## 2 Experimental Setup  
The experimental configuration of the CW single-frequency MgO:PPLN OPO is shown schematically in Figure 1. The nonlinear medium used for the OPO is 5 mol % MgO-doped periodically poled lithium niobate (MgO:PPLN, HC Photonics) with a length of 50 mm and a laser aperture of 8 mm \(\times\) 1 mm. The MgO:PPLN crystal contains seven domain grating periods from 28.5 \(\upmu\)m to 31.5 \(\upmu\)m with 0.5-\(\upmu\)m increments. The crystal is antireflection-coated for the signal wavelength (R \(<\) 1%@1.4-1.7 \(\upmu\)m), idler wavelength (R \(<\) 1%@3-4 \(\upmu\)m), and pump wavelength (R \(<\) 1%@1.064 \(\upmu\)m). The crystal is mounted in a temperature-controlled oven, in which the crystal temperature can be adjusted in the range of 25-200 \({}^{\circ}\)C with a temperature stability of \(\pm\) 0.1 \({}^{\circ}\)C. A simple bow-tie ring cavity is used in the OPO system. The ring cavity consists of two identical curved cavity mirrors (M\({}_{1}\) and M\({}_{2}\)) and two flat mirrors (M\({}_{3}\) and M\({}_{4}\)), which are all made of CaF\({}_{2}\) and are antireflection-coated at the pump wavelength (T \(>\) 98\({}^{\circ}\)@1064 nm) and idler wavelength (T \(>\) 95\({}^{\circ}\)@3-5 \(\upmu\)m), and have high reflectivity at the signal wavelength (R \(>\) 99.8%@1.4-1.7 \(\upmu\)m). The OPO configuration gives a singly resonant OPO, which is resonant for the signal frequency. The two identical curved cavity mirrors (M\({}_{1}\) and M\({}_{2}\)), enclosing the MgO:PPLN crystal, have a 75-mm radius of curvature and are separated by a distance of 120 mm. The nonlinear crystal is placed at the center between the two curved mirrors (M\({}_{1}\) and M\({}_{2}\)). The other two flat cavity mirrors (M\({}_{3}\) and M\({}_{4}\)) are separated by 35 mm. The total resonator length is about 325 mm.  
Figure 1: Configuration of the continuous-wave (CW) single-frequency MgO-doped periodically poled lithium niobate (MgO:PPLN) optical parametric oscillator (OPO).  
The pump source is a continuous-wave single-frequency Nd:YVO\({}_{4}\) laser that produces over 10 W of radiation at 1064 nm. The Nd:YVO\({}_{4}\) laser has an excellent beam quality of M\({}^{2}\) \(\sim\)1.1 and an output of linear polarization, which was described in Reference [21]. When the power is changed, the laser beam characteristics change significantly. In order to maintain a stable output, the Nd:YVO\({}_{4}\) laser is operated at maximum output power in this experiment. A combination of a half-wave plate and a polarizing beam splitter is used as a power attenuator to change the incident pump power. By using the second half-wave plate, the pump polarization is aligned along the crystallographic \(z\)-axis of the MgO:PPLN crystal to utilize the largest nonlinear coefficient \(d_{33}\). The pump beam is mode-matched to the OPO cavity with a series of convex lenses, producing a 1/e\({}^{2}\) waist radius of 60 \(\upmu\)m at the center of the MgO:PPLN crystal. Its waist yields a focusing parameter \(\xi_{\rm{P}}\)\(\sim\)1.1. With the current OPO cavity, the signal beam waist at the center of the MgO:PPLN crystal is about 70 \(\upmu\)m, resulting in optimum mode-matching to the pump (\(\xi_{\rm{S}}\)\(\sim\)\(\xi_{\rm{P}}\)). To enhance the single-frequency operation of the OPO, an uncoated 0.5-mm-thick yttrium aluminium garnet (YAG) plate is used as an intracavity etalon with a free spectral range of 120 GHz. A 45\({}^{\circ}\) flat dichroic mirror M\({}_{5}\) is utilized as a filter to separate the idler from the output beams.