Operation of most lasers is based on the process of stimulated emission of electromagnetic radiation (photons) by atoms and other quantum systems that are in an excited state. The quantum systems excited by the pump energy interact with the passing resonant optical radiation. As a result of such interaction the quantum particles move to a lower-energy state, and the resonant light emission increases (such particle transitions are called “stimulated transitions”, and the radiation emitted as a result of such transition is called “stimulated radiation”).

The term “laser” is an abbreviation standing for Light Amplification by Stimulated Emission of Radiation. In its modern meaning the term is also used to describe devices using stimulated scattering for light amplification. In this case, the light pump energy is converted into energy of amplified light as a result of nonlinear interaction with the amplified light in an active medium.

In its broad sense the term “laser” refers to the three classes of devices:

- Laser generators.

- Optical amplifiers.

- Amplified spontaneous emission sources.

In the narrow sense, the term is applied to the laser generators only.

Any laser-generator consists of the three components – a pumping device, an active medium and a feedback device. For generation to occur the amplification of light by the active medium shall exceed the total losses in the feedback device and other elements of the laser. In this case, the amplitude (intensity) of the light increases after a round-trip, and the laser generation is developed from spontaneous noise.

Optical amplifiers differ from laser generators by the absence of feedback. In some cases it is even necessary to take special measures for reducing parasitic feedback in order to prevent unwanted generation. Amplifiers are designed for the amplification of optical signals and are characterised by the gain factor, gain spectrum, saturation power, noise factor, crosstalk factor, performance and other parameters.

In the absence of anamplified signal at the amplifier input the output radiation is not zero. Part of the spontaneously emitted light passing through the amplifier increases its power. In this mode the amplifier is a source of amplified spontaneous emission.

Laser applications are wide and varied. One of the most important applications is optical communication. Lasers are also widely used as sensors (magnetic field sensor, fibre-optic pressure sensor, etc.). Nonresonant effects of powerful laser radiation on various substances and materials in continuous and pulsed modes are used in technological applications of lasers. The selective action on atoms, ions, molecules and molecular complexes, resulting in photodissociation, photoionisation, photochemical reactions, etc., is used in laser chemistry, laser isotope separation, etc. Ultrashort laser pulses are used to study fast processes, in ultrafast photography, in time-resolved spectroscopy, etc. Ultrastable lasers lie in the basis of optical frequency standards, laser seismographs, gravimetres and other precision measuring instruments. Tunable lasers have revolutionised spectroscopy. In medicine, lasers are used for diagnostic purposes, and as therapeutic and surgical instruments.

The newest laser applications include production and testing of nanomaterials, treatment and testing of surfaces, production of elements for electronic and photonic circuits, including elements based on photonic crystals.

The advent of lasers led to the birth of such new branches of physics as photonics, nonlinear optics, and holography.