Fiber optic transmitters are devices that include an LED or laser source, and signal conditioning electronics, to inject a signal into fiber. The modulated light may be turned on or off, or may be linearly varied in intensity between two predetermined levels.  Light Emitting Diodes (LEDs) have relatively large emitting areas and as a result are not as good light sources as laser diodes.  However, they are widely used for short to moderate transmission distances because they are much more economical.  Laser diodes can couple many times more power to optical fiber than LEDs.  They are primarily used for applications that require the transmission of signals over long distances.