How Does an Optical Fiber Transmit Light?

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. However, some of the light signal degrades within the fiber, mostly due to impurities in the glass. The extent that the signal degrades depends on the purity of the glass and the wavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibers show much less signal degradation -- less than 10 percent/km at 1,550 nm.

A Fiber-Optic Relay System

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 the other ship sees the Morse code message, decodes it into English and sends the message up to the captain. Now, imagine doing this when the ships are on either side of the ocean separated by thousands of miles and you have a fiber-optic communication system in place between the two ships. 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 TransmitterThe 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. The transmitter is physically close to the optical fiber and may even have a lens to focus the light into the fiber. Lasers have more power than LEDs, but vary more with changes in temperature and are more expensive. The most common wavelengths of light signals are 850 nm, 1,300 nm, and 1,550 nm (infrared, non-visible portions of the spectrum). Optical RegeneratorAs 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 1 km) 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 the incoming signal (see this page on fiber amplifiers for more details). Optical ReceiverThe 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.

Testing the Finished Optical Fiber

Testing the Finished Optical Fiber
The finished optical fiber is tested for the following:

• Tensile strength - Must withstand 100,000 lb/in2 or more

• Refractive index profile - Determine numerical aperture as well as screen for optical defects

• Fiber geometry - Core diameter, cladding dimensions and coating diameter are uniform

• Attenuation - Determine the extent that light signals of various wavelengths degrade over distance

• Information carrying capacity (bandwidth) - Number of signals that can be carried at one time (multi-mode fibers)

• Chromatic dispersion - Spread of various wavelengths of light through the core (important for bandwidth)

• Operating temperature/humidity range

• Temperature dependence of attenuation

• Ability to conduct light underwater - Important for undersea cables Once the fibers have passed the quality control, they are sold to telephone companies, cable companies and network providers. Many companies are currently replacing their old copper-wire-based systems with new fiber-optic-based systems to improve speed, capacity and clarity