JP2011034617A - Method for correcting coma aberration of objective lens in optical recording and reproducing device, and optical recording and reproducing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To properly correct coma aberration in an optical recording medium having a plurality of recording layers even if the distribution or a phase of the coma aberration is reversed between one recording layer and the other recording layer when a lens is shifted in the same direction. <P>SOLUTION: An objective lens 14 is shifted in a radial direction of the optical recording medium 2 with respect to an optical axis of an optical pickup, and in order to correct the coma aberration caused by the shift of the lens, the objective lens 14 is tilted with respect to the optical recording medium 2. When tilting the objective lens 14, whether a relation between a position of a radius of an aperture of the objective lens 14 and the coma aberration caused by tracking is positive polarity or negative polarity is determined with respect to the recording layer where laser beams should be focused, and a tilting direction is varied so as to suppress the coma aberration resulting from the shift of the lens on the basis of the relation. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for correcting coma aberration of an objective lens in an optical recording / reproducing apparatus and an optical recording / reproducing apparatus, and in particular, coma aberration of an objective lens when performing recording or reproduction on an optical recording medium provided with a plurality of recording layers. It relates to a correction method.

  An optical pickup of an optical recording / reproducing apparatus is required to accurately irradiate a predetermined position of an optical recording medium with laser light. In order to increase the irradiation accuracy of the laser beam, it is necessary to control the position and orientation of the objective lens, which is a lens provided closest to the optical recording medium, with high accuracy.

  The radial position of the optical recording medium of the objective lens is always corrected by the tracking servo. When the objective lens is shifted by the action of the tracking servo, the optical axis of the laser beam incident on the objective lens does not coincide with the central axis of the objective lens, thereby generating coma aberration. As a method of correcting the coma generated by the lens shift, the elastic characteristics of the suspension wire of the actuator that controls the position of the objective lens are used, and a voltage corresponding to the lens shift amount of the objective lens is applied to the tilt coil to Has been proposed (Patent Document 1). Specifically, since the coma aberration is substantially proportional to the lens shift amount of the objective lens, correction data for correcting the coma aberration according to the lens shift amount of the objective lens is stored in a predetermined memory circuit. The When the lens shift of the objective lens is detected, the objective lens is tilted based on the correction data, and the coma aberration corresponding to the lens shift amount is corrected. Since optical recording media of different standards have different relationships between the magnitude of coma and the lens shift amount of the objective lens, appropriate correction data is stored for each optical recording medium.

JP 2008-97661 A JP 2007-273045 A JP 2007-80432 A

  In recent optical recording media, a plurality of recording layers may be provided, and it is necessary to correct coma aberration appropriately for each recording layer. When coma occurs due to the lens shift of the objective lens, the magnitude of the coma aberration is substantially proportional to the lens shift amount of the objective lens. It is conceivable to change the tilt direction uniformly according to the direction of the shift and change the tilt angle according to the lens shift amount. However, the inventor of the present application has found that even if the lens is shifted in the same direction, the coma aberration distribution or phase may be reversed between one recording layer and another recording layer. For this reason, depending on the recording layer, the tilt direction of the objective lens is opposite to the direction in which the objective lens should originally face, and there is a problem that a correction effect cannot be obtained. The coma aberration is particularly large in the Blu-ray Disc (registered trademark) where spherical aberration correction is indispensable and is a problem that cannot be ignored.

  In an optical recording medium having a plurality of recording layers, the coma aberration distribution or phase is reversed between one recording layer and the other recording layer when the lens is shifted in the same direction. It is an object of the present invention to provide a coma aberration correcting method and an optical recording / reproducing apparatus that can appropriately correct aberrations.

  According to an objective lens coma correction method according to an embodiment of the present invention, there is provided a method for correcting an objective lens generated by lens shift when information is recorded or reproduced by irradiating an optical recording medium having a plurality of recording layers with laser light. This is a correction method for coma aberration. This method shifts the objective lens in the radial direction of the optical recording medium with respect to the optical axis of the optical pickup and corrects the coma caused by the lens shift with respect to the optical recording medium. And tilting. To tilt the objective lens is to determine whether the relationship between the lens aperture radius position of the objective lens and the coma caused by lens shift is positive or negative for the recording layer on which the laser beam is to be focused. And changing the tilt direction so as to suppress the coma generated as a result of the lens shift based on the above relationship.

  When the objective lens is shifted in the radial direction of the optical recording medium with respect to the optical recording medium, coma aberration occurs. At this time, the relationship between the lens opening radius position of the objective lens and the polarity of the coma aberration caused by the lens shift may be positive or negative. In other words, coma aberration distribution or phase inversion may occur between one recording layer and another recording layer. In the former case, if the graph is drawn with the lens aperture radius position of the objective lens as the horizontal axis and the coma aberration amount as the vertical axis, the graph will rise to the right, and in the latter case, the graph will fall to the right. According to the coma aberration correction method of the present invention, by obtaining this relationship in advance, it is possible to select the tilt direction so that the inclination of the graph approaches 0, and effectively the coma aberration for any recording layer. Can be suppressed.

  According to another embodiment of the present invention, an optical recording / reproducing apparatus to which an optical recording medium having a plurality of recording layers can be mounted includes a light source that emits laser light, and a laser beam emitted from the light source. An objective lens capable of focusing on a specific recording layer and capable of lens shift with respect to the optical axis of the optical pickup and tilting with respect to the optical recording medium; Means for determining whether the relationship with the coma aberration amount caused by the lens shift is positive or negative, and, based on the above relationship, in the tilt direction of the objective lens so as to suppress the coma generated as a result of the lens shift. Tilt direction control means for changing the polarity.

  Thus, according to the present invention, in an optical recording medium provided with a plurality of recording layers, the distribution or phase of coma aberration between one recording layer and another recording layer when the lens is shifted in the same direction. It is possible to provide a coma aberration correcting method and an optical recording / reproducing apparatus that can appropriately correct coma even in the case of reverse rotation.

1 is a block diagram schematically showing a configuration of an optical recording / reproducing apparatus according to an embodiment of the present invention. It is the schematic which shows the structure of the optical system of an optical pick-up. It is a conceptual diagram which shows the generation principle of a coma aberration. It is a graph which shows the relationship between the lens aperture radius position of the objective lens in depth position d1, and spherical aberration. It is a graph which shows the relationship between the lens aperture radius position of the objective lens in depth position d2, and spherical aberration. It is a block diagram which shows an example of the control mechanism which performs tilt angle control. It is a block diagram which shows the other example of the control mechanism which performs tilt angle control.

  Embodiments of a coma aberration correcting method and an optical recording / reproducing apparatus according to the present invention will be described below with reference to the drawings.

  First, an optical recording / reproducing apparatus according to an embodiment of the present invention will be described. FIG. 1 is a block diagram schematically showing a configuration of an optical recording / reproducing apparatus according to an embodiment.

  The optical recording / reproducing apparatus 101 includes an optical pickup 1, a spindle motor 112 for rotating the optical recording medium 2, a controller 113 for controlling the operation of the spindle motor 112 and the optical pickup 1, and a laser drive signal to the optical pickup 1. A laser driving circuit 114 for supplying the lens and a lens driving circuit 115 for supplying a lens driving signal to the optical pickup 1 are provided. As will be described later, the optical recording medium that can be mounted on the optical recording / reproducing apparatus 101 of this embodiment includes an optical recording medium 2 having a plurality of recording layers. The optical recording / reproducing apparatus 101 of this embodiment has both the recording and reproducing functions for the optical recording medium, but an apparatus having only one of the functions is also included in the optical recording / reproducing apparatus of the present invention.

  The controller 113 includes a focus servo tracking function, a tracking servo tracking function, a tilt function, a laser control function, and a recording layer management function. When the focus servo tracking function is activated, the information recording surface of the rotating optical recording medium 2 is in focus. When the tracking servo tracking function is activated, the spot of the laser beam is automatically tracked with respect to the eccentric signal track of the optical recording medium 2. When the tilt function is activated, the tilt angle of the objective lens 14 is automatically controlled so that the optical axis direction of the objective lens 14 (described later) faces the normal direction of the optical recording medium 2. Control signals generated by the focus servo tracking function, tracking servo tracking function, and tilt function are sent to the lens driving circuit 115, and the lens driving circuit 115 performs focus control, tracking control, and tilt control. The laser control function generates an appropriate laser drive signal based on the recording condition setting information recorded on the optical recording medium 2 and supplies it to the laser drive circuit 114. The recording layer management function manages which recording layer of the plurality of recording layers is used for recording / reproduction. As will be described later, the recording layer management function can determine whether the relationship between the lens aperture radius position of the objective lens 14 and the amount of coma generated by the lens shift is positive or negative. As will be described later, the controller 113 has a function of controlling the polarity and tilt angle of the objective lens 14 in the tilt direction so as to suppress coma aberration generated as a result of lens shift, as will be described later. These functions may be realized as a circuit incorporated in the controller 113, or may be realized as software executed in the controller 113.

  FIG. 2 is a schematic diagram showing the configuration of the optical system of the optical pickup. The optical pickup 1 is configured so that digital information can be recorded or reproduced on the optical recording medium 2. The optical recording medium 2 may be a DVD (Digital Versatile Disc) -ROM, DVD ± R / RW, or an optical recording medium having a structure and storage capacity equivalent to these, CD (Compact Disc) -ROM, CD -R / RW and an optical recording medium having a structure and storage capacity equivalent to these may be used, and BD (Blu-ray Disc (registered trademark)) or CB that performs high-density recording / reproduction using blue laser light having a wavelength of about 405 nm -It may be an optical recording medium such as HD (China High Density Optical Recording Medium Standard) or the like and dedicated for recording and reproducing.

  The optical pickup 1 includes semiconductor lasers 3 and 4 that are light sources that emit laser light having a predetermined wavelength. The semiconductor laser 3 includes a first light emitting region that emits a laser beam having a wavelength of 650 nm (first laser beam) for recording and reproducing a DVD, and a laser beam having a wavelength of 780 nm for recording and reproducing a CD (second laser beam). A second light emitting region that emits (laser light). These light emitting regions are formed at a predetermined distance and are accommodated in one package. On the other hand, the semiconductor laser 4 emits a laser beam (third laser beam) having a wavelength of 405 nm for recording / reproducing a high-density optical recording medium such as BD or CB-HD.

  In the semiconductor laser 3 and the semiconductor laser 4, the optical axis of the first or second laser light emitted from the semiconductor laser 3 and the optical axis of the third laser light emitted from the semiconductor laser 4 are orthogonal to each other. Is provided.

  A diffraction grating 5 is disposed at a predetermined position on the light emitting side of the semiconductor laser 3. On one side of the diffraction grating 5, the first and second laser beams emitted from the semiconductor laser 3 are respectively converted into three light beams (0th-order main beam and ± 1st-order subbeams; not shown). A diffraction grating pattern optimized to be divided is formed. In the diffraction grating 5, the ± first-order sub beam is condensed at a symmetrical position separated by a predetermined distance in the track line direction around the condensing position of the main beam on the surface (information recording surface) of the optical recording medium 2. The first and second laser beams emitted from the semiconductor laser 3 are divided.

  A diffraction grating 6 is also disposed at a predetermined position on the light emitting side of the semiconductor laser 4. On one side of the diffraction grating 6, the third laser beam emitted from the semiconductor laser 4 is divided into three light beams (0th-order main beam and ± 1st-order subbeams; not shown). An optimized diffraction grating pattern is formed. The diffraction grating 6 is configured such that, on the surface (information recording surface) of the optical recording medium 2, ± first-order sub beams are condensed at symmetrical positions separated by a predetermined distance in the track line direction with the main beam condensing position as the center. The third laser beam emitted from the semiconductor laser 4 is divided.

  A substantially cubic dichroic prism 7 is provided at a position where the laser beam from the semiconductor laser 3 and the laser beam from the semiconductor laser 4 intersect. The dichroic prism 7 transmits the first and second laser beams almost entirely and reflects the third laser beam substantially totally.

  The laser light that has passed through or reflected by the dichroic prism 7 enters the polarization beam splitter 8. About 90% of the laser light incident on the polarizing beam splitter 8 is reflected by the polarizing beam splitter 8 and enters the rising mirror 11. The remaining 10% of the laser beam incident on the polarization beam splitter 8 passes through and enters the front monitor photodetector 15. The front monitor photodetector 15 measures the light intensity of the first to third laser beams emitted from the semiconductor lasers 3 and 4. The outputs of the semiconductor lasers 3 and 4 are adjusted based on the output of the front monitor photodetector 15.

  The laser light incident on the rising mirror 11 is reflected by the rising mirror 11 and enters the collimating lens 9. The laser light is a divergent beam until it enters the collimating lens 9 after being emitted from the semiconductor lasers 3 and 4, but is converted into substantially parallel beam light by the collimating lens 9.

  The laser light that has passed through the collimating lens 9 enters the liquid crystal element 10. The liquid crystal element 10 includes a transparent electrode (not shown) divided into a predetermined shape. The transparent electrode is made of, for example, tin-doped indium oxide (ITO) or tin oxide. When a voltage is applied to each region of the liquid crystal element 10, an appropriate phase difference is given to the passing laser beam, and various wavefront aberrations such as spherical aberration occurring on the optical path can be corrected and optimized.

  The laser beam that has passed through the liquid crystal element 10 enters the quarter-wave plate 12, and the laser beam main beam and ± 1st-order sub-beam (hereinafter referred to as “forward laser beam”) change from linearly polarized light to circularly polarized light. Converted. Further, a temperature compensation element 13 is attached to the movable part, and an expander lens 13 'is integrally formed. The expander lens 13 ′ has a specific lens power according to the wavelength of the light source to be used, changes the wavefront (angle) of incident light on the objective lens 14, and converts the magnification of the objective lens 14. The temperature correction element 13 generates spherical aberration in a direction to cancel the spherical aberration caused by the temperature change of the objective lens 14, and keeps the spherical aberration amount as a whole to a uniform value. In the present embodiment, the temperature correction element 13 is provided as an independent element closer to the light source than the objective lens 14, but the objective lens 14 itself may have this function. The laser light that has passed through the quarter-wave plate 12 enters the objective lens 14 and is focused on the information recording surface of the optical recording medium 2. At the time of recording, the focused laser beam writes a record in a predetermined bit on the information recording surface, and the recording operation is completed.

  During reproduction, the laser beam condensed on the optical recording medium 2 is reflected by the information recording surface and further proceeds in the reverse direction. First, the laser beam reflected by the optical recording medium 2 is converted into a substantially parallel beam by the objective lens 14. The laser light continues to enter the quarter-wave plate 12 and is converted from circularly polarized light into linearly polarized light in a direction orthogonal to the polarization direction of the forward laser light. The laser light that has passed through the quarter-wave plate 12 enters the collimating lens 9 through the liquid crystal element 10 and is converted into a convergent beam. The laser light that has passed through the collimating lens 9 is reflected by the rising mirror 11 and enters the polarizing beam splitter 8. The polarization beam splitter 8 causes the return light from the collimating lens 9 to pass through the internal joint surface and enter the anamorphic lens 16. The anamorphic lens 16 gives astigmatism for detecting a defocus error to the laser light incident from the polarization beam splitter 8 and forms an image on the light receiving element 17. The light receiving element 17 photoelectrically converts the received laser light for each divided light receiving region (not shown) and outputs an electrical signal.

  The objective lens 14 can be moved in the optical axis direction by a focus coil (not shown) connected to the controller 113, and the optical pickup 1 can be moved in the radial direction of the optical recording medium 2 by the tracking coil 21 connected to the controller 113. The optical recording medium 2 can be tilted with a tilt coil 22 connected to the controller 113. Next, a method for correcting coma aberration will be described. Although the present invention can be widely applied to an optical recording medium having a plurality of recording layers, a case where the optical recording medium has two recording layers will be described for the sake of simplicity.

  As described above, even if the objective lens is shifted in the same direction, the distribution or phase of coma aberration may be reversed between one recording layer and another recording layer. One example of such a case is when spherical aberration is corrected. That is, coma aberration distribution or phase reversal is (1) in an optical recording medium having a plurality of recording layers, where the phase of spherical aberration is reversed between one recording layer and another recording layer. 2) It occurs when spherical aberration is corrected, and (3) tracking is performed in that state. First, this phenomenon will be described.

  The phenomenon (1) is as follows. First, as shown in FIG. 3A, a case is considered in which the objective lens 14 is focused at a depth position d3 between the two recording layers 2a and 2b of the optical recording medium 2. A portion from the surface of the optical recording medium 2 to the recording layer 2a is a protective layer 2x, and a protective layer 2y is also provided between the recording layer 2a and the recording layer 2b. Since the recording layers 2a and 2b are thin films of metal, the depth position d1 and the recording layer 2a can be identified on the drawing. Similarly, the depth position d2 and the recording layer 2b can be identified on the drawing. * The laser beam incident on the outer peripheral side of the lens opening of the objective lens 14 has a refraction angle larger than that of the laser beam incident near the center, and therefore is focused before the depth position d3. For this reason, spherical aberration occurs in the objective lens 14.

  Therefore, as shown in FIG. 3B, the objective lens 14 is designed so that the spherical aberration is minimized at the depth position d3. When recording / reproduction is actually performed on the recording layers 2a and 2b, since the depth positions d1 and d2 are in-focus positions, it is necessary to correct spherical aberration again. However, spherical aberration is required at the depth position d3. By correcting the spherical aberration so as to minimize the spherical aberration, the amount of correction of the spherical aberration when focusing on the depth positions d1 and d2 can be reduced.

  FIG. 3C is an enlarged view of a portion A in FIG. 3B, and shows a situation in which recording / reproduction is performed on the recording layers 2a and 2b. In the figure, only the laser beam incident on the outer peripheral side of the lens opening is displayed. Even when the spherical aberration is corrected so that the spherical aberration is minimized at the depth position d3 (FIG. 3B), the laser light is actually refracted on the surface of the optical recording medium 2 (the surface of the protective layer 2x). Should be noted (optical path B3). When recording / reproducing is performed on the recording layer 2a, the focus servo is operated to move the objective lens 14 away from the optical recording medium 2 (optical path B1). As shown in the optical path B1, the laser light is focused on the near side of the depth position d1, diffused again, and enters the depth position d1. As a result, spherical aberration occurs in the beam spot focused on the depth position d1. On the other hand, when recording / reproducing is performed on the recording layer 2 b, the focus servo is activated to move the objective lens 14 closer to the optical recording medium 2. As a result, as shown in the optical path B2 in the figure, the laser light is refracted on the surface of the optical recording medium 2 and reaches the depth position d2 without being focused (the focal point is the depth position with respect to the traveling direction of the laser light). It is ahead of d2.) Even in this case, spherical aberration occurs in the objective lens 14, but the optical path B1 is once converged, whereas the optical path B2 is not converged. For this reason, the phase of the spherical aberration is reversed in the optical path B1 and the optical path B2.

  This will be further described with reference to FIGS. FIG. 4A is a graph showing the relationship between the lens aperture radius position of the objective lens 14 and the spherical aberration at the depth position d1. The horizontal axis is normalized based on the outer peripheral position of the objective lens 14. The wavefront aberration on the vertical axis is shown as a peak value from an ideal wavefront having no phase difference (a surface connecting points having the same phase), and is normalized with the wavelength being 1. The wavefront aberration is generated point-symmetrically or concentrically with respect to the central axis of the objective lens 14, and it can be seen that this wavefront aberration is a spherical aberration. The spherical aberration pattern (convex upward at the lens center position) shown in FIG. 4A is referred to as “positive” spherical aberration in the following description. Similarly, FIG. 5A is a graph showing the relationship between the lens aperture radius position of the objective lens 14 and the spherical aberration at the depth position d2. The phase of the spherical aberration pattern shown in FIG. 5A is reversed in phase from the pattern of FIG. 4A (convex upward at the lens center position). In the following description of the spherical aberration pattern shown in FIG. 5A, it is referred to as “negative” spherical aberration. Thus, the phase of the spherical aberration is reversed between the recording layer 2a and the recording layer 2b. The above is the description of (1).

  Next, the correction of spherical aberration (2) will be described. First, when “positive” spherical aberration occurs, the phase of the laser beam is adjusted by the liquid crystal element 10 so that a staircase-shaped correction pattern as shown by a thin line in FIG. . As a result, the spherical aberration is corrected as shown in FIG. The same applies to the case where “negative” spherical aberration occurs, and the phase of the laser beam is adjusted by the liquid crystal element 10 so that a step-like correction pattern as shown by a thin line in FIG. To do. As a result, the spherical aberration is corrected as shown in FIG.

  Next, the phenomenon (3) will be described. First, consider a case where recording / reproduction is performed on the recording layer 2a. Assume that the objective lens 14 is shifted in the radial direction of the optical recording medium 2 in order to follow the recording target bit or the reproduction target bit of the recording layer 2a in a state where the spherical aberration is corrected as described above. At this time, the correction pattern of the liquid crystal element 10 remains the thin line in FIG. 4A, while the spherical aberration pattern of the objective lens 14 translates in the radial direction as the objective lens 14 shifts. As a result, as shown in FIG. 4C, the point symmetry or concentricity of the wavefront aberration is lost, and the relationship between the lens aperture radius position of the objective lens 14 and the amount of coma generated by the lens shift is linear. . This means that coma has occurred. That is, it can be seen that coma aberration occurs when the lens shift is performed with the spherical aberration corrected. In this figure, the relationship between the lens aperture radius position of the objective lens 14 and the coma aberration amount caused by the lens shift is a positive relationship, that is, a graph rising to the right. This case is referred to as “positive” coma in the following description. FIG. 4C shows an aberration curve when an objective lens shift of +0.1 mm is assumed, and the amount of coma generated is equivalent to −0.081 [mλrms]. This is equivalent to −0.92 [deg] in terms of the tilt amount of the Blu-ray disc.

  The same applies when recording / reproduction is performed on the recording layer 2b. Also in this case, as shown in FIG. 5C, the point symmetry of the wavefront aberration is lost, and the relationship between the lens aperture radius position of the objective lens 14 and the amount of coma generated by the lens shift is linear. However, the relationship between the lens aperture radius position of the objective lens 14 and the amount of coma generated by the lens shift is a negative relationship, that is, a graph with a lower right. This case will be referred to as “negative” coma in the following description. FIG. 5C shows an aberration curve when an objective lens shift of +0.1 mm is assumed, and the amount of coma generated is equivalent to +0.081 [mλrms]. This is equivalent to +0.92 [deg] in terms of the tilt amount of the Blu-ray disc.

  As described above, coma aberration occurs when the lens shift is performed with the spherical aberration corrected, but the phase is correlated with the positive / negative of the phase of the spherical aberration, and when the phase of the spherical aberration is positive, the coma aberration is also increased. When it is positive and the phase of the spherical aberration is negative, the coma aberration is also negative.

  Based on the above phenomenon, in the present embodiment, when the objective lens 14 is tilted with respect to the optical recording medium 2 in order to correct the coma aberration, the amount of coma aberration caused by the lens aperture radius position and the lens shift of the objective lens 14. It is detected in advance whether the relationship is positive polarity or negative polarity. For the detection, it is sufficient to know information on which recording layer is actually recorded and reproduced. This is because, in the present embodiment, when the recording layer 2a is a target for recording / reproduction, the relationship is positive, and when the recording layer 2b is a target for recording / reproduction, the relationship is negative. Because. Note that the above detection is executed by the recording layer management function of the controller 113.

  In the present embodiment, the polarity in the tilt direction is changed based on the above relationship so as to suppress coma generated as a result of lens shift. Specifically, the polarity in the tilt direction is different between when the recording layer on which the laser beam is focused is on the objective lens 14 side with respect to the intermediate depth position d3 and when the recording layer is on the side opposite to the objective lens 14. Change. That is, when the recording layer 2a is a target for recording / reproduction, the inclination of the upward-sloping graph near the lens aperture radius position 0 in FIG. 4C is close to 0 (indicated by an arrow in the figure). When the recording layer 2b is a target for recording / reproduction, the inclination of the downward-sloping graph near the lens aperture radius position 0 in FIG. 5C is close to 0 (indicated by an arrow in the figure). ) To tilt.

  It is desirable to change the tilt angle of the objective lens 14 according to the correction amount of the spherical aberration. This is because the magnitude of coma is approximately proportional to the amount of spherical aberration correction. Here, the magnitude of the coma aberration means the difference between the maximum value and the minimum value of the aberration (“c” in FIGS. 4A and 4C), and the correction amount of the spherical aberration is the aberration conversion value of the correction pattern. ("H" in FIGS. 4 (a) and 5 (c)).

  FIG. 6 shows a block diagram of a control mechanism included in the controller 113 for performing such tilt angle control. The drive voltage applied to the tracking coil 21 from both positive and negative terminals is detected, multiplied by the gain of G1, and added to the drive voltage of the tilt coil 22. A drive voltage corresponding to the tilt amount of the optical recording medium 2 is normally applied to the tilt coil 22. The gain G1 is determined by the sensitivity of the tilt coil with respect to the voltage converted value V (Trk) of the reference movement amount, which is determined by the sensitivity of the tracking coil 21, and the tilt coil necessary for correction (of the coma aberration generated by the reference movement amount). The ratio of the voltage V (Tilt) (V (Tilt) / V (Trk) can be obtained. The tilt angle necessary for correction is calculated from the coma aberration amount generated by the lens shift with respect to the lens shift amount. The aberration amount can be estimated by calculation from the correction amount of the spherical aberration and the lens shift amount, and the gain G1 calculated in this way is the optical pickup to be used, the optical disk to be reproduced, and the reproduction environment (for example, ambient temperature). ) Is changed as necessary to take different values depending on the amount of correction of spherical aberration that varies depending on the). The switches S are provided at both ends of the amplifier circuit G1 so that the polarity of the gain can be changed according to the direction in which the gain is to be performed.

  As described above, in this embodiment, when the objective lens 14 is tilted, the drive voltage of the tilt coil 22 that controls the tilt angle of the objective lens 14 is proportional to the magnitude of coma aberration and in the correction direction of spherical aberration. Accordingly, a voltage multiplied by a gain having a different polarity is superimposed on the driving voltage of the tracking coil that controls the tracking amount of the objective lens 14. The correction direction of the spherical aberration means the direction of the correction pattern by the liquid crystal element 10. Specifically, when correcting the positive spherical aberration shown in FIG. 4A, the correction direction of the spherical aberration is downward near the lens aperture radius position 0, and the negative spherical aberration shown in FIG. 5A is corrected. In this case, the spherical aberration correction direction is upward in the vicinity of the lens aperture radius position 0. If it is known which of the correction patterns is applied, the phase of coma aberration can be predicted. Specifically, in the case of a downward correction pattern, a positive coma aberration (pattern in FIG. 4) is predicted, and in the case of an upward correction pattern, a negative coma aberration (pattern in FIG. 5) is predicted. The sign of the gain is determined.

  FIG. 7 shows a block diagram of a control mechanism included in the controller 113 according to another embodiment. The drive voltage applied to the tracking coil 21 from both positive and negative terminals is detected, multiplied by the gain of G2, and added to the drive voltage of the tilt coil 22. The gain G2 is calculated from the above-described V (Tilt) / V (Trk)) and the spherical aberration correction amount SA ((SA in the equation of FIG. 7) in the liquid crystal element 10, and the spherical aberration correction amount SA has a positive / negative polarity. The value of either positive or negative is set as the correction amount of the spherical aberration by the liquid crystal element increases, and the amount of coma aberration generated during lens shift increases.

  As described above, in the present embodiment, when the objective lens 14 is tilted, a voltage obtained by multiplying the drive voltage of the tilt coil for controlling the tilt angle of the objective lens 14 by the gain proportional to the correction amount of the spherical aberration is used. This is superimposed on the driving voltage of the tracking coil that controls the lens shift amount of the lens 14. According to this method, since the tilt direction is determined by the sign of the correction amount of the spherical aberration, the switch S as in the embodiment shown in FIG. 6 is unnecessary. When the input voltage of the actuator is implemented by digital control, the calculation as in this embodiment is effective.

  In the embodiment described above, the liquid crystal element is used as the spherical aberration correcting means. However, the spherical lens can be corrected by moving the expander lens in the optical axis direction or by correcting the collimator lens in the optical axis direction. Aberrations can be corrected, and even when these means are used, the same effects as those of the above-described embodiment can be obtained.

  The present invention is also applicable to an optical recording medium having three or more recording layers. In that case, the spherical aberration correction performed in advance may be performed with reference to the intermediate position of three or more recording layers.

1 Optical pickup 2 Optical recording medium
2a, 2b Recording layer 3, 4 Semiconductor laser 5, 6 Diffraction grating 7 Dichroic prism 8 Polarizing beam splitter 9 Collimator lens 10 Liquid crystal element 11 Rising mirror 12 1/4 wavelength plate 14 Objective lens 15 Front monitor photodetector 16 Anamorphic lens 17 Light receiving element
21 Tracking coil 22 Tilt coil 101 Optical recording / reproducing device 113 Controller d1, d2, d3 Depth position

Claims (10)

A method for correcting coma aberration of an objective lens caused by lens shift when irradiating a laser beam to an optical recording medium having a plurality of recording layers to record or reproduce information,
Shifting the objective lens in the radial direction of the optical recording medium with respect to the optical axis of the optical pickup;
Tilting the objective lens relative to the optical recording medium in order to correct coma caused by the lens shift;
Have
When the objective lens is tilted, the relationship between the lens opening radius position of the objective lens and the coma aberration caused by the lens shift is positive or negative with respect to the recording layer on which laser light is focused. And controlling the polarity of the tilt direction so as to suppress the coma aberration generated as a result of the lens shift based on the relationship. In advance, the spherical aberration of the objective lens is corrected so as to be minimized at an intermediate depth position among the plurality of recording layers,
The objective lens is tilted when the recording layer on which the laser beam is to be focused is on the objective lens side with respect to the intermediate depth position, and when the recording layer is on the side opposite to the objective lens, The coma aberration correction method according to claim 1, comprising changing a polarity in a tilt direction.   The coma aberration correction method according to claim 2, wherein tilting the objective lens includes changing the tilt angle in accordance with a correction amount of the spherical aberration.   The objective lens is tilted by applying a voltage obtained by multiplying the tracking coil drive voltage corresponding to the lens shift amount of the objective lens by a gain having a polarity different depending on the correction direction of the spherical aberration. The coma aberration correction method according to claim 3, comprising adding and superimposing the driving voltage on the tilt coil for controlling the amount.   To tilt the objective lens, a voltage obtained by multiplying the tracking coil drive voltage corresponding to the lens shift amount of the objective lens by a gain proportional to the correction amount of spherical aberration including positive and negative polarities is obtained. The coma aberration correction method according to claim 3, comprising adding and superimposing on a drive voltage of a tilt coil that controls the tilt amount. An optical recording / reproducing apparatus to which an optical recording medium having a plurality of recording layers can be mounted,
A light source that emits laser light;
An objective lens capable of focusing a laser beam emitted from the light source on a specific recording layer of the optical recording medium, allowing a lens shift with respect to an optical axis of the optical pickup and a tilt with respect to the optical recording medium;
Means for determining whether the relationship between the lens aperture radius position of the objective lens and the amount of coma generated by the lens shift is positive or negative for the recording layer to be focused with laser light;
Based on the relationship, a tilt direction control unit that changes the polarity of the tilt direction of the objective lens so as to suppress coma aberration generated as a result of the lens shift;
Having
Optical recording / reproducing device. Spherical aberration correction means for correcting the spherical aberration of the objective lens so as to be minimized at an intermediate depth position of the plurality of recording layers,
The tilt direction control means includes the tilt direction when the recording layer on which laser light is to be focused is on the objective lens side with respect to the intermediate depth position and when the recording layer is on the side opposite to the objective lens. The optical recording / reproducing apparatus according to claim 6, wherein the direction polarity is changed.   The optical recording / reproducing apparatus according to claim 7, wherein the tilt direction control unit changes the tilt angle of the objective lens in accordance with a correction amount of the spherical aberration. A tilt coil for controlling the tilt angle of the objective lens;
A tracking coil for controlling the lens shift amount of the objective lens;
Have
The tilt direction control means obtains the tilt amount of the objective lens by using a voltage obtained by multiplying a tracking coil drive voltage corresponding to the lens shift amount of the objective lens by a gain having a polarity different depending on the correction direction of the spherical aberration. The optical recording / reproducing apparatus according to claim 8, wherein the optical recording / reproducing apparatus adds and superimposes on a drive voltage of a tilt coil to be controlled. A tilt coil for controlling the tilt angle of the objective lens;
A tracking coil for controlling the tracking amount of the objective lens;
Have
The tilt direction control means obtains a voltage obtained by multiplying a driving voltage of a tracking coil corresponding to a lens shift amount of the objective lens by a gain proportional to a correction amount of spherical aberration including positive and negative polarities, and the tilt of the objective lens. 9. The optical recording / reproducing apparatus according to claim 8, wherein the optical recording / reproducing apparatus adds and superimposes on a drive voltage of a tilt coil for controlling the amount.

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