Optical Pumping: Three-level And Four-level Systems.

A two-level system cannot achieve population inversion through optical pumping. This is because it can only absorb pump photons—and therefore gain energy—when population inversion has not yet been established, meaning optical amplification cannot occur. The fundamental limitation arises from stimulated emission induced by the pump light itself.

A three-level system can achieve population inversion through optical pumping. In this configuration, atoms are excited from the ground state to a higher energy level by pump light with a shorter wavelength (higher photon energy). The excited atoms then relax to an intermediate energy level—the upper laser level—through spontaneous emission or non-radiative processes. From this upper laser level, atoms can undergo stimulated emission and transition back to the ground state.

If the pump intensity is sufficiently high, population inversion can be established between the upper laser level and the ground state. This is possible because the transition to the upper laser level suppresses stimulated emission caused by the pump light. Quasi-three-level gain media will be discussed separately in later sections.

A four-level system, such as Nd:YAG, achieves laser gain much more efficiently. In this case, the lower laser level is located slightly above the ground state. Since atoms in the lower laser level rapidly decay to the ground state—typically through non-radiative transitions—the population of the lower laser level remains extremely low.

As a result, only a moderate population in the third energy level (the upper laser level), generated by relatively modest pump power, is required to achieve population inversion and optical amplification. This makes four-level laser systems significantly easier to operate and more efficient than three-level systems.