Optical head, diffraction device, objective lens, and optical disc apparatus

An optical head comprising:

• a first light source configured to emit a blue-violet laser beam;
• a second light source configured to emit a red laser beam;
• a third light source configured to emit an infrared laser beam;
• a diffraction device, with n, m and k as integers, configured to focus an n-order diffraction light out of diffraction lights with plural orders which are generated by diffracting a laser beam emitted from the first light source onto an information recording surface of a first information recording medium, configured to focus an m-order diffraction light out of diffraction lights with plural orders which are generated by diffracting a laser beam emitted from the second light source onto an information recording surface of a second information recording medium, configured to focus a k-order diffraction light out of diffraction lights with plural orders which are generated by diffracting a laser beam emitted from the third light source onto an information recording surface of a third information recording medium;
• a condensing lens configured to condense the laser beam reflected on the information recording surface of the first information recording medium, reflected on the information recording surface of the second information recording medium, or reflected on the information recording surface of the third information recording medium;
• and
• a light receiving unit configured to receive the laser beam condensed by the condensing lens,
• wherein the k-order diffraction light has a concave lens power,
• the optical path of the laser beam before being reflected on the one of the surfaces of the first, second and third information recording mediums is an advancing path, the optical path of the laser beam after being reflected on the one of the surfaces of the first, second and third information recording mediums is a returning path,
• a position of the optical axis direction in the light receiving unit, in which a diffraction light with the k-order diffraction light in the advancing path and the k-order diffraction light in the returning path, reflected on the information recording surface of the third information recording medium, is condensed by the condensing lens and then the condensed diffraction light
• forms a focal spot in the light receiving unit is different from a position of the optical axis direction in the light receiving unit, in which a diffraction light that is different from the k-order diffraction light in at least one of the advancing path and the returning path, reflected on the surface of the third information recording medium is condensed by the condensing lens and then the condensed diffraction light forms a focal spot in the light receiving unit.