General description

The basic principle of the laser can be summarized as follows: A laser consists essentially of three components 

•   the aktive laser medium, into which the
•   an "energy pump" selectively pumps energy into it, and
•   a resonator that stores part of this energy in the form of electromagnetic waves in a few resonator modes.

The energy pump (e.g. flash lamp, gas discharge or another laser) generates an energy level in the laser medium that is extremely different from the thermal equilibrium. If the pumping power is sufficiently high, the population density N_k(E_k) becomes greater for at least one energy level E_k than the population density N_i(E_i) for an energetically lower level E_i, which is connected to E_k by a permitted transition (inversion). Since in this case the induced emission rate is greater than the absorption rate, light can be amplified as it passes through the active medium. The task of the resonator is now to send light emitted by the atoms of the laser medium activated by the pump through the amplifying medium again by selective optical feedback, thereby making the laser amplifier a self oscillating oscillator. In other words, the resonator stores the light in a few resonator modes, so that in these modes the radiation density becomes very high and the induced emission can become much larger than the spontaneous emission.
 

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