JPS5746334A - Optical information reproducer - Google Patents

Abstract

PURPOSE:To facilitate easy control for both the position and angle of a photo detector, by splitting a reflected beam given from a disk into four beams through a quadrangular pyramid prism to detect both an out-of-tracking signal and an out-of- focus signal. CONSTITUTION:A reflected beam 20 is splitted into four reflected beams 20a-20d through a quadrangular pyramid prism 22 after passing through a detector lens 21. The four ridges of the prism 22 are set in the projected track direction F and its right-angled direction when viewed from the light traveling direction. The beams 20a and 20b are made incident to photodetectors 23a and 23b respectively to obtain an out-of-tracking signal. This is exactly equivalent to the fact that the beams are detected by the 2-split photodetector at the position of the prism 22 although the area to be detected is on the focal plane of the lens 21. A reflected beam 20c declines by 45 deg. to the track direction and is made incident to a photodetector 24 which is set so that its parallel direction is turned toward the optical axis before split of the beam 20. Thus an out-of-focus signal is obtained.