JPH09274188A - Optical modulation element and optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the utilization efficiency of light and to prevent the degradation in orientation characteristics by rubbing the oriented films on the surfaces of the grating-like rugged parts on transparent substrates in the stripe direction of gratings. SOLUTION: The SiON film on the surface of the first glass substrate 1 is etched, by which the grid-shaped rugged parts 2 having a rectangular section are formed. A polyimide film 4 is formed by a spin coating method on the surface of the second glass substrate 3 and is subjected to a rubbing treatment for the purpose of orientation. On the other hand, a polyimide film 5 is similarly formed on the surface of the glass substrate 1 formed with the rugged parts 2 and is subjected to the rubbing treatment in the direction parallel with the strip direction of the rugged parts 2 in such a manner that the rubbing direction of the polyimide film 5 and the stripe direction of the rugged parts 2 are made the same. The glass substrates 1, 3 are disposed to face each other and the circumferences of the respective glass substrates 1, 3 are sealed with an epoxy resin 6. Liquid crystals 7 are vacuum injected between the substrates from a liquid crystal injection port to form the optically anisotropic diffraction gratings. Further, a λ/4 film 8, a photopolymer and a third substrate 9 are laminated and adhered to the glass substrate 3, by which the optical modulation element is obtd.

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 writing and reading optical information on an optical disk such as a CD (compact disk), a CD-ROM, a video disk and a magneto-optical disk. And an optical modulator comprising an optically anisotropic diffraction grating used therein.

[0002]

2. Description of the Related Art Conventionally, as an optical head device for writing optical information on an optical disk or a magneto-optical disk or reading optical information, a signal light reflected from a recording surface of the disk is guided to a detection unit (beam). A prism type beam splitter as an optical component for splitting;
It is known that a diffraction grating or a hologram element is used.

Conventionally, in a diffraction grating or hologram element for an optical head device, a rectangular grating (relief type) having a rectangular cross section is formed on a glass or plastic substrate by a dry etching method or an injection molding method. Was diffracted to give a beam splitting function.

Further, in order to improve the light utilization efficiency of an isotropic diffraction grating having a light utilization efficiency of about 10%, it is conceivable to use polarized light. In order to use polarized light, a prism type beam splitter is combined with a λ / 4 plate, and a forward direction (direction from the light source side to the optical disc side) and a backward direction (direction from the optical disc side to the light source side and the detector side) are used. There was a way to increase efficiency and round trip efficiency.

However, the prism type polarizing beam splitter is expensive, and other methods have been sought. As one method, a method of using a flat plate of birefringent crystal such as LiNbO ₃ and forming an anisotropic diffraction grating on the surface to have polarization selectivity is known. However, the birefringent crystal itself is expensive, and application to the consumer field is difficult.

As described above, the isotropic diffraction grating has a utilization efficiency of about 50% in the forward direction and a utilization efficiency of about 20% in the backward direction.

In order to achieve high light utilization efficiency,
There is known a light modulation element having an optically anisotropic diffraction grating that utilizes the optical anisotropy of liquid crystal by forming unevenness for a diffraction grating on a transparent substrate such as a glass substrate and filling the unevenness with liquid crystal. ing. In this case, if the glass substrate is directly dry-etched to form the irregularities, the etching speed will be slow and unnecessary deposits will be generated, so that a certain depth can be reproducibly reproduced and the surface distribution of the depth can be reduced. It is difficult to form.

Therefore, normally, a transparent thin film such as SiO ₂ or SiON is formed by a vapor deposition method, a plasma CVD method, a reactive dry etching method, or the like to a desired depth to form a glass substrate and a SiO ₂ film or the like. Dry etching is performed with good reproducibility and small surface distribution by using the difference in etching rate. An optically anisotropic diffraction grating is formed by filling the concave and convex portion thus formed with liquid crystal.

[0009]

However, if the liquid crystal is directly filled, the following problems occur. That is, regarding the alignment characteristics of the liquid crystal, the liquid crystal is aligned along the stripe direction of the concave-convex portion (lattice), but the alignment characteristic is often deteriorated due to the surface texture or cross-sectional shape of the concave-convex portion (lattice), and the yield is reduced.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical head device and an optical modulation element which solve the above-mentioned problems, enhance the utilization efficiency of light, can be manufactured at low cost, and prevent the deterioration of the alignment characteristics of liquid crystal. .

[0011]

SUMMARY OF THE INVENTION The present invention provides an optically anisotropic diffraction grating in which a grid-shaped uneven portion is formed on the surface of a transparent substrate, and the concave portion is filled with liquid crystal having optical anisotropy. In the provided light modulation element, there is provided a light modulation element characterized in that a liquid crystal alignment film is provided on the surface of the uneven portion.

Further, the present invention provides an optical head device characterized in that optical information is written and / or read by irradiating an optical recording medium with light from a light source through the light modulation element. .

[0013]

A preferred embodiment of the present invention is characterized in that the liquid crystal alignment film is made of polyimide. Polyimide can easily impart the property of aligning liquid crystals by rubbing.

Another preferred embodiment of the present invention is characterized in that the thickness of the liquid crystal alignment film is 1/3 or less of the light wavelength. By setting the wavelength to 1/3 or less of the light wavelength, it is possible to suppress the reflection of light due to, for example, the refractive index (1.65) of the polyimide alignment film and the refractive index of the glass substrate (about 1.52). ,preferable.

Yet another preferred embodiment of the present invention is characterized in that the liquid crystal alignment film is rubbed. By rubbing, the alignment characteristics of the liquid crystal can be improved.

In still another preferred embodiment of the present invention, the rubbing direction of the liquid crystal alignment film is substantially parallel to the stripe direction of the lattice. With such a configuration, the alignment force due to the stripe of the lattice and the alignment force due to the rubbing are combined, and the alignment characteristic of the liquid crystal can be further enhanced.

The transparent substrate is made of a glass substrate, a transparent plastic substrate or the like, and a grid-like uneven portion having a depth of 1 to 2 μm and a pitch of 2 to 20 μm is formed on the transparent substrate by a photolithography method or a dry etching method.

An alignment film made of, for example, polyimide is provided on the surface of the uneven portion, and by rubbing it along the stripe direction of the uneven portion, the alignment film is aligned in addition to the alignment force by the uneven portion, and the alignment force is increased. The device can be improved, and the yield can be improved.

In this case, the thickness of the fibers of the rubbing cloth is usually much larger than the groove width of the grid-shaped uneven portion, and the bottom of the groove cannot be rubbed, but the convex portions of the grid-shaped rectangular uneven portion are not rubbed. The flat portion on the upper surface and the edge portion of the convex portion are rubbed, and thereby orientation is performed. Since the orientation due to this rubbing is added to the original orientation force due to the lattice,
As a whole, the orientation of the liquid crystal is significantly improved.

[0020]

FIG. 1 shows an embodiment of the present invention. 0.5m thickness
m, 10 × 10 mm square, p-CVD method (plasma CVD method) on the surface of the first glass substrate 1 having a refractive index of 1.52.
Thereby forming a SiON film of 1.4 μm. After that, SiON is formed by photolithography and dry etching.
The film was etched to form a grid-like uneven portion 2 having a rectangular cross section with a depth of 1.4 μm and a pitch of 5 μm.

Specifically, the SiON film is coated with a photoresist by spin coating. Next, a photomask having a predetermined pattern is brought into close contact with the photoresist film, exposed to ultraviolet rays, and subjected to photoresist development processing to form a lattice pattern of the photoresist on the surface of the first glass substrate 1. Using the lattice pattern of the photoresist as a mask, it was formed by dry etching using C ₂ F ₆ gas.

A polyimide film 4 is formed on the surface of the second glass substrate 3 as an alignment film for liquid crystal alignment by spin coating.
nm, and rubbing treatment for alignment was performed.

On the other hand, the polyimide film 5 is formed on the surface of the first glass substrate 1 on which the uneven portion 2 is formed by the same spin coating method.
With a thickness of 60 nm. The polyimide film 5 was rubbed in a direction parallel to the stripe direction of the uneven portion 2.

The surface of the first glass substrate 1 on which the concavo-convex portion 2 is formed faces the surface of the second glass substrate 3 on which the polyimide film 4 is formed, and the rubbing direction of the polyimide film 5 and the concavo-convex portion 2 Two glass substrates were laminated so that the stripe directions (rubbing directions) were the same, and the periphery of the two glass substrates was sealed with epoxy resin 6 except for the liquid crystal injection port. From the liquid crystal inlet, liquid crystal 7 (product name BL002 manufactured by Merck & Co., nematic liquid crystal, ordinary light refractive index = 1.52
5, extraordinary light refractive index = 1.771) was vacuum-injected to form an optically anisotropic diffraction grating. At this time, Δn (difference between ordinary refractive index of liquid crystal and extraordinary refractive index) = 0.246.

A λ / 4 film 8 is laminated and adhered to the surface of the second glass substrate 3 opposite to the polyimide film 4 by a transparent adhesive, and a photopolymer (not shown) for improving wavefront aberration is further formed thereon. ) And the third glass substrate 9 were laminated and adhered to each other to produce a light modulation element. An antireflection film made of a dielectric multilayer film was applied to the light incident part and the light emitting part of the light modulation element.

When a semiconductor laser (provided below the light modulation element in FIG. 1 but not shown) is used as a light source and a P wave having a wavelength of 678 nm (polarization component parallel to the paper surface) is incident, the P wave is generated. Was about 92%. Further, the reflected light (circularly polarized light) from the optical disk (which is provided above the light modulation element in FIG. 1, but not shown) is the λ / 4 film 8.
Change to S wave (polarization component perpendicular to the paper surface), and the S wave is diffracted by the optically anisotropic diffraction grating. The diffraction efficiency of + 1st order diffracted light is 33%, and the diffraction efficiency of -1st order diffracted light is 33%.
Met.

As a result, the forward efficiency was about 92% and the round trip efficiency was about 61% (± first-order diffracted light detection).

[0028]

As described above, according to the present invention, the alignment film made of, for example, polyimide is provided on the surface of the grid-shaped irregularities provided on the transparent substrate, and this is rubbed in the grid stripe direction. Since the alignment force of the alignment film is added to the original alignment force in the stripe direction due to the grid-like irregularities, deterioration of the alignment characteristics due to the influence of the surface properties and cross-sectional shape of the grid is prevented, and the yield is improved and stable and strong. The alignment force is obtained, and the alignment characteristics of the liquid crystal are improved.

[Brief description of drawings]

FIG. 1 is a side sectional view of an optical modulator according to an embodiment of the present invention.

[Explanation of Codes] 1: First glass substrate 2: Concavo-convex portion 3: Second glass substrate 4: Polyimide film 5: Polyimide film 6: Epoxy resin 7: Liquid crystal 8: λ / 4 film 9: Third glass substrate

Claims (6)

[Claims] 1. A light modulation element comprising an optically anisotropic diffraction grating in which a grid-shaped uneven portion is formed on the surface of a transparent substrate and the concave portion of the unevenness is filled with a liquid crystal having optical anisotropy. A light modulation element characterized in that a liquid crystal alignment film is provided on the surface of the uneven portion. 2. The light modulation element according to claim 1, wherein the liquid crystal alignment film is made of polyimide. 3. The light modulation element according to claim 1, wherein the liquid crystal alignment film has a thickness of 1/3 or less of a light wavelength. 4. The light modulation element according to claim 1, 2 or 3, wherein the liquid crystal alignment film is rubbed. 5. The rubbing direction of the liquid crystal alignment film is substantially parallel to the stripe direction of the lattice.
The light modulation element according to any one of the preceding claims. 6. A light which writes and / or reads optical information by irradiating an optical recording medium with light from a light source through the light modulation element according to any one of claims 1 to 5. Head device.