Power conversion efficiency of the laser.

To achieve high power conversion efficiency, several key factors must be considered, especially in optically pumped solid-state lasers:

• The gain medium should be sufficiently long and highly doped to ensure efficient absorption of the pump light.

• For (quasi-)three-level gain media, the gain medium should not be excessively long, as this would lead to strong reabsorption losses.

• The laser cavity losses should be minimized to maintain a low threshold pump power. In addition, the primary cavity loss should come from the output coupler transmission, ensuring high output coupling efficiency so that most generated photons are converted into useful laser output rather than dissipated through parasitic losses.

• Parasitic losses should be reduced as much as possible by using highly reflective mirrors, optical materials with low scattering and absorption losses, and by minimizing the number of optical surfaces within the resonator.

• The resonator should provide an appropriate mode field, and the pump light should be confined to the resonator mode volume to maximize overlap between the pump and laser modes.

• The gain medium should avoid detrimental processes such as parasitic absorption (including excited-state absorption), as well as excitation quenching caused by cross relaxation or upconversion. In addition, the system should maintain a low quantum defect, meaning the ratio of laser photon energy to pump photon energy should be as high as possible.

These considerations demonstrate that optimizing power conversion efficiency requires balancing multiple design factors, often involving subtle engineering trade-offs.