Laser

A pulsed infrared laser beam traveling down a single strand of optical fiber can carry thousands of times the information which can be carried by an electrical signal over copper wire. Not only can a single optical signal travel more rapidly than an electrical signal, but optical signals of different wavelengths, or colors, can travel down a fiber simultaneously, and without interfering. This technique is known as wavelength division multiplexing (WDM). Using WDM, the capacity of an optical network can be increased by a factor number of wavelengths, or channels used—if two channels are used, the network capacity doubles, without the need for an additional fiber; if four channels are used, the network capacity quadruples. To offer a good analogy, a WDM network is like a high-speed superhighway compared to the one-lane country road that is copper wire.

Low-loss optical fiber is lighter, more compact, and less expensive than the more traditional copper wire. Transcontinental and transoceanic optical fiber systems installed for telephone systems use directly modulated diode lasers operating at 1550 nm to generate the signals. Because even optical fiber generates some loss, the signals must be amplified periodically by repeaters, which also use laser technology. In the repeaters, a piece of optical fiber doped with erbium is pumped by a diode laser to optically amplify the original signal, which is launched back into the fiber link to continue its journey.