Fibre Optic

(or Optical Fibre)

Physics - total internal reflection

Any basic optical textbook will describe the concept of "total internal reflection" - where light approaching a boundary between two media of different refractive indices from the "more dense" side at an angle greater than the "critical angle" will be internally reflected with no loss. Extending this model to a "tube of glass", we can see how a beam of light launched into one end may be continually internally reflected, passing down the glass tube and exiting the other end.

A full treatment of the physics behind this phenomenen requires more than simple geometric optics, and involves modelling a wave travelling through a cylindrical waveguide. Nevertheless, the simple model of light "bouncing down the inside of a fibre" is a useful analogy.

In practice, optical fibres are cylindrical glass strands, typically around the size of a human hair, made of two different types of glass, with the inner core having a higher refractive index than the outer. The total internal reflection occurs at the boundary between the two glasses, rather than at the edge of the fibre.

Here endeth the science lesson. It's not really appropriate for this forum.

Uses in entertainment industry - lighting

(not really my field. FO Starcloths. VArious effects. Can someone expand.)

In this arena, the fibre is used simply as a "lightpipe" to deliver light to specific locations from an external source. Low-grade plastic fibre, rather than glass, is common.

Data communications

Fibre optic cabling is used extensively in the data communications and telecoms industries for long-haul (and increasingly for short haul) communications circuits. A modulated light source can be launched into a glass fibre; this signal can then be transmitted over long distances with low attenuation and interference - typically tens or even hundreds of km.

There are two basic types of cables used in fibre communication systems - multimode and single-mode. Physically, they both have a similar outer diameter (typically 125 microns), however while the inner core of a multimode fibre is typically 50 or 62.5 microns in diameter, a single mode fibre will have a 9 micron core. The lower modal dispersion (now that's definitely outwith the context of this forum!) of the single mode fibre means that it is more suitable for long haul and/or high speed applications. However, drivers for single mode use are significantly more expensive, and the smaller core means that splicing and patching requires more skill, and fibre ends are more sensitive to dust and dirt.

See Also:

• Ethernet