How a Laser Works

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The below figure shows schematically how a population inversion can be achieved so that laser action-or lasing us it is called can occur. Atoms from the ground state E₁ are "pumped" up to an excited state E₃, for example by the absorption of light energy from an intense continuous spectrum source that surrounds the lasing material.

The basic three - level scheme for laser operation.

Metastable state E₂ has a greater population than the ground state E₁

From E₃, the atoms decay rapidly to a state of energy E₂. For lasing to occur, this state must be metastable; that is, it must have a relatively long mean life against decay by spontaneous emission. If conditions are right, state E₂ can then become more heavily populated than state E₁, thus providing the needed population inversion. A stray photon of the right energy can then trigger an avalanche of stimulate emission events, resulting in the production of laser light. A number of lasers using crystalline solids (such as ruby) as a lasing material operate in this three-level mode.