# Performance Studies of High-Power Optical Parametric Oscillators Pumped by a Pulsed Fiber Laser
## 3 Experimental Results and Discussion
### Characteristics of Idler Beam  
Based on the comprehensive analysis of the parameters affecting output power, we built the OPO with the highest output power and conversion efficiency. The curvature radius of the output coupler, crystal length and cavity length were \(-\)100 mm, 38 mm and 42 mm, respectively. In the cold cavity, the estimated waist of \(\sim\)150 \(\upmu\)m can be obtained by SNLO. To allow careful mode-matching at high power, the pump beam waist was reduced to 172 \(\pm\) 3.4 \(\upmu\)m by a convex lens with a focal length of 150 mm. The pump beam with a repetition rate of 200 kHz and a pulse width of 150 ns was focused into the center of the MgO:PPLN crystal. To investigate the performance of the OPO, we measured the variation of the output power in the idler beam with the increasing pump power, as shown in Figure 7. As evident, the frequency conversion starts at a threshold pump power of 33 \(\pm\) 1.6 W. With the pump power increasing gradually, the idler power increases accompanied by two implicit bi-stability at 1.0 \(\pm\) 0.1 W and 4.3 \(\pm\) 0.2 W. Until the crystal break, the pump power increases to 79 \(\pm\) 4 W, and a maximum idler power of 8.0 \(\pm\) 0.4 W is obtained. The dash-square line in Figure 7 shows the corresponding optical-optical  
Figure 5: (**a**) Idler power and (**b**) corresponding optical–optical efficiency versus pump power for different curvature radii of output coupler.  
conversion efficiency at different output power. The conversion efficiency has a maximum value of 10.6 \(\pm\) 0.7% and tends to level off when the output power exceeds 7.5 \(\pm\) 0.4 W.  
When the temperature of the oven was 75.0 \({}^{\circ}\)C, the idler spectrum was measured at maximum output power using a Fourier Transform Optical Spectrum Analyzer (OSA205C, Thorlabs, Inc.) with a resolution of ~0.36 nm. As evident from Figure 8a, the idler spectrum centers at 3768.4 nm with a FWHM bandwidth of ~18.6 nm. Using an infrared photodetector (PCI-4 TE-12-1x1, VIGO Photonics) to measure the waveform of the idler beam, the data show its pulse width is ~130 ns, as shown in Figure 8b.  
Figure 6: (**a**) Idler power and (**b**) corresponding optical–optical efficiency versus pump power for different crystal lengths.  
Figure 7: Idler power and corresponding optical–optical efficiency versus pump power.  
To estimate the beam quality M\({}^{2}\) factors of the idler beam, we used a lens with a focal length of 500 mm to focus the beam and measured its diameter around the beam waist by 90/10 knife-edge method. According to the hyperbolic fitting of the data, as shown in Figure 9a, the M\({}^{2}\) factors of the idler beam along the horizontal and vertical directions are \(\sim\)1.84 and \(\sim\)1.69, respectively. We also performed measurements of the output power stability characterized by the OPO by recording long-term average power fluctuations at a wavelength of 3768.4 nm. The results are shown in Figure 9b. While generating an output power of \(\sim\)6.3 W recorded over 7 h, the RMS was better than 1.6%, which mainly attributed to the reliable stabilization of the crystal temperature and the appropriate output coupling optics to emit the intracavity power of the signal beam.