Astigmatic difference correcting method for optical head and apparatus therefor

Claim 1

An astigmatic difference correcting method for an optical head, comprising the steps of: rotating and adjusting a rotational optical member which is arranged between a first laser beam source having an astigmatic difference and a collimator lens to be rotatable around an optical path of a laser beam of a first wavelength emitted from said first laser beam source; causing rotation and adjustment of said rotational optical member to set a minimum difference between focal positions in an X-axis direction and in a Y-axis direction which are perpendicular to the optical path of the laser beam; irradiating the laser beam in which the difference between focusing offset positions is minimum on an optical disk having a recording density having a bit length not more than half a laser wavelength; causing the laser beam of the first wavelength to be incident on and emerge from said optical disk through a dichroic mirror and an objective lens; causing a laser beam of a second wavelength emitted from a second beam source to be incident on and emerge from said optical disk through said dichroic mirror and said objective lens, said dichroic mirror allowing one of the laser beams of the first and second wavelengths to be transmitted through said dichroic mirror and allowing the other to be reflected by the dichroic mirror; and detected a focusing error between said optical disk and said objective lens using a beam of the second wavelength obtained through said dichroic mirror and reflected by said optical disk.