METHOD AND APPARATUS FOR DETECTION OF FOCUS ERROR

PURPOSE: To obtain a focus error signal at a high output and stably by providing a hologram lens, two 3-division lights detectors and a focus error detection circuit. CONSTITUTION: A hologram lens 11 diffracts diffracted lights 15a to 15c from reflected lights 15 which are reflected from an optical disk 1 and focused, and two 3-division lights detectors 12, 13 receive respectively ± primary diffracted lights 15b, 15c from the hologram lens 11. A focus error detection circuit 14 calculates a signal obtained from the detectors 12, 13 to output a focus error signal. -Primary diffracted lights of the diffracted lights from the hologram lens 11 pass through a focusing point of + primary diffracted lights and form an astigmatism rotated at 180 deg.. Accordingly, an error by a tilt of the optical disk 1 and a shift of an objective lens 2 is differentiated to an offset direction, whereby it is possible to obtain a focus error signal in which, if stable, it simultaneously has a high output value.