Energy Level Diagram for Einstein's Processes:

10. Laser Systems and Characteristics of Laser Beam

Different laser systems are designed based on the number of energy levels involved in achieving population inversion, which affects their efficiency and operational mode.

Two-Level Laser System:
- In a simple two-level system ($E_1, E_2$), pumping promotes atoms from $E_1$ to $E_2$.
- However, as $N_2$ increases, the rate of stimulated emission from $E_2$ to $E_1$ and absorption from $E_1$ to $E_2$ become equal.
- It is impossible to achieve population inversion ($N_2 > N_1$) in a stable two-level system under continuous pumping because the maximum $N_2$ can be is $N_{total}/2$. Therefore, two-level systems cannot sustain continuous laser action.
Three-Level Laser System (e.g., Ruby Laser):
- Involves three energy levels: a ground state ($E_1$), a short-lived pump level ($E_3$), and a metastable upper laser level ($E_2$).
- Pumping: Atoms are rapidly pumped from $E_1$ to $E_3$.
- Relaxation: Atoms in $E_3$ quickly undergo non-radiative decay to the metastable $E_2$ state (a state with a relatively long lifetime).
- Population Inversion: Due to the fast decay from $E_3$ to $E_2$ and the longer lifetime of $E_2$, atoms accumulate in $E_2$. If pumping is strong enough, population inversion can be achieved between $E_2$ and $E_1$ ($N_2 > N_1$).
- Lasing: Stimulated emission occurs between $E_2$ and $E_1$.
- Disadvantage: A large amount of pumping power is required because population inversion must be achieved relati