| 摘要 |
PROBLEM TO BE SOLVED: To eliminate the spherical aberration, which is generated when the thickness of a disk is varied, by providing a hologram type ring and using the hologram type lens in which plural light beams from laser beam sources are totally passed in the region within the ring and the specific beams from the sources are diffracted in the region outside the ring. SOLUTION: A hologram type ring lens 35 selectively diffracts incident light beams by wavelengths to prevent the occurrence of the spherical abberation against the light beams converged on the information recording surfaces of a DVD 37 and a CD-R 41. The lens 35 is arranged so that the hologram is positioned between a region A and a region B and the light beams pass the region B of an objective lens 36 are diffracted and no spherical abberation is generated. Thus, the spherical abberation, which is generated when the thickness of a disk is changed, is eliminated, a DVD and a CD-R are made interchangeable, the construction of the hologram lens is made easy and the mass- productivity is improved. |
