Question:
How is it that light from a laser can travel in a small glass
fiber for many miles without regeneration of the original signal?
Replies:
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.
NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.
