Lasers and stimulated emission explorer
Pump atoms in a gain rod, watch population inversion build in the metastable level and see stimulated photons grow into a coherent beam between the mirrors.
Learning goals
- Explain excitation, spontaneous emission and stimulated emission.
- Explain metastable states, pumping, population inversion, threshold, optical feedback, output coupling and cavity modes.
- Relate laser operation to coherence, linewidth, divergence, irradiance, photon quantities and a helium-neon example.
Three-level atoms pumped at 20% between two mirrors. 0% of atoms are in E₂ and 100% in E₁; 0 photons are in the rod.
- Atoms in E₁
- 100 %
- Atoms in E₂
- 0 %
- Population inversion
- no
- Photons between the mirrors
- 0
- Output beam
- 0 photons/s
Try this
0 of 4 doneRaise the pump until a strong beam builds up between the mirrors. (not done yet)
Inversion alone is not enough: the gain must beat the losses. Then each photon makes copies on every pass, all along the axis and in step: coherent light.
Make E₂ short-lived and pump the three-level atoms at 80% or more. (not done yet)
Atoms now leave E₂ as fast as they arrive, so they cannot pile up there: a laser needs a metastable level.
Pump two-level atoms as hard as you can for 15 s. (not done yet)
The pump drives atoms down as often as up, so E₂ never holds more than half of them: no inversion, no laser.
Take the mirrors away with the pump on. (not done yet)
Without feedback each photon crosses the rod once. There is inversion but too little amplification: no beam, only a weak glow in all directions.
You’ve tried every task.