| 摘要 |
An objective optical system for use in an optical pick up apparatus in which recording and/or reproducing information is conducted for a first optical disc equipped with a protective layer having a thickness t<SUB>i </SUB>by using a first light flux having a first wavelength lambda<SUB>1 </SUB>emitted from a first light source and recording and/or reproducing information is conducted for a second optical disc equipped with a protective layer having thickness t<SUB>2 </SUB>(t<SUB>2</SUB>>=t<SUB>1</SUB>) by using a second light flux having a second wavelength lambda<SUB>2 </SUB>(lambda<SUB>2</SUB>>lambda<SUB>1</SUB>) emitted from a second light source, comprising: an optical surface forming a first diffractive structure, wherein the first diffractive structure provides a diffractive operation for the second light flux and doesn't provide a diffractive operation for the first light flux, wherein the objective optical system satisfies the following formula (1): <?in-line-formulae description="In-line Formulae" end="lead"?>0.9<WD<SUB>1</SUB>/WD<SUB>2</SUB><1.1 (1) <?in-line-formulae description="In-line Formulae" end="tail"?> where WD<SUB>1 </SUB>represents a first working distance in recording and/or reproducing the information for the first optical disc and WD<SUB>2 </SUB>represents a second working distance in recording and/or reproducing the information for the second optical disc.
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