How to achieve population inversion?

Course Details

Field of Study: Electronics Engineering
Course Name: Optoelectronics
Subtitle: How to achieve population inversion?

Population Inversion:

  • Must have population inversion to sustain lasing.
  • More molecules in the excited state than in the ground state.
  • Population of molecules is inverted (relative to how the population normally exists).
  • Normally, there are more molecules in the ground state than in the excited state (need > 50 %).

Why is it important?
More molecules in the ground state –> more molecules that can absorb photons
Remember: absorption competes with stimulated emission
More molecules in the excited state –> net gain in photons produced

How to achieve population inversion?
Laser systems: 3-level or 4-Level
4-level is better and easier to sustain population inversion
3-level system: lasing transition is between Ey (excited state) and the ground state
4-level system: lasing transition is between two energy levels (neither of which is ground state)
All you need is to have more molecules in Ey than Ex for population inversion

Example of a 3 level system
3-level-system

Example of a 4 level system
4-level-system

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