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.
Laser diodes (LD) are used in applications in which longer distances and higher data
rates are required. Because an LD has a much higher output power than an LED, it is
capable of transmitting information over longer distances. Consequently, and given
the fact that the LD has a much narrower spectral width, it can provide high-bandwidth communication over long distances.
In applications such as wavelength-division multiplexing in which several wavelengths are being transmitted down the same fiber, the stability of the source becomes critical. This usually requires complex circuitry and feedback mechanisms to detect and correct for drifts in wavelength. The benefits, however, of high-speed transmission using LDs typically outweigh the
drawbacks and added expense.