Abstract:
An aspheric lens includes at least one aspheric lens surface, and an angle of inclination on the aspheric lens surface is smaller than or equal to 65°. The aspheric lens surface is given by the equation 
     
       
         
           
             
               Z 
                
               
                 ( 
                 r 
                 ) 
               
             
             = 
             
               
                 
                   cr 
                   2 
                 
                 
                   1 
                   + 
                   
                     
                       1 
                       - 
                       
                         
                           ( 
                           
                             1 
                             + 
                             K 
                           
                           ) 
                         
                          
                         
                           c 
                           2 
                         
                          
                         
                           r 
                           2 
                         
                       
                     
                   
                 
               
               + 
               
                 Lr 
                 2 
               
               + 
               
                 Ar 
                 4 
               
               + 
               
                 Br 
                 6 
               
               + 
               
                 Cr 
                 8 
               
               + 
               
                 Dr 
                 10 
               
               + 
               
                 Er 
                 12 
               
               + 
               
                 Fr 
                 14 
               
               + 
               
                 Gr 
                 16 
               
               + 
               
                 Hr 
                 18 
               
               + 
               
                 Jr 
                 20 
               
             
           
         
       
     
     where L≠0, c is a curvature, r 2 =x 2 +y 2 , K is a conic integer, and L and A through J are aspheric coefficients.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 from Korean Patent Application No. 10-2008-0134953, filed on Dec. 26, 2008, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present general inventive concept relates to an aspheric lens that can be used as an objective lens in an optical information storage media system recording/reproducing information to/from an information storage medium, and to an optical pickup apparatus including the aspheric lens as an objective lens. 
         [0004]    2. Description of the Related Art 
         [0005]    Generally, an aspheric single objective lens is used to focus light onto an information storage medium in an optical information storage media system. 
         [0006]    The most important factor in determining the storage capacity of an optical information storage medium, e.g., an optical disc, is the size of a light spot. When the size of a light spot is smaller, the size of marks or pits that can be recorded/reproduced is also smaller. Thus, the storage density of an optical disc can be increased. 
         [0007]    To reduce the size of a light spot, a light source emitting a short wavelength light, such as blue laser, and an objective lens with a high numerical aperture (NA) are commonly used. For example, according to the Blue-ray disc (BD) standard, a blue light with an wavelength of approximately 405 nm and an objective lens with a NA of 0.85 are employed. 
         [0008]    However, as the wavelength of light becomes shorter and the NA value of an objective lens becomes higher, an optical system employing the same becomes more vulnerable to various aberrations. 
         [0009]    Therefore, when an aspheric single objective lens with a high NA is used for high density information storage media, such aberrations are compensated for by aspheric lens surfaces. 
         [0010]    An aspheric surface equation generally used for designing an aspheric lens is as follows. 
         [0000]    
       
         
           
             
               
                 
                   
                     Z 
                      
                     
                       ( 
                       r 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         cr 
                         2 
                       
                       
                         1 
                         + 
                         
                           
                             1 
                             - 
                             
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   K 
                                 
                                 ) 
                               
                                
                               
                                 c 
                                 2 
                               
                                
                               
                                 r 
                                 2 
                               
                             
                           
                         
                       
                     
                     + 
                     
                       Ar 
                       4 
                     
                     + 
                     
                       Br 
                       6 
                     
                     + 
                     
                       Cr 
                       8 
                     
                     + 
                     
                       Dr 
                       10 
                     
                     + 
                     
                       Er 
                       12 
                     
                     + 
                     
                       Fr 
                       14 
                     
                     + 
                     
                       Gr 
                       16 
                     
                     + 
                     
                       Hr 
                       18 
                     
                     + 
                     
                       Jr 
                       20 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
         [0011]    In Equation 1, 
         [0000]    
       
         
           
             
               cr 
               2 
             
             
               1 
               + 
               
                 
                   1 
                   - 
                   
                     
                       ( 
                       
                         1 
                         + 
                         K 
                       
                       ) 
                     
                      
                     
                       c 
                       2 
                     
                      
                     
                       r 
                       2 
                     
                   
                 
               
             
           
         
       
     
         [0000]    indicates quadric surfaces such as spherical surfaces or elliptical surfaces, and the high degree (higher-order) terms from r 4  to r 20  are applied for correcting aberrations. As shown in Equation 1, the generally high terms from r 4  to r 20  are used in the case of an aspheric lens. 
         [0012]    In case of an objective lens for BD, the NA is as high as 0.85. Thus, it is necessary to correct aberrations also in the periphery of the lens, which explains the use of high degree terms when designing the shape of an aspheric lens. 
         [0013]    However, when high degree terms are used for designing a lens, variation of an aspheric surface becomes larger. Although this large variation significantly contributes to correct aberrations, the angle of inclination on the surface of the lens also becomes larger. In this case, the permitted axial deviation of the surfaces of the lens becomes smaller. 
       SUMMARY 
       [0014]    The present general inventive concept provides an aspheric lens, which has gently sloped lens surfaces and allows increases in permitted axial deviation, and an optical pickup apparatus employing the aspheric lens as an objective lens. 
         [0015]    Additional aspects and utilities of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept. 
         [0016]    The foregoing and/or other aspects and utilities of the present general inventive concept may be achieved by providing an aspheric lens including at least one aspheric lens surface, wherein an angle of inclination on the aspheric lens surface may be smaller than or equal to 65°. 
         [0017]    The aspheric lens surface may be given by the equation 
         [0000]    
       
         
           
             
               Z 
                
               
                 ( 
                 r 
                 ) 
               
             
             = 
             
               
                 
                   cr 
                   2 
                 
                 
                   1 
                   + 
                   
                     
                       1 
                       - 
                       
                         
                           ( 
                           
                             1 
                             + 
                             K 
                           
                           ) 
                         
                          
                         
                           c 
                           2 
                         
                          
                         
                           r 
                           2 
                         
                       
                     
                   
                 
               
               + 
               
                 Lr 
                 2 
               
               + 
               
                 Ar 
                 4 
               
               + 
               
                 Br 
                 6 
               
               + 
               
                 Cr 
                 8 
               
               + 
               
                 Dr 
                 10 
               
               + 
               
                 Er 
                 12 
               
               + 
               
                 Fr 
                 14 
               
               + 
               
                 Gr 
                 16 
               
               + 
               
                 Hr 
                 18 
               
               + 
               
                 Jr 
                 20 
               
             
           
         
       
     
         [0000]    where L≠0, c is a curvature, r 2 =x 2 +y 2 , K is a conic integer, and L and A through J are aspheric coefficients. 
         [0018]    The aspheric lens may be formed of a plastic material. 
         [0019]    The aspheric lens may be used as an objective lens of an optical pickup for BD. 
         [0020]    The NA (numerical aperture) of the aspheric lens may be 0.85. 
         [0021]    The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing an aspheric lens usable as an objective lens of an optical information storage media system, the aspheric lens including at least one aspheric lens surface which can be determined by equation having a first term corresponding to an elliptical surface characteristic; a second term corresponding to a parabolic surface characteristic: and higher-order terms corresponding to slope characteristics of the aspheric lens surface. 
         [0022]    An angle of inclination on the aspheric lens surface formed the above terms of the equation may be smaller than or equal to 65°. 
         [0023]    The first term may include 
         [0000]    
       
         
           
             
               
                 cr 
                 2 
               
               
                 1 
                 + 
                 
                   
                     1 
                     - 
                     
                       
                         ( 
                         
                           1 
                           + 
                           K 
                         
                         ) 
                       
                        
                       
                         c 
                         2 
                       
                        
                       
                         r 
                         2 
                       
                     
                   
                 
               
             
             , 
           
         
       
     
         [0000]    and the second term may include Lr 2 , wherein L≠0, c is a curvature, r 2 =x 2 +y 2 , and K is a conic integer. 
         [0024]    The second term may include Lr 2 ; and the higher-order terms may include Ar 4 +Br 6 +Cr 8 +Dr 10 +Er 12 +Fr 14 +Gr 16 +Hr 18 +Jr 20 , wherein L≠0, c is a curvature, r 2 =x 2 +y 2 , and L and A through J are aspheric coefficients. 
         [0025]    The first term may include 
         [0000]    
       
         
           
             
               
                 cr 
                 2 
               
               
                 1 
                 + 
                 
                   
                     1 
                     - 
                     
                       
                         ( 
                         
                           1 
                           + 
                           K 
                         
                         ) 
                       
                        
                       
                         c 
                         2 
                       
                        
                       
                         r 
                         2 
                       
                     
                   
                 
               
             
             , 
           
         
       
     
         [0000]    the second term may include Lr 2 , and the higher-order terms may include Ar 4 +Br 6 +Cr 8 +Dr 10 +Er 12 +Fr 14 +Gr 16 +H 18 +Jr 20 , wherein L≠0, c is a curvature, r 2 =x 2 +y 2 , K is a conic integer, and L and A through J are aspheric coefficients. 
         [0026]    An NA (numerical aperture) of the aspheric lens formed according to the above terms of the equation may be 0.85. 
         [0027]    The foregoing and/or other aspects and utilities of the present general inventive concept may be achieved by providing an optical pickup apparatus including a light source to emit light, and an objective lens to transmit the light and receive the light reflected from an disc, wherein the objective lens includes an aspheric lens having an aspheric lens surface which can be determined by equation having a first term corresponding to an elliptical surface characteristic, a second term corresponding to a parabolic surface characteristic, and higher-order terms corresponding to slope characteristics of the aspheric lens surface. 
         [0028]    The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing an optical information storage media system including an optical pickup apparatus having a light source to emit light, and an objective lens to transmit the light and receive the light reflected from an disc, wherein the objective lens includes an aspheric lens having an aspheric lens surface which can be determined by equation having a first term corresponding to elliptical surface, a second term corresponding to a parabolic surface, and higher-order terms corresponding to slopes of the aspheric lens surface, and a control unit to control the optical pickup apparatus to write and read information using the light. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    The above and other features and advantages of the present general inventive concept will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
           [0030]      FIG. 1  is a diagram illustrating an optical pickup apparatus according to an embodiment of the present general inventive concept; 
           [0031]      FIG. 2  is a diagram illustrating an optical information storage media system employing the optical pickup apparatus of  FIG. 1 ; and 
           [0032]      FIGS. 3A and 3B  are diagrams illustrating an aspheric lens of the optical pickup apparatus of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0033]    Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept by referring to the figures 
         [0034]      FIG. 1  illustrates an optical configuration of an optical pickup apparatus  50  according to an embodiment of the present general inventive concept.  FIG. 2  illustrates an optical information storage media system employing the optical pickup apparatus  50  of  FIG. 1 . 
         [0035]    Referring to  FIG. 1 , the optical pickup apparatus  50  includes a light source  11  (e.g., a laser diode) to emit light, an objective lens  17  to collect incident light and focus the collected light onto an information storage medium, that is, an optical disc  10 , and a photo detector  19  which receives light that is irradiated onto the optical disc  10  and reflected thereby. The objective lens  17  is an aspheric lens. Furthermore, the optical pickup apparatus  50  may includes a light path changer  15  (e.g., a polarizing beam splitter) to alternate or guide the travelling path of incident light, a wave plate  13  (e.g., a ¼ wave plate) to change the polarization of the incident light, and a collimating lens  12  to produce parallel light incident onto the objective lens  17 . In  FIG. 1 , reference numeral  18  refers to a detecting lens which collects incident light and forms a suitable size light spot on the photo detector  19  using the collected light. The detecting lens  18  may be an astigmatism lens to detect focus error signals according to astigmatism methods. 
         [0036]    The optical disc  10  may be an optical disc having a single information storage layer or a plurality of information storage layers on one side. The optical disc  10  may be a Blue-ray disc (BD). 
         [0037]    The light source  11  outputs light having a wavelength appropriate for recording/reproducing information to/from the optical disc  10 . For example, the light source  11  may output blue light according to the requirements of the BD standard, that is, blue light with a wavelength of 405 nm or close to 405 nm. 
         [0038]    The objective lens  17  is driven in a focusing direction and/or a tracking direction by an actuator  16 . The objective lens  17  may be formed to have an effective numerical aperture (NA) of 0.85 as required by the BD standard. Furthermore, to ensure compatibility with the standard for BD and HD DVD, that is, compatibility between the standards of the BD DVD and the HD DVD, the object lens  17  may also have an effective NA of 0.65 as required by HD DVD. The actuator  16  may drive the objective lens  17  not only in the focusing direction, but also in the tracking direction. Furthermore, the actuator  16  may also drive the objective lens  17  in the tilting direction. 
         [0039]    According to an embodiment of the present general inventive concept, the aspheric lens can be used as the objective lens  17  and may have at least one aspheric lens surface on which an angle of inclination is smaller than or equal to 65°. The objective lens  17  may be formed of a plastic material or a glass material. 
         [0040]    As illustrated in  FIGS. 3A and 3B , the aspheric lens  17  may have an aspheric lens surface Sb. A reference R is a radius at the vertex on an optical axis, and reference r is a radius (or distance) to a point of the aspheric lens surface Sb from a reference surface at the optical axis. Reference h is a distance from the optical axis and proportional to the radius r. The above-described references are used to determine the aspheric lens  17  which will be described later. 
         [0041]    Reference A indicates the angle of inclination and can be determined by the optical axis and a line (surface or plane) St that is tangential to a point of the aspheric lens surface Sb. 
         [0042]      FIG. 1  illustrates an example of optical configurations of the optical pickup apparatus  50 . The optical pickup apparatus  50  of  FIG. 1  is a separated type optical system having the light source  11  and the photo detector  19  that are separated from each other, which includes the single light source  11  and the single photo detector  19 . The light source  11  may be output light of a single wavelength. Alternatively, the light source  11  may be a multi-type light source which outputs lights of a plurality of wavelengths to ensure compatibility with optical discs of a plurality of formats; e.g., BD and/or HD DVD and conventional DVD. The optical pickup apparatus  50  may further include a hologram optical module (not illustrated) to ensure compatibility with optical discs of a plurality of formats for which lights of different wavelengths are used. Furthermore, the optical configuration of the optical pickup apparatus  50  may be modified variously from that illustrated in  FIG. 1 . 
         [0043]    Referring to  FIG. 2 , an optical information storage media system may be an apparatus having a spindle motor  312  to rotate the optical disc  10 , the optical pickup  50  which is installed to move across the optical disc  10  to record/reproduce information to/from the optical disc  10 , a signal processing unit  100  to detect focus error signals from signals detected by the optical pickup apparatus  50 , a driving unit  307  to drive the spindle motor  312  and the optical pickup apparatus  50 , and a control unit  309  to control focusing servo and tracking servo of the optical pickup apparatus  50 . Reference numeral  352  denotes a turntable, and reference number  353  denotes a clamp to chuck the optical disc  10 . 
         [0044]    Light reflected by the optical disc  10  is detected by the photo detector  19  disposed in the optical pickup apparatus  50  and is converted to electric signals via photoelectric conversion. The signal processing unit  100  receives the electric signals and generates focus error signals (FES). The FES is input to the control unit  309  via the driving unit  307 . The signal processing unit  100  may detect tracking error signals and/or tilting signals based on electric signals from the photo detector  19 . 
         [0045]    The driving unit  307  controls a speed at which the spindle motor  312  rotates, amplifies input signals, and drives the optical pickup apparatus  50 . The control unit  309  sends a focus servo command, a tracking servo command, and/or a tilting servo command, which are adjusted based on signals input by the driving unit  307 , back to the driving unit  307  for the optical pickup  50  to perform focus, track, and/or tilt operations. Furthermore, the control unit  309  controls the light source  11  to output light of an appropriate power according to reproducing/recording modes, and controls the driving unit  307  to apply a signal for defocus driving the objective lens  17  to the actuator  16 , such that a focus offset is applied on the objective lens  17  prior to changing a current mode to the recording mode. At this point, the applied amount of the focus offset is adjusted according to the power of output light. After applying the focus offset for a predetermined time period, the focus offset may be removed. 
         [0046]    An aspheric lens according to an embodiment of the present general inventive concept, which may be applied as the objective lens  17 , includes at least one aspheric lens surface Sb. Thus, both surfaces of the aspheric lens may be aspheric lens surfaces Sb. 
         [0047]    In the aspheric lens according to an embodiment of the present general inventive concept, an aspheric surface equation given by Equation 2 below may be used when designing an aspheric lens surface. 
         [0000]    
       
         
           
             
               
                 
                   
                     Z 
                      
                     
                       ( 
                       r 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         cr 
                         2 
                       
                       
                         1 
                         + 
                         
                           
                             1 
                             - 
                             
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   K 
                                 
                                 ) 
                               
                                
                               
                                 c 
                                 2 
                               
                                
                               
                                 r 
                                 2 
                               
                             
                           
                         
                       
                     
                     + 
                     
                       Lr 
                       2 
                     
                     + 
                     
                       Ar 
                       4 
                     
                     + 
                     
                       Br 
                       6 
                     
                     + 
                     
                       Cr 
                       8 
                     
                     + 
                     
                       Dr 
                       10 
                     
                     + 
                     
                       Er 
                       12 
                     
                     + 
                     
                       Fr 
                       14 
                     
                     + 
                     
                       Gr 
                       16 
                     
                     + 
                     
                       Hr 
                       18 
                     
                     + 
                     
                       Jr 
                       20 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
             
             
               
                 
                   L 
                   ≠ 
                   0 
                 
               
               
                 
                     
                 
               
             
           
         
       
     
         [0000]    where c is a curvature, r 2 =x 2 +y 2 , K is a conic constant, and L and A through J are aspheric coefficients. 
         [0048]    Here, x and y are a radius or distance from the optical axis and used to form the radius r at the point of the aspheric lens surface Sb with respect to the reference surface. 
         [0049]    In other words, the aspheric lens  17  according to an embodiment of the present general inventive concept has an aspheric correction characteristic and can correct a curved surface of an aspheric lens surface, e.g., an elliptical surface, which is obtained according to the term of 
         [0000]    
       
         
           
             
               
                 cr 
                 2 
               
               
                 1 
                 + 
                 
                   
                     1 
                     - 
                     
                       
                         ( 
                         
                           1 
                           + 
                           K 
                         
                         ) 
                       
                        
                       
                         c 
                         2 
                       
                        
                       
                         r 
                         2 
                       
                     
                   
                 
               
             
             , 
           
         
       
     
         [0000]    via a second term (Lr 2 ) indicating a parabolic surface. 
         [0050]    Accordingly, when an aspheric correction is performed by using the second term indicating a parabolic surface (referred hereinafter as the second term), aberration can be corrected better than the case of, for example, an elliptical surface only. Therefore, loads in higher terms, that is, loads in the higher-order terms from term r4 to term r20 are relieved, and thus it also becomes possible to make gentle slope on a surface of the objective lens  17 . 
         [0051]    The aspheric lens according to an embodiment of the present general inventive concept may have an angle of inclination smaller than or equal to 65°, and may be formed of a plastic material. Alternatively, the aspheric lens may be formed of a glass material. As described above, the aspheric lens may be used as an objective lens for an optical pickup apparatus for BD. In this case, the NA of the aspheric lens may be 0.85. 
         [0052]    Table  1  illustrated an example of designing the objective lens  17  according to an embodiment of the present general inventive concept. Data in Table 1 were obtained when the NA of the aspheric lens is 0.85, the effective diameter of the aspheric lens  17  is 3.4 mm, a center thickness of the aspheric lens corresponding to the optical axis is 1.76 mm, the refractive index of a material constituting the aspheric lens is 1.98, the focal length of the aspheric lens is 1.98 mm, and the working distance of the aspheric lens is 0.73 mm. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 First Aspheric Lens Surface 
                 Second Aspheric Lens Surface 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 R (=1/c) 
                 1.6 
                 4.136693 
               
               
                 K 
                 −0.8379427 
                 −20.26923 
               
               
                 L 
                 1.44098E−02 
                 −5.45116E−02 
               
               
                 A 
                 1.80243E−02 
                 8.66518E−02 
               
               
                 B 
                 2.86512E−03 
                 −9.40997E−02 
               
               
                 C 
                 1.91222E−05 
                 5.44710E−02 
               
               
                 D 
                 2.12686E−04 
                 −1.67841E−02 
               
               
                 E 
                 −0.922324E−05 
                 2.12539E−03 
               
               
                 F 
                 0 
                 0 
               
               
                 G 
                 0 
                 0 
               
               
                 H 
                 0 
                 0 
               
               
                 J 
                 0 
                 0 
               
               
                   
               
             
          
         
       
     
         [0053]    As described above, when an aspheric lens surface is corrected via the second term, the angle of inclination on the lens surface of the aspheric lens can be gentle. As a result, the permitted axial deviation can be increased, and thus a manufacturing tolerance can be increased. 
         [0054]    Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the claims and their equivalents.