Physics AS optical fibres question PLEASE HELP Watch

Nayzar
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#1
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Hi.
I always trip up on this question and can never get full marks on it:
Q3c) State and explain why the core of an optical fibre is made as narrow as possible (3 marks)
The AQA A Jan 2010 mark scheme says:

">to reduce multipath or multimode dispersion
>(which would cause) light travelling at different angles to arrive at differenttimes/pulse broadening/merging of adjacent pulses/'smearing'/ poorresolution/lower transmission rate/lower bandwidth/less distance betweenregenerators
>or to prevent light/data/signal loss (from core or fibre)
>(which would cause) signal to get weaker/attenuation/crossover/data to beless secure"

can someone please explain all this to me? I've never heard or multipath/multimode dispersion but im guessing it means the spreading out of different light rays due to entering at different angles. also i dont know how it would prevent signal loss or stop the signal getting weaker. How does light have less chance of spreading out in a thinner core?
I SO CONFUSE
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tutorpace
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The answer comes from the use of optical fibers: information transfer for one. The thinner the more channels in a bundle. Also the attenuation of the light is smaller in a thin fiber . Just to add a little more info, there are also monomode optical fibers that are even thinner (8 to 20 µm). These are useful to transport light pulses, and only allow one spatial mode within the fiber. That means that instead of the typical picture of light bouncing in the walls of the fiber, you get that light can only propagate forward with a certain intensity profile. If they are thicker then light can take many different paths, bouncing at slightly different angles and still being reflected - this means that the time taken for each photon a pulse to go down a fibre is slightly different which smears out the pulse and so reduces the bits/second that can be sent. Thick plastic fibres are used for simply illuminating something because they are cheap, rugged and easy to make, thinner glass fibre is used for short distances and very thin very expensive single mode fibre is used for higher bandwidth long distance. For more info visit
http://en.wikipedia.org/wiki/Optical_fiber
http://physics.tutorpace.com/
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uberteknik
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Wider fibre cores produce a longer path for any given photon to travel as they bounce back and forth along the length of the cable. Since attenuation is a function of distance travelled, a wider cable produces a greater attenuation as the incidence angle increases for any given photon.

Because individual photons travel different distances along the length of the core, they will arrive at the exit point at different times. The energy density of the original coherent light source is therefore spread over a greater arrival time and having travelled varying distances and hence different attenuations.

This cause both spatial dispersion as the photons exit the core at varying angles and also temporal dispersion caused by the different exit times.
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tflanjee
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Basically, different beams of light enter the optical fibre at different angles. This means that different beams of light take different times to go through the optical fibre, as some will undergo total internal reflection more times than others. For example, if a beam goes straight through the optical fibre (this is called the axial path) it won't undergo any total internal reflection. However if a beam goes in at a steep angle, it will be total internally reflected several times, meaning it travels slightly further and takes longer to get through. This is multipath dispersion.
If this happens too much, the beam would take so long to get through that it would arrive at the same time as a beam from the next pulse, causing the pulses to merge into each other, this is called signal smearing.
One option would be to make the time between pulses longer to avoid this, however this would mean less data is transmitted so instead they make the core of the fibre really thin, so that the beam can ONLY go straight down it or at a very small angle, which limits the amount of TIR. These fibres with very thin cores are called monomode fibres.
To my understanding, even when total internal reflection occurs, there's always a bit they gets through and is refracted, just like when light is refracted there is a little bit of light that is reflected from the boundary. this means that having less total internal reflection means less light is lost from the boundary between the core and cladding, so you keep your strong signal. This also makes the signal more secure as the data is not lost from the fibre.
Hope that helps
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Nayzar
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.Ok that makes sense thanks guys. So as an exam answer if I was asked to state 3 reasons for a thin cladding would this do:

>Reduces the number of waves entering the fibre at a high angle of incidence to reduce multipath dispersion.
>Waves are (TI) reflected less times so less energy is lost.
>Waves are (TI) reflected less times, meaning they take straighter and shorter. Therefore signals arrive faster
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lerjj
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(Original post by Nayzar)
.Ok that makes sense thanks guys. So as an exam answer if I was asked to state 3 reasons for a thin cladding would this do:

>Reduces the number of waves entering the fibre at a high angle of incidence to reduce multipath dispersion.
>Waves are (TI) reflected less times so less energy is lost.
>Waves are (TI) reflected less times, meaning they take straighter and shorter. Therefore signals arrive faster
I think you mean a narrow core, rather than thin cladding?

As I understood it the main issue was that you got signal confusion if the length of time it takes one symbol to arrive (from the shortest path to the longest path, because it gets spread out) is longer than the difference in timings between sending each symbol. That puts a limit on the maximum frequency that you can send data at, because it you pulse the light too fast, it all bleeds together due to dispersion effects.

Others seem to be saying stuff about energy though, so that might also be correct (i.e. signal clarity has dropped off due to getting spread out).
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Nayzar
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(Original post by lerjj)
I think you mean a narrow core, rather than thin cladding?

As I understood it the main issue was that you got signal confusion if the length of time it takes one symbol to arrive (from the shortest path to the longest path, because it gets spread out) is longer than the difference in timings between sending each symbol. That puts a limit on the maximum frequency that you can send data at, because it you pulse the light too fast, it all bleeds together due to dispersion effects.

Others seem to be saying stuff about energy though, so that might also be correct (i.e. signal clarity has dropped off due to getting spread out).
yeah i meant core sorry. so the benefits are reducing multipath dispersion/spreading out/merging of signals; reducing energy loss; and increasing speed/rate of energy transfer
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