Fig. 1. Process of sum frequency conversion

Fig. 2. Representation based on adiabatic channel evolution

Fig. 3. Optical path diagram

Fig. 4. Simulation results of the sum frequency generation process. (a) Sum frequency conversion efficiency; (b) spectra of fundamental wave, second-harmonic, third-harmonic

Fig. 5. Influence of deuterium concentration gradient and crystal length on conversion efficiency. (a) Conversion efficiency for different crystal lengths (horizontal axis) and different deuterium-doped concentration gradients (vertical axis); (b) scatter plot corresponding to Fig. 5 (a)

Fig. 6. Sum-frequency efficiency along the direction of optical transmission varies with the pump intensity

Fig. 7. Comparison of sum frequency conversion efficiency between gradient-doped deuterium crystal and KDP crystal