Patent ID: 6859428
Filing Date: 2005-02-22
Classification: G02B,G11B

Abstract:
1. A converging element which converges a first light beam from a first light source used for recording and/or reproduction for a first optical information recording medium having a first transparent plate and converges a second light beam from a second light source used for recording and/or reproduction for a second optical information recording medium having a second transparent plate thicker than the first optical information recording medium, the converging element comprising an inner region near a center axis of the first and second light beams and an outer region far from the center axis and adjacent to said inner region; wherein when the first light beam comes in said inner region and said outer region to be focused onto a recording layer formed on the first transparent plate of the first information recording medium, the first beam is converged on the recording layer such that spherical aberration of the first light beam transmitting said outer region becomes minimum for the first thickness of the first optical information recording medium; and wherein when the second light beam comes in said outer region to be focused onto a recording layer formed on said second transparent plate, the second beam is converged on the recording layer such that spherical aberration of the second light beam transmitting said inner region becomes minimum for a transparent plate having a thickness between the first and second thicknesses.