# Performance Studies of High-Power Optical Parametric Oscillators Pumped by a Pulsed Fiber Laser
## 4 Conclusions  
In conclusion, we presented the experimental characterization of a high average-power, YDFL-pumped signal-resonant OPO module. Owing to the pump-single-pass scheme, the module can be compacted as far as possible. The output power of the OPO affected by pump beam size, waist location, curvature radius of output coupler and crystal length were investigated, respectively. The experimental data shows that a smaller pump size (a waist of \(\sim\)150 \(\upmu\)m) improves the performance of the OPO but increases the risk of crystal damage. In addition, the pump beam should be focused on the crystal's center to improve the conversion efficiency and robustness of the OPO. The conversion efficiency is also influenced by the curvature radius of the output coupler and crystal length. An output coupler with a small curvature radius and a long crystal should be adopted to optimize  
Figure 8: (**a**) Spectrum and (**b**) waveform of idler beam.  
Figure 9: (**a**) Beam-quality measurement and (**b**) long-term power stability at 3768.4 nm.  
the conversion efficiency. A comprehensive analysis of the factors affecting output power is essential to improve optical-optical efficiency for high-power OPOs working at long mid-infrared ranges. It should be noted that the bi-stability phenomenon is extraordinarily ordinary in high-power OPOs. Using MgO:PPLN as the nonlinear crystal, the OPO afforded a maximum MIR power of 8 \(\pm\) 0.4 W at 3768.4 nm with a repetition rate of 200 kHz and pulse width of 130 ns when the pump power was 79 \(\pm\) 4 W. The M\({}^{2}\) factors of the idler beam along the horizontal and vertical directions were \(\sim\)1.84 and \(\sim\)1.69, respectively. Thanks to excellent thermal control and suitable output coupling of the signal beam, a long-term idler power fluctuation of 1.6% RMS was achieved. The output power and conversion efficiency can be further improved by optimizing the repetition rate, pulse width, as well as output coupler transmissivity. In addition, all pump power can be utilized fully to pump two OPOs simultaneously using a non-polarizing beam splitter cube, which can improve correspondingly two times output power. A higher beam quality can be obtained by using an idler-resonant configuration [18; 19; 25] because its mode size is larger than the signal-resonant OPO. Based on the demonstrated performance characteristics, future work will be focused on higher output power at MIR wavelength and a more compact structure.  
Conceptualization, X.L.; methodology, X.L. and J.N.; validation, L.H. and Y.S.; formal analysis, L.H., X.L. and J.N.; investigation, X.L., G.Z. and S.Z.; data curation, L.H. and Y.S.; writing--original draft preparation, L.H. and Y.S.; writing--review and editing, X.L. and J.N.; funding acquisition, X.L. and J.N. All authors have read and agreed to the published version of the manuscript.  
This research was funded by 'National Key R&D Program of China' (Grant No. 2022 YFF0712801), 'National Natural Science Foundation of China' (Grant No. 12192251) and 'National Postdoctoral Program for Innovative Talents' (Grant No. BX201700117).  
Not applicable.  
Data Availability StatementData underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.  
The authors declare no conflict of interest.