Download The structure of a fiber optic cable

Practical No. : 1
Fiber optics
Introduction:
An optical fiber is made up of the core, (carries the light pulses), the cladding
(reflects the light pulses back into the core) and the buffer coating (protects the core and
cladding from moisture, damage, etc.). Together, all of this creates a fiber optic which
can carry up to 10 million messages at any time using light pulses. Fiber optics is the
overlap of applied science and engineering concerned with the design and application of
optical fibers. Optical fibers are widely used in fiber-optic communications, which
permits transmission over longer distances and at higher bandwidths (data rates) than
other forms of communications. Fibers are used instead of metal wires because signals
travel along them with less loss and are also immune to electromagnetic interference.
Fibers are also used for illumination, and are wrapped in bundles so they can be used to
carry images, thus allowing viewing in tight spaces. Specially designed fibers are used
for a variety of other applications, including sensors and fiber lasers.
Light is kept in the core of the optical fiber by total internal reflection. This causes the
fiber to act as a waveguide. Fibers which support many propagation paths or transverse
modes are called multi-mode fibers (MMF), while those which can only support a single
mode are called single-mode fibers (SMF). Multi-mode fibers generally have a larger
core diameter, and are used for short-distance communication links and for applications
where high power must be transmitted. Single-mode fibers are used for most
communication links longer than 550 meters (1,800 ft).
Joining lengths of optical fiber is more complex than joining electrical wire or cable. The
ends of the fibers must be carefully cleaved, and then spliced together either
mechanically or by fusing them together with an electric arc. Special connectors are used
to make removable connections.
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The structure of a fiber optic cable:
1. Core: 8 µm diameter.
2. Cladding: 125 µm diameter.
3. Buffer: 250 µm diameter.
4. Jacket: 400 µm diameter.
History:
As far back as Roman times, glass has been drawn into fibers. Yet, it was not until
the 1790s that the French Chappe brothers invented the first "optical telegraph." It was a
system comprised of a series of lights mounted on towers where operators would relay a
message from one tower to the next. Over the course of the next century great strides
were made in optical science.
In the 1840s, physicists Daniel Collodon and Jacques Babinet showed that light
could be directed along jets of water for fountain displays. In 1854, John Tyndall, a
British physicist, demonstrated that light could travel through a curved stream of water
thereby proving that a light signal could be bent. He proved this by setting up a tank of
water with a pipe that ran out of one side. As water flowed from the pipe, he shone a light
into the tank into the stream of water. As the water fell, an arc of light followed the water
down.
Alexander Graham Bell patented an optical telephone system called the
photophone in 1880. His earlier invention, the telephone, proved to be more realistic
however. That same year, William Wheeler invented a system of light pipes lined with a
highly reflective coating that illuminated homes by using light from an electric arc lamp
placed in the basement and directing the light around the home with the pipes.
Doctors Roth and Reuss, of Vienna, used bent glass rods to illuminate body
cavities in 1888. French engineer Henry Saint-Rene designed a system of bent glass rods
for guiding light images seven years later in an early attempt at television. In 1898,
American David Smith applied for a patent on a dental illuminator using a curved glass
rod.
In the 1920s, John Logie Baird patented the idea of using arrays of transparent
rods to transmit images for television and Clarence W. Hansell did the same for
facsimiles. Heinrich Lamm, however, was the first person to transmit an image through a
bundle of optical fibers in 1930. It was an image of a light bulb filament. His intent was
to look inside inaccessible parts of the body, but the rise of the Nazis forced Lamm, a
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Jew, to move to America and abandon his dream of becoming a professor of medicine.
His effort to file a patent was denied because of Hansell's British patent.
In 1951, Holger Moeller applied for a Danish patent on fiber-optic imaging in
which he proposed cladding glass or plastic fibers with a transparent low-index material,
but was denied because of Baird and Hansell's patents. Three years later, Abraham Van
Heel and Harold H. Hopkins presented imaging bundles in the British journal Nature at
separate times. Van Heel later produced a cladded fiber system that greatly reduced
signal interference and crosstalk between fibers.
Also in 1954, the "maser" was developed by Charles Townes and his colleagues
at Columbia University. Maser stands for "microwave amplification by stimulated
emission of radiation."
The laser was introduced in 1958 as a efficient source of light. The concept was
introduced by Charles Townes and Arthur Schawlow to show that masers could be made
to operate in optical and infrared regions. Basically, light is reflected back and forth in an
energized medium to generate amplified light as opposed to excited molecules of gas
amplified to generate radio waves, as is the case with the maser. Laser stands for "light
amplification by stimulated emission of radiation."
A helium-neon gas laser (He-Ne) is tested in a laboratory setting. The laser tube is
made from lead glass- the same glass used in neon signs. Image courtesy of J&K Lasers.
In 1960, the first continuously operating helium-neon gas laser is invented and
tested. That same year an operable laser was invented which used a synthetic pink ruby
crystal as the medium and produced a pulse of light.
In 1961, Elias Snitzer of American Optical published a theoretical description of
single mode fibers whose core would be so small it could carry light with only one waveguide mode. Snitzer was able to demonstrate a laser directed through a thin glass fiber
which was sufficient for medical applications, but for communication applications the
light loss became too great.
Charles Kao and George Hockham, of Standard Communications Laboratories in
England, published a paper in 1964 demonstrating, theoretically, that light loss in existing
glass fibers could be decreased dramatically by removing impurities.
In 1970, the goal of making single mode fibers with attenuation less then
20dB/km was reached by scientists at Corning Glass Works. This was achieved through
doping silica glass with titanium. Also in 1970, Morton Panish and Izuo Hayashi of Bell
Laboratories, along with a group from the Ioffe Physical Institute in Leningrad,
demonstrated a semiconductor diode laser capable of emitting continuous waves at room
temperature.Military scientists have utilized laser technology for variety of military
applications.
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In 1973, Bell Laboratories developed a modified chemical vapor deposition
process that heats chemical vapors and oxygen to form ultra-transparent glass that can be
mass-produced into low-loss optical fiber. This process still remains the standard for
fiber-optic cable manufacturing.
The first non-experimental fiber-optic link was installed by the Dorset (UK)
police in 1975. Two years later, the first live telephone traffic through fiber optics occurs
in Long Beach, California.
In the late 1970s and early 1980s, telephone companies began to use fibers
extensively to rebuild their communications infrastructure. Sprint was founded on the
first nationwide, 100 percent digital, fiber-optic network in the mid-1980s.
The erbium-doped fiber amplifier, which reduced the cost of long-distance fiber
systems by eliminating the need for optical-electrical-optical repeaters, was invented in
1986 by David Payne of the University of Southampton and Emmanuel Desurvire at Bell
Labratories. Based on Desurvire's optimized laser amplification technology, the first
transatlantic telephone cable went into operation in 1988.
In 1991, Desurvire and Payne demonstrated optical amplifiers that were built into
the fiber-optic cable itself. The all-optic system could carry 100 times more information
than cable with electronic amplifiers. Also in 1991, photonic crystal fiber was developed.
This fiber guides light by means of diffraction from a periodic structure rather then total
internal reflection which allows power to be carried more efficiently then with
conventional fibers therefore improving performance.
The first all-optic fiber cable, TPC-5, that uses optical amplifiers was laid across
the Pacific Ocean in 1996. The following year the Fiber Optic Link Around the Globe
(FLAG) became the longest single-cable network in the world and provided the
infrastructure for the next generation of Internet applications.
Today, a variety of industries including the medical, military, telecommunication,
industrial, data storage, networking, and broadcast industries are able to apply and use
fiber optic technology in a variety of applications.
How Fiber optic works:
A glass tunnel through which the light travels is created. When the light hits the
cladding, it interacts with and reflects back into the core. Because of this design, the light
can “bend” around curves in the fiber and makes it possible to travel further distances
without having to be repeated.
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The light that travels along the fiber is made up of a binary code that pulses “on” and
“off” and determines what information a given signal contains. The advantage of fiber is
that these on/off pulses can be: translated video, computer, or voice data depending on
the type of transmitter and receiver used.
The following figure illustrates the transmission of light within a fiber via total
internal reflection. As long as the incident angle is less than the critical angle, the light
will be totally reflected without attenuation
Simply put, fiber optics is the use of light pulses to transmit data through thin
strands of glass or plastic. However, there's nothing simple about the advantages it
provides over wire cable, once the most common method used to transmit data. Optical
fiber carries more data at a faster pace, is smaller and easier to install, and is less
susceptible to interference, making it more secure. Commercial installation of optical
fiber didn't really begin until the late 1970s. It has since nearly replaced wire cable as the
primary means for long-distance communication
Transmission modes of Optical Fiber Cables
Single mode Transmission
Single Mode cable is a single stand of glass fiber with a diameter of 8.3 to 10 microns
that has one mode of transmission. Single Mode Fiber with a relatively narrow diameter,
through which only one mode will propagate typically 1310 or 1550nm. Carries higher
bandwidth than multimode fiber, but requires a light source with a narrow spectral width.
Synonyms mono-mode optical fiber, single-mode fiber, single-mode optical waveguide,
uni-mode fiber.
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Single Modem fiber is used in many applications where data is sent at multi-frequency
(WDM Wave-Division-Multiplexing) so only one cable is needed - (single-mode on one
single fiber)
Single-mode fiber gives you a higher transmission rate and up to 50 times more distance
than multimode, but it also costs more. Single-mode fiber has a much smaller core than
multimode. The small core and single light-wave virtually eliminate any distortion that
could result from overlapping light pulses, providing the least signal attenuation and the
highest
transmission
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Single mode fiber glass has a much smaller core that allows only one mode of
light to propagate through the core. Single mode fiber has a higher bandwidth and less
loss than Multi-mode fiber and for this reason it is the ideal transmission medium for
many applications. The standard Single mode fiber core is approximately 8-10 um in
diameter. Because of its greater information-carrying capacity, Singlemode fiber is
typically used for longer distances and higher-bandwidth applications.
While is might appear that Multi-mode fibers have higher information carrying capacity,
this is not the case. Singlemode fibers retain the integrity of each light pulse over longer
distances which allows more information to be transmitted. This is why Multi-mode
fibers are used for shorter distances.
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Multimode Transmission
Multi-Mode cable has a little bit bigger diameter, with a common diameters in the
50-to-100 micron range for the light carry component (in the US the most common size is
62.5um). Most applications in which Multi-mode fiber is used, 2 fibers are used (WDM
is not normally used on multi-mode fiber). POF is a newer plastic-based cable which
promises performance similar to glass cable on very short runs, but at a lower cost.
Multimode fiber gives you high bandwidth at high speeds (10 to 100MBS - Gigabit to
275m to 2km) over medium distances. Light waves are dispersed into numerous paths, or
modes, as they travel through the cable's core typically 850 or 1300nm. Typical
multimode fiber core diameters are 50, 62.5, and 100 micrometers. However, in long
cable runs (greater than 3000 feet [914.4 meters), multiple paths of light can cause signal
distortion at the receiving end, resulting in an unclear and incomplete data transmission
so designers now call for single mode fiber in new applications using Gigabit and
beyond.
Facts About Fiber Optics:
Fiber optics were needed because television cables were becoming more capable of
carrying more information than copper wire so computer and telephone companies
needed something to compete. Currently all new undersea cables are made of optical
fibers. Experts say that sometime in the early 21st century, 98% of copper wire will be
replaced by fiber optic cable.
Fiber optic cable installed for copper wire that already needs replacing is less expensive
since it only needs repeaters to amplify the signals running through it every six miles
rather than every mile. Optical fiber phone lines cannot be bugged or tapped.A fiber is
thinner than a human hair.
ADVANTAGES OF FIBER OPTICS
Fiber optic systems have many attractive features that are superior to electrical
systems. These include improved system performance, immunity to electrical noise,
signal security, and improved safety and electrical isolation.
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Greatly increased bandwidth and capacity
Lower signal attenuation (loss)
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Immunity to Electrical Noise
Immune to noise (electromagnetic interference [EMI] and radio-frequency
interference [RFI]
No crosstalk
Lower bit error rates
Signal Security
Difficult to tap
Nonconductive (does not radiate signals)Electrical Isolation
No common ground required
Freedom from short circuit and sparks
Size and Weight
Reduced size and weight cables
Environmental Protection
Resistant to radiation and corrosion
Resistant to temperature variations
Improved ruggedness and flexibility
Less restrictive in harsh environments
Overall System Economy
Low per-channel cost
Lower installation cost
DISADVANTAGES OF FIBER OPTICS
Price - Even though the raw material for making optical fibres, sand, is abundant and
cheap, optical fibres are still more expensive per metre than copper. Although, one fibre
can carry many more signals than a single copper cable and the large transmission
distances mean that fewer expensive repeaters are required.
Fragility - Optical fibres are more fragile than electrical wires.
Affected by chemicals - The glass can be affected by various chemicals including
hydrogen gas (a problem in underwater cables.)
Opaqueness - Despite extensive military use it is known that most fibres become opaque
when exposed to radiation.
Requires special skills - Optical fibres cannot be joined together as a easily as copper
cable and requires additional training of personnel and expensive precision splicing and
measurement equipment
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