Optical head and optical disk apparatus

An information recording/reproducing apparatus, comprising:

• a laser beam source;
• an objective lens for condensing a beam from said laser beam source on an optical disk;
• variable focusing means for moving a focal position of an optical spot condensed by said objective lens;
• tracking means for making the optical spot track a recording track of said optical disk;
• spherical aberration adding means for adding a variable spherical aberration to light condensed by said objective lens;
• coma aberration adding means for adding a variable coma aberration to light condensed by said objective lens;
• a light detector for detecting reflected light from the optical disk;
• an operation circuit for generating a reproducing signal, a focal shift signal and a tracking shift signal from a detection signal of said light detector;
• servo-control means for controlling the variable focusing means, the tracking means, the spherical aberration adding means and the coma aberration adding means;
• and
• a laser drive circuit for driving said laser beam source in accordance with a signal modulated by a record signal,
• wherein said information reproducing apparatus further comprises:
• a detected light dividing device for dividing a light flux in the vicinity of an optical axis of said reflected light into a plurality of partial light fluxes to make incident onto the light detector, and dividing the light flux in a peripheral portion of said reflected light into a plurality of partial light fluxes to make incident onto said light detector,
• wherein said operation circuit generates:
• a first focal shift signal and a first tracking shift signal based on the detection signal of the partial light fluxes derived from said light flux in the vicinity of the optical axis,
• a second focal shift signal and a second tracking shift signal based on the detection signal of the partial light fluxes derived from said light flux of the peripheral portion,
• a spherical aberration signal in accordance with a differences signal between said first and second focal shift signals, and a focal shift signal in accordance with a sum signal, and
• a coma aberration signal in accordance with a difference signal between the first and second tracking shift signals, and a tracking shift signal in accordance with a sum signal, and
• wherein said servo-control means controls said variable focusing means in accordance with said focal shift signal, said tracking means in accordance with said tracking shift signal, said spherical aberration adding means in accordance with said spherical aberration signal, and said coma aberration adding means in accordance with said coma aberration signal.

A servo-control method for making an optical spot condensed by an objective lens track a record track of an optical disk, comprising the steps of:

• detecting a first focal shift signal and a first tracking shift signal by use of an inside light flux containing a light flux center of the reflected light out of reflected light from the optical disk;
• detecting a second focal shift signal and a second tracking shift signal by use of an outside light flux enclosing said inside light flux out of the reflected light from said optical disk;
• controlling a focal position of the optical spot in accordance with a sum signal of said first and second focal shift signals;
• controlling a tracking of said optical spot in accordance with a sum signal of said first and second tracking shift signals;
• adding a spherical aberration to light condensed by said objective lens in accordance with a spherical aberration signal generated based on a difference between said first and second focal shift signals;
• and
• adding a coma aberration to light condensed by said objective lens in accordance with a coma aberration signal generated based on a difference between said first and second tracking shift signals.