Fiber optics
Last updated: May 5, 2009.
The Romans must have been particularly
pleased with themselves the
day they invented lead water pipes around 2000 years ago. At last, they
had
an easy way to carry their water from one place to another.
Imagine what they’d make of modern fiber-optic cables—"pipes" that
can carry telephone calls and emails around the world in a seventh of a
second!
Please note that in some countries, including the UK,
fiber optics is spelled "fibre optics." If you're looking for information online, it's always
worth searching both spellings.
Photo: A section of 144-strand fiber-optic cable.
Each strand is made of optically pure glass and is thinner than a human
hair. Picture by Tech. Sgt. Brian Davidson, courtesy of US Airforce.
What is fiber optics?
We're used to the idea of information travelling in different
ways.
When we speak into a landline telephone,
a wire cable carries the
sounds from our voice into a socket in the wall, where another cable
takes it to the local telephone exchange.
Cellphones work a different
way: they send and receive information using invisible
radio waves—a
technology called wireless because it uses no cables. Fiber optics
works a third way. It sends information coded in a beam
of light down a glass or plastic pipe. It was originally developed
for endoscopes in
the 1950s to help doctors see inside the human body without having to
cut it open first. In the 1960s, engineers found a way of using the
same technology to transmit telephone calls at the speed of light
(186,000 miles or 300,000 km per second).
Photo: Right: You can clearly see the red light shining down these fiber-optic cables. Picture courtesy of NASA Glenn Research Center
(NASA-GRC).
Optical technology
A fiber-optic cable is made up of 100 or more incredily thin strands
of glass or plastic known as optical fibers. Each one is less than a
tenth as thick as a human hair and can carry 10 million telephone calls.
Fiber-optic cables carry information between two places using
entirely optical (light-based) technology. Suppose you wanted to send
information from your computer to
a friend’s house down the street
using fiber optics. You could hook your computer up to a laser, which
would convert electrical information from the computer into a series of
light pulses. Then you’d fire the laser down the fiber-optic cable.
After travelling down the cable, the light beams would emerge at the
other end. Your friend would need a photoelectric cell (light-detecting
component) to turn the pulses of light back into electrical information
his or her computer could understand. So the whole apparatus would be
like a really neat, hi-tech version of the kind of telephone you can
make out of two baked-bean cans and a length of string!
How fiber-optics works
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).
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,
travelling 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 higher refractive index than the core. Light travels slower in
the cladding
than in the core. Any light that tries to leak into the cladding tends
to bend back inside the core.)
Optical fibers carry light signals down them in modes. A mode is
the path that a light beam follows down the fiber. One mode is
simply 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.
Types of fiber-optic cables
The simplest type of optical fiber is called single-mode.
It has a
very thin core about 5-10 microns (millionths of
a meter) in diameter. In a single-mode fiber, all signals travel
straight down the middle without bouncing off the edges (red line in
diagram). Cable TV,
Internet, and telephone signals are generally carried by single-mode
fibers, wrapped together into a huge bundle. Cables like this can send
information over 100 km (60 miles).
Another type of fiber-optic cable
is called multi-mode. Each optical fiber in
a multi-mode cable is about
10 times
bigger than one in a single-mode cable.
This means light beams can travel through the core by following a
variety of
different paths (purple, green, and blue lines)—in other words, in
multiple different modes.
Multi-mode cables can send information only
over relatively short distances and are used (among other things) to
link computer networks together.
Even thicker fibers are used in medical tools called gastroscopes
(or endoscopes),
which doctors poke down people’s throats for detecting illnesses inside
their stomachs. A gastroscope is a thick fiber-optic cable consisting
of many optical fibers. At the top end of a gastroscope, there is an
eyepiece and a
lamp. The lamp shines its light down one part of the cable into the
patient's stomach. When the light reaches the stomach, it reflects off
the stomach walls into a lens
at the bottom of the cable. Then it travels back up another part of the
cable into the doctor's eyepiece. Different sizes of gastroscopes can
be used to inspect different parts of the body. There is also an
industrial version of the tool, called a fiberscope, which can be used
to examine things like inaccessible pieces of machinery in airplane
engines.
Try this fiber-optic experiment!
This nice little experiment is a modern-day recreation of a famous
scientific demonstration carried out by Irish physicist John Tyndall in 1870.
It's best to do it in a darkened bathroom or
kitchen at the sink or washbasin. You'll need an old clear, plastic drinks bottle, a flashlight (torch),
some aluminum foil, and some sticky tape.
- Take the plastic bottle and wrap aluminum foil around the sides,
leaving the top and bottom of the bottle uncovered. If you need to,
hold the foil in place with sticky tape.
- Fill the bottle with water.
- Switch on the flashlight and press it against the base of the
bottle so the light shines up inside the water. It works best if you
press the flashlight tightly against the bottle. You need as much light
to enter the bottle as possible, so use the brightest flashlight you can
find.
- Standing by the sink, tilt the bottle so the water starts to pour
out. Keep the flashlight pressed tight against the bottle. If the room
is darkened, you should see the spout of water lighting up ever so
slightly. Notice how the water carries the light, with the light beam
bending as it goes!
Photo: Seen from below, your water bottle should look like this when it's
wrapped in aluminum foil. Don't cover the bottom of the bottle or light won't be able to get in.
The black object on the right is my flashlight, just before I pressed
it against the bottle. You can already see some of its light shining into the bottom of the
bottle.
