Lasers are devices that generate and amplify highly organised electromagnetic radiation through the physical process of stimulated emission. The word laser is an acronym derived from light amplification by stimulated emission of radiation. A laser normally contains an active or gain medium capable of amplifying light, a mechanism for supplying energy to that medium, and an optical resonator that provides feedback. The gain medium may be a gas, semiconductor, solid material, liquid dye or another engineered system.
Energy supplied to the medium creates a population of atoms, ions, molecules or charge carriers in excited states. Under suitable conditions, an incoming photon can stimulate an excited quantum system to emit another photon with the same frequency, phase relationship, propagation direction and polarisation as the stimulating radiation. Repeated stimulated emission can produce optical amplification. An optical cavity, commonly formed using two mirrors or an equivalent distributed feedback structure, allows light to pass repeatedly through the gain medium. When optical gain exceeds relevant losses, sustained laser oscillation can occur. The resulting output is commonly directional, spectrally narrow and highly coherent compared with ordinary thermal or spontaneous light sources.
The essential physical distinction between laser emission and ordinary light emission is the role of stimulated emission and optical feedback. In spontaneous emission, an excited atom or other quantum system decays without being triggered by a passing photon, producing radiation with properties that are not generally phase aligned with radiation from independent emitters. In stimulated emission, an incoming photon interacts with an excited system and induces emission of a second photon that is correlated with the stimulating field.
For efficient laser operation, the gain medium must contain suitable energy levels and must be driven into a condition called population inversion, in which the relevant upper laser state has a greater population than the lower state in the sense required for net stimulated gain. In thermal equilibrium, such an inversion is generally not present. Pumping supplies energy to create the non equilibrium population distribution. Depending on the laser, pumping can be accomplished using electrical current, another optical source, electrical discharge, chemical reactions or other mechanisms. The detailed energy level structure determines how the laser can be pumped and what wavelengths it can produce.
A laser's optical cavity is central to its operation because it provides feedback and selects modes that can build up through repeated circulation. A simple cavity may consist of two mirrors facing each other, with one mirror highly reflective and the other partially transmitting so that a portion of the intracavity radiation becomes the useful output beam. The cavity supports particular longitudinal modes whose frequencies satisfy the boundary conditions imposed by the cavity length and optical properties.
Transverse modes can also occur and determine the spatial intensity distribution of the beam. The lowest order transverse mode often produces a smooth approximately Gaussian beam profile and is widely used where high beam quality is important. Laser cavities can be designed in many ways, including linear cavities, ring cavities, distributed feedback structures and semiconductor resonators. Cavity design affects threshold, linewidth, mode structure, stability, output coupling and beam quality. In many practical lasers, additional optical components are used to control frequency, polarisation, pulse formation or spatial mode behaviour.
Lasers can operate in continuous wave or pulsed regimes. A continuous wave laser produces an approximately continuous optical output over the relevant operating interval, although its power may still be modulated. Pulsed lasers generate discrete optical pulses and can produce very high peak powers even when their average power is comparatively moderate. Q switching stores energy in the gain medium and releases it in a short high intensity pulse.
Mode locking creates a stable relationship among multiple longitudinal modes so that they combine into extremely short pulses, potentially reaching picosecond or femtosecond durations. Different laser architectures are selected according to the required wavelength, output power, pulse duration, beam quality, efficiency and application. Semiconductor diode lasers are widely used in optical communications, barcode scanners, optical storage and consumer devices. Solid state lasers are important for industrial processing, medicine and research. Gas lasers, including helium neon and carbon dioxide systems, have been used for laboratory, alignment and industrial purposes. Excimer lasers operate in the ultraviolet and have specialised applications including semiconductor manufacturing and some medical procedures.
Laser light is often described as coherent, monochromatic and directional, but these terms require qualification. Coherence refers to stable relationships in phase between different parts of the electromagnetic field. Lasers generally exhibit substantially greater temporal and spatial coherence than thermal sources, but real lasers have finite linewidth and finite coherence length. Monochromatic means consisting of a single frequency, but real lasers always have some spectral width caused by physical and technical mechanisms.
Directionality refers to the relatively small angular divergence of a well collimated laser beam. A laser beam can still spread as it propagates because diffraction imposes fundamental limits on focusing and divergence. Beam quality is often characterised using parameters such as the M squared factor, which compares a real beam with an ideal diffraction limited Gaussian beam. These properties make lasers valuable for applications requiring precise delivery of optical energy, stable optical phase, narrow spectral bandwidth, accurate ranging or controlled interaction with materials.
Lasers are used across science, engineering, medicine, communications, manufacturing and consumer technology. In telecommunications, semiconductor lasers convert electrical information into optical signals that travel through optical fibres. In manufacturing, lasers can cut, weld, drill, mark and process materials because their energy can be concentrated into a small region. In medicine, lasers can be selected for surgical cutting, coagulation, tissue ablation, vision correction and dermatological treatment according to wavelength and tissue absorption characteristics.
In measurement, laser interferometers can detect extremely small changes in distance, displacement and deformation. Laser ranging systems determine distance from the travel time or phase of reflected light, while lidar uses laser pulses to map atmospheric or terrestrial targets. Laser spectroscopy uses narrow and controllable optical frequencies to investigate atoms, molecules and materials. Laser cooling and trapping have enabled highly controlled experiments in atomic physics. In entertainment and displays, low power visible lasers can generate highly directional coloured light. These applications arise from the ability to control wavelength, direction, intensity, temporal structure and coherence.
Laser safety is important because the eye can focus visible and near infrared laser radiation onto a small area of the retina, potentially producing severe injury at power levels that would not appear dangerous from the apparent brightness of the beam. Ultraviolet and infrared radiation can present additional hazards because it may be invisible while still interacting strongly with biological tissue. Hazard classification depends on wavelength, accessible power, exposure duration, beam geometry and other factors.
Reflections from apparently ordinary surfaces can also be hazardous for sufficiently powerful lasers. Industrial and research systems therefore use engineering controls, enclosed beam paths, interlocks, warning systems, appropriate eyewear and controlled operating procedures. Lasers can also create non optical hazards, such as electrical hazards, hot surfaces, fire risks or hazardous materials, depending on the system. Safety requirements are consequently part of laser engineering rather than an optional accessory. The correct precautions depend on the specific laser classification and operating environment.
The modern laser emerged from theoretical work on stimulated emission associated with the development of quantum theory and from twentieth century experimental advances in microwave amplification and spectroscopy. The maser, which amplified microwaves through stimulated emission, established important principles that were later applied at optical frequencies. Early laser demonstrations showed that coherent optical amplification could be achieved using suitable gain media and resonators. Since then, laser technology has expanded into an enormous range of wavelengths, powers, sizes and architectures.
Some modern semiconductor lasers are microscopic structures integrated directly onto electronic and photonic chips, while large high energy lasers can occupy substantial laboratory or industrial systems. The underlying principle remains the same: energy is supplied to a gain medium, stimulated emission provides optical amplification, and an optical feedback mechanism permits selected radiation to build up. Lasers therefore represent a practical application of quantum transitions, electromagnetic wave propagation, resonator physics, materials engineering and precision control of light.
