## 3 Results and Discussion  
We first measured the sum frequency output spectrum at room temperature (T1 = T2 = 25 \({}^{\circ}\)C, \(\Delta\)T = 0 \({}^{\circ}\)C) before setting up a temperature difference at the end faces of the MgO:PPLN. A fiber spectrometer with a resolution of \(\sim\)0.8 nm was used to measure the orange light spectrum. A simultaneous triple-wavelength output at 594.9, 596.9, and 598.6 nm was observed as shown in Figure 2a. The relative intensities of the triple-wavelength operation could be varied to achieve dual-wavelength output by introducing a longitudinal temperature change to the MgO:PPLN crystal. Accordingly, a dual-wavelength operation was achieved at 594.9 and 598.6 nm (Figure 2b) by setting the temperature difference to \(\Delta\)T = 7.1 \({}^{\circ}\)C (T1 \(\sim\)41.2 \({}^{\circ}\)C, T2 \(\sim\)34.1 \({}^{\circ}\)C). This could be attributed to the longitudinal temperature change induced on the crystal that causes a variation of phase matching along the crystal, which either enhances or lowers the phase mismatching. Each incoming wavelength of the fundamental is quasi-phase-matched in a different crystal segment. The bandwidth of orange light would as well be broadened by increasing the temperature gradient of the MgO:PPLN crystal,  
Figure 1: Schematic of the experimental setup. The beams are focused into single-pass MgO:PPLN to produce orange light by sum frequency generation (SFG). LD, laser diode, ASE: amplified spontaneous emission, WDM: wave division multiplexer; C-lens: collimator lens. Inset: close view of crystal gratings with different sections. which has an effect of inducing a chirp within the crystal and allows to achieve several phase matching conditions, hence extending the SFG process to other wavelengths and widens the fundamental wavelength acceptance range of the MgO:PPLN crystal. Accordingly, the fundamental wavelengths that simultaneously meet the condition of energy (1/\(\lambda_{3}\) = 1/\(\lambda_{1}\) + 1/\(\lambda_{2}\)) and momentum conservation (\(\Delta k\)) will be upconverted in the PPLN, resulting in broad wavelength up-conversion. This effect is depicted in Figure 2c, which shows the broadened orange light output when the temperature difference was set at \(\Delta\)T = 22.1 \({}^{\circ}\)C (T\({}_{1}\) \(\sim\)73.0 \({}^{\circ}\)C, T\({}_{2}\) \(\sim\)50.9 \({}^{\circ}\)C) with an effective bandwidth of -4.4 nm. However, when the bandwidth is increased, the conversion efficiency is decreased as a penalty. The calculated temperature profile along the MgO:PPLN crystal is shown in Figure 3 where we have used \(\Delta\)T = 22.1 \({}^{\circ}\)C since it relates to the widest bandwidth of the measured sum frequency up-conversion. It can be observed that shorter thermal contacts yield a linear temperature distribution contrasted with longer thermal contacts. Nevertheless, due to the limitations of our set up, we employed \(\sim\)5 mm thermal contacts in our experiment. We note that it would be attractive to achieve a 'flat-top' spectra of the up-converted wavelengths by precisely adjusting the temperature distribution along the crystal length using several smaller temperature controllers, which may result in bandwidth broadening and also by using shorter thermal contacts. Figure 4 depicts the characteristics of the CW orange light outputs as a function of the product of the fundamental powers \(P_{1}\) and \(P_{2}\). Without applying a temperature gradient, the  
Figure 3: (a) Longitudinal temperature distribution in MgO:PPLN crystal for different contact lengths at the crystal ends; (b) false color image of the top view of the temperature (in \({}^{\circ}\)C) distribution along the PPLN crystal. Figure 2: Measured SFG spectra at (a) \(\Delta\)T = 0 \({}^{\circ}\)C; (b) \(\Delta\)T = 7.1 \({}^{\circ}\)C; and (c) \(\Delta\)T = 22.1 \({}^{\circ}\)C temperature differences. maximum triple-wavelength orange light output power of 9.3 mW was obtained.