Method for generating a focus error signal due to astigmatism and optical pickup device using the same

An optical pickup device comprising:

• an irradiation optics for irradiating laser beams as at least two light spots onto an information recording surface of an optical disc on which a land and a groove are spirally or concentrically formed and scanning said two light spots along the lands or grooves while maintaining the distance between the light spots in such a manner that the distance between the centers of said two light spots in a radial direction of said optical disc is one half of the track pitch;
• first and second photodetectors each including a light receiving surface for receiving reflected light and divided into first to fourth quadrants defined in a tangential direction and in a radial direction from the center of the light receiving surface as at least four elements positioned in a centrally symmetric relationship with respect to the center of the light receiving surface within the first to fourth quadrants;
• a photodetection optics having an astigmatism generating element for applying astigmatism to light reflected from said light spots of the information recording surface and introducing the reflected lights into said first and second photodetectors respectively;
• first and second photodetecting-means for generating first and second preliminary focus error signals respectively each including;
• a first diagonal adding means connected to the first and third quadrant elements on centrally symmetric positions of said photodetector for adding outputs of said elements;
• second diagonal adding means connected to second and fourth quadrant elements on centrally symmetric positions of said photodetector for adding outputs of said elements;
• and
• diagonal differential means connected to said first and second diagonal adding means for generating an output difference between outputs of said diagonal adding means as the first or second preliminary focus error signal;
• and
• diagonal adding means connected to the pair of diagonal differential means of the first and second photodetecting-means for adding outputs of the first and second preliminary focus error signals;
• diffraction grating element.