This document discusses laser diodes and their operation. It describes how population inversion is achieved through optical pumping, allowing for stimulated emission to produce coherent light amplification. A laser diode uses a p-n junction with heavily doped active regions to generate population inversion. Optical feedback is provided by a Fabry-Perot cavity formed between two mirrors to reflect photons through the gain region multiple times, producing lasing. Laser diodes can operate at nanosecond speeds and produce narrow spectral output useful for applications like optical communication and barcoding.