An alexandrite crystal is a broadband tunable laser-amplification-and-gain medium that exhibits excellent performance in the near-infrared band. Because of the continuous development and increasing commercial application of high-power red-light laser diode (LD) technology, LD-pumped alexandrite lasers have gradually attracted the interest of many researchers in the field of all-solid-state lasers. When an LD pumps an alexandrite crystal, a considerable amount of energy absorbed by the alexandrite crystal is converted into the thermal energy of the crystal beside the oscillation laser. This part of heat energy causes the thermal lens effect of the crystal and severely affects the output laser efficiency, stability of the resonant cavity, and quality of the output laser beam. To effectively reduce the influence of the thermal effect on laser output performance, three factors causing the thermal effect are analyzed in this study. The thermal focal lengths of an alexandrite crystal are calculated accurately by employing theoretical and experimental methods. The dips on the input-output curve of the F-P cavity are analyzed and explained based on the specific influence of the thermal focal length on the size of the fundamental laser mode. Finally, a method that can effectively reduce the influence of the thermal effect on the laser output performance in a certain range is developed.

The thermal effect in alexandrite laser is studied theoretically and experimentally by using an alexandrite crystal, 638-nm red-light high-power fiber-coupled LD, and F-P cavity as the laser gain medium, pump source, and research object, respectively. The following steps are performed. First, the three factors causing the thermal effect are analyzed and theoretically studied in detail by establishing the heat conduction model of the crystal, reasonably setting the boundary conditions, and solving the corresponding heat conduction equations, and the theoretical value of the thermal focal length under the corresponding factors is calculated by conducting software simulation. Then, the critical stability condition of the resonator is used to measure the actual thermal focal lengths of the crystal several times by conducting specific experiments, and the average value is taken as the final thermal focal length. The experimental value of the thermal focal length is consistent with the calculated theoretical value. Finally, based on the specific influence of the thermal focal length on the size of the fundamental laser mode, the dips on the input-output curves of the F-P cavity at three different cavity lengths are comprehensively analyzed, and a method that can effectively reduce the influence of the thermal effect on the laser output performance in a certain range is established.

In this study, the thin lens combination formula is applied to calculate the thermal focal length of the crystal and combine the thermal focal lengths generated under three conditions; the combined result, f, is used as the final thermal focal length of the crystal. Then, the calculated value of the thermal focal length becomes closer to the actual value [Fig. 6(a)]. In the actual thermal focal length measurement, an F-P cavity structure is built to measure the thermal focal length of the alexandrite crystal, and the measurement light path is easy to obtain (Fig. 5). Furthermore, the error can be reduced to an acceptable range by averaging multiple measurements. Based on the specific influence of the thermal focal length on the size of the fundamental laser mode(Fig. 9), the dips on the input-output curve of the F-P cavity at three different cavity lengths are explained and analyzed in detail. After the comprehensive thermal focal length, f_min, at the maximum pump power is calculated, the rear cavity length, L₂, can be made smaller than f_min by changing the cavity parameters to stabilze the resonant cavity in the entire pump-power range (Fig. 7). This is important for reducing the influence of the thermal lens effect and improving the performance of the laser.

In the F-P cavity with a small anterior cavity length (L₁), the thermal focal length, f, should be larger than the rear cavity length, when the resonator is in a stable working state. Furthermore, the thermal focal length, f, equals the rear cavity length, L₂, when the resonant cavity is in a critically stable working state. The analysis of the influence of the thermal focal length on the laser output characteristic curve under different pump powers reveals that to stabilize the resonant cavity output in the entire pump-power range, L₂ can be changed after calculating the thermal focal length, f_min, at the maximum pump power.