In the four level laser system, the atoms from ground state E1 are raised to excited state E4 with the help of pumping. From the
In the three level laser system, the atoms are pumped from the ground state E1 to higher state E3 with the help of pumping source.
Coherence can be classified into two ways: a) temporal coherence: consider a light wave traveling along +X axis. Consider two different points A and B