JPH09185837A - Optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make industrial production possible even with incident light of S waves and to provide a device having high light utilization efficiency. SOLUTION: A transparent thin film consisting of SiOx (x≈1.2) having as refractive index of 1.8 is formed on a first glass substrate 1 having a refractive index of 1.52. Projecting parts of 10μm are formed by a photolithography method and a dry etching method thereon, by which rectangular grid-like rugged parts 2 are formed. Liquid crystals 3 (nematic liquid crystal, ordinary refractive inde x=1.5266, extraordinary refractive inde x=1.8181) are implanted into these rugged parts 2.

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. .

[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 has been known to use a diffraction grating or a hologram element.

Conventionally, a diffraction grating or a hologram element for an optical head device has a rectangular grating (relief type) having a rectangular cross section formed on a glass or plastic substrate by dry etching or injection molding. To provide a beam splitting function.

In order to increase the light use efficiency over an isotropic diffraction grating having a light use efficiency of about 10%, it is conceivable to use polarized light. When trying to use polarized light, a prism type beam splitter is combined with a λ / 4 plate to increase the efficiency of the forward path (direction from the light source to the recording surface) and the return path (direction from the recording surface to the detection unit), thereby improving the reciprocating efficiency. There was a way to raise it.

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 deflection selectivity is known. However, the birefringent crystal itself is expensive, and application to the consumer field is difficult. When a lattice is to be formed on LiNbO ₃ by the proton exchange method, it is difficult to form a fine-pitch lattice because protons in the proton exchange solution are easily diffused into the substrate.

As described above, the isotropic diffraction grating has a utilization efficiency of about 50% in the forward path and about 20% in the return path, so that the reciprocation limit is about 10%.

On the other hand, a grating-shaped uneven portion is formed on a transparent substrate, and a liquid crystal is filled therein to form an optically anisotropic diffraction grating. Further, a retardation film is laminated to improve light utilization efficiency. The present applicant has proposed an optical head device using a high hologram (diffraction element) (Japanese Patent Application No. 7-2
59961).

In this case, the optically anisotropic diffraction grating is formed using a transparent substrate made of glass, plastic or the like, which is usually industrially easy to manufacture and inexpensive, and the refractive index of the transparent substrate is 1. It is about 0.5. Since the ordinary refractive index of the liquid crystal is also about 1.5, it was possible to easily realize a structure in which the refractive index of the transparent substrate and the ordinary refractive index of the liquid crystal were almost the same. Although a special optical glass or the like can be used as a transparent substrate having an extraordinary light refractive index (about 1.8) of liquid crystal, it is expensive and not suitable for mass production.

An optically anisotropic diffraction grating is formed by a structure in which the refractive index of a transparent substrate having a grid-like concavo-convex portion and the ordinary refractive index of liquid crystal are substantially equal to each other, and a retardation film such as a λ / 4 plate is combined therewith. When a diffractive element is manufactured, in order to increase the transmittance on the outward path and the diffractive efficiency on the return path, the polarization direction of the light incident on the diffractive element from the light source is orthogonal to the stripe direction of the uneven portion (P wave). ) Must be.

This is because the liquid crystal is usually oriented along the stripe direction of the concavo-convex portion, the ordinary refractive index of the liquid crystal corresponds to the polarized light orthogonal to the stripe direction, and the polarized light parallel to the stripe direction corresponds to the ordinary light refractive index of the liquid crystal. The refractive index of extraordinary light of the liquid crystal corresponds. Therefore, for P waves, the ordinary index of refraction of the liquid crystal and the index of refraction of the transparent substrate are almost the same, so that
Light is transmitted. Since the refractive index of extraordinary light of the liquid crystal and the refractive index of the transparent substrate are different for S waves, the light is diffracted.

The P-wave or S-wave incident from the light source is transmitted through the optical anisotropic diffraction grating, converted into circularly polarized light by the phase difference film, and passed through the aspherical lens (objective lens) to be condensed on the optical recording medium. To be done. The light reflected by the recording surface of the optical recording medium becomes circularly polarized light in the reverse direction, passes through the aspherical lens again, passes through the retardation film, is converted into S waves or P waves, and passes through the optical anisotropic diffraction grating. To do. Therefore, when the incident light is the P wave, the return light is the S wave and is diffracted.
In the case of a wave, the return light is a P wave and is transmitted without being diffracted.

From the above, it was necessary to make the incident light a P wave in order to increase the transmittance of the forward path and the diffraction efficiency of the return path. On the other hand, the polarization direction of the light of the semiconductor laser of the light source has a certain direction with respect to the PN junction surface of the semiconductor, and the diffraction direction of the return light is defined by the stripe direction of the diffraction grating. Therefore, when the incident light is limited to the P wave, there is a problem that the arrangement of the semiconductor laser and the detector is greatly restricted and the degree of freedom in design is limited.

[0013]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems and can industrially produce high light even for incident light having polarized light parallel to the stripe direction of the optically anisotropic diffraction grating. An object is to provide an optical head device having utilization efficiency.

[0014]

The present invention provides an optical head device for writing information and / or reading information by irradiating an optical recording medium with light from a light source through a diffraction element, wherein the diffraction element is And an optical anisotropic diffraction grating in which a liquid crystal having optical anisotropy is filled between a first transparent substrate and a second transparent substrate on the surface of which a grid-like uneven portion is formed, The convex portion of the concave and convex portion is formed by coating the convex portion made of a transparent material on the first transparent substrate at a predetermined cycle, and the refractive index of the first transparent substrate is equal to the ordinary refractive index of liquid crystal. Provided is an optical head device, which is substantially equal to each other and the refractive index of the convex portion is substantially equal to the refractive index of extraordinary light of the liquid crystal.

Further, an optical head device in which the refractive index of the convex portion is higher than that of the first transparent substrate, and the convex portion is formed of SiO _x N _y (0 ≦ x <2, 0 ≦ y <1. 3) An optical head device comprising 3), an optical head device in which the convex portion is composed of a photosensitive organic material, and an optical head device in which the photosensitive organic material is horizontally aligned.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the convex portion having a refractive index larger than that of a first glass substrate (first transparent substrate) having a refractive index of about 1.5 is used. Specifically, Si
O _x N _y (0 ≦ x <2, 0 ≦ y <1.3, refractive index about 1.
5 to 1.9), MgO (refractive index 1.72), PbF ₂
(Refractive index 1.75), Y ₂ O ₃ (refractive index 1.87), and a mixture of Al ₂ O ₃ and ZrO ₂ (refractive index 1.63 to
It is formed using any transparent material selected from 2.05) or a photosensitive organic material such as photosensitive polyimide (refractive index 1.78).

Of the transparent materials, the SiO _x N _y
(0 ≦ x <2, 0 ≦ y <1.3) (For this, SiO _x
(Including 1 ≦ x <2) is preferable because fine processing can be easily performed by a dry etching method.

When the refractive index of the convex portion is about 1.7 to 1.9, it becomes almost equal to the extraordinary light refractive index of the liquid crystal (about 1.8), which effectively functions as a diffraction grating for S waves.
More preferable. As the first transparent substrate having a refractive index almost equal to the ordinary refractive index (about 1.5) of the liquid crystal, a glass substrate,
A plastic substrate or the like can be preferably used.

The optically anisotropic diffraction grating of the present invention is manufactured as follows. First, a film having a thickness of about 1 to 2 μm is formed by a vacuum vapor deposition method, a sputtering method, or the like. Thereafter, by photolithography and dry etching, an uneven portion having convex portions with a refractive index of about 1.8 and having a predetermined period is formed, and processed into a diffraction grating pattern.

The cross-sectional shape of a plane perpendicular to the longitudinal direction of the concave-convex portion may be a symmetrical rectangular shape such as a rectangle or a square, or may be asymmetrical such as a step or a saw. In the case of a left-right asymmetrical shape, one of the ± 1st-order diffracted lights by the optically anisotropic diffraction grating has a higher diffraction efficiency, and only the diffracted light with the higher diffraction efficiency needs to be detected. Is preferable because high light utilization efficiency can be obtained.

Further, the uneven portion may be modified such that the interval between the uneven portion and the uneven portion is given a distribution, and the left and right symmetrical ones and the left and right asymmetrical ones are mixed.

Next, a second glass substrate (second transparent substrate) is prepared, a polyimide alignment film is formed on the surface of the second glass substrate which is in contact with the liquid crystal, and the rubbing direction of the polyimide alignment film is the stripes of the uneven portion. The second glass substrate is laminated and adhered to the first glass substrate according to the direction. At that time, the first
The epoxy resin containing the spherical spacers is applied to the peripheral portions of the glass substrate and the second glass substrate except the opening for injecting the liquid crystal and adhered. Then, liquid crystal is injected from the opening in a vacuum, and the opening is closed with a sealing resin.

As the liquid crystal of the present invention, a polymer liquid crystal, a liquid crystal monomer, a liquid crystal composition and the like can be appropriately used. When a polymer liquid crystal is used as the liquid crystal, after the liquid crystal monomer is injected, the liquid crystal monomer is polymerized by irradiating ultraviolet rays or heating in an aligned state. In that case, since the polymer liquid crystal can be aligned only by the uneven portion, the alignment film may be omitted.

Then, a retardation film (λ / 4) made of a material such as polycarbonate or polyvinyl alcohol is formed on the second glass substrate (on the surface opposite to the concavo-convex portion).
Boards).

In this case, in the outward path (direction from the light source side to the optical recording medium side), the refractive index of the liquid crystal portion corresponding to the S wave from the semiconductor laser incident from below is about 1.8 (extreme refractive index). And the convex portion is also approximately 1.8.
Therefore, the light passes upward without being diffracted.

On the return path (direction from the side of the optical recording medium to the side of the light source), the polarization direction is changed by the retardation film and the P wave is incident. At that time, since the refractive index of the liquid crystal corresponding to the P wave is about 1.5 (ordinary light refractive index) and the refractive index of the convex portion is about 1.8, it functions as a diffraction grating to cause light diffraction. .

Here, reflection occurs at the interface due to the difference in refractive index of 0.3 between the convex portion and the liquid crystal and the first and second glass substrates. The reflectance at the interface is approximately 0.8% per interface. The transmittance in consideration of the reflection loss is 98.4% on the two surfaces, which is a loss of 1.6%.

Also in the return path, reflection by the interface occurs in the optically anisotropic diffraction grating portion. Although it is difficult to accurately estimate this reflection loss, it is assumed that the area of the convex portion is almost half and half the light is reflected at the two interfaces. In that case, the light transmittance is 99.2% and the reflection loss is 0.
It will be 8%. As described above, the total transmittance in the forward and return paths is 97.6%, and the reflection loss is estimated to be 2.4%. However, even in the case of the P-wave input described above, a reflection loss of 1.6% is inevitable, so that the difference from the S-wave input of the present invention is considered to be slight, and there is no practical problem.

It should be noted that a thin film of a transparent material may remain on the bottom of the concave portion of the uneven portion for manufacturing reasons, and the reflection loss at that time is estimated to be about 3.2%. It can be used in various configurations.

In the present invention, the convex portion made of a transparent material may be formed by using photosensitive polyimide, in which case the diffractive element is manufactured as follows. A photosensitive polyimide having a refractive index of about 1.8 is applied on a first glass substrate having a refractive index of about 1.5 to a thickness of about 1 to 2 μm. After that, exposure and development are performed by a photolithography method to form convex portions with a pitch (cycle) of 8 μm to form a grid-shaped concave and convex portion.

Next, a second glass substrate is prepared, a polyimide alignment film is formed on the surface of the second glass substrate which is in contact with the liquid crystal, and the rubbing direction of the polyimide alignment film is aligned with the stripe direction of the concavo-convex portion to form the second glass substrate. The glass substrate of 1 is laminated and adhered to the first glass substrate. At that time, epoxy resin containing a spherical spacer is applied to the peripheral portions of the first glass substrate and the second glass substrate in a portion other than the opening for injecting the liquid crystal and adhered thereto. Then, liquid crystal is injected from the opening in a vacuum, and the opening is closed with a sealing resin.

It is preferable to provide the photo-sensitive organic material such as the photosensitive polyimide with the orientation because the polyimide orientation film of the second glass substrate can be omitted. The orientation is imparted by a horizontal orientation treatment such as a rubbing treatment. After that, a retardation film (λ / 4 plate) made of a material such as polycarbonate or polyvinyl alcohol is laminated on the second glass substrate (on the surface opposite to the uneven portion).

In this case, in the forward path (direction from the light source side to the optical recording medium side), the refractive index of the liquid crystal portion corresponding to the S wave from the semiconductor laser incident from below is about 1.8 (extreme light refractive index). And the convex portion is also approximately 1.8.
Therefore, the light passes upward without being diffracted.

On the return path (direction from the side of the optical recording medium to the side of the light source), the polarization direction is changed by the retardation film and the P-wave is incident. At that time, since the refractive index of the liquid crystal corresponding to the P wave is about 1.5 (ordinary light refractive index) and the refractive index of the convex portion is about 1.8, it functions as a diffraction grating to cause light diffraction. .

In the diffractive element of the present invention, another diffractive grating may be formed on the surface on the light source side, in which case tracking error detection by the three-beam method is preferable.

The pattern of the concave-convex portion (optical anisotropic diffraction grating) in the present invention has a curvature in the diffraction grating plane or a grating interval so that the beam shape of the return light from the optical recording medium has a desired shape. You can also give a distribution to.

In the present invention, when a coating such as a UV curable acrylic resin is provided on the light source side surface of the diffraction element and / or the optical recording medium side surface, unevenness on the surface of the λ / 4 plate or the glass substrate is generated. This is preferable because the resulting wavefront aberration can be reduced. Further, by laminating a glass substrate or a plastic substrate having good flatness on the coating film of the UV-curable acrylic resin or the like, it is possible to significantly reduce the wavefront aberration, which is preferable. Therefore,
Since the light incident / exit surface of the diffraction element is flattened, the wavefront aberration is consequently reduced.

As the light source in the present invention, various solid and gas lasers such as a semiconductor laser, a solid-state laser such as a YAG laser, and a gas laser such as He-Ne can be used, and the semiconductor laser can be made smaller and lighter, continuous oscillation, and maintenance and inspection can be performed. Etc. are preferable. Using a harmonic generator (SHG) in which a semiconductor laser or the like and a non-linear optical element are incorporated in the light source section and using a short-wavelength laser such as a blue laser enables high-density optical recording and reading.

The optical recording medium of the present invention is a medium capable of recording and / or reading information by light. Examples include optical disks such as CDs, CD-ROMs, DVDs (digital video disks), magneto-optical disks, and phase-change optical disks.

[0040]

【Example】

[Example 1] Example 1 is shown in FIG. On the first glass substrate 1 having a size of 10 mm × 10 mm, a thickness of 0.5 mm, and a refractive index of 1.52, a refractive index of 1.8 and a depth of 1.2 μm are formed by a vacuum deposition method.
A transparent thin film of SiO _x (x≈1.2) was formed. Then, a transparent thin film of SiO _x (x≈1.2) is formed into convex portions with a pitch (period) of 10 μm by photolithography and dry etching, and as a result, a lattice having a rectangular cross section in a plane perpendicular to the longitudinal direction. The concave-convex portion 2 was formed.

10 mm × 10 mm square, thickness 0.5 mm,
A second glass substrate 5 having a refractive index of 1.52 was prepared, and a polyimide alignment film 4 was formed on the surface of the second glass substrate 5 in contact with the liquid crystal 3.
The first and second glass substrates 1 and 5 were laminated and adhered so that the rubbing direction was along the stripe direction of the uneven portion 2. At this time, the peripheral portions of the two glass substrates were sealed except for the liquid crystal injection opening.

Specifically, the following was done. An epoxy resin (not shown in FIG. 1) containing a spherical spacer of 4 μm was applied to the peripheral portion of the first glass substrate 1, and the second glass substrate 5 was placed thereon. After that, in a reduced pressure atmosphere, the liquid crystal 3
As a mixed liquid crystal composition (trade name BL009 manufactured by Merck,
Nematic liquid crystal, Δn = 0.915, ordinary light refractive index =
1.5266, extraordinary light refractive index = 1.8181, phase transition temperature to solid-state liquid crystal phase ≦ −20 ° C., phase transition temperature to isotropic phase = 108 ° C.) were injected into the uneven portion 2.
The opening was closed with a sealing resin to produce an optically anisotropic diffraction grating.

Next, a retardation film 7 made of polycarbonate was adhered onto the second glass substrate 5 (the surface opposite to the concavo-convex portion) using a transparent adhesive 6. Further, a UV curable acrylic resin 8 was applied thereon. Further, the third glass substrate 9 was placed thereon, and ultraviolet rays were irradiated to laminate and adhere the third glass substrate 9. Furthermore, for the entire device, an antireflection film is formed on the light incident surface and the light emitting surface,
A diffractive element was produced.

As a result of the above, the transmittance was 95% for the S wave (light having a polarization direction perpendicular to the paper surface in FIG. 1) having a wavelength of 678 nm from the semiconductor laser (not shown in FIG. 1). With respect to P waves (light having a polarization direction parallel to the paper surface) from an optical disc (not shown in FIG. 1), the diffraction efficiency of the first-order diffracted light is 38.4%, and the diffraction efficiency of the -1st-order diffracted light is 35. 4%
Met. The total transmittance of P waves was 96%.

Therefore, the reciprocating efficiency is 0.95 × 0.9
When calculated with 6 × 0.738, it was 67.3%, which was a sufficiently high efficiency for practical use. The wavefront aberration of transmitted light is
It was 0.015λ _rms (root mean square) or less at the center of the light entrance / exit surface of the diffractive element (circular area with a diameter of 2 mm).

Example 2 Example 2 is shown in FIG. The same parts as those in Example 1 are designated by the same reference numerals. 10 mm x 10
A photosensitive polyimide (trade name: Photo Nice, manufactured by Toray Industries, Inc., refractive index: 1.78) having a thickness of 1 mm was formed on a first glass substrate 1 having a square shape of 0.5 mm, a thickness of 0.5 mm and a refractive index of 1.52 by spin coating. It was applied to 0.3 μm. After that, the pitch (cycle) of the photosensitive polyimide film was set to 8 by photolithography.
As a result, projections and depressions of μm were formed, and as a result, projections and depressions 2 having a grid shape having a rectangular cross section in a plane perpendicular to the longitudinal direction were formed. Then, a rubbing process was performed in the stripe direction of the uneven portion 2. Subsequent steps were performed in the same manner as in Example 1 to manufacture a diffraction element.

As a result of the above, the transmittance was 90% with respect to the S wave (light having a polarization direction perpendicular to the paper surface in FIG. 2) having a wavelength of 678 nm from the semiconductor laser (not shown in FIG. 2). The diffraction efficiency of the 1st-order diffracted light is 38% and the diffraction efficiency of the -1st-order diffracted light is 35% with respect to the P wave (light having a polarization direction parallel to the paper surface in FIG. 2) from the optical disc (not shown in FIG. 2). %Met.
The total transmittance of P waves was 91%.

Therefore, the reciprocating efficiency is 0.90 × 0.9.
When calculated with 1 × 0.73, it was 59.8%, which was a sufficiently high efficiency for practical use. The wavefront aberration of the transmitted light was 0.015 λ _rms or less at the center of the light entrance / exit surface of the diffractive element (circular range with a diameter of 2 mm).

[Example 3] The configuration of FIG.
10mm x 10mm square, thickness 0.5mm, refractive index 1.5
On the first glass substrate 1 of No. 2, SiO _x N _y (x = 0.7, y = 0.8) was deposited to a thickness of 1.4 μm by reactive sputtering. The film forming conditions are a substrate temperature of 200 ° C.
Nitrogen gas flow rate 17.4SCCM, oxygen gas flow rate 0.2S
Using a mixed gas of CCM, the gas pressure during film formation is 5 × 10 ^-3
torr.

The formed film had a refractive index of about 1.8 at 680 nm, and almost no absorption was observed at wavelengths of 600 nm or more. After that, SiO _x N _y (x =
0.7, y = 0.8) transparent thin film with a pitch (cycle) of 1
As a result, projections of 0 μm were formed, and as a result, projections and depressions 2 having a grid shape having a rectangular cross section in a plane perpendicular to the longitudinal direction were formed. After that, a polyimide film was applied to both substrates, and a rubbing process was performed in the stripe direction of the uneven portion 2. Subsequent steps were performed in the same manner as in Example 1 to manufacture a diffraction element.

As a result of the above, the transmittance was 80% with respect to the S wave (light having a polarization direction perpendicular to the paper surface in FIG. 2) having a wavelength of 678 nm from the semiconductor laser (not shown in FIG. 2). The diffraction efficiency of the first-order diffracted light is 37% and the diffraction efficiency of the -1st-order diffracted light is 36% with respect to the P-wave (light having a polarization direction parallel to the paper surface in FIG. 2) from the optical disc (not shown in FIG. 2). %Met.
The total transmittance of P waves was 91%.

Therefore, the reciprocating efficiency is 0.80 × 0.9.
When calculated with 1 × 0.73, it was 53.1%, which was a sufficiently high efficiency for practical use. The wavefront aberration of the transmitted light was 0.015 λ _rms or less at the center of the light entrance / exit surface of the diffractive element (circular range with a diameter of 2 mm).

This SiO _x N _y (x = 0.7, y
= 0.8) film has a flow rate ratio of N ₂ O and NH ₃ of 1: 2 even in the plasma CVD method using SiH ₄ , N ₂ O and NH _3.
A film can be formed by setting it to about 4.

[0054]

According to the present invention, even incident light (S wave) having a polarization parallel to the stripe direction of the optical anisotropic diffraction grating is transmitted with a high light transmittance in the outward path and is desired in the return path. It is possible to easily obtain an optical head device having a high light utilization efficiency that diffracts light. Further, the optically anisotropic diffraction grating used for this can be industrially produced with good productivity.

[Brief description of the drawings]

FIG. 1 is a side sectional view of an optical head device of Example 1.

FIG. 2 is a side sectional view of an optical head device of Example 2.

[Explanation of symbols]

 1: First Glass Substrate 2: Concavo-convex Part 3: Liquid Crystal 4: Polyimide Alignment Film 5: Second Glass Substrate 6: Transparent Adhesive 7: Phase Difference Film 8: UV Curable Acrylic Resin 9: Third Glass Substrate

Claims (5)

[Claims] 1. An optical head device for writing information and / or reading information by irradiating an optical recording medium with light from a light source through a diffractive element, wherein the diffractive element has a grid-like uneven portion on its surface. An optically anisotropic diffraction grating filled with a liquid crystal having optical anisotropy between the first transparent substrate and the second transparent substrate on which the convex and concave portions are formed. The first transparent substrate is formed by coating convex portions made of a transparent material at a predetermined cycle. The refractive index of the first transparent substrate is substantially equal to the ordinary refractive index of liquid crystal, and An optical head device characterized in that the index is approximately equal to the extraordinary index of refraction of liquid crystal. 2. The optical head device according to claim 1, wherein the convex portion has a refractive index higher than that of the first transparent substrate. 3. The convex portion is SiO _x N _y (0 ≦ x <2, 0
The optical head device according to claim 1, wherein ≦ y <1.3). 4. The optical head device according to claim 1, wherein the convex portion is made of a photosensitive organic material. 5. The optical head device according to claim 4, wherein the photosensitive organic material is horizontally aligned.