Title of Invention: Objective lens for high-density optical recording medium
Abstract: An objective lens for a high-density optical recording medium, employed as an objective lens for an optical pickup for converging a luminous flux onto an optical recording medium, comprises, successively from the light source side, a first lens L1 made of a biconvex lens having at least one surface formed as an aspheric, and a second lens L2 made of a planoconvex lens having a convex surface directed onto the light source side, and satisfies the following conditional expressions (1) to (3): 0.588 less than xcex22xe2x89xa60.676xe2x80x83xe2x80x83(1) NA1xe2x89xa70.45xe2x80x83xe2x80x83(2) RMS1xe2x89xa60.07xcexxe2x80x83xe2x80x83(3) where xcex22 is the imaging magnification of the second lens L2; NA1 is the numerical aperture of the first lens L1; RMS1 is the RMS of the wavefront aberration of the first lens L1; and xcex is a reference wavelength.
Brief Summary: This application claims the priority of Japanese Patent Application No. 11-002416 filed on Jan. 8, 1999, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an objective lens for a high-density optical recording medium; and, in particular, to an objective lens for a high-density optical recording medium, which is used in an optical pickup apparatus for writing or reading information signals with respect to an optical recording medium such as an optical disk, a magneto-optical disk, an optical card, or the like for which high-density recording is demanded.
2. Description of the Prior Art
In recent years, CD (compact disk), MO (magneto-optical disk), DVD (digital video disk), and the like have come into wider use as an optical recording medium for recording audio information, visual information, data information for computers, and the like.
In optical pickup apparatus for writing or reading information signals with respect to these optical recording media, objective lenses having numerical apertures of 0.45, about 0.5 to 0.6, and 0.6 are in use for CD, MO, and DVD, respectively.
Also, since demands for smaller dimensions, lighter weight, and lower cost are strong, the optical pickup apparatus generally employ an aspherical single-element lens made of a synthetic resin material or glass material as the objective lens for satisfying these demands.
Meanwhile, along with dramatic increases in information recording capacities, there has recently been longing for the emergence of recording media whose density is higher even by a small amount. Therefore, efforts are always under way to improve the recording density of optical recording media.
Here, for improving the recording density of optical recording media, it is necessary to reduce the light-converging spot diameter caused by the objective lens. Letting xcex be the wavelength of a light source, and NA be the numerical aperture of the objective lens, the light-converging spot diameter is represented by the following expression:
kxc3x97xcex/NA
where k is a constant, whereby it is necessary to attain at least one of the shortening of the wavelength of the light source and the increasing of the NA of the objective lens in order to reduce the light-converging spot diameter.
The shortening of the wavelength of the light source has been improved by the shortening of the wavelength of semiconductor lasers and the development of SHG light sources.
On the other. hand, the increasing of the NA of the objective lens has conventionally been in progress by employing an aspherical single-element lens. This technique can achieve an NA of about 0.6 or less. For attaining a further higher NA, however, there has been a limit to the use of aspherical single-element lenses, since single-element lenses become harder to make. For example, the objective lens disclosed in Japanese Unexamined Patent Publication No. 10-123410 has a two-group, two element lens configuration in order to attain an NA of about 0.70.
However, the objective lens disclosed in the above-mentioned publication is harder to design and make as compared with the conventional single-element objective lenses, thus increasing labor and cost. Further, since the method of its evaluation is not easy, the labor and cost for inspecting the product would increase.
In view of the circumstances mentioned above, it is an object of the present invention to provide an objective lens for a high-density optical recording medium, which is easier to design, make, and inspect and can favorably correct aberration, while having a high NA of 0.75 or greater and a two-group, two-element lens configuration.
The objective lens for a high-density optical recording medium in accordance with the present invention is an objective lens for an optical pickup for converging a luminous flux onto an optical recording medium;
the objective lens comprising, successively from a light source side, a first lens made of a biconvex lens having at least one surface formed as an aspheric surface, and a second lens having a convex surface directed onto the light source side, the second lens satisfying the following conditional expression (1):
0.588 less than xcex22xe2x89xa60.676xe2x80x83xe2x80x83(1)
where xcex22 is the imaging magnification of the second lens.
Preferably, the second lens has a spheric surface on the light source side and a planar surface on the optical recording medium side.
Preferably, the first lens is constituted by a lens whose aberration is corrected by itself and satisfies the following conditional expressions (2) and (3):
NA1xe2x89xa70.45xe2x80x83xe2x80x83(2)
RMS1xe2x89xa60.07xcexxe2x80x83xe2x80x83(3)
where
NA1 is the numerical aperture of the first lens;
RMS1 is the RIMS of the wavefront aberration of the first lens; and
xcex is a reference wavelength.
Preferably, both surfaces of the first lens may be made aspheric as well.
Detailed Description: In the following, the objective lens for a high-density optical recording medium in accordance with an embodiment of the present invention will be explained with reference to the drawings.
FIG. 1 is a view showing the configuration of the objective lens for a high-density optical recording medium in accordance with Example 1 as a representative of this embodiment.
As shown in FIG. 1, the objective lens for a high-density optical recording medium in accordance with this embodiment is a lens having a two-group, two-element configuration comprising, successively from the light source side, a first lens L1 made of a biconvex lens having at least one surface formed as an aspheric surface, and a second lens L2 made of a planoconvex lens. Here, the second lens L2 has a spheric surface with a convex surface directed onto the light source side and a planar surface on the optical recording medium side. The aspherical form of the first lens L1 is represented by the following aspherical depth expression: 






  Z
  =
  
    
      
        
          Y
          2
        
        /
        R
      
      
        1
        +
        
          
            1
            -
            
              K
              xc3x97
              
                
                  Y
                  2
                
                /
                
                  R
                  2
                
              
            
          
        
      
    
    +
    
      
        ∑
        
          i
          =
          2
        
        5
      
      ⁢
      
        xe2x80x83
      
      ⁢
      
        
          A
          i
        
        ⁢
        
          Y
          
            2
            ⁢
            i
          
        
      
    
  







where
Z is the length of the perpendicular to the tangential plane (the plane perpendicular to the optical axis) of an apex of the aspheric surface from a point on the aspheric surface at a distance Y from the optical axis;
Y is the distance from the optical axis;
Ai is the aspheric surface coefficient (i=2 to 5);
K is the eccentricity; and
R is the radius of curvature of the aspheric surface near the optical axis.
Here, light from the light source (not depicted) is transmitted through this objective lens and is focused onto the light-converging point of the second lens L2 on the optical recording medium side. The position of the light-converging point is distanced from the second lens L2 by 1 to 2 xcexcm on the optical axis Z, thus being so close to the second lens L2 that it cannot be depicted.
Also, the objective lens in accordance with this embodiment satisfies the following conditional expressions (1xe2x80x2) to (3xe2x80x2):
0.588 less than xcex22xe2x89xa60.676xe2x80x83xe2x80x83(1xe2x80x2)
NA1xe2x89xa70.45xe2x80x83xe2x80x83(2xe2x80x2)
RMS1xe2x89xa60.07xcexxe2x80x83xe2x80x83(3xe2x80x2)
where
xcex22 is the imaging magnification of the second lens L2;
NA1 is the numerical aperture of the first lens L1;
RMS1 is the root mean square (RMS) of the wavefront aberration of the first lens L1; and
xcex is a reference wavelength.
Operations of this embodiment will now be explained.
First, since at least one surface of the first lens L1 is formed as an aspheric surface, the first lens L1 can become an aberration-corrected lens which is usable as an objective lens by itself.
Also, since the second lens L2 is a planoconvex lens, not only the manufacture in terms of the polishing of the lens or the like becomes easier, but also its centering become s easier, whereby the cost of the lens can be lowered.
Conditional expression (1xe2x80x2) defines the imaging magnification xcex22 of the second lens L2 by itself.
If the imaging magnification xcex22 exceeds the upper limit of the expression, then it is necessary to enhance the numerical aperture of the first lens L1 alone in order to enhance the NA of the whole system, whereby it becomes difficult to correct aberration of the first lens L1.
If the imaging magnification xcex22 is below the lower limit of the expression, then it means that the: object point distance becomes longer while the focal length of the second lens L2 is unchanged or that the focal length becomes shorter while the object point distance is unchanged. Here, if the object point distance of the second lens L2 is made longer, it is also necessary to elongate the focal length of the first lens L1, whereby the lens as a whole becomes greater, thus making it difficult to make the lens smaller. If the focal length of the second lens L2 is made shorter, on the other hand, the refracting power of the second lens L2 becomes greater, so that errors in manufacture greatly influence lens performances.
Thus, when the second lens L2 satisfying conditional expression (1xe2x80x2) is combined with the first lens L1, whose aberration is corrected by itself as mentioned above, so as to form a lens having a two-group, two-element configuration, a further higher NA of the lens can be attained.
Conditional expression (2xe2x80x2) defines the numerical aperture of the first lens L1 when used alone. If the first lens L1 satisfies this conditional expression, then it can easily yield, when combined with the second lens L2, a small-size objective lens with a numerical aperture of 0.75 or greater for a high-density optical recording medium.
Namely, letting NA1 be the numerical aperture of the first lens L1 by itself, and xcex22 be the imaging magnification of the second lens L2, the numerical aperture NA of the whole objective lens is represented by the following expression:
NA=NA1/xcex22
Here, if an objective lens having a numerical aperture of 0.75 or greater for an high-density optical recording medium is to be designed while the numerical aperture of the first lens L1 is set to 0.44 which is below the lower limit of conditional expression (2xe2x80x2), then the imaging magnification of the second lens L2 becomes smaller than 0.588, whereby it becomes difficult to make the objective lens smaller as mentioned above.
Preferably, the first lens L1 has a numerical aperture of about 0.6 at the maximum in view of the fact that its manufacture becomes easier if a lens which has conventionally been made is used.
For example, if an objective lens having a numerical aperture of 0.6 is employed as the first lens L1 of the objective lens in accordance with this embodiment, while the second lens L2 is a lens having an imaging magnification of 0.667, then the whole objective lens system has a numerical aperture of 0.9, whereby a high-NA objective lens for a high-density optical recording medium can be obtained.
Conditional expression (3xe2x80x2) is a standard for evaluating wavefront aberration, which is known as Marechal criterion. Lenses satisfying this conditional expression (3xe2x80x2) is referred to as aberration-free lenses in general. When this conditional expression (3xe2x80x2) is satisfied, then the first lens L1 is configured so as to correct aberration by itself.
Since the first lens L1 is a lens whose aberration are corrected by itself, it will be sufficient if only the aberration correction of the second lens L2 is taken into consideration, whereby the labor for designing and evaluating will be alleviated. Also, even when thee first lens L1 is an aspherical lens whose performances are hard to inspect in general, its performance can be inspected by itself, whereby the process thereof is simplified.
The objective lens for a high-density optical recording medium in accordance with Example 1 has the configuration, operations, and effects explained in detail in the foregoing, whereas a lens having an aspheric surface on each side thereof is used as the first lens L1.
Table 1(following) shows the radius of curvature (radius of curvature near the optical axis in the case of an aspheric surface) R (mm) of each lens surface, axial surface space of each lens (center thickness of each lens and air space between neighboring lenses) D (mm), and refractive index N of each lens at a wavelength of 410 nm in Example 1. In Table 1 and Tables subsequent thereto, the numbers referring to the radius of curvature R, axial surface space D, and refractive index N successively increase from the light source side.
The middle part of Table 1 lists the respective values of constants of each aspheric surface indicated in the above-mentioned aspherical depth expression in Example 1; whereas the lower part thereof lists the values of incident luminous flux diameter xcfx86 (mm), focal length f (mm) of the whole lens system, numerical aperture NA of the whole lens system, RMS of the wavefront aberration of the whole lens system, focal length f1 (mm) of the first lens L1, numerical aperture NA1 of the first lens L1, RMS1 of the wavefront aberration of the first lens L1, focal length f2 (mm) of the second lens L2, and imaging magnification xcex22 in Example 1.
As shown in Table 1, Example 1 satisfies all of conditional expressions (1xe2x80x2) to (3xe2x80x2).
The objective lens for a high-density optical recording medium has a configuration substantially the same as that of Example 1 and exhibits operations and effects similar to those of Example 1, whereas a lens identical to that of Example 1 is used as the first lens L1.
Table 2(following) shows the radius of curvature (radius of curvature near the optical axis in the case of an aspheric surface) R (mm) of each lens surface, axial surface space of each lens (center thickness of each lens and air space between neighboring lenses) D (mm), and refractive index N of each lens at a wavelength of 410 nm in Example 2.
The middle part of Table 2 lists the respective values of constants of each aspheric surface indicated in the above-mentioned aspherical depth expression in Example 2; whereas the lower part thereof lists the values of incident luminous flux diameter xcfx86 (mm), focal length f (mm) of the whole lens system, numerical aperture NA of the whole lens system, RMS of the wavefront aberration of the whole lens system, focal length f1 (mm) of the first lens L1, numerical aperture NA1 of the first lens L1, RMS1 of the wavefront aberration of the first lens L1, focal length f2 (mm) of the second lens L2, and imaging magnification xcex22 in Example 2.
As shown in Table 2, Example 2 satisfies all of conditional expressions (1xe2x80x2) to (3xe2x80x2).
FIG. 2 is a view showing the configuration of the objective lens for a high-density optical recording medium in accordance with Example 3.
The objective lens for a high-density optical recording medium has a configuration substantially the same as that of Example 1 and exhibits operations and effects similar to those of Example 1. Here, as the first lens L1, a lens having the same radius of curvature, surface space, and refractive index as those of Example 1 and a diameter greater than that of Example 1 is used.
Table 3(following) shows the radius of curvature (radius of curvature near the optical axis in the case of an aspheric surface) R (mm) of each lens surface, axial surface space of each lens (center thickness of each lens and air space between neighboring lenses) D (mm), and refractive index N of each lens at a wavelength of 410 nm in Example 3.
The middle part of Table 3 lists the respective values of constants of each aspheric surface indicated in the above-mentioned aspherical depth expression in Example 3; whereas the lower part thereof lists the values of incident luminous flux diameter xcfx86 (mm), focal length f (mm) of the whole lens system, numerical aperture NA of the whole lens system, RMS of the wavefront aberration of the whole lens system, focal length f1 (mm) of the first lens L1, numerical aperture NA1 of the first lens L1, RMS1 of the wavefront aberration of the first lens L1, focal length f2 (mm) of the second lens L2, and imaging magnification xcex22 in Example 3.
As shown in Table 3, Example 3 satisfies all of conditional expressions (1xe2x80x2) to (3xe2x80x2).
FIG. 3 is a view showing the configuration of the objective lens for a high-density optical recording medium in accordance with Example 4.
As shown in FIG. 3, the objective lens in accordance with Example 4 is constituted by two lens elements substantially the same as those of Example 3, whereas cover glass 1 for the optical recording medium is disposed on the optical recording medium side of the second lens L2. Employed as the first lens L1 is a lens identical to that of Example 3, and the whole lens system exhibits operations and effects similar to those of Example 1.
The cover glass 1 can be disposed at a position between the optical-recording-medium-side surface of the second lens L2 and the light-converging point.
The cover glass 1 can have various modes. For example, in the case where the back side of the optical recording medium (the opposite side of the medium from the second lens L2) is formed with a signal surface, the transparent substrate of the optical recording medium can be used as this cover glass 1. For example, in the case where the front side of the optical recording medium (the side facing the second lens L2) is formed with the signal surface, a transparent substrate for adjusting the optical path length disposed at a given position between the second lens L2 and the optical recording medium can be used as the cover glass 1. Further, a transparent substrate for protecting the medium, or the like disposed at a given position between the second lens L2 and the optical recording medium can also be used as this cover glass 1 in the case where the front side of the optical recording medium (the side facing the second lens L2) is formed with the signal surface.
Table 4(following) shows the radius of curvature (radius of curvature near the optical axis in the case of an aspheric surface) R (mm) of each lens surface, axial surface space of each lens (center thickness of each lens and air space between neighboring lenses) D (mm), and refractive index N of each lens at a wavelength of 410 nm in Example 4.
The middle part of Table 4 lists the respective values of constants of each aspheric surface indicated in the above-mentioned aspherical depth expression in Example 4; whereas the lower part thereof lists the values of incident luminous flux diameter xcfx86 (mm), focal length f (mm) of the whole lens system, numerical aperture NA of the whole lens system, RMS of the wavefront aberration of the whole lens system, focal length f1 (mm) of the first lens L1, numerical aperture NA1 of the first lens L1, RMS1 of the wavefront aberration of the first lens L1, focal length f2 (mm) of the second lens L2, and imaging magnification xcex22 in Example 4.
As shown in Table 4, Example 4 satisfies all of conditional expressions (1xe2x80x2) to (3xe2x80x2).
FIG. 4 is a view showing the configuration of the objective lens for a high-density optical recording medium in accordance with Example 5.
The objective lens for a high-density optical recording medium in accordance with Example 5 has a configuration substantially the same as that of Example 1 and exhibits operations and effects similar to those of Example 1. Here, in the first lens L1, its surface on the light source side is aspheric, whereas its surface on the optical recording medium side is spheric.
Table 5(following) shows the radius of curvature (radius of curvature near the optical axis in the case of an aspheric surface) R (mm) of each lens surface, axial surface space of each lens (center thickness of each lens and air space between neighboring lenses) D (mm), and refractive index N of each lens at a wavelength of 410 nm in Example 5.
The middle part of Table 5 lists the respective values of constants of the aspheric surface indicated in the above-mentioned aspherical depth expression in Example 5; whereas the lower part thereof lists the values of incident luminous flux diameter xcfx86 (mm), focal length f (mm) of the whole lens system, numerical aperture NA of the whole lens system, RMS of the wavefront aberration of the wavefront aberration of the whole lens system, focal length f1 (mm) of the first lens L1, NA1 is the numerical aperture of the first lens, RMS of the wavefront aberration of the whole lens system, focal length f1 (mm) of the first lens L1, numerical aperture NA1 of the first lens L1, RMS1 of the wavefront aberration of the first lens L1, focal length f2 (mm) of the second lens L2, and imaging magnification xcex22 in Example 5.
As shown in Table 5, Example 5 satisfies all of conditional expressions (1xe2x80x2) to (3xe2x80x2).
FIGS. 5A and 5B are charts showing wavefront aberration of the first lens L1 at a wavelength of 410 nm in Examples 1 to 4 and in Example 5, respectively. As can be seen from FIGS. 5A and 5B, the first lens L1 in each of Examples 1 to 5 is a lens whose aberration are favorably corrected by itself.
FIGS. 6A to 6E are wavefront aberration charts of the whole lens system at a wavelength of 410 nm in Examples 1 to 5, respectively. As can be seen from FIGS. 6A to 6E, the objective lens in accordance with each of Examples 1 to 5 is a lens whose aberration are favorably corrected.
Without being restricted to those of the above-mentioned Examples, the objective lens of the present invention can be modified in various manners. For example, the radius of curvature R of each lens and the lens space (or lens thickness) D can be changed as appropriate.
Though the optical-recording-medium-side surface of the second lens L2 is preferably a planar surface in general from the viewpoint of processability, it may be a surface having a slight curvature as well.
Also, the objective lens in accordance with the present invention is not only applicable to cases where the object point (light source) is at infinity, but also usable as a so-called finite conjugate type lens whose objective point is disposed at a finite distance.
As explained in the foregoing, the objective lens for a high-density optical recording medium in accordance with the present invention has a two-group, two-element configuration in which an aspherical lens conventionally known as a single-element objective lens for an optical pickup is used as the first lens, whereas a lens with a predetermined imaging magnification having a convex surface directed onto the light source side is used as the second lens, whereby a high-NA objective lens can be attained while making its aberration better without increasing the labor and cost for designing, making, and inspecting the lens.

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