Fig. 1. Schematic process of FLW of the LiNbO₃ waveguide couplers (left). Schematic end-face coupling arrangement for investigation of the waveguide couplers at 632.8 nm (right).

Fig. 2. Optical microscope images of the cross sections of (a) the single-line waveguide and waveguide couplers (b) C1, (c) C2, and (d) C3. The measured near-field intensity distributions at 632.8 nm of (e) the single-line waveguide and waveguide couplers (f) C1, (g) C2, and (h) C3 along TM polarization.

Fig. 3. xy-plane beam propagation simulation (RSoft) of 3D waveguide couplers (a) C1 and (c) C2 in LiNbO₃ crystal. Monitored 632.8 nm light guided in each coupler arm along the 3D waveguide couplers (b) C1 and (d) C2.

Fig. 4. (a) Microscopic photograph of the cross section, (b) measured near-field modal profile along TM polarization at 632.8 nm, and (c) simulated modal profile evolution of the 3D waveguide coupler C4.