JPH1048588A - Optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to improve the efficiency of light in a backward path without degrading the light utilization efficiency in a forward path by arranging a first substrate forming rugged parts of optically anisotropic diffraction gratings on the side (optical recording medium side) opposite to a light source. SOLUTION: The optically anisotropic diffraction gratings 2 are formed of optically anisotropic materials, such as liquid crystals, held between the first substrate and a second substrate 9. At this time, the substrate 9 provided with the rugged parts is arranged on the light source 1 side. A phase difference plate 3 is thereafter laminated on the outside surface of the substrate 8. In such a case, the ordinary ray refractive indices of the grating part and the liquid crystal part are nearly equal with respect to the P waves (the light of a polarization direction parallel with the plane of Fig.) from the light source 1 and, therefore, the light transmits these parts in the forward path. In the backward path, the polarization direction is changed by the plate 3 and S waves (perpendicular to the plane of Fig.) are made incident so that the rugged parts function as diffraction gratings and the diffraction of the light takes place. In such a case, a significant difference is not admitted even if the gratings are formed at any of the substrates in the forward path, but the higher diffraction efficiency than the diffraction obtainable when the gratings are disposed on the lights source side is obtd, by providing the substrate 8 on the optical recording medium 6 side with the gratings in the backward path where the light diffracts.

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 optical disks such as CDs (compact disks), CD-ROMs and video disks and magneto-optical disks. About.

[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. This isotropic diffraction grating has a forward path (a direction from a light source to an optical recording medium).
Is about 50%, and the return path (in the direction from the optical recording medium to the photodetector) is about 20%. Therefore, the round trip limit is about 10%.

[0004] For this reason, 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. In order to use polarized light, a prism type beam splitter is combined with a retardation plate such as a λ / 4 plate to form a forward path (a direction from a light source to an optical recording medium) and a return path (a direction from an optical recording medium to a photodetector). There was a method of increasing the efficiency of reciprocation to increase the reciprocating 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 deflection selectivity is known. However, since the birefringent crystal itself is expensive and not suitable for mass production, it is difficult to apply it to the consumer field.
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.

[0006] Japanese Patent Publication No. 63-503102 discloses that
A hologram (diffraction element) with high light utilization efficiency is formed by forming a lattice-shaped uneven portion on a transparent substrate, filling it with liquid crystal to form an optically anisotropic diffraction grating, and further laminating a retardation plate. An optical head device used is described.

[0007]

In the above configuration, the substrate on which the uneven portions are formed has conventionally been arranged on the light source side so that the diffracted light reaches the photodetector straight.

Although the use efficiency of light is improved by doing so, it has been desired to further improve the use efficiency of light.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and writes and / or writes information by irradiating light from a light source onto an optical recording medium through a diffraction element. In the optical head device for reading information, the diffractive element is filled with an optically anisotropic material between a first substrate having a lattice-shaped uneven portion formed on a surface thereof and a flat second substrate. The optical anisotropic diffraction grating is provided, and the refractive index of the concave and convex portions of the first substrate is made substantially equal to one of the ordinary light refractive index and the extraordinary light refractive index of the optically anisotropic material. An optical head device is provided in which a second substrate of the element is arranged so as to come.

An optical head device characterized in that the optically anisotropic material is a nematic liquid crystal, and an optical anisotropic diffraction grating and an optical anisotropic diffraction grating and a device in which the optically anisotropic material is laminated on the optical recording medium side. An optical head device comprising a retardation plate is provided.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, an optically anisotropic diffraction grating is provided, and a flat second substrate of a diffraction element is arranged on the light source side. Thereby, light use efficiency is improved.

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

In FIG. 1, 1 is a light source, 2 is an optically anisotropic diffraction grating, 3 is a phase difference plate, 4 is a diffraction element including an optically anisotropic diffraction grating and a phase difference plate, and 5 is a condenser lens. , 6 denotes an optical recording medium, 7 denotes a photodetector, 8 denotes a first substrate, and 9 denotes a second substrate. The first substrate 8 is a substrate having an uneven portion, and the second substrate is a flat substrate.

FIG. 2 is a sectional view of the optically anisotropic diffraction grating 2.

In FIG. 2, reference numeral 8 denotes a first substrate, and 9 denotes a second substrate.
Reference numerals 11 and 12 denote substrates, 13 denotes a projection, 14 denotes a sealing material for sealing the periphery, and 15 denotes a liquid crystal sealed therein.

In the present invention, a transparent substrate made of glass, plastic, or the like is used as the substrate. Of this substrate, the first
The substrate has an uneven portion on its surface. This uneven portion may be formed by etching, mechanically cutting, or press-forming the substrate itself.

Further, a transparent material film having a desired refractive index may be formed on the substrate so as to have predetermined irregularities.
Specifically, a transparent material film may be formed over the entire surface, and the unevenness may be formed by etching or machining, or the predetermined unevenness may be directly formed by printing or other methods.

The refractive index of a normal transparent substrate made of glass, plastic, or the like is about 1.5, so that it is used as it is when it is matched with the ordinary light refractive index of a liquid crystal, which is a typical example of an optically anisotropic material. it can. When the substrate is directly cut, it can be used as it is. When a transparent material film is laminated on the substrate, a transparent material film having a refractive index almost the same as that of the substrate may be formed.

The refractive index of the substrate is changed to the extraordinary light refractive index of the liquid crystal (1.
(Approximately 7 to 1.8), a transparent substrate having a refractive index close to the refractive index can be used. However, it is preferable to form such a high-refractive transparent material film to form irregularities.
This is because a special optical glass or the like must be used for a high refractive index substrate material, which is expensive and is not suitable for mass production, but a transparent material film can easily form films with various refractive indexes. is there.

These transparent material films are preferably made of SiO _x or SiO _x N _y. By appropriately setting the conditions of vapor deposition or sputtering, the values of x and y are changed so that the desired refractive index is obtained. do it. In particular, these films can be easily finely processed by dry etching.

In this case, specifically, for example,
A transparent material film of about 2 μm is formed on a substrate by a vacuum deposition method, a sputtering method, or the like. After that, the refractive index of 1. was obtained by photolithography and dry etching.
An uneven portion formed of convex portions having a predetermined period of about 5 is processed so as to have a diffraction grating pattern.

The cross-sectional shape of the convex portion in a plane perpendicular to the longitudinal direction may be a symmetrical rectangular shape such as a rectangle or a square, or an asymmetrical shape such as a step shape or a saw shape. In the case of an asymmetric shape, the diffraction efficiency of one of the ± 1st-order diffracted lights by the optically anisotropic diffraction grating is increased, and only the diffracted light having the higher diffraction efficiency needs to be detected.
One detector is preferable because high light use efficiency can be obtained.

An alignment film such as polyimide is formed on the surface in contact with the liquid crystal. This alignment film is preferably rubbed. In that case, the rubbing direction of the alignment film is adjusted to the stripe direction of the projection.

Next, a second substrate, which is a flat substrate, is prepared. The second substrate has an alignment film such as polyimide formed on the surface in contact with the liquid crystal, and the alignment film is subjected to an alignment process by rubbing or the like. The rubbing direction is such that the rubbing direction is parallel to the stripe direction of the convex portion when the rubbing direction is superimposed on the first substrate.

Then, the second substrate is superimposed on the first substrate, and the peripheral substrate is sealed and adhered. Then, liquid crystal is sealed inside.

As the liquid crystal, 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 this case, since the polymer liquid crystal can be aligned only by the concave and convex portions, the alignment film may be omitted. When the polymer is not polymerized, a nematic liquid crystal may be usually used.

In the present invention, an optically anisotropic diffraction grating is formed from the first and second substrates and an optically anisotropic material such as a liquid crystal sandwiched between the substrates. At this time, the second substrate provided with the concave and convex portions is arranged on the light source side.

Thereafter, a retardation plate (λ / 4) made of a material such as polycarbonate or polyvinyl alcohol is formed on the outer surface of the first substrate (the surface opposite to the surface on which the uneven portions are formed).
Boards).

A description will be given of an example in which the refractive index of the lattice-shaped uneven portion is made to substantially match the ordinary light refractive index of the liquid crystal. In this case, the lattice-shaped uneven portion is on the opposite side to the light source. In this case, on the outward path, the lattice portion and the liquid crystal are exposed to P-waves (light having a polarization direction parallel to the plane of FIG. Since the ordinary light refractive indices of the portions are almost equal, light is transmitted as it is.

On the return path, the polarization direction changes due to the phase difference plate, and the light enters as an S-wave. At this time, the refractive index of the liquid crystal corresponding to the S wave is about 1.7 to 1.8 (an extraordinary light refractive index), and the refractive index of the convex portion is approximately 1.5, so that the liquid crystal functions as a diffraction grating. Light diffraction occurs.

At this time, no significant difference is recognized on either substrate when the grating is formed on the forward path, but on the optical recording medium side, the grating is provided on the substrate on the optical recording medium side when diffracting on the return path. A higher diffraction efficiency can be obtained than when a grating is provided.

Although the reason for this is unknown, when a grating is provided on the light source side, the light passes through the liquid crystal (optically anisotropic material) and then reaches the diffraction portion, during which light undergoes some optical change. (Disturbance).

In the case where the refractive index of the lattice-shaped uneven portion is made to substantially match the extraordinary light refractive index of the liquid crystal, the S
When a wave (light having a polarization direction perpendicular to the plane of FIG. 1) is incident, light travels straight on the outward path and diffracts on the return path as described above.

In the diffraction element of the present invention, another diffraction grating may be formed on the surface on the light source side. In this case, tracking errors can be detected by a three-beam method, which is preferable.

In the present invention, the pattern of the concavo-convex portions can be given a curvature in the plane of the diffraction grating or a distribution of the lattice spacing so that the beam shape of the return light from the optical recording medium has a desired shape. .

In the present invention, when a coating such as a UV-curable acrylic resin is provided on the surface of the diffraction element on the light source side and / or the surface of the optical recording medium side, it is caused by unevenness of the surface of the phase difference plate or the substrate. It is preferable because wavefront aberration can be reduced. And its UV
By laminating a glass substrate or a plastic substrate with good flatness on a film of a curable acrylic resin or the like, the wavefront aberration can be remarkably reduced, which is preferable. Therefore, since the light entrance / exit surface of the diffraction element is flattened, the wavefront aberration is reduced as a result.

It is preferable to provide an antireflection coating layer on the light incident surface of the diffraction element. This antireflection coating layer may be formed by vapor deposition, sputtering, or the like, and is usually formed when the substrate is a single substrate. Since the substrate on which the uneven portion is formed has a process of forming the uneven portion, if the anti-reflection coating layer is formed in advance and then passed through such a processing step, there is a problem that the surface is damaged. Cheap.

In the case of the present invention, since the substrate on which the uneven portions are formed is disposed on the side opposite to the light source and the retardation plate is laminated, it is not necessary to form an antireflection coating layer on the substrate itself.
For this reason, manufacture becomes easy. That is, the anti-reflection coating layer on the optical recording medium side may be formed on the surface of the phase difference plate or the third substrate surface laminated thereon, and this does not go through the step of making unevenness, and thus causes problems such as scratches. Hateful.

As the light source in the present invention, various solid and gas lasers such as a solid-state laser such as a semiconductor laser and a YAG laser, and a gas laser such as He-Ne can be used. It is preferable from the point of view. 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 on which information can be recorded and / or read by light. Examples are CDs (compact discs) and CD-ROs.
Optical disks such as M and DVD (digital video disk); magneto-optical disks; phase-change optical disks;

[0041]

【Example】

Example 1 An optical anisotropic diffraction grating having a structure as shown in FIG. 2 was prepared, and an optical head device having a structure as shown in FIG.
10mm x 10mm square, thickness 0.5mm, refractive index 1.5
An SiON-based transparent material film having a refractive index of 1.52 and a depth of 1.4 μm was formed on the glass substrate 11 of No. 2 by a reactive sputtering method.

Thereafter, the transparent material film of SiON is formed into protrusions having a pitch (period) of 5 μm by photolithography and dry etching, and as a result, grid-like protrusions having a rectangular cross section in a plane perpendicular to the longitudinal direction are formed. 13 was formed. A polyimide alignment film was formed on the surface in contact with the liquid crystal 15. The first substrate 8 was manufactured such that the rubbing direction was along the stripe direction of the uneven portion.

10 mm × 10 mm square, 0.5 mm thick,
A second glass substrate 15 having a refractive index of 1.52 is prepared,
A polyimide alignment film was formed on the surface in contact with the liquid crystal 15. The second substrate 9 was manufactured so that the rubbing direction was parallel to the stripe direction of the uneven portion when the rubbing direction was superimposed on the first substrate.

Thereafter, the first substrate 8 and the second substrate 9 were arranged so that the alignment film surfaces face each other, and the periphery was sealed with a sealing material to form empty cells. A nematic liquid crystal having a positive dielectric anisotropy (“B” manufactured by Merck
L009 ", Δn = 0.2915, ordinary light refractive index = 1.5
266, extraordinary refractive index = 1.8181, phase transition temperature to solid liquid crystal phase ≦ −20 ° C., phase transition temperature to isotropic phase = 108 ° C.), sealing the injection port and optically anisotropic A diffraction grating was manufactured.

Next, the retardation plate 3 made of polycarbonate was adhered to the outer surface of the first substrate 8 (the surface opposite to the uneven portion) using a transparent adhesive. Further, a UV curable acrylic resin was applied thereon. In addition, a third glass
The third substrate was placed, and the third substrate was laminated and bonded by irradiating ultraviolet rays. Further, with respect to the entire device, an antireflection film was formed on the light incident surface and the light output surface, and the diffraction element 4 was manufactured.

This diffractive element 4 was arranged such that the second substrate was located on the light source 1 side, and a condenser lens 5 and a photodetector 7 were provided.

As a result, the transmittance was 95.6% on the outward path with respect to the P-wave having a wavelength of 650 nm (light having a polarization direction parallel to the plane of FIG. 1) from the semiconductor laser as the light source. The return light from the optical recording medium 6 passes through the phase difference plate 3 and becomes an S-wave (light in a polarization direction perpendicular to the paper surface) and enters the optically anisotropic diffraction grating. The diffraction efficiency of the first and the first order was 78.5%.

COMPARATIVE EXAMPLE 1 The optically anisotropic diffraction grating has the same structure as in Example 1, except that a retardation plate is laminated on the second substrate side and the first substrate having irregularities is disposed on the light source side. Otherwise, the optical head device was manufactured in the same manner as in Example 1.

In this optical head device, the outward transmittance is 95.
Although it was 5%, which was almost equal to that of Example 1, the diffraction efficiency on the return path was 74.6%, which was about 4% lower than that of Example 1.

[0050]

According to the present invention, the first substrate on which the concave and convex portions of the optically anisotropic diffraction grating are formed is disposed on the side opposite to the light source (on the side of the optical recording medium), so that the light utilization efficiency on the outward path is reduced. The light efficiency on the return path is increased without reduction.

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

[Brief description of the drawings]

FIG. 1 is a front view of an optical head device according to 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: optically anisotropic diffraction grating 3: retardation plate 4: diffraction element 5: condenser lens 6: optical recording medium 7: retardation plate 8: first substrate 9: second substrate

Claims (3)

[Claims] 1. An optical head device for writing and / or reading information by irradiating light from a light source onto an optical recording medium through a diffractive element, wherein the diffractive element has a grid-like uneven portion on its surface. An optically anisotropic diffraction grating filled with an optically anisotropic material is provided between a first substrate on which is formed and a flat second substrate, and the unevenness of the first substrate is refracted. An optical head device wherein the refractive index is made substantially equal to either the ordinary light refractive index or the extraordinary light refractive index of the optically anisotropic material, and the second substrate of the diffraction element is arranged on the light source side. 2. The optical head device according to claim 1, wherein the optically anisotropic material is a nematic liquid crystal. 3. The optical head device according to claim 1, wherein the diffraction element comprises an optically anisotropic diffraction grating and a phase difference plate laminated on the optical recording medium side.