## 3 Experimental Results and Discussion  
The wavelengths of the OPO signal and idler are recorded with a laser spectrum analyzer (EXFO WA-650) combined with a wavelength meter (EXFO WA-1500). When the pump beam passes through a 29.5 \(\upmu\)m grating period of the MgO:PPLN crystal and the crystal temperature is controlled at 120.0 \(\pm\) 0.1 \({}^{\circ}\)C, the idler wavelength is 3.68 \(\upmu\)m (Figure 2) and the corresponding signal wavelength is 1.49 \(\upmu\)m (Figure 3). The wavelengths of the pump (\(\lambda_{\rm{p}}\)), signal (\(\lambda_{\rm{s}}\)), and idler (\(\lambda_{i}\)) waves are in accord with the conservation of energy (\(1/\lambda_{\rm{p}}=1/\lambda_{\rm{s}}+1/\lambda_{\rm{i}}\)).  
Figure 3: Signal wavelength of the MgO:PPLN OPO at temperature T = 120 \({}^{\circ}\)C for the grating period \(\Lambda=29.5\)\(\upmu\)m.  
Figure 2: Idler wavelength of the MgO:PPLN OPO at temperature T = 120 \({}^{\circ}\)C for the grating period \(\Lambda=29.5\)\(\upmu\)m.  
To investigate its longitudinal mode structure, the signal spectral information is monitored by a 1.5 \(\upmu\)m scanning confocal Fabry-Perot (F-P) interferometer, with a free spectral range of 1.5 GHz. As shown in Figure 4, the upper trace is the F-P ramp voltage and the lower trace is the voltage of the signal transmission through the F-P interferometer. Figure 4a shows the F-P spectrum of the signal from the MgO:PPLN OPO without the YAG etalon. To lock the cavity mode and reduce the spectral noise, an uncoated 0.5-mm-thick YAG plate is used as an intracavity etalon inserted in the cavity. By adjusting the angle of the etalon carefully, the signal spectral noise can be reduced. Figure 4b shows the F-P spectrum of the signal from the MgO:PPLN OPO with the YAG etalon. As can be seen, a single-frequency operation of the signal is presented in Figure 4b. According to the energy conservation condition \(\omega_{i}+\omega_{s}=\omega_{p}\), the single-frequency operation of the idler from the MgO:PPLN OPO can be confirmed.  
The idler output power, as a function of the incident pump power, is measured by a power meter (Coherent PM2). Figure 5 shows the measured idler power versus incident pump power. When the YAG etalon is inserted in the four-mirror ring cavity, the oscillated threshold of the OPO is increased from 2 W to 5 W. Without the etalon in the OPO cavity, with a pump power of 10 W from the single-frequency Nd:YVO\({}_{4}\) laser, the 3.68 \(\upmu\)m idler power is 1.3 W emitting from mirror M\({}_{2}\). With an intracavity etalon, the 3.68 \(\upmu\)m idler power is 1.1 W, corresponding to an optical efficiency of 11%.  
The idler beam quality is also measured as a function of the idler power. By making use of the knife-edge method, the idler beam radius as a function of the distance from mirror M\({}_{2}\) is achieved. By using a nonlinear fitting method, the beam quality factor M\({}^{2}\) can be obtained. For a single-frequency idler output power of 1 W at 3.68 \(\upmu\)m, the values of M\({}^{2}\) are measured to be about 1.3 and 1.2 in the horizontal and vertical directions, respectively.  
Figure 4: F-P spectrum of the signal from the MgO:PPLN OPO: **(a)** cavity without etalon; **(b)** cavity with an uncoated yttrium aluminium garnet (YAG) etalon.  
We can tune the signal and idler wavelengths by changing the temperature of the MgO:PPLN crystal. According to the Sellmeier equations [22], the theoretical tuning curves for the seven available grating periods are shown in Figure 6. By shifting the MgO:PPLN crystal to keep the pump beam passing through the different grating periods, and changing the nonlinear crystal temperature between 20 \({}^{\circ}\)C and 200 \({}^{\circ}\)C, the OPO is able to generate idler wavelengths ranging from 2.9 to 4.1 \(\upmu\)m.