## 3 Results  
According to the above experimental parameters, the output characteristics of MOPO at different repetition rates were studied with the transmittance of M1 of 40% and the maximum pump power of 34.5 W (measured by a power meter of OPHIR F150A-BB-26-PPS). We used the Fourier spectrometer (ARCoptix, wavelength range from 1 \(\upmu\)m to 5.6-\(\upmu\)m, spectral resolution of 0.01 ppm with linewidth \(<\) 10 GHz) produced by American Thorlabs Company; the wavelength was synchronously collected at 3.3 \(\upmu\)m and 3.8 \(\upmu\)m, when the repetition rate changed from 20 to 100 kHz, corresponding to the wavelength of the near-infrared output at 1.47 \(\upmu\)m and 1.57 \(\upmu\)m, respectively. The mid-infrared laser spectrum with a repetition rate of 100 kHz is shown in Figure 3. Note that when the repetition rate changed from 20 to 100 kHz, the output wavelength and linewidth showed no obvious variation. The measured maximum output power of the dual-wavelength mid-infrared lasers when the repetition rate changed from 20 to 100 kHz is shown in Figure 4. As can be observed, the power of the 3.84 \(\upmu\)m mid-infrared laser increased gradually with the enhancement of repetition rate; when the repetition rate was 63 kHz, the maximum output powers of 2.11 [EMAIL_ADDRESS] \(\upmu\)m and 4.49 [EMAIL_ADDRESS] \(\upmu\)m were obtained, and the mid-infrared total power was 6.6-W, corresponding to the optical-optical conversion efficiency of 19.1%. With  
Figure 3: The dual wavelength mid-infrared laser spectrum. Figure 2: Polarization structure and phase mismatch of MgO:APLN. the continuous increase in repetition rate, the power of the 3.84 um mid-infrared laser decreased gradually, and the gain competition of two parameter outputs in the cavity became more and more intense; however, for the 3.3 um laser, the change in repetition rate had a gentle effect on its output power. When the repetition rate was less than 10 kHz or higher than 110 kHz, the 3.84 um was not achieved throughout the process because the parametric optical gain of OPO1 (1.57 um, 3.30 um) was higher than that of OPO2 (1.47 um, 3.84 um) in the process of multiple optical parametric oscillation and energy conversion. The unbalanced output powers between 3.30 um and 3.84 um were caused by fierce gain competition and reversal conversion in a cavity. We used a pyroelectric array camera (OPHIR PyrocamIII) to measure the exit beam distribution of 3.30 and 3.84 um. Figures 5 and 6 show the output power and beam qualities of two mid-infrared lasers at different repetition rates. When the repetition rate was 63 kHz, the maximum power of two mid-infrared outputs was realized; meanwhile, the slope efficiency was improved with an increase in the pump power, which proves that under the modulation of the optimal repetition rate, the gain obtained by the two parameter lasers oscillation process was optimally distributed, and the problem of gain competition was  
Figure 4: The output power of mid-infrared lasers at different repetition rates. (**a**) The output power of mid-infrared lasers @20–100 kHz. (**b**) The total power of dual-wavelength @60–70 kHz. effectively solved. It can be seen that choosing the appropriate repetition rate can improve the conversion efficiencies and beam qualities of the mid-infrared outputs. Based on the above experiments, the characteristics of mid-infrared parametric outputs by changing the [EMAIL_ADDRESS] \(\upmu\)m of M1 were studied. The output power of dual-wavelength mid-infrared lasers is shown in Figure 7 when the transmittance @1.57-\(\upmu\)m changed from 0 to 99%. As can be seen from this figure, under the low transmittance of the M1, the 3.84 \(\upmu\)m laser had a smaller effective gain. As the transmittance of the 1.57 \(\upmu\)m laser increased, the oscillation process of the 3.84 \(\upmu\)m laser enhanced gradually.