Abstract:
An optical pickup including: first and second light sources for generating first and second laser beams having different wavelengths corresponding to first and second media having different thicknesses, respectively; an objective lens that has diverging and converging/focusing portions, to focus the first and second laser beams from the first and second light sources onto the first and second media, respectively; a photodetector to receive the first and second laser beams that are emitted from the first and second light sources and reflected, respectively, by the first and second media; and a collimating lens arranged on the optical path between the objective lens, and the first and second light sources. The optical pickup is compatible with both existing digital versatile disks (DVDs), and high-definition (HD)-DVDs, which need a blue light source near 405 nm and an objective lens that has a numerical aperture of 0.6 or more.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Application No. 00-1782, filed Jan. 14, 2000, 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 optical pickup for high-density information writing and reading systems, and more particularly, to an optical pickup capable of reducing chromatic aberration that occurs when a blue light source is employed. 
     2. Description of the Related Art 
     In optical writing and reading systems, the recording density is determined by the size of the focused spot. In general, the size of the focused spot (S) is proportional to the wavelength (λ), and inversely proportional to the numerical aperture (NA), as expressed by formula (1): 
     
       
           S∝λ/NA   (1) 
       
     
     For a higher recording density than that for compact disks (CDs) or digital versatile disks (DVDs), the size of the spot being focused on an optical disk must be further reduced. To reduce the spot size, as can be inferred from formula (1), the wavelength (λ) of a laser beam must be reduced and the NA of an objective lens must be increased. Thus, for high density information recording, a laser beam that has a short wavelength, such as a blue laser, must be employed as a light source and the NA of the objective lens must be maintained at 0.6 or more. 
     On the other hand, coma aberration (W 31 ), which occurs due to a tilting of the optical disk, is associated with the tilt angle (θ) of the disk, the refractive index (n) of the disk substrate, the thickness (d) of the disk substrate, and the NA of the objective lens, as expressed by formula (2):                W   31     =         -     d   2       ·           n   2          (       n   2     -   1     )          sin                 θ                 cos                 θ         (       n   2     -       sin   2        θ       )       5   /   2                NA   3               (   2   )                                
     To ensure an allowable coma aberration W 31  with respect to the tilt of disk for high density recording, there is a tendency to reduce the thickness (d) of the disk substrate. For example, CDs have a thickness of 1.2 mm, and DVDs have a thickness of 0.6 mm. Also, it is likely that the thickness of high-definition DVDs (HD-DVDs), which are recently being developed, will be 0.6 mm or less. In using such thin optical disks for high density recording, the first consideration is the compatibility with existing disks including CDs and DVDs. However, for DVD-recordable (DVD-R) and multi-layered DVDs, it is necessary to use a red light source because of the low reflectivity of these disks with respect to short wavelength light. Therefore, a red light source is used for these disks. With regard to the objective lens, by applying a technique capable of correcting both chromatic aberration caused by the different wavelengths of the light sources, and spherical aberration caused by the difference in thickness of the disks, it is possible to use a common objective lens for both existing disks and HD-DVDs. 
     For an optical pickup for 0.6 mm-thick DVDs, which uses a red light source and an objective lens that has an NA of 0.6, in order to be compatible with 1.2 mm-thick CDs, an additional light source that has a wavelength of 650 nm and an objective lens are employed in the optical pickup using various techniques. These techniques include an annular shielding technique for blocking light passing through the intermediate area between a far axis and near axis areas, a method of controlling the NA of the objective lens by using liquid crystal (LC) shutters, and a method of splitting light using a hologram optical member to form individual focuses onto two disks that have different thicknesses. However, for compact disk recordables (CD-Rs), reflectivity with respect to red light sharply drops, and thus a light source that has a wavelength of 780 nm is necessary. For this reason, use of a DVD indefinite/CD definite optical system providing compatibility between light beams of 780 nm and 650 nm, or use of an annular objective lens that has an annular focus region between the near axis and far axis regions has been suggested. In particular, for a CD definite optical system, the NA of the objective lens is limited and divergent rays are incident on the objective lens, thereby correcting an aberration caused by thickness variations of disks and the objective lens. 
     As previously described, an optical pickup using a short wavelength light source is required for higher density information writing and reading than DVD systems are capable of. As an example, for an optical pickup for HD-DVDs, a laser that has a wavelength shorter than 650 nm (as used for DVDs) is required as a light source. In addition, refractivity of optical material for an optical disk varies sharply at wavelengths shorter than 650 nm, thereby causing excessive aberration. Thus, there is a need for an optical system capable of both effectively reducing the chromatic aberration, and being compatible with existing DVDs. 
     For a DVD-R, the reflectivity with respect to light sources other than a red light source decreases. Thus, for the compatibility with DVD-Rs, a light source that has a wavelength of 650 nm must be used. However, the aberration problem cannot be eliminated from a 400 nm-objective lens just by controlling the degree of divergence of the incident light, which is emitted from the 650 nm-light source and is incident on the objective lens. Thus, a major concern in searching HD-DVDs is an effective chromatic aberration correction technique. 
     According to a conventional method for correcting aberration, as shown in FIG. 1, an aberration correcting lens  3  is interposed between an optical disk  1  and an objective lens  3 . Alternatively, as shown in FIG. 2, a wavelength selecting member  4 , which defines the NA of the objective lens  2  depending on the wavelength of light, is located between the objective lens  2  and the light source (not shown). For the optical pickup shown in FIG. 1, the spherical aberration is corrected by adjusting the distance between the two lenses according to the thickness variation of the optical disk  1 . For the optical pickup shown in FIG. 2, the NA of the objective lens  2  with respect to light that has a longer wavelength is limited by the wavelength selecting member  4 , and its optical elements are arranged such that the light from light sources is divergently incident on the objective lens  2 , thereby correcting spherical aberration due to the thickness variation of optical disks. 
     However, the conventional optical pickup shown in FIG. 1 needs to precisely control the distance between the objective lens  2  and the aberration correcting lens  3 . Thus, an additional actuator is required to accurately control the distance between the two lenses  2  and  3 , and between lens  3  and the disk. This solution complicates the manufacture of the optical pickup, and thus increases the cost. In addition, for the conventional optical pickup shown in FIG. 2, because a divergent light is used, axial distortion of the objective lens with respect to the disks causes distortion of a tracking error. 
     SUMMARY OF THE INVENTION 
     To solve the above problems, it is an objective of the present invention to provide an optical pickup that uses laser beams each that have different wavelengths, and is compatible with different types of optical recording media. 
     It is another objective of the present invention to provide an optical pickup that is capable of being assembled easily with a simple structure, and is compatible with different types of optical recording media. 
     It is still another objective of the present invention to provide an optical pickup that is capable of being manufactured at low cost, and is compatible with different types of optical recording media. 
     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. 
     The objectives of the present invention are achieved by an optical pickup comprising: first and second light sources to generate laser beams, which have different wavelengths, corresponding to first and second media that have different thicknesses, respectively; an objective lens, which has diverging and focusing portions, to focus the laser beams from the first and second light sources onto the first and second media, respectively; at least one photodetector to receive the laser beams that are emitted from the first and second light sources and reflected, respectively, by the first and second media; and a collimating lens arranged on the optical path between the objective lens, and the first and second light sources. 
     Preferably, assuming that the front focal lengths of the diverging and condensing portions of the objective lens are f 1  and f 2 , respectively, and the Abbe&#39;s numbers on the d-line of the optical materials for the diverging and condensing portions of the objective lens are v 1  and v 2 , respectively, the objective lens satisfies the relationship 0&lt;1/(f 1 ·v 1 )+1/(f 2 ·v 2 )&lt;0.005. 
     Preferably, assuming that the focal length of the entire objective lens is f and the focal length of the diverging portion is fn, the objective lens satisfies the relationship −0.8&lt;f/fn&lt;0. 
     Preferably, one surface of the objective lens is an aspheric plane, and the photodetector receives the laser beams originating from both the first and second light sources. 
     Preferably, the first light source generates a blue laser beam, and more preferably, has a wavelength of 405 nm, and the second light source generates a red laser beam, and more preferably, has a wavelength of 650 nm. Preferably, the substrate of the first medium has a thickness less than 0.6 mm and the substrate of the second medium has a thickness of 0.6 mm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objectives and advantages of the present invention will become more apparent by describing, in detail, the embodiments thereof with reference to the attached drawings, in which: 
     FIG. 1 illustrates the objective lens of a conventional optical pickup; 
     FIG. 2 illustrates the objective lens of another conventional optical pickup; 
     FIG. 3 shows the optical arrangement of a first embodiment of an optical pickup according to the present invention; 
     FIG. 4 shows the optical arrangement of a second embodiment of an optical pickup according to the present invention; 
     FIG. 5 is a graph comparing the variation of optimal thickness of a disk substrate for the inventive optical pickup and a conventional optical pickup, with respect to the variation of light wavelength; 
     FIG. 6 is a graph comparing the variation of aberration for the inventive optical pickup and a conventional optical pickup, with respect to the variation of light wavelength; 
     FIG. 7A illustrates the optical path of light that has a wavelength of 405 nm in the optical pickup according to the present invention; 
     FIG. 7B illustrates the aberration with respect to light that has a wavelength of 405 nm in the optical pickup according to the present invention; 
     FIG. 7C illustrates the focus error signal for light that has a wavelength of 405 nm in the optical pickup according to the present invention; 
     FIG. 8A illustrates the optical path of light that has a wavelength of 650 nm in the optical pickup according to the present invention; 
     FIG. 8B illustrates the aberration with respect to light that has a wavelength of 650 nm in the optical pickup according to the present invention; and 
     FIG. 8C illustrates the focus error signal for light that has a wavelength of 650 nm in the optical pickup according to the present invention. 
    
    
     DETAILED 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. 
     A first embodiment of an optical pickup according to the present invention is shown in FIG. 3. A first light source  601  is positioned at the end of the optical axis of an objective lens  200  that faces a medium (optical disk)  100 . A collimating lens  400 , and first, second, and third beam splitters  501 ,  502  and  503  are sequentially arranged on the optical path between the objective lens  200  and the first light source  601 . The collimating lens  400  includes a positive lens  401  with condensing power and a negative lens  402  with diverging power. 
     The third beam splitter  503  transmits the laser beam from the first light source  601  and reflects the laser beam from a second light source  701  toward the medium  100 . As shown in FIG. 3, the second light source  701  is arranged on the optical path of the light reflected from the third beam splitter  503 . 
     The second beam splitter  502  transmits the laser beams from the first and second light sources  601  and  701 , and reflects the light reflected from the medium  100  that originated from the second light source  701 . The light reflected by the medium  100  and in turn by the second beam splitter  502  is condensed by a second condensing lens  802  onto a second photodetector  702 . 
     The first beam splitter  501  transmits all of the laser beams from the first and second light sources  601  and  701 . Also, the first beam splitter  501  reflects the light reflected by the medium  100 , which is originated from the first light source  601 , and transmits the light reflected by the medium  100  which originated from the second light source  701 . The light reflected by the medium  100  and in turn by the first beam splitter  501  is condensed by a first condensing lens  801  onto a first photodetector  602 . 
     The collimating lens  400  collimates the laser beams from the first and second light sources  601  and  701 , and the collimated laser beams are incident on the medium  100  through the objective lens  200 . 
     The objective lens  200  is a doublet lens including a positive portion  202  with a condensing/focusing power, and a negative portion  201  with a diverging power. Assuming that the focal length of the objective lens  200  is f and the focal length of the negative portion  201  is fn, the objective lens  200  satisfies the relationship −0.8&lt;f/fn&lt;0. 
     Also, assuming that the front focal lengths of the negative and positive portions  201  and  202  of the objective lens  200  are f 1  and f 2 , respectively, and the Abbe&#39;s numbers on the d-line of the optical materials for the negative and positive portions  201  and  202  are v 1  and v 2 , respectively, the objective lens  200  satisfies the relationship 0&lt;1/(f 1 ·v 1 )+1/(f 2 ·v 2 )&lt;0.005. In addition, one surface of the objective lens  200  is an aspheric plane. 
     A second embodiment of the optical pickup according to the present invention is shown in FIG. 4. A first light source  601  is positioned at the end of the optical axis of an objective lens  200  that faces a medium  100 . First and third beam splitters  501  and  503  are arranged between the objective lens  200  and the first light source  601 . The beam splitters  501  and  503  are separated by a predetermined distance from each other. A collimating lens  400 , including a positive lens  401  with a condensing power and a negative lens  402  with a diverging power, is arranged between the objective lens  200  and the first beam splitter  501 . 
     The collimating lens  400  collimates laser beams from the first and second light sources  601  and  701 , and the collimated laser beams are incident on the medium  100  through the objective lens  200 . 
     The third beam splitter  503  transmits the laser beam from the first light source  601  and reflects the laser beam from the second light source  701  toward the medium  100 . As shown in FIG. 4, the second light source  701  is arranged on the optical path of the light reflected by the third beam splitter  503 . 
     The first beam splitter  501  transmits the laser beams from the first and second light sources  601  and  701 , and reflects the light reflected by the medium  100 . The light reflected by the medium  100  and in turn by the first beam splitter  501  is condensed by a condensing lens  801  onto a photodetector  602 . 
     As in the first embodiment, the objective lens  200  is a doublet lens including a positive portion  202  with a condensing/focusing power, and a negative portion  201  with a diverging power. Assuming that the focal length of the objective lens  200  is f and the focal length of the negative portion  201  is fn, the objective lens  200  satisfies the relationship −0.8&lt;f/fn&lt;0. In addition, one surface of the objective lens  200  is an aspheric plane. 
     Also, assuming that the front focal lengths of the negative and positive portions  201  and  202  of the objective lens  200  are f 1  and f 2 , respectively, and the Abbe&#39;s numbers on the d-line of the optical materials for the negative and positive portions  201  and  202  are v 1  and v 2 , respectively, the objective lens  200  satisfies the relationship 0&lt;1/(f 1 ·v 1 )+1/(f 2 ·v 2 )&lt;0.005. 
     In the first and second embodiments discussed above, a wavelength selecting filter may also be placed between the objective lens  200  and the collimating lens  400 . The wavelength selecting filter is employed if there is a need for separately controlling the NA of the objective lens  200  with respect to the laser beams generated from the first and second light sources  601  and  701 . For example, if the first light source  601  emits a blue laser beam of 405 nm and needs the objective lens  200  to have an NA of 0.7, and if the second light source  701  emits a red laser beam of 650 nm and needs the objective lens  200  to have an NA of 0.6, the wavelength selecting filter could be used to allow a reduction of the NA of the objective lens  200  to 0.6 for the red laser beam of 650 nm, while transmitting all of the blue laser beam of 405 nm. 
     Table 1 shows the design data for an optical pickup, which uses a blue laser beam of 405 nm and includes a disk that has a 0.4 mm-thick substrate. 
     
       
         
               
               
               
               
             
               
               
               
               
             
               
               
             
               
               
               
               
             
               
             
               
               
             
               
             
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Curvature 
                   
                 Name of 
               
               
                 Surface 
                 Radius 
                 Thickness 
                 Glass 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Object Surface 
                 Infinity 
                 0.000000 
                   
               
               
                 s1 
                 Infinity 
                 0.250000 
                 BK7 
               
               
                 s2 
                 Infinity 
                 16.26320 
               
               
                 s3 
                 Infinity 
                 6.000000 
                 BK7 
               
               
                 s4 
                 Infinity 
                 3.000000 
               
               
                 s5 
                 99.276000 
                 1.000000 
                 FD4 
               
               
                 s6 
                 13.400000 
                 2.000000 
                 BAF10 
               
               
                 s7 
                 −17.230000 
                 5.000000 
               
               
                 s8 
                 2.249482 
                 2.000000 
                 BACD5 
               
             
          
           
               
                   
                 K: −1.099197 
               
               
                   
                 A: 0.540411E-0.2 B: −0.113820E-03 
               
               
                   
                 C: 0.266830E-04 D: −0.287691E-04 
               
               
                   
                 E: 0.502346E-05 F: −0.541172E-06 
               
             
          
           
               
                 s9 
                 −13.997487 
                 0.800000 
                 FEL1 
               
               
                 s10 
                 Infinity 
                 1.508124z1 
               
               
                 s11 
                 Infinity 
                 0.400000z2 
                 ‘CG’ 
               
               
                 s12 
                 Infinity 
                 0.000000 
               
               
                 Image Surface 
                 Infinity 
                 0.000000 
               
             
          
           
               
                 Equation of Aspheric Surface (See Formula(3)) 
               
             
          
           
               
                 Refractive Index/Abbe&#39;s Number on d-line, v 
                 BACD: 1.605256/61.3 
               
               
                   
                 FEL1: 1.568949/45.8 
               
               
                   
                 FD4: 1.805793/27.5 
               
               
                   
                 BAF10: 1.694714/47.2 
               
               
                   
                 BK7: 1.530196/64.2 
               
               
                   
                 ‘CG’: 1.621462/31.0 
               
               
                 Diameter of Entrance Pupil (mm) 
                 3.96 
               
               
                 Wavelength (nm) 
                 405 
               
               
                 Focal Distance of Collimating Lens at 
                 −19.325/11.149 
               
               
                 Negative/Positive Portions, fc1/fc2 (mm) 
               
               
                 Focal Distance of Combined Collimating Lens, 
                 24.994 
               
               
                 fc (mm) 
               
               
                 Σ1/(fl · vi) 
                 0.0002 
               
               
                 fc/fc1 
                 −1.293 
               
               
                 Focal Distance of Objective Lens at 
                 2.491/−7.009 
               
               
                 Negative/Positive Portions, fo1/fo2 (mm) 
               
               
                 Focal Distance of Combined Objective Lens, 
                 3.352 
               
               
                 fo (mm) 
               
               
                 Σ1/(fi · vi) 
                 0.0034 
               
               
                 fo/fo1 
                 −0.478 
               
               
                 Thickness at 650 nm 
                 z1: 1.451248 
               
               
                   
                 z2: 0.6000000 
               
               
                   
               
             
          
           
               
                 (3) 
               
             
          
           
               
                 
                   
                     
                       
                         z 
                         = 
                         
                           
                             
                               ch 
                               2 
                             
                             
                               1 
                               + 
                               
                                 
                                   1 
                                   - 
                                   
                                     
                                       ( 
                                       
                                         1 
                                         + 
                                         k 
                                       
                                       ) 
                                     
                                      
                                     
                                       c 
                                       2 
                                     
                                      
                                     
                                       h 
                                       2 
                                     
                                   
                                 
                               
                             
                           
                           + 
                           
                             Ah 
                             4 
                           
                           + 
                           
                             Bh 
                             6 
                           
                           + 
                           
                             Ch 
                             8 
                           
                           + 
                           
                             Dh 
                             10 
                           
                           + 
                           
                             Eh 
                             12 
                           
                           + 
                           
                             Fh 
                             14 
                           
                         
                       
                     
                             
                     
                         
                     
                   
                 
               
             
          
         
       
     
     where z is depth from the vertex of the surface, h is distance from the optical axis, c is curvature, K is a conic coefficient, and A, B, C and D are aspheric coefficients. 
     FIG. 5 is a graph comparing the change in the optimal disk substrate thickness according to the variation of wavelength for a conventional single objective lens  2  shown in FIG.  1  and the doublet objective lens  200  shown in FIGS. 3-4 where the objective lens  200  is designed based on the data of Table 1. For the conventional single objective lens  2 , the optimal thickness of the substrate tends to decrease with an increase in wavelength in order to correct the chromatic aberration. From FIG. 5, when a single objective lens  2  for a wavelength of 405 nm and a 0.4 mm-thick substrate is employed, the chromatic aberration at a wavelength of 650 nm can be eliminated at a substrate thickness of 0.23 mm. Thus, when reading data from a DVD which has a thickness of 0.6 mm, the chromatic aberration corresponding to the thickness variation (0.4−0.23=0.17 mm) and the spherical aberration corresponding to (0.6−0.4=0.2 mm) will occur. To correct this aberration, the thickness of a HD-DVD must be increased to 0.77 mm (0.6 mm+0.17 mm). However, this result goes against a tendency of reducing the thickness of disks for high density recording. 
     FIG. 6 compares the change in aberration according to the variation of wavelength for a conventional single objective lens  2  and the doublet objective lens  200  according to the present invention, which is designed to correct the chromatic aberration. FIG. 6 indicates that the doublet objective lens  200  can reduce the chromatic aberration by the variation of wavelength to a level of OPD 0.015λ rms  of less. 
     The degree of defocus on the medium according to the wavelength variation for writing and reading operations is inversely proportional to NA 2 , and the focal depth, ±λ/2NA 2 , is 0.563 μm. For the conventional objective lens  2 , the defocus with respect to a wavelength variation of ±1 nm is 0.7 μm, which is beyond the range of the focal depth. Meanwhile, the defocus with respect to a wavelength variation of ±1 nm is 0.45 μm for the doublet objective lens  200 , which is within the range of the focal depth. 
     FIG. 7A illustrates the optical path of the blue laser beam that has a wavelength of 405 nm incident on a disk  100  that has a 0.4 mm-thick substrate in an optical system designed with the specifications in Table 1. FIG. 7B shows the aberration occurring in the optical system of FIG. 7A, and FIG. 7C shows a focus error signal obtained by astigmatism in the optical system of FIG.  7 A. 
     FIG. 8A illustrates the optical path of the red laser beam that has a wavelength of 650 nm incident on a disk that has a 0.6 mm-thick substrate in the optical system designed with the data shown in Table 1. FIG. 8B shows the aberration occurring in the optical system of FIG. 8A, and FIG. 8C shows the focus error signal detected by astigmatism in the optical system of FIG.  8 A. As shown in FIG. 8C, an offset of about 0.5 μm or more occurs, which is possibly corrected using appropriate signal processing circuits. This result shows that one photodetector can be used for both types of disks as illustrated in the second embodiment. 
     Table 2 shows the design data for a conventional doublet collimating lens for light of 650 nm, which has a focal distance of 25 mm. As shown in Table 2, the total-to-negative diverging power ratio of the conventional collimating lens is close to −1. In view of the ratio of the collimating lens  400  according to the present invention, which is shown to be −1.29 in Table 1, the collimating lens  400  according to the present invention has considerably stronger diverging power. Meanwhile, the diverging power of the objective lens  200  is relative weak at 0.48. 
     
       
         
               
               
               
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Curvature 
                   
                   
               
               
                 Surface 
                 Radius 
                 Thickness 
                 Name of Glass 
               
               
                   
               
             
             
               
                 Object Surface 
                 Infinity 
                 Infinity 
                   
               
               
                 s1 
                   14.800000 
                    1.7 
                 BAF3 
               
               
                 s2 
                 −13.480000 
                    1.3 
                 SF11 
               
               
                 s3 
                 −48.995000 
                    3.5 
               
               
                 s4 
                 Infinity 
                    6.000000 
                 BK7 
               
               
                 s5 
                 Infinity 
                    16.025691 
               
               
                 s6 
                 Infinity 
                    0.000000 
               
               
                 Image Surface 
                 Infinity 
                    0.000000 
               
             
          
           
               
                 Refractive Index/Abbe&#39;s 
                 BAF3: 1.579229/46.5 
               
               
                 Number on d-line, v 
                 SF11: 1.776662/25.8 
               
               
                   
                 BK7: 1.514520/64.2 
               
               
                 Wavelength (nm) 
                   650 
               
               
                 Focal Distance of Collimating Lens 
                    12.454/−24.333 
               
               
                 at Negative/Positive Portions, 
               
               
                 fc1 and fc2 (mm) 
               
               
                 Focal Distance of Combined 
                    25.000 
               
               
                 Collimating Lens, fc (mm) 
               
               
                 Σ1/(fc · fi · vi) 
                    0.00013 
               
               
                 fc/fc2 
                  −1.027 
               
               
                   
               
             
          
         
       
     
     The optical pickup according to the present invention is compatible for both existing DVDs, and HD-DVDs, which employ a blue light source near 405 nm and an objective lens that has an NA of 0.6 (note that the specification thereof is not standardized yet). In particular, the optical pickup can achieve the dual-compatibility by using only one objective lens and one photodetector. Also, because the optical pickup according to the present invention has a simple structure, it can be easily manufactured at reduced cost. In addition, the defocusing according to wavelength variation during high-density writing and reading operations can be suppressed within the range of focal depth, which enables high quality signal reproduction. 
     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.