# Performance Studies of High-Power Optical Parametric Oscillators Pumped by a Pulsed Fiber Laser
## 3 Experimental Results and Discussion
### Parameters Affecting Idler Power  
We studied how the pump beam size in the MgO:PPLN crystal affected the optical-optical conversion efficiency. By incorporating a set of plano-convex lenses with varying focal-lengths, the pump beam waist was effectively reduced to 155 \(\pm\) 3.1 \(\upmu\)m, 172 \(\pm\) 3.4 \(\upmu\)m, 201 \(\pm\) 4.0 \(\upmu\)m and 230 \(\pm\) 4.6 \(\upmu\)m. The beam sizes are measured by the laser beam profiler with a beam size accuracy of \(\pm\)2% (LBP2-IR2, Newport Corporation, Irvine, CA, USA, and Franklin, MA, USA). Additionally, the focus point of the beam was consistently positioned at the center of the crystal. At a fixed crystal length of 38 mm and the M2 with a curvature radius of \(-\)100 mm, the estimated waist size was \(\sim\)150 \(\upmu\)m. The output power and corresponding conversion efficiency are summarized in Figure 3. At the range of low pump power, the smaller pump beam has higher power density and better mode matching. It is reasonable that the smaller the pump beam waist, the higher the conversion efficiency. The conversion efficiency at \(\sim\)3.8 \(\upmu\)m is 11.1 \(\pm\) 0.8% with a maximum output power of 7.5 \(\pm\) 0.4 W. However, when the average pump power exceeds 75 \(\pm\) 4 W, the crystal will break easily if the pump beam size is too small, as shown in the dash-square line. When the pump beam waist was 155 \(\pm\) 3.1 \(\upmu\)m, thermal guiding in the MgO:PPLN  
Figure 1: Experimental setup of the signal-resonant OPO based on MgO:PPLN crystal. crystal caused a noticeable step increase in the idler power from 4.7 \(\pm\) 0.2 to 6.8 \(\pm\) 0.3 W, which was called bi-stability in references [23; 24]. Specifically, the open line and filled lines represent the decrease and increase in pump power, respectively. At the range, there is a noticeable difference, which indicates the OPO has two stable states. For the sake of comparing different parameters, only a downward trend of the output power has been exhibited in Figure 3a. A series of slight bi-stability phenomena were also observed, such as the steps from 4.2 \(\pm\) 0.2 to 5.1 \(\pm\) 0.3 W (201 \(\pm\) 4.0 \(\upmu\)m), 4.0 \(\pm\) 0.2 to 5.0 \(\pm\) 0.3 W and 6.1 \(\pm\) 0.3 to 7.8 \(\pm\) 0.4 W (172 \(\pm\) 3.4 \(\upmu\)m). Expressly, the bi-stability could be restrained effectively by expanding the pump beam waist. To find the most appropriate pump beam waist location, the whole linear OPO cavity was moved along the beam propagation. The size of the pump beam waist, crystal length, and curvature radius of M2 were 172 \(\pm\) 3.4 \(\upmu\)m, 38 mm, and -100 mm, respectively. The estimated waist of \(\sim\)145 \(\upmu\)m can be obtained by the SNLO. The distance from the front face of the crystal to the waist varied as \(-\)5 mm, 6 mm, 16 mm and 21 mm. Figure 4 shows the output power and conversion efficiency as functions of the pump power for different waist locations. As shown by the dash-dot line in Figure 4, the output power and conversion efficiency are always the lowest when the waist lies outside the MgO:PPLN crystal. Due to the absorption of optical power, the thermal guiding leads to a surge in the output power, i.e., bi-stability (the dash-square line in Figure 4). It becomes apparent when the pump power exceeds 57 \(\pm\) 3 W, indicating a better overlap between the pump and signal beam is found [22]. Compared to the above locations, the ideal waist location is around the center of the MgO:PPLN because the output power is hardly influenced by thermal guiding and increases gradually as the pump power grows. Figure 2: Characteristics of laser beam: (**a**) spectrum, (**b**) M\({}^{2}\) factors (inset: beam-intensity distribution), (**c**) waveform and (**d**) power stability. The curvature radius of the output coupler M2 also influenced the OPO output power.