The source of the optical signal in an optical transmission system, which can take the form of a fiber optic transmission system (FOTS) or free space optics (FSO) system. Light-emitting diodes (LEDs) are used in multimode fiber (MMF), which usually is glass optical fiber (GOF), but also can be plastic optical fiber (POF).Vertical cavity surface-emitting lasers (VCSELs) also can be used in MMF systems. Laser diodes are used in single-mode fiber (SMF) systems.Virtually all light sources emit signals in the infrared (IR) spectrum.The exception is LEDs used in POF systems, which operate in the red region of the visible light spectrum. A light source essentially is an optical transmitter that is paired with an optical receiver, both of which are connected to electrically based devices or systems. So, the source converts electrons to photons and the detector converts photons to electrons. See also laser diode, LED, light detector, FOTS, FSO, GOF, MMF, POF, SMF, and VCSEL.