The second-harmonic blue light generation of the infrared light in the wavelength region of 830nm is a promising technique to achieve a compact blue light source. For the efficient second-harmonic generation a large nonlinearity, a good phase matching, and a high pump intensity along the long interaction length are required. A device geometry satisfying these requirements is a quasi-phase-matched optical waveguide in $LiNbO_3$ or $LiTaO_3$. The quasi-phase-matching in a channel waveguide is achieved by the domain grating that is fabricated by periodic domain inversion. For an efficient second-harmonic generation, it is essential to form the domain grating without degrading the nonlinear optic coefficient, and to fabricate a optical channel waveguide with low propagation loss.
In this dissertation, efficient fabrication methods of quasi-phase-matched waveguides for blue light generation are investigated in two ferroelectric crystals : $LiNbO_3$ and $LiTaO_3$.
First, a new periodic domain inversion method is proposed using the titanium lateral-diffusion in $LiNbO_3$. A triangular domain inversion is formed beside Ti film(i.e. in the gaps between Ti stripes) rather than below Ti film by using particular Ti diffusion conditions(thick Ti film, low diffusion temperature, and slow cooling rate). It is possible to control the domain-inversion depth without forming any planar inverted layer so as to improve the second-harmonic generation efficiency by maximizing the overlap integral. By controlling the domain-inversion depth and the annealing period of proton-exchanged waveguide, the blue light of 23μW at 439nm is generated with a pump power of 20mW in a 3rd order quasi-phase-matched $LiNbO_3$ device. Its normalized conversion efficiency is 28%/W㎠. Furthermore, the mechanism of Ti diffused domain inversion is investigated for various Ti film diffusion conditions. Namely, three different types of domain inversion depending on the heat treatment time are elucidated, i.e., lithium outdiffused domain inversion, Ti lateral-diffused domain inversion, and Ti indiffused domain inversion in turn. Based on the results of this discovery a possible physical mechanism to explain the formation of Ti diffused domain inversion is proposed.
Second, a new heat-treatment technique using a metal-oxide mask is proposed to fabricate the domain grating without degrading nonlinear optical properties in $LiTaO_3$. The metal-oxide mask promotes the proton indiffusion by suppressing the proton outdiffusion, and forms a deep inversion depth. This heat-treatment technique reduces the amount of initial proton exchange for the same inversion depth, as compared with the heat treatment without a mask. It also prevents the formation of the crystal defects on the proton-exchanged surface that is accompanied by the proton outdiffusion. Consequently, it minimizes the degradation of nonlinear coefficient and scattering loss caused by the initial proton exchange.
An efficient blue light generation device is made by fabricating the domain grating by the heat treatment with metal-oxide mask and forming the waveguide by the proton diffusion with a $SiO_2$ mask. Second-harmonic blue light of 1mW at 429nm is generated with the fundamental wave power of 10.3mW. Its normalized conversion efficiency is 1500%/W㎠, which is the highest value for $LiTaO_3$ waveguide devices reported to date.