JPH09297933A - Optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent decrease in the diffraction efficiency due to temp. change and to realize mass-productivity and a low cost by forming a projecting part on one of two substrates which are disposed at a specified distance and filled with a liquid crystal so that a transparent film of specified inorg. oxide is formed on the projecting part. SOLUTION: This optical anisotropic diffraction grating is composed of a transparent substrate 11 and a substrate 12 having projections 13, and the distance (d) between the two substrates is maintained by adhering the substrates on the sealing part 15 with a sealing material 14. The inside of the substrates is filled with a liquid crystal 16. The projecting part 13 is formed by etching, and a transparent film composed org. oxide of SiOx Ny (wherein (x) and (y) satisfy 1<=x<=2 and 0<=y<=1.33) having the same refractive index as that of the liquid crystal 16 is formed on the projecting part 13. By controlling the ratio of (x) and (y), the refractive index can be controlled and deterioration in optical characteristics such as absorption of light can be prevented. By maintaining the distance (d) between the substrates to <=5μm, changes in wave aberration due to temp. change can be decreased, and the reliability can be improved. Thereby, the obtd. device has high efficiency and excellent productivity and can be produced at a low cost, further, decrease in the diffraction efficiency can be prevented.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a CD (compact disc), a CD-ROM, a video disc, a DVD.
The present invention relates to an optical head device for writing and reading optical information on an optical disk such as (digital video disk) and a magneto-optical disk.

[0002]

2. Description of the Related Art Conventionally, 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 detects signal light reflected from the recording surface of the optical recording medium. It is known that a prism type beam splitter is used as an optical component for guiding (beam splitting) light to a portion and a diffraction grating or a hologram element is used.

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

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 using polarized light, a prism type beam splitter is combined with a λ / 4 plate to increase the efficiency of going (direction from the light source to the optical recording medium) and returning (direction from the optical recording medium to the detecting section) to improve the reciprocating efficiency. There was a way to raise.

However, the prism type polarizing beam splitter is expensive, and other methods have been sought. As one method, a method of using a flat plate of birefringent crystal such as LiNbO ₃ and forming an anisotropic diffraction grating on the surface to have polarization selectivity is known. However, the birefringent crystal itself is expensive, and application to the consumer field is difficult. Further, when the lattice is formed by the proton exchange method, there is a problem that it is difficult to form a lattice having a fine pitch because the protons in the proton exchange liquid diffuse quickly into LiNbO ₃ .

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

[0007]

Therefore, an optically anisotropic diffraction grating in which a grid-shaped uneven portion is formed on the surface of a transparent substrate, and the uneven portion is filled with liquid crystal having optical anisotropy is proposed. However, due to the thermal expansion of the liquid crystal, which is a liquid,
There is a problem that the cell swells and the optical wavefront aberration largely deteriorates on the high temperature side.

When the wavefront aberration is deteriorated, it becomes impossible to converge the laser beam in the shape of the optical disc surface, which makes it difficult to read information.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical head device which solves the above-mentioned problems and improves the light utilization efficiency and can be manufactured at low cost.

[0010]

DISCLOSURE OF THE INVENTION The present invention is an optical anisotropic method in which light from a light source is formed on a surface of a transparent substrate in a grid-like uneven portion, and the uneven portion is filled with a liquid crystal having optical anisotropy. In an optical head device for writing information and / or reading information by irradiating an optical recording medium through a characteristic diffraction grating, the substrate gap at the apex of the convex portion of the optically anisotropic diffraction grating is set to 5 μm or less. An optical head device is provided.

An optical head device characterized in that the uneven portion of the optically anisotropic diffraction grating is formed of SiO _x N _y (1 ≦ x ≦ 2, 0 <y ≦ 1.33), and , The concave and convex portions of the optically anisotropic diffraction grating are formed by providing a transparent film having a refractive index almost equal to that of any of the liquid crystals on the surface of the transparent substrate, and the transparent film is formed in the seal portion. The present invention provides an optical head device characterized in that

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The optically anisotropic diffraction grating used in the optical head device of the present invention has a grid-like uneven portion formed on the surface of a transparent substrate, and the uneven portion is filled with liquid crystal having optical anisotropy. The optically anisotropic diffraction grating has a substrate gap at the apex of the protrusion of the optically anisotropic diffraction grating of 5 μm or less. As a result, even if there is a temperature change, it is unlikely that a partial substrate spacing change will occur, and optical wavefront aberration will not easily deteriorate,
Stable diffraction of light is obtained.

FIG. 1 is a schematic view showing a typical constitution of the optical head device of the present invention.

In FIG. 1, 1 is a light source such as a semiconductor laser, 2 is an optical anisotropic diffraction grating, 3 is a phase difference element such as a quarter wavelength plate, 4 is a condenser lens, and 6 is a photodetector. ing.

The light emitted from the light source 1 is linearly polarized light, for example, P
Although it has polarized light, the optically anisotropic diffraction grating 2 does not work as a diffraction grating and transmits as it is. Then, after passing through the phase difference element 3, the light is focused by the condenser lens 4 and reflected on the surface of the optical recording medium 5, and again passes through the condenser lens 4 and the phase difference element 3 to become S-polarized light, which causes an optical difference. It is incident on the isotropic diffraction grating 2. Then, since the polarization direction is deviated by 90 ° from the outward path, the optically anisotropic diffraction grating 2 functions as a diffraction grating, and the light is diffracted and reaches the photodetector 6.

FIG. 2 is a sectional view of an optically anisotropic diffraction grating used in this optical head device.

In FIG. 2, reference numerals 11 and 12 denote substrates and 13
Is a convex portion provided on a part of the substrate, 14 is a sealing material for adhering two substrates, 15 is a sealing portion, and 16 is a liquid crystal. In addition, d represents a gap between the substrate 11 and the convex portion of the substrate 12.

In the present invention, the gap d is set to 5 μm or less. This makes it possible to obtain a highly reliable diffraction grating with little change in the wavefront aberration even if the temperature changes. It is more preferably 3 μm or less, and particularly preferably 2 μm or less. However, since the liquid crystal is injected, the thickness is preferably 0.5 μm or more.

In the present invention, the optically anisotropic diffraction grating constitutes the diffraction grating by the difference in refractive index between the convex portion of the substrate and the liquid crystal. Since the liquid crystal has a refractive index anisotropy, the liquid crystal can be aligned in a specific direction by subjecting the surface of the substrate to an alignment treatment. By utilizing this, it is possible to diffract or not diffract the polarized light incident on the optically anisotropic diffraction grating.

As the substrate of the present invention, a transparent substrate such as glass or plastic can be used, but the refractive index of glass, polyolefin, polycarbonate or the like is 1.4 to 1.6.
It is preferable to use a transparent substrate of the following degree because it is easy to match the ordinary refractive index of the liquid crystal of about 1.5.

In order to form the uneven portion on this substrate, the substrate itself can be processed into such a shape by etching or mechanical cutting, but a transparent material having a refractive index almost equal to that of any of the liquid crystals. It is preferable to laminate the film on the surface of the transparent substrate. For this, a known transparent film forming method such as a plasma CVD method or a sputtering method can be used.

A mask may be provided at the time of forming the transparent film to form concavities and convexities simultaneously with the formation of the transparent film, or a transparent film may be formed on the entire surface and the concavities and convexities may be formed by a photolithography process. As for the concave and convex portion, only the convex portion may be a transparent film as shown in FIG. 2 and the concave portion may expose the surface of the substrate, or the transparent film may remain thin in the concave portion.
Further, it is preferable that the seal portion is a concave portion because a narrow substrate gap can be easily formed.

The transparent film of the present invention is a transparent film having a refractive index almost equal to that of any one of the liquid crystals, as long as it is less likely to be deteriorated by the liquid crystal to be used or change in the refractive index due to swelling or the like. Can be used. Specifically, SiO
A transparent film of an inorganic oxide such as _x or SiO _x N _y is preferably used.

Particularly, as the transparent film, the SiO _x N _y film (1
It is preferable to use ≦ x ≦ 2, 0 <y ≦ 1.33). When this material is used, a plasma CVD method or a reactive direct current sputtering method is used to obtain a film that is almost equal to the ordinary or extraordinary refractive index of the liquid crystal, is optically good, stable, and highly reliable, and has good reproducibility. It is preferable in that it can be easily formed on the upper surface.

The SiO _x N _y film has an advantage that not only the refractive index can be controlled by the ratio of x and y, but also the deterioration of optical characteristics such as absorption of light is less likely to occur at any ratio.

As a method for forming SiO _x N _y , a plasma CVD method is preferably used, and an O ₂ gas, N ₂ gas and N ₂ O gas are mixed at a predetermined ratio by using a conductive Si substrate as a target. The reactive DC sputtering method in which sputtering is performed in the atmosphere is more preferable in that the film formation rate is higher than that in the plasma CVD method.

Specifically, it is preferable to form the uneven portion as follows. On the surface of a polished substrate such as a glass substrate, plasma CVD method or reactive DC sputtering method should be used to make it close to both the ordinary refractive index of liquid crystal and the refractive index of transparent substrate (both have a refractive index of about 1.5). Then, the ratio of oxygen and nitrogen is adjusted to form a SiO _x N _y film.

After that, a photoresist is coated on the SiO _x N _y film by a spin coating method or the like, a photomask having a predetermined pattern is brought into close contact with the photoresist film, exposed to ultraviolet rays, and a photoresist development process is performed. To form a grid pattern of photoresist on the surface of the transparent substrate. Using the lattice pattern of the photoresist as a mask, CF ₄ , C ₂ F ₆ , C ₃ F
₈ , dry etching is performed using a gas such as CHF ₃ to form a grid-shaped uneven portion for an optically anisotropic diffraction grating having a depth of 1 to 2 μm and a pitch of 2 to 20 μm.

The difference between the refractive index of the transparent substrate and the refractive index of the SiO _x N _y film is preferably within 0.1 in order to prevent undesired reflection and the like at the interface.

As the liquid crystal used in the optically anisotropic diffraction grating of the present invention, known liquid crystals such as nematic liquid crystals and smectic liquid crystals used in liquid crystal display devices, liquid crystal compositions, polymer liquid crystals and the like can be preferably used.

Of this substrate, the inner surface of the substrate on the flat side is subjected to orientation treatment. In particular, it is preferable to provide an alignment film made of a polyimide film or the like and rub the top of the alignment film to form the alignment film. Since it is difficult to perform the alignment treatment on the substrate side provided with the uneven portion, the alignment treatment may be omitted.

By laminating a phase difference element such as a phase difference plate or a phase difference film functioning as a λ / 2 plate or a λ / 4 plate on the optical recording medium side with respect to this optical anisotropic diffraction grating. It is possible to make the polarization directions of the light going direction and the light returning direction orthogonal to each other and to function as an optically anisotropic diffraction grating. A material such as polycarbonate, polyvinyl alcohol, or polyarylate having a thickness of about several tens to several hundreds of μm can be preferably used for the retardation element.

At least one surface of this retardation element is covered with an optically transparent organic resin such as a photopolymer or a thermosetting epoxy resin, or a glass substrate, a plastic substrate or the like having a good flatness through the organic resin. Laminating and adhering the substrates is preferable because it has the advantages of reducing wavefront aberration and improving reliability.

This diffractive element has a second surface on the light source side of the substrate.
The diffraction grating may be formed, and in that case, tracking error detection by the three-beam method is preferable. The second diffraction grating is preferably formed by applying a photopolymer, a photoresist or the like and exposing it to a predetermined pattern, or by directly processing a transparent substrate by a dry etching method.

When the optical head device of the present invention is used for reading, a photodetector for detecting the reflected light from the optical recording medium is usually provided on the light source side, and the reflection on the light receiving surface of the detector. An in-plane curvature may be imparted to the pattern of the optically anisotropic diffraction grating or a distribution may be imparted to the lattice spacing so that the light is condensed in a desired beam (spot) shape. The pattern of the optical anisotropic diffraction grating can be a curvature distribution and a grating interval distribution designed by a computer device, and can be an optimum pattern for a focus error detection method such as SSD (spot size detection) method. As the photodetector, one using a semiconductor element such as a photodiode or a CCD element is preferable because it is small and lightweight and consumes less power.

As the light source of the present invention, a semiconductor laser, YA
Various solid and gas lasers such as 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 downsizing and weight reduction, continuous oscillation, maintenance and inspection. Further, by 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 unit and a short wavelength laser such as a blue laser is used, high density optical recording and reading can be performed.

The optical recording medium of the present invention is a medium capable of recording and / or reading information by light. Examples are CDs (compact discs) and CD-ROs.
Optical discs such as M, video discs, DVDs (digital video discs), magneto-optical discs, and phase change optical discs can be used.

By providing an antireflection film on the light entrance / exit surface of the diffractive element, light loss can be prevented. In that case, it is preferable to use an amorphous fluororesin as the antireflection film because the film can be formed at low cost without using an expensive and large-sized film forming device such as a vapor deposition device.

For example, the alignment treatment is performed so that the liquid crystal is aligned in a direction substantially parallel to the longitudinal direction of the grid-like concavo-convex portion (direction perpendicular to the paper surface in FIG. 1). In this case, for the P wave (polarization component whose polarization direction is parallel to the paper surface in FIG. 1) incident from the semiconductor laser of the light source 1, the liquid crystal and the convex portion have the same refractive index, that is, the optical anisotropic diffraction grating It is transparent to P waves. Therefore, the P wave does not undergo any change and directly enters the quarter-wave plate which is the phase difference element 3, changes into circularly polarized light, passes through the aspherical lens as the condenser lens, and emits almost 100% of the light. Reaches the recording surface of the optical recording medium.

That is, the return light to the semiconductor laser is very small, which is advantageous in terms of return light noise. Furthermore, the high transmittance in the forward path is advantageous in terms of cost in that the writing type optical disk device can perform writing with a semiconductor laser having a relatively low output during writing.

The reflected light reflected by the recording surface and returning through the aspherical lens again passes through the quarter-wave plate and is changed into an S wave having a polarization direction different by 90 °. When the S wave enters the optically anisotropic diffraction grating, it functions as a diffraction grating because the refractive index of the liquid crystal is different from that of the convex portion, and the + 1st order diffracted light has a maximum of about 40% and the −1st order diffracted light has a maximum of 40%.
A diffraction efficiency of about% can be obtained. When the detectors for detecting the + 1st-order diffracted light and the -1st-order diffracted light are arranged in only one of them, a reciprocating efficiency of 40% is obtained, and in the case where they are arranged in both, a total round-trip efficiency of 80% is obtained.

Further, when the uneven portion is formed in a slope (sawtooth shape), a reciprocating efficiency of approximately 70 to 90% and when it is formed in three steps is obtained, a reciprocating efficiency of approximately 81% is obtained.

[0043]

EXAMPLES Examples 1 to 6 of the present invention and comparative examples will be described below.
Shown in

Plasma CVD is performed on one surface of a glass substrate having a thickness of 1 mm, a size of 10 mm × 10 mm and a refractive index of 1.52.
A SiO _x N _y film having a thickness of 1.3 μm was formed by the method. At this time, x and y were about 1.8 and 0.17, respectively. Next, by a photolithography method and a dry etching method, an uneven portion for a diffraction grating having a rectangular cross section with a depth of 1.3 μm and a pitch of 6 μm was formed, and a polyimide alignment film was formed thereon.

A polyimide film was formed on the surface of another glass substrate as an alignment film for liquid crystal alignment, and a rubbing treatment for alignment was performed. The surface of the glass substrate on which the uneven portion is formed is opposed to the surface of the flat glass substrate on which the alignment film is formed, and the rubbing direction of the alignment film and the stripe direction of the uneven portion are the same. Then, two glass substrates were laminated.

At this time, the liquid crystal injection port was removed and the two glass substrates were sealed with an epoxy resin sealing material containing a spherical spacer. The diameter of this spherical spacer is Example 1: 4μ
m, Example 2: 6 μm, Example 3: 8 μm, Example 4:10 μm. After that, liquid crystal (product name BL009 manufactured by Merck & Co., nematic liquid crystal, ordinary light refractive index 1.5266, extraordinary light refractive index 1.8181) was vacuum-injected from the liquid crystal injection port.

A quarter-wave plate is laminated and adhered to the surface of the flat glass substrate opposite to the alignment film by a transparent adhesive, and a photopolymer for improving wavefront aberration is further formed thereon.
The glass substrate of 1 was laminated and adhered to produce a diffraction element. An antireflection film made of a dielectric multilayer film was applied to the entrance and exit of the light from the light source of the diffraction element.

Although the concave portion of the glass substrate provided with the uneven portion was roughly made to the extent that the surface of the glass substrate was exposed by the etching, the portion corresponding to the seal portion was prepared by removing the transparent film by dry etching. did.

The spacer diameter (μm) of the seal portion of these diffractive elements, the substrate gap (μm) at the convex portion, 25 ° C. and 60
Table 1 shows the wavefront aberration (mλrms) at ° C.

When the diffraction element of Example 1 is used, a semiconductor laser is used as a light source, and a P wave having a wavelength of 678 nm (polarized component parallel to the paper surface of FIG. 1) is incident, the transmittance of the P wave is about 9
7%. Further, the reflected light (circularly polarized light) from the optical disk is changed into an S wave (a polarized component perpendicular to the paper surface) by the 1/4 wavelength film, and this S wave is diffracted by the optical anisotropic diffraction grating, and the + 1st order diffracted light The diffraction efficiency is 34%, -1
The diffraction efficiency of the next-order diffracted light was 34%.

As a result, the forward efficiency was about 97% and the round trip efficiency was about 66% (± first-order diffracted light detection).

The diffractive elements of Examples 1 and 2 are highly reliable because the wavefront aberration at 60 ° C. is 30 mλrms or less and the output of the photodetector is stable against temperature changes. 1 has a wavefront aberration of 30 mλ at 60 ° C.
It was excellent at rms or less.

[0053]

[Table 1]

[0054]

INDUSTRIAL APPLICABILITY The present invention makes use of the features of an optical anisotropic diffraction grating using a liquid crystal, which are high in efficiency, excellent in mass productivity, and low in cost. A decrease in diffraction efficiency due to a change can be suppressed, and a stable optical head device in a wide temperature range can be easily 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 typical configuration 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 optical head device of the present invention.

[Explanation of symbols]

 1: Light source 2: Optical anisotropic diffraction grating 3: Phase difference element 4: Condensing lens 5: Optical recording medium 6: Photodetector

Continued Front Page (72) Inventor, Yuzuru Tanabe 1150, Hazawa-machi, Kanagawa-ku, Yokohama, Kanagawa Prefecture Asahi Glass Co., Ltd. Central Research Laboratory

Claims (3)

[Claims] 1. On an optical recording medium, light from a light source is passed through an optical anisotropic diffraction grating in which a grid-shaped uneven portion is formed on the surface of a transparent substrate, and the uneven portion is filled with liquid crystal having optical anisotropy. An optical head device for writing information and / or reading information by irradiating the optical head, wherein the substrate gap at the apex of the convex portion of the optically anisotropic diffraction grating is 5 μm or less. 2. The uneven portion of the optically anisotropic diffraction grating has SiO _x N.
The optical head device according to claim 1, wherein the optical head device is formed by _y (1 ≦ x ≦ 2, 0 <y ≦ 1.33). 3. The uneven portion of the optically anisotropic diffraction grating is formed by providing a transparent film having a refractive index almost equal to that of any of liquid crystals on the surface of the transparent substrate, and the transparent film is provided in the seal portion. The optical head device according to claim 1, wherein the optical head device is not formed.