Optical information system focal point displacement detecting means having Fresnel zone plates with rectilinear grating

A displacement detecting device for an optical head emitting an irradiation light converging on a convergence position onto an irradiated face of an optical recording medium which reflects said irradiation light to provide reflected rays, said device including:

• optical means for converting said reflected rays from said irradiated face into an astigmatic bundle of rays focused on first and second focal point positions;
• and
• detecting means for detecting displacement of said radiation light with respect to said irradiated face on the basis of said first and second focal point positions of the reflected rays;
• said optical means comprising:
• a first group of Fresnel zone plates, each of said plates having a rectilinear grating for converging said reflected rays on said first focal point position;
• and
• a second group of Fresnel zone plates, each of said plates having a rectilinear grating for converging said reflected rays on said second focal point;
• said optical means being integrally formed by said first group of Fresnel zone plates and said second group of Fresnel zone plates;
• and
• said detecting means comprising:
• a photodetector for detecting a focusing shape of said reflected rays focused thereon by said first group of Fresnel zone plates and said second group of Fresnel zone plates and generating a detection signal;
• and
• means for detecting displacement between said convergence position of said irradiation light and said irradiated face on the basis of said detection signal.

A displacement detecting device for an optical head emitting an irradiation light converging on a convergence position onto an irradiated face of an information recording medium which reflects said irradiation light to provide reflected rays, and for detecting displacement between said convergence position and said irradiated face, said displacement detecting device comprising:

• optical means for converging said reflected rays, said optical means including an even number of converging sector pairs, each of said converging sector pairs comprising a pair of converging sectors, said pair of converging sectors being disposed in contrapositive positions with respect to a center point of said optical means and having symmetric shapes, each of said converging sectors comprising Fresnel zone plates being to focus rays incident thereto at nearly the same focal points, each of said converging sector pairs having a focal point different from that of a condensing sector pair adjacent thereto, said Fresnel zone plate having a plurality of rectilinear gratings;
• detecting means having the same number of photodetecting sectors as the number of said converging sectors, said photodetecting sectors being disposed in positions respectively corresponding to said converging sectors, said detecting means receiving said rays focused by said optical means and outputting a detected output according to quantity of said rays;
• and
• computing means for receiving said detected output and generating signals representing a focus displacement and a track displacement of said convergence position of said irradiation light from the irradiated face.