Index Key:  PHY075
Author:     yje
Subject:    LASER light through a fiber
Text:       How is it that light from a laser can travel in a small glass 
fiber for many miles without regeneration of the original signal?

Response #:  1 of 1
Author:      R.C. Winther
Text:        Light may be conveyed down a length of a glass fiber via a 
process called total internal reflection.  Suppose a beam of light is 
traveling in material A and strikes an interface between this material and 
another material B, where these two materials have different indices of 
refraction.  Imagine a line at right angles to the interface passing through 
the point where the beam hits.  The angle between this line and the beam is 
called the angle of incidence.If this angle is 0 degrees, all of the beam is 
transmitted & none is reflected.  Now suppose the light source is moved so 
that the beam strikes more and more obliquely, then the amount of beam 
transmitted decreases, and more and more of it is reflected.  If B's index of 
refraction is smaller than A's, we will eventually reach an angle of incidence 
called the critical angle at which all of the beam is reflected and none 
transmitted.  This is total internal reflection.  If the angle of incidence is 
further increased, the total reflection persists.  This is what happens inside 
the glass fiber.  The beam enters traveling approximately straight down the 
fiber.  If there is a not-too-abrupt bend in the fiber, the beam will hit the 
fiber's inside wall (a glass-air interface) at a large angle of incidence and


be totally reflected.  This may occur thousands of times per meter of fiber.  
And this can be done with ordinary light as well as with laser light.  Laser 
light works better because all parts of its beam start out traveling in 
essentially the same direction.  Even if it is collimated, a "beam" of 
ordinary light will enter the fiber with some range of directions; some of the 
beam will not totally reflect, so some light is transmitted through the walls 
and is lost.