over copper cables

  • Fiber optic cables can be laid in parallel with other supply lines as required. There are no electromagnetic interferences.
  • Because of the optical transmission, there are no interference radiation or mass problems.
  • Losses due to distance due to inductances, capacitances and resistances do not occur.
  • Almost frequency-independent line attenuation of the signals.
  • Transmission rates can be increased almost indefinitely by using several carrier waves with different wavelengths (color spectrum).
  • There are no problems with equipotential bonding with different ground potentials at the cable ends of the floor distributors.

Disadvantages of fiber optics

  • However, fiber optic cables are more expensive than copper cables. The cost of material and the effort involved in assembly are higher. In return, fiber optic cables have significantly lower attenuation and are therefore suitable for long distances.
  • Light pulses simply cannot be temporarily stored in a reasonable manner. Due to the lack of optical signal memories and processing elements, complex optical / electrical and electrical / optical signal conversion must take place.

Technical terms

The Refractive index is the factor by which the speed of light in optical media is lower than in a vacuum.

Fashions are the different paths that the photons of light can follow within or along the fiber. Multimode fibers can support many modes.

The splice is the permanent connection between two fibers. In order to establish a connection between two optical fibers, the two ends must be fused (fusion splice) or glued (adhesive splice).

The insertion of an optical component creates an attenuation of the signal. That is with Insertion loss meant.

Dispersion describes the effect that the injected pulse is temporally extended over the propagation path. The impulse becomes wider. This can lead to overlaps with the previous and following pulses. Transmission errors can occur at high speeds.
In order to get the pulse as pulse-like as possible, no normal LEDs are used to generate light pulses, but laser diodes that can generate a pulse with a spectral width of a few nanometers.

Coupling of light into the fiber

A multimode fiber has several modes. With LED light coupling, all modes of a fiber are excited. LEDs fill the entire fiber core. One speaks of full excitation.
The transferable data rate with LED transceivers is limited. Due to its characteristic switching hysteresis, there is the inertia for the transmission LED. An LED transceiver is not sufficient for Gigabit Ethernet (GbE) or 10 Gigabit Ethernet (10 GbE). Instead, lasers are used to couple light. In contrast to LEDs, lasers only excite a certain number of modes. VCSELs (Vertical Cavity Surface Emitting Lasers), which are used by all well-known manufacturers, have been specially developed for fiber optic cables. When light is coupled in, VCSELs only respond to a few modes and have a wavelength of 850 nm.
VCSEL lasers have several advantages over LEDs:

  • lower attenuation during signal coupling
  • higher transmission performance
  • greater transmission distance
  • longer operating time

However, when laser light is coupled into conventional multimode fibers, interference often occurs in the form of centerline dips. The Centerline Dip is a notch in the refractive index profile in the center of the fiber. Further disturbances can be flat tops and peaks in the refractive index profile.
The laser signal brings a large part of the total power to the fiber center. This results in a deformation of the ideal transmission signal. The result is a higher bit error rate and the resulting poor net data rate and failure of the transmission.
When using components with laser light coupling, it is essential to use laser-optimized light guides.

Overview: fiber optic cables

Cable typeDiameter (core / total)Bandwidth (1 km)application
Multimode with step profile100 to 400 µm / 200 to 500 µm100 MHzDistances under 1 km, patch cables
Multimode with gradient profile50 µm / 125 µm1 GHzLAN, backbone, ATM (655 MHz) in Europe
62.5 µm / 125 µm1 GHzLAN, Backbone, ATM (655 MHz) in the USA
Monomode (singlemode) with step profile9 µm / 125 µm100 GHzNetwork operator

Multimode fiber with step index profile

Multimode fibers with a step profile have an overall diameter of 200 to 500 µm. Several light waves are sent through them at the same time. The signal is reflected hard on the walls of the fiber. The refractive index drops sharply between the core and cladding. This makes the output signal worse. You will e.g. B. used as a connection cable in the patch cabinet.

Multimode fiber with graded index profile

Multimode fibers with a gradient profile have a total diameter of 125 µm. Several light waves are sent through them at the same time. The signal is softly reflected on the walls of the fiber. The refractive index of the core usually decreases in a parabolic manner towards the cladding. The output signal is still very good. They are used to connect buildings or floors.

Single mode fiber / single mode fiber

Singlemode fibers or monomode fibers have an overall diameter of 125 µm. The light waves are passed straight through them. They are used for long distances. The core diameter of a single mode fiber is so small compared to the wavelength of light that only one mode (modes) can propagate. Singlemode fibers require the use of very expensive lasers, which leads to high equipment costs.
Singlemode fibers are optimized for city and access networks. The demands on these fiber optic cables are high. In addition to fibers that are easy to process, broadband performance is required for flexible network design.

Laser-optimized multimode fibers

Fiber typesFast ethernetGigabit Ethernet10 Gigabit Ethernet40 Gigabit Ethernet100 Gigabit Ethernet
OM1 fiber (62.5 / 125 µm)2,000 m (FX)275 m (SX)33 m (SR)not specifiednot specified
OM2 fiber (50/125 µm)2,000 m (FX)550 m (SX)82 m (SR)not specifiednot specified
OM3 fiber (50/125 µm)2,000 m (FX)550 m (SX)300 m (SR)100 m (SR4)100 m (SR10)
OM4 fiber (50/125 µm)2,000 m (FX)1,000 m (SX)550 m (SR)150 m (SR4)150 m (SR10)

Color code of the bundles or fibers

There is the color code from Telcordia (formerly Bellcore) and Deutsche Telekom. Both are used to determine the fiber order.

Color code (bundle / fiber color)
No.Deutsche Telekom Telcordia

Overview: fiber optic technology

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