Fibre Optics - Dispersion
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)