Abstract:
An optical pickup apparatus and an optical recording/reproducing apparatus employing the optical pickup apparatus, which can be adaptive for multiple types of optical discs different in the layout conditions such as recording density, etc., are provided. The optical pickup includes an optical pickup for accessing at least first and second optical recording mediums, the pickup comprising a first light source generating a first light beam of a first wavelength; a second light source generating a second light beam of a second wavelength different from the first wavelength; an optical section transmitting the first and second light beams; and an aperture number controller including a wave selecting member for selecting between the first and second light beams of different wavelengths so as to impinge a light beam with an appropriate aperture number to selectively access one of the at least first and second optical recording mediums.

Description:
[0001]     This application is a Continuation of co-pending application Ser. No. 10/206,357, filed on Jul. 29, 2002, which is a Continuation of co-pending application Ser. No. 09/285,436 (U.S. Pat. No. 6,449,235B1, Issued Sep. 10, 2002), filed on Apr. 2, 1999, and for which priority is claimed under 35 U.S.C. § 120; and this application claims priority of Application Nos. P98-11972, P98-11973, and P98-11974 filed in Korea on Apr. 4, 1998, under 35 U.S.C. § 119. The entire contents of each of these applications are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to an optical recording/reproducing apparatus, and more particularly to an optical pickup apparatus, that is adaptable for various types of optical recording media having different layout conditions such as recording density. Also, the present invention is directed to an optical recording/reproducing apparatus using the optical pickup apparatus.  
         [0004]     2. Description of the Related Art  
         [0005]     Generally, an optical recording/reproducing apparatus for driving disc-type media, such as a compact disc (CD) (which are well known as recording media making use of a laser light beam), records or reproduces data by irradiating a laser beam onto the recording face of a disc while rotating the disc. To this end, the optical recording/reproducing apparatus includes an optical pickup for irradiating a laser beam generated from a light source, such as a semiconductor laser, onto the recording face of the optical disc using optical system devices such as an objective lens.  
         [0006]     Recently, a digital versatile disc (DVD) is now commercially available that is capable of storing a larger amount of information than the conventional CD. The DVD is usually designed for use with a light source having a different number of aperture and a different wavelength from the CD. In this case, the wavelength and the number of aperture of a light beam is related to the size of beam spot. The size of beam spot is selected from the standpoint of minimizing an effect caused by cross-talk between signal tracks on the recording face of the optical disc. Accordingly, since the DVD (with a larger recording density than the CD) has a small track pitch, the size of beam spot must also be smaller than that of the CD. In this case, a scheme making use of a shortened wavelength and an increased aperture number can be considered for reducing the size of beam spot. This approach may be viable since the size of a beam spot is directly proportional to the wavelength of the light beam while being inversely proportional to the number of aperture as seen from the following formula:  
             d   =     k   ⁢     λ   NA               (   1   )             
        wherein d represents the size of beam spot, k is a constant, λ is a wavelength of a light beam, and NA is the number of aperture of an objective lens. It can be seen from the formula (1) that a short wavelength and a large aperture number is used as to obtain a smaller size of beam spot when using a DVD as compared to the CD. For instance, an optical pickup for accessing the CD uses a light beam with a wavelength (λ) of 780 nm and an objective lens with the number of aperture of 0.45, whereas an optical pickup for accessing the DVD uses a light beam with a wavelength (λ) of 650 nm and an objective lens with the number of aperture of 0.6. Also, in the DVD, a light beam is sensitive to the thickness of the disc as the number of aperture of a light beam changes. The depth of the recording face, that is, the depth of the light transmission layer, is set to have a smaller value in a DVD than that of the CD. In other words, a noise component increases so that data cannot be recorded or reproduced due to an increase in the optical aberration when a light is transmitted by means of an objective lens with an aperture number of 0.6 through a light transmission layer with a thickness equal to that of a CD. Thus, the thickness of the light transmission layer in the DVD is set to have a smaller value than that in the CD. For instance, a light transmission layer of the CD has a thickness of 1.2 mm while a light transmission layer of the DVD has half the thickness thereof, that is, a thickness of 0.6 mm.        
 
         [0008]     An optical pickup for changeably accessing both a CD and DVD must include two light sources generating a different wavelengths of light beams and two objective lenses with a different number of aperture. Where an optical pickup is provided with two light sources and two objective lenses, problems occur in that the optical pickup size becomes large, its structure is complicated, and manufacturing costs increases. In order to solve these problems, an optical pickup has been used where it has a single light source and means for appropriately controlling the number of aperture of the objective lens depending on the corresponding disc to thereby access the CD and the DVD.  
         [0009]     For example, Japanese Patent Laid-open Gazette No. Pyung 9-185839 has disclosed an optical pickup that can access two types of optical discs of different thicknesses with light transmission layers by controlling the number of aperture of an objective lens employing a liquid crystal filter and a polarizing filter. The optical pickup controls the number of aperture of the objective lens into two modes by turning the liquid crystal filter on or off depending on whether or not a voltage is applied, to thereby selectively change the polarization characteristic of a light beam generated from a light source, and by allowing the polarizing filter to selectively shut out a portion of the light beam in accordance with the polarization characteristic of a light beam changed by means of the liquid filter.  
         [0010]     Also, Japanese Patent Laid-open Gazette No. Pyung 9-198704 has disclosed an optical pickup that is capable of accessing two types of optical discs by providing two objective lenses with a single lens supporting member in the twin-lens system to thereby switch a position of the objective lens in accordance with a rotation of the lens supporting member.  
         [0011]     A different approach involves blue lasers. Blue lasers generate a significantly lower wavelength of light beam as compared with conventional red laser beams. The blue laser is a light source that is expected to be commercially available in accordance with development of GaN system laser. It is reported that a wavelength band of the blue laser is approximately 400 nm. A next-generation optical disc employing such a blue laser, hereinafter referred to as “HD-DVD”, requires a light source with a different wavelength along with an objective lens with a corresponding number of aperture. Particularly, the HD-DVD requires a larger number of aperture because short wavelength of a blue light beam allows it to have a higher density such that its beam spot size must be smaller than the spot used with the DVD. More specifically, a wavelength (λ) of a light beam and the number of aperture NA applied to each of the HD-DVD, the DVD and the CD have a relationship in the following formula: 
 
λ1&lt;λ2&lt;λ3 
 
NA 1 &gt;NA 2 &gt;NA 3   (2) 
        wherein λ1, λ2 and λ3 are wavelengths of light beams corresponding sequentially to the blue laser disc, the DVD and the CD, and NA 1 , NA 2  and NA 3  are the number of aperture corresponding sequentially to the blue laser disc, the DVD and the CD. Accordingly, the size of beam spots irradiated onto the three types of optical discs has a relationship in the following formula (3) when the above formulas (1) and (2) are applied.  
               d1   &lt;   d2   &lt;   d3     ⁢     
     ⁢         λ   ⁢           ⁢   1     NA1     &lt;     λ   NA2     &lt;       λ   ⁢           ⁢   3     NA3               (   3   )             
    wherein d 1 , d 2  and d 3  represent the size of beam spots irradiated onto the HD-DVD, the DVD and the CD, respectively. As described above, the HD-DVD, the DVD and the CD have a different beam spot size due to a difference in the layout condition such as a recording density, etc. Accordingly, when it is intended to interchangeably access the three types of optical discs with a single optical recording/reproducing apparatus, three light sources and three objective lenses are required in the conventional method. However, when the optical pickup includes three light sources and three objective lenses, it has problems in that its structure becomes complicated and that the manufacturing costs increase. Accordingly, it is necessary to provide an optical pickup apparatus that is capable of accessing at least three types of optical discs with a different layout condition as well as having a minimum of constituent elements.        
 
       SUMMARY OF THE INVENTION  
       [0014]     Accordingly, it is an object of the present invention to provide an optical pickup apparatus that is capable of changeably accessing at least three types of optical discs in a different layout condition.  
         [0015]     A further object of the present invention is to provide an optical recording/reproducing apparatus employing the above-mentioned optical pickup apparatus.  
         [0016]     In order to achieve these and other objects of the invention, an optical pickup apparatus according to one aspect of the present invention includes first and second light sources for generating a different wavelength of light beams; an optical system allowing a light beam generated by any one of the first and second light sources to be selectively irradiated onto the discs and having an aperture number control unit for allowing the number of aperture of the light beam to have a different value depending on an optical disc to be accessed; and a photo detecting unit for detecting a light beam reflected from the accessed optical disc and converting it into an electrical signal.  
         [0017]     An optical recording/reproducing apparatus according to another aspect of the present invention includes the above-mentioned optical pickup apparatus.  
         [0018]     An optical pickup apparatus according to still another aspect of the present invention includes a light source for generating a certain wavelength of light beam; an optical system allowing the light beam to be irradiated onto an optical disc to be accessed, the system having an aperture number control unit for allowing the number of aperture of the light beam to have a different value depending on the optical disc to be accessed; and a photo detecting unit for detecting a light beam reflected from the accessed optical disc and converting the detected light beam into an electrical signal.  
         [0019]     An optical pickup apparatus according to still another aspect of the present invention includes a first light source for generating a first light beam; a second light source for generating a second light beam having a larger wavelength than the first light beam; an optical system allowing any one of the first and second light beams to be selectively irradiated onto an optical disc to be accessed, the system having an aperture number control unit for allowing the number of aperture of the light beam to have any one of a first value and a second value smaller than the first value, depending on the optical disc to be accessed; and a photo detecting unit for detecting a light beam reflected from the accessed optical disc and converting the detected light beam into an electrical signal, whereby the apparatus allows the first light beam to be irradiated with the first value of the aperture number when the optical disc to be accessed is the first optical disc, allows the second light beam to be irradiated with the second value of the aperture number when the optical disc to be accessed is the second optical disc, and allows the second light beam to be irradiated with the second value of the aperture number when the optical disc to be accessed is the third optical disc.  
         [0020]     An optical pickup apparatus according to still another aspect of the present invention includes a first light source for generating a first light beam; a second light source for generating a second light beam having a larger wavelength than the first light beam; an optical system allowing any one of the first and second light beams to be selectively irradiated onto an optical disc to be accessed, the system having an aperture number control unit for allowing the number of aperture of the light beam to have any one of a first value and a second value smaller than the first value, depending on the optical disc to be accessed; and a photo detecting unit for detecting a light beam reflected from the accessed optical disc and converting the detected light beam into an electrical signal, whereby the apparatus allows the first light beam to be irradiated with the first value of the aperture number when the optical disc to be accessed is the first optical disc, allows the first light beam to be irradiated with the second value of the aperture number when the optical disc to be accessed is the second optical disc, and allows the second light beam to be irradiated with the second value of the aperture number when the optical disc to be accessed is the third optical disc.  
         [0021]     An optical pickup apparatus according to still another aspect of the present invention includes a first light source for generating a first light beam; a second light source for generating a second light beam having a larger wavelength than the first light beam; an optical system allowing any one of the first and second light beams to be selectively irradiated onto an optical disc to be accessed, the system having an aperture number control unit for allowing the number of aperture of the light beam to have any one of a first value, a second value smaller than the first value and a third value smaller than the second value, depending on the optical disc to be accessed; and a photo detecting unit for detecting a light beam reflected from the accessed optical disc and converting the detected light beam into an electrical signal, whereby the apparatus allows the first light beam to be irradiated with the first value of the aperture number when the optical disc to be accessed is the first optical disc, allows the first light beam to be irradiated with the second value of the aperture number when the optical disc to be accessed is the second optical disc, and allows the second light beam to be irradiated with the third value of the aperture number when the optical disc to be accessed is the third optical disc.  
         [0022]     An optical pickup apparatus according to still another aspect of the present invention includes a first light source for generating a first light beam; a second light source for generating a second light beam having a larger wavelength than the first light beam; an optical system allowing any one of the first and second light beams to be selectively irradiated onto an optical disc to be accessed, the system having an aperture number control unit for allowing the number of aperture of the light beam to have any one of a first value, a second value smaller than the first value and a third value smaller than the second value, depending on the optical disc to be accessed; and a photo detecting unit for detecting a light beam reflected from the accessed optical disc and converting the detected light beam into an electrical signal, whereby the apparatus allows the first light beam to be irradiated with the first value of the aperture number when the optical disc to be accessed is the first optical disc, allows the second light beam to be irradiated with the second value of the aperture number when the optical disc to be accessed is the second optical disc, and allows the second light beam to be irradiated with the third value of the aperture number when the optical disc to be accessed is the third optical disc.  
         [0023]     According to another aspect of the present invention, there is an optical pickup for accessing at least first and second optical recording mediums, the pickup comprising a first light source generating a first light beam of a first wavelength; a second light source generating a second light beam of a second wavelength different from the first wavelength; an optical section transmitting the first and second light beams; and an aperture number controller including a wave selecting member for selecting between the first and second light beams of different wavelengths so as to impinge a light beam with an appropriate aperture number to selectively access one of the at least first and second optical recording mediums. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     These and other objects of the invention will be apparent from the following detailed description of the embodiments of the present invention with reference to the accompanying drawings, in which:  
         [0025]      FIG. 1  is a schematic view showing the configuration of an optical system in an optical pickup apparatus according to the first embodiment of the present invention;  
         [0026]      FIG. 2  is a schematic view showing the configuration of an optical system in an optical pickup apparatus according to the second embodiment of the present invention;  
         [0027]      FIG. 3  is a plan view of the polarizing plate shown in  FIG. 2 ;  
         [0028]      FIG. 4  is a schematic view showing the configuration of an optical system in an optical pickup apparatus according to the third embodiment of the present invention;  
         [0029]      FIG. 5  is a schematic view showing the configuration of an optical system in an optical pickup apparatus according to the fourth embodiment of the present invention;  
         [0030]      FIG. 6  is a schematic view showing the configuration of an optical system in an optical pickup apparatus according to the fifth embodiment of the present invention;  
         [0031]      FIG. 7  is a plan view of the polarizing plate for wavelength selection shown in  FIG. 6 ;  
         [0032]      FIG. 8  is a schematic view showing the configuration of an optical system in an optical pickup apparatus according to the sixth embodiment of the present invention;  
         [0033]      FIG. 9A  is a schematic view showing the configuration of an optical system in an optical pickup apparatus according to the seventh embodiment of the present invention, and  FIG. 9B  is one example of a wavelength selecting polarizing plate integral with an objective lens according to the present invention; and  
         [0034]      FIG. 10  is a schematic view showing the configuration of an optical system in an optical pickup apparatus according to the eighth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     An optical pickup apparatus is intended for use in accessing three different types of optical discs requiring different sizes of beam spots by a proper combination of the wavelength (λ) of a light beam and the number of aperture NA using the fewest number of different elements.  
         [0036]     First, an optical pickup apparatus according to the present invention can obtain the sizes of beam spots corresponding to the blue laser, the DVD and the CD by utilizing an appropriate combination of two light sources generating a different wavelength (λ) of light beam with an optical system for controlling the number of aperture NA into two modes.  
         [0037]     More specifically, the optical pickup apparatus according to the present invention can obtain first to third beam spot sizes d 1 , d 2  and d 3  corresponding to the blue laser, the DVD and CD by sequentially using the first wavelength (λ1) of about 400 nm and the third wavelength (λ3) of about 780 nm, and the first number of aperture NA 1  of about 0.7 and the third number of aperture NA 3  of about 0.45. In this case, the relationship of the first to third beam spots size d 1 , d 2  and d 3  with respect to the wavelength λ and the number of aperture NA can be expressed by the following formula:  
             d1   =       k   ⁢           ⁢     λ1   NA1     ⁢           ⁢   d2     =       k   ⁢           ⁢     λ3   NA1     ⁢           ⁢   d3     =     k   ⁢           ⁢     λ3   NA3                   (   4   )             
 
         [0038]     It can be seen from the formula (4) that the optical pickup apparatus makes use of the first wavelength λ1 and the first number of aperture NA 1  in accessing the HD-DVD, of the third wavelength λ3 and the first number of aperture NA 1  in accessing the DVD, and of the third wavelength λ3 and the third number of aperture NA 3  in accessing the CD. To this end, an optical pickup apparatus according to the first embodiment of the present invention has a configuration as shown in  FIG. 1 .  
         [0039]     Referring to  FIG. 1 , the optical pickup apparatus includes first and second light sources  12  and  14  for generating a first wavelength (λ1) of light beam and a third wavelength (λ3) of light beam, respectively, a twin objective lens  20  with the first number of aperture NA 1  and the third number of aperture NA 3  for converging an incident light beam onto the recording faces of first to third optical discs  10 A,  10 B and  10 C by its position control, and a photo detector  24  for converting light beams reflected from the first to third optical discs  10 A, 10 B and  10 C into electrical signals. Further, the optical pickup apparatus includes a first beam splitter  18  positioned in the path of light beams emitted from the first and second light sources  12  and  14 , a second beam splitter  19  arranged among the first beam splitter  18 , the twin objective lens  20  and the photo detector  24 , a collimator lens  16  arranged between the first light source  12  and the first beam splitter  18 , and a sensor lens  22  arranged between the photo detector  24  and the second beam splitter  19 .  
         [0040]     In the optical pickup apparatus of  FIG. 1 , the first to third optical discs  10 A,  10 B and  10 C represent the HD-DVD, the DVD and the CD, respectively. The first light source  12  generates a first wavelength (λ1) of light beam using the blue laser while the second light source  14  generates a third wavelength (λ3) of light beam. The collimator lens  16  converts a divergent light beam progressing from the first light source  12  toward the first beam splitter  18  into a parallel light beam to thereby prevent any leakage of light. The light beam from the collimator lens  16  passes through the first beam splitter  18  toward the second beam splitter  19 . A light beam from the second light source  14  progresses toward the second beam splitter  19  and is reflected toward the second beam splitter  19 . The second beam splitter  19  passes a light beam from the first beam splitter  18  toward the twin objective lens  20 . Also, the second beam splitter  19  reflects a light beam reflected from the recording faces of the first to third optical discs  10 A,  10 B and  10 C and progressed over the twin objective lens  20 , via the sensor lens  22 , into the photo detector  24 . In this case, a Dichroic polarizer beam splitter is usually used as the first and second beam splitters  18  and  19 .  
         [0041]     The sensor lens  22  converges a parallel light beam progressing from the second beam splitter  19  toward the photo detector  24  onto the surface of the photo detector  24  to thereby prevent any leakage of the light beam. The photo detector  24  detects a reflective light beam reflected by the recording faces of the first to third optical discs  10 A,  10 B and  10 C and then received by way of the twin objective lens  20 , the second beam splitter  19  and the sensor lens  22 , and converts it into an electrical signal.  
         [0042]     The twin objective lens  20  includes first and second objective lens  20 A and  20 B having the first number of aperture NA 1  of about 0.7 and the third number of aperture NA 3  of about 0.45. The first and second objective lens  20 A and  20 B are installed in a single lens supporting member  20 C which is rotated to position the first and second objective lens  20 A and  20 B. The lens supporting member is driven with an actuator (not shown), which is usually driven by an axis sliding system that allows the lens supporting member  20 C to pivot around a rotation axis. Access to the first to third optical discs  10 A,  10 B and  10 C in such an optical pickup apparatus will be described in detail.  
         [0043]     When the first optical disc  10 C is accessed in the optical pickup apparatus of  FIG. 1 , the first optical source  12  is driven and the first objective lens  20 A with the first number of aperture NA 1  is positioned at a light path by a driving of the twin objective lens  20 . A first wavelength (λ1) of light beam generated at the first light source  12  passes through the first and second beam splitters  18 ,  19  to be incident to the first objective lens  20 A. This incident light beam is converged by the first objective lens  20 A to be irradiated onto the recording face of the first optical disc  10 A with the first beam spot size d 1 .  
         [0044]     When the second optical disc  10 B is accessed, the second optical source  14  is driven and the first objective lens  20 A with the first number of aperture NA 1  is positioned at a light path by a driving of the twin objective lens  20 . A third wavelength (λ3) of light beam generated at the second light source  14  is perpendicularly reflected by the first beam splitter  18  and passes through the second beam splitter  19  to be incident to the first objective lens  20 A. This incident light beam is converged by the first objective lens  20 A to be irradiated onto the recording face of the second optical disc  10 B with the second beam spot size d 2 .  
         [0045]     When the third optical disc  10 A is accessed, the second optical source  14  is driven and the second objective lens  20 B with the third number of aperture NA 3  is positioned at a light path by a driving of the twin objective lens  20 . A third wavelength (λ3) of light beam generated at the second light source  14  is perpendicularly reflected by the first beam splitter  18  and passes through the second beam splitter  19  to be incident to the second objective lens  20 B. This incident light beam is converged by the second objective lens  20 B to be irradiated onto the recording face of the third optical disc  10 C with the third beam spot size d 3 . As described above, a numerical value example of the wavelength (λ) of a light beam and the number of aperture NA corresponding to the first to third optical discs  10 A,  10 B and  10 C is indicated in the following table:  
                                     TABLE 1                               NUMBER OF           WAVELENGTH (λ)   APERTURE (NA)                                1st OPTICAL DISC   400 nm   0.7       (BLUE)       2nd OPTICAL DISC   780 nm   0.7       (DVD)       3rd OPTICAL DISC   780 nm   0.45       (CD)                  
 
         [0046]      FIG. 2  shows the configuration of an optical system in an optical pickup apparatus according to the second embodiment of the present invention. Referring to  FIG. 2 , the optical pickup apparatus includes a single objective lens  30  instead of the twin objective lens  20  shown in  FIG. 1 , and an aperture number controller for controlling the number of aperture of the objective lens  30 , that is, a liquid crystal plate  26  and a polarizing plate  28 .  
         [0047]     In the optical pickup apparatus of  FIG. 2 , each of the first and second light sources  12  and  14  generates a vertical polarized beam having a polarization characteristic moving at the short axis direction with respect to an ellipse, hereinafter referred to as “S wave”, or a horizontal polarized beam having a polarization characteristic moving at the long axis direction with respect to an ellipse, hereinafter referred to as “P wave”. For the sake of convenience, it is assumed in the present invention that the light beams generated at the first and second light sources  12  and  14  are S waves.  
         [0048]     The liquid crystal plate  26  is arranged between the second light source  14  and the first beam splitter  18 , and the polarizing plate  28  is arranged between the second beam splitter  19  and the objective lens  30 . The liquid crystal plate  26  varies the polarization characteristic of a light beam depending on whether or not a voltage has been applied, and the polarizing plate  28  selectively shuts out a portion of the light beam in accordance with a polarizing characteristic of an incident light beam. More specifically, the liquid crystal plate  26  passes an S wave when a voltage is applied, whereas it rotates an S wave at 90° (to be converted into P wave and outputs the converted P wave) when a voltage is not applied. Otherwise, if P wave is generated at the second light source  14 , then the liquid crystal plate  28  converts the P wave into S wave when a voltage is applied while it passes the P wave as it is when a voltage is not applied.  
         [0049]     As shown in  FIG. 3 , the polarizing plate  28  includes a circular non-polarizing area  28 A and a polarizing area  28 B defined around the non-polarizing area  28 A. In this case, the non-polarizing area  28 A of the polarizing plate  28  allows the light beam to be passed toward the objective lens  30  independently of the polarization characteristic of an incident light beam. The polarizing area  28 B passes the light beam when a polarizing direction of an incident light beam is identical to its polarizing direction, whereas it shuts out the light beam when a polarizing direction of an incident light beam is different from its polarizing direction. Usually, such a polarizing plate  28  is integral to the objective lens  30 .  
         [0050]     In  FIG. 2 , the polarizing plate  28  is positioned so that the non-polarizing area  28 A corresponds to the third number of aperture NA 3 , and is formed in such a manner that the polarizing area  28 B shuts out P waves. Accordingly, when a P wave is received, the flux diameter of an incident light beam is controlled by the polarizing area  28 B to control the number of aperture. An access to the first to third optical discs  10 A,  10 B and  10 C in such an optical pickup apparatus will be described in detail.  
         [0051]     When the first optical disc  10 A is accessed in the optical pickup apparatus of  FIG. 2 , the first light source  12  is driven. A first wavelength (λ1) of S wave generated at the first light source  12  is incident to the objective lens  30  by way of the collimator lens  16 , the first and second beam splitters  18 ,  19  and the polarizing plate  28 . This incident light beam is converged by the first objective lens  30  with the first number of aperture to be irradiated onto the recording face of the first optical disc  10 A with the first beam spot size d 1 .  
         [0052]     When the second optical disc  10 B is accessed, the second light source  14  is driven and a voltage is applied to the liquid crystal plate  26 . A third wavelength (λ3) of S wave is generated at the second light source  14 , transmits through the liquid crystal plate  26  applied with a voltage as it is, and is perpendicularly reflected by the first beam splitter  18  to thereby be incident on the objective lens  30  by way of the second beam splitter  19  and the polarizing plate  28 . This incident light beam is converged by the objective lens  30  and irradiated onto the recording face of the second optical disc  10 B with a second beam spot size d 2 .  
         [0053]     When the third optical disc  10 C is accessed, the second light source  14  is driven and a voltage is not applied to the liquid crystal plate  26 . A third wavelength (λ3) of S wave generated at the second light source  14 , converted into a P wave by the liquid crystal plate  26  which is not applied with a voltage, and is perpendicularly reflected by the beam splitter  18  to thereby be incident on the polarizing plate  28  by way of the second beam splitter  19 . The P wave received into the polarizing plate  28  is shut out by the outer polarizing area  28 B and passed by the non-polarizing area  28 A, so that the number of aperture of a light beam is controlled into the third number of aperture NA 3  to be incident to the objective lens  30 . This incident light beam is converged by the objective lens  30  and irradiated onto the recording face of the third optical disc  10 C  
         [0054]     The optical pickup apparatus according to the present invention can obtain first to third beam spots size d 1 , d 2  and d 3  corresponding to the blue laser (HD-DVD), the DVD and the CD, respectively, by utilizing a light beam with a first wavelength (λ1) of about 400 nm and a third wavelength (λ3) of about 780 nm, the fourth number of aperture NA 4  of about 0.37 less than the third number of aperture NA 3  and the first number of aperture NA 1  of about 0.7, as different from the optical pickup apparatus shown in  FIG. 1  and  FIG. 2 . In this case, a relationship among the first to third beam spot sizes d 1 , d 2  and d 3 , the wavelength (λ) and the number of aperture NA can be expressed as the following formula:  
             d1   =       k   ⁢           ⁢     λ1   NA1     ⁢           ⁢   d2     =       k   ⁢           ⁢     λ1   NA4     ⁢           ⁢   d3     =     k   ⁢           ⁢     λ2   NA4                   (   5   )             
 
         [0055]     It can be seen from the formula (5) that, the optical pickup apparatus uses the first wavelength (λ1) and the first number of aperture NA 1  when it accesses the HD-DVD, it uses the first wavelength (λ1) and the fourth number of aperture NA 4  when it accesses the DVD, and it uses the second wavelength (λ2) and the fourth number of aperture NA 4  when it accesses the CD. To this end, the optical pickup apparatus according to the third embodiment of the present invention has a construction as shown in  FIG. 4 .  
         [0056]     The optical pickup apparatus shown in  FIG. 4  includes the same construction elements as the optical pickup apparatus shown in  FIG. 2  except for a second light source  32  generating a second wavelength (λ2) of light beam, a liquid crystal plate  26  arranged at the side of first light source  12  and a polarizing plate  26  having a non-polarizing area  34 A with a dimension corresponding to the fourth number of aperture NA 4 . A detailed explanation of the construction elements that are identical to the optical pickup apparatus in  FIG. 2  will be omitted.  
         [0057]     When the first optical disc  10 A is accessed in the optical pickup apparatus of  FIG. 4 , the first optical source  12  is driven and a voltage is applied to the liquid crystal plate  26 . A first wavelength (λ1) of S wave generated at the first light source  12  is incident on the objective lens  30  by way of the collimator lens  16 , the first and second beam splitters  18 ,  19  and the polarizing plate  34 . This incident light beam is converged by the first objective lens  30  with the first number of aperture NA 1  to be irradiated onto the recording face of the first optical disc  10 A at the first beam spot size d 1 .  
         [0058]     When the second optical disc  10 B is accessed, the first optical source  12  is driven and a voltage is not applied to the liquid crystal plate  26 . A first wavelength (λ1) of S wave generated at the first light source  12  is converted into P wave, and is incident on the polarizing plate  34  by way of the first and second beam splitters  18  and  19 . The P wave received into the polarizing plate  34  is shut out at the outer polarizing area  34 B and passes through the non-polarizing area  34 A, so that the number of aperture NA of a light beam is controlled into the fourth number of aperture NA to be incident on the objective lens  30 . This incident light beam is converged by the objective lens  30  and irradiated onto the recording face of the second optical disc  10 B with a second beam spot size d 2 .  
         [0059]     When the third optical disc  10 C is accessed, the second light source  14  is driven. In this case, the second light source  32  generates a P wave differently from the first light source generating an S wave. A second wavelength (λ2) of P wave generated at the second light source  14  is perpendicularly reflected by the beam splitter  18  to thereby be incident on the polarizing plate  34  by way of the second beam splitter  19 . The P wave received into the polarizing plate  34  is shut out at the outer polarizing area  34 B and passes through the non-polarizing area  28 A, so that the number of aperture of a light beam is controlled into the third number of aperture NA 3  to be incident to the objective lens  30 . This incident light beam is converged by the objective lens  30  and irradiated onto the recording face of the third optical disc  10 C with the third beam spot size d 3 .  
         [0060]     A numerical value example of a wavelength of light beam and the number of aperture corresponding to the first to third optical discs  10 A,  10 B and  10 C is described in the following table.  
                                     TABLE 2                               NUMBER OF           WAVELENGTH (λ)   APERTURE (NA)                                1st OPTICAL DISC   400 nm   0.7       (BLUE)       2nd OPTICAL DISC   400 nm   0.37       (DVD)       3rd OPTICAL DISC   650 nm   0.37       (CD)                  
 
         [0061]     As described above, the optical pickup apparatus according to the present invention can access three types of optical discs that are different in recording density, light transmission layer, and other aspects by creating light beam sizes using two optical sources that generate different wavelengths of light beams and an optical system controlled into two aperture number modes. Also, the optical pickup apparatus according to the present invention can obtain beam spots having sizes that correspond to a HD DVD (blue laser), a DVD and a CD by utilizing an appropriate combination of two light sources generating a different wavelength of light beams with an optical system for controlling the number of aperture of a light beam into three modes.  
         [0062]     More specifically, an optical pickup apparatus according to the present invention can obtain first to third beam spot sizes d 1 , d 2  and d 3  corresponding to the blue laser, the DVD and CD, respectively, using light beams with a first wavelength (λ1) of about 400 nm and a third wavelength (λ3) of about 780 nm, and the first number of aperture NA 1  of about 0.7, the third number of aperture NA 3  of about 0.45 and the fourth number of aperture NA 4  of about 0.37. In this case, a relationship of the first to third beam spots size d 1 , d 2  and d 3  with respect to the wavelength λ and the number of aperture NA can be expressed by the following formula:  
             d1   =       k   ⁢           ⁢     λ1   NA1     ⁢           ⁢   d2     =       k   ⁢           ⁢     λ1   NA4     ⁢           ⁢   d3     =     k   ⁢           ⁢     λ3   NA3                   (   6   )             
 
         [0063]     It can be seen from the formula (6) that the optical pickup apparatus makes use of the first wavelength λ1 and the first number of aperture NA 1  when accessing the HD-DVD, of the first wavelength λ  1  and the fourth number of aperture NA 4  when accessing the DVD, and of the third wavelength λ3 and the third number of aperture NA 3  when accessing the CD. To this end, an optical pickup apparatus according to the fourth embodiment of the present invention has a configuration as shown in  FIG. 5 .  
         [0064]     The optical pickup apparatus in  FIG. 5  includes many of the same elements as the optical pickup apparatus in  FIG. 4  except for a second light source  14  for generating a third wavelength (λ3) of light beam, a polarizing plate  38 A having a non-polarizing area  38 A with a size corresponding to the fourth number of aperture NA 4 , and a twin objective lens  36 .  
         [0065]     In the optical pickup apparatus of  FIG. 5 , the twin objective lens  36  includes first and second objective lens  36 A and  36 B having the first number of aperture NA 1  and the second number of aperture NA 2 , respectively.  
         [0066]     When the first optical disc  10 A is accessed in the optical pickup apparatus of  FIG. 5 , the first optical source  12  is driven and a voltage is applied to the liquid crystal plate  26 , and, simultaneously, the first objective lens  36 A with the first number of aperture NA 1  is positioned at a light path by driving of the twin objective lens  36 . A first wavelength (λ1) of S wave generated at the first light source  12  is incident on the first objective lens  36 A by way of the liquid crystal plate  26 , the collimator lens  16 , the first and second beam splitters  18 ,  19  and the polarizing plate  38 . This incident light beam is converged by the first objective lens  36 A with the first number of aperture NA 1  to be irradiated onto the recording face of the first optical disc  10 A at the first beam spot size d 1 .  
         [0067]     When the second optical disc  10 B is accessed, the first optical source  12  is driven and a voltage is not applied to the liquid crystal plate  26 , and, simultaneously, the first objective lens  36 A with the first number of aperture NA 1  is positioned at a light path by driving of the twin objective lens  36 . A first wavelength (λ1) of S wave generated at the first light source  12  is converted into P wave by the liquid crystal plate  26  in which a voltage is not applied, and is incident on the polarizing plate  38  by way of the first and second beam splitters  18  and  19 . The P wave received into the polarizing plate  38  is shut out at the outer polarizing area  38 B and passes through only the non-polarizing area  38 A, so that the number of aperture NA of a light beam is controlled into the fourth number of aperture NA to be incident on the objective lens  30 . This incident light beam is converged by the first objective lens  36 A and irradiated onto the recording face of the second optical disc  10 B with a second beam spot size d 2 .  
         [0068]     When the third optical disc  10 C is accessed, the second light source  14  is driven and the second objective lens  36 B is positioned at a light path by driving of a twin objective lens  36 . A third wavelength (λ3) of S wave generated from the second light source  14  is perpendicularly reflected by the first beam splitter  18  and is incident on the second objective lens  36 B by way of the second beam splitter  19  and the polarizing plate  38 . The incident light beam is converged by the second objective lens  36 B and irradiated onto the recording face of the third optical disc  10 C with the third beam spot size d 3 .  
         [0069]     A numerical value example of a wavelength (λ) of light beam and the number of aperture NA corresponding to the first to third optical discs  10 A,  10 B and  10 C is described in the following table:  
                                     TABLE 3                               NUMBER OF           WAVELENGTH (λ)   APERTURE (NA)                                1st OPTICAL DISC   400 nm   0.7       (BLUE)       2nd OPTICAL DISC   400 nm   0.37       (DVD)       3rd OPTICAL DISC   780 nm   0.45       (CD)                  
 
         [0070]      FIG. 6  shows the configuration of an optical pickup apparatus according to the fifth embodiment of the present invention. The optical pickup apparatus of  FIG. 6  includes many of the same elements as the optical pickup apparatus shown in  FIG. 5  except for a single objective lens  42  and a wave selecting polarizing plate  40  arranged between the second beam splitter  19  and the objective lens  42  to control the number of aperture of a light beam into three modes.  
         [0071]     In the optical pickup apparatus shown in  FIG. 6 , the wave selecting polarizing plate  40  consists of a non-polarizing area  40 A defined at the center, a polarizing area  40 B defined around the non-polarizing area  40 A, and a wavelength selecting area  40 C defined at the outside of the polarizing area  40 B. The non-polarizing area  40 A of such a polarizing plate passes the light beam toward the objective lens  42  independently of the polarization characteristics of an incident light beam. The polarizing area  40 B and the wavelength selecting area  40 C pass the light beam when the polarized direction of an incident beam is identical to its polarized direction while it shuts out the light beam when the polarized direction of an incident beam is different from its polarized direction. In  FIG. 6 , the polarizing plate  40  has an area in which the non-polarizing area  40 A corresponds to the fourth number of aperture NA 4  and is formed in such a manner that the polarizing area shuts out the P wave. Accordingly, when a P wave is received, a flux diameter is controlled by the polarizing area  40 B to control the number of aperture. In the polarizing plate  40 , the wave selecting area  40 C is coated to shut out a specified direction of polarized beam and a specified wavelength of light beam. In  FIG. 6 , the inner circumference of the wave selecting area  40 C has a size corresponding to the third number of aperture NA 3  and shuts out a light beam with the third wavelength (λ3), i.e., 780 nm. Access to the first to third optical discs  10 A,  10 B and  10 C in such an optical pickup apparatus will now be described in detail.  
         [0072]     When the first optical disc  10 A is accessed in the optical pickup apparatus of  FIG. 6 , the first optical source  12  is driven and a voltage is applied to the liquid crystal plate  26 . A first wavelength (λ1) of S wave generated at the first light source  12  is incident on the objective lens  42  by way of the liquid crystal plate  26  applied with a voltage, the collimator lens  16 , the first and second beam splitters  18 ,  19  and the wavelength selecting polarizing plate  40 . This incident light beam is converged by the objective lens  42  with the first number of aperture NA 1  to be irradiated onto the recording face of the first optical disc  10 A at the first beam spot size d 1 .  
         [0073]     When the second optical disc  10 B is accessed, the first light source  12  is driven and a voltage is not applied to the liquid crystal plate  26 . A first wavelength (λ1) of S wave generated at the first light source  12  is converted into P wave by the liquid crystal plate  26  in which a voltage is not applied, and is incident on the wavelength selecting polarizing plate  40  by way of the first and second beam splitters  18  and  19 . The P wave received into the wavelength selecting polarizing plate  40  is shut out at the polarizing area  40 B and pass through only the non-polarizing area  40 A, so that the number of aperture NA of a light beam is controlled into the fourth number of aperture NA 4  to be incident on the objective lens  42 . This incident light beam is converged by the objective lens  42  and irradiated onto the recording face of the second optical disc  10 B with a second beam spot size d 2 .  
         [0074]     When the third optical disc  10 C is accessed, the second light source  14  is driven. A third wavelength (λ3) of S wave generated at the second light source  14  is perpendicularly reflected by the beam splitter  18  to thereby be incident on the wave selecting polarizing plate  40  by way of the second beam splitter  19 . The third wavelength (λ3) of S wave received into the wave selecting polarizing plate  40  is shut out only at the outermost wavelength selecting area  40 C, so that the number of aperture NA of a light beam is controlled into the third number of aperture NA 3  to be incident to the objective lens  42 . This incident light beam is converged by the objective lens  42  and irradiated onto the recording face of the third optical disc  10 C with the third beam spot size d 3 .  
         [0075]     The optical pickup apparatus according to the present invention can obtain first to third beam spot sizes d 1 , d 2  and d 3  corresponding to the blue laser, the DVD and CD, respectively, using a light beam with a first wavelength (λ1) of about 400 nm and a second wavelength (λ2) of about 650 nm, and the first number of aperture NA 1  of about 0.7, the second number of aperture NA 2  of about 0.6 and the fourth number of aperture NA 4  of about 0.45. In this case, a relationship of the first to third beam spot sizes d 1 , d 2  and d 3  with respect to the wavelength λ and the number of aperture NA can be expressed by the following formula:  
             d1   =       k   ⁢           ⁢     λ1   NA1     ⁢           ⁢   d2     =       k   ⁢           ⁢     λ2   NA2     ⁢           ⁢   d3     =     k   ⁢           ⁢     λ2   NA4                   (   7   )             
 
         [0076]     It can be seen from the formula (7) that the optical pickup apparatus makes use of the first wavelength λ1 and the first number of aperture NA 1  in the case of accessing the HD-DVD, of the two wavelength λ2 and the second number of aperture NA 2  in the case of accessing the DVD, and of the second wavelength λ2 and the fourth number of aperture NA 4  in the case of accessing the CD. To this end, an optical pickup apparatus according to the sixth embodiment of the present invention has a configuration as shown in  FIG. 8 .  
         [0077]     The optical pickup apparatus in  FIG. 8  includes many of the same elements as the optical pickup apparatus in  FIG. 5  except for a second light source  32  for generating a second wavelength (λ2) of light beam, a liquid crystal plate  26  arranged at the side of the second light source  32 , and a twin objective lens  44  consisting of first and second objective lens  44 A and  44 B having the first and second aperture numbers NA 1  and NA 2 , respectively.  
         [0078]     When the first optical disc  10 A is accessed in the optical pickup apparatus of  FIG. 8 , the first optical source  12  is driven and the first objective lens  44 A with the first number of aperture NA 1  is positioned at a light path by a driving of the twin objective lens  44 . A first wavelength (λ1) of S wave generated at the first light source  12  is incident on the first objective lens  44 A by way of the collimator lens  16 , the first and second beam splitters  18 ,  19  and the polarizing plate  38 . This incident light beam is converged by the first objective lens  44 A with the first number of aperture NA 1  to be irradiated onto the recording face of the first optical disc  10 A at the first beam spot size d 1 .  
         [0079]     When the second optical disc  10 B is accessed, the second to light source  32  is driven and a voltage is applied to the liquid crystal plate  26 , and, simultaneously, the second objective lens  44 B with the second number of aperture NA 2  is positioned at a light path by a driving of the twin objective lens  44 . A second wavelength (λ2) of S wave generated at the second light source  32  is incident on the second objective lens  44 B by way of the liquid crystal plate  26  applied with a voltage, the first and second beam splitters  18  and  19  and the polarizing plate  38 . This incident light beam is converged by the second objective lens  44 B and irradiated onto the recording face of the second optical disc  10 B with a second beam spot size d 2 .  
         [0080]     When the third optical disc  10 C is accessed, the second light source  32  is driven and a voltage is not applied to the liquid crystal plate  26  and, simultaneously, the second objective lens  44 B is positioned at a light path by a driving of a twin objective lens  44 . A second wavelength (λ2) of S wave generated from the second light source  32  is converted into the P wave by the liquid crystal plate  26  in which a voltage is not applied and is perpendicularly reflected by the first beam splitter  18 , and thereafter is incident to the polarizing plate  38  by way of the second beam splitter  19 . The P wave received into the polarizing plate  38  is shut out at the outer polarizing area  38 B and is passed through the non-polarizing area  38 A only, so that the number of aperture NA of a light beam is controlled into the fourth number of aperture NA 4  and is incident to the second objective lens  44 B. This incident light beam is converged by the second objective lens  44 B and irradiated onto the recording face of the third optical disc  10 C with the third beam spot size d 3 . A numerical value example of a wavelength (λ) of light beam and the number of aperture NA corresponding to the first to third optical discs  10 A,  10 B and  10 C is described in the following table:  
                                     TABLE 4                               NUMBER OF           WAVELENGTH (λ)   APERTURE (NA)                                1st OPTICAL DISC   400 nm   0.7       (BLUE)       2nd OPTICAL DISC   650 nm   0.6       (DVD)       3rd OPTICAL DISC   650 nm   0.37       (CD)                  
 
         [0081]      FIG. 9A  shows the configuration of an optical pickup apparatus according to the seventh embodiment of the present invention. The optical pickup apparatus of  FIG. 7  includes many of the same elements as the optical pickup apparatus shown in  FIG. 6  except for a second light source  32  for generating a second wavelength (λ2) of light beam, and in particular, comparing to  FIG. 7 , a wavelength selecting polarizing plate  46  in which the inner circumference of the wavelength selecting area  46 B has the second number of aperture NA 2  and the wavelength selecting area  46 C is formed to shut out a second wavelength (λ2) of light beam.  
         [0082]     When the first optical disc  10 A is accessed in the optical pickup apparatus of  FIG. 9A , the first light source  12  is driven and a voltage is applied to the liquid crystal plate  26 . A first wavelength (λ1) of S wave generated at the first light source  12  is incident on the objective lens  42  by way of the liquid crystal plate  26  applied with a voltage, the collimator lens  16 , the first and second beam splitters  18 ,  19  and the wavelength selecting polarizing plate  46 . This incident light beam is converged by the first objective lens  42  with the first number of aperture NA 1  to be irradiated onto the recording face of the first optical disc  10 A at the first beam spot size d 1 .  
         [0083]     When the second optical disc  10 B is accessed, the second light source  32  is driven. A second wavelength (λ2) of S wave generated at the second light source  32  is reflected by the first beam splitter  18  and is incident on the wavelength selecting polarizing plate  46  by way of the second beam splitter  19 . The second wavelength (λ2) of S wave is shut out by the outermost wavelength selecting area  46 C only, so that the number of aperture NA of a light beam is controlled into the second number of aperture NA 2  to be incident on the objective lens  42 . This incident light beam is converged by the objective lens  42  and irradiated onto the recording face of the second optical disc  10 B with a second beam spot size d 2 .  
         [0084]     When the third optical disc  10 C is accessed, the second light source  32  is driven. A second wavelength (λ2) of S wave generated at the second light source  32  is perpendicularly reflected by the beam splitter  18  to thereby be incident to the wave selecting polarizing plate  46  by way of the second beam splitter  19 . The S wave received into the wave selecting polarizing plate  46  is shut out at the polarizing area  46 B including the wavelength selecting area  46 C and passed through the non-polarizing area  46 A only, so that the number of aperture NA of a light beam is controlled into the fourth number of aperture NA 4  to be incident on the objective lens  42 . This incident light beam is converged by the objective lens  42  and irradiated onto the recording face of the third optical disc  10 C with the third beam spot size d 3 . In one example, the wavelength selecting polarizing plate  46  is integral to the objective lens  42 . One example of such integration is shown in  FIG. 9B .  
         [0085]     As described above, the optical pickup apparatus according to the present invention can access three types of optical discs that have different recording densities and light transmission layers using light beam sizes suitable for each disc making use of two optical sources generating different wavelengths of light beams and an optical system controlled into two aperture number modes. Also, the optical pickup apparatus according to the present invention can obtain beam spots having sizes corresponding to a HD DVD (blue laser), a DVD and a CD by utilizing an appropriate combination of a single light source generating the shortest different wavelength of light beams with an optical system for controlling the number of aperture of a light beam into three modes.  
         [0086]     More specifically, the optical pickup apparatus according to the present invention can obtain first to third beam spot sizes d 1 , d 2  and d 3  corresponding to the blue laser, the DVD and CD, respectively, using a light beam with a first wavelength (λ1) of about 400 nm, the first number of aperture NA 1  of about 0.7, the fourth number of aperture NA 4  of about 0.37 and the fifth number of aperture NA 5  of about 0.23 less than the fourth number of aperture NA 4 . In this case, a relationship of the first to third beam spot sizes d 1 , d 2  and d 3  with respect to the wavelength λ and the number of aperture NA can be expressed by the following formula:  
             d1   =       k   ⁢           ⁢     λ1   NA1     ⁢           ⁢   d2     =       k   ⁢           ⁢     λ1   NA4     ⁢           ⁢   d3     =     k   ⁢           ⁢     λ1   NA5                   (   8   )             
 
         [0087]     It can be seen from the formula (8) that the optical pickup apparatus makes use of the first wavelength λ1 and the first number of aperture NA 1  in the case of accessing the HD-DVD, of the first wavelength λ1 and the fourth 25 number of aperture NA 4  in the case of accessing the DVD, and of the first wavelength λ1 and the fifth number of aperture NA 5  in the case of accessing the CD. To this end, an optical pickup apparatus according to the eighth embodiment of the present invention has a configuration as shown in  FIG. 10 .  
         [0088]     Referring now to  FIG. 10 , the optical pickup apparatus includes a light source  12  for generating a first wavelength (λ1) of light beam, a twin objective lens  50  for converging a light beam generated from the light source  12  onto recording faces of first to third optical discs  10 A,  10 B and  10 C, a liquid crystal plate  26  and a polarizing plate  48  arranged between the light source  12  and the twin objective lens  50  to control the number of aperture, and a photo detector  24  for converting light beams reflected from the optical discs  10 A,  10 B and  10 C into electrical signals. Further, the optical pickup apparatus includes a beam splitter  18  arranged among the liquid crystal plate  26 , the polarizing plate  48  and the photo detector  24 , a collimator lens  16  arranged between the liquid crystal plate  26  and the beam splitter  18 , and a sensor lens  22  arranged between the photo detector  24  and the beam splitter  18 . In the optical pickup apparatus of  FIG. 10 , the twin objective lens  50  includes first and second objective lens  50 A and  50 B having the first and fourth number of aperture NA 1  and NA 4 , respectively. The polarizing plate  48  includes a non-polarizing area  48 A having a size corresponding to the fifth number of aperture NA 5 , and a polarizing area  48 B.  
         [0089]     When the first optical disc  10 A is accessed in such an optical pickup apparatus, a voltage is applied to the liquid crystal plate  26  and the first objective lens  50 A with the first number of aperture NA 1  is positioned at a light path by a driving of the twin objective lens  50 . A first wavelength (λ1) of S wave generated at the first light source  12  is incident to the first objective lens  50  by way of the liquid crystal plate  26  applied with a voltage, the collimator lens  16 , the beam splitter  18  and the polarizing plate  48  sequentially. This incident light beam is converged by the first objective lens  50 A with the first number of aperture NA 1  to be irradiated onto the recording face of the first optical disc  10 A at the first beam spot size d 1 .  
         [0090]     When the second optical disc  10 B is accessed, a voltage is applied to the liquid crystal plate  26  and the second objective lens  50 B with the fourth number of aperture NA 4  is positioned at a light path by a driving of the twin objective lens  50 . A first wavelength (λ1) of S wave generated at the light source  12  is incident on the second objective lens  50 B by way of the liquid crystal plate  26 , the beam splitter  18  and the polarizing plate  48 . This incident light beam is converged by the second objective lens  50 B and irradiated onto the recording face of the second optical disc  10 B with a second beam spot size d 2 .  
         [0091]     When the third optical disc  10 C is accessed, a voltage is not applied to the liquid crystal plate  26  and the first and second objective lens  50 A and  50 B are positioned at a light path by a driving of a twin objective lens  50 . A first wavelength (λ1) of S wave generated from the light source  12  is converted into P wave by the liquid crystal plate  26  in which a voltage is not applied, and then is incident on the polarizing plate  48  by way of the beam splitter  18 . The P wave received into the polarizing plate  48  is shut out at the outer polarizing area  48 B and passed through the non-polarizing area  48 A, so that the number of aperture of a light beam is controlled into the fifth number of aperture NA 5  to be incident on the first or second objective lens  50 A or  50 B. This incident light beam is converged by the first or second objective lens  44 A or  44 B and irradiated onto the recording face of the third optical disc  10 C with the third beam spot size d 3 . A numerical value example of a wavelength (λ) of light beam and the number of aperture NA corresponding to the first to third optical discs  10 A,  10 B and  10 C is described in the following table:  
                                     TABLE 5                               NUMBER OF           WAVELENGTH (λ)   APERTURE (NA)                                1st OPTICAL DISC   400 nm   0.7       (BLUE)       2nd OPTICAL DISC   400 nm   0.37       (DVD)       3rd OPTICAL DISC   400 nm   0.23       (CD)                  
 
         [0092]     Accordingly, the optical pickup apparatus according to the present invention can access three types of optical discs that have different recording densities and the light transmission layers using light beam sizes suitable for each disc by utilizing an optical system controlled into the blue laser and three aperture number modes.  
         [0093]     As described above, the optical pickup apparatus according to the present invention can access three types of optical disc having a different layout condition by making use of two light sources generating a different wavelength of light beam and an optical system controlled into two aperture number modes. Also, the optical pickup apparatus according to the present invention can access three types of optical disc having a different layout condition at the light beam sizes suitable for them by making use of two light sources generating a different wavelength of light beam and an optical system controlled into two aperture number mode. Furthermore, the optical pickup apparatus can access three types of optical discs different in the layout condition at the light beam sizes suitable for them making use of the blue laser and an optical system controlled into three aperture number modes. Accordingly, the configuration of the optical pickup capable of changeably accessing a plurality type of optical discs can be not only simplified, but also its manufacturing cost can be reduced. Moreover, the optical pickup apparatus according to the present invention can implement an optical recording/reproducing apparatus capable of the plurality type of optical discs accurately.  
         [0094]     Although the present invention has been explained by the embodiments shown in the drawings described above, it should be understood to the ordinary skilled person in the art that the invention is not limited to the embodiments, but rather that various changes or modifications thereof are possible without departing from the spirit of the invention. Accordingly, the scope of the invention shall be determined only by the appended claims and their equivalents.