Fiber Optics

You hear about fiber-optic cables whenever people talk about
the telephone system, the cable TV system or the Internet.
Fiber-optic lines are strands of optically pure glass as thin
as a human hair that carry digital information over long
distances. They are also used in medical imaging and
mechanical engineering inspection.

What are Fiber Optics?

Fiber optics (optical fibers) are long, thin strands of very
pure glass about the diameter of a human hair. They are
arranged in bundles called optical cables and used to
transmit light signals over long distances.

If you look closely at a single optical fiber, you will see
that it has the following parts:

* Core - Thin glass center of the fiber where the light
travels
* Cladding - Outer optical material surrounding the core
that reflects the light back into the core
* Buffer coating - Plastic coating that protects the
fiber from damage and moisture

Hundreds or thousands of these optical fibers are arranged in
bundles in optical cables. The bundles are protected by the
cable's outer covering, called a jacket.

Optical fibers come in two types:

* Single-mode fibers
* Multi-mode fibers

See Tpub.com: Mode Theory for a good explanation.

Single-mode fibers have small cores (about 3.5 x 10-4 inches
or 9 microns in diameter) and transmit infrared laser light
(wavelength = 1,300 to 1,550 nanometers). Multi-mode fibers
have larger cores (about 2.5 x 10-3 inches or 62.5 microns in
diameter) and transmit infrared light (wavelength = 850 to
1,300 nm) from light-emitting diodes (LEDs).

Some optical fibers can be made from plastic. These fibers
have a large core (0.04 inches or 1 mm diameter) and transmit
visible red light (wavelength = 650 nm) from LEDs.

How Does an Optical Fiber Transmit Light?

Suppose you want to shine a flashlight beam down a long,
straight hallway. Just point the beam straight down the
hallway -- light travels in straight lines, so it is no
problem. What if the hallway has a bend in it? You could
place a mirror at the bend to reflect the light beam around
the corner. What if the hallway is very winding with multiple
bends? You might line the walls with mirrors and angle the
beam so that it bounces from side-to-side all along the
hallway. This is exactly what happens in an optical fiber.

The light in a fiber-optic cable travels through the core
(hallway) by constantly bouncing from the cladding
(mirror-lined walls), a principle called total internal
reflection. Because the cladding does not absorb any light
from the core, the light wave can travel great distances.

However, some of the light signal degrades within the fiber,
mostly due to impurities in the glass. The extent that the
signal degrades depends on the purity of the glass and the
wavelength of the transmitted light (for example, 850 nm = 60
to 75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is
greater than 50 percent/km). Some premium optical fibers show
much less signal degradation -- less than 10 percent/km at
1,550 nm.