Abstract:
An optical pickup for reading information from a recording medium comprises a light illuminating portion for illuminating a plurality of light beams having different wavefronts onto a recording surface of the recording medium to create a plurality of light spots including a first light spot and a second light spot. The first and second light spots at least partially overlap with each other. A detector receives light beams reflected by the recording medium. The detector has a first light-receiving surface for receiving reflected light of the first light spot and a second light-receiving surface for receiving reflected light of the second light spot.

Description:
BACKGROUND OF THE INVENTION 
     This application claims the benefit of Japanese Patent Application No. 10-31992, filed on Jan. 29, 1998, which is hereby incorporated by reference. 
     1. Field of the Invention 
     The present invention relates generally to an optical pickup and an optical disk drive for reproducing video signals, audio signals, and other data from an optical recording medium and more particularly, to an optical pickup in an optical disk drive for reproducing information on a high-density optical disk having a narrowed track pitch. 
     2. Description of the Related Art 
     In recent years, there has been demand for increasing optical disk recording density to facilitate recording vast amounts of information onto a single optical disk. For example, it would be desirable for a full motion picture for high definition television to be stored onto one disk. Attempts have been made to narrow the track on the disk (i.e., decreasing the track pitch), to increase the recording density. However, if an optical disk with a narrowed track pitch is reproduced without reducing the spot size formed on the optical disk, a crosstalk component from adjacent tracks increases, which deteriorates the signal-to-noise ratio of the reproduced signal. 
     A conventional method for removing the crosstalk is to use three beams. These beams are positioned to hit three adjacent tracks, resulting in three spots. Signals produced from the side spots are subtracted from a signal obtained from the center spot. 
     As an example, FIG. 13 illustrates a method using three beams, in which center light spot  4  is made to strike a track  2  to read information from track  2 . In this scheme, other light spots  5  and  6  are positioned to hit tracks  1  and  3 , adjacent to track  2 . The center spot  4  primarily includes a signal from track  2 , but also contains signals leaking from adjacent tracks  1  and  3 . The side spots  5  and  6  mainly contain signals from tracks  1  and  3 , respectively. In this method, the amount of light of the crosstalk component from the adjacent tracks to the light spot  4  is different from the amount of light of the signal components from tracks  1  and  3  contained in the side spots  5  and  6 . Therefore, the amount of light of the signal components contained in the side spots  5  and  6  is reduced to coincide with the crosstalk component from the adjacent tracks to the center spot  4 , and the obtained crosstalk components are subtracted from the signal component contained in the center spot  4 . 
     In FIG. 13, the X-axis direction lies in the direction of tracks and corresponds to the direction of time axis during playback. The Y-axis direction is vertical to the track direction and corresponds to the radial direction of the disk. The light spots  4 ,  5 , and  6  are positioned to hit positions offset from each other, both in the X- and Y-axis directions. Therefore, beams reflected from these spots can travel to mutually spaced detectors on a detector array (not shown). The reflected beams can be separately detected by the detectors spaced from each other. Consequently, the crosstalk component can be removed by subtracting the reflected beams of the side spots  5 ,  6  from the reflected beam originating from the center spot  4 . 
     The light spots  4 ,  5 , and  6  in FIG. 13 are spaced from each other in the direction of the time axis as well as in the radial direction. Therefore, prior to the subtractive processing for removing the crosstalk, the time offsets of the beams reflected from the light spots  4 ,  5 , and  6  must be corrected for the linear velocity of rotation. However, where certain data is sought on the optical disk, such as for a high speed scan, or where data is read at a constant rotational speed, the linear velocity is not kept constant. Therefore, when the crosstalk component is to be removed, it is necessary to correct varying time offsets of the light spots, increasing the difficulty and complexity in removing the crosstalk component. 
     Another method for removing offsets of the light spots on the time axis is to arrange the light spots  4 ,  5 , and  6  to be at the same position in the X-axis direction, but this is not effective. The light spots on the recording surface of an optical disk are focused to the diffraction limit. However, the beams reflected from the disk are not focused to the diffraction limit on the light-receiving surface of the detector because various servo signals must be obtained. Consequently, the spacing between the reflected beams on the light-receiving surface of the detector is narrower than the spacing between the light spots on the recording surface. If the track pitch on the recording surface is narrower, the light spots on the recording surface are closer and overlap with each other. In such a case, the beams reflected from the disk further overlap with each other on the light-receiving surface of the detector. In order to remove the crosstalk component, the beams reflected from the light spots must be detected separately. Since the detector cannot separate the overlapping reflected beams, it is impossible to remove the crosstalk component. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an improved optical pickup and optical disk drive that substantially obviates one or more of the problems due to the limitations and disadvantages of the related art. 
     An object of the present invention to provide an optical disk drive for use with a recording medium recorded with a narrow track pitch that removes crosstalk leaking from adjacent tracks, thus permitting signals to be read from the medium with a good signal-to-noise ratio. 
     According to one aspect of the present invention, there is provided an optical pickup having a light illuminating portion for illuminating a plurality of light beams having different wavefronts onto a recording surface of the recording medium to create a plurality of light spots, including a first light spot and a second light spot, the first and second light spots at least partially overlapping with each other, and a detector for receiving light beams reflected by the recording medium, the detector having a first light-receiving surface for receiving reflected light of the first light spot and a second light-receiving surface for receiving reflected light of the second light spot. 
     According to another aspect of the present invention, there is provided a optical disk drive having a optical pickup including a light illuminating portion for illuminating a plurality of light beams having different wavefronts onto a recording surface of the recording medium to create a plurality of light spots including a first light spot and a second light spot, the first and second light spots at least partially overlapping with each other, and a detector for receiving light beams reflected by the recording medium, the detector having a first light-receiving surface for receiving reflected light of the first light spot and a second light-receiving surface for receiving reflected light of the second light spot, and operation means for obtaining a signal representing information recorded on the recording medium based on a first output signal from the first light-receiving surface and a second output signal from the second light receiving surface. 
     Other objects and features of the invention will appear in the course of the description thereof, which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram illustrating the structure of an optical pickup in accordance with the invention; 
     FIGS. 2A and 2B are diagrams illustrating a holographic diffraction element used in the optical pickup shown in FIG. 1; 
     FIGS. 3A and 3B are diagrams illustrating light beams impinging on the optical disk; 
     FIG. 4 is a diagram illustrating a pattern of the light-receiving surface of the detector and the distribution of the reflected beam impinging on the detector; 
     FIG. 5 is a diagram illustrating another pattern of the light-receiving surface of the detector and the distribution of the reflected beam impinging on the detector; 
     FIG. 6 is a diagram illustrating a holographic diffraction element used in an optical pickup in accordance with another embodiment of the invention; 
     FIG. 7 is a diagram illustrating light beams impinging on the optical disk; 
     FIG. 8 is a diagram illustrating a pattern of the light-receiving surface of the detector and the distribution of the reflected beam impinging on the detector; 
     FIG. 9 is a diagram illustrating a holographic diffraction element for use in an optical pickup in accordance with another embodiment of the invention; 
     FIG. 10 is a diagram illustrating light beams impinging on the optical disk by the holographic diffraction element shown in FIG. 9; 
     FIG. 11 is a plan view of a detector used to detect beams reflected from the light spots shown in FIG. 10; 
     FIG. 12 is a view illustrating a still other holographic diffraction element for use in an optical pickup in accordance with the invention; and 
     FIG. 13 is a view illustrating light spots formed on an optical disk by the conventional device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is shown an optical pickup for use in an optical disk drive in accordance with the present invention. The optical pickup includes a semiconductor laser  11  as a light source, a holographic diffraction element  12  for diffracting the light beam emitted by the laser  11  into zeroth-order light and first-order light, a beam splitter  13  having a semitransparent film for both reflecting the light beam from the semiconductor laser  11  and passing the beam reflected from the optical disk  15  to guide the beams in different directions, an objective lens  14  for focusing the light beams onto the optical disk  15 , and a detector  16  for receiving the beams reflected from the optical disk  15 . 
     The light beam emitted from the semiconductor laser  11  is diffracted by the holographic diffraction element  12 , reflected by the semitransparent film of the beam splitter  13 , and focused onto the recording surface of the optical disk  15  by the objective lens  14 , creating light spots on the surface. The light beams impinging on the optical disk  15  are reflected by the recording surface and hit the detector  16  via the beam splitter  13 . With this structure, the light beams are illuminated onto the optical disk  15 , information is read from the recording surface of the optical disk  15 , and the original information is recreated by a processing circuit (not shown). 
     One example of a pattern on the holographic diffraction element  12  used in the optical pickup shown in FIG. 1 is depicted in FIGS. 2A and 2B. FIG. 2A is a plan view of the diffraction element. FIG. 2B is a side elevation of the element. The holographic diffraction element  12  is positioned such that the X-axis direction of the light beam passing through the diffraction element  12  is tangential to the optical disk  15  on the recording surface of the disk, in the longitudinal direction of the track. The Y-axis direction is in the normal direction, i.e., vertical to the longitudinal direction of the track. 
     As shown in FIG. 2A, the pattern of the holographic diffraction element  12  is composed of fringes that become denser gradually toward the ends from the center in the Y-axis direction. The holographic diffraction element  12  provided with this pattern diffracts the wavefront of the first-order light in the X-axis direction such that the light approaches parallel light, as shown in FIG.  2 B. The diffraction element  12  passes the wavefronts of the zeroth-order light and the first-order light in the Y-axis direction intact. The action of the holographic diffraction element  12  places the light source of the first-order light in the X-axis direction at point A. A virtual image of the light source of the first-order light in the Y-axis direction is located at point B. The positions of the virtual images of the light source in the X- and Y-axis directions are made different in this way. Consequently, the first-order light is a light beam having astigmatism. 
     With the holographic diffraction element  12 , the light beams create light spots  31  and  32  on the optical disk  15  as shown in FIG. 3A, which shows the shapes of the spots formed on the recording surface of the optical disk  15 . FIG. 3B illustrates the light beams impinging on the optical disk  15 . 
     As shown in FIG. 3A, data tracks  1 ,  2 , and  3  are formed substantially parallel to the X-axis direction on the recording surface of the optical disk  15 . Each track consists of an array of pits. The zeroth-order light, which is undiffracted light from the holographic diffraction element  12 , creates the light spot  31  on the recording surface. The holographic diffraction element  12  imparts astigmatism to the first-order light, which forms the light spot  32  on the recording surface. Since the light spot  31  is free of aberrations, this spot is focused close to the diffraction limit on the recording surface. 
     As shown in FIG. 3B, the focal point of the zeroth-order light  17  and the focal point of the first-order light  18  in the X-axis direction are on the recording surface  15 . On the other hand, the focal point of the first-order light  19  in the Y-axis direction is off the recording surface. Therefore, the astigmatism increases the width of the light spot  32  in the Y-axis direction on the recording surface that is at the diffraction limit of the light spot  31 . The spot  32  is elliptical and extends over three tracks. Hence, the beam reflected from the light spot  32  is higher than the beam reflected from the light spot  31  in ratio of the crosstalk component from the adjacent tracks to the total amount of light. 
     The zeroth-order light is light free of aberrations and is uniform in cross section in the X- and Y-axis directions. The zeroth-order light is focused on the recording surface. Therefore, the wavefront of the zeroth-order light, i.e., the equiphase wave surface, is a sphere centered at the recording surface. The scope of this sphere is limited by the aperture of the objective lens. For this reason, the wavefront of the light spot  31  at the focal point of the zeroth-order light is circular. The first-order light has astigmatism and so its wavefront is a sphere centered at the recording surface in the X-axis direction, the scope of the wavefront being limited by the aperture of the objective lens in the same way as the zeroth-order light. In the Y-axis direction, however, the wavefront is a sphere centered at a position deeper than the recording surface. Consequently, the wavefront of the light spot  32  of the first-order light is elliptical. That is, the shape of the wavefront of the first-order light is such that its toric surface is limited by the aperture of the objective lens. Accordingly, the intensity of the first-order light as viewed from the longitudinal direction of the light spot is distributed more widely and mildly than the intensity of the zeroth-order light. Consequently, the light spot  32  is higher than the zeroth-order light in ratio of crosstalk component from adjacent tracks to the total amount of light. 
     The beams reflected from the light spots  31  and  32  are focused by the objective lens  14 , guided to the photodetector  16 , and converted into electric signals. Since this detector  16  is positioned at the focal point in the Y-axis direction in which the major axis of the elliptical light spot  32  lies, the beam reflected from the light spot  31  is out of focus and spread widely. 
     The pattern on the light-receiving surface of the detector  16  and the distribution shape of the reflected beam hitting the detector  16  are shown in FIG.  4 . The detector  16  has three light-receiving surfaces  41 ,  42 , and  43  arrayed in the Y-axis direction. Only the beam  44  reflected from the light spot  31  is made to hit the light-receiving surfaces  41  and  42 . Beam  45  reflected from the light spot  32  and a central portion of the beam  44  reflected from the light spot  31  hit the light-receiving surface  43 . The sum of the outputs from the light-receiving surfaces  41  and  42  is applied to the positive (+) terminal of a subtractor  47 . The output from the light-receiving surface  43  is multiplied by a given integer by an amplifier  46  and applied to the negative (−) terminal of the subtractor  47  and subtracted from the sum of the outputs from the light-receiving surfaces  41  and  42 . 
     Since the amount of light of the adjacent track component contained in the light-receiving surfaces  41  and  42  is different from the amount of light of the adjacent track component contained in the light-receiving surface  43 , an amplification factor k of the amplifier  46  is selected such that the amounts of light of the crosstalk components represented by the signals applied to the positive and negative terminals of the subtractor  47  are coincident with each other. By setting the amplification factor k to its optimum value, the signal component from the center track  2  is obtained as the output of the subtractor  47  from which the adjacent crosstalk component has been removed. Although the reflected beam  44  partially hits the light-receiving surface  43 , the crosstalk component from the adjacent tracks can be reduced accurately by sufficiently focusing the reflected beam  45  and narrowing the light-receiving surface  43 . 
     The amplification factor k may be a constant value determined according to the track pitch. Alternatively, the amplification factor k may be automatically controlled at all times so as to minimize the amount of the crosstalk component. For this method, the amount of light of the crosstalk component that remains in the output from the subtractor  47  is first detected and the output from the subtractor is then fed back to the amplifier to minimize the amount of light of the crosstalk component. For example, the correlation between adjacent tracks can be detected using a method described in Japanese Unexamined Patent Publication No. H09-320200. The amount of the crosstalk component is then found, and the amplification factor k is automatically controlled at all times so as to minimize the amount of the crosstalk component. 
     The pattern on the holographic diffraction element  12  is formed into fringes that become gradually denser toward the ends from the center in the Y-axis direction, as shown in FIG.  2 A. Since it suffices to impart astigmatism to the first-order light, the fringes may become denser toward the center from the ends in the Y-axis direction. 
     Alternatively, the light-receiving surface  43  of the detector  16  shown in FIG. 4 may be split into two light-receiving surfaces  53   a  and  53   b  in the Y-axis direction as shown in FIG.  5 . The outputs from the light-receiving surfaces  53   a  and  53   b  are applied to amplifiers  56   a  and  56   b , respectively. The sum of the outputs from the amplifiers  56   a  and  56   b  are applied to the negative (−) terminal of a subtractor  57 . The sum of the outputs from the light-receiving surfaces  51  and  52  is applied to the positive (+) terminal of the subtractor  57 . This subtractor  57  produces the difference between these two inputs. In this way, the crosstalk component can be removed. 
     Where the optical disk  15  is tilted, the amount of light falling on the light-receiving surface  53   a  will be different from the amount of light falling on the light-receiving surface  53   b . If the optical disk  15  is tilted in a radial direction, the crosstalk component can be removed accurately by adjusting the ratio of the amplification factor of the amplifier  56   a  to the amplification factor of the amplifier  56   b  in accordance with the ratio of the amount of light falling on the light-receiving surface  53   a  to the amount of light falling on the light-receiving surface  53   b . Thus, both side crosstalk components in the Y-axis direction are removed independently. 
     Various known methods can be used for focusing and tracking servo signals in this embodiment. If the direction of astigmatism used for focusing is at an angle of 45 degrees to the tracking direction, a focus error signal can be obtained without being affected by a push-pull tracking error signal. With respect to the first-order light, astigmatism induced by the diffraction element and the astigmatism due to the astigmatism-generating elements for focusing are combined constructively to shift the positions of the focal lines. However, the signals can be separated and detected at the position of one focal line in the same way as in the embodiment described above. 
     Instead of the holographic diffraction element  12  described previously, a holographic diffraction element  61  formed by arranging two astigmatism-generating patterns symmetrically about the center in the Y-axis direction also can be used. The pattern on this diffraction element  61  is shown in FIG.  6 . 
     When the holographic diffraction element  61  is used, two virtual images of the light source of the first-order light which are symmetrical about the actual light source in the Y-axis direction are formed. Therefore, as shown in FIG. 7, two light spots  72  and  73  having astigmatism are formed by the first-order light on the opposite sides of the light spot  71  of the zeroth-order light on the recording surface of the optical disk  15 . 
     The beams reflected from these light spots  71 ,  72 , and  73  can be detected by a detector  81  as shown in FIG.  8 . This detector  81  has three light-receiving surfaces  82 ,  83 , and  84  arrayed in the Y-axis direction. A beam  85  reflected from the light spot  71  hits the light-receiving surface  82 . Beams  86  and  87  reflected from the light spots  72  and  73  fall on the light-receiving surfaces  83  and  84 , respectively. This detector  81  is also located at the position of the focal line in the Y-axis direction in which the major axis of the elliptical light spots  72  and  73  lies in the same way as in the detector  16  described above, and so the beam reflected from the light spot  71  is out of focus and spread widely. The output from the light-receiving surface  82  is applied to the positive (+) terminal of a subtractor  89 . The outputs from the light-receiving surfaces  83  and  84  are multiplied by a constant factor by an amplifier  88  and are applied to the negative (−) input terminal of the subtractor  89  and are then subtracted from the output from the light-receiving surface  82 . 
     In this embodiment, the amount of light of the adjacent track crosstalk component contained in the beam  85  is different from the amount of light of the adjacent track crosstalk component contained in the beams  86  and  87 . Therefore, the amplification factor k of the amplifier  88  is selected such that the amounts of light of the crosstalk components represented by the signals applied to the positive and negative terminals of the subtractor  89  are equal. By setting the amplification factor k to its optimum value, the signal component from the center track  2  is obtained as the output of the subtractor  89  from which the adjacent crosstalk component has been removed. This amplification factor k can be a constant value estimated from the stipulated track pitch. Alternatively, the crosstalk that remains in the output from the subtractor  89  may be detected, and automatic adjustment may always be made to minimize the crosstalk component. 
     If the optical disk  15  is tilted perpendicular to the track direction, the crosstalk component can be removed accurately by removing the two sides of crosstalk components in the Y-axis direction independently by the use of the two light-receiving surfaces  83  and  84 . 
     Since the outputs from the light-receiving surfaces  83  and  84  contain the two sides of crosstalk components in the Y-axis direction, the light-receiving surface  82  may be divided into two parts about the centerline in the Y-axis direction. The outputs from the light-receiving surfaces  83  and  84  may be subtracted from the outputs from the two parts of the light-receiving surface  82 . In this way, the two side crosstalk components may independently be removed. 
     Although the diffraction efficiency of the holographic diffraction element may be made uniform over the whole surface, it is preferable that the diffraction efficiency in the central portion in the Y-axis direction is lowered or the central portion does not diffract. This increases the amount of zeroth-order light, and the amount of the first-order light falling on the center track is decreased. Therefore, only the signal components from the adjacent tracks can be accepted into the first-order light. Consequently, only the crosstalk component from the adjacent tracks contained in the zeroth-order light can be removed with higher accuracy. 
     By using a blazed hologram as the holographic diffraction element, extra diffracted light can be reduced and the efficiency of utilization of light can be enhanced. In addition, optical noise due to extra diffracted light can be reduced. 
     In the description provided above, astigmatism is given to the first-order light by a holographic diffraction element. However, the present invention is not limited to this scheme. For example, the zeroth-order light and the first-order light illuminated onto the detector can be altered as different wavefronts by other methods to vary the focal point on the optical axis to separate the zeroth-order light and the first-order light on the light-receiving surface of the detector and remove the crosstalk component. 
     FIG. 9 shows an example of holographic diffraction element for producing different focal points on the optical axis by a method not utilizing astigmatism. The holographic diffraction element  91  has two patterns  92  and  93  of concentric circles that are at the same position in the X-axis direction but at different positions in the Y-axis direction. These concentric circles  92  and  93  impart convex-lens action to the first-order light. 
     The zeroth-order light and the first-order light transmitted through the holographic diffraction element  91  create three light spots  101 ,  102 , and  103  on an optical disk, as shown in FIG.  10 . The light spot  101  is attributed to the zeroth-order light and is created at the diffraction limit in the same way as in the prior art technique. The light spots  102  and  103  are created by the first-order light diffracted by the two sets of concentric circles, respectively. These spots  102  and  103  are offset with respect to the spot  101  in the Y-axis direction. Since these light spots  102  and  103  are not at the diffraction limit, they are spread more widely than the light spot  101 . 
     Beams reflected from these light spots  101 ,  102 , and  103  can be detected by a detector  111  shown in FIG.  11 . This detector  111  is located at the position of diffraction limit of the beams reflected from the light spots  102  and  103  in the X-axis direction. The detector  111  is divided into three light-receiving surfaces  112 ,  113  and  114 . A beam  115  reflected from the light spot  101  falls on the light-receiving surface  112 . Beams  116  and  117  reflected from the light spots  102  and  103 , respectively, and marginal portions of the reflected beam  115  impinge on the light-receiving surfaces  113  and  114 . Since the reflected beams  116  and  117  focused to the diffraction limit is converged and confined to narrow regions, these beams are detected by the narrow light-receiving surfaces  113  and  114 , respectively. The crosstalk can be reduced by setting the amplification factors and performing a subtractive calculation in the same way as in the above-described process. In this embodiment, however, the adjacent tracks illuminated by the light spots  102  and  103  are longer in the longitudinal direction of track than the center track illuminated by the light spot  101 . As a result, the higher-frequency components of the crosstalk cannot be fully removed. Therefore, it is desired to equalize the output signals from the light-receiving surfaces  112 ,  113 , and  114  for matching their characteristics. 
     In addition to the holographic diffraction element described above, a holographic diffraction element  121  shown in FIG. 12 can be used in removing the crosstalk component. This diffraction element  121  acts like a convex lens or a concave lens producing zeroth-order light and first-order light whose centers of optical axes are made coincident with each other. However, when this holographic diffraction element  121  is employed, higher-frequency components of the crosstalk cannot be fully removed for the same reason as for the holographic diffraction element  91 ; and so it is desirable to equalize the output signals from the corresponding light-receiving surfaces for matching their characteristics. 
     Various methods other than the embodiments described above are conceivable. For example, first-order light showing different annular zonal intensities at the focal point from those of the zeroth-order light may be illuminated. When two kinds of first-order light are used, a beam may be illuminated which has coma and whose light intensity increases toward the ends away from the zeroth-order light in the Y-axis direction. That is, the diffraction element and the detector can be shaped variously, as long as two beams different in wavefront and focal distance are made to overlap with each other and are illuminated, and as long as differently shaped images are created by the reflected beams on their respective light-receiving surfaces of the detector as two separated beams. 
     In the embodiments described above, light beams emanating from one light source are separated using a holographic diffraction element. Instead of the holographic diffraction element, a plurality of light sources may be employed. 
     As described thus far, a plurality of light beams directed to a recording medium such as an optical disk are made to overlap with each other so that cumbersome adjustments of the light beams on the time axis are unnecessary. Hence, it is easy to reduce the effects of the crosstalk component from the adjacent tracks and the S/N of the reproduced signal can be improved. In addition, the overlapping light beams can be spatially separated on the light-receiving surfaces of the detector, because the overlapping light beams have different wavefronts and focal positions on the optical axis. Consequently, where an optical disk with a narrow track pitch is played back, high-quality signals can be reproduced. In this way, the invention is quite effective in increasing the recording density of the optical disk or the like. 
     The features and advantages of the invention are apparent from the description and thus is intended by the appended claims to cover all such features and advantages of the invention. Further, because numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation as illustrated and described. Hence, all suitable modifications and equivalents may be resorted to as falling within the scope of the invention.