## 3 Experimental Results and Discussion  
With a HR mirror replacing the SESAM, CW lasing characteristics of the Er:Lu\({}_{2}\)O\({}_{3}\) ceramic were evaluated at first. The output power as a function of the absorbed pump power is shown in Figure 2. The ceramic laser reached threshold at an absorbed pump power of 1.7 W. Under an absorbed pump power of 9.4 W, a maximum output power of 0.95 W was obtained, corresponding to a slope efficiency with respect to the absorbed pump power of 12.9%. Then we used the SESAM as the SA and cavity end mirror. The average output power of the Er:Lu\({}_{2}\)O\({}_{3}\) ceramic laser in Q-switched mode of operation is presented in Figure 2. It can be seen that the threshold was increased to 2.3 W and the average output power increased almost linearly with respect to the absorbed pump power up to the maximum pump power of 9.4 W. Under the absorbed pump power of 9.4 W, the laser produced an output power of 0.69 W at 2716 nm with a spectrum bandwidth of \(\sim\)0.2 nm, as shown in the inset of Figure 2, corresponding to a slope efficiency of \(\sim\)9.7%. The decreased slope efficiency compared to the CW operation can be attributed to the intrinsic non-saturable losses of the SA. With an infrared detector (PVI-10.6, \(\sim\)1.5 ns response time) monitoring the laser temporal behavior, the real-time laser output pulses were recorded by a digital oscilloscope (Keysight DSO104A, bandwidth: 1 GHz). The measured pulse duration and the pulse repetition rate as a function of the absorbed pump power are presented in Figure 3. The pulse duration was 81 ns near the threshold and shows a slight decrease with an increase in the absorbed pump power, while the repetition rate monotonically increased from 3.1 kHz to 71 kHz as the absorbed pump power varied from 2.3 W to 9.4 W. At an absorbed pump power of 9.4 W, the minimum pulse width of 70 ns was obtained, which is nearly an order of magnitude narrower than previously reported for the PSQ Er:Lu\({}_{2}\)O\({}_{3}\) ceramic laser [29]. Given that the PSQ laser properties related to the SA, the improved performance in the present work can be attributed to the following reasons: low non-saturable loss of the Bragg-reflector-based SESAM itself results in a higher laser gain, while matching the semiconductor band-gap energy with the lasing photon energy further decreases the saturation fluence and the saturation carrier density of the SA, and thus, bleaching the SA is achievable with fewer laser photons, which in turn results in a decreased intracavity loss and an increased laser net gain [30; 31]. Figure 1: Experimental setup for the passively Q-switched Er:Lu\({}_{2}\)O\({}_{3}\) ceramic laser. These cascading effects in the PSQ Er:Lu\({}_{2}\)O\({}_{3}\) ceramic laser result in a dramatic increase in output power and lasing efficiency, and a shorter Q-switched laser pulse width [32]. Figure 4 shows the pulse energy and the corresponding pulse peak power as a function of the absorbed pump power. The pulse energy and the peak power show a monotonic increase with respect to the absorbed pump power. Since the absorber is fully saturated, the pulse energy and peak power do not depend noticeably of the pump power. Under an absorbed pump power of 9.4 W, the pulse energy was found to be \(\sim\)9.8 u\(\parallel\) and corresponds to a peak power of 0.14 kW. To the best of our knowledge, this is the highest peak power reported for the Q-switched Er:Lu\({}_{2}\)O\({}_{3}\) laser [23]. The typical pulse profile and the corresponding pulse train for the maximum output power is shown in Figure 5. Pulse to pulse amplitude fluctuation was estimated to be less than 5%. As a comparison, Table 1 summarizes the results ever produced from the PSQ Er:Lu\({}_{2}\)O\({}_{3}\) lasers. The Er:Lu\({}_{2}\)O\({}_{3}\) ceramic laser in present work provides similar pulse energy and average output power to the crystal laser, but higher pulse peak power and five times decrease in pulse duration [21; 22; 23; 29].