JP3528381B2 - Optical head device - Google Patents

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. Regarding

[0002]

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

Conventionally, in a diffraction grating or hologram element for an optical head device, an isotropic diffraction grating (relief type) having a rectangular cross section is formed on a glass or plastic substrate by a dry etching method or an injection molding method, This provided a function of diffracting light and beam splitting.

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. As one method, there is known a method in which a flat plate of a birefringent crystal such as LiNbO ₃ is used and an optical anisotropic diffraction grating is formed on the surface to provide polarization selectivity. However, the birefringent crystal itself is expensive, and it is difficult to apply it to the consumer field. Further, when the lattice is formed on LiNbO ₃ by the proton exchange method, there is a problem that it is difficult to form a lattice with a fine pitch because the protons in the proton exchange liquid easily diffuse into the LiNbO ₃ substrate.

As described above, the isotropic diffraction grating has a light utilization efficiency of about 50% on the outward path and a light utilization efficiency of 20 on the return path.
%, The limit is about 10% for round trips.

On the other hand, the applicant of the present invention forms a grating-shaped uneven portion on a transparent substrate and fills the uneven portion with a liquid crystal to form an optically anisotropic diffraction grating. It has been proposed to manufacture a device (Japanese Patent Application No. 7-259961). Also in this proposal, the reciprocating efficiency can be improved by combining the retardation sheet (λ / 4 plate).

Conventionally, quartz or plastic has been used as the material for the retardation sheet. Quartz has the advantage of being inert to liquid crystals, but has the problem of being very expensive.

On the other hand, a plastic sheet can be produced at a low cost by subjecting it to a stretching treatment, but a plastic capable of producing such a retardation sheet, such as polyvinyl alcohol, polycarbonate, polyarylate,
Polysulfone, polyether sulfone, acrylic resin and the like have a problem of being dissolved or swollen by liquid crystal. In order to solve these problems, conventionally, a glass substrate was placed on the surface in contact with the liquid crystal, and a plastic retardation sheet was further laminated on the glass substrate, but the yield was reduced due to the increase in the number of steps, and the cost was reduced. There was a problem of increasing.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and provide an optical head device which is inexpensive and has a high light utilization efficiency.

[0011]

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 the diffraction element, wherein the diffraction element is An optically anisotropic diffraction grating formed by forming a grid-shaped uneven portion on the surface of the transparent substrate and filling the uneven portion with a liquid crystal having optical anisotropy,
It has a retardation sheet directly laminated and arranged on a transparent substrate facing the transparent substrate with the liquid crystal interposed therebetween, and the diffraction element has another diffraction grating formed on the surface on the light source side. A characteristic optical head device is provided.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the uneven portion may be optically isotropic or anisotropic. In the case where the uneven portion is optically isotropic, it is possible to form the uneven portion by directly microfabricating a glass or plastic substrate which is easily available and has a refractive index of about 1.5.

The concavo-convex portion may have a left-right symmetrical rectangular shape such as a rectangular shape or a square shape, or a left-right asymmetric shape such as a stepped shape or a saw-shaped shape in a cross section perpendicular to the longitudinal direction. 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, regarding the uneven portion, a change is given such that a distribution is given to the interval between the uneven portion, a left-right symmetrical object and a left-right asymmetric object are mixed, and an uneven part and a convex object are mixed. You may go.

The diffraction element further forms another diffraction grating on the surface on the light source side. In that case, the tracking error can be detected by the three-beam method, which is preferable.

In the diffraction grating pattern of the present invention, the beam shape of the return light from the optical recording medium has a desired shape.
It is also possible to add curvature in the plane of the diffraction grating or to give distribution to the grating interval.

In general, the ordinary refractive index of liquid crystal is lower than the extraordinary refractive index, and the ordinary refractive index is often around 1.5. Therefore, it is easy to realize the condition that the refractive index of the transparent substrate and the ordinary refractive index of the liquid crystal are equal.

It is considered that the liquid crystal to be filled in the concave-convex portion has the major axis direction of liquid crystal molecules aligned along the longitudinal direction (stripe direction) of the concave-convex portion. In this case, the ordinary refractive index of the liquid crystal corresponds to the P wave (light polarized in the direction perpendicular to the longitudinal direction of the concavo-convex portion), and the ordinary refractive index and the refractive index of a transparent substrate such as a glass substrate are substantially equal. . The extraordinary refractive index of the liquid crystal corresponds to the S wave (light polarized in the direction parallel to the longitudinal direction of the uneven portion), and the extraordinary refractive index and the refractive index of the transparent substrate are different. Therefore, the optically anisotropic diffraction grating is optically transparent to P waves, and only transmits light. However, for S waves, the optically anisotropic diffraction grating functions as a diffraction grating.

Therefore, since the retardation sheet is directly laminated on the transparent substrate so as to face the transparent substrate with the liquid crystal interposed therebetween, for example, when P waves are incident from below (see FIG. 1),
Almost 100% of the light is transmitted through the optically anisotropic diffraction grating and becomes circularly polarized light by the retardation sheet. After that, the light passes through the aspherical lens of the objective lens, is reflected by the recording surface such as an optical disk, passes through the aspherical lens again, and again passes through the phase difference sheet, and then becomes an S wave, which is an optically anisotropic diffraction grating. Incident on. In that case,
The optically anisotropic grating functions as a diffraction grating, and is, for example, a rectangular grating that is bilaterally symmetric in a cross section perpendicular to the longitudinal direction of the concavo-convex portion. About 40% in the direction and 40 in the direction of the -1st order diffracted light.
A diffraction efficiency of about% can be obtained.

As described above, the optical anisotropic diffraction grating is easy to integrate and functions as a highly efficient polarization beam splitter. By using such an optical anisotropic diffraction grating, an optical head device with high light utilization efficiency can be realized.

The difference Δn between the extraordinary refractive index of the liquid crystal and the refractive index of the substrate (which is almost equal to the ordinary refractive index of the liquid crystal) multiplied by the depth d of the grating is half the light wavelength (λ / 2). It is known in principle that the best diffraction efficiency can be obtained when Therefore, the depth d of the grating is d = λ /
It may be processed with a value obtained by the formula (2Δn).

The retardation sheet of the present invention can be obtained by subjecting a transparent plastic sheet made of any one of polyvinyl alcohol, polycarbonate, polyarylate, polysulfone, polyether sulfone or acrylic resin to a stretching treatment.

Further, the transparent shielding film formed on the surface of the retardation sheet which is in contact with the liquid crystal is inert to the liquid crystal, has an effect of preventing the liquid crystal from penetrating, and has a refractive index close to that of plastic. The material is formed into a thin layer. From the required chemical properties, SiO ₂ , Al ₂ O ₃ , ZnO,
It is preferable to use an inorganic oxide such as SnO ₂ or ITO. Particularly preferably, SiO ₂ is preferable for reasons such as reliability and film forming cost.

As the method for forming the shielding film on the surface of the retardation sheet, there are vapor deposition, sputtering, chemical vapor deposition,
A sol-gel method or the like is used as appropriate.

The shielding film of the present invention also has the effect of preventing the permeation of gas that causes the generation of bubbles, which is a problem when a plastic substrate is used as the retardation sheet.

An alignment film such as a polyimide film for liquid crystal alignment is
It is provided on the surface of the shielding film on the side in contact with the liquid crystal or on the surface of the transparent substrate on which the irregularities are formed. In that case, it is preferable to provide an alignment film on the surface of the shielding film that is in contact with the liquid crystal, because a horizontal alignment process such as a rubbing process can be performed with a good alignment rate because the surface has no irregularities.

When at least one of the surface on the light source side and the surface on the optical recording medium side of the diffractive element of the present invention is provided with a coating such as a UV curable acrylic resin, a λ / 4 plate or a glass substrate is formed. It is preferable since the wavefront aberration caused by the unevenness of the surface 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 entrance / exit surface of the diffractive element is flattened, the wavefront aberration is consequently reduced.

As the light source of the present invention, a semiconductor laser, YA
Solid-state lasers such as G lasers and gas lasers such as He-Ne can be used, and semiconductor lasers are preferable in terms of size and weight reduction, continuous oscillation, maintenance and inspection. In addition, a harmonic wave generation device (S
HG) and a short wavelength laser such as a blue laser can be used for 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) and CD-RO
Optical discs such as M, video discs, DVDs (digital video discs), magneto-optical discs, and phase change optical discs can be used.

As the liquid crystal used in the present invention, known liquid crystals such as nematic liquid crystal and smectic liquid crystal used in liquid crystal display devices can be used. Further, polymer liquid crystals, liquid crystal monomers, liquid crystal compositions and the like can be used as appropriate.

[0031]

EXAMPLES (Reference Example) FIG. 1 shows an example of the present invention. 10 mm x 10 mm square,
First glass substrate 1 having a thickness of 0.5 mm and a refractive index of 1.52
A rectangular grid-shaped uneven portion 2 having a depth of 1.2 μm and a pitch (cycle) of 10 μm was formed on the top by photolithography and dry etching.

10 mm × 10 mm square, thickness 0.5 mm,
Prepare a second glass substrate 4 having a refractive index of 1.52, and
A retardation sheet (λ / 4 plate) 5 was adhered to the surface with a transparent adhesive. A silica thin film 6 having a thickness of 0.03 μm was previously formed on the surface of the retardation sheet 5 on the liquid crystal 3 side by a sputtering method. Further, a polyimide alignment film 7 was formed on the surface of the retardation sheet 5 on the second glass substrate 4 on the liquid crystal 3 side.

A sealant 8 made of epoxy resin containing a spherical spacer was applied to the peripheral portions of the two glass substrates excluding the liquid crystal injection openings. After that, the concavo-convex portion 2 of the first glass substrate 1 and the polyimide alignment film 7 of the second glass substrate 4 are faced to each other, and the rubbing direction of the polyimide orientation film 7 is the longitudinal direction (stripe direction) of the concavo-convex portion 2. The two glass substrates were overlapped with each other along the line, and pressurized and heated to seal.

Then, in a reduced pressure atmosphere, a mixed liquid crystal (trade name BL009 manufactured by Merck & Co., a nematic liquid crystal, Δn = 0.2) was used.
915, ordinary refractive index = 1.5266) was injected through the opening, and then the opening was closed with a sealing resin. On the light incident surface (surface on the light source side) 11 of the first glass substrate 1 and on the light emitting surface (surface on the optical recording medium side) 12 of the second glass substrate 4,
An antireflection film was provided to manufacture the diffraction element 10.

Table 1 shows the results of using 6 types of retardation sheets 5. Table 1 shows the transmittance (%) of a semiconductor laser (not shown; located below the diffractive element in the figure) serving as a light source 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). ), The diffraction efficiency (%) of the first-order diffracted light with respect to the S-wave (light in the polarization direction perpendicular to the paper) from the optical recording medium (not shown, and located above the diffractive element in the figure), the −first-order diffracted light Diffraction efficiency of (%),
The round trip efficiency (%) is shown.

[0036]

[Table 1]

[0037]

INDUSTRIAL APPLICABILITY In the present invention, the retardation sheet faces the transparent substrate on which the optically anisotropic diffraction grating is formed via the liquid crystal .
Since it is directly laminated on the transparent substrate , the diffraction element can be made thinner and lighter, and the manufacturing process can be simplified. Since the diffraction element further forms another diffraction grating on the surface on the light source side, tracking error detection by the three-beam method can be performed in that case.
Further, when a shielding film such as SiO ₂ is provided on the liquid crystal side surface of the retardation sheet, it is possible to prevent the liquid crystal from penetrating into the retardation sheet, and the liquid crystal reacts with the retardation sheet to cause bubbles in the liquid crystal. Can be prevented.

[Brief description of drawings]

FIG. 1 is a side view of a basic configuration of an optical head device, showing a reference example of the present invention.

[Explanation of symbols]

1: First glass substrate 2: uneven part 3: Liquid crystal 4: Second glass substrate 5: Phase difference sheet 6: Silica thin film 7: Polyimide alignment film 8: Sealing agent 10: Diffraction element 11: Light incident surface 12: Light emission surface

Front page continued (58) Fields surveyed (Int.Cl. ⁷ , DB name) G11B 7/135 G02B 5/18 G03H 1/00

Claims (5)

(57) [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 pattern on a surface of a transparent substrate. Is provided with an optically anisotropic diffraction grating formed by filling the uneven portion with a liquid crystal having optical anisotropy, and directly on a transparent substrate facing the transparent substrate with the liquid crystal interposed therebetween. An optical head device comprising: a retardation sheet that is laminated and arranged; and that the diffraction element has another diffraction grating formed on the surface on the light source side. 2. The optical head device according to claim 1, wherein a transparent shielding film which is inert to the liquid crystal and prevents the liquid crystal from penetrating is formed on a surface of the retardation sheet which is in contact with the liquid crystal. 3. The optical head device according to claim 1, wherein the material of the retardation sheet is any one of polyvinyl alcohol, polycarbonate, polyarylate, polysulfone, polyether sulfone, and acrylic resin. 4. The optical head device according to claim 2, wherein the shielding film is an inorganic oxide. 5. The optical head device according to claim 4, wherein the shielding film is made of SiO _x (1 ≦ x ≦ 2).