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An effective configuration in order to achieve a laser device is to use not only two levels (the level of the laser transition) of an atom but a three level configuration in order to achieve the so called population inversion in the high level from which the stimulus of the incident wave let the atoms emit at the same time.

In order to achive the population inversion we have to pump the electron in the level 1 on to the level 2 via an electromagnetic wave of the right frequency that gives to the atoms the energy to jump on the higher level.
If we have only two levels the electrognetic wave pumps the electrons in the lower level on the higher level with the same rate the wave stimulates the emission from the level 2 to the level 1. We need to “trap” the electron in the higher level.

Using three levels of an atom we can pum the electron in a high level (level 3 ). The electron decays rapidly in an intermediate level (level 2) and stays there until a wave of the right frequency stimulates the decay on level 1.

This procedure needs two different stimulating waves with different frequencies. The first (pumping radiation) has the frequency while the second one (stimulating radiation) has the frequency .

The device will be set in a “ready” state by the inversion of population caused by the pumping wave. When we need the laser emission we let the stimulating wave go through the material and the electrons in the second level decay all at once giving a strong emission of radiation.
In order to fully use the stimulating incident wave in the laser device the active material is located inside mirrors.

These mirrors make a resonant cavity that enhance the radiation emission.

Out of the active material like ruby in modern technology are used also diode lasers.