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Fibre Optics - Dispersion

  1. Jan 15, 2008 #1
    I'm confused as to sources of dispersion in a fibre. Any clarification would be appreciated.

    Modal (Intermodal) dispersion: pulse spreading due to different group velocities of different modes in the fibre. Each mode has a slightly different n-effective hence the different Vg for each mode. Not present in single mode fibres, as there's only one mode. Can also be though of as different ray paths - light travelling down the centre of the fibre will reach the end before light which is TIR'ing its way along.

    Intramodal covers the following;

    Material Dispersion: Spectral width; each spectral component will in general travel with a different group velocity -> difference wavelengths travel at different speeds. Sources of spectral width: LED greater width than a laser, and sidebands caused by modulation.

    Waveguide dispersion: ?? changes in the distribution of light between core and cladding? Dependant on core diameter?

    So with Dispersion Shifted Fibres :- single mode fibre, therefore no modal dispersion. Waveguide dispersion is negative and material dispersion is positive -> therefore at a particular wavelength they cancel -> 0 dispersion. Wave guide dispersion can be shifted so that the 0 occurs at a different wavelength (1550nm)
    Last edited: Jan 15, 2008
  2. jcsd
  3. Jan 15, 2008 #2


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  4. Jan 15, 2008 #3


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    That's cool stuff. What part is confusing? What learning resources do you have available, and what have you read so far on the web?
  5. Jan 15, 2008 #4


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    Doh! dlgoff is quick today!
  6. Jan 15, 2008 #5
    dlgoff - Thanks, that link, along with going through the maths, has helped clarify waveguide dispersion.

    A great book on the subject is Ghatak, Ajoy and K., Thyagarajan. (2000). Introduction to Fibre Optics. Cambridge, UK : Cambridge University Press.
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