JP2003045062A - Optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical head device incorporated with a liquid crystal element which can withstand laser beams emitted from a high power light source and having stable reproducing and recording characteristics. SOLUTION: A liquid crystal element 100 consisting of two transparent substrates sandwiching a liquid crystal layer and superposed on each other and provided with alignment layers formed by obliquely depositing an inorganic material on the surfaces coming in contact with the liquid crystal layer of the two transparent substrates is provided together with a quarter wave plate 5 in the optical path between a beam splitter 2 of the optical head device and an object lens 6 mounted on an actuator 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head device, and more particularly to an optical head device for recording and reproducing information on an optical recording medium such as an optical disk.

[0002]

2. Description of the Related Art A DVD, which is an optical disc, has digital information recorded at a higher density than a CD, which is also an optical disc, and an optical head device for reproducing the DVD is
The wavelength of the light source is 650 nm or 635 nm, which is shorter than 780 nm of CD, or the numerical aperture of the objective lens (N
A) is set to 0.6, which is larger than 0.45 of the CD, to reduce the spot diameter on the optical disc surface.

Further, in the next-generation optical recording, it has been proposed to obtain a larger recording density by making the wavelength of the light emitted from the light source about 400 nm and the NA larger than 0.6. However, due to the shorter wavelength of the light source and the higher NA of the objective lens, the permissible tilt for tilting the optical disk surface from a right angle with respect to the optical axis and the permissible uneven thickness of the optical disk become smaller.

The reason why these allowances become small is that coma aberration occurs when the optical disc is tilted, and spherical aberration occurs when the optical disc is uneven in thickness. This is because it becomes difficult to read the signal due to deterioration. Therefore, it has been proposed to install the liquid crystal element in the optical path and change the wavefront shape of the transmitted light to cancel the above aberration.

Further, as a method of changing the amount of light reaching the optical disk by using the liquid crystal element without changing the output of the light from the light source so much, for example, the liquid crystal in which the polarization state of the light transmitted through the liquid crystal element is changed by the applied voltage. By combining the element and a polarization separation element such as a polarization beam splitter, the amount of light after passing through the polarization beam splitter can be changed.

For example, when a polarization beam splitter is used which transmits almost 100% of light in a certain linear polarization direction (a direction) and reflects almost 100% of light in a linear polarization direction (b direction) orthogonal to the polarization direction. Will be described. When a certain voltage is applied to the liquid crystal element, almost 100% of the incident light passes through the polarization beam splitter when the linear polarization direction of the incident light is set to the a direction. When the voltage applied to the liquid crystal element is adjusted so that the polarization state of the incident light after passing through the element is circularly polarized, the light intensity after passing through the polarization beam splitter is 50%.

By changing the voltage applied to the liquid crystal element in this way, the polarization state of the transmitted light can be changed and the light intensity after passing through the polarization beam splitter can be modulated.

[0008]

However, in the conventional liquid crystal element, the wavelength of the light source is 500 nm or less, especially 400 nm.
This is a problem because it was found that the light would be damaged if the nearby light was continuously irradiated. According to our experiments, the cause of this optical damage is that the polyimide film, which is the most common material used as an alignment film for liquid crystal elements, is deteriorated by light with a wavelength of 500 nm or less, which affects the alignment of liquid crystals. Turned out to be giving.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and collects light emitted from a light source having a wavelength of 500 nm or less and light emitted from the light source onto an optical recording medium. An objective lens for emitting light, and a photodetector for receiving the emitted light that is condensed and reflected by the optical recording medium,
In an optical head device including a liquid crystal element arranged in an optical path between a light source and an optical recording medium or in an optical path between an optical recording medium and a photodetector, the liquid crystal element includes a plurality of transparent liquid crystal layers sandwiching a liquid crystal layer. Provided is an optical head device comprising a substrate, and an alignment film in which an inorganic material is obliquely vapor-deposited is provided on the side of each transparent substrate surface which is in contact with the liquid crystal layer.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The optical head device of the present invention has a wavelength of 5
A light source that emits light of 00 nm or less, an objective lens that collects the light emitted from the light source on an optical recording medium, and a photodetector that receives the emitted light that is collected and reflected by the optical recording medium. An optical head device comprising a liquid crystal element arranged in an optical path between a light source and an optical recording medium.

A light source that emits light having a wavelength of 500 nm or less and has an irradiation energy of 1000 kJ / cm ² or more can be used. Further, the liquid crystal element may be arranged in the optical path between the photodetector and the optical recording medium. The position of the liquid crystal element should be set between the collimator lens and the beam splitter, or between the beam splitter and the objective lens, when there is a collimator lens or a beam splitter between the light source and the optical recording medium. However, the liquid crystal element controls the plane wave, which is preferable.

The liquid crystal device according to the present invention comprises a plurality of transparent substrates sandwiching a liquid crystal layer, and each of the transparent substrate surfaces is provided with an alignment film in which an inorganic material is obliquely vapor-deposited on the side in contact with the liquid crystal layer. There is. By configuring the liquid crystal element in this way, the alignment film made of an inorganic material does not deteriorate even when light with a wavelength of 500 nm or less, particularly around 405 nm is continuously irradiated, and the alignment property of the liquid crystal layer that configures the liquid crystal element is maintained. Does not affect Therefore, when this liquid crystal element is mounted in an optical head device, there is an effect that the wavefront of the incident light does not change even if light is continuously incident on the element.

The number of transparent substrates to be stacked may be two, three, four or more. In any case, a liquid crystal layer is sandwiched between one transparent substrate and another transparent substrate, and an alignment film in which an inorganic material is obliquely vapor-deposited is provided on the surface of the transparent substrate which is in contact with the liquid crystal layer. The number of transparent substrates may be selected according to the purpose of use, and if necessary, a curved surface such as a concave shape or a convex shape may be formed on the surface of the transparent substrate in contact with the liquid crystal layer.

In the following description, two transparent substrates are used. The transparent substrate that constitutes the liquid crystal element,
An optical head device in which a planar electrode is formed so that a voltage can be applied to the liquid crystal layer and the wavefront shape of the transmitted light of the liquid crystal element can be changed is spherical aberration due to thickness deviation of the optical disk or tilt deviation of the optical disk. This is preferable because it can correct coma aberration due to. It is also preferable in that a desired transmitted wavefront shape can be generated without forming an uneven curved surface on the transparent substrate.

An example of the optical head device of the present invention shown in FIG. 1 uses a blue laser having a wavelength of 405 nm as a light source.
It is for recording information on the optical disk 8 which is an optical recording medium and for reproducing the information recorded on the optical disk 8. This optical head device has a semiconductor laser 1 as a light source, collimating lenses 30 and 31, an objective lens 6, an actuator 7 having the objective lens mounted thereon, an optical disk 8, a prism type beam splitter 2, and a photodetector 9. Further, a liquid crystal aberration correction element, which is the liquid crystal element 100 that changes the wavefront of the light emitted from the semiconductor laser 1, is arranged integrally with the quarter-wave plate 5.

As a method of changing the amount of light reaching the optical disk, there is a method of changing the output light amount of the light source. However, depending on the characteristics of the semiconductor laser, noise may increase when the output is reduced. On the transparent substrate constituting the liquid crystal element, a flat electrode is formed so that a voltage can be applied to the liquid crystal layer to change the polarization state of the transmitted light of the liquid crystal element, and a polarization separation element such as a polarization beam splitter is formed. The combined use of the optical head devices is preferable because the amount of light reaching the optical disk can be changed by the voltage applied to the liquid crystal element while keeping the semiconductor laser in a state where the output noise is minimized.

In another example of the optical head device of the present invention shown in FIG. 3, a liquid crystal element 101 for changing the polarization state of the light emitted from the semiconductor laser 1 is used, and the liquid crystal element 101 and the polarization type beam splitter 2 are used. The amount of light transmitted to the optical disc is changed by combining this with. 3 that are the same as those in FIG. 1 indicate the same elements as in FIG.

An example of a liquid crystal element used in the optical head device of the present invention will be described with reference to FIG. 2 which is a schematic sectional view. In the liquid crystal element, transparent conductive films 24 and 24a, which are flat electrodes, are formed on the surfaces of two glass substrates 21 and 21a, which are transparent substrates facing each other, and an inorganic material is formed on the transparent conductive films 24 and 24a. Alignment films 26 and 26a formed by obliquely vapor-depositing materials are stacked, and the glass substrate 2
The periphery of 1, 21a is sealed with a sealing material 22,
A liquid crystal is filled in a space formed by the alignment film 26 and the alignment film 26a to form a liquid crystal layer 23.

Here, Ta ₂ O ₅ , WO ₃ , and Bi ₂ O ₃ can be cited as the inorganic material used for the oblique vapor deposition. Furthermore, SiO _x (0 <x <2) and (1-
y) SiO _x + yZrO ₂ , (1-y) SiO _x + yT
An iO ₂ (0 <x <2, 0 <y <1) oxide or the like may be used. Among these, SiO _x (0 <x
It is preferable to use <2) in order to obtain excellent stability of the alignment state of liquid crystal molecules.

As a method of oblique vapor deposition, for example, when forming an oblique vapor deposition film of SiO _x , in a vacuum vapor deposition apparatus,
A substrate is arranged vertically above the SiO vapor deposition source, the angle formed by the vertical line and the substrate normal is set to 88 to 60 °, and the substrate temperature is set between room temperature and 300 ° C. to perform vacuum vapor deposition of SiO.
The oxygen amount x is adjusted by adjusting the oxygen gas concentration. The angle formed by the vertical line and the substrate normal is 88 to 80 °, which is preferable as the pretilt angle of liquid crystal molecules.
It is preferable to perform the baking at about ° C in order to stabilize the surface condition of the film.

The two substrates thus formed with the orientation films by oblique vapor deposition are arranged such that the orientation films face each other and the two orientation directions form a predetermined angle with a predetermined gap. A peripheral portion of is sealed with a sealing material, and liquid crystal is injected into the gap to form a liquid crystal element.

In the polyimide alignment film, which is most often used in the conventional liquid crystal device, the alignment state of the liquid crystal molecules is largely changed when the laser beam having a short wavelength (for example, 405 nm) is continuously irradiated with high intensity. In the case of using an inorganic material by oblique vapor deposition in (1) as an orientation film, the orientation state does not change, and the transmitted wave front is good. The alignment direction of liquid crystal molecules (the component direction on both substrates of the alignment direction of liquid crystal molecules on both sides of the liquid crystal layer) is preferably parallel alignment when the liquid crystal element is used for aberration correction. When the polarization direction of the incident light is the same, applying a voltage to the liquid crystal layer changes the effective refractive index, which can change the wavefront of the incident light with almost no change in the polarization state of the light. it can.

The above-mentioned liquid crystal element having an alignment film formed by oblique vapor deposition can be used as a liquid crystal aberration correcting element that changes the wavefront shape of transmitted light, and the liquid crystal element whose polarization state changes by transmission and the transmission depending on the polarization direction. It can also be used as an active attenuator or the like that can change the amount of light transmitted to an optical disk by combining with a polarization beam splitter having a different ratio. The liquid crystal element other than the above used in the optical head device has a great effect of improving the optical damage and can be used for this liquid crystal element.

The liquid crystal material used is preferably a nematic liquid crystal used for display applications,
Alternatively, the liquid crystal molecules may be twisted by adding a chiral agent. Also, as the material of the transparent substrate used,
Glass, polycarbonate resin, acrylic resin, epoxy resin, vinyl chloride resin, etc. can be used,
A glass substrate is preferable in terms of durability and the like.

A phase plate such as a quarter-wave plate may be laminated on the liquid crystal element for use. As the material for the phase plate, a birefringent single crystal such as quartz or LiNbO ₃ may be used, or an organic substance film such as polymer liquid crystal or polycarbonate may be used.

[0026]

EXAMPLE The optical head device of this example is provided with a phase correction element for correcting spherical aberration caused by the thickness deviation of the optical disk. The wavelength of the light emitted from the semiconductor laser that is the light source is 405 nm. When the thickness of the optical disk of the objective lens deviates from the designed value, spherical aberration occurs and the signal reading accuracy deteriorates. A phase correction element that corrects this spherical aberration is a liquid crystal element 1 of the optical head device shown in FIG.
It was installed in the 00 position.

This liquid crystal element has the same structure as the sectional view shown in FIG. The liquid crystal used was a nematic liquid crystal, and the alignment direction of the liquid crystal molecules was parallel to the polarization direction of the emitted light before entering the liquid crystal layer so as to change the wavefront of the emitted light from the semiconductor laser.

As shown in FIG. 2, this liquid crystal element has a structure in which a liquid crystal layer 23 is surrounded by a sealing material 22 and sandwiched by alignment films 26, 26a transparent electrode films 24, 24a and glass substrates 21, 21a. In this embodiment, transparent electrodes made of ITO divided into concentric segments shown in FIG. 4 are formed on two transparent glass substrates sandwiching the liquid crystal layer 23 so that a voltage can be applied to the liquid crystal layer 23. did.

On this transparent electrode, as an alignment film for liquid crystal,
50 nm (0.05 μm) thick SiO (purity 99.9)
The 9%) orthorhombic film was vacuum-deposited at a substrate temperature of 80 ° C. by an electron beam heating (EB) method. At this time, vapor deposition was performed such that the angle formed by the substrate normal and the vertical (vertical) line of the vapor deposition source was 85 °. After vapor deposition, in order to obtain good alignment of liquid crystal molecules, this alignment film is heated to 200 ° C. in the atmosphere.
Was fired for 1 hour. A desired voltage was applied to each of the concentric segments so as to correct the spherical aberration due to the thickness deviation of the optical disc.

The NA of the objective lens is 0.85, the thickness of the cover layer up to the reflecting surface of the optical disk is 0.10 mm,
The reproduction characteristics of three types of optical disks of 0.11 mm and 0.09 mm were examined. As the optical head device, an optical disc device having a cover layer thickness of 0.10 mm and adjusted to minimize spherical aberration was used. With a 0.10 mm-thick optical disk, good reproduction characteristics were obtained without applying a voltage to the liquid crystal layer. On the other hand, the reproduction characteristics of the optical discs having the thicknesses of 0.11 mm and 0.09 mm were not good due to the influence of spherical aberration.

The distribution of the voltage applied to the liquid crystal layer is adjusted according to the thickness of each optical disk, and spherical aberration of the opposite sign to the spherical aberration generated by the thickness deviation of the cover layer of the optical disk is generated to generate a spherical aberration on the optical disk. The light showed good condensing characteristics and could improve the reproducing characteristics. Further, even after performing a light irradiation test for 5000 hours with a laser light output of 30 mW, there was no deterioration of the characteristics and the condensing characteristics were good.

[0032]

EFFECTS OF THE INVENTION By using a film obtained by obliquely vapor-depositing an inorganic material as the alignment film of the liquid crystal element mounted in the optical head device of the present invention, the alignment film can withstand irradiation light of high power for a long time, The alignment state of liquid crystal molecules can be stably maintained. Therefore, in the optical head device equipped with this liquid crystal element, the condensing characteristic of the light emitted from the light source does not deteriorate even when used for a long time, and a stable signal reproducing characteristic is obtained when reproducing information on the optical recording medium. Further, good and constant recording characteristics can be obtained at the time of recording information on the optical recording medium.

[Brief description of 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 sectional view showing an example of a liquid crystal element according to the present invention.

FIG. 3 is a conceptual sectional view showing another example of the principle configuration of the optical head device of the present invention.

FIG. 4 is a schematic plan view showing a divided electrode pattern of a liquid crystal element that corrects spherical aberration in the present invention.

[Explanation of symbols]

1: Semiconductor laser 2: Beam splitter 30, 31: Collimating lens 5: quarter wave plate 6: Objective lens 7: Actuator 8: Optical disc 9: Photodetector 100, 101: Liquid crystal element 21, 21a: glass substrate 22: Seal material 23: Liquid crystal layer 24, 24a: transparent conductive film 26, 26a: Alignment film

Continued front page    F-term (reference) 2H049 BA06 BA47 BB03 BB62 BC02                       BC09 BC21                 2H088 EA62 HA29 MA18                 2H090 LA16 MB06                 5D119 AA32 BA01 HA63 JA09

Claims (4)

[Claims] 1. A light source for emitting light having a wavelength of 500 nm or less, an objective lens for converging the light emitted from the light source on an optical recording medium, and the emitted light condensed and reflected by the optical recording medium. An optical head device comprising a photodetector for receiving light and a liquid crystal element arranged in the optical path between the light source and the optical recording medium or in the optical path between the optical recording medium and the photodetector,
The liquid crystal element is provided with a plurality of transparent substrates sandwiching a liquid crystal layer, and an alignment film in which an inorganic material is obliquely vapor-deposited is provided on the side of each transparent substrate surface in contact with the liquid crystal layer. Head device. 2. The inorganic material is SiO _x (0 <x <2).
The optical head device according to claim 1, further comprising: 3. A flat electrode is formed on the transparent substrate so that a voltage can be applied to the liquid crystal layer to change the wavefront shape of the transmitted light of the liquid crystal element. Optical head device. 4. A flat electrode is formed on the transparent substrate so that a voltage can be applied to the liquid crystal layer to change the polarization state of the transmitted light of the liquid crystal element. Optical head device.