OPTICAL HEAD

PURPOSE:To improve the detecting sensitivity for an out-of-focus state and to attain a compact optical system by using a means to detect the out-of-focus state on an information recording medium and a polarized beam splitter set on an optical path together with a means which converts the section spot size of a beam. CONSTITUTION:The laser beam L sent from a light source 33 is condensed on the surface of an optical disk 21 and this reflected light is reflected again by a polarized beam splitter 35. This reflected beam is delivered from an output surface 401 including an optical axis through a prism 41 unified with the splitter 35. Then the reflected beam is transmitted through a projection lens 42 and irradiated to a light detector 43 to be divided into a beam L1 which is delivered from an output surface 402 and detects a track shift and a beam L2 which detects an out-of-focus state through a light detector 45. Therefore a small change is given to the section spot size of a beam through the prism 41 to improve the detecting sensitivity for out-of-focus state. The focal distance of the lens 42 is shirtened since the prism 41 is unified with the splitter 35. This attains a compact optical system with reduction of number of parts and easy assembly and adjustment.