Patent ID: 7012875

Claim:
An optical disk apparatus comprising: an optical head which is constituted by: a semiconductor laser; an objective lens for condensing laser light of said semiconductor laser onto an optical disk; a variable focal point mechanism for varying a focus position of the light condensed in an optical system; a spherical aberration applying mechanism for adding variable spherical aberration to the light condensed in said optical system; a first optical branching element for branching reflection light reflected from said optical disk from said optical system; a lens for condensing the branched reflection light; and a light receiving element for receiving the light condensed by said lens so as to convert the received light into an electric signal; and in which a second branching element is additionally provided in luminous flux of said light, while said second branching element branches reflection light to be branched in such a manner that said second branching element further separates the light to be branched into first luminous flux located in the vicinity of an optical axis and second luminous flux located at a peripheral portion of said optical axis, and both said first luminous flux and said second luminous flux are condensed to said light receiving element; and a calculation circuit for acquiring both a reproduction signal and a focal shift signal from the electric signal derived from said light receiving element; wherein: a first focal shift signal and a second focal shift signal are detected as to each of said first luminous flux and said second luminous flux; said spherical aberration applying mechanism is controlled based upon a subtraction signal obtained by essentially performing a subtraction between said first focal shift signal and said second focal shift signal; and said variable focal point mechanism is controlled based upon a summation signal obtained between said first focal shift signal and said second focal shift signal: said spherical aberration applying mechanism does not apply aberration to outgoing light, but applies aberration to incoming light; and a feedback system is constituted so as to compensate spherical aberration on said optical disk plane in such a manner that said first focal shift signal and said second focal shift signal are detected, and a spherical aberration error which is obtained from a substraction signal by essentially substracting said first and second focal-shift signals is fed back to said spherical aberration applying mechanism.