JPH09288844A - Optical head device and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provided an optical head device in which a high reliability is achieved and a diffraction element holds polymer liquid crystal having optical anisotropy between two transparent substrates and an optical anisotropy analysis element of which the orientation of the polymer liquid crystal changes periodically. SOLUTION: A light beam emitted from a light source 1 is made incident on a beam splitter 2 and a certain portion of the light passes through it just as it is, and is converged by a converging lens 3 to reach an optical recording medium 4. The light reflected from the medium 4 is changed in its optical path at a certain rate through the beam splitter 2 and is made incident to the optical anisotropic diffraction grating 5. This diffraction grating 5 is formed to diffract the light in the direction of the polarization of the incident light, and the light is divided and detected by detectors 6A, 6B, and 6C. The grating 5 is the one that holds polymer liquid crystal having optical anisotropy between two transparent substrates and is an optical anisotropic diffraction element of which the polymer liquid crystal changes periodically. Thus, the diffraction element is allowed to be mass-produced with a good productivity.

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 for reading information on a magneto-optical medium.

[0002]

2. Description of the Related Art Conventionally, an optical polarization element is used in an optical head device for reading optical information from a magneto-optical disk such as a mini disk. As the optical head, a prism type beam splitter is used as an optical component that guides (beam splits) the signal light reflected from the recording surface of the magneto-optical disk, and the emitted light is diffracted through multiple optical polarization elements. The light is divided into light and detected by the detector.

As this light polarizing element, a birefringent single crystal such as LiNbO ₃ which is formed by joining crystal pieces having two different crystal directions is used. However, these single-crystal light polarizing elements are expensive because they use a single crystal and their processing is complicated, and it is difficult to mass-produce them.

[0004]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object thereof is to provide an optical head device suitable for mass production and having high reliability.

[0005]

The present invention is an optical head device for reading information and / or writing information by irradiating an optical recording medium with light from a light source, which comprises a light source and an optical recording medium. A beam splitter that changes the optical path of the light so that the light from the light source is transmitted and the reflected light from the optical recording medium does not return to the light source is arranged, and the reflected light whose optical path is changed is passed through the diffraction grating. In an optical head device that divides a plurality of diffracted lights and detects the light by a detection unit, a diffractive element has a polymer liquid crystal having optical anisotropy sandwiched between two transparent substrates. Provided is an optical head device, which is an optical anisotropic diffractive element whose orientation direction is periodically changed.

Further, electrodes are provided on each of the two transparent substrates of the optically anisotropic diffractive element, and at least one of the two electrodes is formed periodically, and a liquid crystal is applied while applying an electric field to these electrodes. And an optical anisotropic diffractive element in which the alignment direction of the polymer liquid crystal is cyclically changed to form an optical anisotropic device.

Further, the electrodes of at least one of the two transparent substrates of the optically anisotropic diffractive element are patterned, and when an electric field is applied to the electrodes, the liquid crystal is natural in the portions where the electrodes do not face each other. In such an arrangement, the liquid crystal is cured in this state by forcibly aligning the parts where the electrodes face each other, and an optical anisotropic diffractive element in which the alignment direction of the polymer liquid crystal changes periodically is set. The above optical head device is provided.

Further, a mask on which a light-shielding pattern is periodically formed is arranged on the optically anisotropic diffraction element, and an electric field is applied to the entire surfaces of the electrodes of the two transparent substrates, or without applying an electric field. An optical anisotropic diffractive element in which the alignment direction of the polymer liquid crystal is periodically changed by curing the liquid crystal only in the portion exposed to the light by light exposure and then changing the applied state of the electric field to cure the remaining portion. The above optical head device is provided.

That is, by providing the alignment of the polymer liquid crystal with a periodic structure, an optical anisotropic diffraction grating is formed and a light polarizing element is realized.

[0010]

FIG. 1 is a schematic view of an optical head device according to the present invention. In FIG. 1, 1 is a light source, 2 is a beam splitter, 3 is a condenser lens, 4 is an optical recording medium, 5 is an optical anisotropic diffraction grating, and 6A, 6B and 6C are detectors. It should be noted that only light that is effectively used is shown in this drawing for the sake of clarity.

The light emitted from the light source 1 is emitted from the beam splitter 2
However, a certain proportion of the light is transmitted as it is, is condensed by the condenser lens 3 and reaches the optical recording medium 4. The light reflected by the optical recording medium 4 has its optical path changed at a certain ratio by the beam splitter 2 and enters the optical anisotropic diffraction grating 5. In this optical anisotropic diffraction grating 5, a diffraction grating is formed so that the light is diffracted in the polarization direction of the incident light, and the light is divided and detected by the detection units 6A, 6B, 6C.

The beam splitter 2 is a combination of prisms as shown in the figure. A certain proportion of the light from the light source goes straight, and a certain proportion of the reflected light from the optical recording medium has its optical path bent 90 ° and again. The light is bent by 90 °, and the light is detected by the detection unit located at substantially the same position as the light source.

The beam splitter according to the present invention may be arranged so that a certain proportion of the light from the light source is transmitted and a certain proportion of the reflected light from the optical recording medium reaches the detecting portion. For example, the light from the light source may be bent 90 ° toward the optical recording medium, and the reflected light from the optical recording medium may go straight. Also, the bendable angle is not limited to 90 °. The configuration shown in FIG. 1 is advantageous in terms of downsizing the optical head device.

In the present invention, the light emitted from the light source 1 does not pass through the optical anisotropic diffraction grating 5 before reaching the optical recording medium 4. For this reason, the reflected light may exit the beam splitter, pass through the optically anisotropic diffraction grating, and then be redirected by a mirror or prism.

Although three detectors are provided in FIG. 1, the number and positions of the detectors may be such that the number of detectors required by the optical head is arranged at a required position.

In the present invention, the optically anisotropic diffraction grating 5 sandwiches a polymer liquid crystal having optical anisotropy between two transparent substrates, and the orientation direction of the polymer liquid crystal is cyclic. The optical anisotropic diffraction element is changing.

FIG. 2 is a sectional view of the optically anisotropic diffraction element used in the present invention. In FIG. 2, 11A and 11B
Is a transparent substrate such as glass or plastic, 12A, 12B
Is an electrode of In ₂ O ₃ —SnO ₂ (ITO), SnO _2, etc., 13A and 13B are polyimide, polyamide, SiO.
And the like, 14 is a polymer liquid crystal, 15 is its horizontal alignment portion, and 16 is its vertical alignment portion.

In the optically anisotropic diffraction element of the present invention, it is preferable to provide electrodes on two transparent substrates in order to facilitate the manufacture thereof. In particular, it is preferable to provide a periodic electrode on the surface of at least one of the transparent substrates which is in contact with the polymer liquid crystal. Specifically, it is preferable that one electrode is a periodic electrode and the other electrode is a solid electrode, or both are periodic electrodes.

FIG. 3 is a plan view of an example of this periodic electrode. Portions 22 with electrodes and portions 21 without electrodes are alternately arranged periodically. 2 and 3 W
₁ is the width of the electrodes, W ₂ is the width of the portion without electrodes, and W ₃ is the pitch of the electrodes. In the example of FIGS. 2 and 3, the number of electrodes is 2.
The substrates have the same width and both have a stripe-shaped periodic pattern.

A polymer having a periodic alignment structure is obtained by sandwiching an unpolymerized liquid crystal material (liquid crystalline monomer) between two transparent substrates as described above and polymerizing them with an electric field applied to the electrodes. A manufacturing method of forming an optically anisotropic diffraction grating of liquid crystal is preferable. This makes it possible to easily form a polymer liquid crystal in which the alignment state is periodically changed and mass-produce an optically anisotropic diffraction grating with good productivity.

At this time, as the unpolymerized liquid crystal material, when the liquid crystal to be used has a positive dielectric anisotropy, an alignment film that is horizontally aligned is used. Usually, orientation treatment is performed in a specific direction by rubbing or the like. By doing so, the liquid crystal molecules in the portion to which the electric field is applied are aligned parallel to the electric field and perpendicular to the transparent substrate. The part to which no electric field is applied is parallel to the transparent substrate and is aligned along the alignment treatment direction of the alignment film.

FIG. 4 is a schematic diagram showing the alignment state of the liquid crystal under the condition where this electric field is applied. In this example, a nematic liquid crystal having a positive dielectric anisotropy is used, and an electric field of the same polarity is applied to the electrodes 31A, 31B, and 31C on the upper substrate, and similarly, the electrode 32A on the lower substrate. , 32
The electric fields of the same polarity are applied to B and 32C.
Therefore, in the portion 35 where the electrodes face each other, the liquid crystal is vertically aligned. On the other hand, in the portion 36 where the electrodes do not face each other, the liquid crystal is aligned substantially according to the alignment treatment direction of the alignment film.

When a considerably high electric field is applied, the liquid crystal molecules start to rise in the portion 36 where the electrodes do not face each other, but the characteristics are deteriorated in the diffraction grating of the present invention, so that the electrodes normally face each other. In the portion 35 where the liquid crystal is almost vertically aligned, the portion 36 where the electrodes do not face each other
Then, the voltage and its frequency (including direct current or alternating current) may be determined such that the orientation is substantially horizontal and the difference in refractive index between the two portions is large.

When the liquid crystal material to be used has a negative dielectric anisotropy, an alignment film that is vertically aligned is used. Also in this case, the alignment treatment is usually performed in a specific direction by rubbing or the like. By doing so, the portion to which the electric field is not applied is oriented perpendicularly to the transparent substrate, and the portion to which the electric field is applied is oriented parallel to the transparent substrate along the orientation treatment direction.

It is also possible to form a periodic pattern of vertical alignment regions and horizontal alignment regions by a combination of the photolithography method and the rubbing method by utilizing the difference in alignment ability of the alignment film. Also, by alternately applying an electric field,
The electric field distribution can be improved. In this case, the alignment film can be omitted.

FIG. 5 is a schematic view showing the alignment state of the liquid crystal in the state where this electric field is applied. This example also shows an example in which a nematic liquid crystal having a positive dielectric anisotropy is used, and an electric field of opposite polarity is applied to every other electrode 41A, 41B, 41C of the upper substrate, and similarly, the lower substrate. Electrodes 42A, 4
Similarly, an electric field of opposite polarity is applied to every other one of 2B and 42C.

Therefore, in the portion 45 where the electrodes face each other, the liquid crystal is vertically aligned as in the example of FIG. On the other hand, in the portion 46 where the electrodes do not face each other, the liquid crystal is also influenced by the electric field between the adjacent electrodes and is aligned in that direction. Therefore, it is possible to align in a specific direction without providing an alignment film.

According to the above-mentioned method, the liquid crystal material can be solidified while the distribution of the orientation is fixed by fixing the distribution of the orientation and fixing the distribution of the liquid crystal material by polymerizing the whole with heat, ultraviolet rays or the like.

The polymer liquid crystal of the present invention is a polymer produced from such an unpolymerized liquid crystal material (liquid crystalline monomer), as long as the unpolymerized liquid crystal material exhibits liquid crystallinity, and here, it is 0. Any material having a refractive index anisotropy of 02 or more may be used. This refractive index anisotropy is preferably large,
Polymers of 0.1 or higher are preferred. Therefore, the polymer liquid crystal itself does not need to exhibit liquid crystallinity.

The polymer liquid crystal is preferably produced by polymerizing a liquid crystalline monomer by light or heat. In particular, a liquid crystal monomer that can be polymerized by ultraviolet light or visible light is preferable because a polymer liquid crystal can be produced on-site (directly on a substrate) by a photolithography process.

The liquid crystal monomer is a monomer which exhibits liquid crystallinity at room temperature or the temperature during photopolymerization. Liquid crystallinity means that a known liquid crystal phase such as nematic, smectic or cholesteric is shown, but a cholesteric having a short spiral pitch is not suitable for the present invention.

The liquid crystalline monomer is preferably selected from esters such as acrylic acid or methacrylic acid.
It is preferable that one or more phenyl groups, particularly two or three phenyl groups are contained in the alcohol residue forming an ester. Further, the alcohol residue forming an ester may contain one cyclohexyl group. In order to extend the temperature range in which liquid crystal monomers can exist as liquid crystals,
Two or more components can be mixed and used.

In a preferred aspect of the present invention, electrodes are provided on each of the two transparent substrates, and at least one of the two electrodes is formed periodically. With such a configuration, the alignment state of the liquid crystal material when an electric field is applied can be made different between a portion corresponding to the periodically formed divided electrode and a portion where the divided electrode is not formed. The diffraction grating can be easily formed by an electric field.

As in the example of the above figure, both electrodes are periodically formed, and if the two electrodes are symmetrical between the two transparent substrates, this optical anisotropic diffraction grating Both of the ± first-order diffracted lights emitted from can have almost the same diffraction efficiency.

Further, both of the two electrodes may be periodically formed, and the two electrodes may be asymmetrical between the two transparent substrates. Specifically, for example, when viewed in the example of FIG.
A is projected to the electrode 32B side only on the right side of the electrode 32A.
Is made wider than the width of the electrode 31A.

By doing so, in the state where the polymer liquid crystal cell is formed, the center plane located between the two transparent substrates and parallel to the two transparent substrates becomes asymmetric. With such a configuration, each divided electrode of the two electrodes faces each other in a state where their positions and / or sizes are different, and when viewed with a pair of upper and lower divided electrodes, the alignment of the polymer liquid crystal by the divided electrodes The part may be left-right asymmetric.
Accordingly, an optically anisotropic diffraction grating having high diffraction efficiency of any of the ± first-order diffracted lights can be easily formed by an electric field.

In the present invention, as described above, the alignment film is provided on the surface of at least one of the transparent substrates which is in contact with the liquid crystal, and at least one of the alignment films includes alignment films having different alignment forces periodically. It can be done. Even with such a divided alignment film in which the alignment force changes periodically, a distribution can be imparted to the alignment state of the polymer liquid crystal. Furthermore, the alignment state of the polymer liquid crystal in the direction of the period can be left-right asymmetric. Therefore, it is possible to easily form an optically anisotropic diffraction grating having a partially high diffraction efficiency of the ± 1st-order diffracted light.

Further, both alignment films of the two transparent substrates can be divided alignment films. Therefore, in the state where the polymer liquid crystal cell is formed, the alignment state can be asymmetrical with respect to the center plane located at the center between the two transparent substrates and parallel to the two transparent substrates. With such a configuration, when the divided alignment films of the two alignment films are viewed as a pair of upper and lower split alignment films, the alignment portion of the polymer liquid crystal divided alignment films can be left-right asymmetric. Therefore, an optically anisotropic diffraction grating having high diffraction efficiency of any of the ± 1st-order diffracted lights can be easily formed by the alignment film.

Further, in the present invention, a mask having a light-shielding pattern is periodically arranged on the optically anisotropic diffraction element,
While the electric field is applied to the entire surface of the electrodes of the two transparent substrates, the liquid crystal is cured only in the part exposed to the light by light exposure without applying the electric field, and then the remaining part is cured by changing the applied state of the electric field. Thus, it is also possible to form an optically anisotropic diffraction element in which the alignment direction of the polymer liquid crystal is periodically changed.

Further, using such a mask, electrodes are arranged on the outside of the substrate to apply an electric field, or a magnetic field is applied from the outside of the substrate to form a periodic alignment. Good.

The optical head device of the present invention optically functions as follows. Note that the optical anisotropic diffraction grating will be described assuming that an ultraviolet curable unpolymerized nematic liquid crystal having a positive dielectric anisotropy is used to perform ultraviolet curing and configured as shown in FIG.

The light is emitted from the light source and reflected by the optical recording medium,
The optical path is changed by the beam splitter, and the light enters the optically anisotropic diffraction grating. At this time, with respect to S-polarized light (polarized light component perpendicular to the paper surface of FIG. 2), the optically anisotropic diffraction grating has an optically uniform horizontal alignment portion and a vertical alignment portion, that is, both portions are ordinary light refracted. Become a rate. Therefore, the light passes through without any change.

On the other hand, the P-polarized light (polarized light component parallel to the paper surface of FIG. 2) has a different refractive index between the vertically aligned portion and the horizontally aligned portion. That is, the vertical alignment portion has an ordinary refractive index,
The horizontally oriented portion has an extraordinary light refractive index. Therefore, it is recognized as a diffraction grating, and theoretically as + 1st order light is -40%,
40% of diffracted light is obtained as the −1st-order light, and the transmitted light can theoretically be zero. The rest is high-order light.

Therefore, the polarization state of the reflected light from the optical recording medium and its change can be read by, for example, measuring the relative intensity ratio of the transmitted light amount and the ± first-order diffracted light amounts.

In the optically anisotropic diffraction grating of this example, the electrodes have the same width on the two substrates, but the two periodic electrodes provided on the two transparent substrates are arranged at the positions and / or Alternatively, by making the sizes asymmetric, a polymer liquid crystal corresponding to the electrode portion and oriented in a specific direction by the electric field can be formed asymmetrically. Accordingly, an optically anisotropic diffraction grating having high diffraction efficiency for any of the ± 1st-order diffracted lights can be obtained.

The width W ₁ of the electrodes may be set to about half the electrode pitch, but if necessary, 20 to 80%.
May be done to the extent. The electrode pitch W ₃ is 5 to 5
It may be used by appropriately optimizing in the range of about 0 μm.

[0047]

【Example】

Example 1 An ITO solid transparent electrode was formed on one surface of a 120 mm × 120 mm square glass substrate having a thickness of 1 mm,
By photolithography and dry etching, I
The transparent electrode of TO has a width W ₁ = 10 μm and a pitch W ₂ = 20.
Patterning was performed in a stripe shape of μm. An antireflection film was formed on the opposite surface by vapor deposition.

Then, about 60 nm is formed by spin coating.
A polyimide film of a certain degree was formed, and horizontal alignment treatment was performed by rubbing. The two glass substrates thus formed are arranged so that the surfaces on which the transparent electrodes are formed face each other,
The periphery was sealed with a sealing material to form an empty cell having an injection port with a cell gap of 5 μm.

In the empty cell, p- [4-ω-acryloyloxyalkyloxy) phenylcarbonyloxy]
A liquid crystal monomer containing benzonitrile and p- (4-ω-acryloyloxyalkyloxy) benzoic acid-p′-n-alkyloxyphenyl ester as a main component was added with 1 part of benzoin isopropyl ether as a photopolymerization initiator.
% Added composition (acting as a nematic liquid crystal of positive dielectric anisotropy) was injected.

After that, an electric field of 5 V was applied between the electrodes of the two substrates to vertically orient the liquid crystal composition in the portions where the electrodes face each other. After that, the whole is irradiated with ultraviolet rays having a wavelength of 360 nm, and the whole is polymerized and cured while keeping the above alignment, thereby fixing the alignment.

The optical anisotropic diffraction grating thus formed has a refractive index of 1.52 (ordinary refractive index) in the electric field applying section for S-polarized light (light polarized perpendicular to the paper surface of FIG. 2). For the P-polarized light (light polarized in the direction parallel to the paper surface of FIG. 2) with a refractive index of 1.53 (ordinary refractive index) in the applying section, the refractive index of 1.54 (ordinary refractive index) in the electric field applying section A refractive index of 1.66 (extraordinary light refractive index) was obtained at the applying portion, and a refractive index difference of about 0.12 was obtained.

In the optical head device having the configuration of FIG. 1 using this optical anisotropic diffraction grating, when a laser light source having a wavelength of 678 nm is used as a light source, the light transmittance for S-polarized wave incident light is 85%, and the light diffraction The efficiency was 0.5% for both ± 1st order.
The light transmittance with respect to the P-polarized wave incident light was 3.4%, and the light diffraction rates were + 1st order 34% and -1st order 32%.

Example 2 A diffractive element was manufactured in the same manner as in Example 1 except that the structure of the transparent electrode of Example 1 was changed as follows.

The transparent electrode on one glass substrate has a width W ₁ = 1.
The patterning was performed in a stripe shape having a pitch of 0 μm and a pitch W ₂ = 20 μm. The transparent electrode on the other glass substrate has a width W ₁ = 5
The patterning was performed in a stripe shape with a pitch of W ₂ = 20 μm. An optical anisotropic diffraction grating was formed in the same manner as in Example 1 except that an empty cell was formed using such a substrate to form an optical head device. In this optical head device, there was a difference in the efficiency of diffracted light between the + 1st-order diffracted light and the -1st-order diffracted light.

[0055]

According to the present invention, it is possible to easily obtain an optical head device which can be mass-produced and has high light utilization efficiency and high reliability. Further, by appropriately selecting the liquid crystal material, the desired refractive index can be freely set in various ways, and the shape and pitch of the parts having different orientations can be freely selected, so that it is easy to manufacture the optical head device with desired characteristics and arrangement. .

Furthermore, by making the alignment state of the liquid crystal asymmetric between the two transparent substrates, an asymmetric optical anisotropic diffraction grating can be easily formed, and the diffraction efficiency of either ± 1st order diffracted light is high. The diffractive element can be easily manufactured.

The present invention can be applied in various ways within a range that does not impair the effects of the present invention.

[Brief description of drawings]

FIG. 1 is a schematic view of an optical head device according to the present invention.

FIG. 2 is a sectional view of an optically anisotropic diffraction element used in the optical head device of the present invention.

FIG. 3 is a plan view of an example of periodic electrodes of the optically anisotropic diffraction element used in the present invention.

FIG. 4 is a schematic diagram showing an alignment state of liquid crystal in a state in which an electric field is applied to the optically anisotropic diffraction element used in the present invention.

FIG. 5 is a schematic diagram showing an alignment state of liquid crystal in a state in which an electric field is applied to the optically anisotropic diffraction element used in the present invention.

[Explanation of symbols]

 1: Light source 2: Beam splitter 3: Condensing lens 4: Optical recording medium 5: Optical anisotropic diffraction grating 6A, 6B, 6C: Detection unit

Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location G02F 1/1337 G02F 1/1337

Claims (4)

[Claims] 1. An optical head device for reading information and / or writing information by irradiating an optical recording medium with light from a light source, wherein the light from the light source is transmitted between the light source and the optical recording medium. A beam splitter that changes the optical path of the light so that the reflected light from the optical recording medium does not return to the light source is placed, and the reflected light whose optical path is changed is passed through a diffraction grating and detected by dividing it into multiple diffracted lights. In an optical head device for detecting light at a section, a diffractive element sandwiches a polymer liquid crystal having optical anisotropy between two transparent substrates, and the orientation direction of the polymer liquid crystal changes periodically. The optical head device is an optical anisotropic diffractive element. 2. An optical anisotropic diffraction element is provided with electrodes on each of two transparent substrates, and at least one of the two electrodes is formed periodically, and a liquid crystal is applied while applying an electric field to these electrodes. 2. The optical head device according to claim 1, wherein the optically anisotropic diffraction element is formed by hardening the polymer to form a polymer liquid crystal whose orientation direction is periodically changed. 3. The electrodes of at least one of the two transparent substrates of the optically anisotropic diffraction element are patterned, and when an electric field is applied to the electrodes, the liquid crystal is natural in the portions where the electrodes do not face each other. In such an arrangement, the liquid crystal is cured in this state by forcibly aligning the parts where the electrodes face each other, and an optical anisotropic diffractive element in which the alignment direction of the polymer liquid crystal changes periodically is set. The optical head device according to claim 2, wherein the optical head device is formed. 4. A mask on which a light-shielding pattern is periodically formed is arranged on an optically anisotropic diffraction element, and an electric field is applied to the entire surfaces of electrodes of two transparent substrates, or without applying an electric field. An optical anisotropic diffractive element in which the alignment direction of the polymer liquid crystal is periodically changed by curing the liquid crystal only in the portion exposed to the light by light exposure and then changing the applied state of the electric field to cure the remaining portion. The optical head device according to claim 2, wherein the optical head device is formed.