What are the Sources of Dispersion in Fibre Optics?

[-DJL-]

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 traveling 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)

 

[dlgoff]

...wave-guide dispersion depends on the shape, design, and chemical composition of the fiber core. Only 80 percent of the power from a light source is confined to the core in a standard single-mode fiber, while the other 20 percent actually propagates through the inner layer of the cladding. This 20 percent travels at a faster velocity because the refractive index of the cladding is lower than that of the core (Keiser, 1983). Consequently, signals of differing frequencies and wavelengths are dispersed and the pulse becomes indistinguishable (Lerner, 1997).

"www.cmste.uncc.edu/new/papers/Attenuation%20in%20Fiber%20Optics.doc"[/URL]

 

[berkeman]

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?

 

[berkeman]

Doh! dlgoff is quick today!

 

[-DJL-]

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.