Amplification by stimulated emission
Stimulated emission allows excited matter to contribute radiation correlated with an incident field. It differs from spontaneous emission. Amplification draws on energy previously supplied to the material, so additional emitted light does not represent energy created from nothing by an incoming photon.
Pumping, gain and feedback
Energy is supplied to a gain medium so that amplification can overcome absorption and other losses. Optical feedback often returns light through the medium until lasing conditions are met. Solids, gases and other systems provide different transitions and pumping arrangements rather than a single universal device design.
References: [1]
Coherence and beam properties
Laser output commonly has greater directional and spectral control than ordinary lamp light. It is not necessarily one perfectly exact frequency or an infinitely narrow beam. Continuous operation, short pulses and different oscillation modes produce different temporal and spectral properties relevant to particular applications.
References: [1]
From quantum electronics to measurement
The 1964 Nobel Prize recognized Townes, Basov and Prokhorov’s quantum-electronics work associated with the maser–laser principle. Lasers subsequently served communications, spectroscopy and optical-disc reading. Their usefulness comes from controllable radiation properties, not simply from making light brighter than an ordinary lamp.