If nothing is done to mitigate this, parts of the input signal will arrive at the output later than other parts – and this will cause the output signal to be “dispersed” relative to the input signal as illustrated in Figure 1. Parts of the light being injected into the fiber will travel via one mode – other parts via another mode – and so on. So the different modes travel different distances – and hence some tend to travel faster than others. Some modes travel close to the center of the core – others bounce against the outer edges of the core, and these modes travel a longer way than the ones close to the center. The problem is that the different modes follow different paths through the fiber – and these paths are of different lengths. In a “Single Mode” fiber, the core is so small that it will allow only one mode to propagate. In a “Multi Mode” fiber, the core is rather large and may typically allow up to 17 different modes to propagate. Light “modes” are different types of waves carrying the light through the fiber. Modal Dispersion is the most serious of the dispersion types, and hence the most severe “speed limiter”. Modal Dispersion, Chromatic Dispersion and Polarization Mode Dispersion. Dispersion can be described as a “speed limiter”- and the 3 main types are: Hence transmission is limited by: A) The dispersion of the fiber B) The transmission rate, and C) The length of the fiber. Furthermore, dispersion is almost always dependent on the fiber length – the longer the fiber, the greater the dispersion. Since a faster transmission rate requires pulses to be of a shorter duration, this also means that a given level of dispersion will be more harmful to faster transmission rate signals. A measure of that quality is the BER (Bit Error Rate) which states the number of transmission errors relative to the total number of transmitted bits. Once the dispersion grows too large, the broadened pulses will overlap each other and the detector will start misreading the signal, creating errors that will effectively hamper the transmission quality. If the dispersion is small, the detector at the other end of the fiber will still be able to detect the signal correctly. Whenever such a signal is affected by dispersion, the edges of the square pulses will be rounded, and the pulse will be spread out over time. Illustrating such a signal would be a series of square pulses as shown in Figure 1. Digital information is consequently a series of “no light” and “full light” transmitted in a code which a receiver at the other end of the fiber understands and can convert to a digital electrical signal. Such a pulse is created by turning a laser on and off, creating light pulses where “no light” represents a digital “0” – and “full light” represents a digital “1”. Much, but not all, of the traffic traveling through fiber networks takes the form of pulses of laser light. Fiber Optic Coatings, Buffers and Cable Jacketing Materials.Fiber and Services for Electric Cooperatives.Specialty Optical Fibers, Modules and Kits.
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