Fiber Optics

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.

Parts of a single optical fiber
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 or 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).
Some optical fibers can be made from plastic. These fibers have a large core (0.04 inches or 1 mm diameter) and transmit visible red light (wavelength = 650 nm) from LEDs.
Let's look at how an optical fiber works.

How does a fiber optic cable work?

Over the last 20 years or so, fiber optic lines have taken over and transformed the long distance telephone industry. Optical fibers are also a huge part of making the Internet available around the world. When fiber replaces copper for long distance calls and Internet traffic, it dramatically lowers costs.
To understand how a fiber optic cable works, imagine an immensely long drinking straw or flexible plastic pipe. For example, imagine a pipe that is several miles long. Now imagine that the inside surface of the pipe has been coated with a perfect mirror. Now imagine that you are looking into one end of the pipe. Several miles away at the other end, a friend turns on a flashlight and shines it into the pipe. Because the interior of the pipe is a perfect mirror, the flashlight's light will reflect off the sides of the pipe (even though the pipe may curve and twist) and you will see it at the other end. If your friend were to turn the flashlight on and off in a morse code fashion, your friend could communicate with you through the pipe. That is the essence of a fiber optic cable.
Making a cable out of a mirrored tube would work, but it would be bulky and it would also be hard to coat the interior of the tube with a perfect mirror. A real fiber optic cable is therefore made out of glass. The glass is incredibly pure so that, even though it is several miles long, light can still make it through (imagine glass so transparent that a window several miles thick still looks clear). The glass is drawn into a very thin strand, with a thickness comparable to that of a human hair. The glass strand is then coated in two layers of plastic.
By coating the glass in plastic, you get the equivalent of a mirror around the glass strand. This mirror creates total internal reflection, just like a perfect mirror coating on the inside of a tube does. You can experience this sort of reflection with a flashlight and a window in a dark room. If you direct the flashlight through the window at a 90 degree angle, it passes straight through the glass. However, if you shine the flashlight at a very shallow angle (nearly parallel to the glass), the glass will act as a mirror and you will see the beam reflect off the window and hit the wall inside the room. Light traveling through the fiber bounces at shallow angles like this and stays completely within the fiber.
To send telephone conversations through a fiber optic cable, analog voice signals are translated into digital signals . A laser at one end of the pipe switches on and off to send each bit. Modern fiber systems with a single laser can transmit billions of bits per second -- the laser can turn on and off several billions of times per second. The newest systems use multiple lasers with different colors to fit multiple signals into the same fiber.
Modern fiber optic cables can carry a signal quite a distance -- perhaps 60 miles (100 km). On a long distance line, there is an equipment hut every 40 to 60 miles. The hut contains equipment that picks up and retransmits the signal down the next segment at full strength.

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 carrying 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 fiber is less than in copper wire.
• Light signals - Unlike electrical signals in copper wires, light signals from one fiber do not interfere with those of other fibers in the same cable. This means clearer phone conversations or TV reception.
• 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.
• Flexible - Because fiber optics are so flexible and can transmit and receive light, they are used in many flexible digital cameras for the following purposes:
 Medical imaging - in bronchoscopes, endoscopes, laparoscopes
 Mechanical imaging - inspecting mechanical welds in pipes and engines (in airplanes, rockets, space shuttles, cars)
 Plumbing - to inspect sewer lines