Optical head device for optimally detecting a focussing error

Abstract

In an optical head device, a grating element (22) is partitioned into first and second regions (221, 222) by a partition line (23) which passes through its point of intersection with an optical axis of an optical beam generated from a source point (17) and through a particular point. According to this invention, the particular point is determined so as to obtain an optimal focussing error signal within a dynamic range. An optical detector (24) is segmented into three segments by first and second segmentation lines (251, 252) which should preferably form particular angles with a reference plane (35) defined by the source point, the point of intersection, and a middle point between first and second points of convergence of beams diffracted from the first and the second regions. Alternatively, the detector is segmented into first and second sets of segments by a segmentation line on which the first and the second points of convergence are present and which should preferably form a particular angle with the reference plane. According to this invention, such a particular angle is determined so as to obtain the optimal focussing error signal even when the optical beam has a wavelength which is different from its nominal wavelength.