When a photon whose energy matches an atomic transition passes an excited atom, it can trigger the atom to drop to the lower level and emit a second photon that copies the first in wavelength, phase, polarization and direction. This is the process that makes optical gain and coherent laser light possible. It competes with absorption, so net amplification only happens when more atoms sit in the upper level than the lower one.
Below threshold you're just seeing fluorescence, stimulated emission only takes over once the round-trip gain beats the losses.
Also heard as
- induced emission
Do not mix it up with
- ASE: ASE starts from random spontaneous emission that is then amplified, so it lacks the single-mode coherence of a laser beam.
Related terms
Population inversion
The condition where more atoms occupy the upper laser level than the lower one, which a medium needs to give gain.
Gain medium
The material inside a laser or amplifier that amplifies light once it is pumped: a crystal, glass, gas, semiconductor or doped fiber.
ASE
Amplified Spontaneous Emission: random spontaneous photons amplified in a gain medium, a broadband noise background.
Lasing threshold
The pump level at which gain exactly balances resonator losses and laser output begins.