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Homework Help: Fibre optics pulse spreading

  1. Dec 27, 2015 #1
    1. The problem statement, all variables and given/known data

    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 Im reading is about the advantages of total internal reflection in fibre optics.

    2. Relevant equations
    (Hope a moderator does not complain about the use of colours. Its meant to make the reading easier ...... thats why I did it here. Otherwise just just looks too long to read.

    3. The attempt at a solution

    Please help. High school physics level
  2. jcsd
  3. Dec 27, 2015 #2
  4. Dec 27, 2015 #3
  5. Dec 27, 2015 #4
  6. Dec 27, 2015 #5


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    Its not that internal reflection doesn't happen, rather it doesn't need to happen because the rays are curved by the graded refractive index. Remember light is bent when it moves from one refractive index to another.
  7. Dec 27, 2015 #6
    So the light is channeled through the optical fibre by refraction & internal reflection?
    Last edited: Dec 27, 2015
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