Question

what is the mechanism of 3 - level laser

Answer 1

Consider a system consisting of three energy levels E1, E2, E3 with N number of electrons.

We assume that the energy level of E1 is less than than E2 and E3, the energy level of E2 is greater than E1 and less than E3, and the energy level of E3 is greater than E1 and E2.

It can be simply written as E1 < E2 < E3. That means the energy level of E2 lies in between E1 and E3.