OPTICAL HEAD

PURPOSE:To increase the processing speed of reproduction signal with a compact optical head with low cost by using two photodetectors as well as a member which extracts the luminous flux reflected by an information storage medium and transmitted through a projection lens asymmetrically an optical axis. CONSTITUTION:The laser beam sent from a light source 21 is focused toward an information storage medium 27 through an objective lens 26, and this reflected light is made incident on a projection lens 30. The lens 30 consists of a spherical lens 31 and another spherical lens 32 cut partially as adhered together. Thus the lens 30 divides an incident beam into two parts. The 1st beam components 33 is irradiated to a photodetector 36 to detect a tracking shift and to read information signals; while the 2nd beam component 37 detects an out-of-focus state by a photodetector 37. The diameter of the photodetector 36 can be reduced since it condenses at a short focal distance part of the lens 30. This improves the frequency characteristics of a detection signal and increases the signal processing speed. Thus a half mirror can be omitted to realize a compact optical head with reduction of the cost.