Error detecting device for optical head

Claim 1

An error detecting device for an optical head, comprising: optical means for converting a reflected light of a light irradiating a surface to be irradiated into an astigmatic pencil of rays having a first focal point and a second focal point, said optical means having a first focusing means for focusing said reflected light on said first focal point of said astigmatic pencil of rays, and a second focusing member for focusing said reflected light on said second focal point of said astigmatic pencil of rays, said first and second focusing members being made integral; and means for detecting errors of said light irradiating the surface to be irradiated, based on a detecting signal of a spot of said reflected light formed by said first focusing member, and a detecting signal of a spot of said reflected formed by said second focusing member, wherein said optical means is provided by a Fresnel zone plate, said zone plate including; a first, second, third and a fourth Fresnel zone plate member divided in four by two parting lines passing the center of said Fresnel zone plate, said first and said third Fresnel zone plate members opposed to each other across said center has said first focal point, and said second and said fourth Fresnel zone plate members opposed to each other across said center has said second focal point; and said detecting means having: a first, a second, a third and a fourth detecting section for respectively detecting a spot of said reflected light formed by said first, said second, said third and said fourth Fresnel zone plate members, a focusing error computing unit for computing a focusing error signal, based on detecting signals from said first, said second, said third and said fourth detecting sections, and a tracking error computing unit for computing a tracking error signal, based on detecting signals from at least said second and said fourth detecting sections.