###### Abstract  
Despite the importance of improving the efficiency of lasers in order to expand their utility range, the improvement of the efficiency of Ti:sapphire lasers has not progressed due to their high crystal losses. Therefore, we improved the efficiency of CW Ti:sapphire lasers by high-intensity pumping, which is one of the most effective methods of suppressing the efficiency reduction due to losses. Using a easily commercially available 0.25 wt.%, figure of merit (FOM) 200 Ti:sapphire crystal, optics and a pump source, we achieved an optical-to-optical conversion efficiency of 32.4% with a slope efficiency of 42.5% at an incident pump power of 5.0 W which corresponds the maximum pumping intensity of 860 \(\mathrm{kW}/\mathrm{cm}^{2}\). Furthermore, we ensured the reliability of our theoretical analysis by reproducing the experimental results. From this reliable theory, double-pass pumping and increasing the pump power to 25 W resulted in the highest optical-to-optical conversion and slope efficiencies for the incident pump power of 55.9% and 59.6%, respectively, at a high intrinsic residual loss of 4.0%. Even if losses doubled or deviated from the optimum condition for the highest efficiency, the efficiency reduction due to these factors was only a few percent. These results show that with high-intensity pumping, lasers with efficiencies well exceeding half of the quantum limit can be achieved even if all components, including Ti:sapphire crystals, are easily commercially available. T:sapphire laser; high efficiency; high-intensity pumping; low FOM; commercially available crystal +
Footnote †: journal: Applied  
the efficiency of CW lasers close to their quantum efficiency of approximately 70%, which is the efficiency limit. However, the disadvantages of Ti:sapphire crystals in improving laser efficiencies are higher losses and smaller products of the emission cross-section \(\sigma_{\text{em}}\) and fluorescence lifetime \(\tau\). The efficiency reduction due to losses is particularly high in gain media with small products, such as Ti:sapphire crystals. Therefore, usage of low-loss Ti:sapphire crystals with a high figure of merit (FOM) as laser gain media easily enables high efficiency. However, the loss increases with a higher doping concentration of Ti\({}^{3+}\)-ions, and the maximum FOM becomes lower as the doping concentration increases. The loss also does not reach a negligibly low level even when the doping concentration is reduced within the range where the pump absorption is sufficient for lasing [16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. According to our investigation, the maximum FOM of normally commercial crystals is approximately 200 at a Ti\({}^{3+}\)-ion doping concentration of 0.25 wt.%. To purchase low-loss crystals with FOM exceeding 250 (from CRTUR [17], OPTOGAMA [18], CASTECH [19], NEWLIGHT Photonics [20]) or 300 (from Altechna [21], Roditi [22], EKSMA OPTICS [23], Opt City [24], CRYiNK [25]), special orders must be placed with crystal growth companies. This makes them extremely expensive, if available at all. Regarding high-efficiency CW Ti:sapphire lasers, first, in 1991, [PERSON] et al. achieved a high optical-to-optical conversion efficiency of 28.0% and a high slope efficiency of 32.0% by Ar-ion laser-pumping with unknown FOM [9]. The highest optical-to-optical conversion efficiency of 35% and slope efficiency of 40% of Ar-ion-laser-pumped CW Ti:sapphire lasers were achieved by [PERSON] et al. in 1994 [10]. However, they used extremely low-loss commercially unavailable crystals with a high FOM of 1000, which were obtained through collaboration with Union Carbide. Since Union Carbide's products do not currently contain Ti:sapphire crystals, such extremely low-loss crystals would not be available, even if specially ordered [26].