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The laser effect

The word laser is the acronym of Light Amplification by Stimulated Emission of Radiation. 

The reason of this name comes from the fact that when we have some electrons in a higher level than the level of minimum energy an electromagnetic wave of the right frequency may stimulate the decay of the electrons onto a lower level. The amazing thing in this situation is that the emitted electromagnetic wave has the same frequency and phase and direction of the incident stimulating wave.
 

This means that the incident light is highly reinforced because

• the same direction of emitted light leads to a very low spreading of the light ray (collimation)
• the same phase implies that there is no interference between the photons (coherence)

In order to achieve a high powerful stimulated emission we have to pump a large number of electrons in the same level and let them decay on to the lower level together at the same time. 

The problem in this situation is that if we pump some electrons in a higher energy level by stimulated absorption of energy the atoms emit with the same rate as the pumping occurs. We have to “lock” the electrons in a level that doesn’t decay at the same rate as the pumping.