| 摘要 |
An optical pickup apparatus includes a diffractive optical element, and an objective lens that focuses a light beam of a first wavelength lambda1, a light beam of a second wavelength lambda2 and a light beam of a third wavelength lambda3 on a first recording medium, a second recording medium, and a third recording medium, respectively, the wavelengths lambda1, lambda2, and lambda3 being different from each other. The diffractive optical element includes a first diffractive surface that neither diffracts the light beam of the first wavelength lambda1 nor the light beam of the third wavelength lambda3 but diffracts the light beam of the second wavelength lambda2, and a second diffractive surface that neither diffracts the light beam of the first wavelength lambda1 nor the light beam of the second wavelength lambda2 but diffracts the light beam of the third wavelength lambda3, and each of the first and second diffractive surfaces satisfies the following condition inequality: <?in-line-formulae description="In-line Formulae" end="lead"?>Lambda/lambda>=8<?in-line-formulae description="In-line Formulae" end="tail"?> wherein Lambda represents the minimum pitch in the case that the width which generates a phase difference of one wavelength when the closest wavefronts resulting from adjacent steps in each of the diffractive surfaces are linked with each other is defined as one pitch, and lambda represents the wavelength of the diffracted light.
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