LASER Light amplification by stimulated emission of radiation

LASER – Light amplification by stimulated emission of radiation.

• Properties of LASER: 1. Laser light is coherent

2. Laser light is perfectly monochromatic.

3. Laser emits light in one direction i. e. it is unidirectional. 4. Laser is extremely intense. 5. Laser beam is extremely narrow.

Spontaneous emission: - After completion of life time, excited atom comes to ground energy state spontaneously emitting a photon is called as spontaneous emission.

• In spontaneous emission, emitted radiations are random in directions, random in phase i. e. radiations are incoherent. • Stimulated emission: - When atom in excited state and before coming to ground state if it is triggered due to action of an incident photon. The interaction between excited atom & incident photon can trigger the excited atom to makes transition to ground state. This transition generates another photon which is identical to incident photon. This process of forced emission of photon due to incident photon is called as stimulated emission. • In stimulated emission, emitted radiations are in same phase i. e. coherent & monochromatic.

Stimulated emission

• Life time: - The time for which atom remains in excited state is known as life time. • Metastable state : - The life time of atom is very small. For hydrogen it is 10 -8 sec. But there exists such excited states in which life time is greater than 10 -8 sec. This state is called metastable state. • Population inversion: - Populations means number of active atoms occupying an energy state. Generally population of lower energy level is high and that of higher energy level is low. But for stimulated emission, population of higher energy level should be high. Making population of higher energy level more than that of ground state is called population inversion. N 1 – population of ground state, N 2 – population of higher energy state then making N 2>>N 1 is called population inversion.

• Pumping: - The process of raising atoms from lower energy state to higher excited energy state is called s pumping. • Optical pumping: -