## 3 Results and Discussion
The measured and simulated MIR idler energy and o-o conversion efficiency as the functions of the input pump energy is displayed in Figure 3. The black-filled circles and black solid diamonds describe the experimental data recorded by an energy meter (OPHIR, PE10). As depicted in the curves, the measured threshold pump energy of the 30 mm length BGSe-OPO cavity was about 10 mJ, which translated into the axial energy fluence of about 0.035 J/cm\({}^{2}\). With the increase in pump energy, the output pulse energy and o-o conversion efficiency also increased monotonically. At the pump energy of \(\sim\)79.9 mJ, the output pulse energy of the idler wave was 6.07 mJ, which corresponded to a maximum o-o conversion efficiency of \(\sim\)7.6%. Then, as the pump energy rose, the o-o conversion efficiency began to drop while the MIR output pulse energy continued to grow. The idler pulse energy reached its maximum value of 6.76 mJ at a 10 Hz repetition rate with the continuously increasing pump energy to 93.5 mJ, which represented an o-o conversion efficiency of 7.24%, and then the output energy fell gradually for higher pump levels. That means the higher pump energy could not further enhance the output pulse energy. The rollover behavior could be dominantly attributed to the thermal effect due to the absorption of the interaction beams inside the BGSe crystal in the OPO process. The accumulated heat elevated the temperature in the BGSe crystal. In particular, for high energy levels, the rise of temperature shall cause the change of the three waves refractive indices in BGSe crystal, which brought about the phase mismatching, thus leading to a decrease in the idler output energy at 6.45 \(\upmu\)m. The achieved idler output energy and o-o conversion efficiency are almost six and three times higher than the previous result reported in [19], respectively. Furthermore, no optical damage was observed in the BGSe crystal and other optical components during the experiment. Meanwhile, the simulated idler output energy (red solid line) and o-o conversion efficiency (blue solid line) as the functions of the input pump energy are also depicted in Figure 3. It can be seen that the measured data are in close agreement with the calculated results for the pump energy of less than 62 mJ. Afterward, the o-o conversion efficiency in theoretical simulations showed saturation phenomena such as the experimental measurement. However, with the pump energy further increasing to around 93 mJ, the calculated results were a little higher than the measured values. Moreover, no rollover behavior  
Figure 3: Measured and simulated MIR idler output pulse energy at 6.45 \(\upmu\)m and o-o conversion efficiency versus input pump energy at 1064 nm. was exhibited in the theoretical simulation. The deviation could be mainly attributed to ignoring the thermal effect in the numerical model. The pump and signal spectral intensity profiles recorded were monitored simultaneously by an optical spectrometer (Ocean Optics, NIR Quest 256, 800-2500 nm) at the maximum output energy. Both the pump and signal beams were collected in the same place by a coupling fiber. As shown in Figure 4, the measured spectrum consists of two parts with the central wavelength of 1064.3 nm and 1274.5 nm for the pump and the signal of the BGSe-OPO, respectively. Moreover, the energy conversion condition in the OPO process can be written by:  
\[\frac{1}{\lambda_{p}}=\frac{1}{\lambda_{s}}+\frac{1}{\lambda_{i}} \tag{6}\]  
where the relation is \(\lambda_{p}<\lambda_{s}<\lambda_{i}\). The measured pump wavelength \(\lambda_{p}\) was 1064 nm and the signal wavelength \(\lambda_{s}\) was 1274 nm. Thus, the corresponding idler wavelength \(\lambda_{i}\) was retrieved to be 6.45 um. In addition, Figure 5 illustrates the temporal profiles of pump and idler pulses under the maximum output energy. They were measured by InGaAs (DET10A/M, THORLABS, rise time \(<\)1 ns, 200-1100 nm) and HgCdTe (PDAVJ10, THORLABS, 2.0-10.6 um, bandwidth 100 MHz) detectors, respectively.