JP2007018558A - Aberration correcting element, optical head, optical disk device, and aberration correcting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aberration correcting method for suppressing an arrow-shaped aberration generated owing to position shifting of a component of an optical head, an optical head using the same, and an optical disk device. <P>SOLUTION: The optical head 10 converges luminous flux projected from a semiconductor laser 11 on an optical disk 18 by an objective 17 and photodetects the luminous flux reflected by the optical disk 18 by an optical detector 21 through the objective 17, and is equipped with a correcting element 13 in the optical path from the semiconductor laser 11 to the optical disk 18. The correcting element 13 can suppress an aberration W<SB>A</SB>including the arrow-shaped aberration generated in the optical path by imparting an aberration having the opposite polarity to the aberration W<SB>A</SB>. Distortion of a converged spot shape is improved. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an aberration correction element or the like that corrects aberrations of an optical head, and more particularly to an aberration correction element or the like that suppresses sagittal aberration caused by misalignment of components of an optical head.

  DVD recorders equipped with DVD ± RW and DVD-RAM dominate the market. However, as terrestrial digital broadcasting started in 2003 and digital HD (High Definition) TV broadcasting is scheduled to start in 2005, optical discs, which are recording media for video content, are required to have a larger capacity. Yes. In order to realize a large-capacity optical disk, one means is to use a short wavelength light source and a high NA (numerical aperture) objective lens. A blue-violet semiconductor laser (LD) and a high NA objective lens of 0.65 or more are used. The development of optical discs exceeding 20 GB / single side is also being actively conducted.

  In the above optical disk apparatus, the third-order spherical aberration, coma aberration, and astigmatism of the optical system of the optical head can be suppressed to some extent by restricting the manufacturing variation of each optical element. However, when the optical disk apparatus is operated, spherical aberration is generated due to the difference in the substrate thickness of the optical disk, and coma aberration is generated due to the tilt of the optical disk. In addition, the above three types of aberration also occur due to the inclination and displacement of the optical components during the assembly of the optical head. Therefore, as one means for correcting the aberration, a method has been proposed in which a liquid crystal element is inserted in the optical path of the optical head to control the in-plane phase distribution of the light beam incident on the objective lens. For example, Patent Document 1 discloses a means for correcting spherical aberration and coma aberration. Patent Document 2 discloses a correction means for astigmatism.

Japanese Patent Laid-Open No. 9-128785 Japanese Patent Application Laid-Open No. 11-259892

  However, when the optical disk is reproduced, not only the third-order spherical aberration, coma aberration, and astigmatism but also higher-order aberrations exist. FIG. 10 [2] is a diagram showing an example of the two-dimensional intensity distribution of the focused spot after correcting third-order spherical aberration, coma aberration (excluding the combination with arrow aberration described later) and astigmatism. . In the figure, a bright portion indicates a portion with high light intensity, and a dark portion indicates a portion with low light intensity, and the light intensity at the bottom portion of the focused spot is emphasized. From FIG. 10 [2], although the aberration is corrected, the shape of the central portion of the focused spot is a substantially triangular shape, and accordingly, the portion with strong light intensity of the side lobe extends in three directions. Appears. Due to this shape, when the recording information of a predetermined track of the optical disk is read, the recording information (recording pits and recording marks) of both adjacent tracks is also read. Therefore, an unnecessary signal from an adjacent track leaks into a desired reproduction signal, which causes a large problem that causes an increase in the noise component of the reproduction signal. The condensing spot shape distortion is caused by a sagittal aberration which is one of the fifth-order aberrations and a third-order coma aberration combined therewith, and is caused by an inclination of the optical component, a deviation from the optical axis, or the like.

When the polar coordinates on the pupil plane of the objective lens are expressed by (r, φ), the wavefront aberration W (r, φ) on the pupil plane of the objective lens is expressed by the following formula 1.
W (r, φ) = W 11 rcosφ + W 20 r 2 + W 22 r 2 cos 2 φ + W 31 r 3 cosφ + W 33 r 3 cos 3 φ + W 40 r 4 ... (1)

Here, W ₁₁ rcos φ is due to image point movement, W ₂₀ r ² is aberration due to defocus, W ₃₁ r ³ cos φ is third-order coma aberration due to tilt of optical disk, tilt of optical element, misalignment, W ₄₀ r ⁴ is the third-order spherical aberration mainly due to the deviation of the substrate thickness of the optical disk, and W ₂₂ r ² cos ² φ is the third-order astigmatism mainly caused by the optical element in the optical head and the semiconductor laser. Represents each. W ₃₃ r ³ cos ³ φ is an aberration caused by the optical element, and represents a fifth-order arrow aberration as described in Optics (Iwanami Shoten), page 315. Note that W _ij is an aberration coefficient.

The shape of the central portion of the focused spot is as shown in FIG. 10 [2] because the fifth-order arrow aberration W ₃₃ r ³ cos ³ φ and the third-order coma aberration W ₃₁ r ³ cos φ are combined. is due to aberration W _a, it can be expressed by the following equation 2.
W _A = hr ³ (4cos ³ φ-3cosφ) (2)

Here, h is a coefficient. Since the wavefront aberration of Formula 2 is proportional to r ³ , it is small at the center of the light beam and increases rapidly at the periphery of the light beam. On the other hand, the change of the wavefront aberration with respect to the rotation angle φ is as shown in FIG. In the figure, the wavefront aberration on the vertical axis is positive wavefront aberration on the upper side with respect to 0, and negative wavefront aberration on the lower side. From the same figure, at the rotation angles 0, 60, 120, 180, 240, and 300 [deg], the distribution has a wavefront aberration that is positive or negative and has the maximum absolute value. The correction of this wavefront aberration is different from the wavefront aberration caused by spherical aberration, coma aberration, and astigmatism, and is impossible with the correction elements for each aberration described in the background art. Further, Eliminating the inclination and displacement of the optical elements constituting the optical head, the aberration W _A is reduced, the two-dimensional shape of the central portion of the focused spot in a circular, ring-shaped light intensity distribution of the side lobe Can be made substantially uniform. However, when considering mass production, improving the assembly accuracy leads to an increase in the number of assembling steps and time, which is a serious problem in mass production of the optical head.

In view of the above problems, an object of the present invention is to achieve a good light converging spot shape by correcting the aberration W _A where the fifth-order arrow-shaped aberration and third-order coma aberration combined, from the adjacent tracks detecting a reproduction signal with suppressed noise component is to provide an aberration correction element for correcting the aberration W _a is responsible for the distortion of the focused spot shape.

The aberration correction element according to the present invention is used for an optical head that collects a light beam emitted from a light source on an optical disk by an objective lens and receives the light beam reflected by the optical disk by a photodetector through the objective lens. It is. The aberration correction element according to the present invention has an incident surface and the exit surface light flux passes, the incident surface and exit surface, the opposite polarity aberration with respect to aberration W _A containing dart aberration of the light flux which passes through It consists of a curved surface to be given (claim 1). In the prior art, since there is no corrects the aberration W _A containing arrow-shaped aberration, distortion has occurred to the focused spot shape. In contrast, in the present invention, the curved surface providing an aberration of opposite polarity to the aberration W _A containing arrow-shaped aberration, it is possible to suppress the aberration W _A.

. At this time, the curved surface forming the entrance surface and the curved surface forming the exit surface may overlap each other when one of them is translated toward the other (claim 2). In this case, since the optical path difference from the entrance surface to the exit surface is the same everywhere, generation of unnecessary aberrations can be suppressed. It may be composed of a first aberration correction element having an entrance surface and a second aberration correction element having an exit surface. In this case, since the first and second aberration correction elements can be moved separately, various correction methods can be obtained. It may be composed of a single aberration correction element having an incident surface formed on one surface and an output surface formed on the other surface. In this case, the number of aberration correction elements can be reduced. When the XY axis is in the plane perpendicular to the optical axis of the light beam and the Z axis is in the direction of the optical axis, the curved surface is Z = 2πk ₁ (X ² + Y ² ) {X ² −m (X ² + Y ² )} (k ₁ is a coefficient, m is a positive number of 1 or less) is shaped according to, it may be a (claim 5). The curved surface has a shape according to Z = 2πk ₂ (X ⁴ −Y ⁴ ) (k ₂ is a coefficient) when the XY axis is taken in a plane perpendicular to the optical axis of the luminous flux and the Z axis is taken in the direction of the optical axis. (Claim 6). The curved surface takes polar coordinates (r, φ) in a plane perpendicular to the optical axis of the light beam, and Z = Ar ³ {4 cos ³ (π / 2−φ) −3 cos when taking the Z axis in the direction of the optical axis. The shape may conform to (π / 2−φ)} (A is a coefficient). As described above, by the incident and exit surfaces to a particular curved shape, it is possible to provide an aberration of opposite polarity to the aberration W _A.

The aberration correction element according to the present invention is a liquid crystal element that transmits a light beam. The liquid crystal element is divided voltages independently to a plurality of regions capable of applying a predetermined voltage value in each region to provide an aberration of opposite polarity to the aberration W _A containing dart aberration of transmitted light flux is applied (Claim 8). In this case, the plurality of regions may include a single region including the optical axis of the light flux and a region that is an integral multiple of 6 divided in the circumferential direction around the single region. ). The phase of the transmitted light beam in each region changes according to the voltage value applied to each region. By using this, it is possible to provide an aberration of opposite polarity to the aberration W _A. In the conventional art, because there is no corrects the aberration W _A containing arrow-shaped aberration, distortion has occurred to the focused spot shape. In contrast, in the present invention, a liquid crystal element that gives an aberration of opposite polarity to the aberration W _A containing arrow-shaped aberration, it is possible to suppress the aberration W _A.

  An optical head according to the present invention condenses a light beam emitted from a light source on an optical disk by an objective lens, and receives a light beam reflected by the optical disk by a photodetector through the objective lens. An aberration correction element according to the present invention is provided in the optical path (claim 10). By providing the aberration correction element according to the present invention, distortion of the focused spot shape can be suppressed.

  An optical disk apparatus according to the present invention includes an optical head including the aberration correction element according to claim 5 and means for moving the aberration correction element in the X-axis direction (claim 11). An optical head provided with an aberration correction element and means for rotating the aberration correction element around the Y axis (Claim 12), an optical head provided with the aberration correction element according to Claim 7, and aberration correction A device having means for rotating the element around the Z axis (Claim 13), an optical head having the aberration correcting element according to Claim 8 or 9, and a voltage applied to each region of the liquid crystal element Means (claim 14). Since the aberration correction element according to the present invention is provided, distortion of the focused spot shape can be suppressed, so that data read / write errors are reduced.

Aberration correcting method according to the present invention, the aberration correction device according to claim 5, wherein measuring the X-axis aberration W _A while moving in the direction, the aberration W _A fixes the aberration correcting element to the smallest position (claim 15), while rotating the aberration correcting element according to claim 6, wherein about the Y axis to measure the aberration W _a, aberration W _a fixes the aberration correcting element to the smallest angle (claim 16), claim the aberration correcting element 7 according to measured aberration W _a while rotating about the Z axis, the aberration W _a fixes the aberration correcting element to the smallest angle (claim 17), or, according to claim 8 or 9 with the aberration correcting element described, while changing the voltage applied to each region was measured aberration W _a, prescribed in each region a voltage value aberration W _a is the smallest (claim 18).

In other words, the optical head of the present invention is an optical head comprising a light source, an objective lens that condenses the luminous flux emitted from the light source onto an optical disc, and a photodetector that receives the reflected luminous flux of the optical disc. _A correction amount can be continuously selected with respect to the aberration WA that is arranged between the light sources and is generated in the optical path from the light source to the optical disc within a preset range and is a combination of sagittal aberration and coma aberration. It has a correction element. More preferably, firstly, the correction element has two curved surfaces having the same shape as each other, and is composed of two elements, and each of the two elements includes an incident surface, an output surface. Any one of the above is the curved surface, or a single element, and both the incident surface and the exit surface of the element are the curved surface. Second, when the curved surface has an XY axis in a plane perpendicular to the optical axis and a Z axis in the optical axis direction, Z ₁ = 2πk ₁ (X ² + Y ² ) {X ² −m (X ² + Y ² )} (k ₁ is a coefficient, m is a positive number less than 1), or a curved surface according to Z ₂ = 2πk ₂ (X ⁴ −Y ⁴ ) (k ₂ is a coefficient). . The curved surface has polar coordinates (r, φ) in a plane perpendicular to the optical axis, and Z ₃ = Ar ³ {4cos ³ (π / 2−φ) − when the Z axis is taken in the optical axis direction. 3cos (π / 2−φ)} (A: coefficient) may be used. Third, the correction element is a liquid crystal element, a central portion that is close to the optical axis is a single region, and a peripheral portion that is far from the optical axis is a region that is an integral multiple of 6 around the optical axis. It is characterized by being divided into two.

In addition, the optical disk apparatus of the present invention is characterized by having means for controlling the optical head and the correction element. More preferably, firstly, when the correction element has two curved surfaces having the same shape in a concave-convex relationship, and the curved surface shape conforms to Z ₁ and Z ₂ , the control means uses the element as an optical axis. It is characterized in that it includes means for continuously moving in a specific direction perpendicular, or means for continuously rotating the element around an axis in a specific direction perpendicular to the optical axis. Or, the correction element has a shape identical two curved surfaces in the unevenness of each other, in accordance with Z ₃ is curved sectional shape, said control means including means for continuously rotating the the element around the optical axis It does not matter. Second, the correction element is a liquid crystal element, and the control means includes means for applying a voltage to each region of the liquid crystal element.

Further, the aberration correcting method of the present invention, by continuously moving one or both of the two curved surfaces in the correction element having a shape identical two curved surfaces in the unevenness of each other, to correct the aberration W _A It is characterized by that. More preferably, first, when the two curved shapes of the correction element follow Z ₁ and Z ₂ , the element is moved relatively continuously in a specific direction perpendicular to the optical axis, It is characterized by correcting the aberration W _a by continuously rotating around a specific axis perpendicular to the axis. Second, the two curved shapes of the correction element may follow Z ₃ and the element may be continuously rotated around the optical axis. In the aberration correction method of the present invention, the correction element is a liquid crystal element, and the central portion of the element that is close to the optical axis is a single region, and the peripheral portion that is far from the optical axis is is divided into an integer multiple of the area of 6 around the optical axis, it is characterized by correcting the aberration W _a by applying a voltage to the respective regions of the device.

In the optical head of the present invention, when using the correction element having two curved surfaces having the same shape in a concave-convex relationship, the element is continuously moved in a plane perpendicular to the optical axis, or Rotate continuously around an axis perpendicular to the optical axis of the element, or rotate the element continuously about the optical axis. When a liquid crystal element divided into a plurality of regions is used as the correction element, a voltage having an amplitude corresponding to the region is applied to each region of the liquid crystal element. Thus, it is possible to correct the aberration W _A generated in the optical path from the light source to the optical disc.

  Further, in the optical disk apparatus of the present invention, the phase distribution of the transmitted light beam of the correction element is controlled by the control means, so that it is possible to reduce noise components leaking from adjacent tracks included in the reproduction signal of the optical disk by the optical disk apparatus.

In the aberration correction method of the present invention, one or both of the two curved surfaces of the correction element are moved relatively continuously in a direction perpendicular to the optical axis or continuously around an axis perpendicular to the optical axis. Or rotate relatively continuously around the optical axis. Furthermore, only by applying the amplitude of the voltage corresponding thereto for each area of the liquid crystal element is divided into a plurality of regions, can be corrected well simply and accurately aberration W _A.

According to the present invention, by providing an aberration of opposite polarity to the aberration W _A containing dart aberration generated in the optical path from the light source to the optical disc, since the aberration W _A can be suppressed, the distortion of the focusing spot shape Can improve.

  Next, embodiments of the present invention will be described with reference to the drawings. However, the “aberration correction element” in the claims is simply referred to as “correction element”. FIG. 1 is an optical path diagram showing a first embodiment of a correction element and an optical head according to the present invention. Hereinafter, description will be given based on this drawing.

The optical head 10 includes a semiconductor laser 11, a collimator lens 12, a correction element 13, a polarizing beam splitter 14, a ¼ wavelength plate 15, a mirror 16, an objective lens 17, a re-condensing lens 19, and a cylindrical lens 20, and a photodetector. 21 etc. The optical head 10 condenses the light beam emitted from the semiconductor laser 11 on the optical disk 18 by the objective lens 17, and receives the light beam reflected by the optical disk 18 by the photodetector 21 through the objective lens 17. The correction element 13 is provided in the optical path from the semiconductor laser 11 to the optical disk 18. Correction element 13, two correcting element 13A, consists 13B, by providing an aberration of opposite polarity to the aberration W _A containing dart aberration generated in the optical path, it is possible to suppress the aberration W _A. Therefore, distortion of the condensing spot shape is improved.

  This will be described in more detail. The linearly polarized light beam emitted from the semiconductor laser 11 is converted into a parallel light beam by the collimator lens 12 and passes through the correction elements 13A and 13B. The correction elements 13A and 13B can be moved continuously in a plane (XY plane) perpendicular to the optical axis (Z axis). The light beam that has passed through the correction elements 13A and 13B is incident on the polarization beam splitter. Since the emitted light beam of the semiconductor laser 11 is adjusted to be P-polarized light (X-axis direction), it passes through the polarization beam splitter 14. The light beam that has passed through the polarization beam splitter 14 becomes circularly polarized light by the quarter-wave plate 15, the direction of which is bent by 90 degrees by the mirror 16, and is condensed on the recording surface of the optical disk 18 by the objective lens 17. The light beam reflected by the optical disk 18 becomes a parallel light beam again by the objective lens 17, passes through the mirror 16, and becomes linearly polarized light by the quarter wavelength plate 15. The polarization direction is rotated by 90 degrees with respect to the forward light flux and becomes S-polarized light (Y-axis direction), so that it is reflected by the polarization beam splitter 14 and enters the re-condensing lens 19. Then, this light beam is condensed by the re-condensing lens 19, astigmatism is added by the cylindrical lens 20, and enters the photodetector 21. The photodetector 21 is composed of a plurality of light receiving elements (not shown), and a focus and track error signal and a reproduction signal are generated using detection signals from the respective light receiving elements.

  2 is an end view showing the correction element of FIG. 1, FIG. 2 [1] is an end view on the ZX plane, and FIG. 2 [2] is an end view on the YZ plane. Hereinafter, a description will be given based on FIG. 1 and FIG.

The correction element 13A has a flat surface 22a composed of a flat surface and a curved surface 23a composed of a curved surface. The correction element 13B has a flat surface 22b made of a flat surface and a curved surface 23b made of a curved surface. The curved surfaces 23a and 23b have a shape according to 2πk ₂ (X ⁴ −Y ⁴ ) (k ₂ is a coefficient), and are saddle-shaped shapes having different signs of curvature in the X-axis direction and the Y-axis direction. In FIG. 1, the planes 22a and 22b of the correction elements 13A and 13B are arranged to face each other. As a result, the curved surfaces 23a and 23b are curved surfaces having the same shape, which have an unevenness (or fit) relationship with each other.

  In addition, when the correction elements 13A and 13B are not moved in a plane perpendicular to the optical axis (that is, when the central axes of the correction elements 13A and 13B coincide with the optical axis), the curved surfaces 23a and 23b are uneven. Because of the relationship, there is no optical path difference between the light beams from the vicinity of the optical axis passing through the correction elements 13A and 13B to the periphery. Therefore, by arranging the correction elements 13A and 13B in the optical head 10, unnecessary aberration does not occur.

…（３） Next, the principle of correction in this embodiment will be described. As described above, the wavefront aberration W (r, φ) on the pupil plane of the objective lens 17 is expressed as in Expression 1. Wavefront aberration W (r, phi) on the pupil plane of the objective lens 17 standard deviation When W _rms, as described in page 54 of the optical disc technology (Radio Technology Published), W _rms the following formula 3

... (3)

In Expression 3, W ₀ is an average value of W (r, φ), and R is the radius of the objective lens 17. W _rms is used for evaluation of wavefront aberration. If this value is reduced, the influence of the wavefront aberration on the reproduction signal is reduced, and a good reproduction of the optical disk apparatus is possible.

…（４） In this embodiment, it intended to correct the aberration W _A. Therefore, Formula 3 becomes Formula 4 below by Formula 2.

... (4)

In order to reduce W _rms , the absolute value of hr ³ (4 cos ³ φ- ³ cos φ) at each r and φ may be reduced. In the present embodiment, correcting element 13A, and the aberration W _A to incident light of the objective lens 17 by 13B to provide a wavefront aberration of the opposite polarity is performed to reduce the W _rms.

Next, the operation of the correction elements 13A and 13B in this embodiment will be described. The correction elements 13A and 13B shown in FIG. 2 are moved in opposite directions. When the movement direction is the X axis and the movement amount is a ₁ , the wavefront aberration W ₂ of the light beam transmitted through the correction elements 13A and 13B can be expressed by the following equation (5).
W ₂ = 2πk ₂ [{(X + a ₁ ) ⁴ −Y ⁴ } − {(X−a ₁ ) ⁴ −Y ⁴ }] (n−1)
= 16πk ₂ a ₁ X (X ² + a ₁ ² ) (n−1) (5)

Here, n is the refractive index of the correction elements 13A and 13B. Further, when the same polar coordinates as those on the pupil plane of the objective lens 17 are displayed (X = r cos φ, Y = rsin φ), the above formula can be expressed by the following formula 6.
W ₂ = 4πk ₂ a ₁ (n−1) (4r ³ cos ³ φ + 4a ₁ ² rcos φ) (6)

Here, the coefficient k ₂ and the movement amount a ₁ of the correction elements 13A and 13B may be determined so that h = −4πk ₂ a ₁ (n−1). Since the movement amount a ₁ is a value that can be selected continuously, the arrow-shaped aberration component of the aberration W _A within a range set in advance (the first term on the right side of the equation 2) can be corrected accurately. Although the moving amount a ₁ depends on the coefficient k ₂ , 200 [μm] is sufficient. In addition, (the second term on the right side of the equation 2) coma aberration component of the aberration W _A is corrected by tilting the optical disk 18. Note that the second term of Equation 6 remains only with the above correction. Since this term is a component of image point movement, it can be corrected by moving the objective lens 17 in a plane perpendicular to the optical axis direction.

FIG. 10 [1] is a diagram showing an example of the two-dimensional intensity distribution of the focused spot after the above correction. In FIG. 10 [1], as in FIG. 10 [2], the light intensity distribution at the bottom of the focused spot is emphasized. From the figure, the shape of the central part of the substantially triangular shape shown in FIG. 10 [2] is circular, and the sidelobe shape having a strong light intensity in three directions has a substantially uniform light intensity distribution in a ring shape. It has a good condensing spot shape. As a result, it can be seen that the aberration W _A is corrected. In the present embodiment, correcting element 13A, the plane 22a of 13B, although is opposed to 22b, the curved surface 23a, the correction of aberration W _A even is opposed to 23b is possible.

  FIG. 3 is an end view showing a second embodiment of the correction element according to the present invention, FIG. 3 [1] is an end view in the ZX plane, and FIG. 3 [2] is an end view in the YZ plane. . Hereinafter, a description will be given based on FIGS. 1 and 3.

The correction elements 13C and 13D of this embodiment are used in the optical head 10 instead of the correction elements 13A and 13B of the first embodiment. Similarly to the correction elements 13A and 13B, the correction elements 13C and 13D have flat surfaces 22c and 22d and curved surfaces 23c and 23d. The curved surfaces 23c and 23d have a shape according to 2πk ₁ (X ² + Y ² ) {X ² −m (X ² + Y ² )} (k ₁ is a coefficient, m is a positive number less than 1), and two correction elements The planes 22c and 22d of 13C and 13D are arranged facing each other. As a result, the shapes of the curved surfaces 23c and 23d of the two correction elements 13C and 13D are in an uneven relationship with each other and are the same. Further, when the correction elements 13C and 13D are not moved in a plane perpendicular to the optical axis, there is no optical path difference between light beams from the vicinity of the optical axis passing through the correction elements 13C and 13D to the periphery. Therefore, by disposing the correction elements 13C and 13D in the optical head 10, unnecessary aberration does not occur.

Next, the operation of the correction elements 13C and 13D in this embodiment will be described. Similarly to the correction elements 13A and 13B, the correction elements 13C and 13D are continuously moved in directions opposite to each other. When the movement direction is the X axis and the movement amount is a ₂ , the wavefront aberration W ₁ of the light beam that has passed through the correction elements 13C and 13D can be expressed by the following Expression 7. The same polar coordinate display (X = rcosφ, Y = rsinφ) as that on the pupil plane of the objective lens 17 is used.
W ₁ = 2πk ₁ (n−1) {4a ₂ r ³ cos ³ φ + 4a ₂ (1-2 m) r ³ cos φ + 8a ₂ ³ (1-m) r cos φ} (7)

When m is 7/8, Expression 7 can be expressed as the following Expression 8.
W ₁ = 2πk ₁ a ₂ (n−1) (4r ³ cos ³ φ-3r ³ cos φ + a ₂ ² rcos φ) (8)

Here, the coefficient k ₁ and the movement amount a ₂ of the correction elements 13C and 13D may be determined so that h = −2πk ₁ a ₂ (n−1). Since the movement amount a ₂ are continuously selectable value, it is possible to correct the dart aberration component and the coma aberration component of the aberration W _A accurately. Note that the third term of Equation 8 remains only with the above correction. Since this term is a component of image point movement, it can be corrected by moving the objective lens 17 in a plane perpendicular to the optical axis direction.

Further, when m is 1, Expression 7 can be expressed as the following Expression 9.
W ₁ = 2πk ₁ a ₂ (n−1) r ³ (4 cos ³ φ-4 cos φ) (9)

Here, the coefficient k ₁ and the movement amount a ₂ of the correction elements 13C and 13D may be determined so that h = −2πk ₁ a ₂ (n−1). Since the movement amount a ₂ are continuously selectable value, it is possible to correct the dart aberration component and the coma aberration component of the aberration W _A accurately. Further, unlike the case where m is 7/8, aberration due to image point movement is not generated. Although leaving coma aberration component of the aberration W _A, it can be corrected by tilting the optical disk 18.

By performing the above correction, a good condensing spot shape as shown in FIG. 10 [1] can be obtained as in the first embodiment. As in the first embodiment, the correction element 13C, the surface of the curved surface 23c which face the 13D, even 23d, correction of aberration W _A is possible.

  FIG. 4 is an end view showing a third embodiment of the correction element according to the present invention, FIG. 4 [1] is an end view in the ZX plane, and FIG. 4 [2] is an end view in the YZ plane. FIG. 4 [3] is an end view at the time of phase correction on the ZX plane. Hereinafter, description will be given based on FIG. 1 and FIG.

  The correction element 30 is used in the optical head 10 instead of the correction elements 13A and 13B in the first embodiment, and can be rotated with respect to a specific axis perpendicular to the optical axis. The correction element 30 has two curved surfaces 31, 32, and the curved surfaces 31, 32 have the same shape as the curved surfaces 23a, 23b of the correction elements 13A, 13B of the first embodiment, respectively. Further, when the correction element 30 is not rotated around a specific axis perpendicular to the optical axis, there is no optical path difference between light beams from the vicinity of the optical axis passing through the correction element 30 to the periphery. Therefore, by disposing the correction element 30 in the optical head 10, unnecessary aberration does not occur.

Next, a description will be given of a correction method of an aberration W _A in the third embodiment. FIG. 4 [3] is a diagram illustrating an example of a correction method by the correction element 30. From the figure, the correction element 30 is continuously rotated around the Y axis. As a result, the curved surfaces 31 and 32 are relatively displaced in the X-axis direction. Therefore, as in the first embodiment, the wavefront aberration of the light beam that has passed through the correction element 30 can be expressed as Equation 6. Since the inclination angle α is continuously selectable values can be corrected accurately dart aberration component of the aberration W _A. The inclination angle α of the correction element 30 may be 0.2 degrees although it depends on the thickness of the correction element 30. In addition to the correction of the arrow-shaped aberration component, correction of the coma aberration component by tilting of the optical disk 18, and performs a correction of the components of the image point movement by plane movement of the objective lens 17, it is possible to correct the aberration W _A .

Further, the curved surfaces 31 and 32 have the same shape when the curved surface shape m of the second embodiment is 7/8, that is, 2πk ₁ (X ² + Y ² ) {X ² −7/8 (X ² + Y ² )}. If the same shape as a curved surface in accordance with the correction element 30 is rotated by a desired angle around the Y axis, it can be corrected with arrow-shaped aberration component of aberration W _a and coma component. The image point movement component remaining after rotation of the correction element 30 can be corrected by moving the objective lens 17 in a plane perpendicular to the optical axis direction. On the other hand, when the curved surfaces 31 and 32 have a curved surface shape m of 1, that is, the same shape as the curved surface according to −2πk ₁ (X ² + Y ² ) Y ² , the correction element 30 is changed. When rotated by a desired angle around the Y axis, it can be corrected and dart aberration component and components of the image point movement of aberration W _a. The coma component remaining after the rotation of the correction element 30 can be corrected by tilting the optical disk 18.

By performing the correction as described above, a good condensing spot shape as shown in FIG. 10 [1] can be obtained also in the third embodiment. In the third embodiment, the number of correction elements can be reduced as compared with the first and second embodiments. In addition, the correction element of the third embodiment, since the implemented correction of arrow-shaped aberration in slope, as compared with the first and second embodiment is the same amount move two correcting elements, correction of aberration W _A Means are simple. In the first to third embodiments, coma is corrected by tilting the optical disk 18, but coma may be corrected by tilting the objective lens 17 instead.

  FIG. 5 is a perspective view showing a fourth embodiment of the correction element according to the present invention. FIG. 6 is a perspective view of the correction element of FIG. 5 at the time of phase correction. Hereinafter, description will be given based on FIG. 1, FIG. 5 and FIG.

The correction elements 34 and 35 of the present embodiment are used in the optical head 10 instead of the correction elements 13A and 13B in the first embodiment. The correction elements 34 and 35 can be rotated around the optical axis. The correction elements 34 and 35 have curved surfaces 37 and 38 and flat surfaces 36 and 39, respectively. The curved surfaces 37 and 38 are expressed as Ar ³ {4cos ³ (π / 2−φ) −3cos (π / 2−φ)) (A is a polar coordinate on the pupil plane of the objective lens 17 using (r, φ). The shape according to the coefficient. The light beam that has passed through the collimator lens 12 travels in the optical axis (Z-axis) direction. That is, the light beam that has passed through the collimator lens 12 enters from the flat surface 39 of the correction element 34, passes through the curved surfaces 38 and 37, and exits from the flat surface 36 of the correction element 35. At this time, the curved surfaces 37 and 38 are curved surfaces having an uneven shape and the same shape. In addition, when the correction elements 34 and 35 are not rotated around the optical axis (that is, when the curved surfaces 37 and 38 are completely overlapped in the plane), the curved surfaces 37 and 38 are in an uneven relationship with each other. An optical path difference does not occur between the light beams from the vicinity of the optical axis passing through 34 and 35 to the periphery. Therefore, by arranging the correction elements 34 and 35 in the optical head 10, unnecessary aberration does not occur.

Next, a description will be given of a correction method of an aberration W _A in the fourth embodiment. FIG. 6 is a diagram illustrating an example of a correction method using the correction elements 34 and 35. From the figure, the correction elements 34 and 35 are arranged slightly apart in the optical axis (Z-axis) direction. When correcting aberrations W _A, a correction element 34, 35 is continuously rotated in opposite directions around the optical axis. When the rotation angle is θ, the wavefront aberration W ₃ of the light beam transmitted through the correction elements 34 and 35 can be expressed by the following formula 10.

W ₃ = Ar ³ [4 {cos ³ ((π / 2−φ) + θ) −cos ³ ((π / 2−φ) −θ)} − 3 {cos ((π / 2−φ) + θ) − cos ((π / 2−φ) −θ)}] (n−1)
= 2Ar ³ {4sin ³ (π / 2-φ) -3sin (π / 2-φ)} (3-4sin ² θ) sinθ (n−1)
= 2Ar ³ (n−1) (4cos ³ φ-3cosφ) (3-4sin ² θ) sinθ (10)

Here, the coefficient A and the rotation angle θ of the correction elements 34 and 35 may be determined so that h = −2A (n−1) (3−4sin ² θ) sinθ. Since the rotational angle θ is continuously selectable value, it is possible to correct the aberration W _A accurately, it is possible to obtain a good focusing spot shape as shown in FIG. 10 [1]. It is sufficient that the rotation angle necessary for correction is ± 5 degrees, and the correction elements 34 and 35 can be arranged close to each other to such an extent that the rotation angle can be secured. For this reason, there is no practical problem in terms of the arrangement space of the correction elements 34 and 35 in the optical system of the optical head 10. In the present embodiment, although is opposed curved surfaces 37 and 38 of the correcting elements 34, 35, the correction of aberration W _A even is opposed to the plane 36 and 39 it is possible.

The correction of aberrations W _A in the first to fourth embodiments, while observing the condensing spot shape, adjusting each compensation element such that the shape as shown in FIG. 10 [1]. That is, the optimal movement amount is corrected by the correction elements 13A and 13B, the optimal inclination angle is corrected by the correction element 30, and the optimal rotation angle is corrected by the correction elements 34 and 35, and then fixed. Thereby, leakage of unnecessary signals from adjacent tracks to the reproduction signal of the optical disc 18 can be reduced, and the signal quality of the reproduction signal can be improved.

  7 and 8 show a fifth embodiment of the correction element according to the present invention. FIG. 7 [1] is a plan view, FIG. 7 [2] is a cross-sectional view, and FIG. 7 [3] is a waveform diagram of an applied voltage. FIG. 8 [1] is a graph showing the effect, and FIG. 8 [2] is a graph showing the effect. Hereinafter, description will be given based on these drawings.

  The correction element 40 of this embodiment is used in the optical head 10 instead of the correction elements 13A and 13B in the first embodiment. The correction element 40 is a liquid crystal element and has a function of changing the phase distribution in the cross section of the transmitted light beam.

  FIG. 7 [1] is a diagram illustrating an example of a pattern in a plane perpendicular to the optical axis direction of the correction element 40. The region is divided into 13 regions 41 to 53 made of transparent electrodes, and the phase of the light beam 54 transmitted through each region 41 to 53 can be changed. FIG. 7 [2] is a diagram showing an outline of a cross section of the correction element 40. The correction element 40 includes a liquid crystal layer 58 containing liquid crystal molecules sandwiched between alignment films 57 and 59 and further sandwiched between transparent electrodes 56 and 60 and glass substrates 55 and 61. One of the transparent electrodes 56 and 60 is a full-surface electrode, and the other is the 13-divided electrode described above. 7 [3] is a diagram showing a waveform of a voltage applied between the transparent electrodes 56 and 60. By independently applying a voltage as shown in the figure between the transparent electrodes 56 and 60 in the regions 41 to 53, the orientation of the liquid crystal molecules in the liquid crystal layer 58 is changed, and the liquid crystals corresponding to the regions 41 to 53 respectively. The phase of the light beam 54 transmitted through the layer 58 is changed.

Next, a description will be given of a correction method of an aberration W _A in the fifth embodiment. The aberration W _A of the objective lens peripheral region periodically changes with respect to φ, as shown in FIG. 11, the aberration W _A in the vicinity of the optical axis is almost zero. The direction in which φ is 0 [deg] is now the X axis. In order to correct the wavefront aberration, the voltage applied to each region of the correction element 40 is controlled. Specifically, a reference voltage ± V _A is applied to the region 53 where the wavefront aberration is almost zero. Then, a voltage ± V _B is applied to each region so as to give a phase corresponding to a negative wavefront aberration to the transmitted light fluxes of the regions 41, 44, 45, 48, 49, 52, and the regions 42, 43, 46, 47, A voltage ± V _C is applied to each of the transmitted light fluxes 50 and 51 so as to give a phase corresponding to a positive wavefront aberration having the same absolute value and opposite polarity as the areas 41, 44, 45, 48, 49, and 52. To do. At this time, the amplitudes of the applied voltages V _B and V _C are different from the amplitude of V _A , and (V _B −V _A ) or (V) between the transmitted light flux in the regions 41 to 52 and the transmitted light flux in the region 53. _C -V _a) to generate a phase difference proportional, it is possible to correct the aberration W _a by the phase difference.

FIG. 8 [1] shows the state of wavefront aberration of the transmitted light beam in each region when the above phase control is performed. In the same figure, the curve of the wavefront aberration shown in FIG. 11 is also shown. As shown by the thick solid line in the figure, the absolute value of the correction amount of the wavefront aberration in the regions 41 to 52 is the same, but the sign differs depending on the region. 8 [2] is carried out phase control correction element 40 is a diagram showing the wavefront aberration when the aberration W _A is corrected. The dotted line in the figure is the wavefront aberration after correction by the correction element 40. In the same figure, the wavefront aberration of FIG. 11 is also shown. From Figure 8 [2], the correction by the correction element 40, although remaining wavefront aberration is slightly seen that the aberration W _A was suppressed. Thereby, a condensing spot as shown in FIG. 10 [1] can be obtained.

Note that the aberration W _A is, phi is the absolute value at 60 degree intervals are characteristic of the maximum value. However, there is always, phi is no need absolute value is the maximum value of the aberration W _A at 0 °, when the absolute value of the aberration W _A for each optical head is maximized phi is also different. In the peripheral portion where the distance from the optical axis of the correction element 40 is far, the optical head can be divided into 12 areas, that is, an integer multiple of 6, around the optical axis. in accordance with the direction of the different aberrations W _a each, and controls the phase of the transmitted light beam in each of said areas. For example, if φ is aberration W _A is the maximum value at 30 degrees, by applying a voltage ± V _B in regions 41,42,45,46,49,50, region 43,44,47,48,51,52 A voltage ± V _C may be applied to the capacitor.

Correcting element 40 because it is a liquid crystal element, it is possible to correct the aberration W _A thin element than the first to fourth embodiments described above, is beneficial to miniaturization of the optical head. The liquid crystal element can also correct wavefront aberrations such as spherical aberration, coma aberration, and astigmatism by changing the electrode shape. By simply combining the elements for correcting various aberrations with the correction element 40, A small correction element capable of correcting various aberrations can be created.

Further, in the liquid crystal element of the present embodiment, the peripheral region is divided into twelve, but the number of divisions is not limited thereto as long as the number of divisions is an integral multiple of six. In the correction of aberration W _A, if a liquid crystal element which is divided as no practical problem, less than the number of divisions, or not be problematic. Further, it is not always necessary to divide the circumferential region into equal intervals, and the circumferential interval may be changed for each region.

  FIG. 9 is a block diagram showing a first embodiment of an optical disc apparatus according to the present invention. Hereinafter, description will be given based on this drawing.

  The optical disk device 70 according to the present embodiment includes the optical head 10, the control circuit 71, and the optical disk 18. The optical head 10 has the configuration shown in FIG. 1 and the description thereof. The control circuit 71 is a circuit that controls the correction elements 13A, 13B, 13C, 13D, 30, 34, 35, and 40 described in the first to fifth embodiments of the correction element according to the present invention.

  Next, the control method of the correction element in this embodiment will be described. While observing a signal quality index (for example, bit error rate) of the reproduction signal of the optical disc 18, a signal A corresponding to the correction amount is transmitted to each correction element so as to obtain an optimum signal quality. Although not shown, the correction element is mounted on the movable mechanism as necessary. For example, the correction elements 13A, 13B, 13C, and 13D are movable mechanisms that can move in a plane perpendicular to the optical axis, the correction element 30 is a mechanism that can move with respect to a plane perpendicular to the optical axis, and the correction elements 34 and 35 are It is mounted on a mechanism that can rotate around the optical axis. By the signal A transmitted from the control circuit 71, the mechanism portion on which the correction elements 13A, 13B, 13C, 13D, 30, 34, and 35 are mounted operates, and the phase distribution of the transmitted light beam of the correction element changes. Also in the correction element 40, the phase distribution of the transmitted light beam changes according to the voltage applied to each region. Note that the phase distribution of the light beam passing through the correction element may be changed while observing the shape of the condensed spot. As a result, it is possible to correct a desired aberration, and to reduce a noise component leaking from an adjacent track, which is included in the reproduction signal.

In the embodiments according to the present invention described above, the fifth-order dart has been described in the form of correcting aberration W _A with aberration component and the third-order coma aberration components, the fifth-order higher dart aberration component and the third order or more Correction of aberrations having coma aberration components is also possible. In that case, if the correction element has a shape that takes into account the corresponding wavefront aberration, correction can be performed without any problem.

  Further, as an error signal detection method in each embodiment according to the present invention, although not mentioned above, a push-pull method as a track error signal detection method such as a knife edge method or an astigmatism method as a focus error signal detection method. A method applied to a conventional optical head, such as a phase difference method, can be used. Embodiments of the optical head and the optical disc apparatus of the present invention are not limited to recording / reproducing discs such as phase change optical discs and magneto-optical discs, but can be applied to write-once optical discs and read-only optical discs.

Next, the effect of the present invention will be described in detail. The first effect of the present invention, the inclination of the optical elements in the optical head is to be corrected aberration W _A five-order arrow-shaped aberration and third-order coma aberration occurring are combined in such positional deviation. The reason for this is that when the curved surface shape of the correction element having two curved surfaces having the same shape in a concave-convex relationship takes the XY axis in the plane perpendicular to the optical axis and the Z axis in the optical axis direction, Z ₁ = 2πk ₁ (X ² + Y ² ) {X ² −m (X ² + Y ² )} (k ₁ is a coefficient, m is a positive number less than 1), or Z ₂ = 2πk ₂ (X ⁴ −Y ⁴ ) In the case of a curved surface according to (k ₂ is a coefficient), the correction element is moved relatively continuously in a specific direction perpendicular to the optical axis or continuously around an axis in a specific direction perpendicular to the optical axis. be rotated because correcting the aberration W _a. In addition, when the curved surface shape of the correction element having two curved surfaces having the same shape, which are in a concave-convex relationship, takes polar coordinates (r, φ) in a plane perpendicular to the optical axis and takes the Z axis in the optical axis direction , Z ₃ = Ar ³ {4 cos ³ (π / 2−φ) −3 cos (π / 2−φ)} (A: coefficient), the correction element is continuously rotated around the optical axis. Te, because correcting the aberration W _a. In addition, when the correction element is a liquid crystal element having a region divided into a plurality of regions, the phase distribution of the light beam transmitted through each region is changed by applying a voltage with an amplitude corresponding to each region of the liquid crystal element, This is because to correct the aberration W _a.

  The second effect is that the noise component of the reproduction signal in the optical disc apparatus can be reduced. The reason is that by incorporating the correction element in the optical head, the phase distribution of the transmitted light beam of the correction element is controlled by the control means, and the noise component leaking from the adjacent track included in the reproduction signal of the optical disk apparatus can be suppressed. It is.

The third effect is a convenient means for correcting the aberration W _A, and is that good precision. The reason is that when the curved surface shape of the correction element conforms to Z ₁ and Z ₂ , the element is continuously moved in a specific direction perpendicular to the optical axis, or around an axis in a specific direction perpendicular to the optical axis. continuously rotated, if the curved shape of the correction element according to Z _3, because the device need only be relatively continuously rotated around the optical axis. Further, when the correction element is the liquid crystal element, it is only necessary to change the phase distribution of the light beam transmitted through the element.

1 is an optical path diagram showing a first embodiment of a correction element and an optical head according to the present invention. FIG. 2 shows the correction element of FIG. 1, FIG. 2 [1] is an end view in the ZX plane, and FIG. 2 [2] is an end view in the YZ plane. FIG. 3 is an end view showing a second embodiment of the correction element according to the present invention, FIG. 3 [1] is an end view on the ZX plane, and FIG. 3 [2] is an end view on the YZ plane. FIG. 4 is an end view showing a third embodiment of the correction element according to the present invention, FIG. 4 [1] is an end view on the ZX plane, FIG. 4 [2] is an end view on the YZ plane, and FIG. [3] is an end view at the time of phase correction on the ZX plane. It is a perspective view which shows 4th Embodiment of the correction | amendment element based on this invention. It is a perspective view at the time of phase correction of the correction element of FIG. FIG. 7A is a plan view, FIG. 7 [2] is a cross-sectional view, and FIG. 7 [3] is a waveform diagram of an applied voltage, showing a fifth embodiment of the correction element according to the present invention. FIG. 8 [1] is a graph showing the effect, and FIG. 8 [2] is a graph showing the effect. 1 is a block diagram showing a first embodiment of an optical disc apparatus according to the present invention. It is a figure which shows the two-dimensional intensity distribution of the condensing spot formed with the emitted light beam of an objective lens, FIG. 10 [1] is this invention, FIG. 10 [2] is a prior art example. It is a graph showing changes in aberration W _A with respect to the circumferential direction about the optical axis.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Optical head 11 Semiconductor laser 12 Collimator lens 13A, 13B, 13C, 13D, 30, 34, 35, 40 Correction element 14 Polarizing beam splitter 15 1/4 wavelength plate 16 Mirror 17 Objective lens 18 Optical disk 19 Re-condensing lens 20 Cylinder Lens 21 Photodetector 23a, 23b, 23c, 23d, 31, 32, 36, 38 Curved surface 22a, 22b, 22c, 22d, 37, 39 Plane 41, 42, 43, 44, 45, 46, 47, 48, 49 , 50, 51, 52, 53 region 54 light beam 55, 61 glass substrate 56, 60 transparent electrode 57, 59 alignment film 58 liquid crystal layer 70 optical disk device 71 control circuit

Claims (18)

In an aberration correction element used in an optical head that collects a light beam emitted from a light source on an optical disk by an objective lens and receives the light beam reflected by the optical disk by a photodetector through the objective lens,
Having an entrance surface and an exit surface through which the luminous flux is transmitted;
The entrance and exit surfaces is composed of a curved surface to provide an aberration of opposite polarity to the aberration W _A containing dart aberration of the light flux which passes,
An aberration correction element characterized by the above. The curved surface that constitutes the incident surface and the curved surface that constitutes the emission surface are shapes that overlap each other when one of them is translated toward the other.
The aberration correction element according to claim 1. The first aberration correction element on which the incident surface is formed and the second aberration correction element on which the emission surface is formed.
The aberration correction element according to claim 2. A single aberration correction element having the incident surface formed on one surface and the exit surface formed on the other surface,
The aberration correction element according to claim 2. When the XY axis is in the plane perpendicular to the optical axis of the luminous flux and the Z axis is in the direction of the optical axis, the curved surface is Z = 2πk ₁ (X ² + Y ² ) {X ² −m (X ² + Y ² )} (k ₁ is a coefficient, m is a positive number of 1 or less),
The aberration correction element according to claim 1. The curved surface has a shape according to Z = 2πk ₂ (X ⁴ −Y ⁴ ) (k ₂ is a coefficient) when the XY axis is taken in a plane perpendicular to the optical axis of the luminous flux and the Z axis is taken in the direction of the optical axis. Is,
The aberration correction element according to claim 1. The curved surface takes polar coordinates (r, φ) in a plane perpendicular to the optical axis of the luminous flux, and Z = Ar ³ {4cos ³ (π / 2−φ) when the Z axis is taken in the direction of the optical axis. ) -3cos (π / 2-φ)} (A is a coefficient),
The aberration correction element according to claim 1. In an aberration correction element used in an optical head that collects a light beam emitted from a light source on an optical disk by an objective lens and receives the light beam reflected by the optical disk by a photodetector through the objective lens,
A liquid crystal element through which the luminous flux passes,
The liquid crystal element is divided voltage to the plurality of regions capable of applying independently the light flux of arrow-shaped predetermined voltage value to the respective regions so as to provide an aberration of opposite polarity to the aberration W _A which includes an aberration that transmits Is applied,
An aberration correction element characterized by the above. The plurality of regions are composed of a single region including the optical axis of the light flux and a region that is an integral multiple of 6 divided in the circumferential direction around the single region.
The aberration correction element according to claim 8. In an optical head that collects a light beam emitted from a light source on an optical disk by an objective lens and receives the light beam reflected by the optical disk by a photodetector through the objective lens,
In the optical path from the light source to the optical disc, the aberration correction element according to any one of claims 1 to 9 is provided.
An optical head characterized by that. An optical head comprising the aberration correction element according to claim 5;
Means for moving the aberration correction element in the X-axis direction;
An optical disc apparatus comprising: An optical head comprising the aberration correction element according to claim 6;
Means for rotating the aberration correction element about the Y axis;
An optical disc apparatus comprising: An optical head comprising the aberration correction element according to claim 7;
Means for rotating the aberration correction element about the Z axis;
An optical disc apparatus comprising: An optical head comprising the aberration correction element according to claim 8 or 9,
Means for applying a voltage to each region of the liquid crystal element;
An optical disc apparatus comprising: While the aberration correcting element according to claim 5, wherein moving to the X-axis direction to measure the aberration W _A, to fix the aberration correcting element this aberration W _A is the smallest position,
An aberration correction method characterized by the above. The aberration W _A measured aberration correcting element according to claim 6, wherein while rotating about the Y axis, to fix the aberration correcting element to the angle formed by the aberration W _A is the smallest,
An aberration correction method characterized by the above. The aberration W _A measure an aberration correcting device according to claim 7, wherein while rotating about the Z axis, to fix the aberration correcting element to the angle formed by the aberration W _A is the smallest,
An aberration correction method characterized by the above. With the aberration correcting device according to claim 8 or 9, wherein, while changing the voltage applied to each of said areas to measure the aberration W _A, defining a voltage value that this aberration W _A is the smallest in each of said areas,
An aberration correction method characterized by the above.

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