Population inversion is an important concept for lasers optics, as it is essential for light amplification by stimulated emission. Here we explain the concept using a rare-earth-doped optical fibers; however, the principle is general and can be applied for other types of lasers, amplifiers, and superluminescent light sources (e.g. semiconductor laser and SLD).

Figure 1: Schematic of population inversion and amplification of light in rare-earth-doped fiber.
(a) Light absorption without population inversion, and (b) light amplification with population inversion.

Figure 1 schematically shows light absorption and amplification in a rare-earth-doped fiber. The fibers shown in Figure 1(a) and (b) have the same geometry, and are doped with the same amount of rare-earth elements; however the number of ions in the two energy states are different. In Figure 1(a) the number of ions in the lower energy state (N1) are smaller than that of the upper energy state (N2); i.e, N1>N2. On the other hand, in Figure 1(b), N2>N1. Roughly speaking, these two situations correspond to (a) weak pumping, and (b) strong pumping.

If the pumping is not enough and population inversion is not created, the absorption of light (by the ion in the lower energy level) surpasses the amplification and thus the light is attenuated [see Figure 2(a)]. On the other hand, if population inversion is created, amplification by stimulated emission (by the ions in the upper energy state) surpasses absorption and thus the light is amplified [see Figure 2(b)].