Optical focusing control apparatus

Claim 1

An optical focusing control apparatus for performing focusing control of an objective lens which condenses light on a recording medium and allows reflected light returned from the recording medium to pass therethrough, comprising: a condenser lens for condensing the returned light which passes through said objective lens; a beam splitter for splitting light received from said condenser lens into light along first and second optical paths; a first detector disposed closer to said beam splitter than the focal point of the first optical path of said condenser lens for detecting a quantity of light received from said beam splitter along said optical path; a second detector disposed further from said beam splitter than the focal point of the second optical path of said condenser lens for detecting a quantity of light received from said beam splitter along said second optical path; said first detector including a light-receiving surface having a center situated on an optic axis of the first optical path and an area smaller than that of an irradiation pattern of the returned light emerging from said condenser lens; said second detector including a light-receiving surface having a center situated on an optic axis of the second optical path and an area smaller than that of an irradiation pattern of the returned light emerging from said condenser lens; said light-receiving surface in said first and second detector divided into four portions by a straight line, which is obtained by projecting a track direction of said recording medium, and a circle which is defined by a circumference of the area smaller than that of an irradiation pattern of the returned light emerging from said condenser lens, two portions of the four portions of said light-receiving surface in said first and second detector, which are defined by the circle, for producing a photoelectric output indicative of the detected quantity of light; said first and second detectors being disposed in such a manner that the respective photoelectric outputs thereof coincide when said objective lens is focused; a first adder for receiving the photoelectric output from each of the two portions of said light-receiving surface in said first detector and producing a sum of the photoelectric output; a second adder for receiving the photoelectric output from each of the two portions of said light-receiving surface in said second detector and producing a sum of the photoelectric output; a subtracter for producing a difference between the sum of the photoelectric output produced by said first adder and the sum of the photoelectric output produced by said second adder; and said objective lens having its focusing control based on the difference produced by said subtractor.