| 摘要 |
<P>PROBLEM TO BE SOLVED: To enable stable and highly accurate interchangeable recording and reproducing between a BD of base material approximately 0.1mm thick corresponding to a blue light beam and a DVD of base material 0.6mm thick corresponding to a red light beam by using a compound objective lens consisting of hologram and an objective lens. <P>SOLUTION: Lattices having a cross-sectional shape in which steps of height of 0 times, two times, one times, three times of a unit step giving difference of the optical path of approximately one wavelength for a blue light beam facing an optical axis from the outer peripheral side is one period p1 are formed only at an inner peripheral part of a hologram 131. This hologram does not diffracts a blue light beam, but transmits as 0 order diffraction light as it is, also, radiates a read light beam passing through the inner peripheral part as +1 order diffraction light. Thereby, a focal length of a red light beam is made longer than that of a blue light beam, and operation distance is made longer. <P>COPYRIGHT: (C)2004,JPO |
