Characteristics of periodically domain-inverted LiNbO₃ and LiTaO₃ waveguides for second harmonic generation

Characteristics of waveguides with periodically domain-inverted regions in LiNbO₃ and LiTaO₃ for quasi-phase-matched (QPM) second harmonic generation are investigated. The domain-inverted regions in LiNbO ₃ are formed by heat treatment using selective SiO₂ mask and those in LiTaO₃ are formed by heat treatment using selective proton exchange. The domain-inverted region in LiTaO₃ is fabricated up to 2.7 μm depth, which is twice as deep as that in LiNbO₃. The channel waveguides incorporating the regions are fabricated using proton exchange by pyrophosphoric acid. The propagation loss (0.9 dB/cm) of LiTaO ₃ waveguide is much lower than that of LiNbO₃ waveguide (2.9 dB/cm). The second harmonic power generated in LiTaO₃ waveguide by third-order QPM consequently has three time higher conversion efficiency, compared to that in LiNbO₃ waveguide. In domain-inverted LiNbO ₃ waveguide, we have observed optical damage. On the other hand, in domain-inverted LiTaO₃ waveguide, stable blue light (421 nm) has been obtained without optical damage.