## 3 Experimental Setup and Output Characterization
### SFG and SHG Characterization  
The maximum input powers measured before the crystal were P\({}_{1}\) = 229 mW and P\({}_{2}\) = 200 mW. Figure 2a shows the power of SHG light as a function of pump power, for an SHG wavelength of 487 nm. The LD power, P\({}_{1}\) was increased steadily to a maximum value of 229 mW by varying the operating current, and the ASE power P\({}_{2}\) was fixed at 200 mW (maximum). A maximum SHG power of 5.2 mW was measured at 487 nm corresponding to \(\sim\)10%/W conversion efficiency, lower than the expected nonlinear conversion efficiency of \(\sim\)29%/W. Figure 2b illustrates the SFG power as a function of the product of pump (P\({}_{1}\)) and signal (P\({}_{2}\)) powers incident on the crystal. The maximum  
Figure 1: Schematic of the experimental setup. The beams are focused into single-pass MgO:PPLN to generate orange output by sum frequency generation (SFG) and blue output by second harmonic generation (SHG). LD: laser diode; ASE: amplified spontaneous emission; WDM: wave division multiplexer; C-lens: collimator lens; OSA: optical spectrum analyzer. sum-frequency generation of 9.3 mW at 598.8 nm is achieved, resulting in an overall conversion of \(\sim\)20.3%/W, which is also lower than the expected conversion efficiency of \(-\)42%/W. Possible reasons for low conversion efficiencies can be attributed to an inhomogeneous temperature distribution within the crystal and/or poling imperfections in the QPM structure. As for the low SHG conversion efficiency, one predominant reason is using the second-order QPM for the given domain period (10.2 \(\upmu\)m), which can be improved by using the first-order QPM period (5.1 \(\upmu\)m). The conversion efficiencies could be further improved by using a high fundamental power. The temperature of the MgO:PPLN was varied from 25 \({}^{\circ}\)C to 120 \({}^{\circ}\)C. Figure 3 shows the orange and blue laser intensities as a function of temperature. The SFG was always present with high intensity in the region 30-100 \({}^{\circ}\)C. We attributed this to the broadband nature of the ASE source used in this work. The drop in the SFG intensity below 30 \({}^{\circ}\)C and above 100 \({}^{\circ}\)C could be because of a phase-matching condition at \(\lambda_{2}\) wavelength with low intensity. A further decrease in the SFG intensity at 110 \({}^{\circ}\)C may be attributed to a competition between the SFG and SHG processes. The high intensity of blue light could be a result of the QPM condition for SHG satisfied at 110 \({}^{\circ}\)C. When the temperature is set at 25 \({}^{\circ}\)C (Figure 4a, only SFG appears with relatively lower intensity. As the temperature is increased to 60 \({}^{\circ}\)C, the SFG intensity rises to almost twice as the intensity observed at room temperature as seen in Figure 4b. We attribute this to phase matching temperature of the SFG process being near 60 \({}^{\circ}\)C.  
Figure 2: Measured (black squares) output power as a function of the fundamental power for (**a**) second harmonic generation (SHG), and (**b**) sum frequency generation (SFG). The solid curve in (**a**) corresponds to a quadratic fit to the SHG data using \(P_{SHG}\ =\ \eta_{SHG}P_{1}^{2}\) and in (**b**) the solid curve is a quadratic fit for the SFG process using \(P_{SFG}\ =\ \eta_{SFG}P_{1}P_{2}\), where \(\eta_{SHG}\ =\ 10\%/W\) and \(\eta_{SFG}\ =\ 20.3\%/W\) are the SHG and SFG normalized conversion efficiencies determined from power measurements respectively. Other fit parameters are L = 50 mm (crystal length) and \(\omega_{f}\) = 43 \(\upmu\)m. The error bars include the uncertainty in the output power measurements. There exists some competition between the SFG and SHG processes since both processes consume energy from the fundamental lasers [25; 26]. According to Figure 4c,d, we can see that SHG appears along with SFG and at 110 \({}^{\circ}\)C, the SHG intensity reaches its maximum. We estimated a reduction of SFG power by \(\sim\)7% of its maximum (at 60 \({}^{\circ}\)C) when the SHG intensity is at its maximum.