JP4459714B2 - Liquid crystal aberration correction apparatus and optical head - Google Patents

Description

  The present invention relates to a liquid crystal aberration correction apparatus and an optical head for correcting wavefront aberration generated in an optical system such as an optical head, and more particularly to an objective lens simultaneously with correction of spherical aberration caused by a thickness error of a light transmission layer of an optical disk. The present invention relates to a technique for correcting an optical axis deviation aberration caused by an optical axis deviation of the liquid crystal aberration correction apparatus.

  In the correction of spherical aberration using the conventional liquid crystal aberration correction device, when the objective lens moves in the radial direction of the optical disk by the tracking operation and the optical axis of the liquid crystal aberration correction device deviates from the optical axis of the objective lens, the optical axis There is a problem that wavefront aberration due to deviation occurs and the correction effect of spherical aberration correction decreases. As an improvement measure, the coma aberration correction function of the liquid crystal aberration correction apparatus is used to correct the optical axis deviation aberration caused by the deviation of the optical axis of the liquid crystal aberration correction apparatus and the optical axis of the objective lens, thereby reducing the correction effect of the spherical aberration correction. There are things that prevent (see, for example, Patent Document 1 and Patent Document 2).

JP 2002-133697 A (paragraph 0010, FIGS. 1 and 5) JP 2002-358690 A (paragraph 0006, FIG. 6 and FIG. 7)

  However, in the method of correcting the optical axis deviation caused by the deviation between the optical axis of the liquid crystal aberration correcting apparatus and the optical axis of the objective lens by using the coma aberration correcting function, the optical axis deviation aberration is increased when the amount of optical axis deviation increases. There is a problem that the correction cannot be sufficiently performed, and the deterioration of the correction effect of the spherical aberration correction cannot be sufficiently prevented. The reason why such a problem occurs will be described in detail with reference to FIGS. 4 to 15 in the column “Best Mode for Carrying Out the Invention”.

  Therefore, the present invention has been made to solve the above-described problems of the prior art, and the object thereof is optical axis deviation aberration caused by deviation between the optical axis of the objective lens and the optical axis of the spherical aberration corrector. It is an object of the present invention to provide a liquid crystal aberration correction apparatus capable of appropriately correcting the above and an optical head equipped with the liquid crystal aberration correction apparatus.

A liquid crystal aberration correction apparatus according to the present invention is a liquid crystal aberration correction apparatus that corrects wavefront aberration of a light beam toward an objective lens that is supported so as to be displaceable in a predetermined direction orthogonal to an optical axis, the liquid crystal molecule layer, and the liquid crystal molecule A first electrode pattern provided on one surface of the layer, a second electrode pattern provided on the other surface opposite to the first surface of the liquid crystal molecule layer, and the first electrode pattern A voltage application circuit for applying a voltage and applying a voltage having a phase opposite to the voltage applied to the first electrode pattern to the second electrode pattern, wherein the second electrode pattern includes the objective electrode A direction that is outside the effective diameter of the lens and that is disposed on opposite sides of the optical axis of the liquid crystal aberration correction apparatus and orthogonal to the moving direction of the objective lens that is orthogonal to the optical axis of the liquid crystal aberration correction apparatus Have a long shape A first electrode and a second electrode, made of a light transmissive material, are arranged along the moving direction of the objective lens perpendicular to the optical axis of the liquid crystal aberration correcting device, and the from said first electrode Island-like electrodes arranged in the order of the third electrode, the fourth electrode, the fifth electrode, and the sixth electrode toward the second electrode, and higher than the first to sixth electrodes A film-like seventh electrode made of a light-transmitting material having resistance and connecting between the first to sixth electrodes, wherein the third electrode and the sixth electrode correct the liquid crystal aberration. The fourth electrode and the fifth electrode are arranged symmetrically with respect to the optical axis of the liquid crystal aberration correction apparatus, and the fourth electrode and the fifth electrode are arranged symmetrically with respect to the optical axis of the liquid crystal aberration correction apparatus . 1 electrode pattern is made of a light-transmitting material and has an annular shape centering on the optical axis of the liquid crystal aberration correcting device. 8 electrode, made of a light transmissive material, arranged inside the eighth electrode, made of an annular ninth electrode centered on the optical axis of the liquid crystal aberration correction device, and a light transmissive material, The tenth electrode disposed inside the ninth electrode, the eighth electrode, and the ninth electrode are made of a light-transmitting material having higher resistance than the eighth electrode, and the ninth electrode, A film-shaped eleventh electrode connecting the electrodes, and a light-transmitting material having a higher resistance than those of the ninth electrode and the tenth electrode, and the ninth electrode and the tenth electrode And the third to sixth electrodes of the second electrode pattern have the liquid crystal aberration more than the ninth electrode of the first electrode pattern. The first and second electrodes of the second electrode pattern are disposed at positions close to the optical axis of the correction device. The eighth electrode of the first electrode pattern is disposed at a position farther from the optical axis of the liquid crystal aberration correction device than the eighth electrode, and the voltage application circuit is configured to perform the eighth and tenth in the first electrode pattern. The same voltage is applied to the electrodes, and a voltage lower than the voltage applied to the eighth and tenth electrodes is applied to the ninth electrode, and the voltage application circuit includes the second electrode pattern. When correcting the optical axis deviation aberration that occurs when the objective lens is displaced in the direction of the first electrode along the moving direction of the objective lens, the first voltage is applied to the third electrode. , A second voltage lower than the first voltage is applied to the fourth electrode and the sixth electrode, and a third voltage lower than the second voltage is applied to the fifth electrode. And the objective lens is not in the direction of the second electrode. When correcting the optical axis deviation aberration generated in the case of the above, a fourth voltage is applied to the sixth electrode, and a fifth voltage lower than the fourth voltage is applied to the third electrode and the fifth electrode. A voltage is applied, and a sixth voltage lower than the fifth voltage is applied to the fourth electrode .

  According to the present invention, since the refractive index of the liquid crystal molecular layer is changed by the voltage applied to the second electrode pattern for correcting the optical axis deviation aberration, even if the amount of optical axis deviation is large, It is possible to appropriately correct the axial deviation aberration, and to obtain an effect of suppressing a reduction in the correction effect of the spherical aberration correction caused by the optical axis deviation.

The best mode for carrying out the present invention will be described below with reference to the drawings.
<Configuration and operation of optical head>
FIG. 1 is a diagram schematically showing a configuration of an optical head equipped with a liquid crystal aberration correction apparatus 4 according to Embodiment 1 of the present invention. As shown in FIG. 1, the laser light emitted from the laser light source 1 passes through the collimator lens 2, the polarization beam splitter 3, the liquid crystal aberration correction device 4, and the quarter wavelength plate 5 and is condensed by the objective lens 6. Then, the light passes through the light transmission layer 12 of the optical disc 7 and is focused on the optical disc information recording surface 8. The reflected light reflected by the optical disk information recording surface 8 passes through the light transmission layer 12, the objective lens 6, the quarter wavelength plate 5, the liquid crystal aberration correction device 4, and the polarization beam splitter 3, and is collected on the photodiode 10 by the condenser lens 9. Tie a statue to. In the following description or calculation, the laser oscillation wavelength is 405 nm, and the numerical aperture (NA) of the objective lens 6 is 0.85. Further, an apparatus to which the liquid crystal aberration correcting apparatus 4 of the present invention can be applied is not limited to the optical head of FIG. 1, but can be applied to optical heads having other configurations.

  The information recorded on the optical disk information recording surface 8 is reproduced as follows. When the laser light emitted from the laser light source 1 is reflected by the optical disc information recording surface 8, the reflected light is intensity-modulated by the information recorded on the optical disc information recording surface 8. The light intensity of the reflected light is extracted as an electrical signal by the photodiode 10 to reproduce the information recorded on the optical disc information recording surface 8.

  Information is recorded on the optical disk information recording surface 8 as follows. The laser light emitted from the laser light source 1 is intensity-modulated and focused on the optical disc information recording surface 8 by the objective lens 6. Information is recorded by deforming or altering the optical disk information recording surface 8 by heat generated at the focal point by the intensity-modulated laser light.

<Spot diameter and wavefront aberration>
In order to record and reproduce information, the spot diameter of the laser beam is narrowed down to the diffraction limit by the objective lens 6. However, in order to obtain a diffraction-limited spot diameter, the laser beam must converge to the focal point with an ideal spherical wave. The difference between the ideal spherical wave phase and the actual wavefront phase of the laser light is called wavefront aberration. The standard deviation of the wavefront aberration is called WFErms (Wave Front Error root mean square). In general, in order to obtain a diffraction-limited spot diameter, it is considered that the standard deviation WFErms of wavefront aberration is desirably sufficiently smaller than 0.07λ, and more desirably 0.03λ or less. Here, λ is the wavelength of the laser beam.

<Liquid crystal aberration corrector>
FIG. 2 is a diagram schematically showing a cross-sectional structure of the liquid crystal aberration correction apparatus 4 shown in FIG. As shown in FIG. 2, the liquid crystal aberration correction device 4 has a layer (liquid crystal molecule layer) 24 composed of liquid crystal molecules 24a sandwiched between two glass substrates 23a and 23b. Transparent electrodes 22a and 22b for applying an electric field to the alignment films 21a and 21b and the liquid crystal molecules 24a constituting the liquid crystal molecule layer 24 are arranged on the two glass substrates 23a and 23b. When an AC drive voltage (for example, a rectangular wave having a frequency of 1 kHz) is applied between the transparent electrodes 22a and 22b, the liquid crystal molecules 24a are tilted according to the magnitude of the effective value of the applied voltage. The refractive index of the liquid crystal molecular layer 24 is determined by the inclination of the liquid crystal molecules 24a.

  FIG. 3 is a diagram for explaining the principle of aberration correction of the liquid crystal aberration correction apparatus 4 shown in FIG. As shown in FIG. 3, the light beam A emitted from the laser light source passes through the liquid crystal aberration correction device 4. The refractive index change Δn of the liquid crystal aberration correction device 4 is controlled by the voltage applied to the liquid crystal molecular layer 24. If the thickness of the liquid crystal aberration correction device 4 is d, the optical path length of the light beam A changes by Δn · d. Thus, by changing the optical path length of the light beam A, the wavefront aberration can be controlled.

<Spherical aberration correction by liquid crystal aberration correction device>
FIG. 4 is an explanatory diagram of a cross-sectional shape (left side) and spherical aberration (right side) obtained by cutting a light beam (laser light) by a plane orthogonal to the optical axis. Wavefront aberration called spherical aberration is caused by a thickness error of the light transmission layer 12 of the optical disc 7. On the right side of FIG. 4, when the vertical axis is wavefront aberration, the wavefront has a rotationally symmetric shape with respect to the center of the light beam (point 0).

FIG. 5 is a diagram schematically showing the structure of a conventional liquid crystal aberration correction apparatus (comparative example of the present invention) for correcting spherical aberration. As shown in FIG. 5, in the conventional liquid crystal aberration correction apparatus, spherical aberration correction electrodes (upper surface electrodes) 41 to 45 are formed as transparent electrodes (electrodes corresponding to reference numeral 22a in FIG. 2), and transparent electrodes (FIG. 2), coma aberration correction electrodes (lower surface electrodes) 51a, 52b, 53, 52a, 51b are formed. The transparent electrodes 41, 42, and 43 (black portions in FIG. 5) connect the transparent electrodes 41, 42, and 43 using a material having a small resistance value of about 10 ⁻⁸ to 10 ⁻⁷ Ω · m. The transparent electrodes 44 and 45 are made of a material having a sheet resistance of 100Ω / □ or higher.

  6 shows a state in which a rectangular wave is applied to the spherical aberration correction electrode of the conventional liquid crystal aberration correction apparatus shown in FIG. 5, the upper surface electrode potential (upper right), and the effective value of the voltage applied to the liquid crystal molecules (lower right). FIG. A rectangular wave having an amplitude of 1.75V was applied to the upper surface electrodes 41 and 43, and a rectangular wave having an amplitude of 0.35V was applied to the upper surface electrode 42, and the phase was inverted to the lower surface electrodes 51a, 52b, 53, 52a and 51b (reverse) A rectangular wave with a polarity of 1.05V is applied. As a result, the voltage applied to the liquid crystal molecules becomes an effective value of the voltage as shown in the lower right of FIG.

  FIG. 7 shows a cross-sectional shape (upper left) and spherical aberration (upper right) of a light beam cut by a plane orthogonal to the optical axis of the liquid crystal aberration correction device, a planar shape (left middle) of a spherical aberration correction electrode, and a liquid crystal aberration correction device. FIG. 4 is a diagram showing a wavefront aberration (middle right) caused by, and a cross-sectional shape (lower left) obtained by cutting the corrected light beam by a plane orthogonal to the optical axis of the liquid crystal aberration correction apparatus and a corrected spherical aberration (lower right). . In FIG. 7, DD ′ indicates a direction orthogonal to the optical axis of the liquid crystal aberration correction apparatus (the radial direction of the light beam and the radial direction of the optical disk), and 0 indicates the optical axis of the liquid crystal aberration correction apparatus. Indicates the position. As shown in the middle part of FIG. 7, the laser light that has passed through the liquid crystal aberration correction device is added with wavefront aberration generated by the liquid crystal molecular layer of the liquid crystal aberration correction device when a voltage is applied to the spherical aberration correction electrode. . As a result, the spherical aberration is canceled out, and the wavefront aberration can be reduced as shown in the lower right part of FIG.

<Optical axis deviation>
FIGS. 8A and 8B are diagrams showing a deviation between the optical axis of the objective lens 6 and the optical axis of the liquid crystal aberration correction apparatus 4. The objective lens 6 moves following the blurring of the information track on the optical disc in the radial direction of the optical disc during recording and reproduction of the optical disc. That is, the objective lens 6 performs tracking correction for displacing the objective lens 6 in the radial direction of the optical disk by an objective lens actuator 11 including a magnetic coil and a magnet provided in the objective lens holder. Including the assembly error, the optical disk is shown in the inner circumferential direction (that is, inward in the radial direction of the optical disk) shown in FIG. 8A and in the outer peripheral direction of the optical disk shown in FIG. Each moves about several hundred μm. Therefore, due to the movement of the objective lens 6, as shown in FIGS. 8A and 8B, the positional relationship between the optical axis of the liquid crystal aberration correction device 4 and the optical axis of the objective lens 6 is shifted (FIG. 8A ) And (b) are shown as a deviation of 0.4 mm).

<Optical axis deviation aberration>
FIG. 9 shows a cross-sectional shape (upper left) and spherical aberration (upper right) obtained by cutting a light beam at a plane orthogonal to the optical axis of the liquid crystal aberration correction device, a planar shape (left middle) of a spherical aberration correction electrode, and a liquid crystal aberration correction device. FIG. 6 is a diagram showing a wavefront aberration (right middle stage) caused by the above and a spherical aberration after correction (lower stage). The aberration that occurs when the objective lens is displaced is referred to as “optical axis deviation aberration”. This optical axis deviation aberration is caused by a relative positional deviation between the spherical aberration converted to the light beam on the liquid crystal aberration correction device and the aberration given by the liquid crystal aberration correction device due to the deviation of the objective lens. It is. As shown in the upper left part of FIG. 9, an optical axis shift occurs, and the spherical aberration is as shown in the upper right part of FIG. At this time, even if a voltage as shown in the middle left part of FIG. 9 is applied to the spherical aberration correction electrode and wavefront aberration is applied as shown in the middle right part of FIG. 9, the wavefront aberration as shown in the lower part of FIG. 9 remains.

  FIG. 10 is a diagram based on simulation calculation showing the shape of the optical axis deviation aberration that occurs when the objective lens moves in the outer peripheral direction of the optical disc (left side) as a three-dimensional view (right side). FIG. 11 is a diagram based on simulation calculation showing a shape of an optical axis deviation aberration generated when the objective lens moves in the inner peripheral direction of the optical disc (left side) as a three-dimensional view (right side). As shown in FIG. 10 and FIG. 11, the generated optical axis deviation aberration is not a rotationally symmetric shape, so it is shown in a three-dimensional view. The aberration is shown in terms of the aberration of the light beam on the liquid crystal aberration correction apparatus.

<Conventional electrode>
FIG. 12 is a diagram showing the voltage applied to each electrode when the optical axis deviation aberration is corrected by the conventional liquid crystal aberration correction apparatus shown in FIG. As shown in FIG. 12, a rectangular wave (amplitude 1.75 V and amplitude 0.35 V) is applied to the spherical aberration correction electrode, and at the same time, for example, a voltage (phase-inverted amplitude) as shown in FIG. 12 is applied to the coma aberration correction electrode. 1. When the objective lens has an optical axis shift in the outer peripheral direction of the optical disk by adding a rectangular wave of 1.25 V, a phase inverted amplitude of 1.05 V, and a phase inverted amplitude of 0.85 V) Aberration can be corrected. The amplitude of the rectangular wave applied to the coma aberration correction electrode is determined by the direction of displacement of the objective lens and the amount of displacement.

  FIG. 13 shows optical axis misalignment aberration (upper stage), correction aberration (middle stage), and residual aberration (lower stage) when the objective lens is moved in the outer peripheral direction of the optical disk by the conventional liquid crystal aberration correction apparatus shown in FIG. It is a figure based on simulation calculation. In FIG. 13, the left column indicates the optical axis deviation is 0.118 mm in the optical disc outer peripheral direction, the middle column indicates the optical axis offset is 0.236 mm in the optical disc outer peripheral direction, and the right column indicates the optical axis deviation is the optical disc outer periphery. The case of 0.401 mm is shown in the direction.

  FIG. 14 shows the optical axis deviation aberration (upper stage), correction aberration (middle stage), and residual aberration (lower stage) when the objective lens is moved in the inner peripheral direction of the optical disk by the conventional liquid crystal aberration correction apparatus shown in FIG. It is a figure based on the simulation calculation which shows). In FIG. 14, the left column shows the optical axis deviation of 0.401 mm in the optical disk inner circumferential direction, the middle column shows the optical axis deviation of 0.236 mm in the optical disk inner circumferential direction, and the right column shows the optical axis deviation. The case of 0.118 mm in the inner peripheral direction of the optical disc is shown.

  FIG. 15 is a diagram showing a correction effect by the conventional liquid crystal aberration correction apparatus shown in FIG. The horizontal axis of FIG. 15 indicates the amount of optical axis shift (position shift amount), and the vertical axis indicates the standard deviation WFErms of residual aberration. When the amount of deviation of the optical axis is positive, it indicates an optical axis deviation in the outer circumferential direction of the optical disk, and when it is negative, it indicates an optical axis deviation in the inner circumferential direction of the optical disk. For example, when the amount of optical axis deviation is 0.4 mm, the standard deviation WFErms of residual aberration is 0.045λ in the conventional correction method (triangle mark in FIG. 15), and no correction is made (before correction) ) (Indicated by a circle in FIG. 15), but it is insufficient because 0.030λ which is a desirable value is not satisfied.

<Liquid Crystal Aberration Correction Device of the Present Invention>
FIG. 16 is a diagram schematically showing the configuration of the liquid crystal aberration correction apparatus 4 according to Embodiment 1 of the present invention. As shown in FIG. 16, the liquid crystal aberration correction device 4 according to the first embodiment includes a liquid crystal molecular layer 24, spherical aberration correction electrodes 41 to 45 disposed on one surface of the liquid crystal molecular layer 24, and liquid crystal Optical axis misalignment correction electrodes 31, 32 a, 33 b, 35, 32 b, 33 a, 33 b, and 34 are disposed on the other surface of the molecular layer 24. The spherical aberration correction electrodes 41 to 45 shown in FIG. 16 have the same structure as the spherical aberration correction electrode in the conventional liquid crystal aberration correction apparatus.

The optical axis deviation aberration correcting electrodes 31, 32a, 33b, 35, 32b, 33a, 33b, and 34 are transparent electrodes 31, 32a, made of a light-transmissive material having a low resistance (10 ⁻⁸ to 10 ⁻⁷ Ω · m). 32b, 33a, 33b, 34 and the transparent electrode 35 made of a light-transmitting material having a high resistance (sheet resistance of 100Ω / □ or more) that electrically connects the transparent electrodes 31, 32a, 32b, 33a, 33b, 34 Become. Since the transparent electrodes 31 and 34 are disposed outside the effective diameter of the objective lens 6, they may be electrodes made of an opaque conductive material that is not light transmissive. The transparent electrodes 32a, 33b, 32b, and 33a are linearly arranged in the radial direction of the optical disc (the horizontal direction in FIG. 16). The transparent electrode 32a and the transparent electrode 33a are arranged at symmetrical positions around the optical axis position of the liquid crystal aberration correction device 4, and the transparent electrode 33b and the transparent electrode 32b are optical axis positions of the liquid crystal aberration correction device 4. Are arranged at symmetrical positions with respect to the center. In the first embodiment, the transparent electrodes 32a, 32b, 33a, 33b are arranged at positions corresponding to the insides of the opposed ring-shaped aberration correction electrodes 42. The transparent electrodes 31 and 34 are arranged at positions corresponding to the outer sides of the opposed ring-shaped aberration correction electrodes 41. In FIG. 16, the transparent electrodes 31 and 34 have a longer shape in the direction orthogonal to the optical disk radial direction (the tangential direction of the disk track) than the transparent electrodes 32 a, 32 b, 33 a, and 33 b.

  FIG. 17 is a diagram illustrating an example of a voltage applied to the liquid crystal aberration correction apparatus 4 according to the first embodiment illustrated in FIG. In the liquid crystal aberration correction apparatus 4 according to Embodiment 1, for example, a voltage as shown in FIG. 17 is applied to correct the optical axis deviation aberration. In addition to the application of the rectangular wave Vsa1 (amplitude 1.75V) to the spherical aberration correction electrodes 41 and 43 and the application of the rectangular wave Vsa2 (amplitude 0.35V) to the spherical aberration correction electrode 42, for example, optical axis deviation aberration correction The rectangular wave Vd2 (amplitude 1.25V) phase-inverted to the electrodes 34 and 32a, the rectangular wave Vd1 (amplitude 0.85V) phase-inverted to the optical axis deviation aberration correcting electrodes 31 and 32b, and the optical axis deviation aberration correcting electrode 33a. , 33b, a phase-inverted rectangular wave Vd3 (amplitude 1.05V) can be used to correct optical axis deviation aberration when the objective lens moves in the outer peripheral direction and optical axis deviation occurs.

  18A and 18B are diagrams based on simulation calculation showing the voltage applied to the optical axis deviation aberration correction electrode of the liquid crystal aberration correction device 4 and the correction aberration generated by the liquid crystal aberration correction device 4. FIG. As shown in FIG. 18A, in order to correct the optical axis deviation aberration due to the movement of the objective lens 6 in the outer peripheral direction of the optical disc, for example, a rectangle whose phase is inverted to the optical axis deviation aberration correction electrodes 34 and 32a. Wave Vd2 (amplitude 1.25V), rectangular wave Vd1 (amplitude 0.85V) phase-inverted to optical axis deviation aberration correcting electrodes 31, 32b, rectangular wave Vd3 phase-inverted to optical axis deviation aberration correcting electrodes 33a, 33b (Amplitude 1.05V) is added. Further, as shown in FIG. 18B, in order to correct the optical axis deviation aberration due to the movement of the objective lens 6 in the inner peripheral direction of the optical disc, for example, phase inversion is applied to the optical axis deviation aberration correcting electrodes 34 and 33b. The rectangular wave Vd2 (amplitude 0.85V), the rectangular wave Vd1 (amplitude 1.25V) phase-inverted to the optical axis deviation aberration correction electrodes 31 and 33a, and the phase inverted to the optical axis deviation aberration correction electrodes 32a and 32b. A rectangular wave Vd3 (amplitude 1.05V) is applied. In this way, the electrodes for applying the rectangular waves Vd1, Vd2, and Vd3 and the amplitudes of the rectangular waves Vd1, Vd2, and Vd3 are switched in the inner and outer circumferential directions of the optical disk.

  FIG. 19 shows optical axis deviation aberration (upper stage), correction aberration (middle stage), and residual aberration (lower stage) when the objective lens 6 is moved in the outer peripheral direction of the optical disk by the liquid crystal aberration correction apparatus 4 according to the first embodiment. It is a figure based on the simulation calculation shown. In FIG. 19, the left column indicates the optical axis deviation is 0.118 mm in the optical disc outer peripheral direction, the middle column indicates the optical axis offset is 0.236 mm in the optical disc outer peripheral direction, and the right column indicates the optical axis deviation is the optical disc outer periphery. The case of 0.401 mm is shown in the direction. As can be seen from comparison with FIG. 13 (conventional), the residual aberration is small in the case of FIG. 19 (present invention).

  FIG. 20 shows optical axis misalignment aberrations (upper stage), correction aberrations (middle stage), and residual aberrations (lower stage) when the objective lens is moved in the inner circumferential direction of the optical disk by the liquid crystal aberration correction apparatus according to the first embodiment. It is a figure based on the simulation calculation which shows. In FIG. 20, the left column shows the optical axis deviation of 0.401 mm in the optical disc outer peripheral direction, the middle column shows the optical axis deviation of 0.236 mm in the optical disc outer peripheral direction, and the right column shows the optical axis deviation in the optical disc outer periphery direction. The case of 0.118 mm in the direction is shown. As can be seen from comparison with FIG. 14 (conventional), the residual aberration is small in the case of FIG. 20 (present invention).

  FIG. 21 is a diagram showing a correction effect by the liquid crystal aberration correction apparatus 4 according to the first embodiment. The horizontal axis of FIG. 21 indicates the amount of deviation (positional deviation) of the optical axis, and the vertical axis indicates the standard deviation WFErms of residual aberration. When the amount of deviation of the optical axis is positive, it indicates an optical axis deviation in the outer circumferential direction of the optical disk, and when it is negative, it indicates an optical axis deviation in the inner circumferential direction of the optical disk. For example, when the optical axis deviation amount is 0.4 mm, the standard deviation WFErms of residual aberration is 0.045λ in the conventional correction method (triangle mark in FIG. 21), which is a desirable value of 0.030λ. Although not satisfied, the WFErms is 0.026λ in the present invention (indicated by a square in FIG. 21), and therefore satisfies a desirable value of 0.030λ or less. As shown in FIG. 21, when the optical axis deviation amount is 0.4 mm, the standard deviation WFErms of the residual aberration can be reduced to 3/5 as compared with the conventional example. Compared to (circle in FIG. 21), it can be reduced to 2/5.

<Power supply line of the present invention>
FIG. 22 is a plan view schematically showing power supply lines for applying a voltage to the transparent electrodes 32a, 33b, 32b, and 33a of the liquid crystal aberration correcting apparatus 4 according to the first embodiment. In order to apply a voltage to the transparent electrodes 31, 32a, 32b, 33a, 33b, 34, wiring is necessary. In the present invention, since the potential drop using the high-resistance transparent electrode 35 is used, the optical axis position of the liquid crystal aberration corrector 4 is centered on the liquid crystal aberration corrector 4 so as not to adversely affect the potential drop. A point-symmetric wiring structure is adopted. As shown in FIG. 22, the transparent electrode 31 is supplied with power through a power supply line 71, and the transparent electrode 34 is supplied with power through a power supply line 74. The transparent electrode 32a is composed of two semicircular electrodes and is fed by two feeding lines 72a connected to each. Further, the transparent electrode 33 is a single circular transparent electrode, and is fed by a feeder line 73b. The transparent electrode 32b is a single circular transparent electrode, and is fed by the feeder line 72b. Further, the transparent electrode 33a is composed of two semicircular electrodes and is fed by two feeding lines 73a connected to each of them. The portions where the feeder lines 72a, 72b, 73a, 73b are not provided with the high-resistance transparent electrode 35, and the feeder lines 72a, 72b, 73a, 73b are in contact with the high-resistance transparent electrode 35. Not.

<Drive voltage>
FIG. 23 is a diagram showing a circuit configuration for generating a driving AC voltage of the liquid crystal aberration correcting apparatus 4 according to the first embodiment. The rectangular wave generated by the rectangular wave generator 61 becomes a rectangular wave Vsa1 by the amplitude adjustment circuit 62 and is applied to the spherical aberration correction electrodes 41 and 43. The rectangular wave generated by the rectangular wave generator 61 is converted into a rectangular wave Vsa2 by the amplitude adjustment circuit 63 and applied to the spherical aberration correction electrode 42.

  The rectangular wave generated by the rectangular wave generator 61 is reversed in phase by the inverter 64 and becomes a rectangular wave Vd1 by the amplitude adjusting circuit 65 and applied to the optical axis deviation aberration correcting electrode 31. The rectangular wave Vd1 is applied to either the optical axis deviation aberration correcting electrode 32b or 33a by the switching unit 68.

  The rectangular wave reversed in phase by the inverter 64 becomes a rectangular wave Vd2 by the amplitude adjustment circuit 66 and is applied to the optical axis deviation aberration correcting electrode 34. The rectangular wave Vd2 is applied to either the optical axis deviation aberration correcting electrode 32a or 33b by the switching unit 68.

  The rectangular wave reversed in phase by the inverter 64 becomes a rectangular wave Vd3 by the amplitude adjusting circuit 67 and is applied to the optical axis deviation aberration correcting electrodes 33a and 33b or the optical axis deviation aberration correcting electrodes 32a and 32b.

  FIG. 24A shows the relationship between the optical disk thickness error and the voltage effective values of the drive voltages Vsa1 and Vsa2, and FIG. 24B shows the relationship between the optical axis deviation and the voltage effective values of the drive voltages Vd1, Vd2, and Vd3. FIG. The amplitudes of the voltages Vsa1 and Vsa2 are changed according to the thickness error of the optical disc, but the thickness error itself cannot be detected, so that the optical disc playback signal is the best (for example, the amplitude of the playback RF signal is maximized). The amplitude adjustment circuits 62 and 63 are controlled.

  The amplitude adjustment circuits 65, 66, and 67 are controlled based on the output of the position sensor (not shown) of the objective lens 6 (indicating the amount of optical axis deviation and the direction of deviation). The effective values of the output voltages of the amplitude adjustment circuits 65, 66, and 67 are the voltages Vd1, Vd2, and Vd3 in FIG. Further, the position of the objective lens 6 may be detected by detecting a current flowing in a drive coil provided in the objective lens actuator. The switching unit 68 switches based on the output direction of the objective lens position sensor in accordance with the movement direction (optical axis deviation direction) of the objective lens.

  As described above, when the liquid crystal aberration correction apparatus according to the first embodiment is used, the optical axis of the objective lens 6 and the liquid crystal aberration correction apparatus 4 are simultaneously corrected with the spherical aberration caused by the thickness error of the light transmission layer of the optical disk. The optical axis deviation aberration caused by the optical axis deviation can be corrected appropriately.

<Modification>
FIG. 25 is a diagram schematically showing a configuration of a modification of the liquid crystal aberration correction apparatus according to the first embodiment. 25, the same reference numerals are given to the same or corresponding components as those in FIG. The liquid crystal aberration correction apparatus shown in FIG. 25 is different from the case of FIG. 16 in that the inner sides of the long optical axis deviation correction electrodes 31 and 34 are formed in an arc shape. Except for this point, the liquid crystal aberration correction apparatus shown in FIG. 25 is the same as the liquid crystal aberration correction apparatus shown in FIG.

  FIG. 26 is a diagram schematically showing a configuration of another modification of the liquid crystal aberration correction apparatus according to the first embodiment. In FIG. 26, the same reference numerals are given to the same or corresponding components as those in FIG. In the liquid crystal aberration correction apparatus shown in FIG. 26, the inside of the long optical axis deviation aberration correction electrodes 31 and 34 is formed in an arc shape, and the optical axis deviation aberration correction electrodes 32a, 33b, 32b, and 32a are elliptical. This is different from the case of FIG. Except for these points, the liquid crystal aberration correction apparatus shown in FIG. 26 is the same as the liquid crystal aberration correction apparatus shown in FIG.

  In FIG. 16, FIG. 25, and FIG. 26, the transparent electrodes 32a, 33b, 32b, and 32a can be shaped as free curves other than circular, semicircular, elliptical, and semielliptical.

  Further, in the above description, the case where the optical axis deviation aberration correcting electrodes 31, 32a, 33b, 32b, 32a, 34 are arranged on the surface of the liquid crystal molecular layer 24 far from the objective lens 6 has been described. A structure in which the positions of the electrodes 41 to 45 and the electrodes 31, 32a, 33b, 32b, 32a, and 34 are interchanged is also possible.

  Furthermore, in the above description, the liquid crystal molecular layer to which the optical axis deviation aberration correcting electrodes 31, 32a, 33b, 32b, 32a, and 34 apply a voltage, and the liquid crystal molecular layer to which the spherical aberration correcting electrodes 41 to 45 apply a voltage. Is a common liquid crystal molecular layer, but each may be a separate liquid crystal molecular layer.

  In the above description, the case where the spherical aberration correction is performed in the liquid crystal molecular layer is shown, but other optical means for correcting the spherical aberration (for example, the objective lens 6 and the liquid crystal aberration for correcting the optical axis deviation aberration). You may use together with the lens arrange | positioned between correction | amendment apparatuses, its moving mechanism, etc.).

  In addition to the optical head, the present invention can also be applied to an apparatus for correcting aberrations in various optical systems (for example, a laser microscope).

It is a figure which shows schematically the structure of the optical head carrying the liquid-crystal aberration correction apparatus which concerns on Embodiment 1 of this invention. It is a figure which shows roughly the cross-sectional structure of the liquid-crystal aberration correction apparatus shown by FIG. It is a figure for demonstrating the aberration correction principle of the liquid-crystal aberration correction apparatus shown by FIG. It is explanatory drawing of the cross-sectional shape (left side) and spherical aberration (right side) which cut the light ray with the surface orthogonal to the optical axis of a liquid-crystal aberration correction apparatus. It is a figure which shows roughly the structure of the conventional liquid-crystal aberration correction apparatus for spherical aberration correction. FIG. 6 is a diagram illustrating a state in which a rectangular wave is applied to a spherical aberration correction electrode of the conventional liquid crystal aberration correction apparatus illustrated in FIG. 5, an upper surface electrode potential, and an effective value of a voltage applied to liquid crystal molecules. Cross-sectional shape (upper left) and spherical aberration (upper right) of the light beam cut by a plane orthogonal to the optical axis of the liquid crystal aberration correction device, planar shape of the spherical aberration correction electrode (middle left) and wavefront aberration caused by the liquid crystal aberration correction device It is a figure which shows the cross-sectional shape (lower left stage) which cut | disconnected the light ray corrected by the surface orthogonal to the optical axis of a liquid crystal aberration correction apparatus (lower left stage), and the spherical aberration after correction | amendment (lower right stage). (A) And (b) is a figure which shows the shift | offset | difference of the optical axis of an objective lens, and the optical axis of a liquid-crystal aberration correction apparatus. Cross-sectional shape (upper left) and spherical aberration (upper right) of the light beam cut by a plane orthogonal to the optical axis of the liquid crystal aberration correction device, planar shape of the spherical aberration correction electrode (middle left) and wavefront aberration caused by the liquid crystal aberration correction device (Right middle) and spherical aberration after correction (lower). It is a figure which shows the shape of the optical axis deviation aberration which arises when an objective lens moves to the outer peripheral direction of an optical disk (left side) as a three-dimensional figure (right side). It is a figure which shows the shape of the optical axis offset aberration which arises when an objective lens moves to the inner peripheral direction of an optical disk (left side) as a three-dimensional figure (right side). It is a figure which shows the voltage applied to each electrode, when correcting an optical axis offset aberration with the conventional liquid-crystal aberration correction apparatus shown by FIG. FIG. 6 is a diagram showing optical axis misalignment aberration (upper stage), correction aberration (middle stage), and residual aberration (lower stage) when the objective lens is moved in the outer peripheral direction of the optical disk by the conventional liquid crystal aberration correction apparatus shown in FIG. FIG. 6 is a diagram showing optical axis misalignment aberration (upper stage), correction aberration (middle stage), and residual aberration (lower stage) when the objective lens is moved in the inner peripheral direction of the optical disc by the conventional liquid crystal aberration correction apparatus shown in FIG. . It is a figure which shows the correction effect by the conventional liquid-crystal aberration correction apparatus shown by FIG. It is a figure which shows schematically the structure of the liquid-crystal aberration correction apparatus which concerns on Embodiment 1 of this invention. It is a figure which shows an example of the voltage applied to the liquid-crystal aberration correction apparatus which concerns on Embodiment 1 shown by FIG. (A) And (b) is a figure which shows the correction aberration which the voltage applied to the optical axis deviation aberration correction electrode of a liquid crystal aberration correction apparatus, and the liquid crystal aberration correction apparatus generate | occur | produce. FIG. 6 is a diagram illustrating optical axis misalignment aberration (upper stage), correction aberration (middle stage), and residual aberration (lower stage) when the objective lens moves in the outer peripheral direction of the optical disc by the liquid crystal aberration correction apparatus according to the first embodiment. It is a figure which shows the optical axis deviation aberration (upper stage), correction | amendment aberration (middle stage), and residual aberration (lower stage) when an objective lens moves to the inner peripheral direction of an optical disk with the liquid crystal aberration correction apparatus which concerns on Embodiment 1. FIG. It is a figure which shows the correction effect by the liquid-crystal aberration correction apparatus which concerns on Embodiment 1. FIG. FIG. 3 is a diagram schematically showing a power supply line of the liquid crystal aberration correction apparatus according to the first embodiment. 3 is a diagram illustrating a circuit configuration for generating a drive AC voltage of the liquid crystal aberration correction apparatus according to Embodiment 1. FIG. (A) shows the relationship between the optical disc thickness error and the drive AC voltage, and (b) shows the relationship between the optical axis deviation and the drive AC voltage. It is a figure which shows schematically the structure of the modification of the liquid crystal aberration correction apparatus which concerns on Embodiment 1. FIG. It is a figure which shows schematically the structure of the other modification of the liquid-crystal aberration correction apparatus which concerns on Embodiment 1. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser light source, 2 Collimator lens, 3 Polarization beam splitter, 4 Liquid crystal aberration correction apparatus, 5 1/4 wavelength plate, 6 Objective lens, 7 Optical disk, 8 Optical disk information recording surface, 9 Condensing lens, 10 Photodiode, 11 Objective Lens actuator, 12 light transmission layer, 21a, 21b alignment film, 22a transparent electrode, 22b transparent electrode, 23a, 23b glass substrate, 24 liquid crystal molecular layer, 24a liquid crystal molecule, 31-35 optical axis deviation aberration correcting electrode (transparent electrode) 41 to 45 Spherical aberration correction electrode (transparent electrode), 51 to 53 coma aberration correction electrode, 61 rectangular wave generator, 62 amplitude adjustment circuit, 63 amplitude adjustment circuit, 64 inverter, 65 amplitude adjustment circuit, 66 amplitude adjustment circuit, 67 Amplitude adjustment circuit, 68 switching means.

Claims (3)

A liquid crystal aberration correction apparatus that corrects wavefront aberration of a light beam toward an objective lens that is supported so as to be displaceable in a predetermined direction orthogonal to an optical axis,
A liquid crystal molecular layer;
A first electrode pattern provided on one surface of the liquid crystal molecular layer;
A second electrode pattern provided on the other surface facing the first surface of the liquid crystal molecular layer ;
A voltage application circuit for applying a voltage to the first electrode pattern and applying a voltage having a phase opposite to that of the voltage applied to the first electrode pattern to the second electrode pattern ;
The second electrode pattern is
Outside the effective diameter of the objective lens , arranged on opposite sides of the optical axis of the liquid crystal aberration correction apparatus, and orthogonal to the moving direction of the objective lens orthogonal to the optical axis of the liquid crystal aberration correction apparatus A first electrode and a second electrode having a shape elongated in the direction of
A third electrode made of a light transmissive material, arranged along the direction of movement of the objective lens perpendicular to the optical axis of the liquid crystal aberration correction device , and from the first electrode toward the second electrode , Island-like electrodes arranged in the order of the fourth electrode, the fifth electrode, and the sixth electrode ;
A film-like seventh electrode made of a light-transmitting material having a higher resistance than the first to sixth electrodes, and connecting between the first to sixth electrodes;
The third electrode and the sixth electrode are arranged symmetrically with respect to the optical axis of the liquid crystal aberration correction device, and the fourth electrode and the fifth electrode are light beams of the liquid crystal aberration correction device. It is arranged in a symmetrical position with respect to the axis ,
The first electrode pattern is
An annular eighth electrode made of a light transmissive material and centered on the optical axis of the liquid crystal aberration correction device;
An annular ninth electrode made of a light transmissive material, disposed inside the eighth electrode, and centered on the optical axis of the liquid crystal aberration correction device;
A tenth electrode made of a light transmissive material and disposed inside the ninth electrode;
A film-like eleventh electrode made of a light transmissive material having a higher resistance than the eighth electrode and the ninth electrode, and connecting between the eighth electrode and the ninth electrode;
A film-like twelfth electrode made of a light-transmitting material having higher resistance than the ninth electrode and the tenth electrode, and connecting between the ninth electrode and the tenth electrode;
Have
The third to sixth electrodes of the second electrode pattern are disposed closer to the optical axis of the liquid crystal aberration correction device than the ninth electrode of the first electrode pattern, and the second electrode pattern The first and second electrodes of the electrode pattern are arranged at positions farther from the optical axis of the liquid crystal aberration correction device than the eighth electrode of the first electrode pattern,
In the first electrode pattern, the voltage application circuit applies the same voltage to the eighth and tenth electrodes, and applies the same voltage to the eighth and tenth electrodes to the ninth electrode. Apply a voltage lower than the voltage,
In the second electrode pattern, the voltage application circuit corrects an optical axis deviation aberration that occurs when the objective lens is displaced in the direction of the first electrode along the movement direction of the objective lens. Sometimes, a first voltage is applied to the third electrode, a second voltage lower than the first voltage is applied to the fourth electrode and the sixth electrode, and the fifth electrode is applied to the fifth electrode. When a third voltage lower than the second voltage is applied and the objective lens corrects optical axis deviation aberration that occurs when the objective lens is displaced in the direction of the second electrode, 4 is applied, a fifth voltage lower than the fourth voltage is applied to the third electrode and the fifth electrode, and a lower voltage than the fifth voltage is applied to the fourth electrode. 6. A liquid crystal aberration correcting apparatus, wherein a voltage of 6 is applied . The second electrode pattern has a power supply line for applying a voltage to each of the first to sixth electrodes;
The liquid crystal aberration correction device according to claim 1 , wherein the second electrode pattern has a point-symmetric shape about the optical axis of the liquid crystal aberration correction device. A light source;
An objective lens that is supported so as to be displaceable in a predetermined direction, and that focuses the light beam from the light source on the information recording surface of the optical disc;
Is disposed between the light source and the objective lens, and the liquid crystal aberration correcting device according to claim 1 or 2 for correcting the wavefront aberration of a light beam passing through,
An optical head comprising: a light receiving element that detects a light beam reflected by the information recording surface.

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