Featured Application
This paper demonstrates the effectiveness of high-gain by high-intensity pumping in increasing the efficiency of solid-state lasers. First, using simple theory, it is shown that the temperature increase of the gain medium, which reduces the efficiency of quasi-three-level (quasi-four-level) lasers such as Yb lasers, increases proportionally to the pump power, but the increase is less than proportional to the pump intensity. It is shown that even with a microchip only 1 mm thick and without high cooling efficiency (thermal conductance), a Yb:YAG gain medium with normal thermal conductivity, and a normally used copper heat sink, the local temperature increase in the gain medium is low even at high pump intensity if the pump power is low. Second, it is shown both experimentally and theoretically that the laser mode tunability, which is essential for high efficiency, is well-preserved even under extremely strong thermal focusing due to high-intensity pumping. These tunability properties are sufficient for high-laser efficiency so that the thermal lensing effect of high-intensity pumping does not cause any efficiency reduction. The efficiency of conventional solid-state lasers can be improved by the simple technique of high-intensity pumping, which does not require expensive elements, complex configurations, or skilled adjustments, thus contributing to the cost reduction of high-efficiency solid-state lasers.
Abstract
To increase the efficiency of laser oscillators by overcoming losses, the unsaturated gain must be increased. For this purpose, high-intensity pumping, typically higher than 100 kW/cm2, is effective. However, the temperature increase and strong thermo-optic effects of the gain medium have been pointed out as obstacles to high efficiency in solid-state lasers. Therefore, the effect of high pump intensity on the laser mode tunability required for high efficiency is investigated by studying the dependence of the laser threshold on the cavity length using a continuous-wave hemispherical short-cavity laser. The results show that the laser mode can be tuned with sufficient range and precision for high efficiency under various loss conditions regardless of the high pump intensity and are in qualitative agreement with a simple theory. Furthermore, according to the heat transport theory, microchip Yb:YAG, the gain medium of this study, does not have a high cooling efficiency, but the maximum temperature increase is estimated to be only about 12 K despite the high pump intensity of about 110 kW/cm2. This is because it is the pump power, not the pump intensity, that is proportional to the temperature increase, as the maximum pump power is only 900 mW. These results indicate that high-intensity pumping is a promising approach to achieve efficient lasing at low cost.