A fiber optic test source is laser diode or LED used to inject an optical signal into fiber to test the performance of a fiber optic system. Laser optical sources are usually used to test single mode fiber and LED optical sources.  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.