Optical head for an optical disk drive

Abstract

An optical head of the present invention applicable to an optical disk drive includes a light-transmitting phase shifting device. The phase shifting device reduces aberration ascribable to a difference in the thickness of the substrate of an optical disk. This not only insures the effective reproduction of data out of optical disks having different substrate thicknesses, but also guarantees a sufficient quantity of light representative of a reproduced signal and a sufficient lens numerical aperture. The optical head therefore achieves a desirable reproduced signal characteristic.

What is claimed is:  
     
         1 . An optical head comprising:  
 a laser having a wavelength λ;     an objective lens for causing a beam issuing from said laser to be transmitted through a substrate of an optical disk having a preselected thickness and to form a fine spot on a recording surface of the optical disk;     optics for sensing a resulting reflection from the recording surface of the optical disk; and     light-transmitting phase shifting means extending along an annular zone symmetrical with respect to an optical axis of the beam incident to said objective lens, for shifting a phase of the beam by 0.3 λ or less.    
     
     
         2 . An optical head as claimed in    claim 1   , wherein said phase shifting means comprises a dielectric film.  
     
     
         3 . An optical head as claimed in    claim 1   , wherein said phase shifting means is provided on a surface of said objective lens.  
     
     
         4 . An optical head as claimed in    claim 3   , wherein said phase shifting means comprises a dielectric film.  
     
     
         5 . An optical head comprising:  
 a laser having a wavelength λ;     an objective lens for causing a beam issuing from said laser to be transmitted through a substrate of an optical disk having a preselected thickness and to form a fine spot on a recording surface of the optical disk;     optics for sensing a resulting reflection from the recording surface of the optical disk; and     light-transmitting phase shifting means extending along an annular zone symmetrical with respect to an optical axis of the beam incident to said objective lens, for effecting a phase shift selectively variable to zero or to a preselected value other than zero in response to a signal input from outside of said optical head.    
     
     
         6 . An optical head as claimed in    claim 5   , wherein said phase shifting means comprises a dielectric film.  
     
     
         7 . An optical head as claimed in    claim 5   , wherein said phase shifting means is provided on a surface of said objective lens.  
     
     
         8 . An optical head as claimed in    claim 7   , wherein said phase shifting means comprises a dielectric film.  
     
     
         9 . An optical head comprising:  
 a laser having a first wavelength for reproducing data out of an optical disk whose substrate has a first thickness;     a laser having a second wavelength for reproducing data out of an optical disk whose substrate has a second thickness;     a wavelength combiner for causing each of beams issuing from said lasers to propagate in a same direction;     an objective lens for causing each of the beams issuing from said lasers to be transmitted through the substrate of the optical disk having a particular thickness and to form a fine spot on a recording surface of the optical disk;     optics for sensing a resulting reflection from the recording surface of the optical disk; and     light-transmitting phase shifting means extending along an annular zone symmetrical with respect to an optical axis of the beam incident to said objective lens, for effecting a substantially zero phase shift with the beam having the first wavelength, but shifting a phase of the beam having the second wavelength by a value other than zero.    
     
     
         10 . An optical head as claimed in    claim 9   , wherein said phase shifting means comprises a dielectric film.  
     
     
         11 . An optical head as claimed in    claim 9   , wherein said phase shifting means is provided on a surface of said objective lens.  
     
     
         12 . An optical head as claimed in    claim 11   , wherein said phase shifting means comprises a dielectric film.  
     
     
         13 . An optical head comprising:  
 a laser;     an objective lens for causing a beam issuing from said laser to be transmitted through a substrate of an optical disk having a preselected thickness and to form a fine spot on a recording surface of the optical disk;     optics for sensing a resulting reflection from the recording surface of the optical disk; and     a plurality of phase shifting means extending along an annular zone symmetrical with respect to an optical axis of the beam incident to said objective lens, for shifting a phase of each incident beam by a particular amount.    
     
     
         14 . An optical head as claimed in    claim 13   , wherein said plurality of phase shifting means comprise dielectric films.  
     
     
         15 . An optical head as claimed in    claim 13   , wherein said plurality of phase shifting means are provided on a surface of said objective lens.  
     
     
         16 . An optical head as claimed in    claim 15   , wherein said plurality of phase shifting means comprise dielectric films.