Abstract:
A gradient index optical element for use in optical lens such as a camera or microscope is disclosed. The optical element comprises at least one of first metal dopants selected from La, Zr, Sn, In, Y, Ge, Zn, Ba, Ca, As, Sr, Ga and Gd distributed in solvent with a slope, and at least one of second metal dopants selected from Nb, Ta, Ti, Bi, Pb, Sb, Zr, Sn, In, Y and Tl, which is distributed equally in the solvent so as to satisfy following equation as absolute amount; 
     
       |ΔC.sub.f /ΔC.sub.g |·100≦5 
     
     wherein ΔC g  is a difference between maximum value and minimum value of equivalent value of metal oxides of the first metal doping and ΔC f  is a difference between maximum value and minimum value of equivalent value of metal oxides of the second metal doping.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a gradient index optical element applicable to an optical lens such as a camera, a microscope or the like. 
     Such a gradient index optical element has a power (refractive power) in its medium by imparting a distribution of a refractive index to the medium. This power is determined by the refractive index distribution, so that in order to make the power large, the absolute value |Δn| of a difference Δn in the refractive index n must be made large. Heretofore, many attempts of making the absolute value |Δn| large have been done by many searchers. For example, an optical element which is commercially available under the name of SELFOC lens (Trade name) has a large absolute value |Δn| by imparting a concentration gradient of Tl to the medium by ion exchange. Also, a lens of |Δn|≈0.059 was obtained by imparting a concentration gradient of Ag to the medium by a double ion exchange method (Applied Optics, vol. 27, no. 3 (1988) p.496). Moreover, a lens of |Δn|≈0.04 was obtained by imparting a concentration gradient of Pb and K to the medium by the sol-gel method (J.Non-cry.sol.100, 506, 1988). Finally, a lens of |Δn|≈0.03 was obtained by imparting a concentration gradient of Ti or Ge to the medium by the sol-gel method (Elect.Lett.22, 99 (1986), Elect.Lett.22, 1108 (1986)). 
     However, the developments in the gradient index optical element up to the present have mainly involved an approach resulting in increasing the absolute value |Δn|, but measures to decrease the chromatic aberration possessed by the optical elements themselves have lagged behind. In designing optical elements, it is possible to drastically decrease the number of lenses constructing the optical system for a lens system of a camera by using the excellent aberration correction ability of the gradient index optical element, but there is an inconsistency that the chromatic aberration correction of the lens system becomes more difficult as the number of the lenses decreases. In order make a lens system in which the number of lenses is small and the chromatic aberration is corrected, it is important to decrease the chromatic aberration itself generated in each lens. Therefore, the following properties are desired for the medium of a gradient refractive index optical element. 
     In a radial gradient refractive index optical element, the refractive index of the medium differs depending on the position through which a light beam is passing (the distance from the axis), and also the refractive index of the light beam differs. The Abbe number υ d  of the medium may be designated by υ d  =(n d  -1)/(n F  -n C ). If this Abbe number υ d  is constant from the above equation, the dispersion (n F  -n C ) increases as the refractive index n d  increases. Therefore, in order to prevent the chromatic aberration from increasing for an increase of refractive index, it is necessary to increase the Abbe number υ d  as the refractive index n d  is increased (refer to Japanese Patent Laid-Open No. 141,302/91). That is, it is desirable to obtain an optical element having a gradient of concentration distribution in the direction A on the n d  -υ d  graph shown in FIG. 5. 
     The gradient index optical element is then obtained by imparting a gradient of refractive index to the medium on the basis of a gradient of concentration of metal oxide in a glass. The optical property of the glass is determined by its oxide composition. For glass having as its principal component SiO 2 , metal oxides except for Si have a high refractive index as compared with SiO 2  glass, and dispersion becomes large (that is, the Abbe number becomes small), so that if a gradient index optical element is obtained by imparting a concentration gradient to these metal dopants, only the gradient index optical element having a gradient of concentration distribution in the direction B on the n d  -υ d  graph shown in FIG. 5 is obtained. 
     Japanese Patent Laid-Open No. 141,302/91 discloses a gradient index optical element obtained by imparting concentration distributions in opposite directions to each other to the medium with the use of two or more metals, in which the concentration distribution property may be variously changed. However, it is difficult to obtain the gradient index optical element, since the above method of imparting concentration distributions of two metals in opposite directions to each other is limited. That is, heretofore, it is difficult to obtain the optical element having optical properties in the direction A on the n d  -υ d  graph shown in FIG. 5 and desired in the designing of optical devices at the point of material production. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome the above described disadvantages of the conventional gradient index optical elements. 
     It is another object of the present invention to provide a gradient index optical element in which chromatic aberration correction is excellent, that is, the Abbe number becomes large as the refractive index becomes large, or the change in Abbe number is small with any increase of refractive index. 
     According to the present invention, there is provided a gradient index optical element comprising at least one of first metal dopants selected from a metal group G {La, Zr, Sn, In, Y, Ge, Zn, Ba, Ca, As, Sr, Ga and Gd} distributed in a medium with a slope, and at least one second metal dopant selected from a metal group F {Nb, Ta, Ti, Bi, Pb, Sb, Zr, Sn, In, Y and Tl}, which is distributed equally in the medium so as to satisfy the following equation as an absolute amount; 
     
         |ΔC.sub.f /ΔC.sub.g |·100≦5 
    
     wherein ΔC g  is a difference between the maximum value and the minimum value of the absolute amount of metal oxides of the first metal doping and ΔC f  is difference between the maximum value and the minimum value of the absolute amount of metal oxides of the second metal doping. 
     It has been found that the gradient index optical element constructed according to the present invention is excellent in performing chromatic aberration correction. That is, in the radial gradient index optical element, as shown in FIG. 1, it is found that a low occurrence of chromatic aberration can be obtained when the concentration distribution of first metal dopant g has its largest absolute value at the center portion of the optical element and decreases parabolically to the periphery portion and when the concentration distribution of second metal dopant f has a substantially even absolute value. When, as shown in FIG. 2, the concentration distribution of the first metal dopant g is made in the reverse direction, it is recognized that the same effect as described above can be obtained. 
     These effects are exhibited only when the metal dopant f is contained in the optical element, and thus, even when the concentration distribution of the metal selected from the above metal group G is obtained with a certain gradient as the first metal dopant g, if the second metal dopant f selected from the above metal group F is not included, the optical element with the desired optical properties cannot be obtained. 
     According to the present invention, the second metal dopant can be made distributed substantially even as an absolute amount, so that the difficulty in production that the concentrations of two kinds of metal dopants are distributed in the reverse directions to each other can be avoided and thus the gradient index optical element having a low occurrence of chromatic aberration can be produced relatively easily. 
     In selecting metals from metal groups G and F with high effect, particularly, when the optical element has a metal dopant selected from metal group G&#34;{Zr, Sn, In, Y, Ge, Zn, Ga} as the first metal dopant g, the second metal dopant f must be selected from metal group F&#34;{Nb, Ta, Bi, Pb, Sb, Zr, Tl}. Also, when the optical element has at least one metal dopant selected from metal group G&#39;{La, Ba, Ca, As, Sr, Gd} as the first metal dopant g, the second metal dopant f must be selected from metal group F&#39;{Nb, Ta, Ti, Bi, Pb, Sb, Zr, Sn, In, Y, Tl}. Moreover, two or more kinds of metals may be selected from respective metal groups, and a combination thereof may be used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a conception view showing a metal content distribution of a gradient index optical element according to the present invention; 
     FIG. 2 is a conception view showing a metal content distribution of a gradient index optical element according to another embodiment of the present invention; 
     FIG. 3 is an explanatory view showing a relationship between the Abbe number and the refractive index of a gradient index optical element shown in example 19; 
     FIG. 4 is an explanatory view showing a relationship between the Abbe number and the refractive index of a gradient index optical element shown in example 20; and 
     FIG. 5 is an explanatory view showing a preferable and non-preferable relationship between Abbe number and refractive index of a gradient index optical element. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     It is found that the gradient index optical element capable of optionally changing the amount of chromatic aberration can be obtained by variously changing the method of selecting metal dopants and the amount of respective metal dopants. For example, assuming that metal dopants g, f are selected and the maximum amount C g  of first metal dopant g is made constant, the gradient of the optical properties shown in FIG. 5 is liable to slant to the left direction by increasing the content of the second metal dopant f having an even concentration distribution. That is, in FIG. 5, the optical element is liable to exhibit the optical properties in the direction of A. Contrarily, if the amount of second metal dopants f is low, the optical element exhibits the optical properties in the direction of B in FIG. 5. Therefore, in order to obtain the optical element with the desired optical properties, then at least a minimum amount of second metal dopant f must be contained in the optical element. 
     The most suitable content of respective metals is different for the contained metal dopant and the aimed optical properties individually, so that it is difficult to determine the most suitable content easily, but it is desired to have an overall conversion amount C g  of metal oxide of the first metal dopants g of 5-40 mol % and an overall conversion amount C f  of metal oxide of the second metal dopant f of 2-40 mol % when the absolute amount of the first metal dopant g is maximum. 
     The lower content limit of the second metal dopant f is dependent upon its kind, its content and the concentration distribution gradient of metal dopants g, f, as described above, so that it is difficult to determine the lower limit of content of metal dopant f easily, but it is desired to make the lower content limit of every kind of metal dopant f according to the value shown in following Table 1. 
     
                       TABLE 1______________________________________Effective relationship between metal dopant and its contentMetal dopant f         Effective content of metal dopant f______________________________________Nb, Ta, Ti, Bi, Sb, T1         2≦Pb, Zr        5≦Sn, In, Y     10≦______________________________________ (Content is a metal oxide conversion ratio mol % of metal dopant f when the absolute amount of metal dopant g becomes maximum). 
    
     The lower content limit of desirable metal dopant f is changed even by a combination of both metal dopants g and f. The effective relationship between the selected metal dopants from respective metal groups shown by G and F, that is, a combination of these metal dopants, and the content of metal dopants f at this time, is shown in the following Table 2. 
     
                       TABLE 2______________________________________Effective relationship of a combination of metal dopants g and fand the content of metal dopant f                      Effective contentMetal dopant f     Metal dopant g   of metal dopant f______________________________________Nb, Ta, Ti, Bi,     La, Ba, Ca, As, Sr, Gd                      2≦Sb, T1Bi        Y, GeT1        Sn, In, Y, Ge, Zn, GaNb, Ta, Ti, Sb     In, Y, Ge, Ga    5≦Pb        La, Y, Ba, Ca, As, Sr, GdBi        Sn, In, Zn, GaT1        ZrZr        Ba, GdNb, Ti. Sb     Sn, Zn           10≦Bi        ZrPb        In, Ge, Zn, GaZr        Ca, As, SrSn        Sr, GdIn, Y     Ba, Sr, As, GdNb, Sb, Ti     Zr               15≦Zr, In    LaSn        La, Ba, Ca, AsTa, Pb    Sn______________________________________ (Content is a metal oxide conversion ratio mol % of metal dopant f when the absolute amount of metal dopant g becomes maximum). 
    
     In the above Table 2, when a few kinds of metal dopants are selected, the contents of metal oxides are summed up in the metal group to obtain the content of metal dopant f and then to apply to the effective content value of Table 2. Moreover, when a few metals in different columns are selected simultaneously, the effective content value corresponding to the set of metal dopants located in the more upper column is applied as the desirable lower limit of content of metal dopant f. 
     EXAMPLE 1 
     In this example, the content of La 2  O 3  is distributed to decrease radially from the center portion to the periphery portion of the medium in such a manner that the composition of the metal oxide is at the center portion of the medium TiO 2  15 mol %, La 2  O 3  15 mol, SiO 2  70 mol %, and at the periphery portion of the medium La 2  O 3  0 mol %, so that a radial gradient index optical element having an even concentration distribution is obtained without decrease in the absolute amount of TiO 2  at the periphery portion of the medium. 
     The refractive index distribution of a gradient index optical element according to this example was measured and the following properties were obtained: n d  =1.728, υ d  =36.6 at the center portion of the medium, and n d  =1.626, υ d  =33.5 at the periphery portion of the medium. It is found from the above measured properties that the optical element according to the present example has an optical property in the direction of A shown in FIG. 5 such that Δn d  =-0.092, Δυ d  =-3.1 (Δυ d  is the value of υ d  at the periphery portion of the medium minus υ d  at the center portion of the medium). The gradient index optical element according to the present example can be used as a very useful radial refractive index distribution due to excellent chromatic aberration correction. 
     EXAMPLES 2-14 
     These examples obtain a gradient index optical element in which one of the metal dopants is selected from each of metal group G and metal group F, respectively, and the center portion of the medium contains a composition of oxide conversion ratio mol % described in Table 3 which shows a relationship between the composition of metal oxide and optical property, resulting in a concentration distribution of the first metal dopants of metal group G being decreased or increased in the radial shape to the periphery portion of the medium, and thus the concentration of the second metal dopant of metal group F is evenly distributed as an absolute amount. In Table 3, the minus sign of Δn d  in the optical property column denotes a convex shaped distribution in which the refractive index of these optical elements is high at the center portion thereof, and the plus sign of Δn d  in the optical property column denotes a concave shaped distribution in which the refractive index of these optical elements is low at the center portion thereof. The plus sign of the product of Δn d  and Δ υ d  designates that the optical properties of thus obtained glass shows optical properties in the A direction of FIG. 5 and thus it is found that these optical elements are excellent in reducing the amount of chromatic aberration occurrence. 
     
                       TABLE 3______________________________________Composition of metal oxide and optical propertiesG                F            PropertyMetal       Content  Metal   Content                               Δnd                                     Δνd______________________________________Example 2   Ge      20       Ti    10     -0.046                                       -0.8Example 3   Sr      20       Ti    10     -0.052                                       -2.1Example 4   Y       25       Ta    15     -0.067                                       -1.7Example 5   Ba      15       Zr    20     -0.046                                       -1.0Example 6   Ba      15       Pb    15     -0.057                                       -2.7Example 7   La      15       Nb    10     +0.081                                       +1.7Example 8   As      15       Bi    5      -0.084                                       -2.0Example 9   In      15       Sb    13     -0.083                                       -0.1Example 10   Ga      10       Ti    5      -0.064                                       -0.8Example 11   Ca      20       Ti    10     -0.045                                       -1.0Example 12   Ba      15       In    18     +0.039                                       +0.1Example 13   La      20       Sn    35     -0.090                                       -0.1Example 14   Sn      10       Nb    20     -0.025                                       -0.3______________________________________ (Content is a metal oxide conversion ratio mol % of metal dopant f when the absolute amount of metal dopant g becomes maximum) 
    
     EXAMPLES 15-18 
     The above examples 1-14 show the case that only one kind of metal dopant is selected from each metal group G and F, respectively, but two or more kinds of metal dopants may be selected from each respective metal group. 
     Examples 15-18 obtain a radial gradient index optical element in which, as shown in Table 4 which shows a relationship between the composition of metal oxide and the optical properties, one or more kinds of first metal dopant having a convex shaped concentration distribution are selected from metal group G and metal group F, respectively, and the center portion of the medium contains a composition of oxide conversion ratio described in Table 4 resulting in a concentration distribution of the first metal dopants of metal group G being decreased in radial shape to the periphery portion of the medium, and thus the concentration of second metal dopants of metal group F is evenly distributed as an absolute amount. 
     
                       TABLE 4______________________________________Composition of metal oxide and optical propertiesG                F            PropertyMetal       Content  Metal   Content                               Δnd                                     Δνd______________________________________Example 15   Be      20       Pb    15     -0.053                                       -1.5   Ge      5Example 16   La      13       Nb    7.5    -0.068                                       -2.5                    Ti    7.5Example 17   Zr      15       Nb    10     -0.035                                        0.0                    Ti    10Example 18   La      10       Ta    7.5    -0.060                                       -2.1   Ge      10       Ti    7.5______________________________________ (Content is a metal oxide conversion ratio mol % of metal dopant f when the absolute amount of metal dopant g becomes maximum) 
    
     EXAMPLE 19 
     It has been found that the optical properties of the gradient index optical element in which the composition (90-x)SiO 2  --xNb 2  O 5  --0La 2  O 3  of the periphery portion of the medium is changed in accordance with the change of ratio x Nb 2  O 5  evenly present in the glass as an absolute amount, by eluting only La 2  O 3  from the medium being the composition (90-x) SiO 2  --xNb 2  O 5  --10La 2  O 3  of the center portion of the medium as described in Table 5, which shows a relationship between the composition of metal oxide and the optical property. As shown in Table 5 and FIG. 3, Δυ d  becomes large in the negative direction as the addition of Nb is increased. 
     
                       TABLE 5______________________________________Composition of metal oxide and optical propertiesG                F            PropertyMetal       Content  Metal   Content                               Δn.sub.d                                     Δν.sub.d______________________________________Example 19   La      10       Nb    5      -0.085                                        0.7   La      10       Nb    10     -0.060                                       -1.2   La      10       Nb    15     -0.043                                       -1.5   La      10       Nb    20     -0.035                                       -1.8______________________________________ (Content is a metal oxide conversion ratio mol % of metal dopant f when the absolute amount of metal dopant g becomes maximum) 
    
     Therefore, it is found that the optical element obtained by having an even concentration distribution of metal group F is also excellent for chromatic aberration correction and thus the amount of chromatic aberration can be controlled by the amount of second metal dopant of metal group F. 
     EXAMPLE 20 
     The above explanations illustrate the case in which the concentration of second metal dopant of metal group F is evenly distributed as an absolute value, but even when the concentration distribution is slightly deviated from an even distribution, the same effect as described above can be obtained. An example showing such an effect is shown in Table 6 which shows a relationship between the composition of metal oxide and the optical properties. In this example the composition of the center portion of the medium is 65SiO 2  --15TiO 2  --20Y 2  O 3  and the composition of the periphery portion of the medium is 65SiO 2  --14TiO 2  --OY 2  O 3  and then TiO 2  is present with a small concentration gradient as an absolute amount. In FIG. 4, line 1 shows n d  -υ d  property at this time, and this is the same as the property (line 2 in FIG. 4) of the optical element in such a manner that the concentration distribution of TiO 2  is quite even. Therefore, it is found that the optical element having the concentration distribution of second metal dopant of metal group F being slightly deviated from an even distribution has a low amount of chromatic aberration and is excellent in chromatic aberration correction. 
     
                       TABLE 6______________________________________Composition of metal oxide and optical propertiesG                F            PropertyMetal       Content  Metal   Content                               Δnd                                     Δνd______________________________________Center  Y       20       Ti    15     -0.085                                       -2.5portionPeripheral   Y       0        Ti    14     -0.085                                       -2.5portion______________________________________ (Content is a metal oxide conversion ratio mol % of metal dopant f when the absolute amount of metal dopant g becomes maximum) 
    
     In the above described examples, the composition of the pheriphery portion of first metal dopant in metal group G is zero, but even when the composition of the periphery of first metal dopant in metal group G is not zero, the directions of the optical properties are not changed so that in the gradient index optical element the occurrence of chromatic aberration is suppressed. Moreover, when the present invention is applied to an axial gradient index optical element, high effect can be derived.