USRE38943EExpiredUtility

Compound objective lens for optical disks having different thicknesses

80
Assignee: MATSUSHITA ELECTRIC INDUSTRIAL CO LTDPriority: Feb 1, 1993Filed: Sep 27, 2000Granted: Jan 24, 2006
Est. expiryFeb 1, 2013(expired)· nominal 20-yr term from priority
Inventors:Yoshiaki KommaSadao MizunoSeiji Nishino
G02B 5/1871G11B 7/08511G02B 3/10G03F 9/70G02B 21/20G11B 2007/0006G03F 9/7088G02B 5/1814G02B 21/025G03F 9/7069G03F 9/7049G11B 19/12G02B 5/1895G02B 21/18G11B 7/139G02B 5/18G02B 5/32G11B 19/127G11B 2007/13727G11B 7/131G02B 5/1842G11B 7/1353G11B 7/1374G02B 27/4238G02B 5/1876
80
PatentIndex Score
17
Cited by
67
References
123
Claims

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.

Claims

exact text as granted — not AI-modified
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. A compound objective lens according to  claim 1  in which the focal points of the beams of divided light generated by the plural focal point generating means correspond to focal positions differing from each other in an optical axis direction. 
     
     
       20. A compound objective lens according to  claim 1  in which two focal points selected from among the focal points of the beams of divided light generated by the plural focal point generating means correspond to two different positions from one of the first and second substrates. 
     
     
       21. A compound objective lens according to  claim 2  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. 
     
     
       22. A compound objective lens according to  claim 2  in which the focal points generated by the hologram correspond to focal positions differing from each other in an optical axis direction. 
     
     
       23. A compound objective lens according to  claim 2  in which two focal points selected from among the focal points correspond to two positions placed far from a substrate at two different thicknesses. 
     
     
       24. A compound objective lens according to  claim 2  in which a numerical aperture of the lens means is equal to or higher than  0 . 6 . 
     
     
       25. An image optical system, comprising:
 a light source for radiating a beam of incident light of one particular wavelength;    lens means having a first convex surface and a second convex surface opposite to each other, for receiving the beam of incident light, which is radiated from the light source and passes 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 the second substrate at a diffraction limit.   
     
     
       26. An image optical system according to  claim 25  in which the plural focal point generating means is a hologram. 
     
     
       27. An image optical system according to  claim 26  in which a plurality of beams of diffracted light having different diffraction orders are generated from the incident light in the hologram, and the beams of the diffracted light are converged at the focal points in one-to-one correspondence. 
     
     
       28. An image optical system according to  claim 27  in which a beam of transmitted light which agrees with a beam of zero-order diffracted light generated by the hologram is converged at a first focal point of the focal points, and a beam of diffracted light generated by the hologram is converged at a second focal point of the focal points. 
     
     
       29. An image optical system according to  claim 26  in which a grating pattern is formed in the hologram in a concentric circle shape. 
     
     
       30. A compound objective lens according to  claim 26  in which a positional relationship between the lens means and the hologram is fixed. 
     
     
       31. An image optical system according to  claim 30  in which the hologram is formed on a lens surface of the lens means. 
     
     
       32. An image optical system according to  claim 31  in which the hologram is placed on a lens surface of the lens means which has a higher curvature than those of other lens surfaces of the lens means and is placed at a position nearer than those of the other lens surfaces of the lens means. 
     
     
       33. An image optical system according to  claim 31  in which the hologram is placed on a lens surface of the lens means which has a lower curvature than those of other lens surfaces of the lens means. 
     
     
       34. An image optical system according to  claim 25  in which a numerical aperture of the lens means for the incident light converted at one focal point of the focal points differs from that for the incident light converged at another focal point of the focal points. 
     
     
       35. An image optical system according to  claim 34  in which the plural focal point generating means is a hologram. 
     
     
       36. An image optical system according m  claim 35  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. 
     
     
       37. An image optical system according to  claim 36  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. 
     
     
       38. An image optical system according to  claim 35  in which a first diffraction efficiency of the hologram at a first region for the incident light differs from a second diffraction efficiency of the hologram at a second region for the incident light. 
     
     
       39. An image optical system according to  claim 25  in which a far field pattern of the incident light radiated from the light source is distributed to decrease an intensity of the incident light toward a peripheral portion of the beam,
 an intensity of the incident light passing through a central portion of the lens means is two or more times that of the incident light passing through peripheral portion of the lens mean.   
     
     
       40. An image optical system according to  claim 25  in which the focal points generated by the plural focal point generating correspond to focal positions differing from each other in an optical axis direction. 
     
     
       41. An image optical system according to  claim 40  in which the plural focal point generating means is a hologram. 
     
     
       42. An image optical system according to  claim 25  in which two focal points selected from among the focal points correspond to two positions placed far from a substrate at two different thicknesses. 
     
     
       43. An image optical system according to  claim 42  in which the plural focal point generating means is a hologram. 
     
     
       44. An optical head apparatus comprising:
 a light source for radiating a beam or incident light of one particular wavelength;    lens means, having a first convex surface and a second convex surface opposite to each other, for receiving the beam of incident light, which is radiated from the light source and passes 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; and    a photo detector for receiving a plurality of beams of reflected light obtained by reflecting the beams of divided light converged on the information recording planes of the substrates by the plural focal point generating means and outputting an electric signal generated according to intensities of the beans of reflected light.   
     
     
       45. An optical head apparatus according to  claim 44  in which the plural focal point generating means is a hologram. 
     
     
       46. An optical head apparatus according to  claim 45  in which a plurality of beams of diffracted light having different diffraction orders are generated from the incident light in the hologram and the beams of the diffracted light are converged at the focal points in one-to-one correspondence. 
     
     
       47. An optical head apparatus according to  claim 46  in which a beam of transmitted light which agrees with a beam of zero-order diffracted light generated by the hologram is converged at a first focal point of the focal points, and a beam of diffracted light generated by the hologram is convened at a second focal point of the focal points. 
     
     
       48. An optical head apparatus according to  claim 45  in which the hologram is arranged on a plane, and an optical axis of the lens means is not in parallel with a normal line of the plane. 
     
     
       49. An optical head apparatus according to  claim 45  in which a positional relationship between the lens means and the hologram is fixed. 
     
     
       50. An optical head apparatus according to  claim 49  in which the hologram is formed on a lens surface of the lens means. 
     
     
       51. An optical head apparatus according to  claim 50  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. 
     
     
       52. An optical head apparatus according to  claim 50  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. 
     
     
       53. An optical head apparatus according to  claim 45  in which a grating pattern is formed in the hologram in a concentric circle shape. 
     
     
       54. An optical head apparatus according to  claim 53  in which the grating pattern is formed in a first portion of a light-passing area of the hologram corresponding to an aperture of the lens mean, and another grating pattern is non-concentrically formed in a second portion of the light-passing area of the hologram to diffract the incident light. 
     
     
       55. An optical head apparatus according to  claim 45  in which the hologram is a phase modulation type of diffraction device. 
     
     
       56. An optical head apparatus according to  claim 55  in which a phase modulation degree of light passing through the hologram is lower than  2 π radians. 
     
     
       57. An optical head apparatus according to  claim 55  in which the hologram is a relief type of diffraction device. 
     
     
       58. An optical head apparatus according to  claim 57  in which a height H of relief formed in the hologram is set to:
   H<λ/(n(λ)− 1 ),    where a symbol λ denotes a wavelength of the incident light and a symbol n(λ) denotes a refractive index of a material of the hologram for the incident light having the wavelength λ, and    a difference in phase modulation degree of the incident light passing through the hologram is lower than  2 π radians.   
     
     
       59. An optical head apparatus according to  claim 57  in which a grating pattern of the hologram is formed in a step-wise cross sectional shape. 
     
     
       60. An optical head apparatus according to  claim 59  in which the 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 the grating pattern of the hologram varies in a step-wise shape of four stairs for each of blocks, and    a ratio of an etching width of a top stair to a length of the block and another ratio of an etching width of a bottom stair to the length of the block are respectively lowered toward an outer direction of the grating pattern of the hologram.   
     
     
       61. An optical head apparatus according to  claim 59  in which the 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 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 a top stair to a length of the block and another ratio of an etching width of a bottom stair to the length of the 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 blocks,    a height of an upper stair of the outside stairs agrees with that of the second stair,    a height of a lower stair of the outside stairs agrees with that of the third stair, and    a difference in width between the upper and lower stairs is increased toward the outer direction of the grating pattern in the outer portion of the hologram.   
     
     
       62. An optical head apparatus according to  claim 59  in which the 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 the grating pattern of the hologram varies in a step-wise shape of four stairs for each of blocks, and    a ratio of an etching width of a top stair to a length of the block and another ratio of an etching width of a bottom stair to the length of the block are respectively lowered toward an inner direction of the grating pattern of the hologram.   
     
     
       63. An optical head apparatus according to  claim 59  in which the 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 outside stairs for each of blocks,    the four outside 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 a top stair to a length of the block and another ratio of an etching width of a bottom stair to the length of the 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 blocks,    a height of an upper stair of the inside stairs agrees with that of the second stair,    a height of a lower stair of the inside stairs agrees with that of the third stair; and    a difference in width between the upper and lower stairs is increased toward the inner direction of the grating pattern in the inner portion of the hologram.   
     
     
       64. An optical head apparatus according to  claim 55  in which the hologram a phase modulation type of diffraction device made of a liquid crystal cell. 
     
     
       65. An optical head apparatus according to  claim 55  in which the hologram means is a phase modulation type of diffraction device placed on a substrate made of a birefringence material. 
     
     
       66. An optical head apparatus according to  claim 44  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 fat the incident light converged at another focal point of the focal points. 
     
     
       67. An optical head apparatus according to  claim 66  in which the plural focal point generating means is a hologram. 
     
     
       68. An optical head apparatus according to  claim 67  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. 
     
     
       69. An optical head apparatus according to  claim 68  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. 
     
     
       70. An optical head apparatus according to  claim 68  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 axis direction is the same as a height of a stair selected from the stairs except a top stair and a bottom stair.   
     
     
       71. An optical head apparatus according to  claim 67  in which a first diffraction efficiency of the hologram at first region for the incident light differs from a second diffraction efficiency of the hologram at a second region for the incident light. 
     
     
       72. An optical head apparatus according to  claim 71  in which a grating pattern of the hologram is concentrically formed, the hologram is a phase modulation type of diffraction device, and a phase modulation degree in an outer portion of the grating pattern of the hologram is lower than that in an inner portion of the grating pattern of the hologram. 
     
     
       73. An optical head apparatus according to  claim 71  in which a grating pattern of the hologram is concentrically formed, the hologram is a phase modulation type of diffraction device, and a phase modulation degree in an inner portion of the grating pattern of the hologram is lower than that in an outer portion of grating pattern of the hologram. 
     
     
       74. An optical head apparatus according to  claim 44  in which a far field pattern of the incident light radiated from the light source is distributed to decrease an intensity of the incident light toward a peripheral portion of the beam,
 an intensity of the incident light passing through a central portion of the lens means is two or more times that of the incident light passing through a peripheral portion of the lens means.   
     
     
       75. An optical head apparatus according to  claim 44  in which the focal points generated by the plural focal point generating means correspond to focal positions differing from each other in an optical axis direction. 
     
     
       76. An optical head apparatus according to  claim 75  in which the plural focal point generating means is a hologram. 
     
     
       77. An optical head apparatus according to  claim 44  in which two focal points selected from among the focal points correspond to two positions placed inside a substrate at two different thicknesses. 
     
     
       78. An optical head apparatus according to  claim 77  in which the plural focal point generating means is a hologram. 
     
     
       79. An optical head apparatus according to  claim 78  in which the lens means converges a beam of transmitted light obtained by passing the incident light through the hologram on an information recording plane placed far from a substrate surface by a first thickness T 1 , and the hologram converges a beam of diffracted light obtained by diffracting the incident light in the hologram and refracting the incident light in the lens means on another information recording plane placed far from the substrate surface by a second thickness T 2 (T 1 <T 2 ). 
     
     
       80. An optical head apparatus according to  claim 78  in which the hologram converges a beam of diffracted light obtained by diffracting the incident light in the hologram and refracting the incident light in the lens means on another information recording plane placed far from the substrate surface by a first thickness T 1 , and the lens means converges a beam of transmitted light obtained by passing the incident light through the hologram on an information recording plane placed far from a substrate surface by a second thickness T 2 (T 1 <T 2 ). 
     
     
       81. An optical head apparatus according to  claim 44  in which the photo detector is arranged close to the light source. 
     
     
       82. An optical head apparatus according to  claim 44  in which the incident light linearly polarized is radiated from the light source, and
   the optical head apparatus, further comprising:      a polarized beam splitter for totally transmitting beam of light linearly polarized in a first direction and totally reflecting a beam of light linearly polarized at a second direction perpendicular to the first direction; and    a  1 / 4 -λ plate for changing the light transmitting through or reflected by the polarized beam splitter to a beam of light circularly polarized in a rotational direction wherein the incident light linearly polarized in a third direction agreeing with the first or second direction is circularly polarized in a first rotational direction by the  1 / 4 -λ plate, the light circularly polarized in the first rotational direction is converged on the information mediums by the lens means and the plural focal point generating means to form a beam of light circularly polarized in a second rotational direction opposite to the first rotational direction, the light circularly polarized in the second rotational direction is changed to the light linearly polarized in a fourth direction perpendicular to the first direction by the  1 / 4 -λ plate, and the light linearly polarized in the fourth direction is totally reflected by or transmits through the polarized beam splitter to be incident on the photo detector.   
     
     
       83. An optical head apparatus according to  claim 44  in which the photo deflector comprises:
 a servo signal detector for detecting a servo signal included in one of the reflected light; and    another signal detector arranged at a periphery of the servo signal detector for detecting another signal included in another one of the reflected light.   
     
     
       84. An optical head apparatus according to  claim 44  in which the photo detector comprises:
 a signal detector for detecting a focus error signal and a tracking error signal included in the reflected light.   
     
     
       85. An optical head apparatus according to  claim 44 , further comprising:
 reshaping means for reshaping the incident light radiated from the light source, the incident light reshaped being refracted by the lens means.   
     
     
       86. A compound objective lens, comprising a region to produce a focal point on an information plane through a layer, wherein
 the region of the lens is divided into a plurality of regions including at least both of a first region and a second region by dividing the region of the lens depending on differences in a distance from an optical axis of the lens,    the first region being located at a position farther from the optical axis than a position of the second region,    the second region being optimized so that the lens has a numerical aperture NA 2  to produce a focal point through a second layer on an information plane placed at a distance T 2  from a surface of the second layer, and    both of the first region and the second region being optimized so that the lens has a numerical aperture NA 1  (NA 1  is not equal to NA 2 ) to produce a focal point through a first layer on an information plane placed at a distance T 1  (T 1  is not equal to T 2 ) from the surface of the first layer,    wherein the compound objective lens has 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 the optical axis at the first convex surface, and the compound objective lens refracts the beam of incident light and emits a beam of refracted light from the second convex surface, and    the region of the lens receives the beam of incident light not yet refracted by the lens and generates from the incident light a plurality of beams of divided light, and generates a plurality of focal points which are placed on the optical axis on a side facing the second convex surface.   
     
     
       87. A compound objective lens according to  claim 86 , wherein the second region has an optical relief,
 the numerical aperture NA 1  is larger than the numerical aperture NA 2 , and    the distance T 1  is smaller than the distance T 2 .   
     
     
       88. A compound objective lens according to  claim 87 , in which the optical relief is formed concentrically in the second region of the compound objective lens. 
     
     
       89. A compound objective lens according to  claim 87 , in which the optical relief is provided on a side of the compound objective lens opposite to an optical disk. 
     
     
       90. A compound objective lens according to  claim 87 , in which the first region has an optical relief formed therein. 
     
     
       91. A compound objective lens according to  claim 90 , in which the optical relief of the first region is higher in height than the optical relief of the second region. 
     
     
       92. A compound objective lens according to  claim 87 , comprising an objective lens on which the plurality of regions are provided. 
     
     
       93. A compound objective lens according to  claim 87 , comprising an objective lens and a relief lens on which the plurality of regions are provided. 
     
     
       94. A compound objective lens according to  claim 87 , in which the distance T 1  is larger than  0 . 4  millimeter and smaller than  0 . 8  millimeter. 
     
     
       95. A compound objective lens according to  claim 87 , in which a height H of the optical relief formed in the second region is set to:
   H<λ/(,(λ)− 1 ),    
       where a symbol λ denotes a wavelength of a light beam passing through the second region and a symbol n(λ) denotes a refractive index of a material of the optical relief at the wavelength λ of the light beam. 
     
     
       96. A compound objective lens according to  claim 95 , in which a difference in phase modulation degree of the light beam passing through the second region is lower than  2 π radian. 
     
     
       97. A compound objective lens according to  claim 87 , in which the optical relief of the second region is a hologram. 
     
     
       98. A compound objective lens according to  claim 97 , in which the hologram of the second region has a diffraction efficiency lower than  100 %. 
     
     
       99. A compound objective lens according to  claim 97 , in which the hologram of the second region is formed to function as a convex lens. 
     
     
       100. A compound objective lens according to  claim 97 , in which the hologram of the second region has slopes inclining to the same direction as a surface of a convex lens. 
     
     
       101. A compound objective lens according to  claim 97 , in which the plurality of regions include at least one region having no optical hologram. 
     
     
       102. A compound objective lens according to  claim 101 , in which the first region has no hologram. 
     
     
       103. A compound objective lens according to  claim 97 , in which the first region has a hologram. 
     
     
       104. A compound objective lens according to  claim 103 , in which the first region is higher in a diffraction efficiency than the second region. 
     
     
       105. A compound objective lens according to  claim 97 , in which the hologram of the second region is formed into a blazed hologram lens. 
     
     
       106. A compound objective lens according to  claim 105 , in which the hologram is formed into saw-teeth in a section thereof. 
     
     
       107. An optical head apparatus for performing at least one of recording and reproduction of pieces of information on and from an optical disk placed to face the optical head apparatus, comprising
 (i) an optical source for radiating a light beam; and    (ii) a compound objective lens receiving the light beam and comprising a region to produce a focal point on an information plane through a layer, wherein    the region of the lens is divided into a plurality of regions including at least both of a first region and a second region by dividing the region of the lens depending on differences in a distance from an optical axis of the lens, the first region being located at a position farther from the optical axis than a position of the second region,    the second region being optimized so that the lens has a numerical aperture NA 2  to produce a focal point through a second layer on an information plane placed at a distance T 2  from a surface of the second layer, and    both of the first region and the second region being optimized so that the lens has a numerical aperture NA 1  (NA 1  is not equal to NA 2 ) to produce a focal point through a first layer on an information plane placed at a distance T 1  (T 1  is not equal to T 2 ) from the surface of the first layer,    wherein the compound objective lens has 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 the optical axis at the first convex surface, and the compound objective lens refracts the beam of incident light and emits a beam of refracted light from the second convex surface, and    the region of the lens receives the beam of incident light not yet refracted by the lens, and generates from the incident light a plurality of beams of divided light, and generates a plurality of focal points which are placed on the optical axis on a side facing the second convex surface.   
     
     
       108. An optical head apparatus according to  claim 107 , comprising:
 a collimate lens to change the light beam from the optical source into an approximately parallel light; and    an optical detector to receive light from both of the information planes placed at a distance T 1  from the surface of the first layer and placed at a distance T 2  from the surface of the second layer; wherein    the compound objective lens receives the approximately parallel light.   
     
     
       109. An optical head apparatus according to  claim 107  wherein the numerical aperture NA 1  is larger than the numerical aperture NA 2  (NA 1 >NA 2 ) and
 the distances T 1  and T 2  correspond to thicknesses T 1  and T 2  of the first and second layers composed of, respectively, first and second information media included in the optical disk, the thickness T 1  being smaller the thickness T 2  (T 1 <T 21 ).   
     
     
       110. An optical head apparatus according to  claim 109 , comprising:
 a photo detector for detecting the light beam, which is converged at an information recording plane, serving as the information plane, of the first information medium having the thickness T 1  and at an information recording plane, serving as the information plane, of the second information medium having the thickness T 2  by the compound objective lens and is reflected by the first information medium and the second information medium, respectively, to obtain first information recorded in the information recording plane of the first information medium and second information recorded in the information recording plane of the second information medium.   
     
     
       111. An optical head apparatus according to  claim 109 , in which the compound objective lens comprises
 an optical device for minimizing an aberration occurring in the light beam in cases where the light beam passing through the optical device transmits through the first layer of the first information medium having the thickness T 1  and is focused on an information recording plane, serving as the information plane, of the first information medium, and    a ring-shaped band, placed on at least one surface of the optical device, for shifting a phase of the light beam passing through the optical device to reduce a wavefront aberration caused by a difference between the thicknesses T 1  and T 2  of the first and second information media in cases where the light beam passing through the optical device transmits through the second layer of the second information medium having the thickness T 2  and is focused on an information recording plane, serving as the information plane of the second information medium;    the optical head apparatus further comprising 
 a photo detector for detecting the light beam which is converged on the information recording plane of the first information medium having the thickness T 1  and on the information recording plane of the second information medium having the thickness T 2  by the compound objective lens and is reflected by the first information medium and second information medium to reproduce information recorded in the first and second information media, respectively. 
   
     
     
       112. An optical head apparatus according to  claim 109 , in which the compound objective lens comprises
   a phase adjusting device, formed in a ring-band shape, for shifting a part of the light beam radiated from the optical source,      the compound objective lens having a light converging performance so as to converge the light beam radiated from the optical source on an information recording plane serving as the information plane of the first information medium having the thickness T 1  through the layer thereof at a diffraction limit, to converge the light beam, of which the part is shifted by the phase adjusting device, on an information recording-plane, serving as the information plane, of the second information medium having the thickness T 2  or the information recording plane of the first information medium having the thickness T 1  through the layer thereof,    the optical head apparatus further comprises 
 a photo detector for detecting the light beam, which is converged on the information recording plane of the first and second information media each having the thickness T 1  or T 2  by the compound objective lens and is reflected by the first and second information media, respectively, to reproduce information recorded in the first and second information media, respectively. 
   
     
     
       113. An optical head apparatus according to  claim 109 , in which the plurality of regions of the compound objective lens include
 a third region, corresponding to a numerical aperture NA 4  equal to or lower than the numerical aperture NA 2  (NA 4 <NA 2 ), for changing the light beam radiated from the optical source to converge the light beam on an information recording plane, serving as the information plane, of the second information medium having the thickness T 2  through the layer thereof; and    a photo detector for detecting the beam light, which is converged on the information recording plane of the first information medium having the thickness T 1  and on an information recording plane serving as the information plane, of the second information medium having the thickness T 2  by the compound objective lens and is reflected by the first and second information media having the thickness T 1  and T 2  respectively, to reproduce first information recorded in the first information medium and second information recorded in the second information medium, respectively.   
     
     
       114. An optical disk apparatus, comprising;
 ( 1 ) an optical head apparatus for performing at least one of recording and reproduction of pieces of information on and from an optical disk placed to face the optical head apparatus, comprising: 
 (i) an optical source for radiating a light beam; and  
 (ii) a compound objective lens receiving the light beam and comprising: 
 a region to produce a focal point on an information plane through a layer, wherein the region of the lens is divided into a plurality of regions including, at least both of a first region and a second region by dividing the region of the lens depending on differences in a distance from an optical axis of the lens,  
 the first region being located at a position farther from the optical axis than a position of the second region,  
 the second region being optimized so that the lens has a numerical aperture NA 2  to produce a focal point through a second layer on an information plane placed at a distance T 2  from a surface of the second layer, and  
 
  both of the first region and the second region being optimized so that the lens has a numerical aperture NA 1  (NA 1  is not equal to NA 2 ) to produce a focal point through a first layer on an information plane placed at a distance T 1  (T 1  is not equal to T 2 ) from the surface of the first layer, and  
    wherein the compound objective lens has 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 the optical axis at the first convex surface, and the compound objective lens refracts the beam of incident light and emits a beam of refracted light from the second convex surface, and    the region of the lens receives the beam of incident light not yet refracted by the lens, and generates from the incident light a plurality of beams of divided light, and generates a plurality of focal points which are placed on the optical axis on a side facing the second convex surface;    ( 2 ) a moving apparatus for moving the optical head apparatus; and    ( 3 ) a rotating apparatus for rotating the optical disk.   
     
     
       115. An optical disk apparatus according to  claim 114 , comprising: a focus controller to change a distance between the optical disk and the compound objective lens and start focus control when a focus error signal detected from the compound objective lens surpasses a predetermined level so as to perform focus control corresponding to the different distances T 1  and T 2 . 
     
     
       116. An optical disk apparatus according to  claim 114 , wherein the numerical aperture NA 1  is larger than the numerical aperture NA 2  (NA 1 >NA 2 ) and
 the distances T 1  and T 2  correspond to thicknesses T 1  and T 2  of the first and second layers composed of, respectively, first and second information media included in the optical disk, the thickness T 1  being smaller the thickness T 2  (T 1 <T 2 ).   
     
     
       117. An optical disk apparatus according to  claim 116 , wherein
 the optical head apparatus is configured, through the compound objective lens, to read an information signal, a focus error signal and a tracking error signal from the optical disk rotated by the rotating apparatus; the optical disk apparatus further comprising:    connecting means for connecting the rotating apparatus and the moving apparatus with an electric source to supply an electric power to the rotating apparatus and the moving apparatus;    actuating means for actuating the compound objective lens of the optical head apparatus;    focus control means for controlling the actuating means to perform a first focus control of the optical head apparatus corresponding to the thickness T 1  of the first information medium and a second focus control of the optical head apparatus corresponding to the thickness T 2  of the second information medium according to the focus error signal read by the optical head apparatus;    tracking control means for controlling the actuating means to perform a first tracking control of the optical head apparatus corresponding to the thickness T 1  of the first information medium and a second tracking control of the optical head apparatus corresponding to the thickness T 2  of the second information medium according to the tracking error signal read by the optical head apparatus;    detecting means for detecting whether the optical disk has the first information medium having the thickness T 1  or the second information medium having the thickness T 2 ; and    changing means for switching from the second focus and tracking controls performed by the focus control means and the tracking control means to the first focus and tracking controls performed by the focus control means and the tracking control means according to the detection of the detecting means.   
     
     
       118. An optical disk apparatus according to  claim 116 , in which the optical head apparatus is configured, through the compound objective lens, to read an information signal, a focus error signal and a tracking error signal from the optical disk rotated by the rotating apparatus;
 the optical disk apparatus further comprising:    connecting means for connecting the rotating apparatus and the moving apparatus with an electric source to supply an electric power to the rotating apparatus and the moving apparatus;    actuating means for actuating the compound objective lens of the optical head apparatus;    focus control means for controlling the actuating means to perform a first focus control of the optical head apparatus corresponding to the thickness T 1  of the first information medium and a second focus control of the optical head apparatus corresponding to the thickness T 2  of the second information medium according to the focus error signal read by the optical head apparatus; and    tracking control means for controlling the actuating means to perform a first tracking control of the optical head apparatus corresponding to the thickness T 1  of the first information medium and a second tracking control of the optical head apparatus corresponding to the thickness T 2  of the second information medium according to the tracking error signal read by the optical head apparatus.   
     
     
       119. An optical disk apparatus according to  claim 118  in which the compound objective lens of the optical head apparatus is moved in a direction to the optical disk by the moving apparatus, and the compound objective lens of the optical head apparatus is operated to focus the light beam on the first or second information medium by the actuating means under the control of the focus control means to decrease an intensity of the focus error signal to zero in case where the intensity of the focus error signal exceeds a threshold. 
     
     
       120. An optical disk apparatus according to  claim 116 , comprising:
 a photo detector for detecting the light beam which is converged at an information recording plane, serving as the information plane, of the first information medium having the thickness T 1  and at an information recording plane, serving as the information plane, of the second information plane having the thickness T 2  by the compound objective lens and is reflected by the first information medium and the second information medium, respectively;    focus control means for performing a first focus control of the optical head apparatus corresponding to the thickness T 1  of the and a second focus control of the optical head apparatus corresponding to the thickness T 2  according to the light beam detected by the photo detector;    tracking control means for performing a first tracking control of the optical head apparatus corresponding to the thickness T 1  and a second tracking control of the optical head apparatus corresponding to the thickness T 2  according to the light beam detected by the photo detector; and    information detecting means for judging according to the light beam detected by the photo detector, for which the first focus control and the second focus control and the first tracking control and the second tracking control are performed, whether the light beam radiated from the light source is converged at the information recording plane of the first information medium having the thickness T 1  or at the information recording plane of the second information medium having the thickness T 2 , reproducing first information recorded in the information recording plane of the first information medium from the light beam detected by the photo detector in cases where it is judged that the light beam is converged at the information recording plane of the first information medium, and reproducing second information recorded in the information recording plane of the second information medium from the light beam detected by the photo detector in cases where it is judged that the light beam is converged at the information recording plane of the second information medium.   
     
     
       121. An optical disk apparatus according to  claim 116 , in which the plurality of regions of the compound objective lens include:
 a third region which corresponds to a numerical aperture NA 3  satisfying a relationship of NA 2 ≦NA 3 <NA 1  and is unified with the first region of the objective lens through a discontinuous plane,    the optical head apparatus further comprising: 
 a photo detector for detecting the light beam which is converged at an information recording plane, serving as the information plane, of the first information medium having the thickness T 1  and at an information recording plane, serving as the information plane, of the second information medium having the thickness T 2  by the compound objective lens and is reflected therefrom, respectively;  
 focus control means for performing a first focus control of the optical head apparatus corresponding to the thickness T 1  and a second focus control of the optical head apparatus corresponding to the thickness T 2  according to the light beam detected by the photo detector;  
 tracking control means for performing a first tracking control of the optical head apparatus corresponding to the thickness T 1  and a second tracking control of the optical head apparatus corresponding to the thickness T 2  according to the light beam detected by the photo detector; and  
 information detecting means for judging according to the light beam detected by the photo detector, for which the first focus control and the second focus control and the first tracking control and the second tracking control are performed, whether the light beam radiated from the optical source is converged at the information recording plane of the first or second information medium having either of the thickness T 1  or T 2 , reproducing first information recorded in the information recording plane of the first information medium having the thickness T 1  from the light beam detected by the photo detector in cases where it is judged that the light beam is converged at the information recording plane of the first information medium, and reproducing second information recorded in the information recording plane of the second information medium having the thickness T 2  from the light beam detected by the photo detector in cases where it is judged that the light beam is converged at the information recording plane of the second information medium. 
   
     
     
       122. An optical disk apparatus according to  claim 116 , in which the optical head apparatus comprises
 an optical device for minimizing an aberration occurring in the light beam in cases where the light beam passing through the optical device transmits through the layer of the first information medium having the thickness T 1  and is focused on an information recording plane, serving as the information plane, of the first information medium,    a ring-shaped band, placed on at least one surface of the optical device, for shifting a phase of the light beam passing through the optical device to reduce a wavefront aberration caused by a difference between the thicknesses T 1  and T 2  of the first and second information media in cases where the light beam passing through the optical device transmits through the layer of the second information medium having the thickness T 2  and is focused on the information recording plane thereof, and    a photo detector for detecting the light beam, which is converged on the information recording planes of the first and second information media having the thicknesses T 1  and T 2  by the compound objective lens and is reflected by the first and second information media, respectively, to reproduce information recorded in the first and second information media, respectively;    focus control means for performing a first focus control of the optical head apparatus corresponding to the thickness T 1  of the first information medium and a second focus control of the optical head apparatus corresponding to the thickness T 2  of the second information medium according to the light beam detected by the photo detector;    tracking control means for performing a first tracking control of the optical head apparatus corresponding to the thickness T 1  and a second tracking control of the optical head apparatus corresponding to the thickness T 2  according to the light beam detected by the photo detector; and    information detecting means for judging according to the light beam detected by the photo detector, for which the first focus control and the second focus control and the first tracking control and the second tracking control are performed, whether the light beam radiated from the optical source is converged at an information recording plane, serving as the information plane, of the first or second information medium having the thickness T 1  or T 2  reproducing first information recorded in the information recording plane of the first information medium having the thickness T 1  from the light beam detected by the photo detector in cases where it is judged that the light beam is converged at the information recording plane of the first information medium, and reproducing second information recorded in the information recording plane of the second information medium having the thickness T 2  from the light beam detected by the photo detector in cases where it is judged that the light beam is converged at the information recording plane of the second information medium.   
     
     
       123. An optical disk apparatus according to  claim 116 , in which the plurality of regions of the compound objective lens include
 a third region, corresponding to a numerical aperture NA 4  equal to or lower than the numerical aperture NA 2  (NA 4 <NA 2 ), for changing the light beam radiated from the optical source to converge the light beam on an information recording plane, serving as the information plane, of the second information medium having the thickness T 2  through the layer thereof;    a photo detector for detecting the light beam, which is converged on the information recording plane of the first and second information media each having the thickness T 1  or T 2  by the compound objective lens and is reflected by the first and second information media, respectively, to reproduce information recorded in the first and second information media, respectively;    focus control means for performing a first focus control of the optical head apparatus corresponding to the thickness T 1  of the first information medium and a second focus control of the optical head apparatus corresponding to the thickness T 2  of the second information medium according to the light beam detected by the photo detector;    tracking control means for performing a first tacking control of the optical head apparatus corresponding to the thickness T 1  of the first information medium and a second tracking control of the optical head apparatus corresponding to the thickness T 2  of the second information medium according to the light beam detected by the photo detector; and    information detecting means for judging according to the light beam detected by the photo detector, for which the first focus control and the second focus control and the first tracking control and the second tracking control are performed, whether the light beam radiated from the optical source is converged at an information recording plane, serving as the information plane, of the first information medium having the thickness T 1  or at an information recording plane, serving as the information plane, of the second information medium having the thickness T 2 , reproducing first information recorded in the information recording plane of the first information medium from the light beam detected by the photo detector in cases where it is judged that the light beam is converged at the information recording plane of the first information medium, and reproducing second information recorded in the information recording plane of the second information medium from the light beam detected by the photo detector in cases where it is judged that the light beam is converged at the information recording plane of the second information medium.

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