Figure 1
APS/Alan Stonebraker

Figure 1: Schematic energy level diagram of a typical 4-level laser. Level 2 is excited by a pump photon (blue), and in turn populates level 3. De-excitation to level 4 emits laser photons (red). However, level 3 can also transfer energy to a triplet state, which can absorb some of the laser photons by exciting to a higher level.