Abstract:
An optical pickup system includes a first light source emitting first light beams with a first wavelength, a second light source emits second light beams with a second wavelength greater than the first wavelength, a third light source emits third light beams with a third wavelength greater than the second wavelength, a composite prism comprising a first prism facing the first and second light sources, a second prism facing the third light source, and a third prism for receiving the first, second and third light beams from the first and second prisms, the first prism has a surface facing the first and second light sources, the surface defines first and second regions, the second region has an aspherical surface to compensate aberration of the second light beams, a collimating lens and objective lens disposed in a common optical path for transmitting the three light beams from the composite prism.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical pickup system used in an information recording and/or reproducing apparatus, and more particularly to an optical pickup system for accessing different types of optical recording media and an information recording and/or reproducing apparatus employing the same. 
   2. Prior Art 
   In recent years, in order to satisfy ongoing requirements for recording and/or reproducing large quantities of data on recording media, many manufacturers have sought to increase the recording density of recording media. The recording density of a recording medium is determined by the size of a light spot illuminating the medium. Generally, the size of the light spot is proportional to the wavelength of the light, and inversely proportional to the numerical aperture (NA) of an objective lens that focuses the light. Therefore, reducing the wavelength or increasing the NA can increase the recording density of the recording medium. 
   An industry-wide standard relating to a next generation optical disk such as a high definition-digital versatile disk (HD-DVD) has been proposed to satisfy the demand for increased recording density of recording media. The HD-DVD standard employs a laser diode generating a blue laser with a wavelength of 405 nm, an objective lens having an NA of 0.85, and a light transmission protective layer of the optical disk having a thickness of 0.1 mm. 
   It is important to be able to employ a conventional compact disk (CD) and a conventional digital versatile disk (DVD) in an HD-DVD apparatus, because CDs and DVDs are still very popular whereas HD-DVDs are still relatively nascent. However, various optical conditions for recording/reproducing on/from CDs, DVDs, and HD-DVDs are different from each other, as shown in table 1. 
   
     
       
             
             
             
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               CD 
               DVD 
               HD-DVD 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               wavelength 
               780 
               nm 
               650 
               nm 
               405 
               nm 
             
             
               numerical aperture (NA) 
               0.45 
                 
               0.6 
                 
               0.85 
             
           
        
         
             
               recording capacity 
               0.65 
               GB 
               4.7 
               GB 
               more than 
             
           
        
         
             
                 
                 
                 
                 
                 
               20 
               GB 
             
             
               thickness of 
               1.2 
               mm 
               0.6 
               mm 
               0.1 
               mm 
             
             
               protective layer 
             
             
                 
             
           
        
       
     
   
   As can be seen, different optical disks need different objective lenses with different NAs. Therefore in a single conventional HD-DVD apparatus, there are usually three different objective lenses respectively adapted to CDs, DVDs and HD-DVDs. However, this makes the volume of the HD-DVD apparatus unduly large. To avoid this shortcoming, another conventional HD-DVD apparatus with only one objective lens and a wavelength selector has been developed. The wavelength selector changes an effective diameter of the objective lens by means of limiting the luminous flux propagating to the objective lens. With the help of the wavelength selector, the objective lens in the HD-DVD apparatus is suitable for reading and/or reproducing not only with respect to HD-DVDs, but also with respect to DVDs and CDs. 
   An information recording and/or reproducing apparatus employing only one objective lens for accessing three different optical recording media is disclosed in US patent application publication no. 2003/0185136A1. This publication discloses an information recording and/or reproducing apparatus including three laser diodes, three photodiodes, three beam splitters, three condensing lenses, a wavelength-selecting unit, and an objective lens. The three diodes emit three laser beams with different wavelengths, e.g., 405 nm, 650 nm and 780 nm, to be used in recording and reproducing operations for HD-DVDs, DVDs and CDs respectively. The three photodiodes receive the three laser beams reflected from the three different optical disks respectively. The three condensing lenses respectively condense the three laser beams. The wavelength-selecting unit changes the luminous flux of laser beams propagating to the objective lens. The objective lens focuses the three laser beams on the three different optical disks. In this apparatus, the wavelength-selecting unit and the objective lens are in a common optical path for the three laser beams to propagate along. The size of the information recording and/or reproducing apparatus is reduced to a certain extent, because two objective lenses adapted to DVDs and CDs are not required and are omitted. The wavelength-selecting unit changes the effective diameter of the objective lens by means of limiting the luminous flux propagating to the objective lens. Therefore the objective lens can be used in recording and/or reproducing operations for the three different optical disks, with optical aberration being reduced to a certain extent. 
   However, the structure and the size of the information recording and/or reproducing apparatus are relatively complex and large, because numerous optical components are still required. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide an optical pickup system for accessing a plurality of different optical recording media, the optical pickup system being compact. 
   Another object of the present invention is to provide an optical pickup apparatus using the optical pickup system described above. 
   To achieve the first object, an optical pickup system for accessing three different optical recording media in accordance with the present invention is provided. The optical pickup system includes a first light source, a second light source, a third light source, a composite prism, a collimating lens and an objective lens. The first light source emits first light beams with a first wavelength. The second light source emits second light beams with a second wavelength greater than the first wavelength. The third light source emits third light beams with a third wavelength greater than the second wavelength. The composite prism comprises a first prism facing the first and second light sources, a second prism facing the third light source, and a third prism for receiving the first, second and third light beams from the first and second prisms, the first prism has a surface facing the first and second light sources, the surface defines first and second regions, the second region having an aspherical surface to compensate aberration of the second light beams. The collimating lens is disposed in a common optical path for collimating and transmitting the first, second and third light beams from the composite prism. The objective lens is disposed in a common optical path for focusing the first, second and third light beams from the wavelength selector on three different optical recording media. 
   To achieve the second object, an information recording and/or reproducing apparatus employing the optical pickup system for accessing three different optical recording media includes an optical pickup system described above is provided. The information recording and/or reproducing apparatus further includes a drive mechanism for changing a relative position between any one of the three optical recording media and the optical pickup system; and an electrical signal processor for receiving signals output from the optical pickup system and performing calculations on the signals to obtain desired information. 
   Other objects, advantages and novel features will be drawn from the following detailed description of preferred embodiments with the attached drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic, isometric view of an optical pickup system according to a preferred embodiment of the present invention, showing optical paths thereof; 
       FIG. 2  is a schematic diagram of an information recording and/or reproducing apparatus including the optical pickup system of  FIG. 1 , together with an optical recording media; 
       FIG. 3  is an enlarged, schematic top view of a composite prism of the optical pickup system of  FIG. 1 , showing optical paths thereof; 
       FIG. 4  is a schematic, side view of a reflective prism and a collimating lens of the optical pickup system of  FIG. 1 , showing optical paths thereof; 
       FIG. 5  is an enlarged, schematic top view of a wavelength selector of the optical pickup system of  FIG. 1 ; 
       FIG. 6  is a cross-sectional view of the wavelength selector of  FIG. 5  taken along line VI-VI thereof, showing optical paths thereof; 
       FIG. 7  is a schematic, isometric view of an optical pickup system according to a second embodiment of the present invention, showing optical paths thereof; and 
       FIG. 8  is a side view of a reflective prism and a collimating lens of the optical pickup system of  FIG. 7 , showing optical paths thereof. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , an optical pickup system  100  according to a preferred embodiment of the present invention is illustrated. Referring also to  FIG. 2 , the optical pickup system  100  is used in an information recording and/or reproducing apparatus  200  for accessing a plurality of different optical recording media. In  FIG. 2 , one suck optical recording media  300  is shown. The optical recording media  300  may, for example, be an HD-DVD, a DVD or a CD. The optical pickup system  100  includes first, second and third semiconductor modules  11 ,  12 ,  13 , first, second and third diffraction elements  21 ,  22 ,  23 , a composite prism  3 , a reflective prism  4 , a collimating lens  5 , a wavelength selector  6 , and an objective lens  7 . The composite prism  3 , the reflective prism  4 , the collimating lens  5 , the wavelength selector  6  and the objective lens  7  are located in a common optical path (not labeled). 
   The first, second and third semiconductor modules  11 ,  12 ,  13  are positioned side by side and arranged on a same side of the composite prism  3 . The first semiconductor module  11  includes a first light source  11   a  and a first detector  11   b . The first light source  11   a  emits first light beams having a first wavelength of 405 nm, which is suitable for a first optical disk (not shown) such as an HD-DVD. The first detector  11   b  is used to receive the first light beams reflected from the first optical disk. The second semiconductor module  12  includes a second light source  12   a  and a second detector  12   b . The second light source  12   a  emits second light beams having a second wavelength of 650 nm, which is suitable for a second optical disk (not shown) such as a DVD. The second detector  12   b  is used to receive the second light beams reflected from the second optical disk. The third semiconductor module  13  includes a third light source  13   a  and a third detector  13   b . The third light source  13   a  emits third light beams having a third wavelength of about 780 nm, which is suitable for a third optical disk (not shown) such as a CD. The third detector  13   b  is used to receive the third light beams reflected from the third optical disk. 
   The first, second, and third diffraction elements  21 ,  22 ,  23  are located respectively between the first, second, third semiconductor modules  11 ,  12 ,  13  and the composite prism  3  (described as below) and are opposite to the first, second and third light sources  11   a ,  12   a ,  13   a  respectively. 
   Referring also to  FIG. 3 , the composite prism  3  includes first, second and third prisms  31 ,  32 ,  33 . The first and second prisms  31 ,  32  are located on a same side of the third prism  33 . The first prism  31  has three first surfaces labeled  310 ,  311 ,  312 , and a first interface  313 . An angle between the first surfaces  310  and  312  is approximately 45°. The first interface  313  is parallel to the first surface  312 . The first interface  313  has a function of selectively reflecting light beams or permitting light beams to pass therethrough, according to the different wavelengths of the light beams. The first surface  310  is divided into first and second regions  310   a  and  310   b . The second region  310   b  includes an aspherical surface, which has functions of compensating optical aberration of and collimating the second light beams emitted from the second light source  12   a . The second prism  32  has two second surfaces  320 ,  321 . The second surface  320  includes an aspherical surface, which has functions of compensating optical aberration of and collimating the third light beams emitted from the third light source  13   a . The third prism  33  includes three third surfaces  330 ,  331 ,  332 , and a second interface  333 . The second interface  333  is parallel to the third surface  332 . The second interface  333  has a function of selectively reflecting light beams or permitting light beams to pass therethrough, according to the different wavelengths of the light beams. 
   Referring also to  FIG. 4 , the reflective prism  4  is a penta prism, which includes five fourth surfaces  40 ,  42 ,  44 ,  46 ,  48 . The fourth surface  40  is perpendicular to the fourth surface  42 . An angle between respective adjacent fourth surfaces  42  and  44 ,  44  and  46 ,  46  and  48 , and  48  and  40  is 112.5° in each case. In addition, the fourth surfaces  44  and  48  have reflective films thereon. Light is internally reflected by the fourth surfaces  44  and  48 , and then propagates out from the fourth surface  42  into the collimating lens  5 . The net effect is that incoming light entering the fourth surface  40  is deviated 90° by the reflective prism  4 . The collimating lens  5  may be directly attached to the fourth surface  42  of the reflective prism  4 , as illustrated. Alternatively, the collimating lens  5  may be spaced from the fourth surface  42 . 
   Referring also to  FIGS. 5 and 6 , the wavelength selector  6  is located between the collimating lens  5  and the objective lens  7 . The wavelength selector  6  has three concentric portions A, B, C, in that order from a center to a periphery of the wavelength selector  6 . The portion A permits light beams of all wavelengths, such as the first, second and third light beams, to propagate therethrough. The portion B only permits light beams with short and intermediate wavelengths, such as the first and second light beams, to propagate therethrough. The portion C only permits light beams with short wavelengths, such as the first light beams, to propagate therethrough. 
   In the present embodiment, both the collimating lens  5  and the objective lens  7  have optical parameters corresponding to the first wavelength for the first optical disk such as the HD-DVD. 
   When recording information on and/or reproducing information from the first optical disk, the first light beams with the first wavelength of 405 nm emitted by the first light source  11   a  propagate through the first diffraction element  21 , are incident on the first region  310   a , and then pass through the first interface  313  and the first surface  311  of the first prism  31  in sequence. The first light beams propagate to the third surface  330  of the third prism  33 , and propagate through the second interface  333  and the third surface  331  in turn. Then the first light beams are incident on the fourth surface  40  of the reflective prism  4 , are reflected by the fourth surfaces  44 ,  48 , and then propagate to the collimating lens  5 . The collimating lens  5  collimates the first light beams into parallel light beams, and directs the first light beams toward the first optical disk. After propagating through the collimating lens  5 , the first light beams are incident on the wavelength selector  6 . The wavelength selector  6  does not block any of the first light beams, so that the first light beams completely propagate through the wavelength selector  6  and are incident on the objective lens  7 . The first light beams are converged to a light spot (not labeled) on the first optical disk by the objective lens  7 . The first optical disk reflects the first light beams, and the first light beams follow the foregoing optical path. Eventually, the first light beams are refracted by the first diffraction element  21  to the first detector  11   b . The first detector  11   b  converts the first light beams to electrical signals. After this, an electrical signal processor  210  of the information recording and/or reproducing apparatus  200  receives electrical signals and obtains desired information. Furthermore, a drive mechanism  220  of the information recording and/or reproducing apparatus  200  changes a relative position between the first optical disk and the optical pickup system  100 , also based on electrical signals output from the optical pickup system  100 . 
   In the above-described first optical path from the first light source  11   a  to the objective lens  7 , parameters of all the components are in accord with the first optical disk. The objective lens  7  matches the parameters of the first optical disk, such as the wavelength, the numerical aperture and the thickness of the protective layer of the first optical disk. Therefore, the objective lens  7  helps prevent optical aberration from occurring in the optical pickup system  100 . Because the first light beams undergo two reflections in the reflective prism  4 , the optical length of the optical pickup system  100  is shortened. Therefore the size of the optical pickup system  100  is compact. 
   When recording information on and/or reproducing information from the second optical disk, the second light beams with the second wavelength of 650 nm emitted by the second light source  12   a  propagate through the second diffraction element  22 , are incident on the second region  310   b , and then propagate to the first surface  312 , the first interface  313  and the first surface  311  of the first prism  31  in sequence. The second region  310   b  condenses the second light beams and compensates optical aberration. The second light beams propagate to the third surface  330  of the third prism  33 , and pass through the second interface  333  and third surface  331  in sequence. Then the second light beams propagate to the fourth surface  40  of the reflective prism  4 , are reflected by the fourth surfaces  44 ,  48 , and propagate to the collimating lens  5 . The collimating lens  5  collimates the second light beams into parallel light beams, and directs the second light beams toward the second optical disk. The second light beams are then incident on the wavelength selector  6 . The portions A, B of the wavelength selector  6  do not block the second light beams, but the portion C does. Accordingly, the second light beams can partially propagate through the wavelength selector  6 . The second light beams are converged to a light spot (not labeled) on the second optical disk by the objective lens  7 . The second optical disk reflects the second light beams, and the second light beams follow the foregoing optical path. Eventually, the second light beams are refracted by the second diffraction element  22  to the second detector  12   b . The second detector  12   b  converts the second light beams to electrical signals. After this, the electrical signal processor  210  of the information recording and/or reproducing apparatus  200  receives electrical signals and obtains desired information. Furthermore, the drive mechanism  220  of the information recording and/or reproducing apparatus  200  changes a relative position between the second optical disk and the optical pickup system  100 , also based on electrical signals output from the optical pickup system  100 . 
   In the above-described second optical path from the second light source  12   a  to the objective lens  7 , optical aberration is significantly corrected because the second region  310   b  of the second prism  32  has an aspherical surface. The size of the optical system  100  is compact because: (i) the optical length is shortened because the second light beams undergo two reflections in each of the first prism  31  and the reflective prism  4 , and (ii) the optical components such as the composite prism  3 , the reflective prism  4 , the collimating lens  5 , the wavelength selector  6  and the objective lens  7  are shared with the first light beams used to access the first optical disk. 
   When recording information on and/or reproducing information from the third optical disk, the third light beams with the third wavelength of 780 nm emitted by the third light source  13   a  propagate through the third diffraction element  23 , are incident on the second surface  320  of the second prism  32 , and then exit from the second surface  321 . The second surface  320  condenses the third light beams and compensates optical aberration. Then the third light beams are incident on the third prism  33 , and propagate from the third surface  332 , the second interface  333 , and through the third surface  331  in sequence. Then the second light beams are incident on the fourth surface  40  of the reflective prism  4 , are reflected by the fourth surfaces  44 ,  48 , and propagate to the collimating lens  5 . The collimating lens  5  collimates the third light beams into parallel light beams, and directs the third light beams toward the third optical disk. The third light beams are then incident on the wavelength selector  6 . The portion A of the wavelength selector  6  does not block the third light beams, but the portions B, C do. Accordingly, the third light beams can partially propagate through the wavelength selector  6 . The third light beams are converged to a light spot (not labeled) on the third optical disk by the objective lens  7 . The third optical disk reflects the third light beams, and the third light beams follow the foregoing optical path. Eventually, the third light beams are refracted by the third diffraction element  23  to the third detector  13   b . The third detector  13   b  converts the third light beams to electrical signals. After this, the electrical signal processor  210  of the information recording and/or reproducing apparatus  200  receives electrical signals and obtains desired information. Furthermore, the drive mechanism  220  of the information recording and/or reproducing apparatus  200  changes a relative position between the third optical disk and the optical pickup system  100 , also based on electrical signals output from the optical pickup system  100 . 
   In the above-described third optical path from the third light source  13   a  to the objective lens  7 , optical aberration is significantly corrected, because the surface  320  of the second prism  32  is an aspherical surface. The size of the optical pickup system  100  is compact because: (i) the third light beams undergo two reflections in each of the third prism  33  and the reflective prism  4 , and (ii) the optical components such as the composite prism  3 , the reflective prism  4 , the collimating lens  5 , the wavelength selector  6  and the objective lens  7  are shared with the first and second light beams which are used to access the first and second optical disks. 
   Referring to  FIGS. 7 and 8 , an optical pickup system  100 ′ according to a second embodiment of the present invention is illustrated. The only difference between the optical pickup system  100 ′ and the optical pickup system  100  of the first embodiment is that the optical pickup system  100 ′ has a reflective prism  4 ′ instead of the reflective prism  4 . The reflective prism  4 ′ is a triangular prism which has three fourth surfaces  40 ′,  42 ′,  44 ′. Accordingly, the first, second and third light beams are reflected once in the reflective prism  4 ′. 
   Although the present invention has been described with reference to specific embodiments, it should be noted that the described embodiments are not necessarily exclusive, and that various changes and modifications may be made to the described embodiments without departing from the scope of the invention as defined by the appended claims.