Efficient coupling into tapered proton-exchanged LiNbO(3) waveguides fabricated by vertically controlled immersion

Planar LiNbO(3) waveguides with a 5-mm tapered region and a 10-mm homogeneous slab waveguide region were fabricated by a double proton-exchange process. The 4-microm waveguide entrance gives rise to a coupling efficiency as high as 93%; the precise control of the depth profile of the tapered region results in a low 1-dB power transfer loss from the tapered region to the 0.4-microm-depth working region. The techniques developed provide an alternative to prism coupling into planar waveguides in Cerenkov second-harmonic generation applications.