Abstract:
Has an object of providing an optical pickup capable of realizing tracking control or focus control at a higher level of precision even when relative positions of a light source and a light detector are shifted due to a change in the environmental temperature, long-term deterioration or the like. The optical pickup includes a base member; a light source, fixed to the base member, for emitting a laser beam; an objective lens for collecting the laser beam onto an optical disc; a light detector, fixed to the base member, for detecting the laser beam reflected by the optical disc; fixing means for fixing the light source and/or the light detector to the base member; and a light blocking section for blocking a central portion of the laser beam in a circular shape on an optical path between the collimator lens and the light detector.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an optical pickup device capable of reading information from an optical disc and an optical disc drive including such an optical pickup device. 
         [0003]    2. Description of the Related Art 
         [0004]    Currently, optical pickup devices capable of reading information from optical discs such as a BD (Blu-ray disc), a DVD (Digital Versatile Disc) and the like are provided. 
         [0005]    Such an optical pickup device is, for example, as described in Japanese Laid-Open Patent Publication No. 2007-42230. 
         [0006]    In the optical pickup device described in Japanese Laid-Open Patent Publication No. 2007-42230, focus control and tracking control can be performed in order to deal with surface fluctuation of an optical disc. Namely, in a conventional optical pickup device, light (laser beam) emitted from a light source (light emitting diode) is collected onto an optical disc by an objective lens. Then, in the optical pickup device, the light reflected by the optical disc is detected by a light detector. The optical pickup device drives the objective lens based on a signal of the light detected by the light detector. Thus, the optical pickup device can perform focus control and tracking control. 
         [0007]    In the optical pickup device, the light source and the light detector are located on an optical base (member acting as a substrate) such that the light source and the center of the light detector (center of a light receiving element structured to have four areas) are positioned conjugate with respect to the optical system. 
         [0008]    In the optical pickup device described above, the light source and/or the light detector needs to be fixed to the optical base with very highly precise positional alignment of a micrometer order and the fixed state needs to be retained. In the above optical pickup device, the light reflected by an optical disc is received by a light receiving section of about 100 μm square on the light detector. The light receiving section is divided into a plurality of areas, and the light source and the light detector are positionally aligned such that the beam to be detected is incident on a position over a dividing line. By performing various types of arithmetic operation on the amount of light incident on each of the areas, focus control or tracking control on the optical disc is performed. Therefore, if the beam to be detected is shifted even by several micrometers on the light receiving section, the light amount balance among the divided areas is changed and thus the quality of the control signal is deteriorated. In a worst case, the control becomes impossible. Henceforth, a high level of reliability is required for the fixed state after the positional alignment. 
         [0009]    As a fixing method, laser welding or tightening with screws are conceivable. However, in order to meet the demands of simplification, cost reduction, size reduction and the like of the optical pickup device, a simpler structure is needed. Thus, fixation retaining means which has been used most commonly recently is use of an adhesive. 
         [0010]    However, where the light source and/or light detector is fixed to the optical base with an adhesive, the following problems occur. 
         [0011]    Due to a change in the environmental temperature and long-term deterioration, relative positions of the light source and the light detector may be shifted. For example, when the environmental temperature is raised, the adhesive fixed to the optical base is softened. Therefore, the position of the light source is shifted. Similarly, the position of the light detector fixed to the optical base is shifted. 
         [0012]    In such a state, the light beam is not incident on a prescribed position of the light receiving section. Therefore, the precision of tracking control and focus control of the optical pickup device is lowered. Especially, the light (laser beam) emitted from the light source has a property that the intensity thereof has a Gauss distribution (the intensity of the center of the light beam is stronger than the intensity of the periphery of the light beam). Therefore, of the light incident on the light detector, a signal of a portion including the center and the vicinity thereof has a large influence on the optical signal. Namely, the shift of the relative positions of the light source and the light detector leads to the deterioration of servo performance. 
         [0013]    This will be described in more detail. For example, it is assumed that as shown in  FIG. 1 , a laser beam is incident on a position shifted from the center of the light detector. A tracking error signal TE when this occurs can be found by the following expression (1). It is assumed that the light detector includes light receiving areas A through D. 
         [0000]        TE =( A+B )−( C+D )  (1)
 
         [0014]    In order to detect a shift of tracking of the laser beam with respect to the track of the optical disc, a signal of an X portion of the light incident on the light detector as shown in  FIG. 1  needs to be detected. The reason for this is that the light diffracted at an edge of the track of the optical disc has a meaning as a signal for tracking control (as light for determining whether there is a shift of tracking or not as shown in  FIG. 2 ). Namely, when there is a tracking shift with respect to the optical disc, the ratio in the X portion is changed. In this manner, the optical pickup device can perform tracking control. 
         [0015]    However, because the laser beam has a strong intensity at the central portion thereof, the difference of the value of (C+D) with respect to the value of (A+B) is large regardless of the intensity of the light in the X portion. In such a state, highly precise tracking cannot be performed.  FIG. 3  shows an example of signals obtained by the light detector when the light source is shifted from the center of the light detector and when the light source is not shifted from the center of the light detector. As shown in  FIG. 3 , when the light source is not shifted from the center of the light detector, the level of the signal TE is varied around 0. From this, it is understood that information in the portions other than the X portion has no specific problem. By contrast, when the source is shifted from the center of the light detector, the level of the signal TE is varied in an area away from 0. From this, it is understood that information in the portions other than the X portion influences the signal TE. 
       SUMMARY OF THE INVENTION 
       [0016]    The present invention, for solving the above-described problems, has an object of providing an optical pickup device capable of realizing tracking control or focus control at a higher level of precision even when the center of the light source and the center of the light detector are shifted from each other due to a change in the environmental temperature, long-term deterioration or the like. 
         [0017]    An optical pickup device according to the present invention includes a base member; a light source, fixed to the base member, for emitting a laser beam; an objective lens for collecting the laser beam onto an optical disc; a light detector, fixed to the base member, for detecting the laser beam reflected by the optical disc; an adhesive for fixing the light source and/or the light detector to the base member; and a light blocking section for blocking a central portion of the laser beam in a circular shape on an optical path between the collimator lens and the light detector. 
         [0018]    Owing to such a structure, the light detector section can reduce the influence of the light amount of the central portion of the light beam, and even where a relative position of the light source or the light detector is shifted, can suppress the TE signal or the FE signal from being offset. 
         [0019]    Therefore, the optical pickup device, owing to the light detector, can suppress the quality of a signal necessary for tracking control or focus control, especially the sensitivity to the shift of a relative position of the light source or the light detector, from being deteriorated. Namely, highly reliable detection of a control signal is made possible. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a view illustrating problems of the conventional art. 
           [0021]      FIG. 2  is a view illustrating problems of the conventional art. 
           [0022]      FIG. 3  is a view illustrating problems of the conventional art. 
           [0023]      FIG. 4  is a structural view illustrating an optical disc drive according to an embodiment of the present invention. 
           [0024]      FIG. 5  shows a structure and light beams for illustrating an optical pickup device according to an embodiment of the present invention. 
           [0025]      FIG. 6A  is a view illustrating a structure of a waveplate according to an embodiment of the present invention. 
           [0026]      FIG. 6B  is a view illustrating a structure of the waveplate according to an embodiment of the present invention. 
           [0027]      FIG. 7  is a view illustrating a light detector according to an embodiment of the present invention. 
           [0028]      FIG. 8  is a view illustrating deterioration in the light collecting performance of an objective lens. 
           [0029]      FIG. 9  shows a structure and light beams for illustrating an optical pickup device according to a modification of an embodiment of the present invention. 
           [0030]      FIG. 10  is a view illustrating an example of effect provided by an embodiment of the present invention. 
           [0031]      FIG. 11  is a view illustrating another example of effect provided by an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0032]    The present invention can be embodied as an optical pickup device and an optical disc drive including the optical pickup device. Hereinafter, a structure of the optical disc drive will be first described. Then, the optical pickup device mounted on the optical disc drive will be described. 
         [0033]    An optical pickup device  100  according to an embodiment will be described. The optical pickup device  100  can read information from an optical disc when being mounted on an optical disc drive. Before describing a structure of the optical pickup device  100 , a structure of the optical disc drive will be described. 
       &lt;1. Structure of the Optical Disc Drive&gt; 
       [0034]    A structure of an optical disc drive  10  will be described with reference to  FIG. 4 . The optical disc drive  10  (hereinafter, referred to as the “drive  10 ”) is usable for a personal computer, an optical disc player, an optical disc recorder or the like. 
         [0035]      FIG. 4  is a structural view of the drive  10 . The drive  10  includes the optical pickup device  100 , a transport motor  2 , a spindle motor  3 , a driving circuit  4 , a nonvolatile memory  8 , and a control section  90 . 
         [0036]    The transport motor  2  moves the optical pickup device  100  based on an instruction from the driving circuit  4 . 
         [0037]    The spindle motor  3  rotates the optical disc  200  or  300  based on an instruction from the driving circuit  4 . 
         [0038]    The driving circuit  4  controls an operation of a light source provided in the optical pickup device  100 . The driving circuit  4  also controls driving amounts such as a distance by which the optical pickup device  100  is moved by the transport motor  4 , a rotation rate of the spindle motor  3  and the like, based on instructions from the control section  90 . 
         [0039]    The nonvolatile memory  8  holds information necessary for controlling, for example, the optical pickup device  100 . 
         [0040]    The control section  90  controls an operation of the optical disc drive  10 . The control section  90  includes a pre-processing circuit  5 , a control circuit  6 , a central processing circuit  7 , and a system controller  9 . The optical pickup device  100  is electrically connected to the pre-processing circuit  5  as signal processing means and to driving circuit  4  for controlling an operation of an objective lens  108 , a light source  101  and a light source  102  of the optical pickup device  100 . Thus, the optical pickup device  100  sends and receives an electric signal to, and from, the pre-processing circuit  5  and the driving circuit  4 . 
         [0041]    Data which is optically read from the optical disc  200  ( 300 ) is converted into an electric signal by a light detector  111  ( FIG. 5 ) of the optical pickup device  100 . This electric signal is input to the pre-processing circuit  5  via signal connection means not shown. The pre-processing circuit  5  generates servo signals including a focus error signal and a tracking error signal, and also performs waveform equalization of a reproduction signal, slicing of a signal into binary data, and processing of an analog signal such as a synchronous data or the like, based on an electric signal obtained from the optical pickup device  100 . 
         [0042]    A servo signal generated by the pre-processing circuit  5  is input to the control circuit  6 . The control circuit  6  causes an optical spot of the optical pickup device  100  to follow the optical disc  200  ( 300 ) via the driving circuit  4 . The driving circuit  4  is connected to the optical pickup device  100 , the transport motor  2 , and the spindle motor  3 . The driving circuit  4  performs a series of controls including focus control and tracking control by the objective lens  108 , transfer control, spindle motor control and the like by means of digital servo. The driving circuit  4  acts to appropriately drive an actuator coil  112  (coil, magnet or the like) on the objective lens  108 , drive the transport motor  2  for moving the optical pickup device  100  to an inner part or to an outer part of the optical disc  200 , and drive the spindle motor  3  for rotating the optical disc  200  ( 300 ). 
         [0043]    The synchronous data generated by the pre-processing circuit  5  is subjected to digital signal processing by the system controller  9 , and the resultant recording/reproduction data is forwarded to a host computer via an interface not shown. The pre-processing circuit  5 , the control circuit  6  and the system controller  9  are connected to the central processing circuit  7 , and are operated by an instruction from the central processing circuit  7 . A program for defining a series of operations including the control operations described above is stored on a semiconductor device such as the nonvolatile memory  8  as firmware in advance. The control operations include an operation of rotating the optical disc  200  ( 300 ), an operation of moving the optical pickup device  100  to a target position, an operation of forming an optical spot on a target track of the optical disc  200  ( 300 ) and causing the optical spot to follow the track, and the like. Such firmware is read from the nonvolatile memory  8  by the central processing circuit  7  in accordance with the form of the necessary operation. 
         [0044]    In this specification, the pre-processing circuit  5 , the control circuit  6 , the central processing circuit  7 , the nonvolatile memory  8  and the system controller  9  will be collectively referred to as the “control section  90 ”. The pre-processing circuit  5 , the control circuit  6 , the central processing circuit  7 , the nonvolatile memory  8  and the system controller  9  can be realized in the form of a semiconductor chip (IC chip). The driving circuit  4  can be realized in the form of a driver IC. 
       &lt;2. Structure of the Optical Pickup Device&gt; 
       [0045]      FIG. 5  shows a structure of the optical pickup device  100  and also shows a route of the light beam.  FIG. 5  is a schematic view. In  FIG. 5 , a collimator lens  105  and a reflector plate  106  are located such that the polarization axes thereof are at 90 degrees to each other. 
         [0046]    The optical pickup device  100  includes a semiconductor laser  101  for BD, a semiconductor laser  102  for DVD, a plate beam splitter  103 , a cubic beam splitter  104 , the collimator lens  105 , the reflector plate  106 , a waveplate  107 , the objective lens  108 , a hologram  109 , a cylindrical lens  110 , the light detector  111 , and the actuator  112 , which are provided on an optical base  120 . The semiconductor laser  101  for BD, the semiconductor laser  102  for DVD, the light detector  111  and the like are fixed to the optical base  120  with an adhesive  130 . 
         [0047]    The semiconductor laser  101  for BD is formed of a semiconductor material and is capable of emitting a laser beam having an aligned phase and a wavelength of 405 nm. The semiconductor laser  102  for DVD is formed of a semiconductor material and is capable of emitting a laser beam having an aligned phase and a wavelength of 650 nm. 
         [0048]    The plate beam splitter  103  is a plate-like beam splitter. The plate beam splitter  103  is structured so as to, when P polarization is incident thereon, transmit the light, and, when S polarization is incident thereon, reflect the light. 
         [0049]    The cubic beam splitter  104  is a cube-shaped beam splitter. The cubic beam splitter  104  is structured so as to, when P polarization is incident thereon, transmit the light, and, when S polarization is incident thereon, reflect the light. The cubic beam splitter  104  is structured so as to transmit the laser beam from the direction of the plate beam splitter  103 . 
         [0050]    The collimator lens  105  is a lens for, when a laser beam is incident thereon from the semiconductor laser  101  for BD or the semiconductor laser  102  for DVD, converting the laser beam into collimated light. The collimator lens  105  is also a lens for, when the light reflected by the BD  200  or the DVD  300  is incident thereon, collecting the reflected light such that the reflected light is focused on the light detector  111 . 
         [0051]    The reflector plate  106  is a reflective plate for reflecting a light beam. The reflector plate  106  is structured so as to reflect light regardless of the polarization state. 
         [0052]    The waveplate  107  is a waveplate. The waveplate  107  is provided on an optical path between the collimator lens  105  and the objective lens  108 , and has a light blocking section, at a central area thereof, for blocking a central portion of the laser beam in a substantially circular shape. The light blocking section will be described later in detail. 
         [0053]    More specifically, as shown in  FIG. 6A , of an area of the waveplate  107  which is passed by the laser beam, a central circular area acts as a ½λ waveplate for the semiconductor laser  101  for BD and the semiconductor laser  102  for DVD. Still as shown in  FIG. 6A , of the area of the waveplate  107  which is passed by the laser beam, an area other than the central circular area acts as a ¼λ waveplate for the semiconductor laser  101  for BD and the semiconductor laser  102  for DVD. 
         [0054]    The above-described structure of the waveplate  107  can be realized by the following method. The following example regards a structure of the ¼λ plate for the semiconductor laser  101  for BD and the semiconductor laser  102  for DVD. The waveplate  107  acting as a ¼λ plate for the semiconductor laser  101  for BD and the semiconductor laser  102  for DVD can be obtained by bringing two quarts plates together.  FIG. 6B  shows a first quartz plate  1071  and a second quartz plate  1072 , which are the two quarts plates. 
         [0055]    The first quartz plate  1071  and the second quartz plate  1072  are brought together such that optical axes thereof are perpendicular to each other, namely, such that the fast axis and the slow axis match each other. 
         [0056]    Here, it is assumed that the thickness of the first quartz plate is t 1  and the thickness of the second quartz plate is t 2 . When t 1 =t 2 , the total phase shift is 0. By contrast, t 1 ≠t 2 , the thickness difference (t 1 −t 2 ) is the phase shift. 
         [0057]    Now, a structure of the ¼λ plate when the wavelength λ is 500 nm will be discussed. 
         [0058]    The first quartz plate  1071  has an optical axis directed to upper right at an angle of 45 degrees and the thickness t 1  thereof is 0.3135 mm. The second quartz plate  1072  has an optical axis directed to upper left at an angle of 45 degrees and the thickness t 2  thereof is 0.3 mm. In this state, the following expression is fulfilled. 
         [0000]      t 1 −t 2 =Δ(n 1 −n 2 )
 
         [0000]      *1: Δ= ¼λ=0.125 μm 
 
         [0000]      *2: n 1 −n 2 =0.00925 (refractive index of quartz when λ=500 nm)
 
         [0059]    In this manner, the ¼λ plate can be realized. The ½λ plate for the semiconductor laser  101  for BD and the semiconductor laser  102  for DVD can be realized in a similar manner. 
         [0060]    The objective lens  108  is a light collecting lens for the semiconductor laser  101  for BD and the semiconductor laser  102  for DVD. The objective lens  108  is structured such that the NA thereof for the semiconductor laser  101  for BD is 0.85 and the NA thereof for the semiconductor laser  101  for BD is 0.65. The objective lens  108  is driven by the actuator  112 . The actuator  112  drives the objective lens  108  to realize focus control and tracking control on the optical disc. 
         [0061]    The hologram  109  is a diffraction grating. The hologram  109  diffracts a part of light. The light diffracted by the hologram  109  is incident on light receiving sections  1112 ,  1113 ,  1114  and  1115  of the light detector  111  and is used as a sub signal (correcting signal) for a tracking error signal. 
         [0062]    The cylindrical lens  110  is a lens having a cylindrical face and a planar lens. 
         [0063]    The light detector  111  detects the laser beam reflected by the BD  200  (DVD  300 ).  FIG. 7  shows a structure of the light detector  111 . The light detector  111  includes light receiving sections  1111 ,  1112 ,  1113 ,  1114  and  1115  ( FIG. 7 ). 
         [0064]    The light receiving section  1111  is located such that the light transmitted through the hologram  109  and collected by the cylindrical lens  110  is incident thereon. The light receiving section  1111  includes four areas A through D. The four areas are formed of a photodiode for converting the received light into an electric signal. The signal detected by the light receiving section  1111  is input to the control section  90  of the drive  10 . 
         [0065]    Thus, the control section  90  of the drive  10  generates a focus error signal (FE signal), a main signal of tracking error (TE main signal), an RF signal and the like based on the light obtained by the light receiving section  1111 . Such signals can be found by the following expressions. 
         [0000]      FE signal=( A+C )−( B+D )
 
         [0000]      TE main signal=( A+B )−( C+D )
 
         [0000]      RF signal= A+B+C+D   
       &lt;3. Size of the Central Area of the Waveplate  107 &gt; 
       [0066]    Now, the size of the central area of the waveplate  107  is defined. The size of the central portion of the waveplate  107  needs to be defined in consideration of the RF signal. A reason for this is that where the size of the central area of the waveplate  107  is too large, there is a risk that the RF signal component is lost and so the quality of the signal is deteriorated. 
         [0067]    Therefore, in this embodiment, a structure for blocking the laser beam from the semiconductor laser  102  for DVD which has a smaller radius than the laser beam from the semiconductor laser  101  for BD will be considered. Where the size of the light blocking area is set to a level at which a certain level of quality of the RF signal for the DVD light beam is guaranteed, the ratio of the blocked portion of the BD light beam with respect to the entirety thereof is necessarily smaller than that of the DVD light beam because the radius of the BD light beam is larger than that of the DVD light beam in terms of the NA ratio. For this reason, setting the size of the light blocking area to a level at which a certain level of quality of the RF signal for the DVD is guaranteed also guarantees a certain level of quality of the RF signal for the BD. 
         [0068]    In order to provide a structure, where the radius of the laser beam from the semiconductor laser  102  for DVD is φ, the size of the central area of the waveplate  107  is designed such that the radius φm of the central area to be blocked fulfills the condition of φm/φ&lt;0.3. When the above condition is fulfilled, the problems caused by the light blocking that the reproduction signal is distorted by the loss of the RF signal component, jitter is deteriorated and the like can be suppressed. 
         [0069]    As a result of an optical simulation, the optical jitter when the light was not blocked was 2.8%, whereas the optical jitter when the ratio of the blocked light (φm/φ) was 30% was 4.7% and the optical jitter when the ratio of the blocked light was 40% was 6.5%. Reproduction jitter, which is one of main indices of the quality of a reproduction signal is determined by a combination of various factors including circuit noise, laser noise, disc noise and the like in addition to the optical jitter. Therefore, in an area in which the absolute value of the optical jitter is small, even where the jitter value is increased slightly, such an increase does not have a conspicuous influence because of being obscured by the other jitter factors. However, in general, where a single factor of the optical jitter exceeds 5%, this has a conspicuous influence on the entire reproduction jitter. Therefore, it is preferable that the optical jitter is suppressed to 5% or less. 
         [0070]    Henceforth, when the ratio of the blocked light (φm/φ) is set to 0.3 or less, the central portion of the light beam can be blocked while the performance of the waveplate  107  for practical use can be guaranteed. 
       &lt;4. Optical Path in the Optical Pickup Device&gt; 
       [0071]    A path of the light emitted from each of the semiconductor laser  101  for BD and the semiconductor laser  102  for DVD in the optical pickup device  100  will be described with reference to  FIG. 5 . 
         [0000]    &lt;4.1 Path of Light from the Semiconductor Laser  101  for BD&gt; 
         [0072]    From the semiconductor laser  101  for BD, a laser beam is emitted. The laser beam emitted from the semiconductor laser  101  for BD is incident on the cubic beam splitter  104  as S polarization. The cubic beam splitter  104  reflects the laser beam as the S polarization. The laser beam reflected by the cubic beam splitter  104  is converted into collimated light by the collimator lens  105 . The laser beam converted into the collimated light by the collimator lens  105  is incident on the reflector plate  106 . The reflector plate  106  reflects the incident laser beam. The laser beam reflected by the reflector plate  106  is transmitted through the waveplate  107 . By the waveplate  107 , the incident laser beam is caused to have a ½λ phase shift in the central portion thereof and a ¼λ phase shift in an outer peripheral portion thereof. The laser beam having such phase shifts is collected by the objective lens  108 . The light collected by the objective lens  108  is reflected by the BD  200 . 
         [0073]    Next, the laser beam reflected by the BD  200  is transmitted through the objective lens  108  to be converted into collimated light. The laser beam converted into the collimated light by the objective lens  108  is transmitted through the waveplate  107 . By the waveplate  107 , the incident laser beam is further caused to have a ½λ phase shift in the central portion thereof and a ¼λ phase shift in the outer peripheral portion thereof. Namely, as a result of being transmitted through the waveplate  107  in a forward run and a return run, the central portion of the laser beam is caused to have a λ phase shift and the outer peripheral portion of the laser beam is caused to have a ½λ phase shift. 
         [0074]    The laser beam transmitted through the waveplate  107  is transmitted through the reflector plate  106  and is incident on the cubic beam splitter  104 . The cubic beam splitter  104  has a structure for transmitting P polarization but not for transmitting S polarization of the light from the collimator lens  105  and the semiconductor laser  101  for BD. Therefore, the central portion of the laser beam, which is the S polarization, is reflected and the outer peripheral portion of the laser beam, which is the P polarization, is transmitted. 
         [0075]    As a result, the outer peripheral portion of the laser beam transmitted through the cubic beam splitter  104  is incident on the plate beam splitter  103 . The plate beam splitter  103  has a structure for transmitting all the light of the BD wavelength regardless of the type of polarization. The laser beam transmitted through the plate beam splitter  103  is transmitted through the hologram  109  and the cylindrical lens  110  and is incident on the light detector  111 . 
         [0076]    By such a structure, of the light emitted from the semiconductor laser  101  for BD, the central portion of the light is suppressed from being incident on the light detector  111 . 
         [0000]    &lt;4.2 Path of Light from the Semiconductor Laser  102  for DVD&gt; 
         [0077]    From the semiconductor laser  102  for DVD, a laser beam is emitted. The laser beam emitted from the semiconductor laser  102  for DVD is incident on the plate beam splitter  103  as S polarization. The plate beam splitter  103  reflects the laser beam as the S polarization. The laser beam reflected by the plate beam splitter  103  is transmitted through the cubic beam splitter  104  and is converted into collimated light by the collimator lens  105 . The laser beam converted into the collimated light by the collimator lens  105  is incident on the reflector plate  106 . The reflector plate  106  reflects the incident laser beam. The laser beam reflected by the reflector plate  106  is transmitted through the waveplate  107 . By the waveplate  107 , the incident laser beam is caused to have a ½λ phase shift in the central portion thereof and a ¼λ phase shift in an outer peripheral portion thereof. The laser beam having such phase shifts is collected by the objective lens  108 . The light collected by the objective lens  108  is reflected by the DVD  300 . 
         [0078]    Next, the laser beam reflected by the DVD  300  is transmitted through the objective lens  108  to be converted into collimated light. The laser beam converted into the collimated light by the objective lens  108  is transmitted through the waveplate  107 . By the waveplate  107 , the incident laser beam is further caused to have a ½λ phase shift in the central portion thereof and a ¼λ phase shift in the outer peripheral portion thereof. Namely, as a result of being transmitted through the waveplate  107  in a forward run and a return run, the central portion of the laser beam is caused to have a λ phase shift and the outer peripheral portion of the laser beam is caused to have a ½λ phase shift. 
         [0079]    The laser beam transmitted through the waveplate  107  is transmitted through the reflector plate  106  and is incident on the cubic beam splitter  104 . The cubic beam splitter  104  has a structure for transmitting all the light of the DVD wavelength regardless of the type of polarization. 
         [0080]    Of the light incident on the plate beam splitter  103 , the central portion, which is the S polarization, is reflected, and the outer peripheral portion, which is the P polarization, is transmitted. 
         [0081]    The laser beam transmitted through the plate beam splitter  103  is transmitted through the hologram  109  and the cylindrical lens  110  and is incident on the light detector  111 . 
         [0082]    By such a structure, of the light emitted from the semiconductor laser  102  for DVD, the central portion of the light is suppressed from being incident on the light detector  111 . 
         [0083]    An embodiment of the present invention has been described, but the present invention is not limited to this. Other embodiments (modifications) of the present invention will be described, hereinafter. The present invention is not limited to the following embodiments and is applicable to embodiments appropriately modified by a person of ordinary skill in the art. 
         [0084]    In the above embodiment, the central portion of the laser beam is blocked by the waveplate  107  and the cubic beam splitter  104 . However, the present invention is not limited to this, and the light may be blocked by a method shown in  FIG. 9 . 
         [0085]    Unlike the structure in  FIG. 5 , the structure in  FIG. 9  includes a ¼λ plate  117  instead of the waveplate  107  and includes a hologram (for blocking the central portion of light)  119  instead of the hologram  109 . The structures of the ¼λ plate  117  and the hologram (for blocking the central portion of light)  119  will be described. 
         [0086]    The ¼λ plate  117  is a member acting as a ¼λ waveplate for the laser beam from the semiconductor laser  101  for BD or the semiconductor laser  102  for DVD. 
         [0087]    The hologram (for blocking the central portion of light)  119  is a member for blocking the central portion of the laser beam emitted from the semiconductor laser  101  for BD or the semiconductor laser  102  for DVD and reflected by the optical disc. The hologram (for blocking the central portion of light)  119  is obtained by applying a light blocking paint on a central area of a hologram. The light blocking means is not limited to the above-mentioned form and may be in the form of a reflective film, an absorptive film, a diffracting grating or the like, or may be formed of other materials. 
         [0088]    Owing to such a structure, the central portion of the laser beam emitted from the semiconductor laser  101  for BD or the semiconductor laser  102  for DVD can be suppressed from being incident on the light detector  111 . 
         [0089]    The optical pickup devices  100  according to embodiments of the present invention have been described. The optical pickup device  100  includes the optical base  120 , the semiconductor laser  101  for BD, fixed to the optical base  120 , for emitting a laser beam, the objective lens  108  for collecting the laser beam onto the optical disc  200 , the light detector  111 , fixed to the optical base  120 , for detecting the laser beam reflected by the optical disc  200 , an adhesive for fixing the semiconductor laser  101  for BD and the light detector  111  onto the optical base, and the waveplate  107  and the cubic beam splitter  104  for blocking the central portion of the laser beam in a circular shape on the optical path from the semiconductor laser  101  for BD to the light detector  111 . 
         [0090]    Owing to such a structure, the light detector  111  can detect the portion of the laser beam other than the central portion thereof. Therefore, the optical pickup device  100 , owing to the light detector  111 , can suppress the quality of a signal necessary for tracking control or focus control, especially the sensitivity to the shift of a relative position of the light source or the light detector, from being deteriorated. Namely, highly reliable detection of a control signal is made possible. 
         [0091]    Thus, the optical pickup device can improve the reliability of tracking control or focus control. 
         [0092]    The hologram (for blocking the central portion of light)  119  blocks only the laser beam reflected by the optical disc  200 . 
         [0093]    Owing to this, the laser beam reaching the objective lens  108  from the semiconductor  101  for BD is not acted on at all. Therefore, the deterioration in the light collecting performance of the objective lens can be alleviated. 
         [0094]    When a portion of the laser beam is caused to be have a phase shift between the light source and the objective lens or when the laser beam is partially blocked, there is a problem that the light collecting performance is deteriorated. As shown in  FIG. 8 , when the central portion of the light incident on the objective lens is blocked, as compared with when the light is not blocked, the phenomenon occurs that the intensity of the central portion of the optical spot is lowered and the intensity of the side lobe is raised. This increases the influence of the inter-code interference with a previous or subsequent signal or the crosstalk with a left or right track, which lowers the signal quality. 
         [0095]      FIG. 8  also shows a spot of collected light when the central portion of the light is caused to have a phase shift different from that of the outer peripheral portion, instead of being blocked. In this case also, although the influence is alleviated as compared with when the central portion of the light is blocked, it can be seen that the intensity of the central portion of the light is lowered and the intensity of the side lobe is raised. 
         [0096]    However, with a structure in which neither the light blocking nor the phase has influence on the light on the optical path between the light source and the objective lens and the central portion of only the light beam incident on the light detector is blocked, the above-mentioned problems can be overcome. 
         [0097]    Namely, in the structure in this embodiment, it is optimum to provide a light blocking member between the light detector lens and the light detector. 
         [0098]    The waveplate  107  and the cubic beam splitter  104  block the laser beam such that, where the radius of the laser beam at a certain position is φ, the radius of the light to be blocked at that position fulfills the condition of φm/φ&lt;0.3. 
         [0099]    Such an arrangement can suppress the loss of the signal component (RF signal) for reading information recorded on an optical disc and also can suppress the quality of a signal necessary for tracking control or focus control, especially the sensitivity to the shift of a relative position of the light source or the light detector, from being deteriorated. Namely, highly reliable detection of a control signal is made possible. 
         [0100]    The optical pickup device  100  includes the semiconductor laser  102  for DVD in addition to the semiconductor laser  101  for BD. The semiconductor laser  102  for DVD emits a laser beam having a longer wavelength than the semiconductor laser  101  for BD, and the objective lens  108  is structured to collect the laser beam from the semiconductor laser  101  for BD or the semiconductor laser  102  for DVD. The waveplate  107  and the cubic beam splitter  104  blocks the laser beam from the semiconductor laser  102  for DVD such that, where the radius of the laser beam at a certain position is φ, the radius of the light to be blocked at that position fulfills the condition of φm/φ&lt;0.3. 
         [0101]    The radius of the BD light beam is larger than that of the DVD light beam in terms of the NA ratio. Therefore, the ratio of the portion of the BD light beam to be blocked with respect to the entirety thereof is necessarily smaller than the ratio in the case of the DVD. Therefore, where the size of the light blocking area is set to a level at which a certain level of quality of the RF signal for the DVD light beam is guaranteed, a certain level of quality of the RF signal for the BD light beam can also be guaranteed. 
         [0102]    In this manner, a certain level of quality of the RF signals for both of the DVD and the BD can be guaranteed and also the reliability of the control signal can be improved. 
         [0103]    Finally, experimental results regarding the effect of improving the reliability of the control signal provided by blocking the central portion of the beam to be detected will be shown. 
         [0104]      FIG. 10  shows measurement results of an FE offset amount when the central portion of the beam to be detected is blocked by 30% in terms of the ratio of the radius of the light beam. 
         [0105]    Where the central portion is not blocked, when the relative positions of the detector and the beam to be detected are shifted by 10 μm, an FE offset amount of about 15% is caused. By contrast, where the central portion of the beam is blocked by 30%, the FE offset is suppressed to about 10%. It is understood that the sensitivity of the FE offset to the shift of the relative positions of the detector and the beam to be detected is reduced by 30% as compared with the light is not blocked. In this case, no other form of deterioration was found in the FE signal quality including the S-shaped waveform or the leak of the groove-crossing signal. 
         [0106]      FIG. 11  shows measurement results of a TE offset amount of a TE main signal under similar conditions. With these results also, the sensitivity of the TE offset to the shift of the relative positions of the detector and the beam to be detected is reduced by 30% as compared with where the light is not blocked. In this case also, no other harm was found to the TE signal quality regarding the amplitude of the TE signal or the like. Regarding the reproduction jitter, which is an index of the RF signal quality, a range of practically usable values was obtained. 
         [0107]    From the above results, it is considered that where the central portion of the beam to be detected is blocked by 30% in terms of the ratio of the radius of the light beam, the reliability of a control signal against the shift of the relative positions of the light detector and the beam to be detected can be improved by about 30% while the quality of the servo signal and the RF signal is maintained in a range of practically usable values. 
         [0108]    The present invention can be realized as an optical pickup device usable for a player, a recorder and the like. The present invention can also be realized as an optical disc drive of a player, a recorder and the like having such an optical pickup device mounted thereon.