Abstract:
An optical pick-up apparatus includes: a semiconductor laser light source that generates an optical beam; a diffraction grating that divides the optical beam into a main beam and side beams; an optical element that guides the main beam and the side beams that are generated by the diffraction grating onto an optical information medium; and a photodetector that detects a signal from reflected light reflected from the optical information medium. The semiconductor laser light source is configured with a 2-wavelength semiconductor laser device in which a semiconductor laser chip that generates an optical beam having a wavelength of λ 1  and a semiconductor laser chip that generates an optical beam having a wavelength of λ 2  are integrated into one chip or are arranged in proximity to each other. The diffraction grating has one grating period Λ by which each of the optical beams having the wavelength λ 1  and the optical beam having the wavelength λ 2 , which are generated by the 2-wavelength semiconductor laser device, is divided into a main beam and side beams. The apparatus is applicable to two kinds of optical information media having different track pitches.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical pick-up apparatus that is a key component of an optical disc drive apparatus. More particularly, the present invention relates to an optical pick-up apparatus for recording, reproducing and erasing information with respect to both of a group of discs including CD, CD-ROM, CD-R and CD-RW (hereinafter called “CD group disc”) and a group of discs including DVD, DVD-ROM, DVD-RAM, DVD-R and DVD-RW (hereinafter called “DVD group disc”), which have different track pitches from each other. 
   2. Related Background Art 
   A configuration and an operation of a conventional optical pick-up apparatus will be described below, with reference to  FIG. 3 . This apparatus enables information to be recorded, reproduced and erased with respect to a CD group disc  110  and a DVD group disc  111 . Although  FIG. 3  shows both of the CD group disc  110  and the DVD group disc  111  for the purpose of illustration, as a practical matter, only one of these discs is loaded thereon. The CD group disc  110  or the DVD group disc  111  is placed on a common plane P facing an objective lens  109 . On the reverse side of the plane P, recording surfaces  110   a  and  111   a  are provided. Since the CD group disc  110  has a thickness larger than that of the DVD group disc  111 , the recording surface  110   a  of the CD group disc  110  is positioned further from the objective lens  109  than the recording surface  111   a  of the DVD group disc  111 . 
   This apparatus has an infrared semiconductor laser device  101  and a red semiconductor laser device  102 . In optical paths between the laser devices  101  and  102  and the discs  110  and  111 , 3-beam generating diffraction gratings  103  and  104 , a beam splitter  105 , a collimator lens  106 , a beam splitter  107 , a condenser lens  112 , a photodetector  113 , a mirror  108  and the objective lens  109  are arranged. The 3-beam generating diffraction gratings  103  and  104  form three beams from optical beams emitted from the infrared semiconductor laser device  101  and the red semiconductor laser device  102 , respectively. The beam splitter  105  guides the beams that have passed through the 3-beam generating diffraction gratings  103  and  104  onto the common optical axis. The collimator lens  106 , the objective lens  109  and the condenser lens  112  function as a light-gathering device. The beam splitter  107  divides the reflected light from the disc  110  or  111 . 
   This optical pick-up apparatus operates as follows: when recording, reproducing or erasing information with respect to the CD group disc  110 , the infrared semiconductor laser device  101  operates. An optical beam emitted from the infrared semiconductor laser device  101 , which is indicated by a solid line, is diffracted by the 3-beam generating diffraction grating  103  so as to be divided into three optical beams (a main beam as zero-order diffracted light and side beams as±first-order diffracted light). These three optical beams pass through the beam splitter  105 , then are converted from the divergent beams to parallel beams by the collimator lens  106 , and enter into the objective lens  109  via the mirror  108  to be focused onto the CD group disc  110 . Reflected light from the CD group disc  110  passes through the objective lens  109  and the mirror  108 , is directed to a different direction by the beam splitter  107 , and is focused onto the photodetector  113  by the condenser lens  112 . From the main beam and side beams incident on the photodetector  113 , an RF signal, a focus error signal and a tracking error signal are detected. 
   Meanwhile, when recording, reproducing or erasing information with respect to the DVD group disc  111 , the red semiconductor laser device  102  operates. An optical beam emitted from the red semiconductor laser device  102 , which is indicated by a dashed line, is diffracted by the 3-beam generating diffraction grating  104  so as to be divided into three optical beams (a main beam as zero-order diffracted light and side beams as±first-order diffracted light). These three optical beams are directed to a different direction by the beam splitter  105 , then are converted from the divergent beams to parallel beams by the collimator lens  106 , and enter into the objective lens  109  via the mirror  108  to be focused onto the DVD group disc  111 . Reflected light from the DVD group disc  111  passes through the objective lens  109  and the mirror  108 , is directed to a different direction by the beam splitter  107 , and is focused onto the photodetector  113  by the condenser lens  112 . From the main beam and side beams incident on the photodetector  113 , an RF signal, a focus error signal and a tracking error signal are detected. 
   The conventional optical pick-up apparatus shown in  FIG. 3 , however, requires the two semiconductor laser devices, the two diffraction gratings and the beam splitter  105  for guiding the emitted light from the two semiconductor laser devices to the common optical axis in order to be compatible with the CD group disc  110  and the DVD group disc  111 . Therefore, it is difficult to downsize the optical pick-up apparatus. In addition, since a large number of optical components are required, which complicates the positional adjustment among the respective optical components, the cost required for components and for adjusting the components increases significantly as compared with an optical pick-up apparatus dealing with a single type of disc. 
   SUMMARY OF THE INVENTION 
   Therefore, with the foregoing in mind, it is an object of the present invention to downsize an optical pick-up apparatus that records, reproduces or erases information with respect to a CD group disc and a DVD group disc and to reduce the cost for the apparatus. 
   An optical pick-up apparatus of the present invention includes: a semiconductor laser light source that generates an optical beam; a diffraction grating that divides the optical beam into a main beam and side beams; an optical element that guides the main beam and the side beams that are generated by the diffraction grating onto an optical information medium; and a photodetector that detects a signal from reflected light reflected from the optical information medium. The semiconductor laser light source is configured with a 2-wavelength semiconductor laser device in which a semiconductor laser chip that generates an optical beam having a wavelength of λ 1  and a semiconductor laser chip that generates an optical beam having a wavelength of λ 2  are integrated into one chip or are arranged in proximity to each other. The diffraction grating has one grating period Λ by which each of the optical beams having the wavelength λ 1  and the optical beam having the wavelength λ 2 , which are generated by the 2-wavelength semiconductor laser device, is divided into a main beam and side beams. The optical pick-up apparatus is configured so as to record or reproduce information with respect to optical information media having different track pitches and perform tracking control associated with the recording and the reproducing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an optical system of the optical pick-up apparatus according to one embodiment of the present invention. 
       FIGS. 2A and 2B  respectively show a relative positional relationship among track pitches of a CD group disc and a DVD group disc, a main beam and a side beam according to one embodiment of the present invention. 
       FIG. 3  shows an optical system of the conventional optical pick-up apparatus. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   According to the optical pick-up apparatus of the present invention, an optical beam of a first wavelength or an optical beam of a second wavelength is generated by a 2-wavelength semiconductor laser device, and is divided into a main beam and side beams by using a diffraction grating having one grating period. With this configuration, an optical pick-up apparatus that allows recording, reproducing or erasing with respect to a CD group disc and a DVD group disc can be configured with one semiconductor laser device and one 3-beam generating diffraction grating. As a result, the optical pick-up apparatus can be downsized easily. In addition, the number of optical components can be reduced and the adjustment step for the optical pick-up apparatus can be simplified, so that the cost can be reduced. 
   Preferably, in the above configuration, when the optical beam of the wavelength λ 1  is used for recording or reproducing information with respect to a first optical information medium having a track pitch of T 1 , and the optical beam of the wavelength of λ 2  is used for recording or reproducing information with respect to a second optical information medium having a track pitch of T 2 , a relationship indicated by the following formula is satisfied:
 
 L 2=(2 n +1)/(2 m +1)× T 2/ T 1× L 1
 
where L 1  denotes a distance on the first optical information medium between the main beam and the side beam that are generated by diffracting the optical beam of the wavelength of λ 1  by the diffraction grating, L 2  denotes a distance on the second optical information medium between the main beam and the side beam that are generated by diffracting the optical beam of the wavelength of λ 2  by the diffraction grating, and m and n are integers of 0 or more.
 
   Preferably, in the above configuration, assuming that an angle between (a) a line on the first optical information medium, passing through centers of the main beam and the side beam that are generated by diffracting the optical beam of the wavelength of λ 1  by the diffraction grating and (b) a line perpendicular to a track pitch direction of the first optical information medium is α 1  (0&lt;α1&lt;90), and an angle between (a) a line on the second optical information medium, passing through centers of the main beam and the side beam that are generated by diffracting the optical beam of the wavelength of λ 2  by the diffraction grating and (b) a line perpendicular to a track pitch direction of the second optical information medium is α 2  (0&lt;α2&lt;90), a relationship indicated by the following formula is satisfied: 
   
     
       
         
           
             
               
                 α1 
                 = 
                   
                 ⁢ 
                 
                   α2 
                   = 
                   
                     
                       sin 
                       
                         - 
                         1 
                       
                     
                     ⁢ 
                     
                       { 
                       
                         
                           ( 
                           
                             
                               2 
                               ⁢ 
                               m 
                             
                             + 
                             1 
                           
                           ) 
                         
                         × 
                         
                           
                             T1 
                             / 
                             2 
                           
                           / 
                           L1 
                         
                       
                       } 
                     
                   
                 
               
             
           
           
             
               
                 = 
                   
                 ⁢ 
                 
                   
                     sin 
                     
                       - 
                       1 
                     
                   
                   ⁢ 
                   
                     
                       { 
                       
                         
                           ( 
                           
                             
                               2 
                               ⁢ 
                               n 
                             
                             + 
                             1 
                           
                           ) 
                         
                         × 
                         
                           
                             T2 
                             / 
                             2 
                           
                           / 
                           L2 
                         
                       
                       } 
                     
                     . 
                   
                 
               
             
           
         
       
     
   
   In the above configurations, preferably, assuming that a distance between (a) the semiconductor laser chip of the wavelength of λ 1  or the semiconductor laser chip of the wavelength of λ 2  and (b) the diffraction grating is d, where d is a value converted with a refractive index of air, an optical magnification of recording or reproducing information with respect to the first optical information medium is β 1 , and an optical magnification of recording or reproducing information with respect to the second optical information medium is β 2 , relationships indicated by the following formulas are satisfied:
 
β1×tan{sin −1 (λ2/Λ)}−β2×(2 n +1)/(2 m +1)× T 2 /T 1×tan{sin −1 (λ1/Λ)}≈0
 
   
     
       
         
           
             
               
                 d 
                 ≈ 
                   
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                   β1 
                   × 
                   
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                   ⁢ 
                   
                     { 
                     
                       
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                 = 
                   
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                       { 
                       
                         
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                             - 
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                       } 
                     
                     . 
                   
                 
               
             
           
         
       
     
   
   In this way, by appropriately designing the position and the period of the diffraction grating, an optimum optical system can be configured with a single diffraction grating. 
   An optical pick-up apparatus according to one embodiment of the present invention will be described below, with reference to  FIGS. 1 and 2 .  FIG. 1  shows an optical system of the optical pick-up apparatus according to the present embodiment. Although  FIG. 1  shows both of a CD group disc  7  and a DVD group disc  8  for the purpose of illustration, as a practical matter, only one of these discs is loaded on a disc loading unit (not illustrated). A disc is loaded with a reverse side of its recording surface  7   a  or  8   a  facing an objective lens  6 . This apparatus employs a 2-wavelength semiconductor laser device  1 . The 2-wavelength semiconductor laser device  1  is provided with an infrared semiconductor laser chip and a red semiconductor laser chip that are mounted in an integral manner. In optical paths between the 2-wavelength semiconductor laser device  1  and the CD group disc  7  or the DVD group disc  8 , a 3-beam generating diffraction grating  2 , a collimator lens  3  and the objective lens  6  that function as a light-gathering device, a beam splitter  4 , a mirror  5 , a condenser lens  9  and a photodetector  10  are arranged. The beam splitter  4  divides reflected light from the disc. The reflected light divided by the beam splitter  4  is guided onto the photodetector  10  by the condenser lens  9 . 
   The operation of this optical pick-up apparatus will be described below. When recording, reproducing or erasing information with respect to the CD group disc  7 , the infrared semiconductor laser chip of the 2-wavelength semiconductor laser device  1  operates. An infrared optical beam (e.g., wavelength λ 1 =785 nm) emitted from the 2-wavelength semiconductor laser device  1 , which is indicated by a solid line, is diffracted by the 3-beam generating diffraction grating  2  so as to be divided into three optical beams (a main beam as zero-order diffracted light and side beams as±first-order diffracted light). These three optical beams are converted from the divergent beams to parallel beams by the collimator lens  3 , enter into the objective lens  6  via the mirror  5 , and are focused onto the CD group disc  7 . Reflected light from the CD group disc  7  passes through the objective lens  6  and the mirror  5 , is directed to a different direction by the beam splitter  4 , and is focused onto the photodetector  10  by the condenser lens  9 . From the main beam and side beams incident on the photodetector  10 , an RF signal, a focus error signal and a tracking error signal are detected. 
   Meanwhile, when recording, reproducing or erasing information with respect to the DVD group disc  8 , the red semiconductor laser chip of the 2-wavelength semiconductor laser device  1  operates. A red optical beam (e.g., wavelength λ 2 =650 nm) emitted from the 2-wavelength semiconductor laser device  1 , which is indicated by a dashed line, is diffracted by the 3-beam generating diffraction grating  2  so as to be divided into three optical beams (a main beam as zero-order diffracted light and side beams as±first-order diffracted light). These three optical beams are converted from the divergent beams to parallel beams by the collimator lens  3 , and enter into the objective lens  6  via the mirror  5  so as to be focused onto the DVD group disc  8 . Reflected light from the DVD group disc  8  passes through the objective lens  6  and the mirror  5 , is directed to a different direction by the beam splitter  4 , and is focused onto the photodetector  10  by the condenser lens  9 . From the main beam and side beams incident on the photodetector  10 , an RF signal, a focus error signal and a tracking error signal are detected. 
   In accordance with the present embodiment, by setting a period Λ and a position of the 3-beam generating diffraction grating  2  as described later, a positional relationship among the respective optical beams and tracks can be obtained as shown in  FIGS. 2A and 2B . Here, the position of the 3-beam generating diffraction grating  2  is represented by a length d, which is a value converted to air, from an emitting end face of the infrared/red semiconductor laser chip constituting the 2-wavelength semiconductor laser device  1  to the 3-beam generating diffraction grating  2 . 
     FIG. 2A  shows a relationship among a land  11  and a groove  12  on the CD group disc  7  (track pitch: T 1 ) and a main beam  13  and side beams  14  that are focused onto them.  FIG. 2B  shows a relationship among a land  15  and a groove  16  on the DVD group disc  8  (track pitch: T 2 ) and a main beam  17  and side beams  18  that are focused onto them. In either case, when the main beams  13  and  17  are positioned on the grooves  12  and  16 , the side beams  14  and  18  are positioned on the lands  11  and  15 , respectively, and also an angle α between a line passing through the centers of the main beam and the side beams and a line perpendicular to the track pitch direction becomes equal to the both groups of discs. 
   The following describes the period Λ and the arrangement of the 3-beam generating diffraction grating  2  in the present embodiment in detail. When the main beams and the side beams are located on the discs as shown in  FIGS. 2A and 2B , relationships indicated by the following formulas are satisfied, where L 1  denotes a distance between the main beam  13  and the side beam  14 , L 2  denotes a distance between the main beam  17  and the side beam  18 , and m and n are integers of 0 or more:
 
 L 1×sin α=(2 m +1)× T 1/2  (1)
 
 L 2×sin α=(2 n +1)× T 2/2  (2)
 
   Here, although the configuration of the present embodiment shown by  FIG. 2  corresponds to the case where both of m and n are 0, the description will be given using m and n in order to show a more common case also. 
   In addition, assuming that a diffracted angle of the infrared light λ 1  by the 3-beam generating diffraction grating  2  is θ 1  and a diffracted angle of the red light λ 2  by the 3-beam generating diffraction grating  2  is θ 2 , then relationships indicated by the following formulas are satisfied with the period Λ of the 3-beam generating diffraction grating  2 :
 
Λ×sin θ1=λ1  (3)
 
Λ×sin θ2=λ2  (4)
 
   Furthermore, assuming that an optical magnification of recording, reproducing or erasing information with respect to the CD group disc is β 1  and an optical magnification of recording, reproducing or erasing information with respect to the DVD group disc is β 2 , then relationships indicated by the following formulas are satisfied between the distance L 1  or L 2  and the length d from an emitting end face of the infrared/red semiconductor laser chip to the 3-beam generating diffraction grating  2 , where d is a value converted with a refractive index of air:
 
 d ×tan θ1≈β1× L 1  (5)
 
 d ×tan θ2≈β2× L 2  (6)
 
   From these formulas (1) through (6), a relationship between the period Λ (undefined value) of the 3-beam generating diffraction grating  2  and T 1 , T 2 , β 1 , β 2 , m, n, λ 1  and λ 2 , which are defined values, will be determined as follows:
 
β1×tan{sin −1 (λ2/Λ)}−β2×(2 n +1)/(2 m +1)× T 2 /T 1×tan{sin −1 (λ1/Λ)}≈0  (7)
 
   Therefore, the period Λ of the diffraction grating  2  can be determined from the formula (7). 
   Meanwhile, as for L 1 , L 2  and d, firstly an appropriate value for L 1  is selected in terms of the system of the optical drive apparatus. Then, L 2  can be determined from the formulas (1) and (2) as follows:
 
 L 2=(2 n +1)/(2 m +1)× T 2 /T 1 ×L 1  (8)
 
   Moreover, the corresponding d and α can be determined from the formulas (1), (2), (5) and (6) as follows:
 
α=sin −1 {(2 m +1)× T 1/2 /L 1}  (9)
 
   
     
       
         
           
             
               
                 
                   
                     
                       d 
                       ≈ 
                         
                       ⁢ 
                       
                         β1 
                         × 
                         
                           L1 
                           / 
                           tan 
                         
                         ⁢ 
                         
                           { 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 λ1 
                                 / 
                                 Λ 
                               
                               ) 
                             
                           
                           } 
                         
                       
                     
                   
                 
                 
                   
                     
                       = 
                         
                       ⁢ 
                       
                         β2 
                         × 
                         
                           L2 
                           / 
                           tan 
                         
                         ⁢ 
                         
                           { 
                           
                             
                               sin 
                               
                                 - 
                                 1 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 λ2 
                                 / 
                                 Λ 
                               
                               ) 
                             
                           
                           } 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 10 
                 ) 
               
             
           
         
       
     
   
   By using the above-stated relationships regarding the position and the period of the 3-beam generating diffraction grating  2 , an optical pick-up apparatus that allows information to be recorded, reproduced or erased with respect to optical discs having different track pitches can be configured with only one 2-wavelength semiconductor laser device  1  and only one 3-beam generating diffraction grating  2 . As a result, there is no need to use two semiconductor laser devices and two 3-beam generating diffraction gratings as in the conventional case, thus facilitating the miniaturization of the apparatus. 
   In addition, the number of optical components can be reduced and the adjustment step for the optical pick-up apparatus can be simplified, so that the cost can be reduced significantly. 
   The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.