In a four-level scheme, pumping lifts ions to a high band, they relax to the upper laser level, lase down to a lower level well above the ground state, and that level drains rapidly. Because the lower laser level is nearly empty, even a small upper-level population gives inversion and threshold is low. Nd:YAG at 1064 nm is the textbook case; ruby is a three-level laser and Yb and Er are quasi-three-level.
It's a clean four-level laser, so threshold barely moves when the crystal warms up, unlike the Yb disk next door.
Also heard as
- four-level system
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.
YAG
Shorthand for yttrium aluminum garnet, and in practice for the Nd:YAG laser at 1064 nm.
Gain medium
The material inside a laser or amplifier that amplifies light once it is pumped: a crystal, glass, gas, semiconductor or doped fiber.
Lasing threshold
The pump level at which gain exactly balances resonator losses and laser output begins.