A device that emits a highly coherent beam of light. The term stands for “light amplification by stimulated emission of radiation.” A typical laser has an active medium, which emits light, and a cavity structure, which selects certain wavelengths and directions for the emitted light. Lasers convert electrical energy into radiant energy in the visible or infrared parts of the spectrum, emitting light with a small spectral bandwidth. For this reason, they are widely used in fiber-optic communications, particularly as sources for long-haul links.