受激发射（英语：Stimulated emission）是雷射的主要光源。受激发射的光放大（英语：Light Amplification by Stimulated Emission of Radiation）缩写就是“LASER”。受激发射概念是由阿尔伯特·爱因斯坦在他1917年发表的论文《关于辐射的量子理论》中提出的；大约10年后，英国著名物理学家、剑桥大学教授保罗·狄拉克首次实验证明受激发射的存在。

Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level. The liberated energy transfers to the electromagnetic field, creating a new photon with identical phase, frequency, polarization, and direction of travel as the photons of the incident wave. This is in contrast to spontaneous emission which occurs at random intervals without regard to the ambient electromagnetic field. Further calculation can be obtained by statistical mechanics