Fyranivålasrar

Fyranivålasrar, or four-level lasers, denote laser systems that use a four-level energy scheme in which the lasing transition occurs between an upper laser level and a lower laser level that decays rapidly to the ground state. In typical operation, optical or electrical pumping excites atoms or ions from the ground state to a higher pumping level, which quickly decays nonradiatively to the upper laser level. Population inversion is established between the upper laser level and the lower laser level, and stimulated emission produces the laser light as the upper level decays to the lower one. The crucial feature of four-level lasers is that the lower laser level is rapidly depopulated to the ground state, reducing reabsorption and allowing lasing at lower pump power than in three-level systems.

In contrast, a three-level laser uses the ground state as the lower laser level, making threshold higher and efficiency more sensitive to pumping. Four-level designs therefore tend to have lower lasing thresholds and can achieve higher slope efficiency, especially in gases and some solid-state media.

Common examples include the helium-neon laser and many solid-state lasers such as Nd:YAG systems when treated as a four-level scheme. The concept also informs diode-pumped solid-state lasers where the lower laser level is fast-relaxing. Practical operation must consider lifetimes, nonradiative losses, and collisions that can modify the idealized scheme.

Historically, the four-level model helped explain why certain transitions dominate and guided the design of practical, efficient laser sources in the mid-20th century and onward.