## 3 Results and Discussion  
The output characteristics of the 1064 nm free-running laser were analyzed first. The output coupler was a 75% partial transmittance plane mirror, and the rear mirror was a high reflectivity plane mirror. The total cavity length was 280 mm. By adjusting the pumping current, the output energy of the free-running laser was measured for different currents, as shown in Figure 3.  
As shown in Figure 3a, the threshold current was 30 A. The highest output energy (1.45 J) was obtained at a pump pulse duration of 200 \(\upmu\)s, a pumping current of 120 A, and a pump repetition rate of 5 Hz. At a pump pulse duration of 230 \(\upmu\)s, the highest output energy (1.71 J) was obtained with a pumping current of 120 A and a pump repetition rate of 2 Hz, as shown in Figure 3b. Figure 3 also shows that when pump repetition rate was low (2 Hz or 5 Hz,) there was no obvious difference in output energy--but as the pump repetition rate increased, the output energy decreased. This result occurred because higher pump repetition rates caused a higher thermal load, resulting in decreased output energy due to the thermo-optical effect. Figure 3: Output energy _versus_ pumping current with a pumping pulse duration of 200 \(\upmu\)s (**a**) and a pumping pulse duration of 230 \(\upmu\)s (**b**). Figure 2: Cross-section of the LD module: (1) copper heat sink; (2) LD bar array consisting of 30 LD bars; and (3) Nd:YAG rod. During the second step of the experiment, we put the Brewster polarizer (BP), Q-switch, and \(\lambda\)/4 plate into the cavity in proper order and aligned them to be coaxial. We then measured the output characteristics of the 1064 nm Q-switched Nd:YAG laser, as shown in Figure 3.  
As shown in Figure 4a, output energy increased linearly when the pumping current was increased from 40 A to 60 A. When the pumping current increased further, the output energy increased more slowly, and the curve began to saturate. This behavior occurred because when the laser operated at high average power, the depolarization caused by thermally induced birefringence drastically distorted the output beam profile and drove down the output energy. As shown in Figure 4b, the pulse duration decreased when the pumping current was increased from 40 A to 60 A. When the pumping current was further increased, the pulse duration stabilized at 10 ns. Figure 4: Output characteristics of a 1064 nm Q-switched laser _versus_ pumping current: (**a**) output energy (**b**) pulse duration. Figure 5: Output characteristics for different output coupler (OC) transmittance levels _versus_ pumping current: (**a**) output energy (**b**) pulse duration. The output characteristics of different OC transmittance levels (\(T\) = 65%, 70%, 75%, 80%, 85%, 91% and \(>\)98%) were also measured. The parameters were set as follows: a pumping current of 80 A, a pump pulse duration of 200 us, and a pump repetition rate of 5 Hz. The output energy of 1064 nm laser reached no higher than 250 mJ when the transmittance of the output coupler equaled 70% or 91% and no higher than 200 mJ when the transmittance equaled 65%. When the transmittance of the output coupler was above 98%, there was no output energy obtained at all. Output energy _versus_ pulse duration at 75%, 80%, and 85% are shown in Figure 5.  
As shown in Figure 5, the output energy did not differ significantly for any of the three different transmittance percentages. When transmittance was 75%, however, the pulse duration of the laser was shorter than that of the others. To this effect, we selected a plane mirror coated for partial transmittance (\(T\) = 75%) at 1064 nm as the output coupler. The experimental results and theoretical results were compared, and the transmittance of OC (\(T\) = 75%) was very similar to the theoretical results (\(R\) = 20.2%, \(T\) = 79.8%). Notable differences between the results may have been caused by differences in the round-trip cavity loss between the actual value and the value that we assumed (\(L\) = 0.05).