Fiber-optic circuit

Fiber-optic circuit

The path of information travel, usually from one electrical system to another, in which light acts as the information carrier and is propagated by total internal reflection through a transparent optical waveguide. An electrooptic modulator and an optoelectric demodulator are required to convert the electrical signals into light and back again at the transmit and receive ends of the link, respectively.

A fiber-optic circuit, or link, is used for data transmission when a shielded twisted pair or a coaxial cable fails to meet one or more required performance criteria of the system designer. Depending upon fiber type, the distance-bandwidth product of a fiber is tens to thousands of times larger than that of electrical transmission. An optical communication fiber is a nearly perfect waveguide for light, meaning that little or no energy escapes through radiation. Thus, the data traveling in the fiber are secure from eavesdropping, as well as being harmless in or around equipment sensitive to electromagnetic interference. Telecommunication fiber also has a very small diameter, 5–10 micrometers (0.0002–0.0005 in.), which allows telecommunication cables to be fabricated with a much higher packing density. Also, the most common materials used to make the fibers, silica and plastic, are less dense than copper, making the cable lighter. Lastly, since the fiber is a dielectric it can be used in volatile or sensitive environments that require electrical isolation. See Optical communications

The transmitter generally consists of a silicon integrated circuit that converts input voltage levels from a personal computer or a mainframe into current pulses. These, in turn, drive a light-emitting diode (LED). See Integrated circuits, Light-emitting diode

Light from the fiber is focused onto a reverse-biased pn-junction photodiode that generates an electron-hole pair for each photon impinging on or near its active area. Another circuit, usually a silicon integrated circuit, amplifies this electron-hole current and converts it into voltage levels suitable for interfacing with the computer at the receiving end.