Fig. 1. Structure and mode simulation of the LNOI waveguide. (a) Cross-section structure of the LNOI ridge waveguide (H, thickness; W, top width; D, etching depth). Optical mode profiles (energy density) for fundamental TE modes at (b) 1550 nm and (c) 775 nm.

Fig. 2. Experiment setup. A continuous-wave (CW) tunable laser serves as the FW source of the SHG process. The output HW light is measured by a high-sensitivity power meter (PM). Two polarizers are inserted to ensure QPM polarization. PC, polarization controller; MMF, multimode fiber.

Fig. 3. Experimentally observed spectra (blue curves) from two 6-mm-long waveguides with poling periods of (a) 4.1 µm and (b) 3.9 µm, respectively. The red curves are plotted with sinc-square functions.

Fig. 4. Simulated effective refractive index (N_eff) as a function of (a) top width, (b) etching depth, and (c) thickness for all four waves in the waveguide.

Fig. 5. SHG spectrum profile simulated with optimized gradient thickness change (red curve). The blue curve is the SHG spectrum measured in the experiment.