Semiconductor lasers-or diode lasers, as they are often called-are an integral part of modern technology. They are used in numerous devices from laser pointers to high-throughput laser printers to the light sources that power the optical-fiber communication behind the internet. At the fundamental level, a laser is a combination of a resonant optical cavity (such as a pair of mirrors facing each other a given distance apart so that only certain wavelengths can fit neatly between the mirrors) and a gain medium (which amplifies the light at one or more of those wavelengths). Turn up the gain to the point where it balances the losses due to absorption and leakage out of the cavity, and you have a laser.
Gregg M. Gallatin,
"Book Review: Nonlinear Laser Dynamics: From Quantum Dots to Cryptography," Journal of Nanophotonics 6(1), 060202 (24 September 2012). https://doi.org/10.1117/1.JNP.6.060202