| 摘要 |
PURPOSE:To decrease the off-setting quantity of a focus dislocation signal by equalizing an electric signal from respective elements when the areas of two adjoining elements are not equal out of respective elements divided into four for a four-divided light detecting device and the spot on the four-divided light detecting device comes to an approximately circle. CONSTITUTION:A projecting image by a light beam group on the incident surface of an objective lens 4 is a straight line, and passing through a converging lens 6 and a cylindrical lens 7, the projecting image on a four-divided light detecting device 18 is curved. The projecting image by a light beam group 11 of an orthogonal direction to a pit column 9 is also curved on the four-divided light detecting device 18. On the other hand, in a bus direction 14 of the cylindrical lens 7 and a direction 15 orthogonal to it, the light beam group of a 45 deg. direction is not curved and the light beam density in the spot on the four-divided light detecting device 18 comes to unequal. At the time of focusing, a spot pattern on the four-divided light detecting device 18 is round, the output from respective four-divided elements 18a-18d comes to the same, and the point in which the focus dislocating signal comes to zero is approximately central to respective focus surfaces of the front side and the rear side of the astigmatism. Thus, the off-setting of the focus dislocating signal is made smaller. |
