JP4479134B2 - Optical head device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical head device, and more particularly to an optical head device that records and reproduces information on an optical recording medium such as an optical disk.
[0002]
[Prior art]
In recent years, in order to increase the recording density of an optical disc, it has been put into practical use to shorten the wavelength of light emitted from a semiconductor laser, which is a light source, or to increase the numerical aperture (NA) of an objective lens. In the next generation optical recording, it has been proposed to obtain a higher recording density by setting the wavelength of the light source to about 405 nm and the NA to 0.85. However, due to the shorter wavelength of the light source and the higher NA of the objective lens, the allowable amount of variation in the thickness of the optical disk is reduced.
[0003]
The reason why the allowable amount is small is that spherical aberration is generated in proportion to the thickness variation of the optical disk, and therefore, the condensing characteristic of the optical head device is deteriorated and it becomes difficult to read the signal. In addition, in the case of a multilayer recording system in which different layers of the optical disc are used as recording layers, spherical aberration is generated based on the spacing between the layers, and means for correcting the spherical aberration is necessary.
[0004]
The following methods have been proposed as means for correcting spherical aberration. One is a method that mechanically changes the position of the collimating lens according to the amount of spherical aberration that occurs, generates spherical aberration with the objective lens, and cancels the spherical aberration that occurs due to the thickness error of the optical disc (mechanical method). There is. In the case of this mechanical system, there is a drawback that the configuration of the optical head device is complicated or large because a mechanically movable part of the collimating lens is required.
[0005]
As another method, there is a method (electric method) for correcting wavefront aberration by applying a voltage to a phase correction element provided in an optical path between an objective lens and a light source. In the case of this electric method, since there are no machine movable parts compared to the mechanical method, it is excellent in reliability such as reduction in size and weight and vibration. Japanese Patent Laid-Open No. 10-20263 discloses an example of correcting wavefront aberration using a phase correction element. In this example, as a method for correcting coma generated by the tilt of the optical disk, the orientation of the liquid crystal sandwiched between a pair of substrates constituting the phase correction element is controlled by changing the voltage applied to the divided electrodes. Then, the coma aberration generated by changing the wavefront of the transmitted light is corrected.
[0006]
[Problems to be solved by the invention]
However, in the case of the conventional phase correction element, since the amount of change in the wavefront of the transmitted light is the same when focusing on one divided electrode, the obtained wavefront shape is applied to the number of divided electrodes (number of divided electrodes) and the divided electrodes. Depends on the number of signals. Therefore, in order to correct continuously changing wavefront aberration with high accuracy, it is necessary to increase the number of divided electrodes and control with a large number of external signal sources (external power supplies). Problems arise. Therefore, a phase correction element having a simple electrode structure and continuously changing the wavefront of transmitted light with a small number of external power sources has been desired. In particular, it has been desired to correct a region having a large amount of change in wavefront aberration, such as a peripheral portion of spherical aberration, with a continuous wavefront change.
[0007]
[Means for Solving the Problems]
  The present invention has been made to solve the above-described problems, and is provided between a light source, an objective lens for condensing emitted light from the light source on an optical recording medium, and the light source and the objective lens. An optical head device comprising a phase correction element for changing the wavefront of the emitted light and a control voltage generating means for outputting a voltage for changing the wavefront to the phase correction element. A liquid crystal layer sandwiched between a pair of formed transparent substrates is provided, and the electrodes of one of the transparent substrates are arranged concentrically 3HornThe ratio of the sheet resistance value to the low resistance electrode is 1000 or more, which is arranged between the annular low resistance electrode and at least the adjacent low resistance electrode and conductively connects the adjacent low resistance electrodes.And the sheet resistance value is 10 ³ Ω / □ to 10 ⁷ Up to Ω / □And a high resistance electrodeThe three annular low-resistance electrodes are composed of three low-resistance electrodes on the outside, the middle, and the inside that are connected to an external signal source so that different voltages can be supplied. The ratio of the average value of the inner radius and the outer radius of the electrode is 0.7 to 0.85, and the ratio of the difference between the inner radius and the outer radius with respect to the pupil radius of the objective lens is 0.02. Up to 0.14,The electrode of the other transparent substrate includes a divided electrode composed of a plurality of segment electrodes in which a planar electrode is concentrically divided, and the center of the low resistance electrode and the center of the divided electrode are on the optical axis.And the plurality of segment electrodes are divided into a first segment electrode group and a second segment electrode group disposed between adjacent segment electrodes constituting the first segment electrode group. Each group is conductively connected so that different voltages can be applied between the electrodes of the transparent substrate facing each other and each segment electrode group.An optical head device is provided.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a conceptual diagram showing an example of the principle configuration of an optical head device according to the present invention, which records and reproduces information on an optical disc as an optical recording medium. The light emitted from the semiconductor laser 1 as the light source passes through, for example, a hologram type polarization beam splitter 2, then becomes parallel light by the collimator lens 3, passes through the phase correction element 4, and then passes through the quarter-wave plate 5. Then, the light is reflected in the 90 ° direction by the rising mirror 11 and condensed on the optical disk 8 by the objective lens 6 installed in the actuator 7.
[0009]
The condensed light is reflected by the optical disk 8 and sequentially passes through the objective lens 6, the rising mirror 11, the quarter-wave plate 5, the phase correction element 4, and the collimating lens 3 in the reverse order to the polarized light beam. The light is diffracted by the splitter 2 and enters the photodetector 9. When the light emitted from the semiconductor laser 1 is reflected by the optical disc 8, the reflected light is amplitude-modulated by the information recorded on the surface of the optical disc 8, and the recorded information is read as a light intensity signal by the photodetector 9. Can do. When recording information on the optical disk 8, the laser output of the semiconductor laser 1 is modulated and information is recorded on the recording film of the optical disk 8.
[0010]
A voltage is output by the phase correction element control circuit 10 which is a control voltage generating means toward the phase correction element 4 so that, for example, the intensity of the reproduction signal of the optical disk 8 obtained from the photodetector 9 is optimized. The voltage output from the phase correction element control circuit 10 is a voltage to be applied to the electrode of the phase correction element 4 in accordance with a thickness error of the optical disk, a shift of an optical system such as an objective lens, or a spherical aberration generated from a multilayer optical disk.
[0011]
Next, the configuration of the phase correction element used in the present invention will be described with reference to the sectional view of FIG. Glass substrates 21a and 21b are bonded to each other by a sealing material 22 mainly composed of an epoxy resin, for example, to form a liquid crystal cell. Examples of materials other than glass in the substrate to be used include acrylic resins, epoxy resins, vinyl chloride resins, polycarbonates, and the like, and glass substrates are preferable from the viewpoint of durability.
[0012]
The sealing material 22 includes, for example, a glass spacer and, for example, a conductive spacer having a resin surface coated with gold or the like. On the inner surface of the glass substrate 21a, there are an electrode 24a from the inner surface, an insulating film 25 mainly composed of silica and the alignment film 26 in this order, and on the inner surface of the glass substrate 21b, the electrode 24b and silica from the inner surface. An insulating film 25 and an alignment film 26 mainly composed of, for example, are coated in this order. The insulating film 25 is for preventing a short circuit between the electrodes due to the conductive foreign matter, and may be omitted. Further, an antireflection film may be coated on the outer surfaces of the glass substrates 21a and 21b.
[0013]
The electrode 24a is provided with a pattern wiring so that the electrode lead-out portion 27 can be connected to the phase correction element control circuit by a connection line. The electrode 24b is electrically connected to the electrode 24a formed on the glass substrate 21a by the conductive spacer coated with the above-described gold or the like. Therefore, the electrode 24b is connected to the phase correction element by the connection line at the electrode lead-out portion 27. Can be connected to the control circuit.
[0014]
Liquid crystal 23 is filled in the liquid crystal cell. The material of the liquid crystal to be used is nematic liquid crystal used for a liquid crystal display or the like, and uniform homogeneous alignment is preferable so that the polarization does not change depending on the applied voltage. The liquid crystal molecules 28 shown in FIG. 2 are in a homogeneous alignment state aligned in one direction.
[0015]
As a material for the alignment film, it is preferable that the pretilt angle of the liquid crystal molecules 28 is 2 to 10 °. A polyimide film that is rubbed in the horizontal direction parallel to the paper surface of FIG. Is good. In addition, it is preferable to increase the difference between the ordinary light refractive index and the extraordinary light refractive index of the liquid crystal so as to reduce the interval between the liquid crystal cells because the responsiveness can be improved. However, as the distance between the liquid crystal cells becomes smaller, it becomes more difficult to manufacture the liquid crystal cell. Therefore, the difference between the ordinary light refractive index and the extraordinary light refractive index of the liquid crystal is 0.1 to 0.2, and the distance between the liquid crystal cells is about 2 to 5 μm. It is preferable that
The material of the electrodes 24a and 24b is preferably high in transmittance, and a transparent conductive film such as an ITO film, a zinc oxide film, or a tin oxide film may be used. The material, physical properties, formation method, and the like of the electrodes 24a and 24b will be described in detail later.
[0016]
The configuration necessary for the function of changing the wavefront using the phase correction element has been described above. By laminating a plate optical element such as a wave plate or a polarization hologram on the phase correction element 4, the wave plate 5 or the polarization beam splitter is stacked. The phase correction element 4 can have both functions. In this case, the number of optical components constituting the optical head device is reduced, so that assembly and adjustment are simplified, and productivity is improved, which is preferable. When laminating the wave plate, it may be laminated directly on the glass substrate of the phase correction element, or after laminating the wave plate on another glass substrate.
[0017]
In addition, it is preferable that an optical head device in which the phase correction element and the objective lens are integrally formed is used. The reason for this is that when a lens shift occurs in which the objective lens moves in a plane perpendicular to the optical axis due to tracking or the like, and the phase correction element and the objective lens are not integrated, the phase of the spherical aberration generated by the optical disc This is because the wavefront aberration (spherical aberration) generated by the correction element is displaced by the amount of lens shift, and the spherical aberration cannot be corrected appropriately.
[0018]
In order to obtain an optical head device in which the phase correction element and the objective lens are integrally formed, the phase correction element may be fixed to an actuator holding the objective lens. In this case, it is preferable to reduce the weight of the phase correction element or use a light and easy to connect signal leader line such as a wire so as not to affect the control characteristics of the actuator.
[0019]
Next, a method for correcting spherical aberration using the phase correction element of the present invention will be described. FIG. 3 is a plan view showing an example of a voltage drop electrode pattern for correcting spherical aberration, and is formed on one of the electrodes 24a and 24b shown in FIG. Reference numerals 31 to 33 are low resistance electrodes, and 30 is a high resistance electrode. FIG. 4 is a plan view showing an example of a divided electrode pattern for reducing residual aberration. Of the electrodes 24a and 24b, the divided electrode pattern is formed on the electrode on which the voltage drop electrode pattern is not formed. Further, the center of the low resistance electrode and the center of the split electrode are arranged on the optical axis.
[0020]
The hatched portion in FIG. 3 is a high resistance electrode 30 formed of a transparent conductive film, for example, a uniform electrode without division. The black portions are low resistance electrodes 31 to 33 for supplying a voltage to the high resistance electrode 30, and the inner diameter and the outer diameter are arranged concentrically around the optical axis. The high resistance electrode 30 and the low resistance electrodes 31 to 33 are conductively connected. Therefore, a high resistance electrode may be arranged between adjacent low resistance electrodes, and the adjacent low resistance electrodes may be conductively connected. That is, the high resistance electrode may be in contact with the low resistance electrode sandwiching the high resistance electrode.
[0021]
The low resistance electrodes 31 to 33 are connected to a signal source (not shown) outside the phase correction element by wiring and can supply an arbitrary voltage by the signals 1 to 3, respectively. In FIG. 4, reference numerals 41 to 47 are segment electrodes constituting divided electrodes, and reference numerals 48 and 49 are wirings.
[0022]
Sheet resistance ρ of electrode material forming low resistance electrode_LSheet resistance of electrode material forming high resistance electrode against ρ_HRatio of ρ_H/ Ρ_LWhen the value is 1000 or more, since the resistance is lower in the low resistance electrode than in the high resistance electrode, the potential becomes equal in the low resistance electrode. On the other hand, in the plane of the high resistance electrode 30, a weak current flows due to the potential difference between the low resistance electrodes 31 and 32, and 32 and 33, resulting in a voltage drop. Symmetric potential distribution occurs. The electrode material and sheet resistance will be described in detail later.
[0023]
Since the orientation direction of the liquid crystal molecules changes according to the effective voltage (potential difference) between the opposing electrodes, the effective refractive index distribution of the liquid crystal is generated in the high resistance electrode 30 if all the potentials of the opposing divided electrodes are made equal. It almost matches the voltage distribution. Therefore, if the low resistance electrodes 31 to 33 are configured so that the voltage distribution generated in the high resistance electrode 30 corresponds to the wavefront aberration distribution to be corrected, both can be canceled and the wavefront aberration can be corrected.
[0024]
FIG. 5 is a graph showing an example of spherical aberration correction due to wavefront aberration generated by the phase correction element according to the present invention, in which the potentials of the segment electrodes constituting the divided electrodes are all equal. A is a spherical aberration distribution that needs to be corrected due to an optical disc thickness variation or an optical system error, B is a phase difference distribution for correcting spherical aberration generated by the phase correction element, and C is a residual remaining after correction. Aberration distribution. The horizontal axis is the pupil radius centered on the optical axis, and is normalized by the radius of the incident light beam.
[0025]
In order to correct the spherical aberration distribution A, the dimensions of the low resistance electrodes 31 to 33 and the applied voltages of the signals 1 to 3 are appropriately set, so that the spherical aberration distribution is obtained by the phase difference distribution B having the opposite sign and substantially the same shape. A can be offset and corrected. Here, the positions of the region D and the region E in the phase difference distribution B correspond to the positions of the low resistance electrodes 31 (including the inside) and 32 shown in FIG. 3, respectively, and the position of the low resistance electrode 33 is shown in FIG. This is not shown in order to correspond to a position having a pupil radius of 1 or more.
[0026]
When the positions of the low resistance electrodes 31 and 32 are changed, the wavefront aberration distribution B generated is also changed. Therefore, in order to correct with high accuracy, it is necessary to determine the positions and sizes of the low resistance electrodes 31 and 32 so as to match the shape of the spherical aberration distribution A. On the other hand, the radius of the incident light beam on the phase correction element surface is set to 1, whereas the radius of the low resistance electrode 31 is 0.2 to 0.3, and the average radius of the inner radius and the outer radius of the annular low resistance electrode 32 is A value of 0.7 to 0.85 is preferable because spherical aberration can be corrected efficiently. In particular, when the radius of the low resistance electrode 31 is 0.21 and the average radius of the low resistance electrode 32 is 0.74, the maximum correction effect can be obtained, which is extremely preferable.
[0027]
As described above, the case where the divided electrodes are made equipotential has been described. In this case, the residual aberration shown in FIG. 5C remains. Since the magnitude of this residual aberration is proportional to the magnitude of the spherical aberration to be corrected, it cannot be ignored when correcting a large spherical aberration such as an interlayer gap in a multilayer recording disk. Therefore, the effect of reducing residual aberration by the phase correction element in the present invention will be described below.
[0028]
The divided electrode shown in FIG. 4 is composed of a plurality of segment electrodes 41 to 47 that are concentric and obtained by etching a uniform transparent conductive film. As a method of dividing the electrode, for example, a photolithography technique and an etching technique can be used. Since the transparent conductive film is removed by etching between one adjacent segment electrode and another segment electrode, that is, at a division interval, there is no conduction between different segment electrodes. However, in the phase correction element according to the present invention, wiring is made by a transparent conductive film inside the electrodes so that several segment electrodes are conductive, thereby reducing the number of signals.
[0029]
That is, a plurality of segment electrodes are divided into two segment electrode groups, and each segment electrode group is conductively connected so that different voltages can be applied between the electrodes of the transparent substrate facing each other and each segment electrode group. It is configured. This configuration is preferable because the number of control voltage generating means can be reduced.
[0030]
In the example shown in FIG. 4, the segment electrodes 41, 43, 45, and 47 form one segment electrode group, and are conducted by the wiring 49 arranged in the left-right direction in the drawing and are connected to an external signal source. An arbitrary voltage can be applied. Similarly, other segment electrode groups are formed for the segment electrodes 42, 44, and 46. The segment electrodes 42, 44, and 46 are electrically connected by an interconnection 48 and connected to an external signal source.
[0031]
FIG. 6 is a wavefront aberration distribution representing residual aberration when spherical aberration is corrected using the phase correction element of the present invention. Also in this figure, the pupil radius is normalized by the incident light beam radius. The residual aberration is 61 when all the segment electrodes described above are equipotential, and the residual aberration is 62 when different voltages are supplied as the signals 4 and 5 shown in FIG. Here, the regions D41 to D47 correspond to the segment electrodes 41 and 42 to 47, respectively. That is, D41 is a domain 41.
[0032]
By appropriately setting the radius of the segment electrode according to the shape of the residual aberration and generating a potential difference between the segment electrodes, the unevenness of the residual aberration shown in 61 can be smoothed, and the residual aberration can be reduced. Here, the segment electrodes 41, 43, 45, and 47 are set to the same potential, and the segment electrodes 42, 44, and 46 are set to a different potential. However, to further reduce the residual aberration, the number of signals is increased. What is necessary is just to enable it to apply a different voltage for every segment electrode.
As a pattern different from the divided electrode pattern shown in FIG. 4, for example, only the segment electrode 42 can be isolated from other segment electrodes and connected to a new external signal source to be controlled independently, and the number of signals is one. Although it increases, the residual aberration is further reduced.
[0033]
Next, the electrode material of the phase correction element in the present invention will be described. Sheet resistance ρ of electrode material forming low resistance electrode of voltage drop type electrode_LAnd sheet resistance ρ of the electrode material forming the high resistance electrode_HRatio of ρ_H/ Ρ_LTo 1000 or more. ρ_HIs too small, a relatively large current flows through the high-resistance electrode, and a voltage drop may occur in the low-resistance electrode in contact with the high-resistance electrode, making it difficult to obtain the desired voltage distribution. . On the other hand, ρ_HWhen is too large, the electrical resistance of the high resistance electrode is lost, and no potential distribution is generated. Therefore, to satisfy the above conditions, ρ_H10³Ω / □ -10⁷Ω / □ is good. On the other hand, ρ_LSince the allowable resistance range of the high-resistance electrode is increased when it is made as small as possible, it is preferably about 1Ω / □ to 100Ω / □.
[0034]
As a material for the low resistance electrode, a metal material such as copper, gold, aluminum, or chromium is preferable in terms of conductivity and durability, but the transmittance is lowered because light is shielded by the electrode portion. Therefore, it is preferable to use a transparent conductive film. For example, it is preferable to use a transparent conductive film having a small specific resistance, such as an ITO film, because the above-described condition of the ratio between the resistance value of the low resistance electrode and the resistance value of the high resistance electrode is satisfied and the transmittance is high. The wiring material on the electrode lead-out portion 27 for applying a voltage to the low resistance electrode from an external phase correction element control circuit may be a transparent conductive film such as an ITO film, or a metal film such as chromium or nickel. Good. In particular, a metal such as nickel that can be connected by solder is preferable because an external signal line can be easily connected by solder.
[0035]
On the other hand, the material of the high resistance electrode needs to be transparent and have a higher sheet resistance than the material of the low resistance electrode. For example, a zinc oxide film doped with one or more elements such as gallium, aluminum, silicon, yttrium, and indium, a tin oxide film doped with one or more elements such as antimony, indium, and gallium, an ITO film, etc. Is good.
[0036]
A zinc oxide film or a tin oxide film doped with the above elements is preferable because a high resistance film can be obtained more easily than an ITO film. In particular, the zinc oxide film doped with the above elements is a suitable material for the optical head device of the present invention in that it has a high specific resistance and good etching properties, and is excellent in light transmittance and durability. is there. When the ITO film is used for both the low resistance electrode and the high resistance electrode, it is preferable to use films having different specific resistances because the film thickness can be easily controlled.
[0037]
On the other hand, as the material of the transparent conductive film of the divided electrode, tin oxide or zinc oxide doped with ITO, gallium, indium, antimony, or the like can be used as in the voltage drop type electrode. However, it is preferable to use ITO because the film resistance can be lowered and the transmittance is high.
[0038]
The low resistance electrode is composed of three low resistance electrodes, outer, middle and inner, and the ratio of the average value of the inner radius and the outer radius of the intermediate low resistance electrode to the pupil radius of the objective lens constituting the optical head device. The above optical head device has a ratio of 0.7 to 0.85 and the ratio of the difference between the inner radius and the outer radius of the intermediate low resistance electrode with respect to the pupil radius of the objective lens is 0.02 to 0.14 Therefore, the optical head device is preferable because it can appropriately correct the spherical aberration.
[0039]
【Example】
The optical head device of this embodiment includes a phase correction element that corrects spherical aberration caused by fluctuations in the thickness of the optical disk, adjustment errors of the objective lens and the optical system, and the like. The phase correction element 4 is incorporated in the optical head device shown in FIG. 1, and is electrically connected to a phase correction element control circuit 10 having five electric signal sources. The wavelength of the emitted light from the semiconductor laser 1 as the light source is 405 nm, the NA of the objective lens 6 is 0.85, and the pupil diameter is 3 mm. The element structure of the phase correction element of this example is the same as that shown in FIG.
[0040]
A specific resistance of 5 × 10 5 is formed on the surface of a non-alkali glass substrate having a thickness of 0.5 mm, which is a transparent substrate, by magnetron sputtering.^-6An ITO film of Ω · m is formed, and this ITO film is patterned by photolithography and etching techniques, and the voltage drop type low resistance electrodes 31 to 33 shown in FIG. 3 and the division shown in FIG. An electrode was formed. The center of the low resistance electrode and the center of the split electrode are arranged on the optical axis.
[0041]
The sheet resistance value of the low resistance electrodes 31 to 33 was 50Ω / □, the sheet resistance value of the divided electrodes was 300Ω / □, and the interval between the segment electrodes of the divided electrodes was 5 μm. Thereafter, a zinc oxide film doped with gallium is formed on the voltage drop electrode by a magnetron sputtering method to form a transparent high-resistance electrode 30 (specific resistance 5 × 10 5^{− Three}Ω · m). The sheet resistance value of the high resistance electrode 30 is 1 × 10⁵Ω / □, and the ratio of the low resistance electrodes 31 to 33 to the sheet resistance value was 2000: 1. In this embodiment, the electrode 24a in FIG. 2 is a voltage drop type electrode, and the electrode 24b is a divided electrode.
[0042]
Thereafter, an insulating film 25 containing silica as a main component was formed on the surfaces of the electrodes 24a and 24b by spin coating, and an alignment film 26 containing polyimide as a main component was also formed by spin coating. At this time, the two produced alkali-free glass substrates were overlapped to form a cell structure shown in FIG. 2 using an epoxy-based sealing material mixed with a 4 μm glass spacer. Between the glass substrates of this cell, nematic liquid crystal having a difference Δn between ordinary light refractive index and extraordinary light refractive index of 0.15 was injected, and the injection port was sealed to form a liquid crystal cell. The alignment film 26 is rubbed in advance so that the liquid crystal layer 23 has a homogeneous alignment. Further, an antireflection film (not shown) was coated on the outer surface of the glass substrate.
[0043]
  The low resistance electrode 31 has an inner diameter of 0.7 mm and an outer diameter of 0.8 mm, the low resistance electrode 32 has an inner diameter of 2.2 mm and an outer diameter of 2.5 mm, and the low resistance electrode 33 has an inner diameter of 3.2 mm and an outer diameter of 4.2 mm. It has an annular shape centered on the electrode, and the electrode is drawn out by wiring formed of the same transparent conductive film materialPart 27 to the phase correction element control circuit 10, and an arbitrary voltage can be supplied to each low resistance electrode using signals 1 to 3. Here, the ratio of the average value (1.17) of the inner radius and the outer radius of the intermediate low-resistance electrode 32 to the pupil radius (1.5 mm) is 0.78, which is 0.7 and 0.85. The ratio of the difference (0.15) between the inner radius and the outer radius of the intermediate low-resistance electrode 32 to the pupil radius (1.5 mm) is 0.1, 0.02 and 0.14 It was between.
[0044]
On the other hand, the segmented electrodes shown in FIG. 4 have concentric segment electrodes 41 to 47 having diameters of 0.6 mm, 1.0 mm, 1.7 mm, 2.4 mm, 2.6 mm, and 2.9 mm from the inside centering on the optical axis. It is divided into The segment electrodes 41, 43, 45, and 47 are electrically connected by the wiring 49 formed of the same transparent conductive film material, and an equal voltage is supplied by the signal 5 generated by the phase correction element control circuit 10. Similarly, the segment electrodes 42, 44, 46 are also conducted by the wiring 48, and an equal voltage is supplied by the signal 4.
[0045]
The phase correction element in this embodiment is controlled by five electric signals, and the electric signal is a rectangular alternating current with a frequency of 1 kHz, and the signals 1, 2, and 3 for the voltage drop type electrodes, and the signals 4 and 5 for the divided electrodes. The signals have the same phase, and the signals 1, 2, 3 and signals 4, 5 are out of phase by π. Thus, for example, the voltage of signals 1, 2, 3 is V₁[V_rms], Signals 4 and 5 are V₂[V_rms], The voltage applied between the electrodes is V₁+ V₂[V_rms] To drive the liquid crystal molecules.
[0046]
Next, the principle of correcting spherical aberration by the phase correction element in this embodiment will be described. FIG. 5B shows a wavefront aberration (spherical surface) that occurs when the optical disk thickness is 0.01 mm thicker than the designed value of 0.1 mm in an optical system in which the NA of the objective lens is 0.85 and the wavelength of the light source is 405 nm. It is a graph which shows radial direction distribution of (aberration). Since the spherical aberration is axially symmetric, it can be considered that the wavefront aberration shown in FIG. 5B is distributed with the pupil radius 0, that is, the optical axis as a rotationally symmetric axis.
[0047]
In order to correct the spherical aberration caused by the optical disk thickness variation of 0.01 mm by the phase correction element, 1.3 V is applied to the low resistance electrodes 31 and 33._rms1.1V to the low resistance electrode 32_rms, Segment electrodes 41 to 47 with voltage 0.5V_rmsWas applied. Therefore, the effective voltage applied to the liquid crystal layer is 1.8 V at the low resistance electrodes 31 and 33._rms1.6V with low resistance electrode 32_rmsIn the plane of the high resistance electrode 30, the voltage width is 0.2V._rmsA voltage distribution that varies continuously at the same time occurred. Similar to the above description, since the alignment direction of the liquid crystal molecules is distributed according to the voltage, the phase correction element can generate a concentric wavefront change, and its radial distribution is as shown in B of FIG. Become.
[0048]
Here, since the voltages of the signals 1 to 3 are determined in accordance with the magnitude of the spherical aberration A, the spherical aberration A and the spherical aberration B generated by the phase correction element cancel each other. As a result, the wavefront aberration on the optical disk surface is C. It will be reduced. The spherical aberration before correction is about 0.1λ_rmsHowever, as a result of correction using the phase correction element, about 0.018λ_rmsDecreased.
[0049]
On the other hand, when the thickness of the optical disk is as thin as 0.01 mm, 1.1 V is applied to the low resistance electrodes 31 and 33 in order to correct spherical aberration whose polarity is reversed from that of A in FIG._rms1.3V for the low resistance electrode 32_rmsMay be applied. As a result, the wavefront change generated by the phase correction element also takes the form of reversing the sign of B in FIG.
[0050]
Next, in order to improve the residual aberration after correction shown by C in FIG. 5, segment electrodes 41, 43, 45, of the segment electrodes constituting the divided electrodes are not changed without changing the voltage applied to the voltage drop type electrodes. 47 to 0.48V_rms, 0.52V to segment electrodes 42, 44, 46_rmsWas applied. This makes 0.04V for each segment electrode of the split electrode._rmsAs a result, the substantial refractive index distribution of the liquid crystal layer also changes. Reference numeral 62 in FIG. 6 is a graph showing residual aberration when this voltage difference occurs, and is expressed as wavefront aberration. 0.5V equal to all segment electrodes 41-47_rmsIt can be seen that the residual aberration is reduced compared to 61 when applied. At this time, the corrected wavefront aberration of about 0.018λ shown above._rmsIs 0.009λ_rmsAs a result, the recording and reproducing characteristics of information on the optical disc were further improved.
[0051]
As described above, the spherical aberration can be accurately corrected by applying appropriate voltages to the low resistance electrodes 31 to 33 and the segment electrodes 41 to 47. Further, even if the low resistance electrodes 31 and 33 are always applied with the same voltage, the low resistance electrodes 31 and 33 may be connected and connected to one power source. Can be operated with two external signals.
[0052]
【The invention's effect】
As described above, in the optical head device of the present invention, one of the electrodes formed on each of the pair of transparent substrates that constitute the phase correction element with the liquid crystal layer interposed therebetween is arranged in three concentric circles. Mounted with a phase correction element composed of a ring-shaped low resistance electrode and at least a high resistance electrode arranged between the low resistance electrodes, and the other electrode composed of a plurality of segment electrodes divided concentrically .
[0053]
Since the phase correction element having this configuration can generate a wavefront change substantially matching the spherical aberration shape, the optical head device of the present invention equipped with this phase correction element is used to cause an optical disc thickness error. Spherical aberration can be corrected efficiently and good signal light with less noise can be obtained. Furthermore, since it can be operated with fewer external signal sources than in the prior art, a low-cost optical head device can be manufactured.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram showing an example of the principle configuration of an optical head device of the present invention.
FIG. 2 is a cross-sectional view showing an example of the configuration of a phase correction element in the present invention.
FIG. 3 is a plan view showing an example of a voltage drop electrode pattern of a phase correction element according to the present invention.
FIG. 4 is a plan view showing an example of a divided electrode pattern of a phase correction element in the present invention.
FIG. 5 is a graph showing an example of spherical aberration correction by wavefront aberration generated by the phase correction element when the same voltage is applied to the segment electrodes constituting the divided electrodes in the present invention. (A is a spherical aberration that requires correction, B is a wavefront change generated by the phase correction element, C is a wavefront aberration after correction, D is a region corresponding to the low resistance electrode 31, and E is a region corresponding to the low resistance electrode 32).
FIG. 6 is a graph showing an example of spherical aberration correction by wavefront aberration generated by the phase correction element when a plurality of voltages are applied to the segment electrodes constituting the divided electrodes in the present invention. D41 to D47 are regions corresponding to the segment electrodes 41 to 47)
[Explanation of symbols]
1: Semiconductor laser
2: Polarizing beam splitter
3: Collimating lens
4: Phase correction element
5: Quarter wave plate
6: Objective lens
7: Actuator
8: Optical disc
9: Photodetector
10: Phase correction element control circuit
21a, 21b: Glass substrate
22: Sealing material
23: Liquid crystal layer
24a, 24b: Electrodes
25: Insulating film
26: Alignment film
27: Electrode extraction part
28: Liquid crystal molecules
30: High resistance electrode
31-33: Low resistance electrode
41-47: Segment electrodes
48, 49: Wiring
61, 62: Residual wavefront aberration

Claims (1)

A light source, an objective lens for condensing the emitted light from the light source on the optical recording medium, a phase correction element for changing the wavefront of the emitted light provided between the light source and the objective lens, and the wavefront are changed. And a control voltage generator that outputs a voltage to the phase correction element. The phase correction element includes a liquid crystal layer sandwiched between a pair of transparent substrates having electrodes formed on the surface. ,
Electrode of one transparent substrate, a low-resistive electrodes of the three annular are arranged concentrically, for connecting conductive low-resistance electrode adjacent have been arranged at least between the low-resistance electrodes adjacent, low resistance electrode A high resistance electrode having a sheet resistance value ratio of 1000 or more and a sheet resistance value of 10 ³ Ω / □ to 10 ⁷ Ω / □ ,
The three annular low-resistance electrodes are composed of three low-resistance electrodes on the outside, the middle, and the inside that are connected to an external signal source so that different voltages can be supplied. The ratio of the average value of the inner radius and the outer radius of the electrode is 0.7 to 0.85, and the ratio of the difference between the inner radius and the outer radius with respect to the pupil radius of the objective lens is 0.02. Up to 0.14,
The electrode of the other transparent substrate includes a divided electrode composed of a plurality of segment electrodes obtained by concentrically dividing a planar electrode, the center of the low resistance electrode and the center of the divided electrode are on the optical axis , and the plurality The segment electrodes are divided into a first segment electrode group and a second segment electrode group disposed between adjacent segment electrodes constituting the first segment electrode group, and conductive connection is made for each segment electrode group The optical head device is configured so that different voltages can be applied between the electrodes of the opposing transparent substrate and the segment electrode groups .

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