PURPOSE:To widen the temp. latitude of the second harmonic generating element which converts the basic waves of a laser to second harmonic waves and to simultaneously enhance conversion efficiency. CONSTITUTION:Local inversion gratings which have a rectangular section and are reversed in spontaneous polarization direction from a substrate are provided at equal intervals within the optical waveguide on the substrate, by which the conversion efficiency from the basic waves to the second harmonic waves is improved. Further, the substrate and the optical waveguide are constituted of the same material, by which the temp. coeffts. of the refractive index of the basic waves and the second harmonic, waves in the direction normal to the substrate surface are approximately equaled and the deterioration in the conversion efficiency by the phase deviation of the second harmonic waves is prevented.
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