Abstract:
A bifocal optical pickup head is disclosed for use in a optical pickup apparatus having a single objective lens, for generating two different focuses (bifocus) to access a compact disc (CD) and a digital versatile disc (DVD), which the thickness is different. When a beam passes through an optical element disclosed in the present invention, numerical aperture is controlled and spherical aberration is reduced by the optical element due to variations in optical path and radius. Therefore, after beams having different numerical apertures are focused by an objective lens, various focuses can be obtained.

Description:
This application is a division of Ser. No. 09/262,962, filed May 4, 1999, now U.S. Pat. No. 6,236,633. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a bifocal optical pickup head, and more particularly to an optical pickup apparatus used with a single objective lens for accessing a compact disc (CD) and a digital versatile disc (DVD), which are different in thickness 
     A technique of accessing data recorded on a surface of an optical disc such as CD or DVD by an optical pickup head has been known. In reading, a laser beam is focused on a data storage surface of the optical disc and the beam reflected from the optical disc is converted to an electric signal by a photo detector. In writing, the above procedures are inverted. The specifications of CD and DVD are shown in Table 1. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                  Thickness 
                 Track pitch 
                 Wavelength 
                 NA (numerical 
               
               
                   
                 (mm) 
                 (μm) 
                 (nm) 
                 aperture) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                  CD 
                  1.2 
                 1.6  
                 780 
                 0.45 
               
               
                 DVD 
                 0.6 *2 
                 0.74 
                 635-650 
                 0.6  
               
               
                   
               
             
          
         
       
     
     As know from Table 1, in comparison with CD, DVD has a higher storage density. The differences in dimension (especially the storage density difference) result in various reading requirements for an optical pickup apparatus. To access data on a data storage surface of a CD or a DVD, an optical pickup head is a key component in the optical pickup apparatus, and optics is most critical in the optical pickup head. 
     Therefore, to access both CD and DVD by an optical pickup apparatus having a single objective lens, it is necessary to design an optics in which beam spot size is changed on the basis of two different numerical apertures. 
     To meet compatible requirement for CD and DVD, it is necessary to access both CD and DVD with a single optical pickup head, and how to obtain a bifocal optics is a critical technique. 
     Conventional techniques of accessing both CD and DVD with a bifocal optical pickup head are listed as follows: 
     1. Two objective lenses having different focal lengths corresponding to CD and DVD respectively are used, and a driving mechanism is utilized to select one of the objective lenses. Such a design has disadvantages of increased weight of an optical pickup head and increased manufacturing cost. 
     2. A diffraction element is used to achieve bifocal effect. 
     3. A holographic optical element is used for bifocus. However, it suffers from manufacturing difficulty and high manufacturing cost. 
     4. A NA controller is constructed by a liquid crystal display (LCD) shutter such that an objective lens has two NA values, thereby achieving bifocus. However, the orientation of the polarization of a laser source needs to be accurately aligned, and the LCD should be continuously powered. 
     5. An annular objective lens is used in which beams passing through center of the lens are for CD while beams passing though inner and outer rings thereof are for DVD. A similar technique is disclosed in U.S. Pat. No. 5,665,957 in which a hologram lens having a pattern thereon is used for bifocus. However, its manufacturing cost is high and mounting tolerance is low. 
     6. Two light sources and two lens are used. For example, according to U.S. Pat. No. 5,777,970, two laser sources of different wavelengths correspond to respective lenses to provide respective focal lengths and spots of respective diameters. It has disadvantages of high manufacturing cost and complicated structure. 
     7. A zooming aperture is utilized in association with a movable objective lens, as shown in U.S. Pat. No. 5,659,533 and U.S. Pat. No. 5,281,797. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a bifocal optical pickup head having advantages of simple structure, low cost, easy assembly and high tolerance. 
     According to a bifocal optical pickup head of the present invention, to access both CD and DVD, an optical element is used in association with a conventional optical pickup head comprising a laser diode, a beam splitter (or holographic optical element (HOE)), a collimator lens, an objective lens and a photo detector, without adding any other optical elements such as zooming aperture, movable objective lens or dual objective lenses. 
     Another object of the present invention is to provide an optical element adapted to a conventional optical pickup head, holographic optical pickup head or two-source optical pickup head. The optical element splits a single laser beam into two laser beams of different optical paths, which are collimated by a collimator lens and then focused by an objective lens to achieve two focuses. 
     According to the present invention, an optical element disposed in a light path is disclosed. When a beam passes through the optical element, numerical aperture is controlled and spherical aberration is reduced by the optical element due to variations in optical path and radius. Therefore, after beams having different numerical apertures are focused by an objective lens, various focuses can be obtained. 
     When a monochrome beam undergoes two different optical paths, two focuses can be obtained. According to the present invention, a cylinder or a circular recess is provided at center of a prior art optical element, such as three-beam grating or holographic optical element, to split a laser beam into two. An optical path difference is caused by the cylinder or circular recess, and NA of the optical element is controlled, such that the beam completely passing through the optical element is used for DVD and the beam passing through the cylinder or circular recess is used for CD. 
     These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an optical element of a first embodiment; 
     FIG. 2 shows a bifocal optical pickup head of the first embodiment of the present invention; 
     FIG. 3 is a partially enlarged view showing a beam focused on a DVD; 
     FIG. 4 is a partially enlarged view showing a beam focused on a CD; 
     FIG. 5 shows an optical element of a second embodiment; 
     FIG. 6 shows a bifocal optical pickup head of the second embodiment of the present invention in which the optical element is used with a conventional optical pickup head; 
     FIG. 7 shows an optical element of a third embodiment in which the optical element is provided in a three-beam grating; 
     FIG. 8 shows a bifocal optical pickup head of the third embodiment of the present invention in which the optical element is used with a holographic optical pickup head; 
     FIG. 9 shows a bifocal optical pickup head of a fourth embodiment of the present invention in which the combination of an optical element and a three-beam grating is used with a two-source optical pickup head; and 
     FIG. 10 shows a bifocal optical pickup head of a fifth embodiment of the present invention in which an optical element, a three-beam grating and a holographic optical element are used with a two-source optical pickup head. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an optical element  10  having a first refraction surface  11  and a second refraction surface  12  perpendicular to axis of a laser beam in an optical pickup head. The refraction surfaces  11  and  12  are located at different places corresponding to different optical paths and different radiuses of the laser beam. For example, a protruded cylinder  130  is formed at a side of a flat transparent body  13  such as glass plate or quartz plate having a high transmittance. Surface A of the transparent body  13  at the same side with the cylinder  130  is formed as the first refraction surface  11 , while axial end surface B of the cylinder  130  is formed as the second refraction surface  12 . The surface A and the axial end surface B are concentric such that a laser beam will be split into two beams of different optical paths when it passes through the refraction surfaces  11  and  12 . Then the beams are collimated and pass through an objective lens to form two spots of different focuses. 
     Preferably, the radial section of the cylinder  130  has a shape depending on sectional shape of the laser beam to enhance the efficiency thereof. The optical element  10  is manufactured by photolithography to perform pattern transfer, followed by etching wherein a desired etched depth can be obtained by controlling the etching time. Of course, injection molding or the likes can be employed instead. 
     FIG. 2 shows a bifocal optical pickup head of a first embodiment of the present invention. The bifocal optical pickup head comprises: a laser diode  20  for generating a laser beam for accessing an optical disc  8  (CD or DVD); an optical element  10  for splitting the laser beam emitted from the laser diode  20  into two beams of different optical paths; a beam splitter cube  30  for separating the two laser beams of different optical paths from a reflection beam from the optical disc  8  (CD or DVD); a collimator lens  40  for collimating the beams as a parallel beam; an objective lens  50  for focusing the parallel beam including the two laser beams of different optical paths on a data storage surface of the optical disc  8  (CD or DVD); a photo detector  61  for converting the reflection beam from the optical disc  8  (CD or DVD) into an electric signal; and a condensing lens  60  for focusing the reflection beam from the beam splitter cube  30  on the photo detector  61 . 
     As clear from FIG. 2, the laser beam for accessing the optical disc  8  (CD or DVD) is emitted from the laser diode  20  through optical element  10  where it is split into two laser beams of different optical paths. Then, these two beams pass through the collimator lens  40  and the objective lens  50 , and are focused on the data storage surface of the optical disc. These two beams passes through the first refraction surface  11  and the second refraction surface  12 , respectively. One of the beams having higher efficiency is used for DVD  81  (referring to FIG.  3 ), while the other which passes through only the cylinder  130  is used for CD  82  (referring to FIG.  4 ). Therefore, by the aid of the optical element  10  of the present invention, to access both CD  82  and DVD  81 , the collimator lens  40  and the objective lens  50  are employed and no other optical devices such as zooming aperture, movable objective lens or dual objective lenses are necessary. 
     To align center of the cylinder  130  with the laser beam center can easily assemble the optical element  10 . Further, since the beam reflected from the optical disc surface passes through the condensing lens  60  to the photo detector  61  via the beam splitter cube  30  and does not returned to the optical element  10 , the assembly tolerance is higher. 
     FIG. 5 shows another preferred embodiment of an optical element  10   a  wherein a cylindrical recess  131  is formed on surface of a flat transparent body  13 . Axial end surface C of the cylindrical recess  131  acts as second refraction surface  12  like axial end surface B in FIG.  1 . 
     FIG. 6 shows a bifocal optical pickup head of a second embodiment of the present invention in which how to use an optical element  10  of the present invention with a conventional optical pickup head is disclosed. The bifocal optical pickup head comprises: a laser diode  20  for emitting a laser beam for accessing an optical disc  8  (CD or DVD); a three-beam grating  70 ; an optical element  10 ; a beam splitter  30 ; a collimator lens  40 ; an objective lens  50 ; a condensing lens  60 ; and a photo detector  61 . 
     In this embodiment, the optical element  10  can be formed as shown in FIG. 1 or FIG. 5, and can be directly integrated on back of a three-beam grating  70  or the like. 
     A holographic optical pickup head shown in FIG. 8 comprises: a laser diode  20  for emitting a laser beam for accessing an optical disc  8  (CD or DVD); an optical element  10  as shown in FIG. 1 or  5  for splitting the laser beam emitted from the laser diode  20  into two beams of different optical paths; a holographic optical element (HOE)  80  for separating the two laser beams of different optical paths from a reflection beam from the optical disc  8  (CD or DVD); a collimator lens  40  for collimating the laser beams passing through the holographic optical element  80  as a parallel beam; an objective lens  50  for focusing the parallel beam including the two laser beams of different optical paths on a data storage surface of CD  82  or DVD  81 ; and a photo detector  61  for converting the reflection beam from the holographic optical element  80  into an electric signal. 
     In FIG. 9, a two-source optical pickup head is shown in which an optical element  10  is integrated with a three-beam grating  70  (as shown in FIG.  7 ). As shown, two laser diodes, i.e., first laser diode  20   a  and second laser diode  20   b,  which generate laser beams of different wavelengths, are used to access different optical discs  8  (CD or DVD) respectively. The laser beam emitted from the first laser diode  20   a  passes through the combination of the optical element  10  and the three-beam grating  70 , a collimator lens  40  and an objective lens  50 , and then is focused on a data storage surface of the optical disc  8 . The laser beam emitted from the second laser diode  20   b  passes through a first beam splitter cube  30   a,  the collimator lens  40  and the objective lens  50 , and then is focused on the data storage surface of the optical disc  8 . The laser beam reflected from the data storage surface of the optical disc  8  passes through a second beam splitter cube  30   b  and a condensing lens  60 , and then is focused on a photo detector  61  where the laser beam representing a photo signal is converted into an electric signal. 
     In the example shown in FIG. 9, if the first laser diode  20   a  emits a laser beam having wavelength of 780 nm, the inner portion of the laser beam is used to access CD, and another laser beam having wavelength of 650 nm from the second laser diode  20   b  is used to access DVD. Therefore, an optical pickup head capable of selectively accessing CD or DVD by the objective lens  50  is formed. If the first laser diode  20   a  is a SHG laser with wavelength of 820 nm, the inner portion of the laser beam accesses CD while the laser beam of second harmonic with wavelength of 41 nm accesses DVD. 
     FIG. 10 shows a preferred embodiment in which a three-beam grating  70 , an optical element  10  and a holographic optical element  80  are combined together to form an integrated optical element  90 , which is used with a two-source optical pickup head. In this embodiment, a first laser diode  20   a  with wavelength of 780 mn and a second laser diode  20   b  with wavelength of 650 nm are for CD and DVD, respectively. 
     A laser beam emitted from the first laser diode  20   a  passes through the integrated optical element  90 , a collimator lens  40  and an objective lens  50 , and then is focused on a data storage surface of an optical disc  8 . The laser beam reflected from the data storage surface of the optical disc  8  follows the opposite optical path, and is focused by the holographic optical element (HOE)  80  in the integrated optical element  90  on a photo detector  61  where the laser beam representing a photo signal is converted into an electric signal. A laser beam emitted from the second laser diode  20   b  passes through a beam splitter cube  30   a,  the collimator lens  40  and the objective lens  50 , and then is focused on the data storage surface of the optical disc  8 . The laser beam reflected from the data storage surface of the optical disc  8  follows the opposite optical path, and is focused by a holographic optical element (HOE)  80   a  on another photo detector  61   a  where the laser beam representing a photo signal is converted into an electric signal. 
     According to the present invention, to access both CD and DVD, it is unnecessary to add any other optical elements such as zooming aperture, movable objective lens or dual objective lenses. The present invention provides advantages of simple structure, low cost, easy assembly and high tolerance. Besides, the optical element of the present invention is adapted to a conventional optical pickup head, holographic optical pickup head or two-source optical pickup head. 
     While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims. 
     
       
         
               
             
               
               
               
               
             
           
               
                   
               
               
                 INDEX 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 optical element 
                 10a 
                 optical element 
               
               
                 11 
                 first refraction surface 
                 12 
                 second refraction surface 
               
               
                 13 
                 transparent body 
                 130 
                 cylinder 
               
               
                 131 
                 cylindrical recess 
                 20 
                 laser diode 
               
               
                 20a 
                 first laser diode 
                 20b 
                 second laser diode 
               
               
                 30 
                 beam splitter cube 
                 30a 
                 first beam splitter cube 
               
               
                 30b 
                 second beam splitter cube 
                 40 
                 collimator lens 
               
               
                 50 
                 objective lens 
                 60 
                 condensing lens 
               
               
                 61 
                 photo detector 
                 61a 
                 photo detector 
               
               
                 70 
                 three-beam grating 
                  8 
                 optical disc 
               
               
                 80 
                 holographic optical element 
                 80a 
                 holographic optical element 
               
               
                 81 
                 DVD 
                 82 
                 CD 
               
               
                 90 
                 integrated optical element