Alexander Graham Bell, the American inventor best
known for developing the telephone, first attempted to communicate using light
around 1880. However, light wave communication did not become feasible until
the mid-twentieth century, when advanced technology provided a transmission source,
the laser, and an efficient medium, the optical fiber. The laser was invented
in 1960 and, six years later, researchers in England discovered that silica
glass fibers would carry light waves without significant attenuation, or loss
of signal. In 1970, a new type of laser was developed, and the first optical
fibers were produced commercially.
In a fiber optic communications system, cables made of optical fibers
connect datalinks that contain lasers and light detectors. To transmit
information, a datalink converts an analog electronic signal—a telephone
conversation or the output of a video camera—into digital pulses of laser
light. These travel through the optical fiber to another datalink, where a
light detector reconverts them into an electronic signal.
Raw Materials
Optical fibers are composed primarily of silicon dioxide (SiO 2 ),
though minute amounts of other chemicals are often added. Highly purified
silica powder was used in the now-outmoded crucible manufacturing method, while
liquid silicon tetrachloride (SiCl 4 ) in a gaseous stream of pure
oxygen (02) is the principal source of silicon for the vapor deposition method
currently in widespread use. Other chemical compounds such as germanium
tetrachloride (GeCl 4 ) and phosphorus oxychloride (POC1 3 )
can be used to produce core fibers and outer shells, or claddings, with
function-specific optical properties.
Because the purity and chemical composition of the glass used in optical
fibers determine the most important characteristic of a fiber—degree of
attenuation—research now focuses on developing glasses with the highest
possible purity. Glasses with a high fluoride content hold the most promise for
improving optical fiber performance because they are transparent to almost the
entire range of visible light frequencies. This makes them especially valuable
for multimode optical fibers, which can transmit hundreds of discrete light
wave signals concurrently.
After the solid glass preform is prepared, it is
transferred to a vertical drawing system. In this system, the preform is first
heated. As it does so, a gob of molten glass forms at its end and then falls
away, allowing the single optical fiber inside to be drawn out.
The fiber then proceeds through the machine, where its diameter is checked, a
protective coating is applied, and it is cured by heat. Finally, it is wound on
a spool.
The Manufacturing
Process
Both the core and the cladding of an optical fiber are made of highly
purified silica glass. An optical fiber is manufactured from silicon dioxide by
either of two methods. The first, the crucible method, in which powdered silica
is melted, produces fatter, multimode fibers suitable for short-distance
transmission of many light wave signals. The second, the vapor deposition
process, creates a solid cylinder of core and cladding material that is then
heated and drawn into a thinner, single-mode fiber for long-distance
communication.
There are three types of vapor deposition techniques: Outer Vapor Phase
Deposition, Vapor Phase Axial Deposition, and Modified Chemical Vapor
Deposition (MCVD). This section will focus on the MCVD process, the most common
manufacturing technique now in use. MCVD yields a low-loss fiber well-suited
for long-distance cables.
Modified Chemical Vapor
Deposition
- 1 First, a
cylindrical preform is made by depositing layers of specially formulated
silicon dioxide on the inside surface of a hollow substrate rod. The
layers are deposited by applying a gaseous stream of pure oxygen to the
substrate rod. Various chemical vapors, such as silicon tetrachloride
(SiCl 4 ), germanium tetrachloride (GeCl 4 ), and
phosphorous oxychloride (POC1 3 ), are added to the stream of
oxygen. As the oxygen contacts the hot surface of the rod—a flame
underneath the rod keeps the walls of the rod very hot—silicon dioxide of
high purity is formed. The result is a glassy soot, several layers thick,
deposited inside the rod. This soot will become the core. The properties
of these layers of soot can be altered depending on the types of chemical vapors
used.
- 2 After the
soot is built up to the desired thickness, the substrate rod is moved
through other heating steps to drive out any
A typical optical fiber cable usually includes several
optical fibers around a central steel cable. Various protective layers are
applied, depending on the harshness of the environment where the cable will be
situated.
moisture and bubbles trapped in the soot layers. During
heating, the substrate rod and internal soot layers solidify to form the boule
or preform of highly pure silicon dioxide. A preform usually measures 10 to 25
millimeters (.39 to .98 inch) in diameter and 600 to 1000 millimeters (23.6 to
39.37 inches) in length.
Drawing the fibers
- 3 The solid
preform is then automatically transferred to a vertical fiber drawing
system. The machines that make up a typical vertical drawing system can be
two stories high and are able to produce continuous fibers up to 300
kilometers (186 miles) long. This system consists of a furnace to melt the
end of the preform, sensors to monitor the diameter of the fiber being
pulled from the preform, and coating devices to apply protective layers
over the outer cladding.
- 4 The preform
first passes through a furnace, where it is heated to about 3600 degrees
Fahrenheit (about 2000 degrees Celsius). Next, a drop of molten glass
called a "gob" forms at the end of the preform, much like a
droplet of water that collects at the bottom of a leaky faucet. The gob
then falls away, and the single optical fiber inside is drawn out of the preform.
As the optical fiber is pulled from the preform, the material in the
original substrate rod forms the cladding, and the silicon dioxide
deposited as soot forms the core of the optical fiber.
- 5 As the
fiber is drawn out, measuring devices monitor its diameter and its
concentricity, while another device applies a protective coating. The
fiber then passes through a curing furnace and another measuring device
that monitors diameter, before being wound on a spool.
Quality Control
Quality control begins with the suppliers of the chemical compounds used
as the raw materials for the substrate rods, chemical reactants, and fiber
coatings. Specialty chemical suppliers provide detailed chemical analyses of
the constituent compounds, and these analyses are constantly checked by
computerized on-stream analyzers connected to the process vessels.
Process engineers and highly trained technicians closely watch the sealed
vessels as preforms are being created and fibers drawn. Computers operate the
complex control schemes necessary to manage the high temperatures and high
pressures of the manufacturing process. Precise measurement devices
continuously monitor fiber diameter and provide feedback for control of the
drawing process.
The Future
Future optical fibers will come from ongoing research into materials with
improved optical properties. Currently, silica glasses with a high fluoride
content hold the most promise for optical fibers, with attenuation losses even
lower than today's highly efficient fibers. Experimental fibers, drawn from
glass containing 50 to 60 percent zirconium fluoride (ZrF 4 ), now
show losses in the range of 0.005 to 0.008 decibels per kilometer, whereas
earlier fibers often had losses of 0.2 decibels per kilometer.
In addition to utilizing more refined materials, the producers of fiber
optic cables are experimenting with process improvement. Presently, the most
sophisticated manufacturing processes use high-energy lasers to melt the
preforms for the fiber draw. Fibers can be drawn from a preform at the rate of
10 to 20 meters (32.8 to 65.6 feet) per second, and single-mode fibers from 2
to 25 kilometers (1.2 to 15.5 miles) in length can be drawn from one preform.
At least one company has reported creating fibers of 160 kilometers (99 miles),
and the frequency with which fiber optics companies are currently retooling—as
often as every eighteen months—suggests that still greater innovations lie
ahead. These advances will be driven in part by the growing use of optical
fibers in computer networks, and also by the increasing demand for the
technology in burgeoning international markets such as Eastern Europe, South
America, and the Far East.
