## 3 Results and Discussion  
The input-output characteristics in Q-switched operation are shown in Figure 2. The maximum output energies of 22.0 mJ/pulse, 19.1 mJ/pulse, 17.7 mJ/pulse and 15.1 mJ/pulse were obtained with no saturation when the repetition rates were 100 Hz, 150 Hz, 200 Hz and 250 Hz, respectively. According to the theoretical analysis of transient thermal distribution of repetitively pumped laser by [PERSON], the temperature in the center of the laser rod increases with the increase of the repetition rate [24]. Serious heat accumulation at higher repetition rate will broaden the fluorescent spectrum and shorten the upper-level lifetime, leading to the decrease of laser output. The higher pump current was not tested to avoid the damage to the Nd:YAG crystal. The lasing thresholds were about 45 A for different repetition rates. The spatial profile of pump beam at the front surface of BGSe crystal was measured as shown in the inset of Figure 2 (linear in spatial dimensions and color scale). The 1064 nm pump beam diameter was 2.5 mm (vertical direction) \(\times\) 1.0 mm (horizontal direction). The spot size in horizontal direction was shorter than in vertical direction due to the aberration caused by the Brewster polarizer. The input-output characteristics of the generated mid-infrared wave at 4.06 \(\upmu\)m with different repetition rates are shown in Figure 3a. With the increase of repetition rate, the LIDT decreased rapidly. When the repetition rate was above 150 Hz, damage on the surface of the BGSe crystal was observed with pump energy higher than 9 mJ/pulse (corresponding to peak intensity of about 20 MW/cm\({}^{2}\)). For higher repetition rate of 500-1000 Hz, optical damages were much more intense, which limited the further improvement in the repetition rate of the BGSe-OPO. The LIDT is lower than our recent report on the low repetition rate (10 Hz) experiment [25]. The thermal effect caused by the extra crystal absorption to the pump wave and the relatively low beam quality from the side-pumped laser were other possible reasons. Figure 1: Schematic of BaGa\({}_{4}\)Se\({}_{7}\)-OPO. The inset shows the photograph of the BaGa\({}_{4}\)Se\({}_{7}\) crystal and the transmittance curve (Adapted with permission from Ref. [23]. Copyright © 2019, American Chemical Society). The maximum output energy of 1.28 mJ/pulse was achieved at pump energy of 10 mJ/pulse with a repetition rate of 100 Hz. The maximum output average power was 250 mW with a repetition rate of 250 Hz. A slight increase in the OPO threshold was observed with the increase of the repetition rate. The OPO thresholds were measured to be 2.65 mJ/pulse, 3.34 mJ/pulse, 3.54 mJ/pulse and 3.61 mJ/pulse for the repetition rate of 100 Hz, 150 Hz, 200 Hz and 250 Hz, respectively. Figure 3b shows the conversion efficiencies of the BGSe-OPO with different repetition rates. The maximum conversion efficiency of 12.8% was obtained for the repetition rate of 100 Hz. For different repetition rates of 100 Hz, 150 Hz, 200 Hz and 250 Hz, the slope efficiencies were measured to be 16.4%, 17.0%, 14.4% and 16.6%, respectively. No saturation phenomenon was observed because the low pump peak intensity was limited by the LIDT. Further optimization to pump beam quality could improve the output characteristics of the BGSe-OPO. The output energy and conversion efficiency of BGSe-OPO decreased with the increase of repetition rate due to the severe thermal effect. It is especially significant for the mid  
Figure 3: (a) The input-output characteristics of the BGSe-based DP-SRO with different repetition rate; (b) The conversion efficiency of the BGSe-based DP-SRO with different repetition rate. Figure 2: Input-output characteristics of side-pumped Nd:YAG laser. Inset: spatial profile of pump beam at the front surface of BGSe crystal. infrared crystal with poor thermal conductivity like AgGaS\({}_{2}\) (1.4 W m\({}^{-1}\) K\({}^{-1}\)), AgGaSe\({}_{2}\) (1.0 W m\({}^{-1}\) K\({}^{-1}\)) and BaGa\({}_{4}\)Se\({}_{7}\) (0.56 W m\({}^{-1}\) K\({}^{-1}\)) [11, 18].