F i be r - Opt i c Medium

Fiber optics is a method of carrying information using optical fibers. An optical fiber is a thin strand of glass or plastic that serves as the transmission medium over which information is sent. It thus fills the same basic function as a copper cable carrying a telephone conversation, computer data, or video. Unlike the copper cable, however, the optical fiber carries light instead of electrons. In so doing, it offers many distinct advantages that make it the transmission medium of choice for applications ranging from telephone calls, television, and machine control.

The basic fiber-optic system is a link connecting two electronic circuits. Figure 1 shows a simple fiber-optic link.

There are three basic parts to a fiber-optic system:

•   Transmitter : The transmitter unit converts an electrical signal to an optical signal. The light source is typically a light-emitting diode, LED, or a laser diode. The light source performs the actual conversion from an electrical signal to an optical signal. The driving circuit for the light source changes the electrical signal into the driving current.

•   Fiber-optic cable : The fiber-optic cable is the trans- mission medium for carrying the light. The cable includes the optical fibers in their protective jacket.

•   Receiver: The receiver accepts the light or photons and converts them back into an electrical signal. In most cases, the resulting electrical signal is identical to the original signal fed into the transmitter. There are two basic sections of a receiver. First is the  detector that converts the optical signal back into an electrical signal. The second section is the output circuit, which reshapes and rebuilds the original signal before passing it to the output.

Depending on the application, the transmitter and receiver circuitry can be very simple or quite complex. Other components that make up a fiber-optic trans- mission system, such as couplers, multiplexers, optical amplifiers, and optical switches, provide the means for building more complex links and communications networks. The transmitter, fiber, and receiver, how- ever, are the basic elements in every fiber-optic system.

Beyond the simple link, the fiber-optic medium is the fundamental building block for optical communications. Most electrical signals can be transported optically. Many optical components have been invented to permit signals to be processed optically without electrical conversion. Indeed, one goal of optical communications is to be able to operate entirely in the optical domain from system end to end.

F IGUR E 1     Basic building blocks of a fiber-optic syste