Point 12 of 27 — Lesson 1: Laser Physics, Population Inversion and the Helium-Neon LaserQuestion bank: 1 questions (1 with pictures)
12Laser action step by step
The pump raises many atoms of the active medium to excited levels, and, through a metastable level, produces a population inversion.
Some atoms decay spontaneously, emitting photons in all directions. Photons that do not travel along the axis leave the sides of the tube and are lost.
Photons travelling along the axis stimulate other excited atoms to emit identical photons. These photons are reflected by the mirrors and pass again and again through the medium, causing new stimulated emission each time. The number of identical photons grows (amplification).
When the beam becomes strong enough, a fraction leaves through the semitransparent mirror as the laser beam. The rest remains in the cavity to continue stimulating atoms.
Meanwhile the pump keeps refilling the excited level, so the process continues.
Why is the resonant cavity needed?
It makes the photons pass through the medium many times, which increases the number of stimulated emissions (amplification) and also selects only the photons that travel along the axis, thereby giving a narrow, parallel, coherent beam.
Why must the active medium be in population inversion for a laser, but not for an ordinary lamp? In a lamp, light comes from spontaneous emission and needs no amplification.
In a laser, amplification by stimulated emission must beat absorption; this is possible only if excited atoms outnumber unexcited ones.
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