A laser is a light source that amplifies light inside a material so the emitted photons reinforce one another, producing a controllable, intense beam. The name reflects the key mechanism—stimulated emission—where light triggers more light of the same kind, unlike ordinary lamps that emit many unrelated waves in many directions.

Laser light is called coherent because its waves keep a predictable relationship over space and time. Spatial coherence helps the beam stay narrow and focus to a tiny spot for high intensity tasks like cutting. Temporal coherence means a well-defined color (narrow spectrum) and stable phase over distance, important for precision measurement and communications.

In stimulated emission, an excited atom or molecule is hit by a photon with the right energy and responds by emitting a second photon that matches the first in wavelength, phase, and direction. Because the new photon is a “copy,” repeated events make the light increasingly ordered, enabling optical amplification rather than random glow.

Thermal sources and most spontaneous emission produce photons with many colors, phases, and directions, so the output is broad and noisy. A laser uses the same quantum transitions but favors stimulated emission, which preferentially amplifies one optical wave. This is why lasers can be bright, directional, and spectrally pure compared with typical lighting.

To amplify strongly, many particles must remain excited long enough to be triggered by passing photons. Useful laser materials provide metastable excited states—levels with relatively long lifetimes—so excitation can accumulate instead of disappearing immediately. This “storage” of energy makes it practical to build up the conditions needed for strong amplification.

Amplification requires more particles in an upper energy level than in a lower one for the chosen transition, called population inversion. With inversion, stimulated emission events outnumber absorptions, so light grows as it passes through the medium. Real lasers typically use three-level or higher schemes because simple two-level systems struggle to sustain inversion efficiently.

Most lasers share three ingredients: a gain medium that can amplify light, a pump that supplies energy to excite the medium, and optical feedback that sends light through the medium repeatedly. A common feedback structure is a two-mirror resonator, where one mirror reflects strongly and the other lets a controlled fraction exit as the usable beam.

The resonator acts like a selective amplifier: only light patterns (modes) that fit the cavity well and experience enough gain survive repeated round trips. Competing waves are suppressed by losses, while supported modes build quickly. This positive-feedback behavior narrows the output spectrum and shapes the beam, making laser emission stable and highly directional.

Lasing begins at a threshold where gain just exceeds total losses from mirrors, scattering, and absorption. Above threshold, output rises, but stronger stimulated emission depletes the excited population, reducing the available gain. The system settles when gain effectively equals losses, creating a steady operating point in continuous-wave lasers.

Many lasers naturally emit a Gaussian-like beam that minimizes divergence for a given beam size, which is why it can be focused to a small spot. High-power systems may support multiple transverse patterns, and optics can reshape beams into flatter “tophat” profiles or special forms like Bessel beams and vortices for machining, trapping, or imaging.

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Q-switching creates short, intense pulses by deliberately keeping cavity losses high while pumping builds a large stored inversion. When losses are rapidly reduced, the cavity “Q” rises and the stored energy is dumped into a brief pulse. This concentrates energy in time, producing peak powers far higher than the average output.

Mode locking generates extremely short pulses by forcing many cavity longitudinal modes to maintain fixed phase relationships, so they add constructively at regular intervals. The pulse repetition rate is set by the cavity round-trip time, producing a coherent pulse train. Broad-gain media can support femtosecond pulses used in timing, spectroscopy, and ultrafast science.

Because laser beams are bright, focusable, and controllable, they power fiber-optic and free-space communications, barcode scanners, optical disc reading, and precision manufacturing like lithography, cutting, and welding. In sensing, lidar measures distance and maps environments. In medicine, lasers enable fine control for eye surgery, skin treatments, and targeted ablation.

Laser hazards are managed by safety classes based on accessible emission and typical injury risk. Lower classes are safe under normal use, often due to enclosure or low power, while higher classes can damage eyes and skin quickly and may be hazardous even from scattered light. Proper labeling, eyewear, and controlled areas are essential for high-power systems.