Abstract:
A high-density focusing objective lens having a high numerical aperture (NA), which can be easily manufactured by an existing technique, and an optical pickup using the objective lens for high-density recording. The objective lens includes a first transmitting portion placed at a relatively near-axis region from the optical axis to divergently transmit an incident light beam; a first reflecting portion to divergently reflect the incident light beam, the first reflecting portion faces the first transmitting portion; a second reflecting portion, formed at a relatively far-axis region around the first transmitting portion that focuses and reflects the light reflected from the first reflecting portion; and a second transmitting portion, formed at a relatively far-axis region around the first reflecting portion, that refracts and transmits the light beam focused by the second reflecting portion as a peripheral light beam, wherein the maximum angle α between the optical axis and the peripheral light beam satisfies the condition of 30° ≦α≦65°. The optical pickup further includes the previously mentioned objective lens as a second objective lens, which is optionally placed into the optical path between a first objective lens and an optical medium. As a result, the NA of the objective lens unit can be increased up to 0.85 while the working distance d 2  is maintained to be 0.2 mm.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Application No. 99-41767, filed Sep. 29, 1999, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an objective lens having a high numerical aperture (NA) for high-density optical focusing, and an optical pickup using the objective lens, and more particularly, to an objective lens for high-density optical focusing, that can be manufactured using existing techniques to have a NA high enough for high-density optical focusing, and an optical pickup using the objective lens for high-density recording. 
     2. Description of the Related Art 
     Assuming that a single objective lens is used in an optical system, the objective lens for use in recording data on and reproducing data from an optical disk has a maximum NA of 0.6 due to limitations caused by the manufacturing process. As a result, it is impossible to reduce an allowable error below an aberration of 0.07λ rms . Examples of a conventional objective lens and an optical pickup using the conventional objective lens are shown in FIGS. 1 and 2. 
     Referring to FIG. 1, the conventional optical pickup for recording and/or reproducing information is designed to enable a high-density recording of 20 gigabytes on an optical disk  1 . The optical pickup includes a light source  11  having a wavelength of 400 nm, a grating  19  for diffracting and transmitting the incident light beam, a first polarization beam splitter (PBS)  21  for altering the light path according to a polarization direction, a λ/4 plate  23  for guiding a circular polarized light beam to the optical disk  1 , an objective lens unit  50  having an NA of 0.85, a second PBS  27  for transmitting or reflecting the incident light beam reflected from the optical disk  1  and the first PBS  21 , a first photodetector  31  for receiving the light beam that has passed through the second PBS  27  and detecting an information signal from the incident light beam, and a second photodetector  37  for receiving the light beam reflected from the second PBS  27  and detecting an error signal therefrom. 
     A collimating lens  13 , a beam shaping prism  15 , and a λ/2 plate  17  are arranged on an optical path between the light source  11  and the grating  19 . The collimating lens  13  collimates the incident light beam, the beam shaping prism  15  shapes the incident light beam, and the λ/2 plate  17  delays the phase of the incident light beam. Another λ/2 plate  25 , which delays the phase of the incident light beam, is arranged on the optical path between the first PBS  21  and the second PBS  27 . A first condensing lens  29 , which condenses the incident parallel light beam, is arranged between the second PBS  27  and the first photodetector  31 . A second condensing lens  33 , which condenses the incident parallel light beam, and an astigmatism lens  35 , which causes astigmatism, are disposed between the second PBS  27  and the second photodetector  37 . A third condensing lens  39  condenses the light beam emitted from the light source  11  and reflected from the first PBS  21 , and a monitoring photodetector  41  monitors the optical power of the light source  11  from the light beam condensed by the third condensing lens  39 . 
     The objective lens unit  50  includes an objective lens  51  to focus the incident light beam and a semi-spherical lens  55 , which is arranged between the objective lens  51  and the optical disk  1 , to increase the NA of the objective lens unit  50 . 
     The objective lens unit  50  uses the semi-spherical lens  55  to further increase the NA of  25  the objective lens unit  50  beyond the NA of 0.6 of the objective lens  51 . Referring to FIG. 2, the NA of the semi-spherical lens  55  is proportional to the product of sin θ, wherein θ is the maximum incident angle of light onto the semi-spherical lens  55 , and the refractive index n of the semi-spherical lens  55 . Thus, the NA of the objective lens unit  50  can be increased up to 0.85 using a semi-spherical  1  lens  55  and an objective lens  51 . 
     In order to reduce the size of a light spot focused on the optical disk  1  with such a high NA in the conventional optical pickup as shown in FIG. 1, the working distance d 1  between the optical disk  1  and the semi-spherical lens  55  must be as small as 0.1 mm. However, such a small distance d 1  hinders the optical disk  1 , preventing it from stably seating on a turntable (not shown) and from rotating during operation. In addition, if the objective lens moves in the focusing direction within the range of ±0.7 mm, which is beyond the working distance d 1 , the focusing servo control must be precisely controlled within the range of 10 nm. Thus, it is difficult to manufacture the optical pickup on a mass production scale. 
     SUMMARY OF THE INVENTION 
     To solve the above problems, it is an objective of the present invention to provide an objective lens for high-density focusing that has a high numerical aperture (NA) and ensures a sufficient working distance with respect to a recording medium, and an optical pickup using the objective lens. 
     Additional objects and advantages of the invention 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 invention. 
     According to an aspect of the present invention, there is provided an objective lens comprising a first transmitting portion placed at a relatively near-axis region from the optical axis, that divergently transmits an incident light beam, a first reflecting portion that divergently reflects the incident light beam and faces the first transmitting portion, a second reflecting portion formed at a relatively far-axis region around the first transmitting portion that focuses and reflects the light beam reflected from the first reflecting portion, and a second transmitting portion formed at a relatively far-axis region around the first reflecting portion that refracts and transmits the light focused by the second reflecting portion as a peripheral light beam, wherein a maximum angle α between the optical axis and the peripheral light beam satisfies the condition of 30° ≦α≦65°. 
     According to another aspect of the present invention, there is provided an optical pickup comprising a light source for emitting a light beam, an optical path changing means for changing the traveling path of an incident light beam, an objective lens unit for focusing the incident light beam to form a light spot on the optical disk, and a photodetector for receiving the incident light beam reflected from the optical disk and passed through the objective lens and the optical path changing means. The objective lens unit comprises a first objective lens for focusing the incident light beam from the optical path changing means and a second objective lens arranged on the optical path between the first objective lens and the optical disk, to further focus the light beam condensed by the first objective lens, the second objective lens comprising a first transmitting portion formed at a region relatively near to the optical axis to divergently transmit the incident light beam, a first reflecting portion arranged facing the first transmitting portion, to divergently reflect the incident light beam, a second reflecting portion formed at a relatively far-axis region around the first transmitting portion, to focus the light beam reflected from the first reflecting portion toward the optical disk, and a second transmitting portion formed at a relatively far-axis region around the first reflecting portion, that diffracts and transmits the light beam condensed by the second reflecting portion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objectives and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
     FIG. 1 shows the optical arrangement of a conventional optical pickup using an objective lens unit having a semi-spherical lens for high-density focusing; 
     FIG. 2 shows the optical arrangement of the conventional objective lens unit shown in FIG. 1; 
     FIG. 3 shows the optical arrangement of an objective lens for high-density focusing according to an embodiment of the present invention, which is designed to accommodate a parallel incident light beam; 
     FIGS. 4 and 5 illustrate the optical arrangement of other embodiments of the objective lens for high-density focusing according to the present invention, which are designed to accommodate a condensing incident light beam; 
     FIG. 6 shows the optical arrangement of an optical pickup using the high-density focusing objective lens according to an embodiment of the present invention; 
     FIG. 7 shows the optical arrangement of the main portions in the optical pickup of FIG. 6 when a relatively thin optical disk is used; and 
     FIG. 8 shows the optical arrangement of the main portions in the optical pickup of FIG. 6 when a relatively thick optical disk is used. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now made in detail to the present preferred embodiments of the present invention, 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 invention by referring to the figures. 
     Referring to FIG. 3, an objective lens  155  for high-density focusing according to the present invention includes a first transmitting portion  156  that divergently transmits an incident light beam, a first reflecting portion  157  arranged facing the first transmitting portion  156 , to divergently reflect the incident light beam, a second reflecting portion  158  arranged around the first transmitting portion  156 , to condense the light beam reflected by the first reflecting portion  157 , and a second transmitting portion  159  to refract and transmit the light beam reflected from the second reflecting portion  158 . 
     The second transmitting portion  159  is positioned facing the optical disk  100 . The first and second reflecting portions  157  and  158 , and the first and second transmitting portions  156  and  159  are designed such that the working distance d 2  between the second transmitting portion  159  and an optical disk  100  is larger than the working distance d 1  (shown in FIG. 2) in a conventional optical pickup. 
     Preferably, the first transmitting portion  156 , which removes optical field aberration, has a concave curvature. Also, the first transmitting portion  156  is designed with spherical and aspherical surfaces to minimize aberration. The first reflecting portion  157  has a convex reflecting surface for reflecting the incident light beam at a maximum angle, which maintains a high NA of 0.6 or more. The reflecting surface of the first reflecting portion  157  is formed to be convex toward the first transmitting portion  156 . The second reflecting portion  158 , which has a concave reflecting surface for minimizing optical aberration such as spherical aberration and coma aberration, reflects the incident light beam reflected by the first reflecting portion  157  toward the second transmitting portion  159 . Conversely, when a light beam travels back through the objective lens  155  after having been reflected from the optical disk  100 , the second reflecting portion  158  reflects the incident light beam from the second transmitting portion  159  toward the first reflecting portion  157 . The second transmitting portion  159  has a planar surface, and thus can be easily processed. Preferably, a space enclosed by the first transmitting portion  156 , the second reflecting portion  158 , the first reflecting portion  157 , and the second transmitting portion  159  is filled with an optical material having a refractive index n different from that of air. This difference enables the light reflected from the second reflecting portion  156  to be focused through the second transmitting portion  159  toward an optical disc  100 . 
     FIG. 3 illustrates an example of the objective lens  155  for focusing a parallel incident light beam on the optical disk  100 . For the parallel incident light bean, it is preferable that the optical disk  100  has a thickness of 0.4 mm or less, but more preferably, of 0.1 mm, so as to overcome coma aberration and astigmatism that occur within an objective lens having a high NA. 
     Preferably, to form a light spot suitable for reproduction from the optical disk  100 , a diameter of the first transmitting portion  157  and an outer diameter of the second transmitting portion  159  satisfy condition (1) below. This condition shields the light beam transmitted through transmitting portion  159  from the light beam incident to the first reflecting portion  157 , so that the effect of a spherical aberration can be sharply reduced and the size of the light spot can be minimized.              0.1   &lt;       diameter                 of                 first                 reflecting                 portion       outer                 diameter                 of                 second                 reflecting                 portion       &lt;   0.5           (   1   )                                
     FIGS. 4 and 5 show the optical arrangement of objective lens  155  used with a condensing incident light beam according to an embodiment of the present invention. Each objective lens  155  shown in FIGS. 4 and 5 have the first and second transmitting portions  156  and  159 , and the first and second reflecting portions  157  and  158 , like the objective lens described with reference to FIG.  3 . However, the design data are different from that of the objective lens  155  shown in FIG.  3 . 
     Referring to FIG. 4, a predetermined condensing light beam entering the first transmitting portion  156  is divergently reflected from the first reflecting portion  157 , and focused by the second reflecting portion  157  to form a high-density light spot on the optical disk  100 . 
     Referring to FIG. 5, a condensing light beam enters the first transmitting portion  156  at a greater incident angle than that shown in FIG.  4 . The incident light beam condenses and spreads out again while passing through the first transmitting portion  156 , and then is divergently reflected by the first reflecting portion  157 . The diverging light beam is then reflected and focused by the second reflecting portion  157  to form a high-density light spot on the optical disk  100 . 
     To minimize the size of the light spot focused on the optical disk  100  with the increased working distance d 2 , it is preferable that, in the objective lens  155  shown in FIGS. 3-5, the maximum angle α between the optical axis and the peripheral light beam emitted from the second transmitting portion  159  after having passed through the first transmitting portion  156  and been reflected by the first and second reflecting portions  157  and  158  satisfies condition (2). 
     
       
         30°≦α≦65°  (2) 
       
     
     Two examples of the optical data for the objective lens  155  having the above configuration are shown in Tables 1 and 2. 
     Tables 1 and 2 show the design data of the objective lens  155  shown in FIG. 3 used with parallel incident light beam when the working distance d 2  is 1.1 mm and 0.2 mm, respectively. Table 3 shows the aspherical coefficients of the aspherical surfaces listed in Tables 1 and 2. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Radius 
                 Thickness 
                 Refractive 
                   
               
               
                   
                 (mm) 
                 (mm) 
                 Index 
                 Dispersion 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 First transmitting surface 
                 ∞ 
                  2.586105 
                 1.526 
                 50 
               
               
                   
                 (aspherical 
               
               
                   
                 surface 1) 
               
               
                 First reflecting surface 
                 0.38706 
                 −2.586105 
                 1.526 
                 50 
               
               
                 Second reflecting surface 
                 3.42277 
                  2.655523 
                 1.526 
                 50 
               
               
                   
                 (aspherical 
               
               
                   
                 surface 2) 
               
               
                 Second transmitting 
                 ∞ 
                  1.100000 
                 — 
                 — 
               
               
                 surface 
               
               
                 Disk 
                 ∞ 
                  0.100000 
                 1.583 
                 50 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Radius 
                 Thickness 
                 Refractive 
                   
               
               
                   
                 (mm) 
                 (mm) 
                 Index 
                 Dispersion 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 First transmitting surface 
                 ∞ 
                  0.574577 
                 1.526 
                 50 
               
               
                 First reflecting surface 
                 0.08600 
                 −0.574577 
                 1.526 
                 50 
               
               
                 Second reflecting surface 
                 0.76177 
                  0.590000 
                 1.526 
                 50 
               
               
                   
                 (aspherical 
               
               
                   
                 surface 3) 
               
               
                 Second transmitting 
                 ∞ 
                  0.198714 
                 — 
                 — 
               
               
                 surface 
               
               
                 Disk 
                 ∞ 
                  0.100000 
                 1.583 
                 50 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Aspherical 
                   
                   
                   
                   
                   
               
               
                 coefficient 
                 K 
                 A 
                 B 
                 C 
                 D 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Aspherical surface 1 
                  0.000000 
                  0.281823E+01 
                 −0.244324E+03 
                 0.757918E+04 
                 −0.962123E+05 
               
               
                 Aspherical surface 2 
                 −0.257566 
                  0.341730E−03 
                 −0.232088E−04 
                 0.735984E−05 
                 −0.176553E−05 
               
               
                 Aspherical surface 3 
                 −0.086297 
                 −0.782050E−02 
                 −0.147428+00  
                 0.646999E+00 
                 −0.347888E+01 
               
               
                   
               
             
          
         
       
     
     Using the objective lens  155  having the above configurations according to the present invention, the objective lens  155  maintains the working distance d 2  to be 0.2 mm and 1.1 mm, respectively, has a NA of 0.6 or more, and eliminates the problem of lens-to-disk interference. 
     The objective lens having a high NA according to the present invention is applicable to miniature optical systems for microscopes, exposure apparatuses for use in the manufacture of semiconductor devices, and mastering apparatuses for manufacturing disks. 
     FIG. 6 shows an embodiment of an optical pickup using the objective lens  155  that includes a light source  110  for emitting a light beam, an optical path changer for changing the traveling path of the incident light beam, an objective lens unit  150  for focusing the incident light beam to form a light spot on an optical disk  100 , and a photodetector  125  for receiving the light reflected from the optical disk  100  to detect information and error signals. The optical pickup shown in FIG. 6 is compatible with an optical disk  101  having a thickness of 0.4 mm, or a high recording density of about 20 gigabytes, a digital versatile disk (DVD)  103  having a thickness of 0.6 mm, and a compact disk (CD, not shown) having a thickness of 1.2 mm. 
     The light source  110  may be a semiconductor laser for emitting light of a short wavelength of about 400 nm. The optical path changer includes a polarization beam splitter (PBS)  115  for transmitting or reflecting the components of the incident light beam according to their polarization, a λ/4 plate  117  arranged on the optical path between the PBS  115  and the optical disk  100 , for delaying the phase of the incident light beam. A collimating lens  113  for collimating the incident light beam may further be arranged on the optical path between the light source  110  and the PBS  115 . 
     The optical lens unit  150  includes a first objective lens  151  having an NA of 0.6 suitable for the DVD  103 . A second objective lens  155  optionally placed on the optical path between the first objective lens  151  and the optical disk  100 , and a variable diaphragm  119  arranged on the optical path before the first objective lens  151 . 
     As shown in FIG. 7, when the high-density optical disk  101 , which is relatively thin is used, the objective lens unit  150  uses both the first and second objective lenses  151  and  155  to form a light spot on the high-density optical disk  101 . However, if a relatively thick DVD  103  is used, the second objective lens  155  is removed from the objective lens unit  150  so that the incident light beam is focused onto the DVD  103  using the first objective lens  151  alone. Here, placing the second objective lens  155  into and removing it from the optical path can be achieved using a rotary-type or solenoid-type driving motor. If the rotary type driving motor is used, the second objective lens  155  is mounted on a rotary plate rotated by the motor, and then the rotation of the rotary plate is controlled to place or remove the second objective lens  155  to or from the optical path. Slidable solenoid-type driving motor driving techniques are well known to those skilled in the art, and thus description thereof will be omitted. 
     The first objective lens  151  has a NA of 0.6 and is used to form a light spot on the DVD  103  having a thickness of 0.6 mm. Preferably, the first objective lens  151  has an annular shielding type configuration, such as those disclosed in U.S. Pat. Nos. 5,665,957; 5,822,135; 5,909,424; and 5,987,924, incorporated herein by reference. As for such an annular shielding type objective lens, the focal position is separately adjusted for a near-axis region and a far-axis region, so that the objective lens is compatible with a CD (not shown) having a thickness of 1.2 mm. 
     The second objective lens  155  has the same configuration as that of the objective lens  155  shown in FIGS. 3 through 5. Thus, the second objective lens  155  includes the first transmitting portion  156  for divergently transmitting the incident light beam, the first reflecting portion  156  arranged facing the first transmitting portion  156 , for divergently reflecting the incident light beam, the second reflecting portion  158  formed around the first transmitting portion  156 , for focusing the light beam reflected by the first reflecting portion  157 , and the second transmitting portion  159  for reflecting and transmitting the light beam reflected by the second reflecting portion  158  as a peripheral light beam. Preferably, the maximum angle a between the optical axis and the peripheral light beam incident on the optical disk  100  satisfies condition (2) above. The first and second transmitting portions  156  and  159 , and the first and second reflecting portions  157  and  158  have the same configuration and function as those of the objective lens  155  of FIGS. 3 through 5. 
     For the optical pickup shown in FIGS. 6-8, the incorporation of the second objective lens  155  into the optical path enables the objective lens unit  150  to both have a high NA of 0.85. In addition, the optical pickup has a lower NA of 0.6 using the first objective lens  151  alone. As a result, a light spot can be accurately formed with a high-density on the high-density optical disk  101 . Preferably, the high-density optical disk  101  has a thickness of 0.4 mm or less, more preferably, 0.1 mm or less, to account for the coma aberration and astigmatism. 
     For recording information on or reproducing information from the high-density optical disk  101 , the variable diaphragm  119 , which is a wavelength selective variable diaphragm, focuses the incident light beam through its narrow central region on the first transmitting portion  156 . Meanwhile, for recording information on or reproducing information from the DVD  103 , the incident light beam passes through a large portion of the variable diaphragm  119  to form a light spot on the DVD  103 . 
     The photodetector  125 , which receives the incident light beam reflected by the optical disk  100  and passed through the PBS  115 , is divided into a plurality of portions for independent photoelectric conversion. The configuration of such a photodetector  125  is well known to one skilled in the art, and thus the description thereof will be omitted. 
     A holographic optical element (HOE)  121 , which diffracts and transmits the incident light beam, splits the light beam into an error signal and an information signal. A condensing lens  123  condenses the light from the HOE  121 . The HOE  121  and the condensing lens  123  are disposed along the optical path between the PBS  115  and the photodetector  125 . 
     The objective lens having the above structure according to the present invention is advantageous in that the interference between the objective lens and the optical disk can be eliminated with an increased working distance d 2  of 0.2 mm or 1.1 mm at a high NA of 0.6 or more. The objective lens according to the present invention can be used as a lens for microscopes, exposure apparatuses for use in the manufacture of semiconductor devices, and mastering apparatuses for use in the manufacture of disks with a high NA, thereby minimizing the size of the optical system. 
     In the optical pickup according to the present invention, a second objective lens can be selectively disposed along the optical path while the working distance d 2  is maintained at 0.2 mm, so that the NA of the objective lens unit including the first objective lens of the optical pickup can be increased to 0.8. Also, the interference with the optical disk can be eliminated when information is recorded on or reproduced from a high-density optical disk having a thickness of 0.4 mm or less. Another advantage of the optical pickup is its compatibility with DVD or CD by selective placement or removal of the second objective lens into or away from the optical path. 
     While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made thereto without departing from the spirit and scope of the invention as defined by the appended claims.