JP2003014914A - Diffraction element and optical pickup device assembled with the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a diffraction element having high diffraction efficiency and high latitude for the design and to provide an optical pickup device in which the element is assembled. SOLUTION: The diffraction element is to be adhered with an adhesive layer to other optical elements. The diffraction element has a diffraction grating consisting of a dielectric material formed on a substrate, with the refractive index of the dielectric material higher than the refractive index of the adhesive which forms the adhesive layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diffraction element and an optical pickup device incorporating the same, and more specifically,
The present invention relates to a diffraction element used as a hologram diffraction element and an optical pickup device incorporating the same.

[0002]

2. Description of the Related Art A diffractive element can realize various optical functions and miniaturization by using a computer hologram method. Therefore, many optical pickup devices using a diffraction element have been proposed.

FIG. 5 shows an optical pickup device disclosed in Japanese Unexamined Patent Publication No. 8-329544. In FIG.
A Si substrate 102 is provided inside the package 101, and a semiconductor laser 103 as a light emitting element and photodetectors 104 and 105 as light receiving elements are provided on the Si substrate 102.
106 and a polarization separation prism 110 are arranged. A hologram diffractive element 108 is formed on the surface of the transparent substrate 107 facing the semiconductor laser 103, and a polarizing prism 109 is adhered to the opposite surface by an adhesive. The polarization prism 109 has a beam splitter 109a and a mirror 109b.

The light emitted from the semiconductor laser 103 passes through the hologram diffraction element 108 and the polarization prism 109, and is condensed on the magneto-optical recording medium 112 by the objective lens 111. The light reflected by the magneto-optical recording medium 112 is
After passing through the objective lens 111 again, a part of the beam is reflected by the beam splitter 109a of the polarizing prism 109 and is incident on the polarizing prism 110 via the mirror 109b. The rest is transmitted through the beam splitter 109a and is incident on the hologram diffractive element 108. To do.

The light incident on the polarization separation prism 110 is 2
It is separated into two different polarization components (P component and S component), the amount of received light of each component is detected by the photodetector 106, and the magneto-optical signal is reproduced based on this. The light incident on the hologram diffraction element 108 is diffracted by the hologram diffraction element 108, the amount of received light is detected by the photodetectors 104 and 105, and a servo signal is generated based on the detection result.

As a concrete structure of the hologram diffraction element 108, a relief type or a refractive index modulation type as shown in FIGS. 6 and 7 is generally used. In the relief type diffraction element 108 of FIG. 6, the convex portion 121 and the concave portion 122 are formed by engraving the surface of the substrate 120 by etching or by injection molding of plastic using a mold. The refractive index modulation type diffractive element 108 of FIG. 7 has a high refractive index portion 126 formed on the surface of a substrate 125 by an ion exchange method or the like. These diffractive elements 108 are usually used in contact with the boundary of the air layer, that is, with the diffraction grating surface exposed.

[0007]

[Problem to be Solved by the Invention] Hologram diffraction element 1
The grating pitch of 08 is proportional to the wavelength of the light source (semiconductor laser 103) and inversely proportional to the diffraction angle. Therefore, at the present time when the wavelength of the light source is shortened, it is necessary to dispose the hologram diffraction element 108 as far as possible from the semiconductor laser 103 and the photodetectors 104 and 105 to prevent the grating pitch from becoming too small. For example, in order to shorten the wavelength of the light source in FIG. 5, the polarizing prism 1 of the transparent substrate 107 is used.
It is conceivable to form the hologram diffractive element 108 on the surface opposed to 09 so that the hologram diffractive element 108 is separated from the semiconductor laser 103 and the photodetectors 104 and 105.

The conventional optical pickup device is a so-called 1
Although the beam method is used, if the beam method is changed to the three-beam method having excellent tracing performance, it is necessary to dispose a light splitting grating between the hologram diffraction element 108 and the light source. In this case, to avoid the complexity of the structure,
As described above, it is necessary to form the hologram diffraction element 108 on the surface of the transparent substrate 107 facing the polarization prism 109 and further to form the light splitting grating on the surface of the transparent substrate 107 facing the semiconductor laser 103.

For example, the relief type diffraction element 108 of FIG.
When is used as a hologram diffraction element, the concave portion 122 of the diffraction element 108 is filled with the optical adhesive used for adhesion with the polarization prism 109. Since the optical adhesive usually has a refractive index close to that of the transparent substrate 107, the convex portion 1
There is almost no phase difference between the light passing through 21 and the light passing through the recess 122. Therefore, the desired diffraction efficiency may not be obtained. In this case, in order to obtain the desired phase difference, it is necessary to form the concave portions 122 and the convex portions 121 as very deep concaves and convexes, which increases the time and labor required for producing the diffraction element.

Further, the refractive index modulation type diffractive element 108 of FIG.
In the case of using, the groove portion like the relief type diffraction element 108 in FIG. 6 is not buried in the adhesive even if it is bonded to another member, but the ion and the like inside the substrate 125 spread in the lateral direction. Therefore, it is difficult to manufacture a diffractive element having a grating pitch of 8 μm or less, and an optical pickup device using such a hologram diffractive element has great design restrictions.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a diffraction element having a high diffraction efficiency and a high degree of freedom in design, and an optical pickup device using the same. And

[0012]

According to the present invention, there is provided a diffraction element which is adhered to another optical element via an adhesive layer, the diffraction element including a diffraction grating made of a dielectric material formed on a substrate. A diffractive element is provided, characterized in that the refractive index of the dielectric is higher than the refractive index of the adhesive forming the adhesive layer.

That is, the phase difference between the light that has passed through the diffraction grating made of a dielectric material and the light that has not passed through this diffraction grating is
Since it can be set with a high degree of freedom based on the characteristics of the dielectric material forming the dielectric, the thickness of the dielectric, and the refractive index of the adhesive, high diffraction efficiency can be obtained.

In the present invention, the dielectric is TiO ₂ , Ta ₂ O ₅ ,
Since it is made of one or more substances selected from SiN, SiON, CeO ₂ , and ZrO ₂ , the diffraction grating can be easily formed by the sputtering method or the vapor deposition method, and the formed diffraction grating is It is chemically stable. By making the refractive index of the substrate and the refractive index of the adhesive substantially equal, reflection at the interface between the adhesive and the substrate is suppressed, and the light utilization efficiency is improved. Since the diffraction grating has the antireflection layer between the dielectric and the substrate and on the upper side of the dielectric opposite to the substrate, reflection at the interface between the diffraction grating and the substrate and at the interface between the adhesive layer and the diffraction grating is suppressed. , The light utilization efficiency is improved.

The diffraction grating is composed only of a dielectric material, and the reflected light generated at the boundary between the dielectric material and the substrate and the reflected light generated at the boundary between the dielectric material and the adhesive layer have opposite phases to each other. The refractive index n and the thickness t of the dielectric may be set. Specifically, the diffraction pattern is composed of only a dielectric, and the refractive index n of the dielectric and the thickness t of the dielectric are t = 2.
mλ / (2n) where m is an integer and λ is the wavelength of light, so that the diffractive element has a simple and easy-to-manufacture structure while maintaining the interface between the substrate and the diffraction grating or the adhesive layer and the diffraction grating. Since the reflection at the interface is suppressed, the light utilization efficiency can be improved while suppressing the manufacturing cost.

According to the present invention, it is provided with the light source, the condensing optical system for condensing the light emitted from the light source on the optical recording medium, and the photodetector for receiving the reflected light from the optical recording medium. ,
There is provided an optical pickup device in which the diffraction element of the present invention is arranged on an optical path from the light source to the condensing optical system or on an optical path from the condensing optical system to the photodetector. In the optical pickup device of the present invention, the degree of freedom in design is widened, and further miniaturization and higher performance are possible.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below based on the embodiments shown in the drawings. The present invention is not limited to this. FIG. 1 shows a schematic structure of a diffractive element according to an embodiment of the present invention.

Referring to FIG. 1, a diffractive element 10 according to the present invention.
Is a substrate 1, a diffraction grating 2 having a predetermined pattern formed on the substrate 1, and a diffraction grating 2 formed on the substrate 1 so as to cover the diffraction grating 2 and bond another optical element 3 to the substrate 1. The adhesive layer 4 is formed. The substrate 1 is not particularly limited, and can be usually used in this field, such as glass,
Examples thereof include optical plastics such as polymethylmethacrylate (PMMA) and polycarbonate (PC), thermally oxidized silicon, and quartz. When glass or optical plastic having a refractive index of about 1.5 is used as the substrate 1, there is almost no difference in the refractive index with the adhesive layer 4, and unnecessary reflection occurs at the interface with the adhesive layer 4. It is difficult and preferable.

The diffraction grating 2 is made of a dielectric material having a refractive index higher than that of the adhesive layer 4, for example, TiO ₂ (n = 2.
4), Ta ₂ O ₅ (n = 2.0), SiN (n = 2.0), Si
ON (n = 1.9), CeO ₂ (n = 2.4), ZrO ₂ (n =
It is composed of a dielectric material composed of 2.2). The thickness of the dielectric, that is, the thickness t of the diffraction grating 2 in this example is set so as to satisfy the relationship represented by the following equation. t = 2mλ / (2n) (1) where m is an integer, λ is the wavelength of light, and n is the refractive index of the dielectric.

The optical element 3 is not particularly limited, and can be generally used in this field. For example, it is used for audio and image files, text files, external memory devices for computers, etc. It constitutes an optical system of a magneto-optical pickup of an optical recording / reproducing apparatus capable of repeatedly recording and reproducing information by an optical effect. The adhesive layer 4 is made of glass or an optical adhesive having a refractive index almost equal to that of optical plastic (refractive index n is about 1.4 to 1.8). As such an optical adhesive, for example, "UV-21
00 "[trade name; manufactured by Daikin Industries, Ltd.] (n = 1.4
77), "BANI-100V" [trade name; manufactured by Maruzen Oil Co., Ltd.] (n = 1.755), "TFC7700"
[Product name: manufactured by Toshiba Silicone Co., Ltd.] (n = 1.40
4) etc. are mentioned.

The phase difference φ between the light passing through the portion where the diffraction grating 2 is formed and the light passing through the portion where the diffraction grating 2 is not formed determines the diffraction efficiency of the diffraction element 10.
The phase difference φ in the diffraction element 10 is expressed by the following equation. φ = 2πt (n-ng) / λ (2) where t is the thickness of the dielectric, that is, the diffraction grating 2, n is the refractive index of the diffraction grating 2, ng is the refractive index of the adhesive layer 4, and λ
Is the wavelength of light.

For example, if the dielectric material is Ta ₂ O ₅ , the refractive index n = 2.0, so that the refractive index ng = 1.
5 is about 0.5, and air (refractive index = 1.
0) and glass (refractive index ≈1.5) have almost the same difference in refractive index. Therefore, the thickness t of the diffraction grating 2 can be suppressed to a thickness substantially equal to that of the relief type diffraction element 108 of FIG. 6 used with a boundary with air. Since such an adhesive layer 4 firmly bonds the diffractive element 10 according to the present invention to the other optical element 3 and sufficiently secures the difference in refractive index between the adhesive layer 4 and the diffraction grating 2, it is desirable. The diffraction efficiency of can be obtained.

As a method of forming the diffraction grating 2, there is an etching method in which a dielectric is uniformly formed on the substrate 1 by using a sputtering method or the like and unnecessary portions are removed by etching to obtain a desired pattern. . Further, a lift-off method or the like may be applied in which a negative pattern is formed on the substrate 1 with a resist, a dielectric is formed thereon by a sputtering method or an evaporation method, and then an unnecessary portion is removed together with the resist. In each of these methods, the width of the line and the space between the lines can be patterned to 0.5 μm or less,
It is possible to fabricate a diffraction grating with a pitch of up to 1 μm.

Further, TiO ₂ and Ta ₂ O forming the above-mentioned dielectric material
₅ , SiN, SiON, CeO ₂ , and ZrO ₂ can be formed into a thin film by a sputtering method or a vapor deposition method, and are chemically stable and industrially useful.

FIG. 2 is a partially enlarged view of the diffraction grating 2.
As shown in FIG. 2, when light is transmitted from the lower side to the upper side in the drawing with respect to the diffraction element 2 of the present invention, the diffraction grating 2 and the substrate 1
Fresnel reflection occurs at the interface with. At that time, the amplitude reflectance Rs is n for the refractive index of the dielectric, ns for the substrate 1.
Rs = (ns−n) / (ns + n) (3) where the refractive index of the adhesive layer 4 is ng.

Similarly, Fresnel reflection occurs at the interface between the diffraction grating 2 and the adhesive layer 4. Amplitude reflectance Rg at that time
Is given by Rg = (n-ng) / (ng + n) (4).

Here, if the refractive index of the substrate 1 and the refractive index of the adhesive layer 4 are both set to approximately the same value of about 1.5, the amplitude reflectances Rs and Rg are Rs≈-Rg (5) Becomes Here, since the thickness t of the diffraction grating 2 satisfies the expression (1), the reflected lights from the two interfaces have almost the same intensity and have opposite phases, and thus cancel each other out.
As a result, reflection at the diffraction grating 2 is suppressed, light utilization efficiency is improved, and ghost suppression is realized.

The wavelength λ of light and the refractive index n of the adhesive layer 4 are
Given g, phase difference φ and integer m, equations (1) and (2) result in a one-element linear equation of n, and t and n are uniquely determined.

FIG. 3 shows a schematic structure of a diffractive element according to another embodiment of the present invention. In FIG. 3, in the diffractive element 20 according to the present invention, the diffraction grating 2 includes the antireflection layers 6a and 6a.
Although it is different from the diffractive element 20 in that it is composed of b and the dielectric 5, the other configurations are common. The antireflection layer 6 a is provided between the substrate 1 and the dielectric 5, and the antireflection layer 6 b is provided on the dielectric 5.

Diffraction grating 2 having antireflection layers 6a and 6b
As a method of forming the film, an antireflection material such as a mixture of SiO ₂ (n = 1.5) and SiN ₂ is uniformly formed on the substrate 1 by using a technique such as a sputtering method, and then, The dielectric 5 is uniformly formed on the substrate 1 by using a sputtering method or the like,
Further, as in the case of forming the antireflection layer 6a on the dielectric pattern, after forming the antireflection layer 6b, an unnecessary portion is removed by etching to form a desired diffraction grating 2. The diffraction grating 2 can be formed by applying the above-mentioned sputtering method, vapor deposition method, lift-off method, or the like, similarly to the diffraction element 10.

In the diffractive element 20, the refractive index nas of the antireflection layer 6a and the thickness tas thereof are as follows, where n is the refractive index of the dielectric 5, the refractive index of the substrate 1 is ns, and the wavelength of light is λ. n · ns) ^1/2 (6) tas = λ / (4 · nas) (7).

Similarly, the refractive index nag of the antireflection layer 6b and its thickness tag are nag = (n, where n is the refractive index of the dielectric material 5, ng is the refractive index of the adhesive layer 4, and λ is the wavelength of light. .Ng) ^1/2 (8) tag = λ / (4 · nag) (9).

The phase difference φ between the light transmitted through the portion where the diffraction grating 2 is formed and the light transmitted through the portion where the diffraction grating 2 is not formed determines the diffraction efficiency of the diffraction element 20. The phase difference φ of the diffractive element 20 is φ = 2π {t (n-ng) where t is the thickness of the dielectric 5, n is the refractive index, ng is the refractive index of the adhesive layer 4, and λ is the wavelength of light. ) + Tas (nas-ng) + tag (nag-ng)} / λ (10).

In the diffractive element 20, the antireflection inviting layers 6a, 6
By providing b, it is possible to suppress the reflection loss due to the difference in the refractive index between the dielectric 5 and the light transmissive substrate 1 and the reflection loss due to the difference in the refractive index between the dielectric 5 and the adhesive layer 4. The light utilization efficiency is improved and ghosts due to reflected light can be suppressed.

FIG. 4 shows the diffractive element 1 according to the invention.
An optical pickup device using 0 (or 20) is shown.
In FIG. 4, in an optical pickup device 50, a Si substrate 11 is provided inside a package 51, on which a semiconductor laser 12 as a light emitting element, photodetectors 13, 14, 15 as a light receiving element, and polarization separation. Prism 16
Are arranged. The diffraction grating 10 of the diffractive element 10 (or 20) is formed on the main surface of the substrate 1 facing the polarizing prism 19, and the substrate 1 is bonded to the polarizing prism 19 via an adhesive layer 4. Since the specific structure of the diffraction grating 2 has been described above, the description thereof will be omitted.

The light emitted from the semiconductor laser 12 passes through the diffraction element 10 (20) and the polarization prism 19, and is condensed on the magneto-optical recording medium 21 by the objective lens 17.
The light reflected by the magneto-optical recording medium 21 passes through the objective lens 17 again, and the beam splitter 1 of the polarization prism 19
A part of the beam is reflected by 9a, passes through a mirror 19b and is incident on a polarization splitting prism 16 and the rest is a beam splitter 19a.
The light passes through a and enters the diffraction grating 2. The light incident on the polarization splitting prism 16 is split into two different polarization components,
The amount of received light of each component is detected by the photodetector 15, and the magneto-optical signal is reproduced based on this. The light incident on the diffractive element 10 (20) is diffracted by the diffractive element 10 (20), the amount of received light is detected by the photodetectors 13 and 14, and a servo signal is generated based on this.

In the optical pickup device 50 according to the present invention, the distance between the diffraction grating 2 and the semiconductor laser 12 or the distance between the diffraction grating 2 and the photodetectors 13 and 14 can be separated by the thickness of the substrate 1. , The diffraction angle can be reduced. Therefore, the pitch of the diffraction grating 2 can be kept wide even if the wavelength of the light source becomes short. Further, even if the optical pickup device according to the present invention is changed to the three-beam method having excellent tracing performance, a desired light splitting grating is simply provided on the surface of the diffraction element 10 (20) facing the semiconductor laser 12. Because trace performance can be obtained,
High performance can be achieved without increasing the number of parts.

[0038]

In the diffractive element of the present invention, the phase difference between the light that has passed through the diffraction grating made of a dielectric material and the light that has not passed through this diffraction grating is determined by the characteristics of the dielectric material constituting the dielectric material and the dielectric material. Since it can be set with a high degree of freedom based on the thickness of the adhesive and the refractive index of the adhesive, a high diffraction efficiency can be obtained. Since the diffraction grating is sealed with an adhesive, it is free from contamination such as dust. In the optical pickup device using the diffraction element of the present invention, the degree of freedom in design is expanded,
Further miniaturization and higher performance are possible.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of a diffractive element according to the present invention.

FIG. 2 is a partially enlarged view of the diffraction element shown in FIG.

FIG. 3 is a sectional view showing another embodiment of the diffractive element according to the present invention.

FIG. 4 is a sectional view of an optical pickup device using the diffraction element according to the present invention.

FIG. 5 is a sectional view of an optical pickup device according to a conventional example.

FIG. 6 is a cross-sectional view of a relief type diffraction element according to a conventional example.

FIG. 7 is a cross-sectional view of a conventional refractive index modulation type diffraction element.

[Explanation of symbols]

1 substrate 2 diffraction grating 3 Other optical elements 4 Adhesive layer (adhesive) 5 Dielectric 6a Antireflection layer 6b Antireflection layer 10 Diffraction element 19 Polarizing prism (other optical elements) 20 diffraction element 50 Optical pickup device

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 2H049 AA03 AA13 AA33 AA37 AA43                       AA45 AA48 AA51 AA57 AA64                       AA68                 2K009 AA04 BB06 CC42 DD03 DD04                       DD12                 5D119 AA38 AA43 JA03 JA13 JA22                       JA24 JA46

Claims (7)

[Claims] 1. A diffractive element bonded to another optical element via an adhesive layer, wherein the diffractive element has a diffraction grating made of a dielectric material formed on a substrate, and the refractive index of the dielectric material is bonded. A diffractive element having a refractive index higher than that of an adhesive forming the agent layer. 2. The dielectric is TiO ₂ , Ta ₂ O ₅ , SiN, SiON,
The diffractive element according to claim 1, further comprising one or more substances selected from CeO ₂ and ZrO ₂ . 3. The diffractive element according to claim 1, wherein the refractive index of the substrate and the refractive index of the adhesive are substantially equal to each other. 4. The diffraction element according to claim 1, wherein the diffraction grating has an antireflection layer between the dielectric and the substrate and on the upper side of the dielectric opposite to the substrate. 5. The diffraction grating is composed only of a dielectric material, and the reflected light generated at the boundary between the dielectric material and the substrate and the reflected light generated at the boundary between the dielectric material and the adhesive layer have mutually opposite phases. The diffraction element according to claim 1, wherein the refractive index of the dielectric and the thickness of the dielectric are set. 6. The refractive index n of the dielectric and the thickness t of the dielectric are: t = 2mλ / (2n) where m is an integer and λ is the wavelength of light. The diffractive element according to one of 7. A light source, a condensing optical system for condensing light emitted from the light source onto an optical recording medium, and a photodetector for receiving reflected light from the optical recording medium. The optical path leading to the condensing optical system or the optical path reaching the photodetector from the condensing optical system.
An optical pickup device in which the diffractive element according to item 1 is arranged.