Focusing error detecting device in head assembly for optical disc

Claim 1

A focusing error detecting device in a head assembly for an optical disc, including optical waveguides formed by a thin film for conduction of light emitted from a light source; a focusing diffraction grating provided on said thin-film waveguide layer for directing the light advancing through the waveguide layer toward an information recording surface of the optical disc and focusing it on said information recording surface; first and second light receiving diffraction gratings for conducting the light reflected by said information recording surface again into said thin-film waveguide layer; a first photo detector which receives the light introduced into said thin-film waveguide layer through said first light receiving diffraction grating and produces a first electric signal proportional to the intensity of the light; and a second photo detector which receives the light introduced into said thin-film waveguide layer through said second light receiving diffraction grating and produces a second electric signal proportional to the intensity of the light, said first light receiving diffraction grating having an optical characteristic that the conversion efficiency to a guided beam becomes maximum when a focused spot of the light focused by said focusing diffraction grating is in a position farther than said information recording surface, and said second light receiving diffraction grating having an optical characteristic that the conversion efficiency to a guided beam becomes maximum when said focused spot is in a position nearer than said information recording surface.