| 摘要 |
An objective optical system is formed of a diffractive optical element with a diffractive surface formed on a planar 'virtual surface' (i.e., a surface that would be planar but for the diffractive structure) and an objective lens for focusing three collimated light beams of three different wavelengths at three different numerical apertures onto desired positions of three different recording media with substrates of different thicknesses, such as an AOD, a DVD, and a CD, that introduce different amounts of spherical aberration in the focused beams. The objective optical system provides compensating spherical aberration to the three light beams while keeping equal the distance between the diffractive optical element and the objective lens, and focuses second-order diffracted light of one wavelength and first-order diffracted light of the other two wavelengths. An optical pickup device includes the objective optical system, the recording media, and a light source supplying the three light beams.
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