What are the Sources of Dispersion in Fibre Optics?

Click For Summary

Discussion Overview

The discussion revolves around the sources of dispersion in fibre optics, including modal, material, and waveguide dispersion. Participants explore the implications of these types of dispersion in both single-mode and multimode fibres, as well as the effects of design and composition on waveguide dispersion.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant describes modal dispersion as pulse spreading due to different group velocities of modes in the fibre, noting its absence in single-mode fibres.
  • Material dispersion is discussed in terms of spectral width, with different wavelengths traveling at different speeds, influenced by the light source used.
  • Waveguide dispersion is mentioned as being dependent on the distribution of light between the core and cladding, with questions raised about its relationship to core diameter.
  • Another participant adds that waveguide dispersion is affected by the shape, design, and chemical composition of the fibre core, noting that a portion of the light propagates through the cladding, which can lead to pulse indistinguishability.
  • There is a mention of dispersion-shifted fibres, where waveguide dispersion is negative and material dispersion is positive, potentially cancelling each other at a specific wavelength.
  • A participant expresses gratitude for a resource that helped clarify waveguide dispersion, suggesting that mathematical understanding is important for grasping the concepts discussed.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and confusion regarding the sources of dispersion, indicating that multiple views and interpretations exist without a clear consensus on all aspects of the topic.

Contextual Notes

Some discussions involve assumptions about the definitions of dispersion types and their dependencies on various factors, such as the design of the fibre and the characteristics of the light source.

Who May Find This Useful

This discussion may be useful for students and professionals in optics, telecommunications, and materials science, particularly those interested in the technical aspects of fibre optics and dispersion phenomena.

-DJL-
Messages
2
Reaction score
0
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? dependent 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:
Engineering news on Phys.org
...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]
 
Last edited by a moderator:
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?
 
Doh! dlgoff is quick today!
 
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.
 

Similar threads

Engineering Optical Fibres: Wavelength for Zero net Dispersion
  • · Replies 3 ·
Replies
3
Views
2K
How does graded-index material reduce pulse spreading in optical fibres?
  • · Replies 5 ·
Replies
5
Views
2K
Multipath dispersion in fibre optic cable
  • · Replies 1 ·
Replies
1
Views
8K
Undergrad Why can't we use opaque object in optical fibres?
  • · Replies 8 ·
Replies
8
Views
2K
Optics: dispersive power of diffraction grating
  • · Replies 10 ·
Replies
10
Views
5K
Dispersion and the dependences of refractive indexes
  • · Replies 1 ·
Replies
1
Views
3K
Collimating light from Optical fibre problem
  • · Replies 1 ·
Replies
1
Views
3K
Graduate Fiber Optics Bandwidth Question
  • · Replies 2 ·
Replies
2
Views
1K
Graduate Modes of Propagation in an Optical Fibre
  • · Replies 3 ·
Replies
3
Views
7K
Light Emitting Diode (LED) Driving 10km Waveguide
  • · Replies 1 ·
Replies
1
Views
2K