Laser

A laser is a device that generates or amplifies light through stimulated emission of radiation. Its name originated as an acronym for “light amplification by stimulated emission of radiation.” Unlike most conventional light sources, lasers can produce highly directional beams with strong optical coherence and a narrow range of wavelengths. These properties make them important tools in optics, communications, manufacturing, and scientific measurement. Laser radiation is not necessarily visible, and lasers need not operate at high power. (educationalgames.nobelprize.org)

Physical principles

Laser operation draws on quantum mechanics. An atom, molecule, or other active system can absorb energy and enter an excited state. It may subsequently release that energy as a photon. In spontaneous emission, the emission occurs without an incident light wave triggering it; in stimulated emission, suitable incoming radiation induces a transition that adds light to the stimulating optical field. This provides the mechanism for amplification. (educationalgames.nobelprize.org)

For a transition between two energy levels, the photon energy satisfies $E_2-E_1=h\nu$, where $h$ is the Planck constant and $\nu$ is the optical frequency. In a conventional laser, amplification requires population inversion: an appropriately weighted excess population in the upper laser level relative to the lower one. External pumping maintains this nonequilibrium condition, counteracting the tendency of the material to absorb light rather than amplify it. (catalogimages.wiley.com)

Components and operation

A typical laser combines a gain medium, an energy source, and an optical resonator. The gain medium supplies optical amplification; pumping transfers energy into it through light, an electrical discharge, or other excitation mechanisms. In a simple resonator, mirrors return radiation through the medium repeatedly, creating optical feedback. A partially transmitting mirror allows some circulating light to emerge as the output beam. (educationalgames.nobelprize.org)

Oscillation begins when amplification compensates for losses within the resonator, including the light extracted as useful output. This condition defines the lasing threshold. Above threshold, stimulated emission depletes the excited population, while pumping replenishes it. The medium and resonator together influence which optical frequencies and spatial patterns can persist. Thus, the output depends on both the emitting material and the design of the surrounding optical system. (catalogimages.wiley.com)

Beam properties

Coherence describes correlations in the phase of a light wave across space or over time. Spatial coherence enables a beam to be focused tightly, while temporal coherence supports measurements involving differences in optical path length. Many lasers also have narrow spectral linewidths, making them useful where a well-defined frequency is required. These properties are related but are not identical. (catalogimages.wiley.com)

Laser beams still spread through diffraction; they are not perfectly parallel. Some semiconductor lasers have substantial divergence and require additional optics for collimation. Nor is every laser strictly monochromatic: ultrashort pulses involve multiple spectral components. The practical advantage is the ability to concentrate and control radiation, rather than an inherently large total energy output. (catalogimages.wiley.com)

Principal types

Lasers are commonly classified by their gain medium:

• Solid-state lasers use active ions in crystalline or glass hosts. Ruby was the medium of the first functioning laser; neodymium-doped yttrium aluminium garnet, or Nd:YAG, commonly emits near 1,064 nanometres.
• Gas lasers include helium–neon lasers, familiar for red output near 633 nanometres, and carbon dioxide lasers, with prominent infrared emission near 10.6 micrometres.
• Semiconductor lasers generate light in semiconductor structures. Their compact construction supports applications such as optical-disc readers and communication systems.
• Dye lasers use organic dye solutions and can offer wavelength tuning over a range rather than emission at a single fixed wavelength. (osha.gov)

Continuous and pulsed emission

Continuous-wave lasers provide sustained output, whereas pulsed lasers emit discrete bursts. Pulse duration, repetition rate, and energy per pulse are separate characteristics: a short pulse can have high peak power without a correspondingly high average power. These distinctions matter when comparing laser systems and their interactions with materials. (osha.gov)

Q-switching allows energy to accumulate in the gain medium while resonator losses suppress oscillation, then rapidly lowers those losses to release an intense pulse. Mode locking establishes fixed phase relationships among optical modes, producing regular trains of much shorter pulses, including picosecond and femtosecond pulses. The two techniques therefore control pulse formation through different mechanisms. (photonics.creol.ucf.edu)

Historical development

Albert Einstein formulated stimulated emission in 1917. During the 1950s, work on the maser, which amplifies microwave radiation, established a practical foundation for optical devices. Charles Townes and Arthur Schawlow analysed the optical-maser concept in 1958. In 1960, Theodore Maiman demonstrated the first functioning laser, using ruby to generate pulses of red light. (educationalgames.nobelprize.org)

The 1964 Nobel Prize in Physics was awarded to Townes, Nikolay Basov, and Aleksandr Prokhorov for fundamental work in quantum electronics that led to masers and lasers. Subsequent developments extended laser technology to semiconductor devices, precision spectroscopy, and the cooling and trapping of atoms. (educationalgames.nobelprize.org)

Applications and hazards

Lasers transmit information through optical fibres, read optical discs, and support barcode scanning. In industry, focused beams perform cutting, drilling, welding, and marking. In spectroscopy, controlled wavelengths probe matter; in interferometry, phase-sensitive measurements reveal minute differences in optical path length. LIGO uses laser interferometers to detect gravitational waves through changes in the interference of light returning from perpendicular arms. (osha.gov)

Laser hazards depend on wavelength, exposure duration, power, and beam geometry. Direct or reflected radiation can damage eyes or skin, and powerful systems may present fire hazards. Associated equipment can introduce electrical hazards, while material processing can produce hazardous fumes. Product classifications describe accessible radiation under specified conditions; an enclosed device can contain a substantially more powerful internal laser than its external classification suggests. (osha.gov)