| 摘要 |
Disclosed is such an optical pickup controlling an aberration even if optical axes of an objective lens and another optical system are shifted from each other. The optical pickup is a device for recording or reproducing an information signal by irradiating light along a track of a rotating optical disk, including a light source, a beam expander for changing a parallelism of light emitted from the light source, and an objective lens 33 for collecting the light emitted from the beam expander along the track of the optical disk and for irradiating the collected light thereonto, wherein the beam expander is comprised of a concave lens 31 and a Fresnel lens 32 , and a focal distance f<SUB>n </SUB>of the diffraction lens exists in a range satisfying the following inequality: <?in-line-formulae description="In-line Formulae" end="lead"?>f<SUB>1</SUB><F<SUB>n</SUB><F<SUB>2</SUB>,<?in-line-formulae description="In-line Formulae" end="tail"?> where <?in-line-formulae description="In-line Formulae" end="lead"?>f<SUB>1</SUB>=((lambda<SUB>2</SUB>/lambda<SUB>1</SUB>)-1)f<SUB>0</SUB>v<SUB>0</SUB><?in-line-formulae description="In-line Formulae" end="tail"?> <?in-line-formulae description="In-line Formulae" end="lead"?>f<SUB>2</SUB>=(1-(lambda<SUB>2</SUB>/lambda<SUB>1</SUB>))b<?in-line-formulae description="In-line Formulae" end="tail"?> <?in-line-formulae description="In-line Formulae" end="lead"?>b=-f<SUB>0</SUB>(f<SUB>0</SUB>+Delta)/Delta<?in-line-formulae description="In-line Formulae" end="tail"?> lambda<SUB>1 </SUB>and lambda<SUB>2 </SUB>are a designed lower limit wavelength and a designed upper limit wavelength, respectively, f<SUB>0 </SUB>and v<SUB>0 </SUB>are a focal distance and a partial dispersion of the objective lens, respectively, and Deltais a difference between focal distances corresponding to the wavelengths lambda<SUB>1 </SUB>and lambda<SUB>2</SUB>.
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