Focusing error detecting device in head assembly for optical disc

摘要

A focusing error detecting device used in a head assembly for recording information to a recording surface of an optical disc or reading out information already recorded, the focusing error detecting device including a diffraction grating for focusing light which has been introduced from a light source into a thin-film waveguide layer, onto the recording surface, and two light receiving diffraction gratings for conducting the light reflected by the recording surface again into the waveguide layer. The two light receiving diffraction gratings have optical characteristics which differ from each other, depending the position of a focused spot of the light focused on the recording surface by the focusing diffraction grating. Therefore, by comparing the intensity of light from one light receiving diffraction grating with that from the other light receiving diffraction grating, there can be seen a positional relationship between the focused spot and the recording surface.