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The light used in optical fiber networks is one type of electromagnetic energy. A nanometer is
one billionth of a meter (0.000000001 meter) in length.
Wavelengths that are not visible to the human eye are used to transmit data over optical fiber.
These wavelengths are slightly longer than red light and are called infrared light. Infrared
light is used in TV remote controls. The wavelength of the light in optical fiber is either 850
nm, 1310 nm, or 1550 nm. These wavelengths were selected because they travel through
optical fiber better than other wavelengths.
When a light strikes the interface between two transparent materials, the light divides into two
parts. Part of the light ray is reflected back into the first substance, with the angle of reflection
equaling the angle of incidence. The remaining energy in the light ray crosses the interface
and enters into the second substance.
The Law of Reflection states that the angle of reflection of a light ray is equal to the angle of
incidence.
The part of an optical fiber through which light rays travel is called the core of the fiber.
Light rays can only enter the core if their angle is inside the numerical aperture of the fiber.
Likewise, once the rays have entered the core of the fiber, there are a limited number of
optical paths that a light ray can follow through the fiber. These optical paths are called
modes. If the diameter of the core of the fiber is large enough so that there are many paths that
light can take through the fiber, the fiber is called “multimode” fiber. Single-mode fiber has a
much smaller core that only allows light rays to travel along one mode inside the fiber.
Every fiber-optic cable used for networking consists of two glass fibers encased in separate
sheaths. One fiber carries transmitted data from device A to device B. The second fiber
carries data from device B to device A. The fibers are similar to two one-way streets going in
opposite directions. This provides a full-duplex communication link. Just as copper twistedpair uses separate wire pairs to transmit and receive, fiber-optic circuits use one fiber strand to
transmit and one to receive. Typically, these two fiber cables will be in a single outer jacket
until they reach the point at which connectors are attached.
Until the connectors are attached, there is no need for twisting or shielding, because no light
escapes when it is inside a fiber. This means there are no crosstalk issues with fiber. It is very
common to see multiple fiber pairs encased in the same cable. This allows a single cable to be
run between data closets, floors, or buildings. One cable can contain 2 to 48 or more separate
fibers. With copper, one UTP cable would have to be pulled for each circuit. Fiber can carry
many more bits per second and carry them farther than copper can.
Usually, five parts make up each fiber-optic cable. The parts are the core, the cladding, a
buffer, a strength material, and an outer jacket.
The core is the light transmission element at the center of the optical fiber. All the light
signals travel through the core. A core is typically glass made from a combination of silicon
dioxide (silica) and other elements. Multimode uses a type of glass, called graded index glass
for its core. This glass has a lower index of refraction towards the outer edge of the core.
Therefore, the outer area of the core is less optically dense than the center and light can go
faster in the outer part of the core. This design is used because a light ray following a mode
that goes straight down the center of the core does not have as far to travel as a ray following
a mode that bounces around in the fiber. All rays should arrive at the end of the fiber together.
Then the receiver at the end of the fiber receives a strong flash of light rather than a long, dim
pulse.
Surrounding the core is the cladding. Cladding is also made of silica but with a lower index of
refraction than the core. Light rays traveling through the fiber core reflect off this core-tocladding interface as they move through the fiber by total internal reflection. Standard
multimode fiber-optic cable is the most common type of fiber-optic cable used in LANs. A
standard multimode fiber-optic cable uses an optical fiber with either a 62.5 or a 50-micron
core and a 125-micron diameter cladding. This is commonly designated as 62.5/125 or 50/125
micron optical fiber. A micron is one millionth of a meter (1µ).
Surrounding the cladding is a buffer material that is usually plastic. The buffer material helps
shield the core and cladding from damage. There are two basic cable designs. They are the
loose-tube and the tight-buffered cable designs. Most of the fiber used in LANs is tightbuffered multimode cable. Tight-buffered cables have the buffering material that surrounds
the cladding in direct contact with the cladding. The most practical difference between the
two designs is the applications for which they are used. Loose-tube cable is primarily used for
outside-building installations, while tight-buffered cable is used inside buildings.
The strength material surrounds the buffer, preventing the fiber cable from being stretched
when installers pull it. The material used is often Kevlar, the same material used to produce
bulletproof vests.
The final element is the outer jacket. The outer jacket surrounds the cable to protect the fiber
against abrasion, solvents, and other contaminants. The color of the outer jacket of multimode
fiber is usually orange, but occasionally another color.
Infrared Light Emitting Diodes (LEDs) or Vertical Cavity Surface Emitting Lasers (VCSELs)
are two types of light source usually used with multimode fiber. Use one or the other. LEDs
are a little cheaper to build and require somewhat less safety concerns than lasers. However,
LEDs cannot transmit light over cable as far as the lasers. Multimode fiber (62.5/125) can
carry data distances of up to 2000 meters (6,560 ft).
Single-mode fiber consists of the same parts as multimode. The outer jacket of single-mode
fiber is usually yellow. The major difference between multimode and single-mode fiber is that
single-mode allows only one mode of light to propagate through the smaller, fiber-optic core.
The single-mode core is eight to ten microns in diameter. Nine-micron cores are the most
common. A 9/125 marking on the jacket of the single-mode fiber indicates that the core fiber
has a diameter of 9 microns and the surrounding cladding is 125 microns in diameter.
An infrared laser is used as the light source in single-mode fiber. The ray of light it generates
enters the core at a 90-degree angle. As a result, the data carrying light ray pulses in singlemode fiber are essentially transmitted in a straight line right down the middle of the core.
This greatly increases both the speed and the distance that data can be transmitted.
Because of its design, single-mode fiber is capable of higher rates of data transmission
(bandwidth) and greater cable run distances than multimode fiber. Single-mode fiber can
carry LAN data up to 3000 meters. Multimode is only capable of carrying up to 2000 meters.
Lasers and single-mode fibers are more expensive than LEDs and multimode fiber. Because
of these characteristics, single-mode fiber is often used for inter-building connectivity.
Glass used in optical fibers:
Heavy metal fluoride glass is an extremely transparent glass being developed for use in
optical fibers that transmit infrared light. I optical fibers infrared light transmits better over
distance than visible light.