1. Provides +ve feedback pf photons

2. Selects the direction in which the ligght is to be amplified

3. Builds up the photon density

1. Pumping

2. Population inversion

3. Spontaneous emission

4. Optical feedback and light amplification

5. Light Oscillation

Optical Pumping

Electrical Discharge

Direct Conversion

3 level pumping: 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.

4 level pumping: In the four level laser system, the atoms from ground state E1 are raised to excited state E4 with the help of pumping.