Optical head including multiple photo detectors for reading signals and error signals for servoing from both a read-only recording medium and a magneto-optical recording medium

An optical head device movable along the radius of a disc-shaped optical recording medium for recording and/or reproducing information signals on or from the disc-shaped optical recording medium via a light beam, said device comprising:

• a light source;
• a diffraction grating arranged on a light path of a light radiated from said light source for splitting a light beam radiated from said light source into at least three light beams;
• an objective lens for converging the light beams radiated from the light source and transmitted through said diffraction grating on a signal recording surface of said disc-shaped optical recording medium;
• driving means for driving said objective lens along the direction parallel to its optical axis and within a plane perpendicular to said optical axis;
• a beam splitter for splitting a light beam incident via said objective lens from the light beam radiated from said light source,
• photodetector means having a first photodetector having a light-receiving surface divided by two boundary lines into four sections, second and third photodetectors arrayed in a direction parallel to one of said boundary lines on both sides of said first photodetector and fourth and fifth photodetectors arrayed in a direction perpendicular to said one of said boundary lines on both sides of said first photodetector;
• a splitting optical element arranged between said photodetector means and said beam splitter on an optical axis of a light beam incident thereon via said objective lens, said splitting optical element splitting each of the three light beams from said diffraction grating into three more light beams;
• and
• a focusing error detection optical element arranged between said photodetector means and said beam splitter on an optical axis of a light beam incident thereon via said objective lens, said focusing error detection optical element causing changes in the shape of a spot on the light receiving surface of said first photodetector in accordance with displacement of the disc-shaped optical recording medium in a direction perpendicular to the in-plane direction of the disc-shaped optical recording medium.