JPS647330A - Focus controller - Google Patents

Abstract

PURPOSE:To attain stable focus control by using a detection signal of a photodetector having detection areas split into three by a reference line in parallel with a generator and detection areas split into two by a reference line at a right angle to the generator so as to control the focus of the covered light onto a recording medium. CONSTITUTION:Astigmatism lenses 18, 20 are arranged on a different position on the optical axis of a cylindrical lens 17 and its generator is arranged in a direction orthogonal to the generator of the lens 17 and the optical axis of the optical system. Moreover each end side face of the lenses 18, 20 is arranged on the optical axis of the lens 17 in a way that the faces are in contact with each other. Thus, the laser beam incident from the lens 17 is split by the lenses 18, 20 and then radiated onto the photodetector 19. The detector 19 is provided with photodetector cells 19a-19f split into three in parallel with the generator of the lenses 18, 20 and split into two in a direction at a right angle thereto. The focus of the laser beam L to the optical disk 1 is controlled by detection signals of the cells 19a-19f. Thus, stable focus control is applied.