JPH11211905A - Production of diffracting element and optical head device using the same diffracting element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarization type diffracting element with which diffraction efficiency is improved and in-plane distribution is made uniform. SOLUTION: On one of opposed two substrates 2, an oriented processing film 11 is formed and between the opposed substrates 2, a spacer 12 for keeping the interval constant and a liquid crystal to be a high molecular liquid crystal thin film 1, are held while orienting the liquid crystal. After the liquid crystal is hardened into high molecular liquid crystal thin film 1 by light radiation, one substrate is detached, a grating having a rugged end face is formed on the high molecular liquid crystal thin film 1, and the rugged parts are filled with isotropic media. Thus, the monomer of the liquid crystal is satisfactorily stabilized and oriented, further, since a means for providing the desired film thickness is adopted, the produced high molecular liquid crystal film is stabilized in a refraction factor as well and further, the film thickness can be made uniform as well.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing a diffraction element and an optical head device using the diffraction element.

[0002]

2. Description of the Related Art Conventionally, a diffraction element using a polymer liquid crystal has been prepared as follows. That is, an orientation treatment was performed on a transparent substrate surface such as glass, and a liquid crystal was thinly applied on the transparent substrate surface and cured by photopolymerization to obtain a polymer liquid crystal thin film. The polymer liquid crystal thin film was formed with a lattice having an uneven cross section by performing dry etching or the like, and the uneven portion was filled with an isotropic medium (referred to as an optical isotropic medium).
Since the polymer liquid crystal thin film has birefringence, this diffraction element becomes a polarization diffraction element having a different diffraction efficiency depending on the polarization direction of incident light.

If the refractive index (n) of an isotropic medium of a diffraction element
_s) is equal to the ordinary refractive index of the polymer liquid crystal film is a birefringent medium (n _o) or the extraordinary refractive index (n _e), the diffraction element to the polarization of the same refractive index direction does not function.
For example, when the n _s = n _o, the diffraction element is transmitted showed high transmittance does not diffract the polarization of the ordinary refractive index direction. On the other hand, the diffraction element functions for polarized light in the extraordinary light refractive index direction, and high diffraction efficiency can be obtained.

When this polarization type diffraction element is used in an optical head device, a quarter-wave plate is inserted between the diffraction element and an optical disk as an optical recording medium, and a straight line passes through the diffraction element. The reciprocation efficiency can be increased by rotating the direction of polarized light by 90 degrees between the forward path and the return path.

However, when a liquid crystal, which is a monomer state before polymerization, is applied to a substrate that has been subjected to an alignment treatment when forming a thin film of the polymer liquid crystal, it is difficult to stabilize the obtained alignment state. If the orientation state is not stable, a substantial birefringence state after polymerization changes, the refractive index is not stabilized, a desired diffraction efficiency cannot be obtained, and a diffraction element cannot be obtained with a high yield. In addition, it is difficult to control the thickness of the polymer liquid crystal, and if the thickness varies, the diffraction efficiency varies, leading to a decrease in the manufacturing yield of the diffraction element.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a method for manufacturing a diffraction element using a polymer liquid crystal thin film and an optical head device using this diffraction element. It is a new offer.

[0007]

SUMMARY OF THE INVENTION The present invention relates to a method for manufacturing a diffraction element in which a lattice having an uneven cross section is formed in a polymer liquid crystal thin film, and the uneven lattice portion is filled with an isotropic medium. At least one of the two substrates is subjected to an alignment treatment, and a spacer and a liquid crystal to be a polymer liquid crystal thin film are sandwiched between the opposed substrates, and the liquid crystal is aligned and cured to form a polymer liquid crystal thin film. And then removing at least one substrate to form a lattice having an uneven cross section on the polymer liquid crystal thin film, and filling the uneven lattice portion with an isotropic medium. I will provide a.

Further, the present invention provides the above-described method for producing a diffraction grating, wherein the two substrates facing each other are subjected to the alignment treatment, and then the mold release treatment is performed on the facing surface of at least one of the substrates. Further, an optical head device that guides light emitted from a semiconductor laser to an optical recording medium, diffracts reflected light from the optical recording medium with a diffraction element, and detects the light with a photodetector,
An optical head device using a diffraction element manufactured by the above manufacturing method as a diffraction element is provided.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, when an alignment process is performed on the opposing surfaces of two opposing substrates, that is, the surface in contact with the liquid crystal, it may be applied to one substrate or to two substrates. However, it is preferable to apply it to two substrates because the molecular alignment of the liquid crystal can be easily controlled. Typical examples of the alignment treatment used here include a rubbing treatment on a substrate surface or a polyimide film adhered to the surface thereof, a polymer film of a polyamide film or an inorganic film such as SiO ₂ , or an oblique vapor deposition treatment of SiO. is there.

The liquid crystal for forming the polymer liquid crystal thin film used in the present invention is a composition such as a monomer, an oligomer or other reactive compound exhibiting liquid crystallinity. Anything can be used. In the following description, the monomer of the liquid crystal will be described as being representative thereof.

A spacer is arranged between the two opposing substrates, the substrates are opposing each other at a predetermined interval, a liquid crystal monomer is injected into the gap, and the liquid crystal monomer is polymerized by a curing means. Cures to a polymer liquid crystal thin film.

Means for curing the liquid crystal monomer include:
There are methods such as irradiation with light such as visible light and UV (ultraviolet) light, and a method by heating. In particular, a curing method of irradiating light such as visible light and UV light is preferable because it can be performed directly on the substrate. Therefore, description will be made here assuming that the monomer of the liquid crystal is polymerized and cured by light irradiation. In order to clarify that the liquid crystal is not polymerized because it is not polymerized,
Instead of "monomer of liquid crystal".

FIG. 1 shows an example of a process for producing a polymer liquid crystal thin film.
Shown in FIG. 1 (a) shows a case in which two
FIG. 1B is a side view showing two substrates, and FIG.
FIG. 1C is a side view showing a state in which a liquid crystal monomer is injected between two substrates and irradiated with light such as UV light to convert the liquid crystal monomer into a polymer liquid crystal thin film. FIG. It is a side view which shows a mode that one board | substrate was removed. In the figure,
1 is a polymer liquid crystal thin film, 2 is a substrate, 11 is an alignment treatment film, 1
2 is a spacer.

As a method of injecting the monomer of the liquid crystal, a vacuum injection method may be employed, or the method may be carried out at atmospheric pressure by a method utilizing a capillary phenomenon. In this case, prior to this, a spacer is sprayed on the first substrate in advance, and then
The second substrate may be stacked. After the mixture of the spacer and the liquid crystal monomer is dropped on the first substrate, the second substrate may be laminated and pressed.

Further, after the spacers are scattered on the first substrate, the monomer of the liquid crystal may be dropped, and the second substrate may be laminated and pressed to reduce the thickness. As the substrate, a transparent glass plate, a plastic plate, or the like can be used, but a glass plate is preferable because of its excellent hardness and durability.

Means for keeping the distance constant between the opposing substrates and a liquid crystal monomer to be a polymer liquid crystal thin film are sandwiched. As the monomer of the liquid crystal used, it is preferable to select from esters such as acrylic acid or methacrylic acid. Among them, an acrylic acid-based material which is a material (side-chain high-molecular liquid crystal) which adds a reactive group to a monomer of a liquid crystal and polymerizes to form a polymer is preferable. In addition to the high birefringence of the material itself, this polymer liquid crystal has the excellent characteristic that the monomer of this liquid crystal reacts sensitively to the alignment treatment of the substrate and easily increases the birefringence after polymerization. Have.

If the thickness of the polymer liquid crystal thin film has a distribution, the diffraction efficiency varies as described above, which is not preferable. Therefore, a spacer is used as a means for keeping the distance between the opposing substrates constant. As this spacer,
Spherical, granular, or fibrous spacers made of inorganic or plastic materials such as glass, alumina, and silica having the rigidity and durability can perform their functions, but glass spacers are particularly preferred because they have excellent rigidity and durability. Usually, the thickness of the polymer liquid crystal thin film is 1 to 5 μm.

As this glass, a spherical SiO ₂ glass is preferable, and the number thereof is 1 cm when the diameter is 5 μm or less.
^{Two to two} hundred thousand are good. If the number is less than 10, it is difficult to keep the distance between the substrates constant. If the number is more than 200,000, light scattering is caused, and stray light may not be generated when the optical head device is mounted. Preferably, the number is from 1,000 to 10,000, and in this case, the above problem can be effectively avoided.

For light irradiation performed after aligning the liquid crystal monomer, visible light or UV light is used as described above, but UV light is preferable for efficient curing. By performing light irradiation in this manner, the monomer of the liquid crystal can be cured while maintaining the alignment state.

After curing, at least one of the two opposing substrates is removed (FIG. 1C). In this case, in order to prevent the polymer liquid crystal thin film from adhering to the substrate to be removed, it is preferable to perform a release treatment on the inner surface of the substrate to be removed, that is, the surface in contact with the monomer of the liquid crystal. When the alignment treatment is performed on the inner surface of one of the two substrates opposed to each other, the release treatment is performed on the inner surface of the substrate that has not been subjected to the alignment treatment. It is preferable in increasing the value. As the release agent used for the release treatment, a fluorosilane type, a fluorinated polymer having a fluorinated aliphatic ring structure, or the like can be used.

The polymer liquid crystal thin film thus manufactured is
A grating having a concave-convex section is formed, and the concave-convex portion is filled with an isotropic medium to obtain a diffraction element. That is, a lattice having an uneven cross section is formed on the polymer liquid crystal thin film by an etching method using photolithography or a pressing method using a mold having a lattice shape. Uneven grating portion the cross corrugated grating is formed, the refractive index is filled with equal isotropic medium to the ordinary refractive index of the polymer liquid crystal film (n _o) or the extraordinary refractive index (n _e) To cure. As the isotropic medium, for example, a photopolymerization type acrylic resin or epoxy resin can be used.

By selecting the refractive index in this manner, the diffraction element may or may not function as a diffraction grating depending on the polarization direction of light passing through the diffraction element. That is, the incident if the refractive index is filled into the concave-convex portion equal isotropic medium to the ordinary refractive index of the polymer liquid crystal film (n _o), the light linearly polarized in a direction giving the ordinary refractive index of the diffraction element in the diffraction element In this case, there is no difference in refractive index between the polymer liquid crystal thin film and the isotropic medium, and no diffraction effect occurs.

However, when light that is linearly polarized in a direction that gives an extraordinary refractive index at an angle of 90 degrees to this direction is incident on the diffraction element, the refractive index difference between the polymer liquid crystal thin film and the isotropic medium is increased. There is a diffraction effect. Extraordinary light refractive index ( _ne )
The same applies to the case where the uneven portion is filled with an isotropic medium equal to.

[0024] There is also a method of filling the isotropic medium polymer ordinary refractive index of the liquid crystal film (n _o) also extraordinary refractive index (n _e) in the not equal refractive index to the uneven portion. In this case, when the optical disc as the optical recording medium has birefringence, there is an effect that the intensity of the signal light reflected by the optical disc is not reduced by combining the optical disc with the diffraction element shown in FIG.

The diffraction element manufactured by the manufacturing method of the present invention is
The polymer molecules have good orientation in one direction of the molecular axis, so the refractive index is uniform and high in the diffraction element, and the uniform thickness of the polymer liquid crystal film provides high and uniform diffraction efficiency. It has excellent characteristics that it can be used.

Further, as shown in FIG. 2, which is a side view showing an example of the diffraction element according to the present invention, it is also possible to form the diffraction element without completely removing the concave portion of the polymer liquid crystal thin film 1 on the substrate 2 while leaving it on the substrate 2. it can. In this case, when light passes through the remaining polymer liquid crystal thin film, the polarization direction may change depending on the incident angle or the like. Therefore, as shown in FIG. 4 which is a side view showing another example of the diffraction element according to the present invention, the polymer liquid crystal thin film 1 in the concave portion is completely removed, that is, the polymer liquid crystal thin film 1
To expose the surface of the substrate 2. 2 and 4, reference numeral 3 denotes an isotropic medium.

In FIG. 5, which is a side view showing another example of the diffraction element according to the present invention, a quarter-wave plate 10 is provided integrally with the diffraction element. 1, 2 and 3 are the same as above.

Further, the diffraction element manufactured by the manufacturing method of the present invention is mounted on an optical head device. That is, the present invention is used as a diffractive element of an optical head device for guiding outgoing light from a semiconductor laser to an optical recording medium, diffracting reflected light from the optical recording medium with a diffractive element, and detecting this diffracted light with a photodetector. A diffraction element manufactured by the manufacturing method is used.

FIG. 3 is a side view schematically showing the case where the diffraction element of the present invention is used as a hologram beam splitter in an optical head device. Light emitted from a semiconductor laser 5 as a light source passes through a diffraction element 4 as a hologram beam splitter, and is condensed on an optical disk 8 by an objective lens 7, and reflected light from the optical disk 8 passes through the objective lens 7 again. The light is diffracted by the hologram beam splitter and reaches the light receiving element 6.

By inserting the quarter-wave plate 10 between the hologram beam splitter and the optical disk 8, the polarization direction of the linearly polarized light emitted from the semiconductor laser 5 can be rotated by 90 degrees in the forward path and the return path. Accordingly, the transmittance is high for the light in the forward polarization direction, and the diffraction efficiency is high for the light in the return polarization direction, so that the light use efficiency can be increased.

[0031]

Embodiments of the present invention will be described below with reference to the drawings. FIGS. 1A, 1B and 1C show side views of each process of the method for producing a polymer liquid crystal thin film.

First, as shown in FIG. 1A, a polyimide thin film is formed on the inner surfaces of two glass substrates 2 having a diameter of 3 inches and opposed to each other. A rubbing orientation treatment was performed in one direction to obtain an orientation treatment film 11. A fluorosilane-based release agent was applied (not shown) on the alignment treatment film 11 of one of the two glass substrates 2 facing each other.

The other substrate was made of SiO ₂ having a diameter of 2.0 μm.
After scattering spherical spacers 12 made of 6000 at a density of 6000 pieces / cm ² , the two substrates were bonded together such that the distance between the inner surfaces of the two substrates became 2 μm. Next, as shown in FIG.
An acrylic acid-based liquid crystal monomer was injected into the gap between the two substrates by a vacuum injection method.

In this state, when the alignment state of the liquid crystal was observed, it was observed that the molecular axis of the monomer of the liquid crystal was aligned in the rubbing direction, and it was confirmed that the liquid crystal was in a good alignment state. Next, UV light of 600 mJ was passed through a glass substrate.
The liquid crystal monomer was photopolymerized and cured by irradiation with light to form a polymer liquid crystal thin film 1. The alignment state of the polymer liquid crystal was uniform and excellent.

Further, as shown in FIG. 1C, the substrate subjected to the release treatment was removed. As a result, the polymer liquid crystal of the substrate that had not been subjected to the release treatment remained on the substrate without being peeled off, and the polymer liquid crystal thin film 1 could be formed on the substrate side.
The orientation was as good as before the substrate was removed. The thickness of the polymer liquid crystal thin film 1 was about 2 μm, and a uniform thickness as designed was obtained.

Thereafter, a lattice having an uneven cross section (FIG. 4) having a longitudinal direction parallel to the rubbing direction is formed on the polymer liquid crystal thin film 1 as a birefringent material on the substrate by an etching method using photolithography. It was filled with an acrylic isotropic medium 3 and cured.

At this time, the refractive index of the isotropic medium 3 was equal to the ordinary light refractive index of the polymer liquid crystal thin film 1. Then, this substrate was cut into an outer dimension of 4 mm × 4 mm to obtain a hologram beam splitter as a polarizing diffractive element.

The hologram beam splitter thus manufactured was incorporated as the diffraction element 4 of the optical head device shown in FIG. The hologram beam splitter mounted on this optical head device shows a high transmittance of 95% for linearly polarized light emitted from the semiconductor laser 5 which is light on the outward path, and reflects the light on the optical disk 8 which is light on the return path to generate 1%. / 4 wavelength plate 10
And a high ± 1st-order diffraction efficiency of 36% was shown for the outgoing light and polarized light whose polarization direction was rotated by 90 degrees.

Therefore, 0.95 × 0.36 = 0.3
4, that is, a high light use efficiency of 34% was obtained, and the diffraction efficiency of this hologram beam splitter was uniform in the plane.

[0040]

According to the manufacturing method of the present invention, the monomer of the liquid crystal is oriented in a stable and stable manner, and a means for making the film thickness desired is adopted. The thin film has a stable and high refractive index and can have a uniform thickness. Therefore, the diffraction element manufactured by the manufacturing method of the present invention in which an isotropic medium is filled in a lattice having an uneven cross section manufactured using this polymer liquid crystal thin film has a high diffraction efficiency and a uniform in-plane diffraction efficiency. It has the following features.

Further, according to the present invention, a polymer liquid crystal thin film having a uniform thickness can be easily obtained, so that the production yield of the diffraction element can be improved. Further, by incorporating the diffraction element manufactured by the manufacturing method of the present invention into an optical head device, the optical head device becomes a device having high light use efficiency and stable diffraction efficiency.

[Brief description of the drawings]

FIG. 1 is a side view showing an example of a process for producing a polymer liquid crystal thin film according to the present invention. (A) A side view showing a substrate facing at a predetermined interval. (B) A side view showing a state in which a liquid crystal monomer is injected between opposing substrates to make a polymer liquid crystal thin film.
(C) A side view showing a state in which one substrate is removed from an opposing substrate.

FIG. 2 is a side view showing an example of the diffraction element according to the present invention.

FIG. 3 is a side view schematically showing a head device according to the present invention.

FIG. 4 is a side view showing another example of the diffraction element according to the present invention.

FIG. 5 is a side view showing another example of the diffraction element according to the present invention.

[Explanation of symbols]

 1: Polymer liquid crystal thin film 2: Substrate 3: Isotropic medium 4: Diffractive element (hologram beam splitter) 5: Semiconductor laser 6: Light receiving element 7: Objective lens 8: Optical disk 10: 1/4 wavelength plate 11: Alignment processing Membrane 12: Spacer

Claims (3)

[Claims] In a method for manufacturing a diffraction element, a lattice having an uneven cross section is formed in a thin film of a polymer liquid crystal and an isotropic medium is filled in the uneven lattice portion, at least one of two opposing substrates is provided. After subjecting the facing surface to an orientation treatment, a spacer and a liquid crystal to be a polymer liquid crystal thin film are sandwiched between the opposed substrates, and the liquid crystal is oriented and cured to form a polymer liquid crystal thin film. A method for manufacturing a diffraction element, comprising: removing the polymer liquid crystal thin film to form a lattice having an uneven cross section, and filling the uneven lattice portion with an isotropic medium. 2. The method of manufacturing a diffraction grating according to claim 1, wherein after performing the alignment process on the two opposing substrates, a mold release process is performed on a facing surface of at least one of the substrates. 3. An optical head device for guiding light emitted from a semiconductor laser to an optical recording medium, diffracting reflected light from the optical recording medium by a diffraction element, and detecting the light with a photodetector, wherein the diffraction element is used. Or an optical head device using a diffraction element manufactured by the manufacturing method according to 2.