A device used in an optical transmission system to detect an optical signal generated by a light source and propagating through a medium. A light detector essentially is an optical receiver that is paired with an optical transmitter, 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. Detectors take several forms. A positive-intrinsic-negative (PIN) diode detector is paired with a light-emitting diode (LED) or vertical cavity surface-emitting laser (VCSEL) light source over a multimode fiber (MMF) in a fiber optic transmission system (FOTS) or a free space optics (FSO) system. An avalanche photodiode (APD) is paired with a laser diode in a single-mode fiber (SMF) system. See also APD, FOTS, FSO, laser diode, LED, light source, PIN, and VCSEL.