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. The E3 is called pumping state. The life time of atoms is least in the energy level E3. It means E3 is unstable stable state and here atoms stay for 10-9 (ten power -9) to 10-8 seconds.
Atoms make transition from level E3 to E2. Energy level E2 is the metastable state (having life time 10-5 to 10-3 seconds). Thus, population of atoms become more in the energy state E2 as compared to E1.
The stimulated emission occurs between E2 and E1 producing laser. E2 is known as upper laser level (ULL) and E1 is known as lower laser level (E1).
The example of three level laser is ruby laser.
Drawback of three level laser system:
In the three level laser system, the terminal level is ground level and hence more than half of the atoms are to be transferred to level E2. This requires more pumping power. If the difference of atoms in two levels is small, the power required is also small. But the presence of large number of atoms in level E2 give rise to large number of spontaneous radiationless transitions. This energy is usually carried by lattice photons due to which the efficiency of three level laser system is less.
Reference: This article is referred from my authored book “optics and lasers” having ISBN 81-272-2948-2. In case of any doubt, post in the comment section.