Understanding Lasers: From Principles to Types

Laser definition and optical amplification principle

A laser is a device that emits light through optical amplification based on stimulated emission of electromagnetic radiation. The term “laser” comes from “light amplification by stimulated emission of radiation.” Unlike ordinary light sources, lasers produce coherent light, which allows beams to be tightly focused, remain narrow over long distances, and have well-defined temporal and spectral properties. The optical amplification principle relies on exciting a gain medium so that it achieves population inversion (more particles in an excited state than in a lower-energy state). When a photon with the right energy passes through the medium, it can trigger stimulated emission: the medium emits a new photon that matches the incoming photon in wavelength, phase, and polarization direction. With sufficient pumping and an optical resonator (feedback via mirrors), stimulated emission repeats many times, causing the light intensity to grow until cavity losses and gain saturation balance. The minimum pump power needed to start sustained laser action is the lasing threshold.

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Coherence: spatial and temporal coherence

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