JP4337191B2 - Optical head device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical head device that records and reproduces optical information of an optical recording medium such as an optical disk.
[0002]
[Prior art]
A DVD, which is an optical disk, records digital information at a higher density than a CD, which is also an optical disk, and an optical head device for reproducing a DVD has a light source (semiconductor laser) wavelength of 650 nm, which is shorter than 780 nm of the CD. Alternatively, the beam spot diameter of the laser beam is reduced to about half of the CD by setting it to 635 nm, or setting the numerical aperture (NA) of the objective lens to 0.6 which is larger than 0.45 of CD.
[0003]
Furthermore, in the next generation optical recording, a higher recording density can be obtained by setting the wavelength of the light source to about 400 nm and the NA to 0.6 or more.
However, when the wavelength of the light source is shortened and the NA of the objective lens is increased, the amount of wavefront aberration (mainly coma aberration) increases due to the occurrence of optical disc tilt in which the optical disc surface is tilted at right angles to the optical axis. The characteristics deteriorate and it becomes difficult to read signals on the optical disk surface.
[0004]
In high-density recording, several methods have been proposed in order to increase the allowable amount of the optical head device for optical disc tilt.
One is a method of adding a movable axis so that the objective lens is tilted according to the detected tilt angle with respect to the objective lens actuator that is normally driven by two axes. However, this additional method has a problem that the structure of the actuator is complicated.
[0005]
The other is a method in which a phase correction element is provided between the objective lens and the light source. With this correction method, it is possible to widen a tilt margin, which is a permissible amount of tilting by simply incorporating an element into the optical head device without significantly modifying the actuator. An example of this correction method is Japanese Patent Laid-Open No. 10-20263 for correcting optical disc tilt. This is by applying a voltage according to the optical disc tilt angle to the partial electrode formed by dividing the electrode of the phase correction element to change the effective refractive index of the birefringent material such as liquid crystal inside the phase correction element, The coma aberration due to the optical disc tilt is corrected by the generated phase shift distribution.
[0006]
[Problems to be solved by the invention]
Conventionally proposed phase correction elements have electrode patterns (the shape of each partial electrode and the overall shape) so that optimal aberration correction can be performed when the central axis of the objective lens coincides with the optical axis. Designed. Therefore, when a shift occurs in the objective lens in the radial direction (radial direction of the optical disk) due to the tracking servo of the optical disk, a positional shift occurs between the aberration distribution that requires correction and the electrode pattern formed on the phase correction element. As a result, the aberration correction function deteriorates.
[0007]
In order to improve the functional deterioration of aberration correction based on the objective lens shift, there is a method in which a phase correction element is mounted on an actuator and is driven integrally with the objective lens. However, in this method, it is necessary to change the design of the actuator in accordance with the phase correction element to be mounted, and on the other hand, there are difficulties such as restrictions on the size and weight of the phase correction element that can be mounted on the actuator.
Further, the phase correction element has a problem in a method for miniaturization, a wiring method for a signal lead line, and the like.
[0008]
[Means for Solving the Problems]
The present invention has been made to solve the problems described above, the light source and the objective lens for converging on an optical recording medium the light emitted from the light source, the light source and the objective lens a phase correction element for correcting the wavefront aberration in it is placed in the optical path to change the phase distribution of the wave front of the outgoing light on the optical recording medium between a voltage for changing the phase distribution on the phase correcting element the an optical head apparatus and a control voltage generator for providing said phase correction device is provided with a sandwiched liquid crystal layer between a pair of transparent substrates having a transparent electrode each of which is formed with said substrate One or more of the transparent electrodes are divided into a plurality of partial electrodes so that different voltages can be applied, and the value of the wavefront aberration caused by the tilt of the optical recording medium across the optical axis is extremely small. Take value, before Partial electrodes are arranged corresponding to at least two parts of the electrodes, and the partial electrodes corresponding to the respective extreme values are further divided into five pieces which are divided in a direction substantially perpendicular to the direction connecting the two extreme values. Among the five microelectrodes constituting the partial electrode corresponding to the extreme value, the contour shape of the region formed by the three microelectrodes arranged in succession is substantially equal, and the phase There is provided an optical head device characterized in that a different voltage can be applied to each of the microelectrodes in accordance with a deviation amount between an optical axis of a correction element and an optical axis of the objective lens. Further, the transparent electrode provides the above optical head device made of ITO.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The phase correction element mounted on the optical head device of the present invention includes a pair of transparent substrates on which electrodes are formed and a liquid crystal sandwiched between the substrates so that wavefront aberration can be corrected. A voltage can be applied to each portion in accordance with the amount of deviation between the optical axis of the correction element and the optical axis of the objective lens. Here, the configuration that can be applied to each part means that the electrode is divided into a plurality of partial electrodes or the like, and a terminal for voltage supply is appropriately provided so that a desired voltage distribution can be obtained in a film-like electrode having a large sheet resistance And so on.
With the above configuration, in the optical head device of the present invention, it is not necessary to mount a phase correction element on the actuator, and wavefront aberration can be corrected without being restricted by size, weight, or the like.
[0010]
The optical head device of the present invention shown in FIG. 1 is for reproducing information recorded on an optical recording medium 8 which is an optical disk such as a CD or a DVD. The light emitted from the semiconductor laser 1 as the light source passes through the polarization beam splitter 2 of the hologram, for example, becomes a parallel light beam by the collimating lens 3, passes through the phase correction element 4 and the quarter wavelength plate 5, and then enters the actuator 7. The light is condensed on the optical recording medium 8 by the installed objective lens 6. The phase correction element 4 in the present invention will be described later.
[0011]
The condensed light is reflected by the optical recording medium 8 and sequentially passes through the objective lens 6, the quarter-wave plate 5, the phase correction element 4, and the collimating lens 3, and then diffracted by the polarization beam splitter 2. Then, the light enters the photodetector 9. When the light emitted from the semiconductor laser 1 is reflected by the optical recording medium 8, the amplitude is modulated by the pits formed on the recording medium, and this light reads the recorded information as a light intensity signal by the photodetector 9. be able to.
[0012]
Further, a tracking error signal corresponding to the deviation from the pit row of the light spot position collected on the optical recording medium 8 can be obtained from the divided photodetector constituting the photodetector 9. By controlling the actuator 7 by the tracking control circuit 12, the objective lens 6 is adjusted in the radial direction (radial direction) including the light spot position of the optical disc so that the light is always condensed on the track.
[0013]
The polarization beam splitter 2 includes, for example, a polarization hologram, and strongly diffracts light having a polarization component in the maximum direction of optical anisotropy (refractive index difference) of the polarization hologram. The phase correction element 4 is controlled in accordance with the tilt signal of the optical disk detected by the tilt sensor 10 by the phase correction element control circuit 11.
[0014]
Next, the configuration of the phase correction element in the present invention will be described with reference to FIG. A transparent lower substrate 21a and upper substrate 21b using non-alkali glass having a thickness of 0.5 mm are bonded to each other with a sealing material 22 mainly composed of an epoxy compound to form a cell. A glass spacer and a conductive spacer with a gold coating on the surface of the resin are contained.
[0015]
On the cell inner surface of the lower substrate 21a, a transparent electrode 24a, for example, an insulating layer 25a mainly composed of silica and an alignment film 26a are formed in this order from the substrate side. Similarly, a transparent electrode 24b, an insulating layer 25b, and an alignment film 26b are formed on the cell inner surface of the lower substrate 21b.
[0016]
In the phase correction element of the present invention, ITO is used as the transparent electrode. However, a metal thin film or the like is not limited to ITO as long as it has optical transparency and conductivity. The transparent electrode 24a is patterned (divided) into a plurality of partial electrodes, and is further patterned so that it can be connected to a wiring from a power source that is a control voltage generator at the electrode lead-out portion 27.
Further, the transparent electrode 24b is in electrical contact with a ground electrode (not shown) formed on the lower substrate 21a by the conductive spacer described above, and the power is supplied from the electrode lead-out portion 27 in the same manner as the partial electrode of the transparent electrode 24a. Can be connected with the wiring.
[0017]
The cell is filled with liquid crystal 23. For example, nematic liquid crystal with a refractive index anisotropy (difference between ordinary light refractive index and extraordinary light refractive index) of 0.15 is used. Further, the interval (cell interval) between the lower substrate 21a and the upper substrate 21b is, for example, 3 μm. As means for obtaining a desired phase shift amount, the birefringence of the liquid crystal is used, and it is desirable to increase the refractive index anisotropy of the liquid crystal material and to reduce the cell interval because the responsiveness can be improved. However, the smaller the cell interval, the more difficult it becomes to produce the cell. Therefore, it is desirable that the refractive index anisotropy of the liquid crystal is 0.1 to 0.2, and the cell interval is about 2 to 5 μm.
[0018]
Moreover, after spin-coating a polyimide on the transparent board | substrate and making it into the polyimide film by baking, the orientation force was provided by the rubbing method and it was set as the orientation film. However, the alignment film is also produced by oblique vapor deposition of silica in addition to the method of rubbing an organic thin film such as polyimide with a cloth as described above. In the phase correction element of the present invention, the liquid crystal molecules 28 are aligned in the left-right direction in the figure as shown in FIG.
[0019]
Hereinafter, the phase correcting element in the present invention will be described in detail in comparison with the conventional element. FIG. 3 shows the wavefront aberration distribution within the effective pupil diameter when the NA of the objective lens is 0.6, and most of the wavefront aberration caused by the optical disc tilt is a coma aberration component. Numbers in the figure, for example, −150 nm to 150 nm represent the magnitude of wavefront aberration, and a positive sign indicates a state in which the wavefront (phase) is advanced, and a negative sign indicates a state in which the wavefront is delayed. Therefore, if the light focused on the optical disc can form in advance an aberration distribution having a phase opposite to that of the wavefront aberration in FIG. 3 by the phase correction element, the wavefront aberration caused by the optical disc tilt can be canceled. This phase correction element cancels the wavefront aberration according to the following principle.
[0020]
The phase difference in FIG. 4 is represented by Δn · d, where Δn is the refractive index of the liquid crystal 23 sensed by the transmitted light based on the application of 3 V, and d is the cell spacing. When the voltage applied is 0V, the liquid crystal molecules 28 as shown in FIG. 2 are almost parallel to the substrate surface, n represents approximately equal to the liquid crystal of the extraordinary refractive index n _e. When the applied voltage is increased, the liquid crystal molecules are aligned in the vertical direction. However, up to about 6 V, the liquid crystal molecules are not completely aligned vertically, and the inclination of the liquid crystal molecules changes depending on the voltage.
[0021]
Therefore, if the voltage applied to the phase correction element is controlled, the phase difference ΔP shown in Formula 1 can be given according to the alignment direction of the liquid crystal molecules.
ΔP = Δn · d (2π / λ) Equation 1
Here, λ is the wavelength of the light source. The distribution of the wavefront aberration generated by the optical disc tilt is as shown in FIG. 3. The transparent electrode of the phase correction element is patterned as shown in FIG. 5, for example, and the voltage is applied to each partial electrode according to the amount of generated aberration. If a phase shift distribution similar to the wavefront aberration distribution can be formed by applying, wavefront aberration can be canceled.
[0022]
Here, for comparison with the phase correction element in the present invention, an electrode pattern of a conventional phase correction element will be described with reference to FIG.
In the wavefront aberration distribution of FIG. 3, the partial electrodes 51 and 54 have a positive phase amount and the partial electrodes 52 and 55 have a negative phase amount with respect to the partial electrode 33 of FIG. Therefore, if 3V is applied to the partial electrode 53, a voltage of 3V or higher is applied to the partial electrodes 51 and 54, and an appropriate voltage of 3V or lower is applied to the partial electrodes 52 and 55, the wavefront aberration can be reduced. A phase difference distribution corresponding to the distribution can be formed.
[0023]
Here, in the case of the phase correction element having the conventional electrode pattern as shown in FIG. 5, the positional deviation of ΔS occurs between the optical axis of the objective lens and the optical axis of the phase correction element due to tracking control or the like, as shown in FIG. This is equivalent to translating the wavefront aberration distribution by ΔS with respect to the phase correction element. When ΔS is less than ± 0.1 mm, the influence is small because it is smaller than the size of the electrode pattern (diameter 4 mm), but ΔS increases and the aberration improvement effect by the phase correction element decreases.
[0024]
Therefore, in the phase correction element according to the present invention, an electrode pattern is formed so that the phase correction function can be more effectively exhibited even when the objective lens shift occurs.
[0025]
In this method, partial electrodes are arranged corresponding to at least two portions of the electrodes, respectively, in which the value of wavefront aberration caused by optical disc tilt takes an extreme value, that is, a maximum and a minimum. Here, the extreme value is assumed to be in a certain region (size of the partial electrode). The partial electrodes corresponding to the respective extreme values are further divided into three or more microelectrodes so as to correct the deviation between the optical axis of the phase correction element and the optical axis of the objective lens. . The microelectrode is preferably divided in a direction substantially orthogonal to the direction connecting the extreme value and the extreme value in order to correct the deviation.
[0026]
Here, a case will be described in which, for example, two partial electrodes giving extreme values are each divided into, for example, five microelectrodes. 6 shows the voltage distribution applied to the partial electrodes and microelectrodes formed on the phase correction element when there is no objective lens shift, and FIG. 7 shows the voltage distribution when the lens shift occurs in the right direction of FIG. FIG. 8 shows a voltage distribution when a lens shift occurs in the left direction of FIG. The electrode pattern of FIG. 6 is composed of three partial electrodes (62a, 62b, 62c) and ten microelectrodes (60a, 60b, 60c, 60d, 60e, 61a, 61b, 61c, 61d, 61e). These electrodes are connected to the wiring of the phase correction element control circuit, which is a control voltage generator, so that a voltage can be applied independently. In this example, the diameter of the circle in the region formed by all three partial electrodes and ten microelectrodes is 4 mm.
[0027]
First, the operation of the phase correction element when there is no objective lens shift will be described with reference to FIG. In this case, the voltages V ₁ is applied to the microelectrode 60b, 60c, 60d and the partial electrode 62c, microelectrodes 60a, 60e, 61a, the voltage V ₂ to 61e and partial electrode 62b is applied, microelectrode 61b, 61c, A voltage V ₃ is applied to 61d and the partial electrode 62a.
[0028]
Portion of the microelectrode 60b, 60c, voltages V ₁ region of the contour formed by 60d, and microelectrodes 61b, 61c, the contour of the voltage V ₃ region formed by 61d, the electrode pattern is a conventional example (FIG. 5) It almost coincides with the outer shape of the electrodes 52 and 54. Therefore, if V ₁ , V ₂ , and V ₃ are applied as V ₁ > V ₂ > V ₃ or V ₁ <V ₂ <V ₃ according to the optical disc tilt angle as described above, the same as the conventional example. Wavefront aberration can be corrected.
[0029]
Next, the operation of the phase correction element when there is an objective lens shift will be described. When the objective lens shift occurs in the right direction in FIG. 6, the distribution of wavefront aberration is shifted to the right with respect to the phase correction element. Therefore, if the above-described microelectrodes 60b, 60c, and 60d regions and the microelectrodes 61b, 61c, and 61d regions, which are regions for performing phase shift, can move following the shift of the distribution of wavefront aberration due to the objective lens shift, Even when a lens shift occurs, the wavefront aberration should be corrected appropriately.
[0030]
Here, for example, when there is no objective lens shift in FIG. 6, the voltage V ₁ applied to the region formed by the microelectrodes 60b, 60c, 60d is formed by the hatched portions 70c, 70d, 70e of the horizontal lines in FIG. Application is made to a region (region corresponding to the microelectrodes 60c, 60d, and 60e in FIG. 6). Further, the voltage V ₃ applied to the region formed by the microelectrodes 61b, 61c, 61d in the absence of the objective lens shift of FIG. 6 is applied to the region formed by the hatched portions 71c, 71d, 71e of the intersecting line in FIG. (A region corresponding to the microelectrodes 61c, 61d, 61e in FIG. 6).
[0031]
The partial electrodes 72a and 72c apply voltages V ₃ and V ₁ respectively regardless of the objective lens shift, and apply V ₂ to the remaining 72b, 70a, 70b, 71a and 71b. The region formed by the microelectrodes 70c, 70d, and 70e has a contour shape substantially equal to the region formed by the microelectrodes 70b, 70c, and 70d, and the phase generated by the objective lens shift whose center of gravity is 0.3 mm. It is designed to be equal to the amount of movement of the distribution. The objective lens shift amount in the optical head device of this example is 0.4 mm at the maximum, and the wavefront aberration can be appropriately corrected at any lens shift position by designing the center of gravity shift by 0.3 mm.
[0032]
Therefore, when the objective lens shift of a predetermined amount or more is performed by the tracking control unit, the state shown in FIGS. 6 and 7 is switched, so that even if a large objective lens shift occurs, a substantially appropriate wavefront aberration correction can be performed. it can.
On the other hand, even when the left lens shift of FIG. 6 occurs, microelectrodes 80a, as shown in FIG. 8, 80b, voltages V ₁ to 82c, microelectrodes 81a, 81b, the voltage V ₃ to 82a, microelectrodes 80c it can be carried out 80d, 80e, 81c, 81d, by applying a voltage V ₂ to 81e and the partial electrode 82b, a substantially appropriate aberration correction similarly.
[0033]
Therefore, the partial electrode made of a microelectrode has a shape corresponding to the extreme value partial shape of the wavefront aberration, and the position of the center of gravity of the partial electrode made of the microelectrode is the optical axis of the phase correction element and the optical axis of the objective lens. It is preferable that the microelectrode is formed so as to change according to the amount of deviation.
[0034]
FIG. 9 compares the objective lens shift characteristics of wavefront aberration when the optical disk is tilted by 1 ° with an objective lens NA of 0.6 and an optical disk thickness of 0.6 mm, compared with the conventional phase correction element and the phase correction element of the present invention. Showed. As described above, when the electrode pattern corresponding to the objective lens shift amount of 0.3 mm is designed and the objective lens shift of 0.2 mm or more occurs, the partial electrode and the minute electrode to which the voltage is applied are switched. As a result, within the above-mentioned objective lens shift range, the wavefront aberration was reduced as compared with the conventional example, and the signal reading accuracy was improved.
[0035]
【The invention's effect】
As described above, in the optical head device of the present invention, the wavefront aberration due to the optical disc tilt can be corrected by the phase correction element, and even when the objective lens shift occurs, it is not necessary to mount the phase correction element on the actuator. A wavefront aberration correction characteristic can be obtained.
In addition, since it is not necessary to mount the phase correction element on the actuator, the phase correction element and the actuator can be designed independently, so that the degree of freedom in designing these optical components can be greatly improved.
[0036]
In the above description, correction of wavefront aberration caused by tilting of the optical disc has been described. However, wavefront aberration caused by positional deviation and tilt of the optical components of the optical head device can be corrected by the same correction principle.
For example, by configuring the electrode of the phase correction element and applying an appropriate voltage so as to correct wavefront aberration caused by the tilt of the objective lens generated during assembly (component equivalent to wavefront aberration generated when tilting the optical disk), Even with an optical head device mounted with the objective lens tilted, information can be recorded / reproduced appropriately.
Therefore, in the optical head device of the present invention, the incorporation of the phase correction element eliminates the need for complicated mechanical adjustments of the optical components such as the tilt adjustment of the objective lens, thereby simplifying the assembly process.
[Brief description of the drawings]
FIG. 1 is a principle configuration diagram showing an example of an optical head device of the present invention.
FIG. 2 is a schematic cross-sectional view of a phase correction element in the present invention.
FIG. 3 is a diagram showing a wavefront aberration distribution generated by a radial tilt of an optical disc.
FIG. 4 is a graph showing voltage dependence of a phase difference generated by a phase correction element in the present invention.
FIG. 5 is a schematic diagram showing an example of an electrode pattern of a phase correction element of a conventional example.
FIG. 6 is a schematic diagram showing a voltage distribution applied to an example of electrodes (partial electrodes, microelectrodes) formed on the phase correction element in the present invention when there is no objective lens shift.
7 is a schematic diagram showing a voltage distribution applied when an objective lens shift occurs in the right direction in the electrode of FIG. 6;
8 is a schematic diagram showing a voltage distribution applied when an objective lens shift occurs in the left direction in the electrode of FIG. 6. FIG.
FIG. 9 is a graph showing the dependency of the objective lens shift on the wavefront aberration of each of the conventional example and the phase correction element according to the present invention.
[Explanation of symbols]
1: Semiconductor laser 3: Collimating lens 4: Phase correction element 5: 1/4 wavelength plate 6: Objective lens 8: Optical recording medium 10: Tilt sensor 23: Liquid crystal 24a, 24b: Transparent electrode 27: Electrode extraction portions 51-55 62a, 62b, 62c: partial electrodes 60a, 60b, 60c, 60d, 60e, 61a, 61b, 61c, 61d, 61e: microelectrodes

Claims (2)

A light source;
An objective lens for converging a light emitted from the light source onto an optical recording medium,
A phase correction element for correcting the wavefront aberration is installed in the optical path to change the phase distribution of the wave front of the outgoing light on the optical recording medium between the light source and the objective lens,
An optical head apparatus and a control voltage generator for supplying a voltage for changing the phase distribution on the phase correcting element,
The phase correction element includes a pair of transparent substrates each formed with a transparent electrode and a liquid crystal layer sandwiched between the substrates,
One or more of the transparent electrodes are divided into a plurality of partial electrodes so that different voltages can be applied to each other, and the value of the wavefront aberration caused by the tilt of the optical recording medium across the optical axis takes an extreme value. The partial electrodes are arranged corresponding to at least two portions of the electrodes, respectively, and the partial electrodes corresponding to the extreme values are further divided in a direction substantially perpendicular to the direction connecting the two extreme values. It consists of 5 microelectrodes,
Of the five microelectrodes constituting the partial electrode corresponding to the extreme value, the contour shape of the region formed by the three microelectrodes arranged in succession is substantially equal,
An optical head device configured to be able to apply a different voltage to each of the microelectrodes according to a deviation amount between an optical axis of the phase correction element and an optical axis of the objective lens. The optical head device according to claim 1, wherein the transparent electrode is made of ITO .

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