Patent ID: 7639591
Filing Date: 2009-12-29
Classification: G11B

Abstract:
1. A light detecting apparatus using a differential astigmatic method to focus 0th order light on any information surfaces of a multilayer optical disc medium, the 0th order light being among 0th order light, positive higher-order diffracted light of 1st or higher order, and negative higher-order diffracted light of 1st or higher order, all of which are generated by diffracting a laser beam, the apparatus comprising: a first photodetector that includes a light-receiving area for receiving the reflected light of the 0th order light from an information surface where the 0th order light should be focused on; a second photodetector that is adjacent to the first photodetector leaving a predetermined space therebetween, the second photodetector including a light-receiving area for receiving the reflected light of the positive higher-order diffracted light from the information surface where the 0th order light should be focused on; and a third photodetector that is adjacent to the first photodetector leaving a predetermined space therebetween, the third photodetector including a light-receiving area for receiving the reflected light of the negative higher-order diffracted light from the information surface where the 0th order light should be focused on, wherein the light-receiving areas of the second and the third photodetectors include other-order light receiving areas that receive the reflected light of the 0th order light from an information surface where the 0th order light should not be focused on, portions of the other-order light receiving areas of the second and the third photodetectors include light reception prohibited areas that prohibit the reception of the reflected light of the 0th order light from the information surface where the 0th order light should not be focused on.