Separation-type optical information recording and reproducing apparatus producing offset-eliminated focus error signal

Claim 1

An optical information recording and reproducing apparatus, comprising: (a) a stationary unit which is fixed with respect to an optical record disk in which at least one circular information track is formed; and (b) a movable unit which is arranged movably in a radial direction of the information track in the optical record disk, said stationary unit comprising a light source for emitting a light beam, a first beam splitting means for directing the light beam emitted from the light source to the movable unit and directing a light beam transmitted from the movable unit in a direction which is different from a direction toward the light source, a second beam splitting means for separating the light beam emanating from the first beam splitting means into two sub-beams, a first photodetecting means having a least two light receiving regions arranged in a tangetial direction of the information track, a second photodetecting means having at least two light receiving elements arranged in the tangetial direction of the information track, and a focusing error detecting optical element arranged in the first sub-beam emanating from the second beam splitting means, said focusing error detecting optical element introducing a change in a light amount distribution of the first sub-light beam in the tangential direction; said movable unit comprising an objective lens for projecting the light beam emanating from said first beam splitting means onto the optical record disk as a light spot; and (c) a calculating circuit means for deriving a focus error signal from a first signal supplied from the first photodetecting means and including a signal component representing the focus error and a second signal supplied from said second photodetection means and having a signal component representing a change in the light amount distribution in the tangential direction due to factors other than the focus error, wherein said calculating circuit means comprises a first differential amplifier for deriving a first difference signal between the output signals supplied from the first photodetecting means, a first adder for deriving a first sum of the output signals supplied from the first photodetecting means, a first divider for deriving as said first signal a first division signal between the first difference and sum signals, a second differential amplifier for deriving a second difference signal between the output signals supplied from the second photodetecting means, a second adder for deriving a second sum of the output signals supplied from the second photodetecting means, a second divider for deriving as said second signal a second division signal between the second difference signal and the sum signal, and a third differential amplifier for deriving a third difference signal between said first and second division signals as the actual focus signal.