13-07-2013, 12:05 PM
Optical Fibers?
[attachment=56598]
What are Optical Fibers?
Optical Fibers are fibers of glass, usually about 120 micrometers in diameter, which are used to carry signals in the form of pulses of light over distances up to 50 km without the need for repeaters. These signals may be coded voice communications or computer data.
History
Interest in the use of light as a carrier for information grew in the 1960's with the advent of the laser as a source of coherent light. Initially the transmission distances were very short, but as manufacturing techniques for very pure glass arrived in 1970, it became feasible to use optical fibers as a practical transmission medium. At the same time developments in semi-conductor light sources and detectors meant that by 1980 worldwide installation of fiber optic communication systems had been achieved.
Multimode
Although it may seem from what we have said about total internal reflection that any ray of light can travel down the fiber, in fact, because of the wave nature of light, only certain ray directions can actually travel down the fiber. These are called the "Fiber Mode". In a multimode fiber the fiber supports many different modes. This is shown in the diagram below.
Graded Index Fiber
In graded index fiber rays of light follow sinusoidal paths. This means that low order modes, i.e. oblique rays, stay close to the center of the fiber, high order modes spend more time near the edge of core. Low order modes travel in the high index part of the core and so travel slowly, whereas high order modes spend predominantly more time in the low index part of the core and so travel faster. This way, although the paths are different lengths, all the modes travel the length of the fiber in tandem, i.e., they all reach the end of the fiber at the same time. This eliminates multimode dispersion and reduces pulse spreading.
Graded Index fiber has the advantage that it can carry the same amount of energy as multimode fiber. The disadvantage is that this effect takes place at only one wavelength, so the light source has to be a laser diode, which has a narrow line width.
[attachment=56598]
What are Optical Fibers?
Optical Fibers are fibers of glass, usually about 120 micrometers in diameter, which are used to carry signals in the form of pulses of light over distances up to 50 km without the need for repeaters. These signals may be coded voice communications or computer data.
History
Interest in the use of light as a carrier for information grew in the 1960's with the advent of the laser as a source of coherent light. Initially the transmission distances were very short, but as manufacturing techniques for very pure glass arrived in 1970, it became feasible to use optical fibers as a practical transmission medium. At the same time developments in semi-conductor light sources and detectors meant that by 1980 worldwide installation of fiber optic communication systems had been achieved.
Multimode
Although it may seem from what we have said about total internal reflection that any ray of light can travel down the fiber, in fact, because of the wave nature of light, only certain ray directions can actually travel down the fiber. These are called the "Fiber Mode". In a multimode fiber the fiber supports many different modes. This is shown in the diagram below.
Graded Index Fiber
In graded index fiber rays of light follow sinusoidal paths. This means that low order modes, i.e. oblique rays, stay close to the center of the fiber, high order modes spend more time near the edge of core. Low order modes travel in the high index part of the core and so travel slowly, whereas high order modes spend predominantly more time in the low index part of the core and so travel faster. This way, although the paths are different lengths, all the modes travel the length of the fiber in tandem, i.e., they all reach the end of the fiber at the same time. This eliminates multimode dispersion and reduces pulse spreading.
Graded Index fiber has the advantage that it can carry the same amount of energy as multimode fiber. The disadvantage is that this effect takes place at only one wavelength, so the light source has to be a laser diode, which has a narrow line width.