Abstract:
A two-group two-element lens system for use in an optical information device includes a first lens adapted to be positioned closer to a light source and having a flat surface which is symmetric with respect to an optical axis thereof and a convex surface which is symmetric with respect to the optical axis, the first lens being made of a medium having a refractive index distribution along the optical axis, and a second lens adapted to be positioned closer to an image and having a spherically convex surface closer to the light source, the second lens having a positive refracting power as a whole and made of a medium having a uniform refractive index.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a lens system for use in an optical information device in which a laser beam is employed to record and reproduce information. 
     2. Description of the Relevant Art 
     Various optical information devices such as optical disc systems, magnetooptic systems, optical card systems, and the like employ an optical lens system comprising a number of lenses for focusing a divergent light beam emitted from a light source such as a semiconductor laser onto the recording surface of a recording medium. 
     Such lens systems for use in optical information devices are required to have various aberrations such as spherical aberration, coma, etc. to be properly corrected and also to be small in size and light in weight so that a lens actuator for driving the lens system has a higher level of performance for focusing and tracking In addition, it is important that lens systems be less costly to manufacture in order to make optical information devices incorporating the lens systems available relatively inexpensively in the market. Known optical lens systems include a collimator lens for rendering diverging light rays parallel and several homogeneous glass lenses for focusing the light beam onto a recording surface. For example, an optical lens system including a single aspherical lens or a single lens with a radial refractive index distribution is known as disclosed in U.S. Pat. Nos. 4,613,212, 4,457,590, and 4,701,032. 
     Generally, optical information processing systems require that information recording tracks or bits be closely spaced in order to record and reproduce as large an amount of information as possible. Therefore, a lens in such systems, particularly an objective lens, needs to have a diffraction limit performance. The diffraction limit performance of a lens is generally determined by the wavelength of light passing therethrough and a numerical aperture (NA) of the lens. The greater the NA of a lens, the smaller the spot of a light beam focused by the lens. It is therefore important for lenses for use in optical information recording and reproducing systems to have a large NA. If conventional optical lens systems have an NA or 0.5 or more, then the number of lenses required is increased, aberrations are difficult to correct, and strict tolerances are required. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a lens system which comprises a small number of lenses and is small in size and light in weight. 
     A lens system according to the present invention comprises two lenses for focusing a diverging or parallel-ray light beam emitted from a light source onto the surface of an information recording medium. The refracting power of the lens system is borne by the two lenses to achieve a high NA. 
     One of the two lenses which is closer to the light source comprises a planoconvex lens having one surface which is flat and symmetric with respect to the optical axis thereof and the other surface which is convex and symmetric with respect to the optical axis, the lens having an axial refractive index distribution. The lens is made of a medium having a refractive index distribution along the optical axis for well correcting aberrations. With the lens being made of such a medium with a refractive index distribution, the lens is given a degree of design freedom, i.e., a refractive index distribution, for aberration correction, as well as a curvature, a thickness, and a refractive index as with an ordinary lens. 
     The distribution of refractive indexes n of the lens along the optical axis may be expressed by: 
     
         n(Z)=n.sub.o -kZ                                           (1) 
    
     where Z is a distance along the optical axis in a positive direction from an origin where the convex surface and the optical axis intersects toward the flat surface, n o  is the refractive index at the vertex of the convex surface, and k is a positive constant. Then, the spherical aberration produced on the convex surface can be corrected dependent on the value of k. Since the amounts of coma, astigmatism, and other aberrations produced by the convex surface vary dependent on the value of k, the axial refractive index distribution is highly effective in correcting aberrations. 
     The aberrations are greatly corrected by expressing the function n(Z) as the simplest equation (1). However, the aberrations can sufficiently be corrected even if the value of n(Z) may have an error of about±0.005 deviating from the value determined by the equation (1). According to the present invention, the function n(Z) is in the range: 
     
         (n.sub.o -kZ)-0.005≦n(Z)≦(n.sub.o -kZ)+0.005 (2) 
    
     The axial refractive index distribution expressed by the formula (2) may not necessarily extend across the entire thickness of the lens. Stated otherwise, the aberrations can sufficiently be corrected by expressing the range of distances Z to which the axial refractive index distribution is to be given, as 0≦Z≦Z s , and by selecting the value of Z s  in the range of: 
     
         0.01≦Z.sub.s /Z.sub.o ≦1                     (3) 
    
     where Z o  is the thickness of the lens on the optical axis. If the flat surface of the lens closer to the light source were convex or concave, degrees of freedom in designing the lens would be increased. However, the cost of manufacture of the lens would also be increased since it would be necessary to align the direction of the refractive index distribution with the optical axes of the surfaces of the lens when the lens is manufactured. For this reason, this surface of the lens is made flat. Inasmuch as one surface of the lens is flat, if the above ratio is outside of the range (3), the aberrations will not be largely affected irrespective of whether there is an axial refractive index distribution or not. Therefore, in the range of Z s  ≦Z, the refractive index may be constant or an axial refractive index distribution may be given for some reasons when producing a refractive index distribution material. 
     A refractive index difference along the optical axis is represented by k.Z s . If the value of k.Z s  were below 0.001, substantially no aberrations could be corrected, and if the value of k.Z s  exceeded 0.2, it would be extremely difficult to produce a refractive index distribution material. Therefore, k.Z s  should be in the range of 0.001≦k.Z s  ≦0.2, and preferably 0.01≦k.Z s  ≦0.12. 
     The refractive index n o  at the vertex of the convex surface should preferably range from 1.50 to 1.75 in order to produce a refractive index distribution material with ease, and n o  -k.Z s  should preferably be 1.45 or higher. 
     Where the lens closer to the light source has a spherically convex surface closer to the light source and a flat surface closer to an image, the value of Z s  which defines the range to which the axial refractive index distribution is to be given may be set to the value Z eff  (FIG. 31) of Z of a point P where a light ray emitted from a point on the optical axis and passing through the outermost edge of an entrance pupil of the lens system intersects with the convex surface of the lens closer to the light source. Therefore, the range of Z in which the formula (2) is effective is as follows: 
     
         0≦Z≦Z.sub.eff                                ( 4) 
    
     In this case, the value of the refractive index difference k.Z eff  along the optical axis is in the range of: 
     
         0.01≦k.Z.sub.eff ≦0.08                       (5) 
    
     and the refractive index n o  at the vertex of the convex surface is in the range of: 
     
         1.55≦n.sub.o ≦1.75                           (6) 
    
     It is suitable for the refractive index n(Z) in the range Z eff  &lt;Z to be in the range of: 
     
         1.45≦n(Z)≦1.80                               (7) 
    
     in view of the ease with which a refractive index distribution material or medium is produced. For the reasons described above, the refractive index may be constant in the range defined by the formula (7), or a refractive index distribution may be given in that range along the optical axis dependent on conditions in which a refractive index distribution material is produced. 
     The other lens of the lens system, i.e., the lens closer to the image or the image-side lens, is a lens having a positive refracting power and made from a medium which has a uniform refractive index. This lens serves to increase the refracting power of the lens system and the NA thereof. In designing the lens, a refractive index, a thickness, and the radii of curvatures of lens surfaces may be employed as degrees of freedom for correcting the aberrations. It is preferable from the standpoint of manufacturing the lens that both surfaces of the lens be spherical or flat and symmetric with respect to the optical axis. In order to well correct spherical aberration and increase the refracting power, the surface of the lens closer to the light source is made spherically convex. The surface of the lens which is closer to the image can be made spherically concave to assist in correcting spherical aberration and coma. The lens should preferably be made of general less costly and durable optical glass having a refractive index ranging from about 1.45 to 1.90, or optical plastics. 
     According to the present invention, the image-side lens has a refractive index n, in the range of: 
     
         1.45≦n,                                             (8) 
    
     Since the image-side lens needs to have a large refracting power so as to increase the NA of the entire lens system, the condition of 0.3=K 2  /K should preferably be met where K is the refracting power of the entire lens system and K 3  is the refracting power of the image-side lens. Thus, the paraxial focal length f 2  (=1/K 2 ) of the image-side lens and the paraxial focal length f (=1/K) of the entire lens system should satisfy the following relationship: 
     
         f.sub.2 /f≦10/3 
    
     but should more preferably satisfy the following relationship: 
     
         f.sub.2 /f≦3                                        (9) 
    
     The above and further objects, details and advantages of the present invention will become apparent from the following detailed description of preferred embodiments thereof, when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1, 4, 7, 10, 13, 16, 19, 22, 25, and 28 are schematic axially cross-sectional views of lens systems according to first through tenth embodiments, respectively, of the present invention; 
     FIGS. 2, 5, 8, 11, 14, 17, 20, 23, 26, and 29 are graphs showing the spherical aberrations (SA) and the offenses against the side condition (OSC) of the lens systems of the first through tenth embodiments; 
     FIGS. 3, 6, 9, 12, and 15 are graphs illustrating wavefront aberrations (RMS-OPD) with respect to image heights of the lens systems of the first through fifth embodiments; 
     FIGS. 18, 21, 24, 27, and 30 are graphs showing the curvatures of field of the lens systems of the fifth through tenth embodiments; and 
     FIG. 31 is a schematic view showing a minimum distance Z eff  required for a refractive index distribution in a lens closer to a light source. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Lens systems according to first through tenth embodiments of the present invention are illustrated respectively in FIGS. 1, 4, 7, 10, 13, 16, 19, 22, 25, and 28. In each of these embodiments, light rays emitted from a light source 1 pass through an aperture stop 2, a lens 3 closer to the light source 1, and a lens 4 closer to an image, and are focused on a recording surface 5A of an information recording medium 5 such as an optical disk or the like. The illustrated position of the aperture stop 2 in each of the embodiments is by way of example only, and the aperture stop 2 may be located in other positions. 
     The following Table 1 shows the distances between the light source 1, the lens 3, the lens 4, and the information recording medium 5, and the thicknesses on the optical axis of the lenses 3, 4 and the information recording medium 5. 
     
                       TABLE 1______________________________________Embodiment    t.sub.1  t.sub.2                    t.sub.3                           t.sub.4                                t.sub.5                                       t.sub.6______________________________________1        13.182   1.400  1.465  2.294                                1.499  1.2302        13.182   1.400  2.743  2.067                                1.498  1.2303        ∞  1.150  0.029  0.910                                1.802  1.2304        ∞  1.150  0.447  0.802                                1.799  1.2305        ∞  1.300  0.003  0.911                                1.600  1.2306        ∞  1.500  0.150  1.650                                1.660  1.2007        ∞  1.500  0.100  1.653                                1.610  1.2008        ∞  1.450  0.200  1.785                                1.059  1.2009        ∞  1.460  0.233  1.401                                1.503  1.20010       ∞  1.550  0.100  1.713                                1.849  1.200______________________________________ 
    
     In Table 1, 
     t 1  : the distance between the light source 1 and the lens 3 on the optical axis; 
     t 2  : the thickness of the lens 3 on the optical axis; 
     t 3  : the distance between the lenses 3, 4 on the optical axis; 
     t 4  : the thickness of the lens 4 on the optical axis; 
     t 5  : the distance between the lens 4 and the information recording medium 5 on the optical axis; and 
     t 6  : the thickness of the information recording medium 5 on the optical axis. 
     These distances and thicknesses are indicated in mm. 
     The value ∞ of the distance t 1  indicates that the light rays applied to the lens 3 are parallel. The lens systems with the value=of the distance t 1  are so-called a focal lenses which require a collimator lens for making light rays from the light source parallel. The values of t 2  indicates the values of Z o . 
     The radii of the aperture stops 2 and the radii of curvature of the lens surfaces in the above embodiments are given in the following Table 2: 
     
                       TABLE 2______________________________________Embodiment    R.sub.0  R.sub.1                    R.sub.2 R.sub.3                                 R.sub.4______________________________________1        2.000    ∞                    -5.556  3.196                                 12.1762        2.000    ∞                    -5.555  3.521                                 ∞3        2.000    5.688  ∞ 2.484                                 4.5844        2.000    6.556  ∞ 2.474                                 5.5545        2.000    4.545  ∞ 3.407                                 ∞6        2.5      7.132  ∞ 2.839                                 4.5667        2.5      7.098  ∞ 2.806                                 4.5448        2.5      7.345  ∞ 2.504                                 4.0939        2.0      4.715  ∞ 2.613                                 3.57410       2.5      7.461  ∞ 2.782                                 4.508______________________________________ 
    
     In Table 2, 
     R 0  : the radius of the aperture stop 2; 
     R 1  : the radius of curvature of the surface of the lens 3 which faces the light source; 
     R 2  : the radius of curvature of the surface of the lens 3 which faces the image; 
     R 3  : the radius of curvature of the surface of the lens 4 which faces the light source; and 
     R 4  : the radius of curvature of the surface of the lens 4 which faces the image. 
     These radii are indicated in mm. 
     The constants n o , k in the equation (1) representing the refractive index distribution of the lens 3, the range Z s  or Z eff  along the optical axis which requires the refractive index distribution, and the refractive index n 1  and the value of k.Z eff  in the range Zs≦Z, for the embodiments are given in the following Tables 3a and 3b: 
     
                       TABLE 3a______________________________________Embodiment n.sub.0  k          Zs   n.sub.1______________________________________1          1.699120 0.2478     0.400                               1.600002          1.715040 0.2876     0.400                               1.600003          1.615000 0.057143   0.350                               1.5950004          1.635000 0.114286   0.350                               1.5950005          1.728419 0.228114   0.500                               1.614362______________________________________ 
    
     
                       TABLE 3b______________________________________Embodiment  n.sub.0 k          Z.sub.eff                               k · Z.sub.eff______________________________________6           1.63408 0.06582    0.453                                0.029827           1.63583 0.07045    0.455                                0.032058           1.70000 0.094275   0.439                               0.04149           1.53000 0.038      0.445                               0.016910          1.75000 0.069858   0.431                               0.0301______________________________________ 
    
     In Tables 3a and 3b, n o  and n 1  are dimensionless, the values of k are indicated in mm -1 , and the values of Z s , Z eff  are indicated in mm. 
     The refractive index n 3  of the lens 4, the refractive index n 5  of the information recording medium 5, and the numerical aperture (NA) on the image side in the embodiments are given in the following Table 4: 
     
                       TABLE 4______________________________________Embodiment n.sub.3       n.sub.5 NA______________________________________1          1.780000      1.53    0.502          1.775367      1.53    0.503          1.780000      1.53    0.554          1.779417      1.53    0.555          1.506396      1.53    0.556          1.79883       1.51509 0.607          1.79883       1.51509  0.6118          1.79883       1.51509 0.709          1.79883       1.51509 0.5010         1.61656       1.51509 0.55______________________________________ 
    
     Calculated values of spherical aberration (SA) and offense against the sine condition (OSC) in the first through tenth embodiments are shown in FIGS. 2, 5, 8, 11, 14, 17, 20, 23, 26, and 29, respectively. Calculated values of wavefront aberration (RMS-OPD) at the wavelength of 780 mm) with respect to image height in the first through fifth embodiments are illustrated in FIGS. 3, 6, 9, 12, and 15, respectively. Meridional curvatures of field (M) and sagittal curvatures of field (S) in the sixth through tenth embodiments are shown in FIGS. 18, 21, 24, 27, and 30, respectively. 
     In the sixth through tenth embodiments, the material of the information recording medium 5 is optical glass (BK7) with n d  =1.51633 and v d  =64.1. For designing purpose, the wavelength of light applied to the lens systems is 632.8 nm, and Z is indicated in mm. 
     In the sixth embodiment, the refractive index distribution n(Z) of the lens 3 can be approximated by: 
     
         n(Z)=1.63408-0.05536Z-0.0500Z.sup.2 +0.04737Z.sup.3 +0.02654Z.sup.4 
    
     in the range of 0≦Z≦0.77, by: 
     
         n(Z)=1.59276 
    
     in the range of 0.77&lt;Z&lt;1.33, and by 
     
         n(Z)=1.59276-0.00038.(Z-1.33)+0.15385.(Z-1.33).sup.2 -0.12912.(Z-1.33).sup.3 +0.02654.(Z-1.33).sup.4 
    
     in the range of 1.33≦Z≦1.50. 
     In the seventh embodiment, the refractive index distribution n(Z) of the lens 3 can be approximated by: 
     
         n(Z)=1.63583-0.06852Z-0.01326Z.sup.2 +0.00530Z.sup.3 +0.05524Z.sup.4 
    
     in the range of 0≦Z≦0.75, by: 
     
         n(Z)=1.59222 
    
     in the range of 0.75≦Z≦1.35, and by 
     
         n(Z)=1.59222-0.00415 (Z-1.35)+0.16124 (Z-1.35).sup.2 -0.16042·(Z-1.35).sup.3 +0.05524 (Z-1.35).sup.4 
    
     in the range of 1.35≦Z≦1.50. 
     In the eighth embodiment, the refractive index distribution n(Z) of the lens 3 can be approximated by: 
     
         n(Z)=1.70000-0.094275Z 
    
     in the range of 0≦Z≦0.45 and by: 
     
         n(Z)=1.65758 
    
     in the range of 0.45≦Z≦1.45. 
     In the ninth embodiment, the refractive index distribution n(Z) of the lens 3 can be approximated by: 
     
         n(Z)=1.53000-0.038Z 
    
     in the range of 0≦Z≦0.46 and by: 
     
         n(Z)=1.51252 
    
     in the range of 0.46&lt;Z≦1.46. 
     In the tenth embodiment, the refractive index distribution n(Z) of the lens 3 can be approximated by: 
     
         n(Z)=1.75000-0.069858Z 
    
     in the range of 0≦Z≦0.45, and by: 
     
         n(Z)=1.71856 
    
     in the range of 0.45&lt;Z≦1.55. 
     The paraxial focal length f of the entire lens system, the paraxial focal length f 1  of the lens 3, and the paraxial focal length f 2  in each of the sixth through tenth embodiments are given in the following Table 5: 
     
                       TABLE 5______________________________________Embodiment f (mm)  f.sub.1 (mm)                         f.sub.2 (mm)                                f.sub.2 /f______________________________________6          4.165   11.248     6.597  1.5847          4.089   11.164     6.455  1.5798          3.566   10.493     5.387  1.5119          4.013    8.897     7.383  1.84010         4.539    9.948     8.550  1.884______________________________________ 
    
     According to the present invention, as can be understood from the above embodiments thereof, a bright two-group, two-element lens system is provided which has an NA of 0.50 or higher. The lens system of the invention is small in size, light in weight, and inexpensive to manufacture since it has fewer lenses than conventional lens systems having lenses made of a homogeneous medium. 
     Although there have been described what are at present considered to be the preferred embodiments of the present invention, it will be understood that the invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments are therefore to be considered in all aspects as illustrative, and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description.