Optical head device and optical disk playback system

An optical head device comprising:

• a light source;
• an objective lens that forms from a light beam radiated from the light source, a beam focused on an information recording layer of an optical disk;
• a light receiving device that has a plurality of light reception surfaces to receive light reflected from the optical disk, the light receiving device converting a quantity of the light received on the respective light reception surfaces into electrical signals to detect an information reproduction signal of optical disk and/or its error detection signal;
• and
• an aperture control means that makes an exit pupil diameter of the objective lens for the reflection light larger than an entrance pupil diameter of the objective lens for the light beam;
• said optical head device detecting, using the light receiving device, reflection light within an area larger than the entrance pupil diameter, and produces a reproduction signal by summing up or differentiating between the electrical signals outputted from the light receiving device, wherein the light receiving device includes a
• first light reception surface that receives a peripheral portion of the reflection light in area
• a second light reception surface that receives a central portion of the reflection light, and
• a signal-level converting means that is capable of varying respective signal levels of electrical signals from the first light reception surface and the second light reception surface,
• and the light receiving device produces the reproduction signal by summing up or differentiating between the electrical signal outputted through the signal-level converting means and the electrical signal outputted through the signal-level converting means from the second light reception surface.

An optical head device comprising:

• A light source;
• an objective lens that forms from a light beam radiated from the light source, a beam focused on an information recording layer of an optical disk;
• a light receiving device that has a plurality of light reception surfaces to receive light reflected from the optical disk, the light receiving device converting a quantity of the light received on the respective light reception surfaces into electrical signals to detect an information reproduction signal of optical disk and/or its error detection signal;
• and
• an aperture control means that makes an exit pupil diameter of the objective lens for the reflection light larger than an entrance pupil diameter of the objective lens for the light beam;
• said optical head device detecting, using the light receiving device, reflection light within an area larger than the entrance pupil diameter, and produces a reproduction signal by summing up or differentiating between the electrical signals outputted from the light receiving device, wherein the aperture control means includes
• an aperture control element that has an inner side area and an outer side area which are divided by a boundary that is an approximate circle with its diameter being the effective entrance pupil diameter determined by the design of the objective lens, and
• an analyzer that is placed between the aperture control element and the objective lens, and transmits only light polarized in one direction,
• wherein the outer side area comprises a half-wavelength plate that rotates the polarization plane of the light beam by 90 degrees.