Two basic light sources are used for fiber optics: laser diodes (LD) and light-emitting diodes (LED). Each device has its own advantages and disadvantages as listed in this table.
Fiber optic sources must operate in the low-loss transmission windows of glass fiber. LEDs are typically used at the 850-nm and 1310-nm transmission wavelengths, whereas lasers are primarily used at 1310 nm and 1550 nm.
LEDs are typically used in lower-data-rate, shorter-distance multimode systems because of their inherent bandwidth limitations and lower output power. They are used in applications in which data rates are in the hundreds of megahertz as opposed to GHz data rates associated with lasers.
Two basic structures for LEDs are used in fiber optic systems: surface-emitting and edgeemitting as shown in following Figure.
Becuase of its specialtiys, makes LED are used different with LDs.
In surface-emitting LEDs the radiation emanates from the surface. LEDs typically have large numerical apertures, which makes light coupling into single-mode fiber difficult due to the fiber’s small N.A. and core diameter. For this reason LEDs are most often used with multimode fiber.
LEDs are used in lower-data-rate, shorter-distance multimode systems because of their inherent bandwidth limitations and lower output power.
The output spectrum of a typical LED is about 40 nm, which limits its performance because of severe chromatic dispersion.
LEDs operate in a more linear fashion than do laser diodes. This makes them more suitable for analog modulation. Notice that the LED has a more linear output power, which makes it more suitable for analog modulation.
Often these devices are pigtailed, having a fiber attached during the manufacturing process. Some LEDs are available with connector-ready housings that allow a connectorized fiber to be directly attached. They are also relatively inexpensive. Typical applications are local area networks, closed-circuit TV, and transmitting information in areas where EMI may be a problem.
