Fiber-optic data lines use an optical signal to transmit information. Through a process known as total internal reflection, a pulse of light is held within the optical fiber. As the light travels in a zig-zag pattern down the length of the fiber line, it becomes attenuated. Attenuation is a decrease in the strength of the pulse of light that reaches the far end of the fiber. A fiber-optic repeater overcomes attenuation by restoring the pulse of light back to its original strength before sending it out on the next leg the network line.
In fiber-optic networks, very thin filaments of glass wire transmit pulses of light. These light pulses are in the near-infrared wavelengths because this wavelength has the lowest attenuation rate. At network switches, these incoming pulses of light are translated into an electronic binary signal. This data signal can then be transmitted to individual computers.
Using a fiber-optic repeater every 28-43 miles (45-70 km), the data signal can be transmitted for great distances. Some of the longest fiber-optic lines cross the Atlantic Ocean. Repeaters require electricity, so conventional electrical wires must still be made available to each repeater.
Old analog signals used amplifiers to extend the distance of a signal. Amplifiers, however, had the unwanted effect of amplifying electrical noise as well as the original signal. Fiber-optic repeaters, on the other hand, remove noise that has entered a signal. This is because digital signals can be electronically separated from unwanted noise. Unlike analog signals, even a weak and distorted fiber signal can be cleaned up and sent further down the network line.
As an optical signal travels, it has a natural tendency to alter its shape. This phenomenon is called dispersion, a change in the velocity of light with the wavelength of light. Put more simply, a narrow pulse of light becomes broader the farther it travels. A fiber-optic repeater has the ability to restore the natural shape of the light pulse. After being restored by the repeater, the signal is re-transmitted to the next fiber-optic section.
Fiber-optics have many benefits over other methods of transmitting data. The glass fibers do not conduct electricity, so they are unaffected by electromagnetic disturbances or lighting storms. In addition, the amount of information that a single fiber-optic wire can carry is greater than copper wire or wireless links. In theory, a single fiber-optic line can carry 50 billion voice conversations on a single beam of light, although this limit has not been achieved in practice.
A fiber-optic repeater does not have the ability to distinguish pulses of light that are of different wavelengths. This limits the ability of a repeater to re-transmit dense optical information. Light information of multiple wavelengths can be transmitted over longer distances through the use of erbium-doped fiber amplifiers instead of repeaters. These amplifiers have the ability to boost the strength of individual wavelengths of light.