Fiber-optic lines have
revolutionized long-distance phone calls, cable TV and the Internet. See more electronic parts pictures.
Introduction to How Fiber Optics Work
You hear about fiber-optic cables whenever
people talk about the telephone system,
the cable TV system or the Internet. Fiber-optic lines are strands
of optically pure glass as thin as a human hair that carry digital information over long
distances. They are also used in medical imaging and mechanical engineering
inspection.
In this article, we will show you how these
tiny strands of glass transmit light and the fascinating way that these strands
are made.
What are Fiber Optics?
Fiber optics (optical fibers) are long, thin strands of
very pure glass about the diameter of a human hair. They are arranged in
bundles called optical cables and used to transmit light signals over long distances.
If you look closely at a single optical fiber,
you will see that it has the following parts:
·
Core - Thin glass center of
the fiber where the light travels
·
Cladding - Outer optical material
surrounding the core that reflects the light back into the core
·
Buffer coating - Plastic coating that protects the fiber from damage and
moisture
Hundreds or thousands of these optical fibers
are arranged in bundles in optical cables. The bundles are protected by the
cable's outer covering, called a jacket.
Optical fibers come in two types:
·
Single-mode fibers
·
Multi-mode fibers
See Tpub.com: Mode Theory for a good explanation.
Single-mode fibers have small cores (about 3.5 x 10-4 inches or 9 microns in diameter) and transmit infrared laser light (wavelength = 1,300 to 1,550 nanometers).Multi-mode fibers have larger cores (about 2.5 x 10-3 inches or 62.5 microns in diameter) and transmit infrared light
(wavelength = 850 to 1,300 nm) from light-emitting diodes (LEDs).
Some optical fibers can be made from plastic. These fibers have a large core (0.04 inches
or 1 mm diameter) and transmit visible red light (wavelength = 650 nm) from
LEDs.
Let's look at how an optical fiber works.
Diagram of total internal reflection in an
optical fiber
How Does an Optical Fiber Transmit
Light?
Suppose you want to shine a flashlight beam
down a long, straight hallway. Just point the beam straight down the hallway --
light travels in straight lines, so it is no problem. What if the hallway has a
bend in it? You could place a mirror at the bend to reflect the light beam
around the corner. What if the hallway is very winding with multiple bends? You
might line the walls with mirrors and angle the beam so that it bounces from
side-to-side all along the hallway. This is exactly what happens in an optical
fiber.
The light in a fiber-optic cable travels
through the core (hallway) by constantly bouncing from the cladding
(mirror-lined walls), a principle called total internal reflection. Because the cladding does not absorb any
light from the core, the light wave can travel great distances.
However, some of the light signal degrades within the fiber, mostly due to impurities in
the glass. The extent that the signal degrades depends on the purity of the
glass and the wavelength of the transmitted light (for example, 850 nm = 60 to
75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50
percent/km). Some premium optical fibers show much less signal degradation --
less than 10 percent/km at 1,550 nm.
A Fiber-Optic Relay System
To understand how optical fibers are used in
communications systems, let's look at an example from a World War II movie or
documentary where two naval ships in a fleet need to communicate with each other
while maintaining radio silence or on stormy seas. One ship pulls up alongside the
other. The captain of one ship sends a message to a sailor on deck. The sailor
translates the message into Morse code (dots and dashes) and uses a signal
light (floodlight with a venetian blind type shutter on it) to send the message
to the other ship. A sailor on the deck of the other ship sees the Morse code
message, decodes it into English and sends the message up to the captain.
Now, imagine doing this when the ships are on
either side of the ocean separated by thousands of miles and you have a
fiber-optic communication system in place between the two ships. Fiber-optic
relay systems consist of the following:
·
Transmitter - Produces and encodes
the light signals
·
Optical fiber - Conducts the light signals over a distance
·
Optical regenerator - May be necessary to boost the light signal (for long
distances)
·
Optical receiver - Receives and decodes the light signals
Transmitter
The transmitter is like the sailor on the deck of the sending
ship. It receives and directs the optical device to turn the light
"on" and "off" in the correct sequence, thereby generating
a light signal.
The transmitter is physically close to the
optical fiber and may even have a lens to focus the light into the fiber.
Lasers have more power than LEDs, but vary more with changes in temperature and
are more expensive. The most common wavelengths of light signals are 850 nm,
1,300 nm, and 1,550 nm (infrared, non-visible portions of the spectrum).
Optical Regenerator
As mentioned above,
some signal
loss occurs when the light
is transmitted through the fiber, especially over long distances (more than a
half mile, or about 1 km) such as with undersea cables. Therefore, one or more optical
regenerators is spliced along the
cable to boost the degraded light signals.
An optical regenerator consists of optical
fibers with a special coating (doping). The doped portion is "pumped" with a laser. When the degraded signal comes into the
doped coating, the energy from the laser allows the doped molecules to become
lasers themselves. The doped molecules then emit a new, stronger light signal
with the same characteristics as the incoming weak light signal. Basically, the
regenerator is a laser amplifier for the incoming signal.
Optical Receiver
The optical receiver is like the sailor on the deck of the
receiving ship. It takes the incoming digital light signals, decodes them and
sends the electrical signal to the other user's computer, TV or telephone (receiving ship's captain). The receiver uses
a photocell or photodiode to detect the light.
Advantages of Fiber Optics
Why are fiber-optic systems revolutionizing
telecommunications? Compared to conventional metal wire (copper wire), optical
fibers are:
Less expensive - Several miles of optical cable can be made
cheaper than equivalent lengths of copper wire. This saves your provider (cable
TV, Internet) and you money. Thinner - Optical fibers can be drawn to smaller diameters than copper
wire. Higher
carrying capacity -
Because optical fibers are thinner than copper wires, more fibers can be
bundled into a given-diameter cable than copper wires. This allows more phone
lines to go over the same cable or more channels to come through the cable into
your cable TV box. Less signal degradation - The loss of signal in optical fiber is less
than in copper wire. Light signals - Unlike electrical signals in copper wires, light signals from
one fiber do not interfere with those of other fibers in the same cable. This
means clearer phone conversations or TV reception. Low power - Because signals in optical fibers degrade
less, lower-power transmitters can be used instead of the high-voltage
electrical transmitters needed for copper wires. Again, this saves your
provider and you money. Digital signals - Optical fibers are ideally suited for carrying digital
information, which is especially useful in computer networks. Non-flammable - Because no electricity is passed through
optical fibers, there is no fire hazard. Lightweight - An optical cable weighs less than a
comparable copper wire cable. Fiber-optic cables take up less space in the
ground. Flexible - Because fiber optics are so flexible and can transmit and
receive light, they are used in many flexible digital cameras for the following purposes:
·
Medical imaging - in bronchoscopes, endoscopes, laparoscopes
·
Mechanical imaging - inspecting mechanical welds in pipes and engines (in airplanes, rockets, space shuttles, cars)
·
Plumbing - to inspect sewer
lines
Because of these advantages, you see fiber
optics in many industries, most notably telecommunications and computer
networks. For example, if you telephone Europe from the United States (or vice
versa) and the signal is bounced off a communications satellite, you often hear an echo on the line.
But with transatlantic fiber-optic cables, you have a direct connection with no
echoes.
MCVD process for
making the preform blank
Image courtesy Fibercore Ltd.
How Are Optical Fibers Made?
Now that we know how fiber-optic systems work
and why they are useful -- how do they make them? Optical fibers are made of
extremely pure optical glass. We think of a glass window as transparent, but the thicker the
glass gets, the less transparent it becomes due to impurities in the glass.
However, the glass in an optical fiber has far fewer impurities than
window-pane glass. One company's description of the quality of glass is as
follows: If you were on top of an ocean that is miles of solid core optical fiber
glass, you could see the bottom clearly.
Making optical fibers requires the following
steps:
1.
Making a preform glass cylinder
2.
Drawing the fibers from the preform
3.
Testing the fibers
Making the Preform
Blank
The glass for the
preform is made by a process called modified chemical vapor deposition(MCVD).
In MCVD, oxygen is bubbled through solutions
of silicon chloride (SiCl4), germanium chloride (GeCl4) and/or other chemicals.
The precise mixture governs the various physical and optical properties (index
of refraction, coefficient of expansion, melting point, etc.). The gas vapors
are then conducted to the inside of a synthetic silicaor quartz tube (cladding) in a special lathe. As the lathe turns, a torch is moved up and
down the outside of the tube. The extreme heat from the torch causes two things
to happen:
Lathe used in
preparing the preform blank Photo courtesy
Fiber core Ltd.
·
The silicon and
germanium react with oxygen, forming silicon dioxide (SiO2) and germanium
dioxide (GeO2).
·
The silicon dioxide
and germanium dioxide deposit on the inside of the tube and fuse together to
form glass.
The lathe turns continuously to make an even coating
and consistent blank. The purity of the glass is maintained by using
corrosion-resistant plastic in the gas delivery system (valve blocks, pipes,
seals) and by precisely controlling the flow and composition of the mixture.
The process of making the preform blank is highly automated and takes several
hours. After the preform blank cools, it is tested for quality control (index of refraction).
Drawing Fibers from
the Preform Blank
Once the preform blank
has been tested, it gets loaded into a fiber drawing tower.
Diagram of a fiber
drawing tower used to draw optical glass fibers from a preform blank
The blank gets lowered into a graphite furnace
(3,452 to 3,992 degrees Fahrenheit or 1,900 to 2,200 degrees Celsius) and the
tip gets melted until a molten glob falls down by gravity. As it
drops, it cools and forms a thread.
The operator threads the strand through a
series of coating cups (buffer coatings) and ultraviolet light curing ovens
onto a tractor-controlled spool. The tractor mechanism slowly pulls the fiber
from the heated preform blank and is precisely controlled by using a laser
micrometer to measure the
diameter of the fiber and feed the information back to the tractor mechanism.
Fibers are pulled from the blank at a rate of 33 to 66 ft/s (10 to 20 m/s) and
the finished product is wound onto the spool. It is not uncommon for spools to
contain more than 1.4 miles (2.2 km) of optical fiber.
Testing the Finished
Optical Fiber
The finished optical
fiber is tested for the following:
Finished spool of
optical fiber
Photo courtesy Corning
·
Tensile strength - Must withstand 100,000 lb/in2 or more
·
Refractive index profile - Determine numerical aperture as well as screen for optical
defects
·
Fiber geometry - Core
diameter, cladding dimensions and coating diameter are uniform
·
Attenuation - Determine the extent
that light signals of various wavelengths degrade over distance
·
Information carrying capacity (bandwidth) - Number of signals that can be carried at one time
(multi-mode fibers)
·
Chromatic dispersion - Spread of various wavelengths of light through the core
(important for bandwidth)
·
Operating temperature/humidity range
·
Temperature dependence of attenuation
·
Ability to conduct light underwater - Important for undersea cables
Once the fibers have passed the quality
control, they are sold to telephone companies, cable companies and network
providers. Many companies are currently replacing their old copper-wire-based
systems with new fiber-optic-based systems to improve speed, capacity and
clarity.
Total internal reflection in an optical fiber
Physics of Total Internal Reflection
When light passes from a medium with one index of refraction (m1) to another medium with a lower index of
refraction (m2), it bends or refracts away from an imaginary line perpendicular to the surface (normal line). As the angle of the beam through m1 becomes
greater with respect to the normal line, the refracted light through m2 bends
further away from the line.
At one particular angle (critical angle), the refracted light will not go into m2,
but instead will travel along the surface between the two media (sine [critical angle]
= n2/n1 where n1 and n2 are
the indices of refraction [n1 is greater than n2]). If the beam through m1 is
greater than the critical angle, then the refracted beam will be reflected
entirely back into m1 (total internal reflection), even though m2 may be
transparent!
In physics, the critical angle is described
with respect to the normal line. In fiber optics, the critical angle is
described with respect to the parallel axis running down the middle of the
fiber. Therefore, the fiber-optic critical angle = (90 degrees - physics
critical angle).
In an optical fiber, the light travels through
the core (m1, high index of refraction) by constantly reflecting from the
cladding (m2, lower index of refraction) because the angle of the light is
always greater than the critical angle. Light reflects from the cladding no
matter what angle the fiber itself gets bent at, even if it's a full circle!
Because the cladding does not absorb any light
from the core, the light wave can travel great distances. However, some of the
light signal degrades within the fiber, mostly due to impurities in the glass.
The extent that the signal degrades depends upon the purity of the glass and
the wavelength of the transmitted light (for example, 850 nm = 60 to 75
percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50
percent/km). Some premium optical fibers show much less signal degradation --
less than 10 percent/km at 1,550 nm.
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