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Optical
Media
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).
Additional
Resources on the Internet
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