The document discusses four-level lasers, explaining the process of light amplification through stimulated emission and the concept of population inversion. It details the energy levels involved, the differences between three-level and four-level lasers, and provides examples like the helium-neon gas laser. Four-level lasers are noted for their lower lasing threshold, higher efficiency, and ability for continuous operation.

1. FOUR LEVEL LASER
SOUSAN
A274153121006
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2. INTRODUCTION
➢Light amplification by stimulated emission of radiation
➢A device that generates an intense beam of coherent
monochromatic light (or other electromagnetic radiation) by
stimulated emission of photons from excited atoms or
molecules.

3. POPULATION INVERSION
➢Population inversion is the state of a system in which the population
of a particular higher energy state is more than that of a specified
lower energy state.
➢A system in which population inversion is achieved is called an active
system. The state of the system in which population inversion is
achieved is called the negative temperature state.

4. ➢Based on the mechanism in which population
inversion is achieved:
1. Three-level laser
2. four-level laser

5. FOUR LEVEL LASER
➢Four energy level energies
➢E1, E2, E3, E4
➢Populations
➢N1, N2, N3, N4
➢The energies are:
➢E1 < E2 < E3 < E4
➢The level E3 and E2 are intermediate levels
➢E3 = Upper metastable, E2 = Lower metastable

7. ➢The atoms are pumped from ground level E1 to excited state E4
➢The atoms decay by a fast, non-radiate transition(Ra) to level E3
➢At this stage, level E3 is virtually vacant
➢The lifetime of laser transition from E3 to E2 is long in comparison
to Ra
➢Population inversion between level E3 and level E2 is established as
soon as the pump energy is applied to the system
➢Laser gain is realized as the spontaneous photon of energy initiate
chain of stimulated emission
➢Atoms of E2 lose their excess energy by mostly non- radiative
transition (Rb) to level E1

9. Differences between 3 Level laser and Four
level laser

10. EXAMPLES OF FOUR LEVEL LASER
➢Helium Neon gas laser
➢Electric pumping excites helium atoms to an excited state
whose energy is roughly same as upper short lived state in the
neon atom
➢The sole purpose of the helium atoms is to exchange energy
with neon atoms via collisional excitation

11. ADVANTAGES OF FOUR LEVEL LASERS
➢The lasing threshold of a four-level laser is lower
➢The efficiency is higher
➢Required pumping rate is lower
➢Continuous operation is possible

12. THANK YOU