BROADBAND WAVELENGTH PLATE HAVING BRAZED DIFFRACTION GRATING, OPTICAL PICKUP USING SAME AND MANUFACTURING METHOD OF THE BRAZED DIFFRACTION GRATING

摘要

PROBLEM TO BE SOLVED: To provide a wavelength plate which allows phase difference to reliably become 90° over the whole broadband, and to provide an optical pickup using the wavelength plate.

SOLUTION: A diffractive optical element made by laminating diffraction gratings on both of principal planes of the substrate of the thickness t having birefringence is constituted so as to satisfy the following relations: Γ1=2π/λ1×(ne'1-no)×t/cosψ1, Γ2=2π/λ2×(ne'2-no)×t/cosψ2, wherein diffraction angles with respect to wavelengths λ1, λ2 (λ1<λ2) of the diffraction grating are ψ1, ψ2, respectively, ordinary refractive index of the substrate having the birefringence is no, extraordinary refractive index of the substrate having the birefringence with respect to the traveling direction of light rays of wavelengths λ1, λ2 are ne1', ne2', respectively, and the phase differences of the diffractive optical element with respect to the wavelengths λ1, λ2 are Γ1, Γ2.

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