In thermal equilibrium the lower level of any transition is always more populated, so light is absorbed. Pumping energy into the medium can push more atoms into the upper level than remain in the lower one, and then stimulated emission outpaces absorption and light is amplified. A simple two-level system cannot be inverted by optical pumping alone, which is why practical lasers use three- or four-level schemes.
Pockels cell holds the cavity closed for 200 microseconds so the rod can build up population inversion before we dump it.
Also heard as
- inversion
Related terms
Stimulated emission
Emission of a photon by an excited atom, triggered by an incoming photon and identical to it in color, phase and direction.
Four-level laser
A laser whose lower laser level sits above the ground state and empties quickly, so inversion is easy to reach.
Pump
The energy source that excites a gain medium, or the act of supplying it, usually with diodes, flashlamps or current.
Gain medium
The material inside a laser or amplifier that amplifies light once it is pumped: a crystal, glass, gas, semiconductor or doped fiber.
Q-switching
Producing short, intense laser pulses by holding cavity losses high while energy builds up, then suddenly dropping them.