by Chris Woodford. Last updated: October 23, 2020.
The Romans must have been particularly
pleased with themselves the day they invented lead 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 right around the world in a seventh of a
Photo: Light pipe: fiber optics means sending light beams down thin strands of plastic or glass by making them bounce repeatedly off the walls. This is a simulated image. 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.
What is fiber optics?
We're used to the idea of information traveling 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
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
(normally that's 186,000 miles or 300,000 km per second in a vacuum,
but slows to about two thirds this speed in a fiber-optic cable).
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 Air Force.
A fiber-optic cable is made up of incredibly thin strands
of glass or plastic known as optical fibers; one cable can have as few as two
strands or as many as several hundred. Each strand is less than a
tenth as thick as a human hair and can carry something like 25,000 telephone calls,
so an entire fiber-optic cable can easily carry several million calls.
The current record for a "single-mode" fiber (that's explained below) is
178 terabits (trillion bits) per second—enough for 100 million Zoom sessions
(according to fiber expert Jeff Hecht)!
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 traveling 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
Artwork: Total internal reflection keeps light rays bouncing down the inside of a fiber-optic 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.)
Types of fiber-optic cables
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.
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 (yellow 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
bigger than one in a single-mode cable.
This means light beams can travel through the core by following a
different paths (yellow, orange, blue, and cyan 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 a medical tool called a gastroscope
(a type of endoscope),
which doctors poke down someone's throat for detecting illnesses inside
their stomach. 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. Other types of endoscopes work the same
way and 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
Uses for fiber optics
Shooting light down a pipe seems like a neat scientific
party trick, and you might not think there'd be many practical applications for
something like that. But just as electricity can power many
types of machines, beams of light can carry many types of
information—so they can help us in many ways. We don't notice just
how commonplace fiber-optic cables have become because the
laser-powered signals they carry flicker far beneath our feet, deep
under office floors and city streets. The technologies that use
it—computer networking, broadcasting, medical scanning, and
military equipment (to name just four)—do so quite invisibly.
Photo: Working on fiber-optic cables. Picture by Nathanael Callon, courtesy of
US Air Force.
Fiber-optic cables are now the main way of carrying information over long distances because
they have three very big advantages over old-style copper cables:
- Less attenuation: (signal loss) Information travels roughly 10 times further before it needs amplifying—which makes fiber networks simpler and cheaper to operate and maintain.
- No interference: Unlike with copper cables, there's no "crosstalk" (electromagnetic interference) between optical fibers, so they transmit information more reliably with better signal quality
- Higher bandwidth: As we've already seen, fiber-optic cables can carry far more data than copper cables of the same diameter.
You're reading these words now thanks to the
Internet. You probably chanced upon this page with a search engine
like Google, which operates a worldwide network of giant data centers
connected by vast-capacity fiber-optic cables (and is now trying to
roll out fast fiber connections to the rest of us). Having clicked on
a search engine link, you've downloaded this web page from my web
server and my words have whistled most of the way to you down more
fiber-optic cables. Indeed, if you're using fast fiber-optic
broadband, optical fiber cables are doing almost all the work every
time you go online. With most high-speed broadband connections, only
the last part of the information's journey (the so-called "last
mile" from the fiber-connected cabinet on your street to your house
or apartment) involves old-fashioned wires. It's fiber-optic cables,
not copper wires, that now carry "likes" and "tweets" under
our streets, through an increasing number of rural areas, and even
deep beneath the oceans linking continents. If you picture the
Internet (and the World Wide Web that rides on it) as a global
spider's web, the strands holding it together are fiber-optic cables;
according to some estimates, fiber cables cover
over 99 percent of the Internet's total mileage,
and carry over 99 percent of all international communications traffic.
The faster people can access the Internet, the
more they can—and will—do online. The arrival of
broadband Internet made possible the phenomenon of cloud computing
(where people store and process their data remotely, using online
services instead of a home or business PC in their own premises). In
much the same way, the steady rollout of fiber broadband (typically
5–10 times faster than conventional DSL broadband, which uses
ordinary telephone lines) will make it much more commonplace for
people to do things like streaming movies online instead of watching
broadcast TV or renting DVDs. With more fiber capacity and faster
connections, we'll be tracking and controlling many more aspects of
our lives online using the so-called Internet of things.
But it's not just public Internet data that
streams down fiber-optic lines. Computers were once connected over
long distances by telephone lines or (over shorter distances) copper
Ethernet cables, but fiber cables are increasingly the preferred
method of networking computers because they're very affordable, secure,
reliable, and have much higher capacity. Instead of linking its
offices over the public Internet, it's perfectly possible for a
company to set up its own fiber network (if it can afford to do so)
or (more likely) buy space on a private fiber network. Many private
computer networks run on what's called dark fiber, which
sounds a bit sinister, but is simply the unused capacity on another
network (optical fibers waiting to be lit up).
The Internet was cleverly designed to ferry any
kind of information for any kind of use; it's not limited to carrying
computer data. While telephone lines once carried the Internet, now
the fiber-optic Internet carries telephone (and Skype) calls instead.
Where telephone calls were once routed down an intricate patchwork of
copper cables and microwave links between cities, most long-distance
calls are now routed down fiber-optic lines. Vast quantities of fiber were laid from the 1980s onward; estimates vary wildly, but the worldwide total is believed to be several hundred million kilometers (enough to cross the United States about a million times). In the mid-2000s, it was estimated that as much as 98 percent of this was unused "dark fiber"; today, although much more fiber is in use, it's still generally believed that most networks contain anywhere from a third to a half dark fiber.
Photo: Fiber-optic networks are expensive to construct (largely because it costs so much to dig up streets). Because the labor and construction costs are much more expensive than the cable itself, many network operators deliberately lay much more cable than they currently need. Picture by Chris Willis courtesy of US Air Force.
Back in the early 20th century, radio and
TV broadcasting was born from a relatively simple idea: it was
technically quite easy to shoot electromagnetic waves through the air
from a single transmitter (at the broadcasting station) to thousands of antennas on people's homes. These days, while radio still beams through the air, we're just as likely to get our
TV through fiber-optic cables.
Cable TV companies pioneered the transition from
the 1950s onward, originally using coaxial cables (copper cables with a sheath of metal screening wrapped around them to prevents crosstalk interference), which carried just a handful of analog TV signals. As more and more people connected to cable and the networks started to offer
greater choice of channels and programs, cable operators found they
needed to switch from coaxial cables to optical fibers and from
analog to digital broadcasting. Fortunately, scientists
were already figuring out how that might be possible; as far back as 1966,
Charles Kao (and his colleague George Hockham) had done the math, proving how a single optical fiber cable might
carry enough data for several hundred TV channels (or several hundred
thousand telephone calls). It was only a matter of time before the
world of cable TV took notice—and Kao's "groundbreaking achievement" was properly recognized
when he was awarded the 2009 Nobel Prize in Physics.
Apart from offering much higher capacity, optical
fibers suffer less from interference, so offer better signal (picture
and sound) quality; they need less amplification to boost signals so
they travel over long distances; and they're altogether more cost
effective. In the future, fiber broadband may well be how
most of us watch television, perhaps through
systems such as IPTV (Internet Protocol Television), which uses the
Internet's standard way of carrying data ("packet switching") to
serve TV programs and movies on demand. While the copper telephone
line is still the primary information route into many people's homes,
in the future, our main connection to the world will be a high-bandwidth fiber-optic
cable carrying any and every kind of information.
Medical gadgets that could help doctors peer
inside our bodies without cutting them open were the first proper
application of fiber optics over a half century ago. Today,
gastroscopes (as these things are called) are just as important as
ever, but fiber optics continues to spawn important new forms of
medical scanning and diagnosis.
One of the latest developments is called a lab on
a fiber, and involves inserting hair-thin fiber-optic cables, with
built-in sensors, into a patient's body. These sorts of fibers are
similar in scale to the ones in communication cables and thinner than
the relatively chunky light guides used in gastroscopes. How do they
work? Light zaps through them from a lamp or laser, through the part
of the body the doctor wants to study. As the light whistles through
the fiber, the patient's body alters its properties in a particular
way (altering the light's intensity or wavelength very slightly,
perhaps). By measuring the way the light changes (using techniques
such as interferometry),
an instrument attached to the other end of
the fiber can measure some critical aspect of how the patient's body
is working, such as their temperature, blood pressure, cell pH,
or the presence of medicines in their bloodstream. In other words,
rather than simply using light to see inside the patient's body, this
type of fiber-optic cable uses light to sense or measure it instead.
Photo: Fiber optics on the battlefield. This Enhanced Fiber-Optic Guided Missile (EFOG-M) has an infrared fiber-optic camera mounted in its nose so that the gunner firing it can see where it's going as it travels. Picture courtesy of
It's easy to picture Internet users linked
together by giant webs of fiber-optic cables; it's much less obvious
that the world's hi-tech military forces are connected the same way.
Fiber-optic cables are inexpensive, thin, lightweight, high-capacity,
robust against attack, and extremely secure, so they offer perfect
ways to connect military bases and other installations, such as
missile launch sites and radar tracking stations. Since they don't
carry electrical signals, they don't give off electromagnetic
radiation that an enemy can detect, and they're robust against
electromagnetic interference (including systematic enemy "jamming"
attacks). Another benefit is the relatively light weight of fiber
cables compared to traditional wires made of cumbersome and expensive
copper metal. Tanks, military airplanes, and
helicopters have all
been slowly switching from metal cables to fiber-optic ones. Partly
it's a matter of cutting costs and saving weight (fiber-optic cables weigh nearly 90
percent less than comparable "twisted-pair" copper cables). But
it also improves reliability; for example, unlike traditional cables
on an airplane, which have to be carefully shielded (insulated) to protect
them against lightning strikes, optical fibers are completely immune
to that kind of problem.
Who invented fiber optics?
- 1840s: Swiss physicist Daniel Colladon
(1802–1893) discovered he could shine light along a water pipe. The water carried the light by
- 1870: An Irish physicist called John Tyndall
(1820–1893) demonstrated internal reflection at London's Royal Society. He shone light into a
jug of water. When he poured some of the water out from the jug, the
light curved round following the water's path. This idea of "bending
light" is exactly what happens in fiber optics. Although Colladon is
the true grandfather of fiber-optics, Tyndall often earns the credit.
- 1930s: Heinrich Lamm and Walter Gerlach, two
German students, tried to use light pipes to make a gastroscope—an
instrument for looking inside someone's stomach.
- 1950s: In London, England, Indian physicist
Narinder Kapany (1926–) and British physicist
Harold Hopkins (1918–1994)
managed to send a simple picture down a light pipe made from thousands of glass fibers.
After publishing many scientific papers, Kapany earned a reputation as
the "father of fiber optics."
- 1957: Three American scientists at the University of Michigan, Lawrence Curtiss, Basil Hirschowitz, and Wilbur Peters, successfully used fiber-optic technology to make the world's first gastroscope.
- 1960s: Chinese-born US physicist Charles Kao (1933–2018) and his colleague George Hockham realized that impure glass was no use for long-range fiber optics. Kao suggested that a fiber-optic cable made from very pure glass would be able to carry telephone signals over much longer distances and was awarded the
2009 Nobel Prize in Physics for this ground-breaking discovery.
- 1960s: Researchers at the Corning Glass Company made the first fiber-optic cable capable of carrying
- ~1970: Donald Keck and colleagues at Corning found ways to send signals much further (with less loss) prompting the
development of the first low-loss optical fibers.
- 1977: The first fiber-optic telephone cable was laid between Long Beach and Artesia, California.
- 1988: The first transatlantic fiber-optic telephone cable, TAT8, was laid between the United States, France, and the UK.
- 2020: According to TeleGeography, there are currently around 406 fiber-optic submarine cables
(carrying communications under the world's oceans), stretching a total of
1.2 million km (0.7 million miles).
That's an increase on 2019's figure of 378 cables, though the total distance covered is apparently still the same.