JP4085527B2 - 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 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 0.6 or more. However, as the wavelength of the light source becomes shorter and the NA of the objective lens becomes higher, the allowable amount of tilt in which the optical disc surface is tilted from the right angle with respect to the optical axis and the allowable amount of uneven thickness of the optical disc are reduced.
[0004]
These tolerances are reduced because coma aberration occurs when the optical disc is tilted, and spherical aberration occurs when the optical disc is uneven in thickness. Due to the difficulty of reading. In high-density recording, several methods have been proposed in order to increase the allowable amount of the optical head device with respect to tilt and uneven thickness of the optical disk.
[0005]
As one method, there is a method in which an axis for tilting is added to an objective lens actuator that normally moves in the biaxial direction of the tangential direction and the radial direction of the optical disc so that the objective lens is tilted according to the detected tilt angle. is there. However, this additional method has problems that spherical aberration cannot be corrected and that the structure of the actuator is complicated.
[0006]
Another method is to correct wavefront aberration by a phase correction element provided between the objective lens and the light source. In this correction method, it is possible to increase the allowable amount of tilt and the allowable amount of unevenness of the optical disc by simply incorporating an element into the optical head device without making a major modification to the actuator.
[0007]
For example, Japanese Patent Laid-Open No. 10-20263 discloses the above correction method for correcting the tilt of an optical disc using a phase correction element. This is because a voltage is applied to a divided electrode formed by dividing an electrode on a substrate sandwiching a birefringent material such as liquid crystal that constitutes a phase correction element. In this method, the refractive index is changed according to the tilt angle of the optical disk, and the coma aberration generated by the tilt of the optical disk is corrected by the phase change of the transmitted light generated by the change of the refractive index.
[0008]
[Problems to be solved by the invention]
In the above-described phase correction element, it is necessary to form a plurality of divided electrodes for applying a voltage to the birefringent material in order to generate a phase change of transmitted light. Since there is no electrode in the gap between the divided electrodes and no voltage can be applied, the electric field distribution is non-uniform at the boundary between the gap and the divided electrode when a voltage is applied, and the alignment of the liquid crystal molecules becomes non-uniform. As a result, the refractive index is not constant near the boundary between the gap and the divided electrode due to the application of voltage, and the amount of light incident on the optical recording medium is reduced because light is diffracted at a portion where the refractive index distribution exists. Signal reading accuracy decreases.
[0009]
[Means for Solving the Problems]
The present invention has been made to solve the above-described problems, and includes a light source, an objective lens for condensing the emitted light from the light source on the optical recording medium, and the emitted light between the light source and the objective lens. An optical head device including a phase correction element that changes a wavefront of the optical head device, wherein the phase correction element includes an anisotropic optical medium sandwiched between a pair of transparent substrates, and the anisotropic The product Δn · d of the difference Δn between the ordinary light refractive index and the extraordinary light refractive index of the optical medium and the thickness d in the light transmission direction of the anisotropic optical medium is 0.92 μm or more, and at least one of the pair of substrates A plurality of divided electrodes arranged at intervals are formed on one transparent side, the interval between the divided electrodes is 5 μm or less, and a control signal for generating a control signal for changing the wavefront is generated in the phase correction element Signal generating means, and To provide an optical head and wherein the difference in transmittance when applying the same voltage in the range of 0~6V is within 2% between.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a conceptual cross-sectional view 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 DVD. The light emitted from the semiconductor laser 1 as a light source is, for example, a hologram type polarized beam. After passing through the splitter 2, it becomes parallel light by the collimating lens 3, is reflected in the 90 ° direction by the rising mirror 11, passes through the phase correction element 4, the quarter-wave plate 5, and is then optical disc 8 by the objective lens 6. Focused on top. Here, the pair of substrates constituting the phase correction element 4 are both transparent.
[0011]
The condensed light is reflected by the optical disk 8 and sequentially passes through the objective lens 6, the quarter-wave plate 5, the phase correction element 4, and the collimating lens 3 in the reverse order, and then diffracted by the polarizing beam splitter 2. The light enters the detector 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.
[0012]
The polarization beam splitter 2 includes, for example, a polarization hologram, and strongly diffracts light having a polarization component in an anisotropic direction (a direction having a difference in refractive index) and guides the light to the photodetector 9. For example, a control signal is generated (output) by the phase correction element control circuit 10 which is a control signal generation unit toward the phase correction element 4 so that the intensity of, for example, a reproduction signal of the optical disk obtained from the photodetector 9 is optimized. . The control signal output from the phase correction element control circuit 10 is an electric signal corresponding to the tilt amount of the optical disk or the shift amount of the objective lens, and becomes a substantially changing voltage applied to the divided electrodes of the phase correction element 4. .
[0013]
The rising mirror 11 reflects the light emitted from the semiconductor laser 1 in the direction of approximately 90 ° and makes it incident on the optical disk. In order to reduce the thickness of the optical head device (direction perpendicular to the surface of the optical disk 8). It is an indispensable optical component. Usually, a glass surface with a highly reflective film such as Al deposited thereon is used.
[0014]
Next, the configuration of the phase correction element used in the present invention will be described with reference to the drawings. FIG. 2 is a cross-sectional view of the phase correction element in the present invention.
As the anisotropic optical medium, an optical crystal such as lithium niobate or a liquid crystal can be used. Here, it demonstrates as what uses a liquid crystal.
[0015]
Glass substrates 21a and 21b are bonded by a sealing material 22 to form a liquid crystal cell. The sealing material 22 contains a glass spacer and a conductive spacer having a resin surface coated with gold or the like. The inner surfaces of the liquid crystal cells of the glass substrates 21a and 21b are coated with electrodes 24a and 24b and an insulating film 25 and an alignment film 26 mainly composed of silica, and an antireflection film is coated on the outer surfaces of the liquid crystal cells. Has been.
[0016]
The electrode 24a is wired in a pattern so that it can be connected to the phase correction element control circuit by a connection line at the electrode lead-out portion 27. Further, the electrode 24b is electrically connected to the electrode formed on the glass substrate 21a by the conductive spacer described above, and can be connected to the phase correction element control circuit by the connection line at the electrode lead-out portion 27 like the electrode 24a. Liquid crystal 23 is filled in the liquid crystal cell, and the liquid crystal molecules 28 shown in FIG. 2 are in a homogeneous alignment state aligned in one direction. The material of the electrodes 24a and 24b is preferably high in transmittance, and a transparent conductive film such as ITO may be used.
[0017]
As the material of the alignment film, it is sufficient 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 the figure, or a silica film is obliquely deposited. Good. The liquid crystal material is preferably a nematic liquid crystal used for display applications and may be twisted by adding a chiral agent.
[0018]
When using the birefringence of the liquid crystal as a means to obtain the desired wavefront change (phase change) amount, the response between the liquid crystal cells should be reduced by increasing the difference between the ordinary and extraordinary refractive indices of the liquid crystal. Can be high. 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 desirable to do.
In the case of the optical head device shown in FIG. 1, since light passes through the phase correction element 4, it is desirable that the electrodes 24a and 24b have high transmittance, and a transparent conductive film such as ITO may be used. In this case, the phase correction element 4 is used as a transmissive element.
[0019]
However, one of the electrodes 24a and 24b can be made using a material having high reflectivity such as Al or Cr, and the phase correction element 4 can be used as a reflective element. In this case, the phase correction element 4 is installed at this position instead of the rising mirror 11 shown in FIG. If the first electrode (for example, the electrode 24a) on which light is incident is made a transparent electrode divided with high transmittance and the other electrode (for example, the electrode 24b) is made high-reflectance, the phase correction element 4 The incident light passes through the transparent electrode 24a and the liquid crystal and is reflected by the electrode 24b, and then passes again through the liquid crystal and the transparent electrode 24a toward the optical disc 8.
[0020]
If the phase correction element is used as the reflection type, the rising mirror 11 in FIG. 1 can be replaced with the phase correction element 4, which is desirable because the number of parts is reduced. However, since the light incident on the phase correction element 4 passes through the liquid crystal twice at an incident angle of approximately 45 °, it is necessary to set a liquid crystal cell interval (the thickness of the liquid crystal layer in the liquid crystal cell) different from that in the transmissive type. There is.
[0021]
Next, effects obtained by using the phase correction element of the present invention will be described. FIG. 3 is a schematic plan view showing an example of the electrode pattern of the divided electrodes of the phase correction element according to the present invention. The electrode 24a of FIG. 2 is divided into five divided electrodes 31 to 35 using a photolithography technique or the like. . The light condensed on the optical recording medium by the objective lens passes through the divided electrodes 31 to 35 and the liquid crystal 23 when passing through the phase correction element. The divided electrodes 31 to 35 are formed of, for example, an ITO film. Since the gap between the divided electrodes corresponding to the solid line in the figure is removed by etching or the like, the divided electrodes 31 to 35 can be set to different voltages. .
[0022]
FIG. 4 is a schematic cross-sectional view in the vicinity of the gap between the divided electrodes of the phase correction element in the present invention and a graph showing a substantial refractive index distribution. FIG. 4A is a schematic cross-sectional view in the vicinity of the gap between two divided electrodes. Transparent electrodes 41a and 41b, insulating films 42, and an alignment film 43, which are divided electrodes, are formed on the surface of the glass substrate 40. FIG. Yes. There is a gap having a width of W between the transparent electrodes 41a and 41b, and no voltage can be applied to the liquid crystal in the gap region from the outside.
[0023]
When a voltage is applied to the transparent electrodes 41a and 41b, an electric field is generated in the liquid crystal from the transparent electrode on the glass substrate surface, so that the liquid crystal molecules 44 are aligned almost perpendicularly to the surface of the glass substrate. The direction of the electric field and the electric field strength are represented by the direction and density of the electric field lines 45. Only the leakage electric field from the transparent electrodes 41a and 41b contributes to the external electric field that determines the orientation of the liquid crystal molecules in the gap region.
[0024]
However, when the gap width W is larger than the liquid crystal cell interval, the strength of the leakage electric field becomes weak at the gap except for the vicinity of the boundary with the transparent electrode, so even if a voltage is applied to the transparent electrodes 41a and 41b. It is considered that the alignment direction of the liquid crystal molecules hardly changes. At this time, since the liquid crystal molecules are in different alignment states near the transparent electrode of the gap and inside the gap, the substantial refractive index distribution perceived by the incident light to the phase correction element becomes non-uniform.
[0025]
FIG. 4B is a graph showing a substantial refractive index distribution near the gap between two divided electrodes. In the gap portion, the liquid crystal molecules are substantially parallel to the glass substrate surface, and the substantial refractive index is higher than that of the transparent electrode portion. A phase difference of θ = 2πΔφ · d / λ is generated when a liquid crystal cell interval is d and a wavelength of incident light is λ, where Δφ is a substantial refractive index difference of liquid crystal between the gap portion and the gap portion of the transparent electrode.
[0026]
When Δφ is increased by applying a voltage to the transparent electrodes 41a and 41b in FIG. 4A and θ becomes about π, the incident light is diffracted near the boundary between the gap and the transparent electrode. The typical transmittance (the portion that is transmitted without being diffracted) decreases. The electric field strength in the gap between the divided electrodes caused by the leakage electric field is inversely proportional to the second to third power of the gap width W. In the present invention, by reducing the gap width W, the effect of the leakage electric field is strengthened, and the refractive index change in the gap between the divided electrodes can be reduced.
[0027]
Therefore, when the gap width W is not small, Δφ is increased by application of voltage and θ becomes about π and the transmittance is reduced. However, when the gap width W is small, the refractive index change in the gap is small and the diffraction effect is reduced. As a result of the weakening, the substantial transmittance is improved.
[0028]
In general, when the gap width W is reduced, the leakage electric field in the gap becomes stronger, so that the decrease in transmittance due to diffraction is reduced. However, the patterning accuracy of a transparent conductive film such as ITO becomes stricter as the gap width W is reduced. Therefore, it becomes difficult to produce a divided electrode. Therefore, it is preferable to form the divided electrodes so that the minimum gap width is obtained in the range where the yield of the phase correction element is good. For example, the effective voltage in which the normal liquid crystal element is used is within the range of 0V to 6V, and should be 7 μm or less. Is realistic.
[0029]
Further, when the product of the difference Δn between the ordinary light refractive index and the extraordinary light refractive index of the liquid crystal and the liquid crystal cell interval d is 0.5 μm or more, the result that the leakage electric field strength increases by setting the gap width W to 7 μm or less is Δφ. Since the phase difference θ becomes smaller than π, the diffraction effect is weakened and the transmittance is improved, which is preferable. Further, when the product is 0.5 μm or more and the gap width W is 5 μm or less, the leakage electric field strength is further increased. As a result, the phase difference θ becomes very small and the transmittance is further improved.
[0030]
The wavefront of the light emitted from the light source is changed by the phase correction element to correct coma aberration, spherical aberration and astigmatism, which are wavefront aberrations.
The electrode pattern shown in FIG. 3 is an example mainly for correcting the coma aberration. For example, even when the optical disk is tilted, the coma aberration is corrected by outputting an appropriate control signal to the phase correction element. A reproduction signal can be obtained. Further, spherical aberration and astigmatism can be corrected by making the electrode pattern as shown in FIG. 5 with the same configuration (FIGS. 1 and 2) and principle (FIG. 4) as described above.
[0031]
FIG. 5A is a schematic plan view showing an example of an electrode pattern of the divided electrodes when correcting spherical aberration. When this electrode pattern is used, even if the thickness of the optical disk changes, a good reproduction signal can be obtained by transmitting an appropriate control signal to the phase correction element.
FIG. 5B is a schematic plan view showing an example of the electrode pattern of the divided electrodes when astigmatism is corrected. When this electrode pattern is used, astigmatism generated by the semiconductor laser and other optical components can be corrected, and a good reproduction signal can be obtained.
[0032]
As described above, an electrode pattern corresponding to the wavefront aberration to be corrected is formed, and the gap between the divided electrodes is set to 7 μm or less, whereby a phase correction element having a substantially high transmittance can be obtained.
[0033]
【Example】
In this embodiment, the information recorded on the optical disk 8 is reproduced by the optical head device shown in FIG. The optical head device incorporates a transmission type phase correction element 4 having the configuration shown in the cross-sectional view of FIG. 2, and a signal obtained by the photodetector 9 is processed by the phase correction element control circuit 10 and the optical disk 8 is processed. An electric signal corresponding to the amount of tilt in the radial direction is generated to drive the phase correction element 4.
[0034]
The phase correction element 4 has a liquid crystal cell structure including non-alkaline glass substrates 21 a and 21 b having a thickness of 0.5 mm and a sealing material 22 mainly composed of epoxy, and the glass contained in the sealing material 22. The distance between the liquid crystal cells is 4.6 μm due to the manufactured spacer. The inside of the liquid crystal cell is filled with nematic liquid crystal having a difference between ordinary light refractive index and extraordinary light refractive index of 0.2, and is shown in FIG. 2 by a polyimide alignment film 26 applied to the surfaces of the glass substrates 21a and 21b. Liquid crystal molecules 28 are aligned in the left-right direction on the paper surface.
[0035]
An insulating film 25 and ITO electrodes 24a and 24b are formed between the alignment film 26 and the glass substrate, and the electrodes 24a and 24b are connected to the phase correction element control circuit 10 at the electrode lead-out portion 27 by connection lines. ing. The electrode 24b is a uniform electrode without division, whereas the portion of the electrode 24a facing the electrode 24b is divided into divided electrodes 31 to 35 shown in FIG. 3 by a photolithography technique.
[0036]
The outermost circumference of these divided electrodes is a circle having a diameter of 4 mm, and the phase corresponding to the voltage value applied to each of the divided electrodes 31 to 35 when the light incident on the objective lens is transmitted through the region of this electrode. A shift is received in each area of light. In the present embodiment, a voltage of 2.5 V is always applied to the divided electrode 33, and a voltage of 2 to 3 V is applied to the divided electrodes 31, 32, 34, and 35 in accordance with the tilt amount of the optical disk. A good reproduction signal could be obtained from the optical disk.
[0037]
FIG. 6 is a graph showing the applied voltage dependence of the light transmittance of the phase correction element used in this example, and the gap width W between the divided electrodes in this example is 5 μm. The light transmittance of a conventional gap width of 10 μm as a reference example is also shown. The configuration of the conventional phase correction element is the same as that of the present embodiment, but the gap width between the divided electrodes is different. The light source used for the measurement is a semiconductor laser having a wavelength of 650 nm.
The transmittance in FIG. 6 is shown as a percentage of the amount of incident light at each voltage with respect to the amount of incident light on the objective lens at a voltage of 0V.
[0038]
In the reference example, with a voltage of 2.5 V applied, the transmittance was reduced by about 9% due to scattering in the gap between the divided electrodes. However, in the case of the phase correction element of this example, the reduction was only about 2%. .
[0039]
【The invention's effect】
As described above, in the optical head device of the present invention, by setting the gap width between the divided electrodes on the substrate constituting the liquid crystal phase correction element to 7 μm or less, it is within the range normally used in the liquid crystal element. A high light transmittance can be obtained at an arbitrary applied voltage, and thus a good reproduction signal can be obtained.
[Brief description of the drawings]
FIG. 1 is a conceptual cross-sectional view of a principle configuration of an optical head device of the present invention.
FIG. 2 is a cross-sectional view of a phase correction element in the present invention.
FIG. 3 is a schematic plan view showing an example of an electrode pattern of divided electrodes of a phase correction element in the present invention.
FIG. 4 is a schematic cross-sectional view in the vicinity of a gap between divided electrodes of a phase correction element and a graph showing a substantial refractive index distribution in the present invention, and (a) is a schematic view in the vicinity of a gap between two divided electrodes. It is sectional drawing, (b) is a graph which shows substantial refractive index distribution of the gap vicinity between two division electrodes.
FIG. 5 is a schematic plan view showing an example of an electrode pattern of divided electrodes of a phase correction element in the present invention, where (a) mainly corrects spherical aberration and (b) mainly astigmatism. This is a case where aberration is corrected.
FIG. 6 is a graph showing the applied voltage dependence of the light transmittance of the phase correction element used in this example.
[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 disk 9: photodetector 10: phase correction element control circuit 21a, 21b: Glass substrate 22: Sealing material 23: Liquid crystal 24a, 24b: Electrode 25, 42: Insulating film 26, 43: Alignment film 27: Electrode extraction portion 28, 44: Liquid crystal molecules 31-35: Split electrode 40: Glass substrate 41a 41b: transparent electrode 45: lines of electric force

Claims (1)

An optical head device comprising a light source, an objective lens for condensing the light emitted from the light source on an optical recording medium, and a phase correction element for changing the wavefront of the emitted light between the light source and the objective lens. And
The phase correction element includes an anisotropic optical medium sandwiched between a pair of substrates at least one of which is transparent,
The product Δn · d of the difference Δn between the ordinary light refractive index and the extraordinary light refractive index of the anisotropic optical medium and the thickness d in the light transmission direction of the anisotropic optical medium is 0.92 μm or more,
A plurality of divided electrodes arranged at intervals are formed on at least one transparent side of the pair of substrates, the interval between the divided electrodes is 5 μm or less, and further for changing the wavefront to the phase correction element An optical head device comprising control signal generating means for generating a control signal, wherein a difference in transmittance when applying the same voltage in the range of 0 to 6 V between the divided electrodes is within 2%.

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