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Three Level Pumping

Four Level Pumping

An atom residing in the lower energy level E1 may absorb the incident photon of energy E2 – E1= hv and jump to the excited state E2 .

A photon of energy hv=E2-E1 can induce / trigger the excited atom to make a downward transition releasing the excess energy in the form of photon

The excited atom in the state E2 may return to lower state E1 on its own out of natural tendency to attain minimum potential energy condition by releasing excess energy in the form of photon

Temporal Coherence

Spatial Coherence

A pair of optically plane parallel mirrors enclosing active medium in between them is called as optical resonator.

A medium in which light gets amplified is called active medium. The medium may be solid, liquid or gas.