Optical pickup apparatus and optimal light spot focusing method

A method of optimal focusing of a light spot by which seeking of an optimal focal point of a light spot is possible, the method comprising:

• dividing a light beam radiated from a light source onto a recording medium and reflected from the recording medium into first through fourth light regions in directions corresponding to tangential and radial directions of a recording medium and dividing the first through fourth light regions into an inner light receiving region and an outer light receiving region in a direction corresponding to the radial direction to detect signals;
• obtaining a subtracting signal between a sum signal of ones of the detected signals of the outer light receiving regions and a sum signal of ones of the detected signals of the inner light receiving regions;
• and
• focusing a light spot onto a position where a size of the differential signal is minimized.

An optical pickup apparatus comprising:

• a light source to emit a light beam having a wavelength corresponding to a recording medium;
• an objective lens to focus the light beam to form a light spot on a recording surface of the recording medium;
• a diffraction member to divide and diffract the light beam reflected by the recording medium and having passed through said objective lens to be incident thereon, said diffraction member to divide the reflected light into five light regions, said diffraction member comprising
• a first diffraction region,
• second through fifth diffraction regions arranged in a tangential direction corresponding to a tangential direction of the recording medium and a radial direction of the recording medium,
• wherein the first diffraction region is disposed between an adjacent pair of the second through fifth diffraction regions in the radial direction and the first diffraction region has a same width in the tangential direction as another adjacent pair of the second through fifth diffraction regions in the tangential direction;
• and
• a photodetector having first through fifth light receiving portions to receive corresponding portions of the light beam reflected by the recording medium and diffracted by the first through