Question

What is meant by population inversion and optical pumping?

Answer 1

Usually, the number of atoms in the lower energy state is more than the number of atoms in the excited state. To emit photons which are coherent, the number of atoms in the highest state must be greater than the number of atoms in the lower energy state. In other words, population of atoms in the higher energy state must be larger than the population of atoms in the higher energy state must be larger than the population of atoms in the lower energy state. The process of making the population of atoms in the higher energy state more than that of lower energy state is known as population inversion.
The method by which population inversion is obtained is called optical pumping.