| 摘要 |
PURPOSE:To suppress deterioration in detection ability of signals of focus error caused by minute displacement of a light detector due to changes in ambient temperature, etc. by connecting electrically light receiving areas in diagonal positions out of four light receiving areas of a quartered light detector, taking the sum, and providing a differential detector that takes difference in summed electric signals. CONSTITUTION:Two similar semicircular light spots of the same direction and equal quantity of light are formed on the quartered light detector 7 by a convex lens 5a and a reflective optical element 6, and the position of the quartered light detector 7 is set so as to make the quantity of light received by four light receiving areas equal. Sum of electric signals generated in diagonal receiving areas of the quartered light detector 7, and their difference is found by a differential amplifier 8. When an information recording medium 3 is displaced to get near an objective 2, divergent light enters the convex lens 5a. Thus, two convergence points caused by the convex lens 5a are deviated in the direction away from the reflective optical element 6, and light sports S and T are formed on the quartered light detector 7. At this time, a negative focus error signal can be detected from an output of the differential amplifier 8. |
