JP4384248B2 - Optical pickup and optical information recording apparatus or reproducing apparatus using the same - Google Patents

Description

  The present invention records an information signal on the optical disc by a light spot irradiated on a recording surface of an optical information recording medium (for the sake of simplicity, this optical information recording medium will be referred to as an optical disc). Alternatively, the present invention relates to an optical pickup having a function of reproducing already recorded information and an optical information recording or reproducing apparatus equipped with the optical pickup.

In general, an optical pickup detects a focus error signal and a tracking error signal in order to correctly irradiate a focused spot on a predetermined recording track in an optical disc, and controls the position of the objective lens using these error signals. It has become. Of these, as a tracking error signal detection method, a three-spot method, a push-pull method, a differential push-pull method, and the like are known as representative detection methods. (Hereinafter, for the sake of simplicity, this differential push-pull method is referred to as a conventional DPP method for convenience.)
In particular, the conventional DPP method can detect a highly sensitive tracking error signal with a relatively simple optical system, and can reliably eliminate the tracking error signal offset caused by the displacement of the objective lens or the tilt of the optical disk. In recent years, this method is widely used mainly for optical pickups compatible with write-once or rewritable optical discs. (For example, see Patent Document 1)
Here, the principle of detecting a tracking error signal by the conventional DPP method will be briefly described. In an optical pickup using the conventional DPP method, for example, as shown in FIG. 2, a diffraction grating 2 is disposed in the optical path between the semiconductor laser light source 1 and the half mirror 3. The diffraction grating 2 generally has linear grating grooves that are engraved at regular intervals with a constant period as shown in FIG. 3, for example, and a zero-order light beam and ± first-order diffraction light from a light beam emitted from the semiconductor laser light source 1. A function of diffracting and separating at least three light beams in total is provided. These three luminous fluxes are individually condensed through the half mirror 3, the collimating lens 4 and the objective lens 5, respectively, and as shown in the diagram on the left side of FIG. Condensed spots 100, 101, 102 are formed. At this time, the three focused spots 100, 101, 102 are irradiated position intervals δ in the radial direction of the optical disk 10, that is, in the direction perpendicular to the guide groove 11 periodically provided on the recording surface of the optical disk 10. The diffraction grating is rotated and adjusted around the optical axis so that it coincides with the period of the guide groove 11 (for the sake of simplicity, this guide groove period will be referred to as the track pitch for the sake of simplicity). The irradiation position is adjusted by such means. Then, the optical disk reflected light flux of each condensing spot reaches the objective lens 5, the collimating lens 4, and the half mirror 3 again. A part of the light quantity passes through the half mirror and then enters the photodetector 20 through the detection lens 6.

  For example, as shown in the diagram on the right side of FIG. 4, the photodetector 20 is provided with three two- or four-divided light receiving surfaces 20a, 20b, and 20c. The detection light spots 200, 201, and 202 are formed respectively. Then, the photoelectric conversion signals from the respective light receiving surfaces are subtracted by subtracters 50a, 50b, and 50c, respectively, so that tracking error signals by a push-pull method are used for each of the detection light spots 200, 201, and 202 (the description is simplified below). For convenience, this signal is simply referred to as a push-pull signal.).

  At this time, the detection light spot corresponding to the main light spot 100 condensed on the optical disk 10 is set to 200, and the detection light spots corresponding to the sub-spots 101 and 102 are set to 201 and 202, respectively. When the push-pull signals to be expressed by Sa, Sb, and Sc are apparent from the positional relationship of the focused spots 100, 101, and 102 on the optical disc, the phases of the push-pull signals Sa, Sb, and Sc are shifted from each other by about 180 degrees. Yes. That is, the push-pull signals Sa and Sb and Sa and Sc are output with their signal waveforms in opposite phases. (Sb and Sc have the same phase.) Therefore, even if the sum signal of Sb and Sc is subtracted from this signal Sa, the signal component is not canceled and can be amplified in reverse.

  On the other hand, when the displacement of the objective lens or the tilt of the optical disk occurs, a predetermined offset component is generated in each push-pull signal due to this, but this offset component is obviously independent of the focused spot position on the disk surface. All of Sa, Sb, and Sc are generated with the same polarity. Therefore, when the subtraction process as described above is performed, only the offset component included in each push-pull signal selectively cancels out, resulting in a good tracking error signal in which only the offset component is completely removed or greatly reduced. Can be detected.

  That is, for example, the right push-pull signals Sb and Sc in FIG. 4 are added by the adder 51, and the signal after the addition is appropriately amplified by the amplifier 52, and then the main light spot 100 is pushed by the subtractor 53. By subtracting from the pull signal Sa, the offset component included in the push-pull signal Sa is completely removed or greatly reduced, and a good tracking error signal in which only the amplitude is amplified is output.

  The above is a schematic description of the detection principle of the conventional DPP method. Since this conventional DPP method is a known technique disclosed in the following Patent Document 1 and the like, further detailed explanation is omitted.

  As described above, the conventional DPP method can detect a highly sensitive tracking error signal with a relatively simple detection optical system, and completely eliminates the offset component of the tracking error signal caused by the displacement of the objective lens or the tilt of the optical disk. It has the feature that it can be removed or greatly reduced, and it is very effective as a tracking error signal detection method for optical pickups that are especially suitable for optical disks with periodic guide grooves such as write once and rewritable optical disks. is there.

  However, this conventional DPP method also has practical problems as described below. That is, in the conventional DPP method, as described above, it is necessary to match the irradiation position interval δ of the three focused spots irradiated on the optical disc to one half of the track pitch Tp of the optical disc. Accordingly, a tracking error signal cannot be detected satisfactorily for an optical disc whose track pitch deviates greatly from twice the focused spot interval δ.

  For example, there are DVD-RAM, DVD-R, DVD-RW, and the like as write-once or rewritable DVD optical disks whose demand is rapidly increasing. Of these, DVD-RAM has a track pitch of about 1. There are two types: .48 μm and a recording capacity of about 2.6 GB (RAM1) and a track pitch of about 1.23 μm and a recording capacity of about 4.7 GB (RAM2). On the other hand, the track pitch of DVD-R and DVD-RW is 0.74 μm, which is exactly half the track pitch of DVD-RAM1 disc.

  In recent years, there has been a strong demand for practical use of an optical pickup that can be applied to all of the recording or reproduction of these plural types of optical disks with a single unit. However, in the conventional DPP method, for example, if the arrangement interval of the three converging spots is adjusted so that an optimal tracking error signal can be detected by a DVD-RAM disc, the DVD-R disc or the DVD-RW disc can be used. Since the arrangement interval of the three focused spots substantially coincides with the track pitch of the disk itself, it becomes difficult to detect a tracking error signal by the conventional DPP method. In other words, when trying to support a plurality of types of optical discs having different track pitches with the same optical pickup, it becomes difficult to detect a good tracking error signal using the conventional DPP method depending on the type of the disc. There is a case.

Recently, a good tracking error signal can always be detected for an optical disc having any track pitch without relying on the disc track pitch difference while taking advantage of the conventional DPP method. A new tracking error signal detection method has been proposed. (For example, see Patent Document 2 etc.)
The tracking error signal detection method disclosed in Patent Document 2 can detect a tracking error signal with a detection optical system that is substantially the same as the conventional DPP method described above, but the only difference is that a light beam emitted from a laser light source is used. As a diffraction grating that diffracts and separates into three light beams, for example, as shown in FIG. 5, the phase of the periodic structure of the grating groove formed on the substantially half surface 27 has the periodic structure of the grating groove formed on the other approximately half surface 28. The point is that a diffraction grating 26 having a special periodic structure shifted by about 180 degrees with respect to the phase is used. The specific detection principle of this method is described in detail in the following Patent Document 2 and is omitted here. However, the diffraction grating 26 having such a special periodic structure grating groove pattern is used as the diffraction grating shown in FIG. When arranged at the same position as the grating 2, as shown in the left diagram of FIG. 6, the three focused spots irradiated on the optical disk 10 are placed on the same guide groove 11 unlike the conventional DPP system. Even if it arrange | positions so that it may irradiate, the detection corresponding to each of the push pull signal Sa detected from the detection light spot 200 corresponding to the main light spot 100 and the sub-spots 101 and 102 as shown in the figure on the right side of FIG. The push-pull signals Sb and Sc detected from the light spots 201 and 202 are output with their signal waveforms in opposite phases. (Sb and Sc have the same phase.) Accordingly, a tracking error signal exactly the same as in the conventional DPP method can be detected by an arithmetic circuit exactly the same as in the conventional DPP method.

  As described above, the above method can detect a tracking error signal by the differential push-pull method similar to the conventional DPP method even if three focused spots are simultaneously irradiated on the same guide groove. It is possible to detect a tracking error signal from which the tracking offset is satisfactorily removed without depending on the difference in track pitch.

  As described above, the new tracking error signal detection method disclosed in Patent Document 2 (in order to simplify the description and clarify the gist of the present invention, this tracking error signal detection method is referred to as an inline DPP method). Multiple types having different track pitches by using the same arithmetic circuit as the conventional DPP method and eliminating the track pitch dependency of the optical disk, which has been a problem with the conventional DPP, while taking advantage of the conventional DPP method This optical disc has the great advantage that a good tracking error signal can always be detected with the same optical pickup.

JP 7-272303 A Japanese Patent Laid-Open No. 9-81942

  However, the inline DPP method has the following serious problems in practice.

  That is, in the in-line type DPP method, when the objective lens is displaced in a direction substantially orthogonal to the recording track of the disc (hereinafter, this direction will be referred to as a tracking direction for simplification), it is detected according to the amount of displacement. There arises a problem that the amplitude of the tracking error signal is greatly reduced. Such characteristic deterioration of the tracking error signal due to the displacement of the objective lens in the tracking direction is generally referred to as the visual field characteristic of the tracking error signal. The inline DPP method has a visual field characteristic of the tracking error signal as compared with the conventional DPP method. Remarkably bad. For this reason, in order to obtain a practical tracking error signal using the conventional inline DPP method, the allowable displacement width of the objective lens must be limited to a very narrow range, which greatly impedes the practical performance of the optical pickup. It was the actual situation.

  In addition, no known document has been mentioned at all about the problem of deterioration in visual field characteristics of such an in-line DPP method, and therefore no improvement means is disclosed.

  An object of the present invention is to improve the tracking error signal visual field characteristic degradation problem which is the biggest problem in the inline DPP method while maintaining the advantages of the inline DPP method as described above, in view of the above situation, It is possible to detect a practical tracking error signal in which the tracking offset is always well removed for a plurality of types of optical discs with different track pitches, with little dependence on the track pitch difference and tracking direction displacement of the objective lens. It is to provide a new tracking error signal detection means. Another object of the present invention is to provide an optical pickup or optical information recording / reproducing apparatus using the new tracking error signal detecting means. In particular, the invention is effective for improving the versatility and reliability of the tracking error signal detecting means.

In order to achieve the above object, in the present invention, first, a laser light source, a light branching means for branching the laser light beam emitted from the laser light source into at least three light beams, and the three light beams are condensed on the recording surface of the optical disk. Are arranged so as to receive the reflected light from the optical disc of the three focused spots and the three focused spots on a light receiving surface divided into two or more parts. In an optical pickup equipped with a photo detector,
The light branching means is a diffraction grating that is divided into at least three regions of first, second, and third and has a predetermined periodic structure in each region, wherein the first region is the second region And the third region, and the second region includes a periodic structure having a phase that is approximately 90 degrees different from the phase of the periodic structure provided in the first region. In the region 3, a diffraction grating having a periodic structure having a phase that is approximately 180 degrees different from the phase of the periodic structure provided in the second region is provided.

  Further, the three focused spots irradiated on the recording surface of the optical disc are substantially zero in the direction substantially orthogonal to the guide grooves periodically provided on the recording surface of the optical disc or substantially the guide groove period. Arranged at intervals of integer multiples.

  Further, as an optical information recording apparatus or reproducing apparatus, a predetermined arithmetic process is applied to the above-described optical pickup and signals output from the respective light receiving surfaces of the photodetector provided in the optical pickup, thereby obtaining a difference. At least a tracking error signal detection device having a function of detecting a tracking error signal by a dynamic push-pull method is provided.

  Further, as an optical information recording apparatus or reproducing apparatus, the laser beam is branched into a main beam and at least two or more sub-beams, and a part or all of the push-pull signals for each of the plurality of sub-beams are added. A differential push-pull signal generation circuit that amplifies at a predetermined amplification factor K and generates a differential push-pull signal by subtracting from the push-pull signal related to the main light flux, and is provided in the optical information recording medium The amplification factor K is variable according to the difference in the arrangement interval of the guide grooves.

  As described above, according to the present invention, the amplitude deterioration of the tracking error signal accompanying the displacement of the objective lens and the remaining off-track amount are satisfactorily improved in recording and reproduction of a plurality of types of optical disks having different track pitches. Since a practical tracking error signal can be detected, an optical pickup having high versatility and high reliability and an optical information recording or reproducing apparatus using the optical pickup can be realized.

  FIG. 1 is a schematic configuration diagram showing an example of an optical pickup according to the present invention.

  In the figure, 1 is a semiconductor laser light source, 3 is a half mirror or beam splitter, 4 is a collimating lens, 5 is an objective lens, 6 is a detection lens, 10 is an optical disk, and 20 has a light receiving surface that is divided into a plurality of predetermined patterns. It is a photodetector. The objective lens 5 is fixed in the lens holder 15 and is moved in two directions, a direction along the optical axis (hereinafter referred to as a focus direction) and a tracking direction by a two-dimensional actuator 25 configured by a predetermined magnetic circuit. It comes to drive.

  Disposed between the semiconductor laser 1 and the half mirror 3 is a special diffraction grating 30 used in the present invention. The laser light emitted from the semiconductor laser 1 is diffracted and separated into at least three light beams (not shown) of zero-order light and ± first-order diffracted light by the special diffraction grating 30, and then reflected by the half mirror 3 to collimate. It reaches the objective lens 5 through the lens 4 and is focused on the recording surface of the optical disc 10 by the objective lens 5 to form three focused spots.

  At this time, the three focused spots 100, 101, and 102 focused on the recording surface of the optical disc 10 are guided grooves 11 periodically provided on the optical disc 10 as shown in the left side of FIG. Are arranged in a substantially straight line so that they are simultaneously irradiated onto exactly the same guide groove.

  Then, the disc reflected light of each condensing spot follows the optical path substantially the same as the forward path in reverse, reaches the half mirror 3 through the objective lens 5 and the collimating lens 4, and then a part of the light quantity passes through the half mirror 3. Then, the light passes through the detection lens 6 and enters a predetermined light receiving surface provided in the multi-segment photodetector 20. An objective lens position control signal such as a focus error signal and a tracking error signal and information recorded on the recording surface of the optical disk 10 from a detection signal obtained from each light receiving surface in the multi-segment photodetector 20 through a predetermined arithmetic circuit. A signal or the like is detected. At this time, as for the tracking error signal, as shown in the diagram on the right side of FIG. 8, subtracters 50a, 50b, 50c and an adder are added to the signals detected on the respective light receiving surfaces 20a, 20b, 20c of the multi-segment photodetector 20. 51, the amplifier 52, the subtractor 53, and the like are detected through the same arithmetic circuit as in the conventional DPP system.

  In the examples shown in FIGS. 4, 6, and 8, the photodetector has a light receiving surface that is divided into two at least in a direction perpendicular to the radial direction of the disk, that is, in a direction corresponding to the so-called tangential direction of the disk. The push-pull signal of each focused spot is detected from the difference between the output signals from each of the two divided light receiving surfaces. In general, the push-pull signal is generally detected from the difference signal of the output signals from the light receiving surface divided into two in the direction corresponding to the radial direction of the disk. However, since the examples shown in FIGS. 4, 6, and 8 all employ an astigmatism method as a focus error signal detection method, the light spot on the light receiving surface of the detector has an intensity distribution. Has rotated about 90 degrees around the optical axis. For this reason, the push-pull signal is detected from the difference between the output signals from the light receiving surface divided in two in the direction corresponding to the tangential direction of the disk, as shown in each figure. The features of the detector light-receiving surface arrangement when combining the focus error signal detection means using the astigmatism method and the tracking error signal detection means using the push-pull method or the differential push-pull method as described above are already known. Since the contents are not directly related to the present invention, further detailed description is omitted.

  By the way, as described above, the first embodiment of the present invention shown in FIGS. 1 and 8 has almost the same optical system configuration and optical system configuration as the conventional optical pickup using the inline DPP method already introduced by FIG. Although it is a condensing spot arrangement, the grating pattern of the special diffraction grating 30 provided between the semiconductor laser 1 and the half mirror 3 is particularly different.

  FIG. 7 is a perspective view showing a grating pattern of the diffraction grating 30 used in the present invention. A grating groove is formed on the grating surface of the diffraction grating 30 at a constant period, but the grating surface is divided into at least three regions by dividing lines orthogonal to the direction in which the grating groove is engraved as shown in the figure. It is divided. That is, the optical disk 10 is divided into at least three regions 31, 32, and 33 in a direction corresponding to the tracking direction. Of these, the central region 32 has a predetermined width W. Further, the phase of the grating grooves periodically provided in the region 31 adjacent to the region 32 is shifted by +90 degrees with respect to the phase of the grating grooves periodically provided in the central region 32. That is, the lattice grooves provided in the region 31 are displaced from the lattice grooves provided in the central region 32 by about a quarter of the lattice groove period. On the other hand, the phase of the grating grooves provided in the region 33 adjacent to the opposite side is shifted by −90 degrees with respect to the phase of the grating grooves provided in the central region 32. That is, the lattice groove provided in the region 33 is approximately 4 times the lattice groove period on the opposite side of the lattice groove provided in the region 31 with respect to the lattice groove provided in the central region 32. The arrangement is shifted by one minute. Therefore, the lattice grooves provided in the region 31 and the region 33 are out of phase with each other by 180 degrees in phase, that is, a half of the lattice groove period.

  The ± 1st-order diffracted light beam diffracted and separated by the diffraction grating 30 having such a special grating pattern is subjected to a predetermined modulation on the wavefront (equal phase surface) of the light wave. FIG. 9 shows this state. For example, when a light beam 130 having a planar wavefront (equal phase surface) 140 is incident on the diffraction grating 30, the + 1st order diffracted light beam 151 diffracted and separated by the diffraction grating 30 and − The wavefronts (equal phase planes) 161 and 162 of the first-order diffracted light beam 152 have stepped wavefront shapes that are shifted in three steps by a quarter of the wavelength λ, that is, by 90 degrees in phase, as shown in the figure. At this time, the width of the central region of the wavefront shifted in three steps is equal to the width W of the central region of the special diffraction grating 30 as shown in the figure. Further, as shown in the figure, the wavefronts 161 and 162 have a wavefront shape in which the phase relationship is reversed and the unevenness is reversed. On the other hand, the 0th-order light beam 150 that passes through the grating as it is is not affected by the grating pattern, and thus has the same planar wavefront (equal phase surface) 160 as the incident light beam 130. In the above description, the case where the light beam 140 incident on the special diffraction grating 30 is a parallel light beam having a planar wavefront (equal phase surface) has been described for the sake of simplicity. However, as in the embodiment of FIG. Of course, even when the divergent light beam emitted from the semiconductor laser light source 1 directly enters the diffraction grating 30, that is, when the light beam incident on the special diffraction grating 30 has a substantially spherical wavefront shape. The ± first-order diffracted light beams diffracted and separated by the special diffraction grating 30 are subjected to the same phase modulation as described above, and have a stepped wavefront shape shifted in three stages.

  Next, if a ± 1st order diffracted light beam with a wavefront (equal phase surface) shifted in three steps using a special diffraction grating divided into three regions as described above is used, why is the conventional inline type The reason why the visual field characteristic of the tracking error signal is improved as compared with the conventional inline DPP method even if the tracking error signal is detected by the same means as in the DPP method will be described below.

  First, the tracking error signal detection principle of the inline DPP method will be briefly described.

As described above, in the conventional in-line type DPP method, a special diffraction grating having two divided regions as shown in FIG. 5 is used as a diffraction grating for diffracting and separating the laser beam emitted from the semiconductor laser light source 1 into three beams. A diffraction grating 26 is used. When the laser beam is diffracted and separated by such a special diffraction grating, the ± 1st-order diffracted beam has a two-step stepped wavefront in which the left and right steps are shifted by a half of the wavelength λ, that is, by 180 degrees in phase. In this state, the light is condensed on the recording surface of the optical disc through a collimator lens, an objective lens, etc. while maintaining this state. (This focused spot is referred to as a sub light spot for convenience.)
On the other hand, the wavefront of the 0th order light passing through the special diffraction grating is not subjected to any phase modulation, and after passing through a collimating lens, enters the objective lens as a parallel light beam having a planar wavefront, and the ± 1st order diffracted light flux In the same manner as in FIG. (This focused spot is referred to as a main light spot for convenience.)
By the way, when the laser beam is condensed on the optical disk recording surface on which the guide groove is periodically formed, the reflected beam is diffracted by the guide groove of the disk and separated into at least a zero-order beam and ± first-order diffracted light. In the pupil plane of the objective lens, the 0th-order light and the ± 1st-order diffracted light travel in an overlapping manner while being shifted from each other by a predetermined distance, and reach the light receiving surface of the photodetector. In the region where the 0th order light and the ± 1st order diffracted light are superimposed on the light receiving surface, the light intensity becomes a predetermined light intensity due to the interference effect between the 0th order light and the ± 1st order diffracted light. At this time, the light intensity is determined depending on the relative difference in phase between wavefronts of the superposed 0th order light and ± 1st order diffracted light. Then, the phase difference between the 0th-order light wavefront and the ± 1st-order diffracted light wavefront sequentially changes depending on the relative position of the focused spot with respect to the disk guide groove.

  FIG. 10 shows the detection light spot on each light receiving surface when the disk reflected light flux of the main light spot and the sub light spot reaches the detector light receiving surface in the conventional inline type DPP method using the two-part special diffraction grating. The phase relationship between the wavefronts of the 0th order light and the ± 1st order diffracted light which are diffracted and separated by a guide groove of the disk and are shifted by a predetermined distance, and the light intensity brightness state due to the interference effect caused by the phase relationship FIG. 6 is a diagram schematically showing the classification according to the relative positional relationship between the focused spot on the optical disc and the guide groove of the disc.

  In FIG. 10, first, focusing on the main light spot, in the detection light spot 200 on the light receiving surface, the 0th-order light and the ± 1st-order diffracted light generated by diffracting and separating by the guide groove of the disc as described above are respectively a fixed distance. However, the wave fronts are both substantially flat. Then, for example, as shown in the figure, when the focused spot on the optical disc is in the middle of the disc guide groove 11 (state name (A)) or in the middle between the guide grooves (state name (C)), it is generated by the disc guide groove. The 0th-order light wavefront and the ± 1st-order diffracted lightwavefront have a phase difference of +90 degrees or −90 degrees from each other, but the phase relationship of each wavefront also changes sequentially as the condensed spot gradually shifts from there. A state in which the light spot has reached a position shifted by a quarter of the guide groove period (state name (B)) or a state in which the light spot has reached a position shifted by a quarter of the guide groove period (state) In the name (D), one of the wavefronts of the ± first-order diffracted light generated by the disk guide grooves has a phase difference of 0 degrees with respect to the wavefront of the 0th-order light and the other has a phase difference of 180 degrees. . Moreover, in the state (B) and the state (D), the relationship of the phase difference is reversed between the left and right (the overlapping region of the + 1st order diffracted light and the 0th order light and the overlapping region of the −1st order diffracted light and the 0th order light).

  For this reason, the change in light intensity between bright and dark in the overlapping region of the + 1st order diffracted light and the 0th order light and in the overlapping region of the −1st order diffracted light and the 0th order light changes continuously depending on the relative positional relationship between the focused spot on the disc and the disc guide groove. Moreover, the aspect of the change is completely reversed in the left and right overlapping regions. Therefore, a tracking error signal by a so-called push-pull method can be detected by detecting the change in light intensity separately on the left and right with a photodetector divided into at least two and outputting the differential signal.

  On the other hand, as described above, in the case of the conventional inline type DPP method, the sub-light spot has a two-step staircase wavefront in which the wavefronts in the light flux are 180 degrees out of phase with each other. Even if the light is reflected and reaches the light receiving surface of the photodetector, it is held. In exactly the same way as the main light spot, ± 1st order diffracted light is diffracted and separated by the optical disc guide groove and superimposed on the 0th order light while being shifted to the left and right by a certain distance. The light intensity changes between bright and dark depending on the relative phase shift amount between the + 1st order diffracted light wavefront and the 0th order diffracted light wavefront. Moreover, the average phase difference of the wavefront of the 0th-order light is defined as ± first-order diffracted light generated by the disk guide groove by the relative positional relationship between the focused spot on the disk and the guide groove (for example, state names (A) to (D)). This change is exactly the same as the main light spot.

  However, in the case of the sub-light spot, since it has a two-step staircase wavefront that is 180 degrees out of phase with each other as described above, the relative positional relationship between the focused spot on the disc and the guide groove (for example, the state name) Even if the average phase difference between the wavefronts of the ± 1st order diffracted light and 0th order light determined by (A) to (D)) is exactly the same as that of the main light spot, the resulting contrast in the left and right interference regions The change in light intensity is completely reversed with respect to the case of the main light spot as shown in the figure. This means the following. That is, in the case of an inline type DPP, even if the relative positions of the main light spot and the sub light spot with respect to the disc guide groove are exactly the same, the output push-pull signal is output with the signal waveform phase completely inverted. It means that. In the in-line type DPP method, the same tracking error signal as in the conventional DPP method can be obtained even if the condensing spot arrangement is such that the main light spot and the sub light spot are simultaneously irradiated onto the same guide groove as shown in FIG. This is due to the above reason.

  Next, consider the case where the objective lens is displaced by a predetermined displacement amount S in the tracking direction, that is, the radial direction of the optical disk in the inline type DPP method as described above. In this case, as a matter of course, as shown in FIG. 11, a positional shift corresponding to the distance S occurs between the boundary line between the region 27 and the region 28 in the special diffraction grating 26 and the central optical axis of the objective lens 5. . As a result, the wavefront 161 of the + 1st order or −1st order diffracted light beam diffracted and separated from the special diffraction grating 26 has a relative shift between the boundary line where the phase shifts stepwise and the central optical axis of the objective lens, which is 180 degrees. There is a difference in the widths of the left and right wavefronts with a phase difference of. Further, the wavefront 161 of the + 1st order or −1st order diffracted light beam is reflected by the disk to be converted into a wavefront 171 of the disk reflected light beam. The wavefront 171 of the disk reflected light beam has the central optical axis of the objective lens as the axis of symmetry. Since it has a wavefront shape obtained by reversing the wavefront 161 of the disk incident light beam in an axisymmetric manner, the light is also emitted in a state where the width of the left and right wavefronts with a phase difference of 180 degrees is different from that of the wavefront 161 of the disk incident light beam. Incident on the detection surface.

FIG. 12 shows that when the objective lens is displaced as described above, the widths of the left and right wave fronts having a phase difference of 180 degrees are different, resulting in an asymmetric wave front shape with respect to the central optical axis of the objective lens. When the sub-light spot formed by condensing the first-order diffracted light beam reflects the disk and reaches the light receiving surface 20b (or 20C) of the photodetector to become the detection light spot 201 (or 202), this detection light Due to the phase relationship between the wavefronts of the 0th order light and the ± 1st order diffracted light that are diffracted and separated by a guide groove of the disk in the spot 201 (or 202) and are shifted by a predetermined distance, and the interference effect caused by the phase relationship. FIG. 5 is a diagram schematically showing the light intensity contrast by classifying according to the relative positional relationship between a focused spot on an optical disc and a guide groove of the disc.
As apparent from comparison between FIG. 12 and FIG. 10, for example, in a state (state name (B) or (D)) where the condensing spot is at a position shifted by a quarter of its period with respect to the disc guide groove. In the case of FIG. 10 where there is no displacement of the objective lens, the phase difference between the wavefronts of the 0th order light and the ± 1st order diffracted light diffracted and separated by the disc guide groove over the entire area in the left and right overlapping areas is 0 degree or 180 degrees. Since it is unified, the light intensity of the part is unified to the dark part or the bright part.
On the other hand, as shown in FIG. 12, when the objective lens displacement causes a difference in the width of the wavefront having a phase difference of 180 degrees as described above, the left and right superimpositions are applied even in the case of the state name (B) or (D). Even within the region, a region where the phase difference between the wavefronts of the 0th order light and the ± 1st order diffracted light has changed from 0 degree to 180 degrees or from 180 degrees to 0 ° appears, and as a result, a part of the bright part is a dark part However, some bright parts appear in the dark part. As a result of the partial inversion of light and darkness in the region, the push-pull signal obtained from the difference signal of the signals detected from the left and right detection surfaces clearly decreases in modulation. This is the main cause of deterioration in the visual field characteristics of the tracking error signal in the conventional inline DPP method using the two-part special diffraction grating compared to the conventional DPP method.

In contrast, in the present invention, as described above, a diffraction grating having a region divided into three is used as a special diffraction grating. When such a diffraction grating is used, the + 1st order diffracted light beam that is diffracted and separated by the diffraction grating and incident on the optical disk and reflected from the optical disk is shifted in three stages by 90 degrees as shown in FIGS. Has a stepped wavefront. (Note that in FIG. 13, the wavefront of the −1st order diffracted light beam is not shown in particular, but as is clear from FIG. 9, the wavefront unevenness of the −1st order diffracted light beam is reversed with respect to the + 1st order diffracted light beam. (It becomes a wave front.)
FIG. 14 shows a case where a wavefront having a three-step staircase phase shift as described above is used in the radial direction of the objective lens as shown in FIG. + 1st order diffracted light beam whose wavefront shape is asymmetrical with respect to the central optical axis of the objective lens due to the displacement of is focused on the recording surface of the optical disc as a sub-light spot by the objective lens, and is further reflected on the optical disc to detect light 0th order light and ± 1st order diffracted light that are diffracted and separated by a guide groove of the disk and shifted by a predetermined distance in the detection light spot 201 when reaching the light receiving surface 20b of the detector The relative phase relationship between the focused spot on the optical disc and the guide groove on the optical disc shows the phase relationship of the wavefront of the optical disc and the light intensity contrast due to the interference effect caused by the phase relationship Is a view schematically showing classified according to.
14 and FIG. 12, it is clear that the interference regions on the left and right sides are different between the case of FIG. 14 using the three-part special diffraction grating of the present invention and the case of FIG. 12 using the conventional two-part special diffraction grating. The situation of light and dark light intensity distribution is clearly different. For example, in the present invention shown in FIG. 14, the region where the light and darkness is partially reversed in the state name (B) or (D) in FIG. And ± 1st order diffracted light has a phase difference of ± 90 degrees, which is an intermediate light intensity between the bright part and the dark part. In the above description, the + 1st order diffracted light beam of the ± 1st order diffracted light beams diffracted and separated by the special diffraction grating 30 is described. As a result, the push-pull signal detected from each sub-light spot using the present invention has a lower rate of modulation degree reduction with respect to the objective lens displacement than the conventional in-line DPP method using a two-split special diffraction grating. As a result, the visual field characteristic of the tracking error signal by the differential push-pull method is greatly improved.

  FIG. 15 shows the tracking direction of the objective lens, that is, the radial direction of the disc when a DVD-RAM 1 disc (recording capacity 2.6 GB, guide groove period 1.48 μm) is reproduced with an optical pickup having the following predetermined parameters. The relationship between the amount of displacement and the amplitude of a tracking error signal detected by the inline DPP method (hereinafter, for the sake of simplicity, this tracking error signal is referred to as a DPP signal), that is, the visual field characteristics of the DPP signal was obtained by computer simulation. It is a result. When the displacement of the objective lens is taken on the horizontal axis, the conventional in-line DPP method using a two-part special diffraction grating is used on the vertical axis, and the DPP signal amplitude when the objective lens displacement is zero is 100% The relative DPP signal amplitude is taken. In the figure, three types of visual field characteristics are shown for the case of using the inline DPP method of the present invention using a three-part special diffraction grating in addition to the conventional inline DPP method. These three types are obtained by changing the width W of the central region 32 of the three-part special diffraction grating 30 in the direction corresponding to the radial direction of the disk. This width W is applied to the central region in the light beam immediately before entering the objective lens. It is calculated for three types of 0.3 mm, 0.5 mm, and 0.8 mm in terms of the corresponding wavefront width.

The main parameters of the optical pickup used in the computer simulation are as follows:
(1) Laser light wavelength: 660 nm
(2) Magnification: about 6.3 times (3) Objective lens NA: about 0.64
As apparent from FIG. 15, when the in-line type DPP method of the present invention using the three-part special diffraction grating is used, the objective lens displacement is smaller than that of the conventional in-line type DPP method using the two-part special diffraction grating. It can be seen that the DPP signal amplitude in the vicinity of zero is reduced, but the DPP signal amplitude reduction ratio with respect to the objective lens displacement is also kept small. That is, the visual field characteristics are improved. It should be noted that the decrease in the amplitude of the DPP signal when the objective lens displacement is near zero is not particularly problematic as long as the output DPP signal is amplified at a predetermined amplification rate via an appropriate amplifier.

  Next, FIG. 16 shows an inline DPP method when a DVD-RAM 2 disk (recording capacity 4.7 GB, guide groove period 1.23 μm) is reproduced with an optical pickup having the same parameters as those used in the computer simulation of FIG. It is the result of having calculated | required the visual field characteristic of the obtained DPP signal amplitude by computer simulation. The vertical axis, horizontal axis, and parameters in the figure are exactly the same as those in FIG. As is apparent from FIG. 16, even when a DVD-RAM2 disk is reproduced, the inline DPP method of the present invention using the three-part special diffraction grating is more conventional than the conventional in-line DPP method using the two-part special diffraction grating. It can be seen that the visual field characteristics are clearly improved.

  As can be seen from FIGS. 15 and 16, in the in-line DPP method of the present invention using a three-part special diffraction grating, the center of the special diffraction grating can be used when reproducing either an optical disk of DVD-RAM1 or DVD-RAM2. It can be seen that the lower the width of the partial area, that is, the larger W in the figure, the smaller the rate of decrease in the DPP signal amplitude with respect to the objective lens displacement. However, when examined in detail, if the width of the central region of the special diffraction grating is extremely widened, the adverse effect that the distortion of the waveform of the DPP signal increases conversely appears. As a result of proceeding with detailed calculations and examinations, the width of the central region is converted to the width W of the wave front corresponding to the central region in the light beam immediately before entering the objective lens, and the aperture diameter of the objective lens is calculated. It has been found that it should be set between 10% and 40%, preferably between 20% and 30%.

  Although the DVD-R / RW disc is not particularly shown, even if the inline DPP method using the three-part special diffraction grating of the present invention is used, it is exactly the same as the conventional DPP method. Visual field characteristics can be obtained.

By the way, in the computer simulations shown in FIGS. 15 and 16, in order to simplify the calculation, the main light spot (the focused spot on the optical disk formed from the zeroth-order light beam that passes through the special diffraction grating as it is) and two subs Assuming that the sum of the detected light amounts of the light spot (the focused spot on the optical disc formed from ± 1st order diffracted light beams diffracted and separated by a special diffraction grating) is the same,
DPP signal = [push-pull signal obtained from the main light spot] −
[Sum of push-pull signals obtained from two front and rear sub-light spots]
As calculated. However, in the actual optical pickup, the light quantity of the main light spot and the sub light spot is clearly different due to the difference in diffraction efficiency between the 0th order light and the ± 1st order diffracted light in the special diffraction grating. There is a large difference in the amplitude of the push-pull signal obtained from the spot. Therefore, when outputting a DPP signal with an actual optical pickup, a predetermined amplification factor K is introduced into the push-pull signal obtained from the sub light spot,
DPP signal = [push-pull signal obtained from the main light spot] −
K × [sum of push-pull signals obtained from two front and rear sub-light spots]
Is generally. Moreover, if we further analyze the contents of this K value,
K = K1 × K2
Among these, K1 is expressed by the following equation.
K1 = [detected light amount obtained from the main light spot]
÷ [Sum of detected light intensity obtained from two sub-light spots before and after]
On the other hand, K2 is a predetermined compensation coefficient for compensating for the difference between the modulation degree of the push-pull signal obtained from the main light spot and the modulation degree of the push-pull signal obtained from the sub light spot.

  In general, in the conventional DPP method and the conventional in-line DPP method using a two-split special diffraction grating, the modulation degree of the push-pull signal obtained from the main light spot and the modulation degree of the push-pull signal obtained from the sub light spot are the same modulation. K2 is ignored, that is, K2 = 1 forcibly and K = K1.

  However, in the in-line DPP method using the three-split special diffraction grating of the present invention, there is a clear difference between the degree of modulation of the push-pull signal obtained from the main light spot and the degree of modulation of the push-pull signal obtained from the sub light spot. (In general, the push-pull signal modulation degree obtained from the main light spot> the push-pull signal modulation degree obtained from the sub light spot.) For this reason, it is desirable to set a predetermined value of 1 or more as K2, and K2 It has been found that the optimum value of f varies depending on the track pitch (guide groove period) of the disk.

  FIG. 17 shows the objective lens displacement amount and the residual off-track amount of the DPP signal when the DVD-RAM 1 disk is reproduced (the tracking servo pull-in point of the DPP signal detected from the optical pickup, that is, the zero cross point and the actual guide groove center. The relationship of (shift amount) is calculated by changing the value of K2. Note that the parameters of the optical pickup used for the calculation are the same as those used in the computer simulations of FIGS.

  As is apparent from FIG. 17, if K2 = 1.0 as in the conventional case, a large amount of off-track remains when the objective lens is displaced, but in proportion to K2 that is gradually increased. It can be seen that the residual off-track amount gradually decreases. However, as a result of the detailed calculation, if the value of K2 is increased too much, the distortion of the DPP signal waveform increases as in the case where the width of the central region of the three-part special diffraction grating is increased. As a result, it was found that the value of K2 could not be increased unnecessarily. As a result of examination, when a DVD-RAM 1 disc is reproduced, the optimum value of K2 is about 1.4 to 1.6.

  FIG. 18 shows the relationship between the objective lens displacement amount and the residual off-track amount of the DPP signal when the value of K2 is changed when a DVD-RAM2 disc is reproduced. Even when reproducing the DVD-RAM 2, if the value of K2 is changed, the amount of residual off-track accompanying the displacement of the objective lens changes, but in the case of the DVD-RAM 2, the optimum K2 is about 1.2 to 1.4. If K2 is set in this range, the residual off-track amount can be made substantially zero when the objective lens displacement is between +0.3 mm and -0.3 mm.

  On the other hand, when a DVD-R / RW disc (guide groove period 0.74 μm) is reproduced, it is not particularly shown, but if the optimum K2 value is set to 1.0 as before, the objective lens displacement Is between +0.3 mm and -0.3 mm, the residual off-track amount can be made substantially zero.

  Even in the case of the inline type DPP method using the conventional two-split special diffraction grating shown in FIG. 5, the amount of residual off-track can be reduced well by making the optimum K2 value variable depending on the type of the disk. be able to.

  The above is the outline of the principle of the present invention. Next, another embodiment of the present invention will be described. The embodiment shown in FIGS. 1 and 8 is arranged so that the main light spot and the two front and rear sub-light spots simultaneously irradiate the same guide groove of the optical disk, but of course not limited thereto. If the distance between the main light spot and the sub light spot in the radial direction of the disk is an integral multiple of the track pitch of the disk, that is, the guide groove period, the DPP signal can be detected by the same means as in the inline DPP method of the present invention. For example, when a DVD-RAM 1 disk is played back, as shown in FIG. 19A, the distance between the main light spot 100 and the sub light spots 101 and 102 in the disk radial direction is approximately 1.48 μm for one period of the disk guide groove 11. If they are set to match, when the DVD-R / RW disc is reproduced with this condensing spot arrangement, the main light spot 100 and the sub light spots 101 and 102 are formed in the guide groove 11 as shown in (2) of FIG. The light is condensed at the same time directly above the separate guide grooves separated by two periods, and the tracking error signal can be detected by the inline DPP method. At this time, since the guide groove period of the DVD-RAM2 disk is 1.23 μm, when the DVD-RAM2 disk is reproduced with the above-mentioned condensing spot arrangement, when the main light spot is in the middle of the guide groove, The light spot slightly deviates from immediately above the adjacent guide groove, but the amount of deviation is small, and does not cause a particularly serious problem for tracking error signal detection by the inline DPP method. Needless to say, the distance between the main light spot 100 and the sub light spots 101 and 102 in the radial direction of the disk is not limited to the above value. For example, a guide groove period of 1.48 μm for a DVD-RAM 1 disk and a DVD It is also possible to set the RAM2 disk to about 1.36 μm, which is the intermediate value of the guide groove period of 1.23 μm.

  Next, another embodiment of the present invention will be described with reference to FIG. FIG. 20 is a diagram showing another embodiment of the optical pickup using the present invention. The optical components shown in the figure are the same as those in the first embodiment of the present invention shown in FIG. The parts are given the same numbers.

  In the first embodiment of the present invention shown in FIG. 1, the special diffraction grating 30 is arranged in the optical path between the semiconductor laser light source 1 and the half mirror 3, but the special diffraction grating 30 is arranged in this position. It is not limited, and it may be placed anywhere in the optical path between the light source and the objective lens. For example, in the embodiment shown in FIG. 20, the special diffraction grating 30 is disposed immediately below the objective lens 5 and fixed in the same lens holder 15 as the objective lens 5, so that the two-dimensional actuator 25 and the objective lens 5 together. It comes to drive. With such a configuration, even if the objective lens 5 is displaced, the relative positional relationship between the objective lens 5 and the special diffraction grating 30 does not change, so that the deterioration of the visual field characteristics due to the displacement of the objective lens as described above can be further improved. .

  As shown in FIG. 20, when the special diffraction grating 30 is arranged directly below the objective lens 5, the special diffraction grating 30 emits the semiconductor laser light source 1 and the optical disk 10 together with the forward laser beam incident on the optical disk 10. The return laser beam reflected and reaching the light detector 20 also passes through. For this reason, when a diffraction grating having normal diffraction characteristics is used, the return beam is also diffracted to generate unnecessary stray light, which may deteriorate the performance of the optical pickup. In order to avoid such a problem, for example, a light beam having a specific polarization direction is diffracted with a predetermined diffraction efficiency, but a light beam having a polarization direction orthogonal to the polarization direction passes through the light beam without generating diffracted light. By using a polarizing diffraction grating having the characteristic to be used as the special diffraction grating 30 and disposing the quarter-wave plate 40 in the optical path between the special diffraction grating 30 and the objective lens 5, And means for automatically making the polarization directions of the return beam perpendicular to each other may be used.

By the way, all of the embodiments described so far are configured so that the optical components constituting the optical pickup are independently arranged, but it is a matter of course that the present invention is not limited to such an optical pickup. Absent. For example, in the embodiment shown in FIG. 21, a so-called semiconductor laser module in which the semiconductor laser light source 1, the photodetector 20, and the like are housed in the same casing 41 is used. (In addition, about each optical component shown in this figure, the same number is attached | subjected to the same optical component as the other Example of this invention shown in FIG. 1 and FIG. 20.)
In the embodiment of FIG. 21, a transparent substrate 42 having a predetermined thickness is disposed at a position corresponding to a window when a laser beam emitted from a semiconductor laser light source is emitted to the outside of the casing 41, and the inside of the casing 41 is sealed. It has stopped. The upper surface of the transparent substrate 42 has a function of separating the optical path of the outgoing light beam emitted from the semiconductor laser light source toward the optical disk 10 and the optical path of the returning light beam reflected from the optical disk 10 and guiding the backward light beam to the photodetector 20. A hologram element 43 is arranged. The special diffraction grating 30 used in the present invention is disposed on the lower surface of the transparent substrate 42.

  As described above, the optical pickup using the semiconductor laser module in which the semiconductor laser light source, the photodetector, and the like are housed in one casing has an advantage that the optical pickup can be reduced in size and thickness.

  In FIG. 21, the specific shape and arrangement of the light receiving surface of the photodetector 20 are not particularly shown, but it is a matter of course that the tracking error signal can be detected by the differential push-pull method. Any configuration of the semiconductor laser module may be used.

  Of course, the present invention is not limited to the optical pickups shown in FIGS. 1, 20, and 21, but uses tracking error signal detection means of a differential push-pull method. The present invention can be applied to any configuration of optical pickups.

  Finally, an embodiment relating to an optical information reproducing apparatus or an optical information recording / reproducing apparatus equipped with the optical pickup of the present invention is shown in FIG.

  Reference numeral 60 denotes an optical pickup having a configuration as shown in the embodiment of FIG. 1, FIG. 20, or FIG. The optical pickup 60 is provided with a mechanism capable of sliding its position in the radial direction (inner and outer peripheral directions) of the optical disc 10, and position control is performed in accordance with an access control signal from the access control circuit 72.

  A predetermined laser drive current is supplied from the laser lighting circuit 76 to the semiconductor laser light source in the optical pickup 60, and laser light is emitted with a predetermined light amount. Various servo signals and information signals detected from a predetermined photodetector in the optical pickup 60 are sent to a servo signal generation circuit 74 and an information signal reproduction circuit 75. The servo signal generation circuit 74 generates a focus error signal and a tracking error signal from these detection signals, and drives the two-dimensional actuator in the optical pickup 60 via the actuator drive circuit 73 based on the focus error signal and the tracking error signal. Control takes place. The information signal reproduction circuit 75 reproduces the information signal recorded on the optical disc 10 from the detection signal.

  Part of the signals obtained by the servo signal generation circuit 74 and the information signal reproduction circuit 75 are sent to the control circuit 70. The control circuit 70 is connected to a laser lighting circuit 76, an access control circuit 72, a spindle motor drive circuit 71, and the like. Control of the light emission amount of the semiconductor laser in the optical pickup 60, control of the access direction and position, and the optical disc 10 are performed. The rotation control of the spindle motor 77 that rotates the motor is performed. A disc discriminating circuit (not shown) for discriminating the type of the optical disc from the signals obtained by the servo signal generating circuit 74 and the information signal reproducing circuit 75 is provided in the control circuit 70. From the result, for example, the amplification gain (corresponding to K2) of the sub push-pull signal (= the push-pull signal obtained from the sub light spot) of the DPP signal generation circuit (not shown) provided in the servo signal generation circuit 74, for example. ) Etc. can be controlled automatically.

It is the schematic block diagram which showed one Example of the optical pick-up using this invention. It is the schematic block diagram which showed the example of the optical pick-up using the conventional DPP system. It is the perspective view which showed an example of the diffraction grating used by the conventional type DPP system. It is a condensing spot arrangement diagram on an optical disc and a schematic connection diagram of a detection system in a conventional DPP method. It is the perspective view which showed an example of the special diffraction grating used with an in-line type DPP system. It is a condensing spot arrangement diagram on an optical disc and a schematic connection diagram of a detection system in an inline type DPP method. It is the perspective view which showed an example of the special diffraction grating used by this invention. It is one Example of the condensing spot arrangement | positioning figure on the optical disk in this invention, and the schematic connection diagram of a detection system. It is the perspective view which showed the wavefront shape of the diffracted light beam and the transmitted light beam at the time of using the special diffraction grating of this invention. It is the schematic which showed the example of the wave front shape and intensity distribution of the light spot on the detector surface in a conventional in-line DPP system. FIG. 6 is a schematic diagram illustrating a wavefront cross-sectional shape of a ± first-order diffracted light beam when an objective lens is displaced in a conventional inline DPP method. It is the schematic which showed the example of the wave front shape and intensity distribution of the light spot on a detector surface when an objective lens is displaced by the predetermined amount by the conventional in-line DPP method. It is the schematic which drawn the wavefront cross-sectional shape of the ± 1st-order diffracted light beam when the objective lens is displaced in the present invention. It is the schematic which showed the example of the wave front shape and intensity distribution of the light spot on a detector surface when an objective lens is displaced by predetermined amount in this invention. It is the diagram which showed the relationship between the objective-lens displacement amount and DPP signal amplitude relative value at the time of reproducing | regenerating a DVD-RAM1 disk using this invention or the conventional in-line DPP system. It is the diagram which showed the relationship between the objective-lens displacement amount and DPP signal amplitude relative value at the time of reproducing | regenerating a DVD-RAM2 disk using this invention or the conventional in-line DPP system. It is the diagram which showed the relationship between the objective-lens displacement amount at the time of reproducing | regenerating DVD-RAM1 disk using this invention, and the amount of residual offtracks after servo drawing. It is the diagram which showed the relationship between the objective-lens displacement amount at the time of reproducing | regenerating a DVD-RAM2 disk using this invention, and the amount of residual offtracks after servo drawing. It is the optical spot schematic arrangement | positioning figure which showed another Example regarding the condensing spot arrangement | positioning on the optical disk in this invention. It is the schematic block diagram which showed another one Example of the optical pick-up using this invention. It is the schematic block diagram which showed another one Example of the optical pick-up using this invention. 1 is a block diagram showing an embodiment of an optical information reproducing or recording / reproducing apparatus equipped with an optical pickup of the present invention.

DESCRIPTION OF SYMBOLS 1 Semiconductor laser light source, 5 Objective lens, 10 Optical disk, 11 Guide groove, 20 Photo detector, 26 2 division | segmentation special diffraction grating, 30 3 division | segmentation special diffraction grating, 100 Main light spot on optical disk surface, 101 * 102 Optical disk surface sub Light spot, 200/201/202 Light spot on detection surface

Claims (2)

  A laser light source that emits laser light;
  A diffraction grating that diffracts laser light emitted from the laser light source and branches the light into at least three light beams;
  An objective lens for condensing the laser beam emitted from the laser light source onto the optical disc;
  A detector for detecting reflected light from the optical disc;
Have
  The diffraction grating is divided into at least three regions of a first region, a second region, and a third region having a predetermined periodic structure;
  The first region is disposed between the second region and the third region;
  The phase of the periodic structure of the second region is approximately 180 degrees different from the phase of the periodic structure of the third region,
  The phase of the periodic structure of the first region is different from both the phase of the periodic structure of the second region and the phase of the periodic structure of the third region.   A laser light source that emits laser light;
  A diffraction grating that diffracts laser light emitted from the laser light source and branches the light into at least three light beams;
  An objective lens for condensing the laser beam emitted from the laser light source onto the optical disc;
  A detector for detecting reflected light from the optical disc;
Have
  The diffraction grating is an optical pickup in which a region having a phase difference between the two regions and the periodic structure is formed between two regions having a phase difference of approximately 180 degrees from each other.

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