JP4449239B2 - Optical head device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical head device for recording / reproducing 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 wavelength of 650 nm or 635 nm, which is shorter than 780 nm of the CD. The numerical aperture (NA) of the objective lens is set to 0.6, which is larger than 0.45 of CD, so that the spot diameter focused on the optical disk surface is reduced.
[0003]
Further, 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 400 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 tolerance of the thickness error of the optical disk becomes smaller.
[0004]
The reason why these allowances are small is that spherical aberration occurs when an optical disc thickness error occurs, so that the light condensing characteristic of the optical head device deteriorates, making it difficult to read signals. Further, in the case of a multi-layer 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 this aberration correction function is necessary.
[0005]
The following methods have been proposed as means for correcting spherical aberration in high-density recording. One is to generate another spherical aberration by mechanically changing the position of the collimating lens according to the amount of spherical aberration, and to cancel the spherical aberration generated on the disk side using this other spherical aberration (cancellation method) ) This canceling method requires a mechanically movable part of the collimating lens, and thus has a drawback that the configuration of the optical head device is complicated or large.
[0006]
As another method, there is a method (correction method) in which wavefront aberration is corrected by a phase correction element provided in an optical path between an objective lens and a light source. In the case of this correction method, there is no mechanical movable part, and it can be easily introduced into a small optical head device. Japanese Patent Laid-Open No. 10-20263 discloses an example of a correction method for correcting the tilt of an optical disk using a phase correction element. In this example, each of a pair of substrates constituting a phase correction element with a birefringent material such as liquid crystal interposed therebetween has a divided electrode formed by dividing the electrode. Then, a voltage is applied to each divided electrode to change the substantial refractive index of the birefringent material according to the tilt angle of the optical disk, and the phase (wavefront) of the transmitted light generated by the change in the refractive index. The coma aberration generated by the tilt of the optical disk is corrected by the change.
[0007]
[Problems to be solved by the invention]
However, in the conventional phase correction element, in order to correct the wavefront aberration by changing the wavefront of the light emitted from the light source, the electrodes included in the phase correction element are divided into a plurality of voltages, which are different control signals. It is necessary to apply.
Therefore, in order to obtain a desired wavefront shape, a large number of electrodes, wirings, and external signal sources (power supplies) are required, and problems such as complicated element configuration and complicated equipment due to the use of a large number of external signal sources (power supplies). Will occur. On the other hand, there has been a demand to reduce the number of electrodes, wirings, and external signal sources (power supplies) as much as possible.
[0008]
If attention is paid to one electrode, the amount of change in the wavefront is the same, so it is difficult to change it continuously. In particular, it has been desired to continuously change a region where the amount of change in wavefront aberration, such as a peripheral portion of spherical aberration, is large.
Furthermore, since an external signal cannot be applied to the region between the divided electrodes, it may cause a decrease in light transmittance due to light scattering or the like. Therefore, it has been desired to reduce the number of divided electrodes as much as possible to reduce the number of regions between the electrodes.
[0009]
[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, the phase correction element being a pair of transparent A liquid crystal layer sandwiched between the substrates is provided, and electrodes for applying a voltage to the liquid crystal layer are formed on the surfaces of the pair of substrates, respectively, and at least one of the electrodes is transparent and has a sheet resistance value of 10 ^Three Ω / □ -10 ⁸ A high-resistance electrode of Ω / □, a low-resistance electrode having a resistance value that is 1/1000 of that of the high-resistance electrode, a disk-shaped first low-resistance electrode, an annular second low-resistance electrode, and at least a circular opening The three low-resistance electrodes are arranged concentrically with the optical axis as the center, and are arranged in the first, second, and third order from the center toward the periphery. And at least each low resistance electrode is conductively connected by a high resistance electrode. A voltage is applied to each of the first low-resistance electrode, the second low-resistance electrode, and the third low-resistance electrode, and the light emitted from the light source has a light flux radius on the phase correction element surface. The ratio with the radius of the first low resistance electrode is 0.2 to 0.3, and the ratio with the average radius of the second low resistance electrode is 0.7 to 0.85. The ratio to the inner radius is 1.0 or more An optical head device is provided.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an example of the principle configuration of the optical head device of the present invention. The optical head device shown in FIG. 1 is for reproducing information recorded on an optical disk 8 such as a CD or a DVD. Light emitted from a semiconductor laser 1 as a light source is, for example, a hologram type polarization beam splitter 2. Then, the light is converted into parallel light by the collimating lens 3, transmitted through the phase correction element 4, transmitted through the quarter-wave plate 5, reflected by the rising mirror 11 in the 90 ° direction, and installed in the actuator 7. The light is condensed on the optical disk 8 by the objective lens 6. 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 disk 8, the reflected light is amplitude-modulated by the information recorded on the surface of the optical disk, and the recorded information can be read as a light intensity signal by the photodetector 9. it can.
[0011]
A voltage is output by the phase correction element control circuit 10 which is a control voltage generation means toward the phase correction element 4 so that, for example, the intensity of the reproduction signal of the optical disk obtained from the photodetector 9 is optimized. The voltage output from the phase correction element control circuit 10 substantially changes to be applied to the electrodes of the phase correction element 4 in accordance with the thickness error of the optical disk, the deviation of the optical system such as the objective lens, or the spherical aberration generated from the multilayer disk or the like. Voltage.
[0012]
Next, the configuration of the phase correction element used in the present invention will be described with reference to FIG. The glass substrates 21a and 21b are bonded by a sealing material 22 whose main component is, for example, an epoxy resin 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, but glass substrates are preferable from the viewpoint of durability.
[0013]
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. An antireflection film may be coated on the outer surface of the liquid crystal cell.
[0014]
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.
[0015]
FIG. 2 does not show that the electrode 24b and the electrode 24a are in contact with the sealing material 22, but the electrode 24b and the electrode 24a are in contact with the sealing material parallel to the paper surface, and both electrodes are electrically connected through the conductive spacer. ing. A liquid crystal layer 23 is filled inside the liquid crystal cell. The material of the liquid crystal layer to be used is preferably 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.
[0016]
As a material for the alignment film, it is preferable that the pretilt angle of the liquid crystal molecules 28 is 2 to 10 °, and a polyimide film is rubbed in the horizontal direction parallel to the paper surface of FIG. 2 or a silica film is obliquely deposited. 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.
[0017]
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.
[0018]
Further, the phase correction element 4 can be laminated with a diffraction grating or a dichroic aperture limiting layer for changing the beam diameter according to the wavelength of the light source, and further on the outer surfaces of the glass substrates 21a and 21b. A diffraction grating, a dichroic aperture limiting layer, or the like can also be directly formed. In these cases, productivity is improved as compared with the addition of individual parts, which is preferable. When laminating the wave plate, the wave plate may be directly bonded to the glass substrate on the optical disk side, or the wave plate may be bonded to another glass substrate, and the other glass substrate may be laminated.
[0019]
Next, a method for correcting spherical aberration using the phase correction element of the present invention will be described. At least one of the electrodes 24a and 24b shown in FIG. 2 is a divided electrode (described in detail below) subjected to division for correcting spherical aberration, and the other electrode may be a uniform electrode that is not divided. It is also possible to use an electrode with other divisions for correcting aberrations other than spherical aberration such as coma and astigmatism. In the case of electrodes with other divisions, one phase correction element can correct both spherical aberration and coma aberration, or both spherical aberration and astigmatism. In the following description, it is assumed that the electrode 24a is a spherical aberration correction electrode and the electrode 24b is a uniform common electrode.
[0020]
FIG. 3 is a schematic plan view showing an example of an electrode pattern of the phase correction element of the present invention formed on one surface of the substrate in order to correct spherical aberration. The hatched portion is a high resistance electrode 30 formed of a transparent conductive film, for example, a uniform electrode without division, and the black paint portions are low resistance electrodes 31 to 33 for applying a voltage to the high resistance electrode 30. . That is, the disk-shaped first low resistance electrode is the low resistance electrode 31, the annular second low resistance electrode is the low resistance electrode 32, and the third low resistance electrode whose inner shape is a circle is the low resistance electrode 33. is there.
[0021]
These three low-resistance electrodes, low-resistance electrode 31, low-resistance electrode 32, and low-resistance electrode 33, are arranged concentrically around the optical axis (not shown), and are arranged in this order from the center toward the periphery. . Further, the low resistance electrodes 31 and 32 and the 32 and 33 are conductively connected by the high resistance electrode 30. 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.
[0022]
FIG. 4 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. A is a spherical aberration distribution that requires correction caused by an optical disc thickness error, an optical system error, and the like. A wavefront aberration distribution for correction generated by the phase correction element, and C is a wavefront aberration distribution after correction. The horizontal axis is the pupil radius centered on the optical axis, and the radius of the incident light beam is 1. Here, the radius of the incident light beam is set to the same size as the pupil radius, and the pupil radius and the radius of the incident light beam are used in the same meaning. When the signals 1 to 3 are set to appropriate voltages in order to correct the spherical aberration distribution A, the wavefront aberration distribution B by the phase correction element continuously changes according to the magnitude of the applied voltage. As shown in FIG. 4, when the wavefront aberration distribution B generated by the phase correction element has the same sign and the same sign as the spherical aberration A, they are canceled out and the wavefront aberration is corrected as shown in the corrected wavefront aberration C. Is corrected.
[0023]
The positions of the regions D and E in the wavefront aberration distribution B of the phase correction element in FIG. 4 correspond to the positions of the low resistance electrodes 31 and 32 shown in FIG. A region corresponding to the low-resistance electrode 33 corresponds to a position having a pupil radius of 1 or more in FIG. 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.
[0024]
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.
[0025]
Sheet resistance ρ of electrode material forming low resistance electrode _L And sheet resistance ρ of the electrode material forming the high resistance electrode _H Ratio of ρ _L / Ρ _H Is less than 1/1000. ρ _L / Ρ _H When the current is large and exceeds 1/1000, a relatively large current flows through the high resistance electrode, and a voltage drop occurs in the low resistance electrode in contact with the high resistance electrode, making it difficult to obtain a desired voltage distribution. May be. Therefore, as the sheet resistance of the high-resistance electrode material is higher than that of the low-resistance electrode material, it is easier to continuously change the potential between adjacent low-resistance electrodes, and a desired potential distribution can be obtained. ρ _L / Ρ _H Is a condition for obtaining a desired potential distribution.
[0026]
But ρ _H If is too large, the conductivity of the high resistance electrode is lost and potential distribution does not occur. _H 10 ^Three Ω / □ -10 ⁸ Ω / □ is good. On the other hand, ρ _L Is preferably about 0.1Ω / □ to 10Ω / □, since the allowable resistance range of the high-resistance electrode is increased when it is made as small as possible.
[0027]
The resistance value ρ of the low resistance electrodes 31 to 33 is set so as to satisfy the above conditions. _L And the resistance value ρ of the high resistance electrode 30 _H Is set to be equipotential because the resistance is low in the low resistance electrode, but the potential distribution in the surface of the high resistance electrode 30 is continuous due to the voltage drop generated between the low resistance electrodes 31 and 32 and 32 and 33. To change. Since the orientation direction of the liquid crystal molecules changes according to the voltage value, an effective refractive index distribution of the liquid crystal that is substantially equal to the voltage distribution is generated. As a result, a wavefront aberration distribution as shown in FIG. 4B is generated.
[0028]
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 relatively low specific resistance, such as an ITO film, because the above-described ratio of the resistance of the low resistance electrode to the resistance 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. . 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.
[0029]
On the other hand, the material for the high resistance electrode needs to be transparent and have a higher sheet resistance than the material for 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. 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.
[0030]
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. In general, the resistivity of an ITO film can be changed by the film forming method. For example, a specific resistance of 1 × 10 can be obtained by using an ITO film of a low resistance electrode by a magnetron sputtering method used in a liquid crystal display or the like. ^-6 Using a method such as dip coating, in which the high resistance electrode is used for electrothermal glass, etc., the specific resistance is 1 × 10. ^-Four It is preferable to form so as to be about ˜1Ω · m.
[0031]
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.
[0032]
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, in order not to affect the control characteristics of the actuator, it is preferable to reduce the weight of the phase correction element, or to use a light and easy to connect signal leader line such as a wire.
[0033]
【Example】
The optical head device of this example includes a phase correction element that corrects spherical aberration caused by an optical disc thickness error, an objective lens size manufacturing error, an optical system adjustment error, and the like. When the thickness of the optical disk deviates from the designed value, the objective lens generates spherical aberration and the signal reading accuracy is lowered. A phase correction element for correcting this spherical aberration was incorporated as the phase correction element 4 of the optical head device shown in FIG. However, the phase correction element control circuit 10 is improved for the phase correction element of this example. The wavelength of the emitted light of the semiconductor laser 1 that is a light source is 405 nm, and a collimated lens 3 forms a parallel light beam. The NA of the objective lens 6 is 0.85, and the effective pupil diameter is 3 mm. Therefore, in this example, a case where the spherical aberration of light having a light beam diameter of 3 mm (radius 1.5 mm) is corrected will be described.
[0034]
The element structure of the phase correction element of this example is the same as that shown in FIG. 2, and the electrode pattern is the same as that shown in FIG. Specific resistance 1 × 10 on the surface of an alkali-free glass substrate having a thickness of 0.5 mm by magnetron sputtering. ^-Five An ITO film of Ω · m was formed, and this ITO film was patterned by photolithography and etching techniques to form the uniform common electrode 24b and the low resistance electrodes 31 to 33 shown in FIG.
[0035]
Thereafter, a specific resistance of 1 × 10 is applied on the low resistance electrodes 31 to 33. ^-2 A transparent high resistance electrode 30 was formed by forming an Ω · m ITO film by dip coating. After that, 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 resistance value of the sheet of the high resistance electrode is 1 × 10. ^Five Ω / □, and the resistance value of the low resistance electrode was 1/1000 of this value.
[0036]
The two produced substrates were overlapped to form a cell structure via an epoxy sealant mixed with a 4 μm glass spacer. A nematic liquid crystal having a difference Δn between ordinary light refractive index and extraordinary light refractive index of 0.15 was injected between the substrates of the cell to form a liquid crystal cell. The alignment film was rubbed in advance so that the liquid crystal layer became homogeneous alignment, and an antireflection film was coated on the surface opposite to the liquid crystal layer of the glass substrate.
[0037]
The low resistance electrode 31 has a disk shape with a diameter of 0.63 mm (radius 0.315 mm), the low resistance electrode 32 has an annular shape with an inner diameter of 2.22 mm and an outer diameter of 2.42 mm, and the average diameter of the annular ring is 2.32 mm ( The low-resistance electrode 33 has an annular shape with an inner diameter of 3.1 mm (inner radius of 1.55 mm), and the thickness of the annular ring is 0.6 mm. All the concentric circles are centered on the optical axis. Is arranged. Therefore, the ratio to the beam radius was 0.21, 0.77, and 1.08, respectively. The low resistance electrodes 31 to 33 are respectively connected to external signal sources by wiring, and an arbitrary voltage can be supplied to the low resistance electrodes by the signals 1 to 3, respectively.
[0038]
The principle of correcting spherical aberration by the phase correction element of this example will be described below. FIG. 5 shows wavefront aberration (spherical aberration) that occurs when the optical disk thickness is 0.01 mm thicker than the designed value of 0.1 mm in an optical system with an objective lens NA of 0.85 and a light source wavelength of 405 nm. FIG. When the optical disk is thicker than the design value, the phase of the central portion of the effective pupil (the region described as 0 to 40 nm on the inner side) and the peripheral portion of the effective pupil (the region described as 0 to 40 nm on the outer side) are both The phase of the sandwiched intermediate portion (the region described as 80 to 120 nm) is advanced, and when the thickness is thin, the phase of the intermediate portion is delayed.
[0039]
In order to correct the spherical aberration caused by the optical disk thickness error of 0.01 mm by the phase correction element, 2.3 V was supplied to the low resistance electrodes 31 and 33 and 2.0 V was supplied to the low resistance electrode 32. Here, 0 V is applied to the electrode 24b.
The high resistance electrode 30 generates a voltage distribution according to the voltage difference between the low resistance electrodes. Similar to the above description, as a result of the substantial refractive index distribution of the liquid crystal generated by the voltage distribution, the phase correction element can generate a concentric phase change, and its radial distribution is as shown in FIG. become.
[0040]
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 changed to C. Reduce as follows. In this example, the spherical aberration before correction is about 0.1λ. _rms However, as a result of correction using the phase correction element, about 0.018λ _rms Decreased.
[0041]
On the other hand, when the optical disk is as thin as 0.01 mm, 2.0 V is applied to the low-resistance electrodes 31 and 33 and 2. What is necessary is just to apply 3V. As a result, the phase change generated by the phase correction element also takes the form of reversing the sign of B in FIG. As described above, the spherical aberration shown in FIG. 5 can be corrected by applying an appropriate voltage to the low resistance electrodes 31, 32, and 33. In addition, even if the low resistance electrodes 31 and 33 are always applied with the same voltage, they do not greatly affect the optical characteristics. Therefore, the low resistance electrodes 31 and 33 may be electrically connected to one power source, and in this case, operated with two signals. be able to.
[0042]
As described above, by using the optical head device of this example, it was possible to satisfactorily correct the spherical aberration caused by the thickness unevenness of the optical disk. Further, in the conventional phase correction element in which the electrodes are divided, the light scattering generated in the vicinity of the region where the voltage between the electrodes cannot be applied is suppressed to be low in this example. As a result, the transmittance is improved by 3%. Furthermore, since it was possible to operate with fewer external signal sources than in the past, an optical head device could be manufactured at a low cost.
[0043]
【The invention's effect】
As described above, in the optical head device of the present invention, at least one of the electrodes formed on each of the pair of substrates constituting the phase correction element by sandwiching the liquid crystal is arranged in three concentric circles. A low resistance electrode and a high resistance electrode for conductively connecting the low resistance electrodes are formed. With this configuration, the phase correction element can cause a continuous phase (wavefront) change in the light emitted from the light source. Therefore, the optical head device can efficiently correct the spherical aberration due to the optical disk thickness error, and the noise. A good signal light with a small amount can be obtained.
[Brief description of the drawings]
FIG. 1 is a conceptual cross-sectional view 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 schematic plan view showing an example of an electrode pattern of a phase correction element in the present invention.
FIG. 4 is a graph showing an example of spherical aberration correction by wavefront aberration generated by a phase correction element according to the present invention (A is spherical aberration that requires correction, B is wavefront aberration generated by a phase correction element, and C is a wavefront after correction). Aberration, D is a region corresponding to the low resistance electrode 31, and E is a region corresponding to the low resistance electrode 32).
FIG. 5 is a diagram showing spherical aberration when an optical disk thickness error of 0.01 mm occurs in an optical system in which the NA of an objective lens is 0.85 and the wavelength of a light source is 405 nm.
[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

Claims (3)

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. An optical head device comprising a control voltage generating means for outputting a voltage to the phase correction element,
The phase correction element includes a liquid crystal layer sandwiched between a pair of transparent substrates, and electrodes for applying a voltage to the liquid crystal layer are formed on the surfaces of the pair of substrates, respectively, and at least one of the electrodes is transparent. The disk-shaped first low resistance is a high resistance electrode having a sheet resistance value of 10 ³ Ω / □ to 10 ⁸ Ω / □, and a low resistance electrode having a resistance value of 1/1000 or less of the high resistance electrode. An electrode, an annular second low resistance electrode, and a third low resistance electrode having at least a circular opening;
The three low resistance electrodes are arranged concentrically with the optical axis as the center, are arranged in the first, second, and third order from the center toward the periphery, and at least the low resistance electrodes are between the high resistance electrodes. It is conductively connected by,
A voltage is applied to each of the first low resistance electrode, the second low resistance electrode, and the third low resistance electrode,
The ratio of the light emitted from the light source to the radius of the light beam on the surface of the phase correction element is 0.2 to 0.3, and the ratio of the average radius of the second low resistance electrode is 0. a .7～0.85 optical head device the ratio of the inner radius of the third low-resistance electrode is characterized in der Rukoto 1.0 or more. 2. The optical head device according to claim 1 , wherein a voltage having the same potential is applied to the first low resistance electrode and the third low resistance electrode .   3. The optical head device according to claim 1, wherein the phase correction element and the objective lens are integrally formed.

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