## 3 Results  
A comparison of CW laser performances with 5%, 8% and 20% transmissions OCs of the Er:Y\({}_{2}\)O\({}_{3}\) medium was first made. Figure 2 shows dependences of output powers on the absorbed pump power. The absorbed pump power was calculated by multiplying the incident pump power by the absorption efficiency, which was measured under non-lasing condition (without existence of the laser resonator) to be \(\sim\)90%. The threshold pump powers for the 5%, 8% and 20% transmission OCs were \(\sim\)1.1 W, \(\sim\)1.4 W, and \(\sim\)3 W, respectively. At pump powers of less than 14 W, the output powers increased linearly with the pump power for all three OCs, and the 8% OC yielded the highest slope efficiency of 10.2% with respect to absorbed pump power. When the pump power was above 14 W, the laser became slightly less efficient due to the increased thermally induced losses. Nevertheless, an output power of over 1.8 W was obtained at 19 W of pump power with the 8% transmission OC. According to the laser properties with different output couplers, the distributed cavity losses (excluding the output coupling loss) were calculated to be \(\sim\)1.67% cm\({}^{-1}\) using the well-known Findlay-Clay method, comprising the thermally induced losses and the losses of the cavity mirrors and ceramic sample (\(\sim\)1.2% cm\({}^{-1}\) estimated though the in-line transmission spectrum of a 1.3-mm-long Er:Y\({}_{2}\)O\({}_{3}\) sample). The attainable output power should be readily increased by improving the cooling system and reducing passive losses of the ceramic sample. The Q-switched operation performances were studied with the same output couplers. An HgCdTe detector (PVM-10.6, VigoSystem S. A) with a rise time of 1.5 ns and an oscilloscope (DSO104A, Keysight) with a bandwidth of 1 GHz were utilized to measure the pulse characters. The threshold for Q-switched operation was the same as that for the CW operation. Not too far above the threshold, stable pulse trains were observed for all three OCs. The pulse characters were then optimized by changing the distance between the center of the mechanical switch and the optical axes of the cavity, and the optimal distance was found to be \(\sim\)50 mm. The pulse duration and pulse energy as functions of the pulse repetition frequency for all three OCs are shown in Figures 3 and 4. With the 5% and 8% transmission OCs, single pulses with 40 ns duration and 30.8 \(\mu\)J energy, and 37 ns duration and 46.1 \(\mu\)J energy were obtained at a 0.26 kHz pulse repetition frequency (corresponding to the rotating speed of  
Figure 2: Continuous-wave (CW) output power vs. absorbed pump power with different output couplers. 130 rounds per seconds and a slit number of 2) under a pump power of 3.2 W. With the increase in repetition frequency, the pulse duration increased while the pulse energy decreased. Nevertheless, pulse durations of less than 50 ns were obtained in the range of 0.26-4 kHz with both OCs. When further increasing the pump power, multi-pulse operation occurred. With the 20% transmission OC, shorter pulse duration and larger pulse energy were achieved. This is because a higher output coupling can delay the occurrence of multi-pulse operation in mechanically Q-switched lasers. In this case, multi-pulse operation occurred when the pump power was above 10 W. Under 10 W of pump power, the pulse duration increased from 27 to 38 ns, whereas the pulse energy decreased from 80.8 to 27.5 \(\upmu\)J with increasing the repetition frequency from 0.26 to 4 kHz, corresponding to \(\sim\)3 kW peak power at 0.26 kHz repetition frequency. The optical-optical conversion efficiencies in the Q-switched mode could be effectively improved using a larger plate carved with more slits but the same turn-off area and/or a motor with higher rotating speed to make the laser operate at a higher repetition rate. Improvement in the Q-switched efficiencies is also achievable through optimizing the pump wavelength to avoid the excited state absorption of the upper laser level during the pumping process.