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Old Dec 27th 2015, 11:33 AM   #1
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Fibre optics High school text

The text book is talking about pulse spreading/dispersion in optical fibres.

Book says: "The spreading of the pulse is due to two dispersion effects:

(i) Chromatic dispersion occurs because the refractive index of the fibre is different for different frequencies (colours) of light ........ the pulse will have 'spread'".

Don't really understand how refractive index can be different for different frequencies. Isn't it meant to be that different frequencies are refracted by different amounts by the same refractive index material?

(ii) "If the width of the fibre is larger than the wavelength of the light there will be alternative paths (called 'modes') along the fibre. Each path is a slightly different length so the pulse arrives spread over a longer time.

This multi-mode dispersion can be reduced by using a graded-index material for the fibre core.

Here, the refractive index of the core material reduces gradually from the centre to the edge". The result of this is that the light travels more quickly towards the edge of the fibre, compensating for the extra distance it has to travel. The refraction occurs gradually as the light moves towards the edge of the fibre, so the light is always bent back towards the centre of the fibre and total internal reflection does not happen".

Don't really understand how total internal reflection does not happen. The whole topic I'm reading is about the advantages of total internal reflection in fibre optics.

Please help
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Old Dec 28th 2015, 02:54 PM   #2
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Don't really understand how refractive index can be different for different frequencies. Isn't it meant to be that different frequencies are refracted by different amounts by the same refractive index material?
The refractive index can be stated in terms of deviations as sini/sinr.
This is the usual high school definition.

More advanced theory shows that this is the same as the ratio of the velocities in the respective media.

Don't really understand how total internal reflection does not happen. The whole topic I'm reading is about the advantages of total internal reflection in fibre optics.
Light rays in a linear optic fibre are a series of straight lines, totally internally reflected when they hit the boundaries.
Rays normally incident at the end travel in one forwards direction down the cable, but skew rays obliquely incident also have a sideways component which becomes a 'line' helix as it winds its way down the cable.

This mode is called the step index fibre.

Your text refers to a graded index where the composition of the fibre varies so the index and therfore the velocities vary as described.
This cause the rays to adopt a curved path in the cable as they are passing through a medium of continually changing index.
This is rather like the distinction between bicfocals and varifocal lenses in spectacles.

The maths is way above high school so I'm afraid it's difficult to describe.
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Old Dec 29th 2015, 04:59 AM   #3
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Just to make sure the first question is answered appropriately: the index of refraction of a material depends on the frequency of light - this is what makes a prism able to spread the colors that make up white light into bands of color. The higher frequency colors (indigo and violet), have higher frequency than the lower frequency colors (red, orange), and so refract less through the glass of the prism. As Hero suggests, light with a higher refractive index (red) travels slower through the medium than light with a higher refractive index (violet), and consequently as a pulse made of multiple frequencies of light travels down a fiber the pulse will spread out. Hence the pulse shape becomes less distinctive (i.e. it "disperses") as it ravels down the fiber.

As for the second part, I think the phrase "total internal reflection does not happen" is misleading. Perhaps what they mean to say is that the light is curved back towards the center of the fiber not by a step change of refractive index at the outer edge of the fiber but rather by a continuous variation of refractive index from the center to the outer edge.
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