## 3 Experimental Results and Discussion
### Multiwavelength Visible Lasers Generation  
By utilizing the Stokes lasers obtained above and keeping the pumping energy at 120 mJ, we calculated the possible visible wavelengths from the KDP crystal and their corresponding phase-matching angles, as presented in Table 1. A total of 28 spectral lines combinations are possible after second order frequency mixing of the aforementioned seven Stokes lasers. Furthermore, 16 visible wavelengths were experimentally detected by the spectrometer (Ocean Optics HR4000 with a minimum FWHM resolution of 0.02 nm), with tuning of the \(\theta\)-angle of the KDP crystal, also as shown in Table 1. We can see that all angles of the experimentally detected wavelengths are from 41.8\({}^{\circ}\) to 44.9\({}^{\circ}\). Since some Stokes lasers are too weak to generate SHG/SFG with large signals, some red lasers cannot be detected. Moreover, some potential SHG and SFG lasers from 687.1 to 747 nm require too large of an incident angle for the limited aperture of the KDP crystal, and therefore have not been detected. Considering that the KDP crystal is placed at the focus of the Stokes laser beams, laser beams in the crystal first converge and then diverge. Thus, even when the horizontal angle \(\theta\) of the KDP crystal is set to a fixed value around 42\({}^{\circ}\), the SHG and SFG processes of multiwavelength Stokes lasers can occur simultaneously. For different \(\theta\) of the KDP crystal, the energies and spectra of visible lasers are shown in Figure 4. As \(\theta\) increases, the spectrum of the output lasers shifts from the yellow waveband to the red waveband. Initially, when \(\theta\) is 40.3\({}^{\circ}\), only three visible wavelengths can be detected with low energy. In this case, the 588.6 and 589 nm wavelengths are too close to distinguish in the measured spectra. As the angle of KDP crystal increases, more distinct wavelengths appear with increased output energy. Once \(\theta\) is greater than 42\({}^{\circ}\), the red waveband becomes detectable. As \(\theta\) is further increased to 43.5\({}^{\circ}\), 11 distinct visible lasers can be obtained simultaneously. Moreover, once \(\theta\) angle is increased to 45.8\({}^{\circ}\), the longest visual wavelength of 658.48 nm is detected. All 16 detected visible wavelengths from 579.48 to 658.48 nm are presented in Figure 4a. Determined by the slit size and the grating type, the optical resolution of HR4000 spectrometer we used in this study is \(\approx\)0.26 nm (FWHM). However, the 1064 nm seed laser is a Q-switch laser with a narrow linewidth of \(\approx\)0.2 nm, which is close to resolution of the spectrometer. And the measured spectral widths of the detected 16 visible wavelengths from 579.48 to 658.48 nm are \(\approx\)0.26 nm. In fact, some of the aforementioned 28 calculated wavelengths may have materialized, but with such low energy to report with scientific confidence. Finally, if the 1064 nm laser was delivered into the KDP with other Stokes lasers, a richer spectral band could be achieved. The relationship between the total output energy of all the resulting visible lasers and the rotating angle of KDP is shown in Figure 5. It shows that maximum output energy of 8.87 mJ is achieved when \(\theta\) is tuned at 42\({}^{\circ}\), which corresponds to an optical conversion efficiency of 20.1%. In addition, the total optical efficiency from the 1064 nm to visible lasers is approximately 7.4%. Furthermore, when \(\theta\) is larger than 42\({}^{\circ}\), the total output energy decreases, not only due to phase  
Figure 3: Output Stokes energy (black line) and optical conversion efficiency (red line) of Stokes lasers in comparison to the energy of the 1064 nm pumping laser. mismatch of some output visible lasers, but also because the fundamental lasers for SHG and SFG processes are high-order Stokes lasers with lower energy.