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We fabricate 50-mm-long direct-bonded quasi-phase-matched LiNbO3 ridge waveguides for difference frequency generation in the 3-μm wavelength range. Conversion efficiency of 40%/W is achieved using a 1-μm-band pump and a 1.55-μm-band signal, and a 0.26-mW output is obtained. We also use the device to demonstrate methane gas detection at around 3.3μm.

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We have devised a novel device structure for a multiple quasi-phase-matched wavelength converter. Optimized continuous phase modulation of a periodic domain structure makes possible multichannel pumping with minimum loss of efficiency. Using the device, we demonstrate variable and simultaneous wavelength conversion of wavelength-division multiplexed signals.

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A relation between B-site transition metal and ferroelectricity is investigated in A BO3 perovskite-type oxides. We focus on intensity of the covalent bonds between the B-site atom and the oxygen octahedron and evaluate Harrison's covalency parameter α. The density functional theory within the local density approximation is adopted to obtain the valence electron energy levels of free atoms from which the covalency α is calculated. Strong correlation between the covalency α and the Curie temperature (T C) is found. As the covalency α increases, T C of ferroelectricity arises. It is also noted that the hybridization between Pb 6p and O 2p is crucial for the high ferroelectricity of PbTiO3.

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