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Radio, microwaves, radar, visible light, x-rays, and
gamma rays seem to be very different things. However,
they are all types of electromagnetic energy. If all the
types of electromagnetic waves are arranged in order
from the longest wavelength down to the shortest
wavelength, a continuum called the electromagnetic
spectrum is created. Light is a form of electromagnetic
radiation.
Optical fiber is the most frequently used medium for the
longer, high bandwidth, point-to-point transmissions
required on LAN backbones and on WANs. Using optical
media, light is used to transmit data through thin glass
or plastic fiber. Electrical signals cause a fiber-optic
transmitter to generate the light signals sent down the
fiber. The receiving host receives the light signals and
converts them to electrical signals at the far end of
the fiber
When a ray of light (the incident ray) strikes the shiny
surface of a flat piece of glass, some of the light
energy in the ray is reflected. Law of Reflection states
that the angle of reflection of a light ray is equal to
the angle of incidence.
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. if the incident ray is not at an exact
90-degree angle to the surface, then the transmitted ray
that enters the glass is bent. The bending of the
entering ray is called refraction.
A light ray that is being turned on and off to send data
(1s and 0s) into an optical fiber must stay inside the
fiber until it reaches the far end. The ray must not
refract into the material wrapped around the outside of
the fiber. The refraction would cause the loss of part
of the light energy of the ray. A design must be
achieved for the fiber that will make the outside
surface of the fiber act like a mirror to the light ray
moving through the fiber. If any light ray that tries to
move out through the side of the fiber were reflected
back into the fiber at an angle that sends it towards
the far end of the fiber, this would be a good “pipe” or
“wave guide” for the light waves.
The laws of reflection and refraction illustrate how to
design a fiber that guides the light waves through the
fiber with a minimum energy loss. The following two
conditions must be met for the light rays in a fiber to
be reflected back into the fiber without any loss due to
refraction:
• The core of the optical fiber has to have a larger
index of refraction (n) than the material that surrounds
it. The material that surrounds the core of the fiber is
called the cladding.
• The angle of incidence of the light ray is greater
than the critical angle for the core and its cladding.
When both of these conditions are met, the entire
incident light in the fiber is reflected back inside the
fiber. This is called total internal reflection, which
is the foundation upon which optical fiber is
constructed. Total internal reflection causes the light
rays in the fiber to bounce off the core-cladding
boundary and continue its journey towards the far end of
the fiber. The light will follow a zigzag path through
the core of the fiber.
A fiber that meets the first condition can be easily
created. In addition, the angle of incidence of the
light rays that enter the core can be controlled.
Restricting the following two factors controls the angle
of incidence:
• The numerical aperture of the fiber – The numerical
aperture of a core is the range of angles of incident
light rays entering the fiber that will be completely
reflected.
• Modes – The paths which a light ray can follow when
traveling down a fiber.
By controlling both conditions, the fiber run will have
total internal reflection. This gives a light wave guide
that can be used for data communications.
The optical paths that a light ray can follow through
the fiber 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. 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. Multimode fiber (62.5/125) can carry data
distances of up to 2000 meters (6,560 ft).Infrared Light
Emitting Diodes (LEDs) or Vertical Cavity Surface
Emitting Lasers (VCSELs) are two types of light source
usually used with multimode fiber
Single-mode fiber consists of the same parts as
multimode. 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. 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
single-mode fiber are essentially transmitted in a
straight line right down the middle of the core. Because
of its design, single-mode fiber is capable of higher
rates of data transmission (bandwidth) and greater cable
run distances than multimode fiber. Because of these
characteristics, single-mode fiber is often used for
inter-building connectivity.
Most of the data sent over a LAN is in the form of
electrical signals. However, optical fiber links use
light to send data. Something is needed to convert the
electricity to light and at the other end of the fiber
convert the light back to electricity. This means that a
transmitter and a receiver are required.The transmitter
receives data to be transmitted from switches and
routers. This data is in the form of electrical signals.
The transmitter converts the electronic signals into
their equivalent light pulses.The transmitters (light
sources) can be lighted and darkened very quickly to
send data (1s and 0s) at a high number of bits per
second.
At the other end of the optical fiber from the
transmitter is the receiver. The receiver functions
something like the photoelectric cell in a solar powered
calculator. When light strikes the receiver, it produces
electricity. The first job of the receiver is to detect
a light pulse that arrives from the fiber. Then the
receiver converts the light pulse back into the original
electrical signal that first entered the transmitter at
the far end of the fiber. Now the signal is again in the
form of voltage changes. The signal is ready to be sent
over copper wire into any receiving electronic device
such as a computer, switch, or router. The semiconductor
devices that are usually used as receivers with
fiber-optic links are called p-intrinsic-n diodes (PIN
photodiodes). |
