Periodic domain-inversion of lithium niobate without a planar inverted layer

In a quasi-phase-matched (QPM) nonlinear optical waveguide, the efficiency of second harmonic generation (SHG) is proportional to the square of effective nonlinear coefficient determined by the overlap between the waveguide modes involved in SHG and the domain-inversion grating. Thus the domain-inversion grating should be fabricated to have the maximum overlap between the guided modes and the grating, after selecting a proper waveguide geometry for the modes of both the fundamental wave and second-harmonic wave to be tightly overlapped. For that purpose, it is necessary to control the domain-inversion depth in a QPM LiNbO₃ waveguide. In the conventional Ti-indiffused domain inversion of LiNbO₃, the depth of the domain-inversion grating may be controlled by the Ti-film thickness and the Ti-indiffusion conditions (i.e. temperature and time). For the control of the domain-inversion depth it prefers the Ti diffusion only in the depth direction. However, the lateral diffusion of the Ti film and the outdiffusion of Li₂O occur at the Ti-free region simultaneously with the diffusion of Ti film in the depth direction. In this paper, we report a new periodic domain-inversion method for a QPM LiNbO₃ waveguide, which is amenable to the control of depth to maximize the efficiency of SHG. Because the method does not form the planar domain-inverted layer, it is effective to the case that the guided modes are well-confined near the surface