AQA AS physics - Pulse broadening and absorption

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FuryBall
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#1
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Hello,

In the AQA AS spec, it says "understand the principles and consequences of pulse broadening and absorption". Could someone give an explanation of what I need to know? I can't find anything on it!


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Callicious
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Hello.

Okay, so...

Absorption
The more transparent a material that makes up the core, the less light is absorbed. The less light that is absorbed, the farther a pulse with a given energy can travel. Per unit distance, a fraction will be absorbed, so it is a sort of exponential decay, except the more transparent the material is the far lower the decay rate. The consequence is that as light is absorbed, less of the pulse remains, to a point where it decays completely or can't be received. We need to minimise this, hence why a very pure transparent material is used in an optical fibre.

Pulse Broadening
Material and Modal: material broadening happens to refractive indices of the material used, i.e. light travels at different 'speeds' in the material due to the optical path lengths for a given distance varying, i.e. the light may travel one metre in distance but the true optical path length is optical index * distance of material. In the end, the pulse is stretched, and thus broadened. This is why monochromatic light is used, i.e. all the pulse travels at the same speed, thus no broadening into a spectrum i.e. as for white light. Modal dispersion, i.e. multipath, causes pulse broadening. This happens due to light being able to take multiple paths. There are infinite numbers of possible paths, but by reducing the width of a fibre, one reduces the average difference in the lengths of these different paths, thus reducing the difference in path lengths at the end of the length of fibre: this reduces the broadening of the pulse, thus making a more coherent signal.

Both can be rectified by adding repeaters, which increases latency in the signal.

Hope I helped!
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soldat123456
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cheers. Thanks for the help
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Qxi.xli
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(Original post by Callicious)
Hello.

Okay, so...

Absorption
The more transparent a material that makes up the core, the less light is absorbed. The less light that is absorbed, the farther a pulse with a given energy can travel. Per unit distance, a fraction will be absorbed, so it is a sort of exponential decay, except the more transparent the material is the far lower the decay rate. The consequence is that as light is absorbed, less of the pulse remains, to a point where it decays completely or can't be received. We need to minimise this, hence why a very pure transparent material is used in an optical fibre.

Pulse Broadening
Material and Modal: material broadening happens to refractive indices of the material used, i.e. light travels at different 'speeds' in the material due to the optical path lengths for a given distance varying, i.e. the light may travel one metre in distance but the true optical path length is optical index * distance of material. In the end, the pulse is stretched, and thus broadened. This is why monochromatic light is used, i.e. all the pulse travels at the same speed, thus no broadening into a spectrum i.e. as for white light. Modal dispersion, i.e. multipath, causes pulse broadening. This happens due to light being able to take multiple paths. There are infinite numbers of possible paths, but by reducing the width of a fibre, one reduces the average difference in the lengths of these different paths, thus reducing the difference in path lengths at the end of the length of fibre: this reduces the broadening of the pulse, thus making a more coherent signal.

Both can be rectified by adding repeaters, which increases latency in the signal.

Hope I helped!
PRSOM
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