Question

How to find a steady state population of a energy level in laser formation?

Answer 1

Population Inversion 

Population inversion is the process of achieving greater population of higher energy state as compared to the lower energy state. Population inversion technique is mainly used for light amplification. The population inversion is required for laser operation.

Consider a group of electrons with two energy levels E1 and E2.

E1 is the lower energy state and E2 is the higher energy state.
N1 is the number of electrons in the energy state E1. 
N2 is the number of electrons in the energy state E2. 
The number of electrons per unit volume in an energy state is the population of that energy state.



Population inversion cannot be achieved in a two energy level system. Under normal conditions, the number of electrons (N1) in the lower energy state (E1) is always greater as compared to the number of electrons (N2) in the higher energy state (E2).

N1 > N2

When temperature increases, the population of higher energy state (N2) also increases. However, the population of higher energy state (N2) will never exceeds the population of lower energy state (N1).

At best an equal population of the two states can be achieved which results in no optical gain.

N1 = N2

Therefore, we need 3 or more energy states to achieve population inversion. The greater is the number of energy states the greater is the optical gain.

There are certain substances in which the electrons once excited; they remain in the higher energy level or excited state for longer period. Such systems are called active systems or active media which are generally mixture of different elements.

When such mixtures are formed, their electronic energy levels are modified and some of them acquire special properties. Such types of materials are used to form 3-level laser or 4-level laser.

Answer 2

Explanation:

How to find a steady state population of a energy level in laser formation?