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

An optical pickup device including an astigmatism generating element for applying astigmatism to light reflected from an information recording surface of an optical disc, comprising:

• a photodetector including a light receiving surface for receiving the reflected light and divided into first to fourth quadrants defined in a tangential direction and in a radial direction from the center of said light receiving surface, and at least four elements positioned in a centrally symmetric relationship with respect to the center of said light receiving surface within the first to fourth quadrants;
• first diagonal adding means connected to 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;
• 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 a preliminary focus error signal;
• first non-diagonal adding means connected to the first and second elements along the radial direction of said photodetector for adding outputs of said elements;
• second non-diagonal adding means connected to the third and fourth elements along the radial direction of said photodetector for adding outputs of said elements;
• first non-diagonal differential means connected to said first and second non-diagonal adding means for calculating the difference between outputs of said non-diagonal adding means;
• third non-diagonal adding means connected to the second and third elements along the tangential direction of said photodetector for adding outputs of said elements;
• fourth non-diagonal adding means connected to the fourth and first elements along the tangential direction of said photodetector for adding outputs of said elements;
• second non-diagonal differential means connected to said third and fourth non-diagonal adding means for calculating the difference between outputs of said adding means;
• multiplying means connected to said first and second non-diagonal differential means for multiplying outputs of said non-differential means to generate a correction signal;
• and
• correcting means connected to said diagonal differential means and said multiplying means for calculating the difference between said preliminary focus error signal and said correction signal to generate a focus error signal.