Optical information storage device having phase compensating mechanism and polarization plane rotating mechanism

An optical system for an optical information storage device, comprising:

• an optical source;
• an optical head having an objective lens for focusing light from said optical source onto a recording surface of an optical recording medium;
• a first photodetector for detecting a reproducing signal from reflected light from said optical recording medium;
• a second photodetector for detecting a tracking error signal and a focusing error signal from said reflected light;
• a beam splitter for separating said reflected light into a first beam directed toward said first photodetector and a second beam directed toward said second photodetector;
• a phase compensating mechanism provided between said beam splitter and said first photodetector for compensating for a phase difference of said first beam, said phase compensating mechanism including a phase plate and a support member for supporting said phase plate so that said phase plate is rotationally adjustable about an axis perpendicular to an optical path of said first beam, said phase plate having a thickness such that a phase difference larger than the sum of the maximum values of variations in phase difference in all optical components constituting an optical system through which said reflected light passes is given to said first beam when the angle of incidence of said first beam on said phase plate is 0°;
• beam separating means provided between said phase compensating mechanism and said first photodetector for separating said first beam into two beams having different polarization planes;
• and
• polarization plane rotating means for rotating a polarization plane of said first beam incident on said beam separating means.

An optical information storage device comprising:

• an optical source;
• an optical head having an objective lens for focusing light from said optical source onto a recording surface of an optical recording medium;
• a first photodetector for detecting a reproducing signal from reflected light from said optical recording medium;
• a second photodetector for detecting a tracking error signal and a focusing error signal from said reflected light;
• a beam splitter for separating said reflected light into a first beam directed toward said first photodetector and a second beam directed toward said second photodetector;
• a phase compensating mechanism provided between said beam splitter and said first photodetector for compensating for a phase difference of said first beam;
• a Wollaston prism provided between said phase compensating mechanism and said first photodetector for separating said first beam into two beams having different polarization planes;
• polarization plane rotating means for rotating a polarization plane of said first beam incident on said beam separating means;
• and
• a condenser lens for focusing said first beam on said first photodetector;
• wherein said phase compensating mechanism includes a phase plate and a support member for supporting said phase plate so that said phase plate is rotationally adjustable about an axis perpendicular to an optical path of said first beam, and
• wherein said Wollaston prism, said condenser lens, and said first photodetector have a common optical axis shifted from a center of an optical path of said first beam incident on said phase plate by a distance equal to a shift amount of said first beam generated in rotationally adjusting said phase plate so as to compensate for the phase difference of said first beam.