HOLOGRAM, CONDENSING OPTICAL SYSTEM FORMED BY USING HOLOGRAM AS WELL AS OPTICAL HEAD DEVICE OPTICAL DISK DEVICE HAVING THIS CONDENSING OPTICAL SYSTEM

摘要

PROBLEM TO BE SOLVED: To decrease the light quantity of the light quantity of unnecessary diffracted light by setting the diffraction efficiency of zero order diffracted light and +1st order diffracted light higher than the diffraction efficiency of the diffracted light of any other order and specifying the respective values of the diffraction efficiency. SOLUTION: A hologram lens 107 is so designed that the diffraction efficiency of its +1st order diffracted light 64 is <100% and that the lens has sufficient strength even to the zero order diffracted light (transmitted light) 61 of a light beam 3. This zero order diffracted light 61 is condensed to the position of the recording surface 38a of an information recording medium 51 having a small substrate thickness t2 by an objective lens 4. The +1st order diffracted light 64 is condensed to the position of the recording surface 38b of an information recording medium 5 having a large substrate thickness t1. In such a case, the diffraction efficiency of the zero order diffracted light 61 and the -1st order diffracted light 64 is higher than the diffraction efficiency of the diffracted light of any other order and the values of the diffraction efficiency of the zero order diffracted light 61 and the 1st order diffracted light 64 are both >=30%. The diffraction efficiency of one of the +2nd order diffracted light and the -1st order diffracted light is <=2%.