Photo: Fiber-optic cables are thin enough to bend, taking the light
signals inside in curved paths too. Picture courtesy of NASA Glenn
Research Center (NASA-GRC).Outdoor optical fiber cable
Light
travels down a fiber-optic cable by bouncing repeatedly off the walls.
Each tiny photon (particle of light) bounces down the pipe like a
bobsleigh going down an ice run. Now you might expect a beam of light,
traveling in a clear glass pipe, simply to leak out of the edges. But if
light hits glass at a really shallow angle (less than 42 degrees), it
reflects back in again—as though the glass were really a mirror. This
phenomenon is called total internal reflection. It's one of the things
that keeps light inside the pipe.
The other thing that keeps light in
the pipe is the structure of the cable, which is made up of two
separate parts. The main part of the cable—in the middle—is called the
core and that's the bit the light travels through. Wrapped around the
outside of the core is another layer of glass called the cladding. The
cladding's job is to keep the light signals inside the core. It can do
this because it is made of a different type of glass to the core. (More
technically, the cladding has a lower refractive index.)
Optical
fibers carry light signals down them in what are called modes. That
sounds technical but it just means different ways of traveling: a mode
is simply the path that a light beam follows down the fiber. One mode is
to go straight down the middle of the fiber. Another is to bounce down
the fiber at a shallow angle. Other modes involve bouncing down the
fiber at other angles, more or less steep.
Artworks: Above: Light
travels in different ways in single-mode and multi-mode fibers. Below:
Inside a typical single-mode fiber cable (not drawn to scale). The thin
core is surrounded by cladding roughly ten times bigger in diameter, a
plastic outer coating (about twice the diameter of the cladding), some
strengthening fibers made of a tough material such as Kevlar®, with a
protective outer jacket on the outside.
