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November 2005
Have you noticed our Fiber Project 2005?
If you’ve been driving around the Blackduck area, no doubt you’ve seen all the
activity this summer. Our project includes three main “installations”. The most
visible activity is the fiber optic cable that is being installed underground.
Why is fiber optic cable so important to our future? Read on.
How Does 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.
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 of 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
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).
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.
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 other ship sees the Morse code message,
decodes it into English and sends the message up to the captain.
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.
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 1km)
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
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 carry 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 is less than in copper wire.
● 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.
Because of these advantages, you see fiber optics in many industries, most notably telecommunications and computer networks.
The above was taken from an article by Craig C. Freudenrich, Ph.D.
Article 1, 2, 3
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