Objective lens for optical disks having different thicknesses

Abstract

A compound objective lens is composed of a hologram lens or transmitting a part of incident light without any diffraction to form a beam of transmitted light and diffracting a remaining part of the incident light to form a beam of first-order diffracted light, and an objective lens for converging the transmitted light to form a first converging spot on a front surface of a thin type of first information medium and converging the diffracted light to form a second converging spot on a front surface of a thick type of second information medium. Because the hologram selectively functions as a concave lens for the diffracted light, a curvature of the transmitted light differs from that of the diffracted light. Therefore, even though the first and second information mediums have different thicknesses, the transmitted light incident on rear surface of the first information medium is converged on the its front surface, and the diffracted light incident on a rear surface of the second information medium is converged on the its front surface. That is, the compound objective lens has two focal points.

1. A compound objective lens, comprising:
 lens means, having a first convex surface and a second convex surface opposite to each other, for receiving a beam of incident light of one particular wavelength passing through an optical axis at the first convex surface, refracting the beam of incident light and emitting a beam of refracted light from the second convex surface; and    plural focal point generating means for receiving the beam of incident light not yet refracted by the lens means, generating from the beam of incident light a plurality of beams of divided light including a first beam of divided light and a second beam of divided light, converging the beams of divided light at a plurality of focal points which are placed on the optical axis on a side facing the second convex surface of the lens means on condition that the first beam of divided light transmits through a first substrate and is converged on an information recording plane placed at a first distance T 1  from a surface of the first substrate at a diffraction limit and that the second beam of divided light transmits through a second substrate and is converged on an information recording plane placed at a second distance T 2  (T 1 ≠T 2 ) from a surface of the second substrate at a diffraction limit.    
     
     
       2. A compound objective lens according to  claim 1  in which the plural focal point generating means is a hologram generating from the incident light as the beams of divided light a plurality of beams of diffracted light having different diffraction orders. 
     
     
       3. A compound objective lens according to  claim 2  in which the first beam of divided light is a beam of transmitted light which agrees with a beam of zero-order diffracted light generated by the hologram, and the second beam of divided light is a beam of first-order diffracted light generated by the hologram. 
     
     
       4. A compound objective lens according to  claim 2  in which
 the hologram is a phase modulation relief type of diffraction device,    a grating pattern of the hologram has a step-wise cross section and is formed in a concentric circle shape,    the grating pattern of the hologram is concentrically partitioned into a plurality of blocks,    a phase modulation degree of the incident light passing through the grating pattern of the hologram varies in a step-wise shape of four stairs for each of the blocks, and    a ratio of an etching width of a top stair to a length of the corresponding block and another ratio of an etching width of a bottom stair to the length of the corresponding block are respectively lowered toward an outer direction of the grating pattern of the hologram.    
     
     
       5. A compound objective lens according to  claim 2  in which the hologram is a phase modulation relief type of diffraction device,
 a grating pattern of the hologram is formed in a concentric circle shape and is concentrically partitioned into a plurality of blocks,    a phase modulation degree of the incident light passing through an inner portion of the grating pattern of the hologram varies in a step-wise shape of four inside stairs for each of the blocks,    the four inside stairs are composed of a top stair, a second stair, a third stair and a bottom stair in that order,    a ratio of an etching width of the top stair to a length of the corresponding block and another ratio of an etching width of the bottom stair to the length of the corresponding block are respectively lowered toward an outer direction of the grating pattern in the inner portion of the hologram,    another phase modulation degree of the incident light passing through an outer portion of the grating pattern of the hologram varies in a step-wise shape of two outside stairs for each of the blocks, and    a difference in width between the outside stairs is increased toward the outer direction of the grating pattern in the outer portion of the hologram.    
     
     
       6. A compound objective lens according to  claim 2  in which
 the hologram is a phase modulation relief type of diffraction device,    a grating pattern of the hologram has a step-wise cross section and is formed in a concentric circle shape,    the grating pattern of the hologram is concentrically partitioned into a plurality of blocks,    a phase modulation degree of the incident light passing through the grating pattern of the hologram varies in a step-wise shape of four stairs for each of the blocks, and    a ratio of an etching width of a top stair to a length of the corresponding block and another ratio of an etching width of a bottom stair to the length of the corresponding block are respectively lowered toward an inner direction of the grating pattern of the hologram.    
     
     
       7. A compound objective lens according to  claim 2  in which
 the hologram is a phase modulation relief type of diffraction device,    a grating pattern of the hologram is formed in a concentric circle shape and is concentrically partitioned into a plurality of blocks,    a phase modulation degree of the incident light passing through an outer portion of the grating pattern of the hologram varies in a step-wise shape of four inside stairs for each of the blocks,    the four inside stairs are composed of a top stair, a second stair, a third stair and a bottom stair in that order,    a ratio of an etching width of the top stair to a length of the corresponding block and another ratio of an etching width of the bottom stair to the length of the corresponding block are respectively lowered toward an inner direction of the grating pattern in the outer portion of the hologram,    another phase modulation degree of the incident light passing through an inner portion of the grating pattern of the hologram varies in a step-wise shape of two inside stairs for each of the blocks, and    a difference in width between the inside stairs is increased toward the inner direction of the grating pattern in the inner portion of the hologram.    
     
     
       8. A compound objective lens according to  claim 2  in which the hologram is a phase modulation type of diffraction device and is made of a liquid crystal cell. 
     
     
       9. A compound objective lens according to  claim 2  in which the hologram is a phase modulation type of diffraction device and is placed on a substrate of a birefringence material. 
     
     
       10. A compound objective lens according to  claim 2  in which a positional relationship between the lens means and the hologram is fixed. 
     
     
       11. A compound objective lens according to  claim 10  in which the hologram is formed on a lens surface of the lens means. 
     
     
       12. A compound objective lens according to  claim 11  in which the hologram is placed on a lens surface of the lens means of which a curvature is higher than those of other lens surfaces of the lens means. 
     
     
       13. A compound objective lens according to  claim 11  in which the hologram is placed on a lens surface of the lens means of which a curvature is lower than those of other lens surfaces of the lens means. 
     
     
       14. A compound objective lens according to  claim 1  in which a numerical aperture of the lens means for the incident light converged at one focal point of the focal points differs from that for the incident light converged at another focal point of the focal points. 
     
     
       15. A compound objective lens according to  claim 2  in which a grating pattern is formed in a first portion of a light-passing area of the hologram corresponding to an aperture of the lens means, and any grating pattern is not formed in a second portion of the light-passing area of the hologram. 
     
     
       16. A compound objective lens according to  claim 15  in which a phase of the incident light passing through the second portion of the light-passing area of the hologram almost agrees with an average value of phases of the incident light passing through the first portion of the light-passing area of the hologram. 
     
     
       17. A compound objective lens according to  claim 15  in which the grating pattern is formed in a step-wise shape having a plurality of stairs,
 a surface height of the second portion of the light-passing area of the hologram in an optical direction is the same as a height of a stair selected from the stairs except a top stair and a bottom stair.    
     
     
       18. A compound objective lens according to  claim 1  in which a numerical aperture of the lens means is equal to or higher than 0.6. 
     
     
       19. An objective lens operable to lead a light beam to at least two kinds of information media having a substrate of different thicknesses T 1  and T 2 , comprising:
   a refractive lens which has a first convex surface, and a diffractive element which has a second convex surface located opposite to the first convex surface of the refractive lens;        wherein the objective lens has a numerical aperture of NA 1  corresponding to a substrate thickness T 1 ,        the diffractive element receives an approximately parallel beam not yet refracted by the refractive lens and diffracts the beam to divide the beam into divided beams in order to converge one of the divided beams corresponding to a substrate thickness of T 2  with a numerical aperture of NA 2   ( NA 1 >NA 2 , T 1 <T 2   ) ; and        wherein focal points are placed on an optical axis on a side facing the first convex surface of the refractive lens.     
     
     
       20. An objective lens according to  claim 19 , wherein the diffractive element is associated with the first convex surface and the second convex surface. 
     
     
       21. An objective lens according to  claim 19 , wherein the diffractive element is an optical relief. 
     
     
       22. An optical head apparatus operable to perform at least one of recording and reproduction of pieces of information on and from an optical disk placed to face the optical head apparatus, the optical disk composing the information media, comprising:
 ( i )  an optical source operable to radiate a light beam; and      ( ii )  the objective lens according to    claim 19   .     
     
     
       23. An optical disk apparatus comprising:
   an optical head apparatus according to    claim 22   ;        a rotating device operable to rotate the at least two kinds of information media; and        a moving device operable to move the optical head apparatus.     
     
     
       24. An objective lens operable to lead a light beam to at least two kinds of information media having a substrate of different thicknesses T 1  and T 2 ,
   wherein the objective lens has a first region and a second region,        the first region is located at a position farther from an optical axis than a position of the second region;        the second region is provided with a diffractive element;        the second region has a numerical aperture NA 2  to produce a focal point through the substrate of thickness T 2 ;        the first and the second regions of the objective lens have a numerical aperture NA 1  to produce a focal point through the substrate of thickness T 1 ; and        wherein NA 1 >NA 2  and T 1 <T 2 .     
     
     
       25. A focus control method to perform focus control on at least two kinds of information media having a substrate of different thicknesses T 1  and T 2  using an optical lens according to  claim 24 , the focus control method comprising:
   a step to detect a signal from the information media;        a step to decide the thickness of the substrate based on the signal; and        a step to perform the focus control corresponding to the decided thickness.     
     
     
       26. An optical disk apparatus to perform focus control on at least two kinds of information media having a substrate of different thicknesses T 1  and T 2  using an optical lens according to  claim 24 , comprising:
   an information detector to detect a signal from the information medium; and        a controller to decide the thickness of the substrate based on the signal and to perform the focus control corresponding to the thickness T 1  or T 2  based on the decided thickness.     
     
     
       27. An objective lens according to  claim 24 , wherein the NA 1  is larger than  0 . 6 . 
     
     
       28. An objective lens according to  claim 24 , wherein the T 1  is smaller than  0 . 8 . 
     
     
       29. An objective lens according to  claim 24 , wherein the first region or the second region has at least one region having no optical relief. 
     
     
       30. An optical head apparatus operable to perform at least one of recording and reproduction of pieces of information on and from an optical disk placed to face the optical head apparatus, the optical disk composing the information media, comprising:
 ( i )  an optical source operable to radiate a light beam; and      ( ii )  the objective lens according to    claim 24   .     
     
     
       31. An optical head apparatus according to  claim 30 , further comprising an optical detector to receive light from the information media having the substrate of different thicknesses T 1  and T 2 . 
     
     
       32. An optical disk apparatus comprising:
   an optical head apparatus according to    claim 31   ;        a rotating device operable to rotate the at least two kinds of information media; and        a moving device operable to move the optical head apparatus.     
     
     
       33. An optical disk apparatus comprising:
   an optical head apparatus according to    claim 30   ;        a rotating device operable to rotate the at least two kinds of information media; and        a moving device operable to move the optical head apparatus.     
     
     
       34. An image reproducing apparatus comprising:
   an optical disk apparatus according to    claim 33   ; and        a displaying unit.     
     
     
       35. A voice reproducing apparatus comprising:
   an optical disk apparatus according to    claim 33   ; and        a voice reproducing unit.     
     
     
       36. An information processing apparatus comprising:
   an optical disk apparatus according to    claim 33   ;        an input apparatus; and        a central processing unit.