A four-level laser system offers significant advantages over a three-level system, primarily in terms of efficiency and performance.
In a three-level laser system, the lasing medium must be pumped to the third energy level, which is the upper state for laser emission. After the photon emission occurs, the system returns to a lower energy level, which is only a little above the ground state. This means that a considerable amount of energy is needed to pump the system, and there’s often population inversion challenges, making it less efficient.
In contrast, a four-level laser system can be pumped to a higher energy level than in a three-level system. This extra level allows the laser to efficiently maintain a population inversion between the upper and lower lasing levels. The upper state of a four-level system is above the third energy level, meaning after the photon emission, the system quickly falls to the ground state, leaving more of the medium in the lasing state at any given time. This results in improved efficiency since less energy is required to maintain the lasing action.
Additionally, the four-level system generally allows for lower thresholds for lasing, which means they can operate more effectively and are often more robust in various conditions. These benefits make four-level laser systems preferable in many applications, particularly in solid-state and semiconductor lasers.