JP3509399B2 - Optical head device - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to writing optical information on an optical recording medium such as a CD (compact disc), a CD-ROM, a video disc, a magneto-optical disc, or the like.
The present invention relates to an optical head device for reading optical information.

[0002]

2. Description of the Related Art Conventionally, as an optical head device for writing or reading optical information on an optical recording medium such as an optical disk or a magneto-optical disk, a signal light reflected from the recording surface of the disk is sent to a detecting section. It is known that an optical component that uses a prism type beam splitter and an optical component that uses a diffraction grating or a hologram are used as optical components for guiding light (beam splitting).

A diffraction grating or hologram for an optical head device is formed by forming a rectangular grating (relief type) having a rectangular cross section on a glass or plastic substrate by a dry etching method, an injection molding method or the like, and diffracting light by this beam. It had a split function.

Further, when it is attempted to improve the light utilization efficiency over the isotropic diffraction grating whose light utilization efficiency is about 10%, it is conceivable to use polarized light. When trying to use polarized light, by combining a prism type beam splitter with a λ / 4 plate, the efficiency of the forward path (the direction from the light source to the recording surface) and the return path (the direction from the recording surface to the detection section) is increased to improve the reciprocating efficiency. There was a way to raise it.

However, the prism type polarization beam splitter is expensive, and another method has been sought. Using a flat substrate of a birefringent crystal such as LiNbO ₃ as a method, forming an anisotropic diffraction grating on the surface to have a polarization selective
It is known how to However, since the birefringent crystal itself is expensive and the productivity is poor, it was difficult to apply it to the consumer field.

In addition, LiNbO _{3 is produced} by the proton exchange method.
There is also a problem that it is difficult to form a lattice having a fine pitch when the lattice is formed on the substrate, because the protons in the proton exchange liquid easily diffuse into the substrate.

As described above, the isotropic diffraction grating has a utilization efficiency of about 50% on the outward path and a utilization efficiency of about 20% on the return path.

On the other hand, a lattice-shaped convex portion is formed on the first substrate, a flat substrate is used as the second substrate, and liquid crystal is filled between them to form an optically anisotropic diffraction grating. In addition, there is known an optical head device using a hologram (diffraction element) having a high light utilization efficiency in which a retardation element is further laminated (Japanese Patent Laid-Open No. 63-503102).

[0009]

In this case, since the diffractive element is a liquid crystal cell, it can usually be easily manufactured industrially. In this case, if the optically anisotropic diffraction grating is formed using a substrate made of glass, plastic, or the like, the transparent substrate has a refractive index of about 1.5. The ordinary refractive index of liquid crystal is also 1.
Since it is about 5, it is possible to easily realize a structure in which the refractive index of the transparent substrate and the ordinary refractive index of the liquid crystal are substantially the same. On the contrary, a special optical glass or the like can be used as the substrate having a refractive index close to the extraordinary light refractive index of liquid crystal (about 1.8).

An optical anisotropic diffraction grating is formed with a structure in which the refractive index of the substrate on which the lattice-shaped convex portions are formed and the ordinary refractive index of the liquid crystal are substantially the same, and a retardation element such as a λ / 4 plate is combined therewith. When a diffractive element is manufactured, the forward transmittance is increased,
In order to increase the diffraction efficiency in the return path, the polarization direction of the light incident on the diffraction element from the light source needs to be light (P wave) orthogonal to the longitudinal direction of the convex portion of the grating.

This is because the liquid crystal is usually oriented along the longitudinal direction of the convex portion of the lattice, and the ordinary refractive index of the liquid crystal corresponds to the polarized light orthogonal to the longitudinal direction of the convex portion, and the convex portion The extraordinary refractive index of the liquid crystal corresponds to the polarized light parallel to the longitudinal direction of. Therefore, because for the P-wave ordinary refractive index and the refractive index of the convex portion of the substrate of the liquid crystal was approximately equal, about 10
0% light is transmitted. On the other hand, because for the S-wave liquid crystal extraordinary refractive index and the refractive index of the convex portion of the substrate is that Do different, so that the light is diffracted.

The P wave or S wave incident from the light source passes through the optical anisotropic diffraction grating, is converted into circularly polarized light by the phase difference element, and is condensed on the optical recording medium through the condenser lens.

The light reflected by the recording surface of the optical recording medium becomes circularly polarized light in the reverse direction, passes through the condenser lens again, passes through the phase difference element, and is converted into S wave or P wave. Pass through the grid. Therefore, when the incident light is the P wave, the return light is the S wave and is diffracted. When the incident light is the S wave, the return light is the P wave and is not diffracted and is transmitted.

Therefore, in order to increase the transmittance on the outward path and the diffraction efficiency on the return path, it was necessary to make the incident light a P wave. 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 longitudinal direction of the convex portion of the diffraction grating. Accordingly, incident light to be limited to the P-wave severely constrained to the semiconductor laser and the arrangement of the photodetector, there is problem that the degree of freedom in design Ru limited.

The present invention solves the above-mentioned problems and is industrially applicable to incident light having a polarization parallel to the longitudinal direction of the optically anisotropic diffraction grating, or to light having an arbitrary polarization direction. It is an object of the present invention to provide an optical head device which can be efficiently produced and has high light utilization efficiency.

[0016]

According to the present invention, in an optical head device in which a diffractive element is arranged between a light source and an optical recording medium, a diffractive element, a phase difference element and a lattice-shaped convex portion are formed on the surface. And an optically anisotropic diffraction grating formed by filling a liquid crystal between a flat first substrate and a flat second substrate, the convex portion of the first substrate of the optically anisotropic diffraction grating being provided. The refractive index is substantially equal to either the ordinary or extraordinary refractive index of the liquid crystal, and the liquid crystal alignment direction on the first substrate on which the convex portions are formed is the direction along the longitudinal direction of the lattice, and the flatness is flat. There is provided an optical head device characterized in that the liquid crystal alignment direction on the second substrate is a diffractive element which is not parallel to the liquid crystal alignment direction on the first substrate.

The diffractive element is arranged on the second substrate and on the first substrate.
The liquid crystal alignment directions are substantially orthogonal to each other on the substrate, and a transparent material film is laminated on the surface of the first substrate of those diffractive elements, and the convex portion is formed of the transparent material. An optical head device characterized by being formed of a film. Further, the transparent material film on the surface of the first substrate of the diffractive element is formed of SiO _x N _y (0 ≦ x <2, 0 ≦ y <
An optical head device comprising: 1.3).

[0018]

1 is a schematic diagram showing the basic structure of an optical head device according to the present invention. In FIG. 1, 1 is a light source such as a semiconductor laser, 2 is an optical anisotropic diffraction grating,
Reference numeral 3 is a retardation element such as a λ / 4 plate. Reference numeral 4 denotes a diffractive element, which is composed of the optically anisotropic diffraction grating 2 and the phase difference element 3. Reference numeral 5 is a condenser lens for condensing light, 6 is an optical recording medium, and 7 is a photodetector.

FIG. 2 is a sectional view of the optically anisotropic diffraction grating used in the present invention. In FIG. 2, 11 is a first substrate provided with a convex portion, 12 is a flat second substrate, 13 is the convex portion, 14 is a sealing material for sealing the periphery, and 15 is a liquid crystal arranged between them. .

In the present invention, the protrusions on the first substrate may be formed by the substrate itself, or a transparent material film may be formed on the surface of the substrate to form the protrusions. When the refractive index of the convex portion is made to match the ordinary refractive index of the liquid crystal, the ordinary glass substrate has a refractive index of about 1.5, so that it can be usually used as it is. When a transparent material film is formed on this substrate, a glass substrate having a refractive index of about 1.5 and a refractive index substantially equal to that of the glass substrate is also used.

As the transparent material film, various organic or inorganic transparent material films having a refractive index of the ordinary liquid crystal to be used can be used. Among them, SiO _x N _y (0 ≦ x
<2, 0 ≦ y <1.3) (for this, SiO _x (1 ≦ x <
(Including 2)) is preferable because fine processing can be easily performed by a dry etching method.

In the optically anisotropic diffraction grating of the present invention, this substrate itself or the transparent material film formed on the surface thereof is processed into a predetermined shape to form a grid-shaped convex portion. In this case, the depth, pitch, and shape of the protrusions may be determined according to the desired diffraction characteristics. The convex portion may be formed by etching or may be formed by depositing a transparent material at a predetermined position.

In the present invention, when the convex portion is formed by using the transparent material film formed on the surface, it is produced, for example, as follows. First, a transparent material film is formed on a substrate having a thickness of about 1 to 2 μm by a vacuum deposition method, a sputtering method or the like. After that, a photolithography method and a dry etching method are used to form a grating having convex portions with a refractive index of about 1.5 and having a predetermined period, and processed to form a diffraction grating pattern.

The cross-sectional shape in a plane perpendicular to the longitudinal direction of the convex portion can be rectangular as shown in FIG. 2 may also rectangular symmetrical shape, such as square, stepped, or in the form of asymmetrical saw-like shape . In the case of a left-right asymmetrical shape, the diffraction efficiency of any one of the ± first-order diffracted lights by the optically anisotropic diffraction grating is high. Therefore, the diffraction efficiency higher diffracted light only detecting them if yo have field coupling, i.e., in the case of one light detector, since the utilization efficiency of the high light can be obtained.

In the present invention, an alignment film of polyimide or the like is usually formed on the surface of the first substrate which is in contact with the liquid crystal, and the liquid crystal alignment direction of this alignment film is aligned with the longitudinal direction of the convex portion. The alignment of the liquid crystal may be performed by rubbing or another method such as oblique vapor deposition, but the rubbing method is preferable in terms of productivity.

Next, a second glass substrate (second substrate) is prepared, an alignment film made of polyimide or the like is formed on the surface of the second glass substrate which is also in contact with the liquid crystal, and the rubbing direction of the alignment film is set to the first direction.
Rubbing is performed in a direction different from the rubbing direction of the substrate. In this case, the liquid crystal alignment directions of the two substrates are made different, but it is particularly preferable to make them orthogonal to each other.

In this way, the second glass substrate is laminated and adhered to the first glass substrate while making the liquid crystal orientation different. At that time, a sealing material such as an epoxy resin containing a spacer is applied to the peripheral portion of the first glass substrate and the second glass substrate at 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.

[0028] In this sealing process, the liquid crystal injection step, the opening sealing step is not limited to the above process, it may be a more time-line cormorants Engineering liquid crystal injection and sealing bonding.

As the liquid crystal used in the present invention, a polymer liquid crystal, a liquid crystal monomer, a liquid crystal composition and the like can be appropriately used.
In the case of an ordinary liquid crystal composition, a nematic liquid crystal used in an ordinary TN type liquid crystal may be used. It is preferable to use a liquid crystal having a large refractive index anisotropy because a large diffraction angle can be obtained. Specifically, Δn ≧ 2, particularly Δn ≧ 2.
5 is preferable.

When a polymer liquid crystal is used as the liquid crystal, the liquid crystal monomer may be polymerized by injecting the liquid crystal monomer and then irradiating it with ultraviolet rays or heating it in an aligned state. In that case, since the polymer liquid crystal can be aligned only by the convex portion,
The alignment film may be omitted if the liquid crystal is aligned in a specific direction. When this polymer liquid crystal is used, it is sufficient that the polymer liquid crystal is polymerized while maintaining the orientation of the liquid crystal monomer, and thus the orientation does not need to change after the polymer liquid crystal is formed.

The liquid crystal is oriented so that the two substrates have different liquid crystal orientation directions. In this case, it is preferable that the liquid crystal alignment directions of the two substrates are substantially orthogonal to each other. Therefore, the liquid crystal used is 90 °
It is preferable to mix the chiral material so as to have a twist or a twist of 90 ° + 180 ° × n (n is an integer of 0 or more).

A retardation element typified by a λ / 4 plate is laminated on the optical anisotropic diffraction grating manufactured in this manner to manufacture a diffraction element. This retardation element converts light that has passed through the optically anisotropic diffraction grating into circularly polarized light. This retardation element is arranged on the side opposite to the light source, that is, on the optical recording medium side. As this retardation element, a known retardation film made of a material such as polycarbonate or polyvinyl alcohol can be used.

In this case, in the present invention, in the optically anisotropic diffraction grating, the first glass substrate on which the convex portion is formed is located on the side opposite to the light source, and the retardation element is laminated on the first glass substrate. To do so.

Thus, the lattice-shaped convex portions (the longitudinal direction is shown in FIG.
Depth direction) is on the side opposite to the light source, and nematic liquid crystal with positive dielectric anisotropy is used, and the twist angle of the liquid crystal is 9
The refractive index of the lattice-shaped convex portions of the first substrate is set to 0 °, and the refractive index of the ordinary liquid crystal is made substantially equal to that of the liquid crystal. The operation in this case will be described with reference to FIG.

In the outward path (direction from the light source side to the optical recording medium side), with respect to the S wave from the semiconductor laser,
The plane of polarization is rotated by 90 ° by the optically anisotropic diffraction grating, and becomes a P wave (having polarization in a direction parallel to the paper surface) in the grating portion (inner surface of the first substrate) after passing through the liquid crystal layer. At this time, for the P wave, the refractive index of the grating-shaped convex portion and the refractive index of the liquid crystal portion (corresponding to the ordinary light refractive index because they are in the short axis direction of the liquid crystal molecules) are substantially equal to each other. Instead, light is transmitted as it is.

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 phase difference element and the S-wave is incident on the optical anisotropic diffraction grating. then,
Since the refractive index of the liquid crystal corresponding to the S wave corresponds to the extraordinary light refractive index, unlike the refractive index of the grating-shaped convex portion (which is almost equal to the ordinary light refractive index), it functions as a diffraction grating to cause light diffraction. .

Another aspect of the present invention is one in which the refractive index of the lattice-shaped convex portions of the first substrate is made substantially equal to the refractive index of extraordinary light of the liquid crystal. Also in this case, since the diffracted light must reach the photodetector, the longitudinal direction of the lattice-shaped convex portions in FIG. 1 is perpendicular to the paper surface of FIG.
This can be applied to P-wave input from a conventional semiconductor laser.

In this case, this convex portion is made substantially equal to the extraordinary light refractive index of the liquid crystal (for example, a refractive index of about 1.8). This may be performed by directly processing a glass substrate having a high refraction, or by laminating a transparent material film on the substrate.

As the transparent material film, various organic or inorganic transparent material films having a refractive index of extraordinary light of the liquid crystal to be used can be used. Among them, SiO _x N _y (0 ≦
x <2, 0 ≦ y <1.3 (for this, SiO _x (1 ≦ x
<Including <2)) is preferable because fine processing can be easily performed by a dry etching method.

The optically anisotropic diffraction grating of the present invention may be formed by directly processing a glass substrate having high refraction, but it is preferably formed by laminating a transparent material film on the substrate. In this case, the following process is performed. To make. First, a film having a thickness of 1 to 2 μm and a refractive index of about 1.8 is formed on a substrate by a vacuum vapor deposition method, a sputtering method or the like. After that, by photolithography and dry etching, a grating having convex portions with a predetermined period is formed, and processed into a diffraction grating pattern.

The cross-sectional shape of the convex portion in the lateral direction may be a symmetrical rectangular shape such as a rectangle or a square as shown in FIG. 2, or may be a stepwise shape, a sawtooth shape or the like, which is asymmetrical. . 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. Therefore, if only one detector is used and only the diffracted light with the higher diffraction efficiency is detected. When good, high light utilization efficiency is obtained.

In the present invention, an alignment film such as polyimide is usually formed on the surface of the first substrate which is in contact with the liquid crystal, and the liquid crystal alignment direction of this alignment film is the same as in the case of combining with the above ordinary refractive index. Are aligned with the longitudinal direction of the convex portion.

Next, a second glass substrate (second substrate) is prepared, an alignment film is formed in the same manner as described above, and liquid crystals are sealed therein to manufacture an optical anisotropic diffraction grating. A diffractive element is manufactured by laminating a retardation element represented by a λ / 4 plate on the optical anisotropic diffraction grating manufactured in this manner.

Also in this case, in the present invention, in the optical anisotropic diffraction grating, the first glass substrate on which the convex portion is formed is located on the side opposite to the light source, and the retardation element is laminated on the first glass substrate. To be done.

Thus, the grid-shaped convex portions (the longitudinal direction is shown in FIG.
Depth direction) is on the side opposite to the light source, and nematic liquid crystal with positive dielectric anisotropy is used, and the twist angle of the liquid crystal is 9
At 0 °, the refractive index of the lattice-shaped convex portions of the first substrate is made approximately equal to the refractive index of extraordinary light of the liquid crystal, and the P wave from the semiconductor laser (having a polarization in the direction parallel to the paper surface) is incident on this. The operation in that case will be described with reference to FIG.

In the outward path (direction from the light source side to the optical recording medium side), with respect to the P wave from the semiconductor laser,
The plane of polarization is rotated by 90 ° in the optically anisotropic diffraction grating, and becomes an S wave (having a polarization in a direction perpendicular to the paper surface) at the grating portion (inner surface of the first substrate) after passing through the liquid crystal layer. At this time, with respect to the S wave, the refractive index of the grating-shaped convex portion and the refractive index of the liquid crystal portion (corresponding to extraordinary light refractive index because they are in the long axis direction of the liquid crystal molecules) are substantially equal, and thus function as a diffraction grating Without passing through, the light is transmitted as it is.

On the return path (direction from the optical recording medium side to the light source side), the polarization direction is changed by the phase difference element and the P-wave is incident on the optical anisotropic diffraction grating. then,
Since the refractive index of the liquid crystal corresponding to the P wave corresponds to the ordinary light refractive index, unlike the refractive index of the grating-shaped convex portion (which is almost equal to the extraordinary light refractive index), it functions as a diffraction grating, and light diffraction occurs. .

In the present invention, in the above description, the alignment directions between the upper and lower substrates are orthogonal to each other.
In the present invention, an angle other than 90 ° can be used. If the polarization direction of the incident light has an arbitrary angle such as 30 °, 45 °, 60 ° with the longitudinal direction of the convex portion of the grating, the liquid crystal may be twisted at the same angle.

As a result, the plane of polarization of the incident light is rotated,
By bringing the polarization plane so that it is always orthogonal or parallel to the longitudinal direction of the grating on the grating formation surface, it is possible to realize an optical head device having high reciprocal efficiency with respect to light incident in an arbitrary polarization direction.

In the diffraction element of the present invention, another diffraction grating may be formed on the light source side substrate, that is, the second substrate side,
In that case, both diffraction for the photodetector and diffraction for tracking error detection by the three-beam method can be realized by one diffraction element.

The pattern of the convex portions of the 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 add a distribution.

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 a good flatness on the coating film of the UV curable acrylic resin or the like, the wavefront aberration can be remarkably reduced. Since the light entrance / exit surface of the diffractive element is flattened, the wavefront aberration is consequently reduced.

[0053] The semiconductor laser as the light source in the present invention, can be solid-state laser, using a gas <br/> lasers such as He-Ne, such as YAG laser, a semiconductor laser is smaller and lighter, continuous wave, terms of maintenance and inspection Is preferred. Harmonic generator (SH with a semiconductor laser etc. and a nonlinear optical element incorporated in the light source)
Using G), the use of short-wavelength laser such as a blue laser, Ru can 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 are CD (Compact Disc), CD-ROM, D
Examples thereof include optical discs such as VD (digital video disc), magneto-optical discs, and phase change optical discs.

[0055]

[Example] [Example 1] 10 mm x 10 mm square, thickness 0.5 mm, refractive index 1.5
On the first glass substrate 11 of No. 2 by a reactive sputtering method, SiO _x N having a refractive index of 1.52 and a thickness of 1.4 μm.
A transparent material film of _y (x≈1.8, y≈0.17) was formed.

[0056] Then, by photolithography and dry etching, lattice SiO _x N transparent material film pitch of _y (cycle) a convex portion of 10 [mu] m, resulting cross-sectional shape in a plane perpendicular to the longitudinal direction a rectangular -Shaped convex part 1
Formed 3. A polyimide alignment film was formed on the surface in contact with the liquid crystal. The rubbing direction was set to be along the longitudinal direction of the convex portion (the vertical direction in the drawing of FIG. 2).

[0057] 10mm × 10mm square, a thickness of 0.5mm,
A second glass substrate 12 having a refractive index of 1.52 was prepared, and a polyimide alignment film was formed on the surface of the second glass substrate 12 in contact with the liquid crystal.
The rubbing direction was orthogonal to the longitudinal direction of the convex portion (parallel direction to the drawing of FIG. 2). Next, the first glass substrate and the second glass substrate were overlapped with each other so that the orientation directions thereof were orthogonal to each other, and the peripheral portion was sealed except for the liquid crystal injection opening.

Specifically, the following was done. Epoxy resin containing a spherical spacer of 8 μm was applied to the peripheral portion of the second glass substrate, and the first glass substrate was placed thereon. Then, a mixed liquid crystal composition as a liquid crystal in a reduced pressure atmosphere (product name “BL009” manufactured by Merck & Co., nematic liquid crystal, Δn =
0.2915, ordinary light refractive index = 1.5266, extraordinary light refractive index = 1.1811, phase transition temperature to solid liquid crystal phase ≦ −20
℃, phase transition temperature to isotropic phase = 108 ℃)
Was injected. The opening was closed with a resin for sealing to produce an optically anisotropic diffraction grating.

Then, a polycarbonate retardation element was adhered to the outer surface of the first glass substrate (the surface opposite to the surface provided with the convex portion) with a transparent adhesive. Further, a UV-curable acrylic resin was applied on it. Further, a third glass substrate was placed thereon and irradiated with ultraviolet rays to laminate and adhere the third glass substrate to the retardation element. Further, for the entire element, an antireflection film was formed on the light incident surface and the light emitting surface to fabricate a diffractive element.

This diffractive element had a transmittance of 85% with respect to the S wave having a wavelength of 678 nm (light having a polarization direction perpendicular to the paper surface in FIG. 1) from the semiconductor laser. The diffraction efficiency of the first-order diffracted light was 25% and the diffraction efficiency of the -1st-order diffracted light was 26% for the S wave (light having a polarization direction perpendicular to the paper surface) on the return path from the optical disc.

Therefore, the reciprocating efficiency is 0.85 × 0.5.
When calculated with 1, it was 43%, which was a sufficiently high efficiency for practical use. The wavefront aberration of the transmitted light is 0.0 at the center of the light entrance / exit surface of the diffractive element (circular range with a diameter of 2 mm).
It was 15λ _rms (root mean square) or less.

Example 2 The rubbing direction of the second substrate is 45 ° with respect to the first substrate.
A diffractive element was manufactured in the same manner as in Example 1 except that the liquid crystal twist angle was 45 °.

Incident light having a wavelength of 678 nm from the semiconductor laser and having an intermediate polarization direction between the S wave and the P wave (perpendicular to the rubbing direction of the second substrate) was used. The diffractive element had a transmittance of 78% with respect to the incident light. The diffraction efficiency of the first-order diffracted light was 25% and the diffraction efficiency of the -1st-order diffracted light was 24% with respect to the S wave (light having a polarization direction perpendicular to the paper surface) on the return path from the optical disk.

Therefore, the reciprocating efficiency is 0.78 × 0.4.
When calculated with 9, it was 38%, which was a sufficiently high efficiency for practical use. The wavefront aberration of the transmitted light is 0.0 at the center of the light entrance / exit surface of the diffractive element (circular range with a diameter of 2 mm).
It was 15λ _rms or less.

Example 3 10 mm × 10 mm square, thickness 0.5 mm, refractive index 1.5
On the first glass substrate 11 of No. 2 by a reactive sputtering method, SiO _x N having a refractive index of 1.8 and a thickness of 1.4 μm.
A transparent material film of _y (x≈0.7, y≈0.8) was formed.

Then, a transparent material film of SiO _x N _y is formed into convex portions with a pitch (cycle) 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 is formed. -Shaped convex part 1
Formed 3. A polyimide alignment film was formed on the surface in contact with the liquid crystal. The rubbing direction was set to be along the longitudinal direction of the convex portion (the vertical direction in the drawing of FIG. 2).

[0067] 10mm × 10mm square, a thickness of 0.5mm,
A second glass substrate 12 having a refractive index of 1.52 was prepared, and a polyimide alignment film was formed on the surface of the second glass substrate 12 in contact with the liquid crystal.
The rubbing direction was orthogonal to the longitudinal direction of the convex portion (parallel direction to the drawing of FIG. 2). Next, the first glass substrate and the second glass substrate were overlapped with each other so that the orientation directions thereof were orthogonal to each other, and the peripheral portion was sealed except for the liquid crystal injection opening.

Specifically, the following was done. Epoxy resin containing a spherical spacer of 8 μm was applied to the peripheral portion of the second glass substrate, and the first glass substrate was placed thereon. Then, the same mixed liquid crystal composition as in Example 1 was injected as liquid crystal in a reduced pressure atmosphere. The opening was closed with a resin for sealing to produce an optically anisotropic diffraction grating.

Then, a polycarbonate retardation element was adhered to the outer surface of the first glass substrate (the surface opposite to the surface provided with the protrusions) with a transparent adhesive. Further, a UV-curable acrylic resin was applied on it. Further, a third glass substrate was placed thereon and irradiated with ultraviolet rays to laminate and adhere the third glass substrate to the retardation element. Further, for the entire element, an antireflection film was formed on the light incident surface and the light emitting surface to fabricate a diffractive element.

This diffractive element had a transmittance of 86% with respect to a P wave having a wavelength of 678 nm (light having a polarization direction parallel to the paper surface in FIG. 1) from a semiconductor laser. The diffraction efficiency of the first-order diffracted light was 26% and the diffraction efficiency of the -1st-order diffracted light was 27% for the P wave (light having a polarization direction parallel to the paper surface) on the return path from the optical disk.

Therefore, the reciprocating efficiency is 0.86 × 0.5.
It was 46% when calculated by 3, and a sufficiently high efficiency was obtained in practical use. The wavefront aberration of the transmitted light is 0.0 at the center of the light entrance / exit surface of the diffractive element (circular range with a diameter of 2 mm).
It was 15λ _rms or less.

[0072]

According to the present invention, since the liquid crystal is twisted, the incident light from the semiconductor laser can be used as an optical head device for P-waves and S-waves, and also for light having an arbitrary polarization direction therebetween. , High light utilization efficiency is obtained. 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 diagram showing a basic configuration of an optical head device of the present invention.

FIG. 2 is a sectional view of an optically anisotropic diffraction grating used in the present invention.

[Explanation of symbols]

1: Light source 2: Optical anisotropic diffraction grating 3: Phase difference element 4: Diffraction element 5: Condensing lens 6: Optical recording medium 7: Photodetector 11: First substrate 12: second substrate 13: convex part 14: Seal material 15: Liquid crystal

Claims (4)

(57) [Claims] 1. An optical head device having a diffractive element disposed between a light source and an optical recording medium, wherein the diffractive element is flat with a retardation element and a first substrate having a lattice-shaped convex portion formed on the surface thereof. And an optical anisotropic diffraction grating filled with liquid crystal between the second substrate, and the refractive index of the convex portion of the first substrate of the optical anisotropic diffraction grating is the ordinary optical refraction of the liquid crystal. Index or extraordinary light refractive index, and the liquid crystal alignment direction on the first substrate on which the convex portions are formed is the direction along the longitudinal direction of the lattice, and the liquid crystal alignment on the flat second substrate. An optical head device characterized by being a diffractive element whose direction is not parallel to the liquid crystal alignment direction of the first substrate. 2. The optical head device according to claim 1, wherein the liquid crystal alignment directions of the diffractive element are substantially orthogonal to each other on the second substrate and the first substrate. 3. The optical head device according to claim 1, wherein a transparent material film is laminated on the surface of the first substrate of the diffractive element, and the convex portion is formed of the transparent material film. . 4. The transparent material film on the surface of the first substrate of the diffractive element is made of SiO _x N _y (0 ≦ x <2, 0 ≦ y <1.3). Optical head device.