Optical fibres are transforming the way we communicate with each other. A
single pair of such fibres, each about as thick as a human hair, can relay
hundreds or thousands of telephone calls between cities at nearly the speed of
light
IN THE past few years, telecommunications companies have increasingly turned
to optical fibres to build long-distance networks spanning the world. Undersea
fibre-optic cables already cross the Atlantic and Pacific oceans, and many
more are planned for the future.
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During the next decade or two, the tiny glass thread may bring the world
into our living room. Large telecommunications companies are promising new
services. Video telephones, better quality cable television and access to
central video libraries are real possibilities. Others include interactive
services that retrieve information from remote computers, shopping and banking
from home, and meters read remotely. All are made possible because fibres can
carry much more information than any type of conventional cable.
Of course, none of this will be cheap. In America many experts believe it
could cost well over a thousand dollars to bring fibres into one home. The
cost is falling already but replacing existing systems will probably only
happen if people find the new services attractive.
Nature of light
Key to developments
A LIGHT ray travels freely through air and space, and unless it is
reflected by something, it travels in straight lines. This is fine for sending
messages between two points in sight of each other on a clear day. Alexander
Graham Bell, a Scottish inventor who settled in America, devised the
Photophone in 1880. It sent voices through the air on beams of light just as
his earlier invention, the telephone, sent voices through electrical wires.
Bell was excited by the Photophone, but it did not prove practical. Rain,
snow, clouds and fog can block light, and most points are not in sight of each
other. The same problems frustrated researchers in their efforts to
communicate through the air after lasers were invented in 1960. Optical
communications needed something to guide light signals from place to place, in
the same way that wires carry electrical signals. That is the role of optical
fibres.
Optical fibres are made of very transparent glass or plastic, and typically
between 0.125 and 0.5 millimetre in diameter. The fibres contain at least two
layers: an inner core which carries the light, and an outer cladding, which
confines the light in the core. By contrast, the glass fibres used in
fibreglass, and the plastic fibres used in clothing, are made from a single
layer because their optical properties are not important.
Guiding light in an optical fibre depends on how light travels through
different media. Light travels at a constant 300 000 kilometres per second in
a vacuum, but it moves more slowly through a denser material. The degree at
which it slows is measured by the refractive index, which equals the speed of
light in a vacuum divided by the speed of light in the material. The
refractive index of a vacuum equals 1; that of air is only slightly larger,
while a typical glass fibre has a refractive index of 1.5, meaning that it can
carry light at 200 000 kilometres per second.
Light waves are bent, or refracted, as they pass between materials with
different refractive indexes. The amount of bending depends on the refractive
index and the angle at which light strikes the surface.
Sometimes light cannot leave the material with higher refractive index. If
it strikes the surface at a large enough angle (measured from the normal 鈥� a
line perpendicular to the surface), it is reflected back into the material.
The critical angle for what is called total internal reflection depends on the
difference in refractive indexes. It is 42掳 for glass (with a refractive
index of 1.5) surrounded by air.
An optical fibre guides light by trapping it using total internal
reflection. Although total internal reflection can occur at a glass-air
surface, unclad glass fibres are not practical, because light can leak out
wherever the fibre touches something. Typical glass fibres have a core with
refractive index about 1 per cent larger than the cladding. The critical angle
at that boundary is 82掳, meaning that light must strike the surface at a
glancing angle in order to be guided along the fibre. However, light can be
guided around corners because the bends are large compared with the diameter
of the fibre. Because total internal reflection directs all light back into
the core, the only losses in a fibre which has a cladding come from absorption
and scattering in the fibre core itself.
Charles Vernon Boys made the first thin glass fibres in 1887, but clad
optical fibres were not developed until the 1950s. Early developers assembled
fibres in bundles to transmit images. As long as the fibres form the same
pattern at both ends, an image that enters the bundle at one end will appear
unchanged at the other, with each fibre carrying one tiny part of the picture.
The fibres can be kept in line by moulding them all into a single solid chunk,
but this makes the bundle rigid and inflexible. To make flexible bundles, a
single fibre is wound many times around into a loop, which is glued together
in one spot. Cutting through the glued part of the bundle produces a flexible
fibre bundle as long as the circumference of the loop, with fibres arranged in
the same way at either end.
These fibre bundles can peer into otherwise unreachable places. This has
made them particularly valuable in medicine, where doctors routinely look
inside the human body with fibre-optic endoscopes. We can use similar probes
to check machinery. If imaging is not needed, fibre bundles can serve as
鈥渓ight pipes鈥� to concentrate light on small, hard-to-reach places, focus light
in certain directions, or to form bright, illuminated signs.
However, the most widespread use of fibre optics is in communications. A
single fibre carries an optical signal from a transmitter to a receiver. Pairs
of fibres provide two-way communications. They can carry signals farther and
faster without amplification than wires, for telephones, televisions, and
computer communications.
The technology has come a long way very quickly: in the late 1970s
engineers developed fibre-optic components for commercial telephone networks.
Today, companies prefer to use fibre optics to transmit many telephone signals
between switching centres. Many telephone companies in Britain, Europe,
America and Japan are testing fibre-optic connections that will carry new
communication services to homes.
Communicating via fibre
Making light of it all
IN a typical fibre-optic communication system, a transmitter generates an
optical signal, which it channels into an optical fibre. The fibre carries the
signal to its destination, where a receiver converts the optical input into
the electrical format needed by whatever is attached to the system.
To generate an optical signal, the transmitter must convert electrical
signals into light. The electrical signal controls circuits within the
transmitter which, in turn, control the current passing through light-emitting
semiconductor devices, such as a semiconductor laser. The amount of current
passing through the device controls how much light it generates. Like
electrical signals, optical signals may be in analogue format, which varies in
intensity, or in digital format, which codes information as a series of
digital bits.
The light which emerges from a small area of the semiconductor device must
be channelled into the core of an optical fibre. Initially this seemed
difficult because both areas are tiny 鈥� only a few micrometres wide and a
fraction of a micrometre high for semiconductor lasers and only about 9
micrometres across for the fibres normally used with them. Engineers had
similar problems when they tried to guide light between the tiny cores of two
fibres.
Channelling light into optical fibres has proved to be easier than early
developers had feared. Engineers have developed fibre-optic connectors, joints
which fit together so perfectly that they automatically align optical fibres.
The margins for error are much less than in connectors for electrical wires.
Engineers have also perfected techniques for splicing together fibres, welding
or gluing them together so their cores match precisely.
Optical fibres cannot be left unprotected in a communication system.
Individual fibres are slightly thicker and stiffer than human hairs. A
protective plastic layer adds to their thickness and stiffness, but bare
fibres are hard to handle (or even to pick up), and they could be damaged by
poor handling or moisture. Like wires, most optical fibres are encased in
plastic tubes as part of a cable. Connectors are mounted on the cable sheath,
which protects the fibre from the pull applied during installation. It also
keeps out moisture, and shields the fibre from other hazards, including
burrowing rodents which gnaw on buried cables.
At the receiver end, the fibre brings optical signals to a photodetector,
which generates an electrical signal corresponding to the optical input. Other
electronic circuits then process the raw electronic signal, amplifying it and
attempting to remove interference, or noise. That signal may go directly to an
electronic device, such as a telephone, or it may need decoding if it is being
used to carry TV signals.
Fibre characteristics
The great attractions
FOR communications, the main advantage of optical fibre systems is that
they can transmit more information over greater distances than wires. The
thick wires that transmit electricity to your home have little resistance, and
thus lose little power, but they cannot carry useful information much faster
than 50 cycles per second (60 in North America and Japan). Other types of
metal cable can carry information at high speeds, but not over long distances
because they have high resistance. Optical fibres can carry a large amount of
information without losing much power.
We measure information in digital communications by the number of bits per
second passing a point. Signal loss is measured in decibels per kilometre,
with 1 decibel equal to about a 20 per cent reduction in signal strength,
leaving 80 per cent (or 0.8) of the signal remaining. Decibels compare power
levels: 1 decibel=10脳(log10 (power in/power out)).
Losses compound, so after two decibels of loss, about 64 per cent of the
signal remains (0.8脳0.8 equals 0.64).
Most optical fibres are made from special glass, extremely pure silicon
dioxide (SiO2), with small amounts of other materials, such as
germanium or boron, added to change slightly the refractive index. Signal
losses are only very small and depend on the wavelength of the light. At a
wavelength of 1300 nanometres, a typical loss is about half a decibel per
kilometre, meaning that about 90 per cent of the input signal remains after
one kilometre, and only 10 per cent is lost. At a wavelength of 1550
nanometres, typical losses are approximately a quarter of a decibel per
kilometre, so that means about 95 per cent of the input remains after
travelling a kilometre.
For typical fibre-optic systems this means that signals can be sent between
about 50 and 60 kilometres at a wavelength of 1300 nanometres, and between 120
and 150 kilometres at a wavelength of 1550 nanometres.
These values are inherent in the glass that makes up the fibre. Losses
could be reduced only by switching to different materials. Researchers have
tested some such materials, including glasses containing zirconium, fluorine,
and a few other elements. But they have yet to prove that they are practical
for use in optical fibres.
The amount of information you need to carry is the other major factor that
determines how well a fibre-optic system works. The limits are easiest to see
for digital systems, where the main problem is the spreading or dispersion of
pulses as they travel through the fibre. This increases with the length of
fibre, until it blurs the distinctions between successive pulses, limiting how
many bits can be sent per second. Two effects contribute to the dispersion:
slight differences in the distances that light travels through the same fibre,
and slight differences in the speed of light in the fibre at different
wavelengths.
It is clear that light rays that are conducted down the centre of the fibre
core do not have to travel as far as those which bounce back and forth within
the core. The difference is large enough to become important when a lot of
data are passing through. To avoid such dispersion, the core of the fibre must
be made so small that it can carry light in only what is known as single mode.
This means the diameter must be about 9 micrometres for transmission at 1300
nanometres. Single mode fibres have become standard in telecommunications
because they can carry a large amount of data.
Some dispersion remains even in single-mode fibres because the refractive
index of glass (and other materials) varies with the wavelength of the light
travelling through the glass, causing an effect called chromatic dispersion.
All light sources emit a range of wavelengths. The differences in refractive
indexes means that light of some wavelengths is able to travel faster than
others, so the pulse spreads out. Dispersion increases with the length of the
fibre, the magnitude of the dispersion gradient and the range of wavelengths.
The larger the pulse spreading, the fewer bits per second can pass through the
fibre.
Fortunately, in standard glass fibres the refractive index changes very
little with light of wavelength near 1300 nanometres, making chromatic
dispersion small. Most existing fibre-optic systems operate at that
wavelength. Fibres can also be made with zero dispersion at a wavelength of
1550 nanometres, where they lose less of the signal, and such fibres will be
used for long-distance and submarine cables in the future.
Fibre optics have special advantages because they carry optical rather than
electronic signals. Stray electromagnetic fields from such sources as
lightning strikes, electric power lines, and spark plugs in cars do not affect
optical signals, but can interfere with electrical signals on wires.
Electrical signals produce currents in nearby conductors, so people can tap
wires without being detected. Optical fibres are more difficult to tap,
because we must do something to the fibre to extract an optical signal. These
advantages mean that fibre optics are ideal for transmitting data between
military locations, and for communicating in electrical power plants.
Fibre-optic applications
Complex networks
TELEPHONE systems have made the most extensive use of fibre-optic
communications. They use optical fibres as part of the network that relays
telephone calls around the world.
At the moment, the telephone in your home is linked to the network by wires
which carry a single conversation at audio frequencies between 1 and 4
kilohertz. These wires run to a switching centre or remote concentrator, where
they are merged with other signals to form what is called a multiplexed
signal. They are then routed through electronic switches to their destination.
In modern European phone networks, voice signals are digitised, at a rate of
64 000 bits per second, before being multiplexed.
Thirty such digitised conversations can be merged, by sending or
interleaving one bit at a time from each to generate one signal at about 2
million bits (megabits) per second. In the same way, signals may be
concentrated to even higher rates, carried by a pair of optical fibres, one
transmitting in each direction. The standard rates used in Europe are:
Voice channels Digital rates
1 64 000 bits/second
30 2 megabits/second
120 8 megabits/second
480 34 megabits/second
1920 140 megabits/second
3680 565 megabits/second
7360 1200 megabits/second
14 700 2400 megabits/second
Optical fibres are the only medium used to carry telephone signals above
100 megabits per second on land, and are widely used at the lower rates.
British Telecom uses fibres to carry up to 2400 megabits per second (2.4
gigabits per second). Telephone networks in the US and Japan already have
tested optical fibres carrying up to 5000 megabits per second. At the moment,
most systems operate at a wavelength of 1.3 micrometres, and can carry signals
about 50 kilometres before they need to be amplified.
National fibre-optic telecommunication networks are being linked up by
international ones. Submarine fibre-optic cables now span the Atlantic and
Pacific oceans as well as the Mediterranean and the English Channel.
Improvements in fibre technology threaten to make communications satellites
obsolete for telephone connections on routes, such as that between London and
New York, which carry millions of calls.
The next step is to extend optical fibre communications all the way to
homes. They can carry telephone, cable television, digital data, stereophonic
music, and other communication services. Many countries are experimenting with
鈥渨ired鈥� homes, but technical details remain to be worked out.
Systems currently being designed and built generally do not bring optical
fibre all the way to homes, but stop at a node serving 10 to 500 homes. At
that node, the optical signals are converted into electrical format, and
delivered to homes over the pairs of wires used for telephone service, the
coaxial cables used for cable television, or some combination of the two.
British Telecom is testing a scheme that can transmit one to four video
channels in a special digital format over telephone wires. Viewers would
select what channels they wanted to receive from a large number of possible
programs. In the US, the leading approach is to transmit hundreds of digitised
video channels through coaxial cables to all subscribers; the Time Warner
Cable Co. is using this technology in a prototype 鈥淔ull Service Network鈥� to
begin service in Orlando at the end of 1994. The differences are not as
dramatic as they sound; users of each system would select from a large number
of possible channels, but the signals would be delivered differently.
Some key questions remain about economics. Optical fibre links to homes now
cost more than conventional telephone or cable television wiring, but the
difference is shrinking. Many telephone and cable companies believe that they
can save money, at least in the short run, by running fibres only to
distribution nodes, and serving individual homes with wires. However, the
Japanese telephone giant, Nippon Telegraph and Telephone, still plans to
install fibres to homes. The biggest question is how much will people pay for
the new services that optical fibres can offer, such as playing video
programmes from stored libraries.
The greatest obstacles for the wholesale introduction of fibre-optic
systems in Britain and the US have been regulations which limit the ability
of telephone and cable-television companies to offer services that compete
with each other. However this is changing rapidly. Some American companies
have circumvented the regulations by offering cable television where they do
not provide telephone service. Most American observers believe existing
regulations will soon be abandoned so phone and cable companies can compete
directly in what most expect to become a multi-billion dollar market.
Future regulations and markets may be hard to predict, but the technical
trends are clear. In a little over a decade, fibre optics have become the most
important method of transmitting many types of communication between fixed
points. Some areas of technology have matured; for example, researchers have
cut the amount of signal that is lost from fibre optics to such a degree that
there is little room for improvement without shifting from silica glass to new
materials. However, dramatic progress continues in other areas. Optical
amplifiers can now increase the strength of optical signals without first
converting them into electrical form. Thus, a signal from one source could be
split, amplified, and split again to serve many people. New transmitters and
receivers can operate to higher data rates and bandwidths, allowing fibres to
carry more information and services. These developments will increase the use
of fibre optics throughout the network of global communications.
Analogue and digital transmission
ELECTRICAL and optical signals can be processed in either analogue or
digital format. Analogue signals vary continuously with time, so a plot of
intensity over time shows a wavy line. If the signal represents sound, the
wave amplitude represents 鈥� or is an 鈥渁nalogue鈥� of 鈥� the intensity of the
sound.
Digital signals encode the same information as a series of 1s and 0s, so
that there are two states for the signal at any given time 鈥� either on or off.
In a digital code, a series of bits represents the signal intensity averaged
out over a brief interval (often the time required for the transmission of
those bits).
A code of 2 bits can represent 22, or 4, states 鈥� on, off and two
intermediate states. These states represent digitally the strength of the
signal at one instant.
The amount of information carried by digital and analogue systems are
measured in different units. Digital systems are measured by the number of
bits per second that can pass a point. Analogue transmission is measured as
bandwidth, the range of frequencies (in hertz) that can pass undistorted
through the system. The units are not exactly comparable.
Signals can be converted between digital and analogue formats. It takes 64 000
bits per second to represent the 3000 hertz bandwidth of an analogue telephone
signal, but an ordinary analogue phone line cannot handle that much digital
data. However, digital signals are less vulnerable to interference (鈥渘oise鈥�).
Noise added to an analogue signal is amplified along with the signal, but
digital systems ignore noise as long as it does not overwhelm the signal. So
record players that can carry analogue signals reproduce noise and scratches
on records, but digital CD players do not pick up fingerprints on discs.
Telecommunication systems have turned to digital transmission because noise is
lower and digital electronics are easier to make than analogue types.
Further reading
Jeff Hecht, Understanding Fiber Optics 2nd ed. (Sams Publishing,
Indianopolis, 1993); Edward L. Safford Jr, The Fiber Optics and Laser
Handbook, Tab Books, Blue Ridge Summit, Pennsylvania.