Optical disk, aberration correction method and optical disk apparatus

An aberration correction method, comprising:

• a first step of forming, in a particular region of a recording face of an information recording layer of an optical disk wherein the recording face of the information recording layer on which a very small spot for recording or playing back information is formed is covered with a transparent substrate, a particular pattern wherein a plurality of pit trains having periods different from each other are disposed alternately;
• a second step of moving a condenser for forming the very small spot along an optical axis of said condenser to play back the particular pattern to determine amplitude variations of playback signals of the pit trains with respect to a focus error amount;
• a third step of determining focus offset amounts each of which provides a maximum amplitude from the amplitude variations of the playback signals of the pit trains determined in the second step;
• and
• a fourth step of adjusting the angle with which a light beam incoming to said condenser diverges or converges so that the difference between the focus offset amounts of the pit trains determined in the third step may be minimized to correct the spherical aberration of the very small spot which appears in accordance with a displacement of the thickness value of said transparent substrate from a prescribed value.

An optical disk apparatus, comprising:

• a condenser movable along an optical axis thereof for forming a very small spot for recording or playing back information on a recording face of an information recording layer of an optical disk wherein the recording face of the information recording layer is covered with a transparent substrate;
• an aberration correction element for adjusting the angle with which a light beam incoming to said condenser diverges or converges;
• a photo-detector for detecting reflected light from a pit formed on the recording face of the information recording layer;
• and
• control means for controlling the adjustment of the diverging or converging angle of the incoming light beam by said aberration correction element so that the amplitude of a playback signal, outputted from said photo-detector, of a particular pattern formed in a particular region of the recording face of the information recording layer from which a spherical aberration of the very small spot occurring in accordance with a displacement of the thickness of the transparent substrate from a prescribed value can be detected may be a predetermined amplitude.