Optical head and optical disc apparatus

An optical head unit comprising:

• a first light source which outputs light with a first wavelength;
• a second light source which outputs light with a second wavelength different from the wavelength of the light from the first light source;
• an object lens which focuses light outputted from the first and second light sources and overlapped in an optical path on the recording/reproduce surface of a recording medium, and captures light reflected on the recording/reproduce surface of a recording medium;
• a diffraction element which is provided on an optical path toward the object lens, and has a diffraction pattern to give a fixed diffraction characteristic, in a part of the light reflected on the recording/reproduce surface;
• a first photodetector which detects light given a fixed diffraction characteristic by the diffraction pattern of the diffraction element, and outputs a first signal;
• a second photodetector which detects light passing through an area different from the diffraction pattern of the diffraction element, and outputs a second signal;
• and
• a signal processing unit which generates a component to compensate an offset of the object lens in the radial direction on the recording/reproduce surface based on the output of the first photodetector, and generates a component to correct the position of the object lens in the radial direction on the recording/reproduce surface and to correct the distance of the object lens to the recording/reproduce surface based on the output of the second photodetector.

An optical disc apparatus comprising:

• an optical head unit having a first light source which outputs light with a first wavelength;
• a second light source which outputs light with a second wavelength different from the wavelength of the light from the first light source;
• an object lens which focuses light outputted from the first and second light sources and overlapped in an optical path on the recording/reproduce surface of a recording medium, and captures light reflected on the recording/reproduce surface of a recording medium;
• a diffraction element which is provided on an optical path toward the object lens, and has a diffraction pattern to give a fixed diffraction characteristic, in a part of the light reflected on the recording/reproduce surface;
• a first photodetector which detects light given a fixed diffraction characteristic by the diffraction pattern of the diffraction element, and outputs a first signal;
• a second photodetector which detects light passing through an area different from the diffraction pattern of the diffraction element, and outputs a second signal;
• and a signal processing unit which generates a component to compensate an offset of the object lens in the radial direction on the recording/reproduce surface based on the output of the first photodetector, and generates a component to correct the position of the object lens in the radial direction on the recording/reproduce surface and to correct the distance of the object lens to the recording/reproduce surface based on the output of the second photodetector;
• and
• a recorded information reproduce unit which reproduces information recorded on the recording/reproduce surface of the recording medium from an output of the second photodetector.