JP4218240B2

JP4218240B2 - Optical head device

Info

Publication number: JP4218240B2
Application number: JP2001383198A
Authority: JP
Inventors: 光生大澤; 浩一村田
Original assignee: Asahi Glass Co Ltd
Current assignee: AGC Inc
Priority date: 2001-12-17
Filing date: 2001-12-17
Publication date: 2009-02-04
Anticipated expiration: 2021-12-17
Also published as: JP2003185819A

Description

【０００１】
【発明の属する技術分野】
本発明は、回折素子および光ヘッド装置に関し、特に光ヘッド装置については光ディスクの情報の記録および再生を行う光ヘッド装置に関する。
【０００２】
【従来の技術】
光を回折する回折格子やホログラム素子は、通常ガラスやプラスチックの基板表面に断面形状が周期的な凹凸格子を設けることにより作製する。このような回折素子では波長に依存する回折効率はほぼ格子の深さのみで決まる。このため、複数の波長の光に対して回折効率を自由に制御することができない問題があった。
【０００３】
また、光ディスクの情報の再生や記録を行う光ヘッド装置においては、光ディスクの多様化により用いるレーザ光の波長は、７９０ｎｍ（ＣＤ系）、６５０ｎｍ（ＤＶＤ系）および４０５ｎｍ（次世代ＤＶＤ系）３種類の波長が検討されている。これら光ディスクの情報を同一の光ヘッド装置で再生するためには複数の波長の半導体レーザを搭載する必要がある。
【０００４】
一方光ヘッド装置において用いられる回折素子としては、トラッキング用の３ビーム発生用の回折素子、光ディスクにより反射した戻り光を光検出器に導くために光路を変えるホログラム素子など、さまざまな用途で用いられる。
【０００５】
このような複数波長の光で用いる光ヘッド装置において、各波長により回折効率を最適化することのニーズは多い。しかしながら、前述のように通常の回折素子では回折効率の波長依存性は、格子の深さのみでほぼ決まるため所望の回折効率を各波長ごとに得ることは困難であった。
【０００６】
【発明が解決しようとする課題】
本発明の目的は、上記従来技術の欠点を解決し、回折効率の波長依存性を制御できる回折素子を提供することにある。また、本発明の回折素子を複数波長を用いた光ヘッド装置に用いることにより上記従来技術の欠点を解決し、設計の自由度を広げた光ヘッド装置を提供することにある。
【０００７】
【課題を解決するための手段】
本発明は、光源と、前記光源からの出射光を光記録媒体へ集光するための対物レンズと、集光され光記録媒体によって反射された出射光を検出するための光検出器とを備える光ヘッド装置において、前記光源と前記対物レンズとの間の光路中に回折素子が配置され、前記回折素子は、透明基板上に形成された周期的な凹凸部を有する回折格子を含み、前記回折格子の前記凹凸部の凸部は、前記透明基板面に平行に配された光学多層膜を備えるとともに、凹部は固体または液体からなる充填材によって充填されており、前記回折格子は、前記透明基板面に垂直に入射する前記光源からの出射光の波長が７９０ｎｍにおいて１次回折効率が発現せずにほぼ透過し、波長が６４０〜６７０ｎｍにおいて１次回折効率が約６％となる光ヘッド装置を提供する。
【０００８】
また、前記光学多層膜が、交互に重ねられたＳｉＯ_２膜およびＴｉＯ_２膜を備えている上記の光ヘッド装置を提供する。
【００１０】
【発明の実施の形態】
本発明の回折素子の一例の概念的断面図を図１に示す。図１において、１は光学多層膜であり、透明基板３上に基板面に平行に形成された複数の層からなり凸部を構成する。図１では簡単のために２種の材料による５層の膜となっているが、膜の材料、種類および層数は所望の回折効率が得られるように設計すればよい。２は透明基板とは屈折率の異なる充填材であり、透明基板３上の光学多層膜１による凸部の間すなわち凹部を充填してもよいし、凹部のみならず凸部にも充填してもよい（図２参照。符号の意味は、図１と同じ。）。
【００１１】
充填材は固体または液体からなり、例えばＡｕ、Ａｌなどの金属材料、ＳｉＯ_２、ＺｒＯ_２、ＭｇＦ_２などの酸化物およびフッ化物材料、Ｈ_２Ｏなどの無機物材料や、アクリル樹脂、エポキシ樹脂などの有機物材料、さらに、液晶などの複屈折性材料などが使用できる。また、透明基板としては、ガラス、プラスチックなどの基板を使用できる。
【００１２】
光学多層膜の材料としては、ＳｉＯ_２、ＳｉＯＮ、ＺｒＯ_２，ＴｉＯ_２、Ａｌ_２Ｏ_３、ＣｅＯ_２、ＭｇＯ、ＭｇＦ_２などの無機物材料を用いることができる。また、有機物材料であってもよい。多層膜の形成方法としては、真空蒸着法、スパッタ法などを挙げることができる。
【００１３】
また、透過波面収差を改善するために、図３および図４に示すようにガラスまたは樹脂製の表面がフラットなカバー４を回折素子の充填材側に設けることが好ましい。
【００１４】
図５には、本発明の回折素子を透明基板３の表面と裏面との両面に形成した例を示す。１１は第２の光学多層膜であり、１２は第２の充填材である。ここでは、両面に形成する回折素子の回折効率の波長依存性や、格子ピッチなどを変化させることによる回折方向の調整など、設計の自由度を増加できて好ましい。
【００１５】
また、この回折素子と他の光学素子とを積層して一体化してもよい。他の光学素子としては、例えば、波長板、通常の回折格子、偏光性回折格子、液晶を用いた光学素子、位相制御素子などである。これらの光学素子と本発明の回折素子とを一体化して一体化回折素子とすることにより、ひとつの一体化回折素子でありながら複数の機能を有するとともに、光ヘッド装置に組み込んだ場合光学部品点数を削減できて好ましい。
【００１６】
本発明の回折素子における光学多層膜の構成材料の第１設計例を表１に示す。
【００１７】
【表１】

【００１８】
例えばガラスからなる透明基板上に、最下層と最上層のＡｌ_２Ｏ_３膜を除いて、ＳｉＯ_２膜とＴｉＯ_２膜とが１３層交互に重ねられ、Ａｌ_２Ｏ_３膜を加えると合計１５層の光学多層膜とされ、光学多層膜は周期的な凹凸形状に加工されて、格子が作製した後、屈折率１．９３の充填材により格子の凹部が充填され回折素子が構成された（図１を参照）。
【００１９】
この回折素子における１次の回折効率の波長依存性の例を図６に示す。図６には、比較例として従来の表面凹凸型の回折格子の１次の回折効率も示した。このように、本発明の光学多層膜により凹凸状の回折格子を形成したものは、波長３５０ｎｍから９００ｎｍまでの光に対してほぼ理論限界の４０％近い回折効率を示すことがわかる。なお、表１に用いたＳｉＯ_２膜、ＴｉＯ_２膜、Ａｌ_２Ｏ_３膜の屈折率は、それぞれ異なり順に１．４７、２．２７、１．５６であった。ただし成膜条件により屈折率が設計値と若干異なる場合には、膜厚を調整する。
【００２０】
本発明の回折素子における光学多層膜の構成材料の第２設計例を表２に示す。
【００２１】
【表２】

【００２２】
上記と同様例えばガラスからなる透明基板上に、ＳｉＯ_２膜とＴｉＯ_２膜とが交互に重ねられ２９層の光学多層膜とされ、光学多層膜は周期的な凹凸形状に加工されて、格子を作製した後、屈折率１．５１の充填材により格子の凹部が充填され回折素子が構成された（図２を参照）。
【００２３】
この回折素子における１次の回折効率の波長依存性の例を図７に示す。図７には比較例として従来の表面凹凸型の回折格子の回折効率も示した。このように、本発明の光学多層膜に凹凸状の回折格子を形成したものは、波長７９０ｎｍの光はほとんど回折せず透過し、波長６５０ｎｍの光は約６％回折することが分かる。さらに、６００ｎｍから６７０ｎｍまでの広い波長帯域にわたり回折効率の変化がほとんどないことが分かる。なお、表２に用いたＳｉＯ_２膜、ＴｉＯ_２膜の屈折率は、それぞれ異なり順に１．４８、２．３２であった。ただし上記と同様、成膜条件により屈折率が設計と若干異なる場合には、膜厚を調整する。
【００２４】
上記の回折素子を、例えばＤＶＤ系（波長６５０ｎｍ）とＣＤ系（波長７９０ｎｍ）光ディスク用の光源を有する光ヘッド装置に搭載した場合について説明する。例えば、図８に示すようにコリメートレンズ２４と光源２１、２２との間に、本発明の回折素子２３を配置する。
【００２５】
この回折素子を図５を参考にして説明する。例えば第１の光学多層膜１および第１の充填材２よりなる第１の回折部分（透明基板３の上側）は、波長６５０ｎｍの光を約６％回折し、波長７９０ｎｍの光をほぼ１００％透過するように設計し、第２の多層膜１１および第２の充填材１２よりなる第２の回折部分（透明基板３の下側）は、波長７９０ｎｍの光を約６％回折し、波長６５０ｎｍの光をほぼ１００％透過するように設計する。これにより、回折素子２３は各波長の３ビームを効率よく発生できる。
【００２６】
回折素子２３によって３ビームに分割された各光源からの光は、コリメートレンズ２４、ビームスプリッタ２５を透過し、対物レンズ２６により光記録媒体２７に集光される。光記録媒体２７にて反射された光は、対物レンズ２６、ビームスプリッタ２５の順にもどり光検出光学系２８により検出される。回折素子２３により得られた各波長の光の３ビームは、トラッキングに使用できる。
【００２７】
また、光通信に使用される波長９５０ｎｍ〜１７００ｎｍの近赤外光に対しても本発明の回折素子を使用でき、特に波長１５５０ｎｍ帯の光を使用する波長分割多重通信の構成部品として使用できる。例えば広い波長帯域で回折効率が一定の回折素子を使用したビームスプリッタや、回折効率の波長分散を調整したゲインイコライザなどを挙げることができる。
【００２８】
【実施例】
本実施例を図９に基づいて説明する。透明基板である石英ガラス基板３１の一方の表面にフォトリソグラフィとエッチングの技術によりレリーフ型の第１の回折格子３２を作製した。格子ピッチは２０μｍ、デューティ比は１／２、深さは約１．４μｍとし、波長６５０ｎｍの光は回折せず透過し、波長７８０ｎｍの光は、１０％程度回折するようにした。
【００２９】
また、レリーフ型の回折格子の凹凸部上に真空蒸着法により波長６５０ｎｍおよび波長７９０ｎｍの光に対する反射防止膜を形成した。このレリーフ型の回折格子は、波長６５０ｎｍの光をほとんど回折せず０次光の回折効率（すなわち透過率）が約９０％、一方波長７９０ｎｍの光を１次回折光として約１０％回折し、０次光の回折効率が約５５％であった。
【００３０】
この石英ガラス基板３１のレリーフ型の回折格子３２を作製した面とは反対側の面には、真空蒸着法により表２に示した光学多層膜である誘電体多層膜を成膜した。上記と同様にして、フォトリソグラフィとエッチングの技術により、誘電体多層膜凸部３３を有する第２の回折格子を作製した。格子ピッチは２０μｍ、デューティ比は１／２とし、エッチングは基板の石英ガラスが露出した段階で止め、誘電体多層膜のみ凹凸になるようにした。
【００３１】
ガラス基板の一方の面に、真空蒸着法により波長６５０ｎｍおよび波長７９０ｎｍの光に対する反射防止膜を形成してカバーガラス３５を作製した。第２の回折格子の凹部に屈折率１．５１のアクリル系の接着剤３４を充填材として充填し、上記カバーガラス３５をこの上に重ねて接着剤３４によりカバーガラス３５を接着して、図９に示したような回折素子を作製した。
【００３２】
この回折素子の透過率および回折効率を調べたところ、波長６５０ｎｍの光に対しては、０次光の回折効率が約７０％、１次光の回折効率が約６％、一方波長７９０ｎｍの光に対しては０次光の回折効率が約５４％、１次光の回折効率が約９％であった。そして、いずれの波長の光についても効率よく３ビームを得ることができた。
【００３３】
この回折素子を、図８に示した光ヘッド装置中の回折素子２３の位置に搭載した。光源２１として、ＤＶＤ用の光源である波長６５０ｎｍのＬＤ、光源２２としてＣＤ用の光源である波長７９０ｎｍのＬＤを使用したところ、７９０ｎｍの光、および６５０ｎｍの光を効率よく各々３ビームに分けることができ、さらに光ヘッド装置の光利用効率を高めことができた。そして、この３ビームは光ヘッド装置のトラッキングに使用され、光ディスクからの情報の再生時に良質の再生特性が得られた。
【００３４】
【発明の効果】
以上説明したように、本発明の回折素子は透明基板上に周期的に重ねた酸化物などの光学多層膜を用いて凹凸状の回折格子を形成したものであり、この回折素子は回折効率の波長依存性をほとんど一定にするなど、回折効率を制御できる。
【００３５】
また本発明の回折素子を複数波長の光を用いる光ヘッド装置に搭載することで設計自由度が高く、かつ光利用効率の高い光ヘッド装置を得ることができる。
【図面の簡単な説明】
【図１】本発明の回折素子の一例を示す側方断面図。
【図２】本発明の回折素子の他の例を示す側方断面図。
【図３】図１の回折素子に透明基板を重ねた、本発明の回折格子を示す側方断面図。
【図４】図２の回折素子に透明基板を重ねた、本発明の回折格子を示す側方断面図。
【図５】本発明の回折素子の別の例を示す側方断面図。
【図６】表１に示した光学多層膜の設計例により得られる回折素子における、1次の回折効率の波長依存性を示すグラフ。
【図７】表２に示した光学多層膜の設計例により得られる回折素子における、1次の回折効率の波長依存性を示すグラフ。
【図８】本発明の回折素子を搭載した光ヘッド装置一例を示す概略的側面図。
【図９】実施例における回折素子の概略的側方断面図。
【符号の説明】
１、１１：光学多層膜
２、１２：充填材
３：透明基板
４：カバー
２１、２２：光源
２３：回折素子
２４：コリメートレンズ
２５：ビームスプリッタ
２６：対物レンズ
２７：光記録媒体
２８：光検出光学系
３１：石英ガラス基板
３２：回折格子
３３：誘電体多層膜凸部
３４：接着剤
３５：カバーガラス[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a diffraction element and an optical head device, and more particularly to an optical head device that records and reproduces information on an optical disk.
[0002]
[Prior art]
A diffraction grating or a hologram element for diffracting light, the usual substrate surface of glass or plastic sectional shape made work by providing a periodic concavo-convex grid. In such a diffraction element, the diffraction efficiency depending on the wavelength is almost determined only by the depth of the grating. For this reason, there has been a problem that the diffraction efficiency cannot be freely controlled for light of a plurality of wavelengths.
[0003]
Further, in an optical head device for reproducing and recording information on an optical disk, the wavelengths of laser light used for diversification of optical disks are 790 nm (CD system), 650 nm (DVD system), and 405 nm (next-generation DVD system). Wavelengths of In order to reproduce the information on these optical disks with the same optical head device, it is necessary to mount semiconductor lasers having a plurality of wavelengths.
[0004]
On the other hand, the diffraction element used in the optical head device is used for various purposes such as a diffraction element for generating three beams for tracking and a hologram element for changing the optical path in order to guide the return light reflected by the optical disk to the photodetector. .
[0005]
In such an optical head device used for light of a plurality of wavelengths, there are many needs for optimizing the diffraction efficiency for each wavelength. However, as described above, in a normal diffraction element, the wavelength dependence of the diffraction efficiency is almost determined only by the grating depth, and it is difficult to obtain a desired diffraction efficiency for each wavelength.
[0006]
[Problems to be solved by the invention]
An object of the present invention is to provide a diffractive element that solves the above-mentioned drawbacks of the prior art and can control the wavelength dependence of diffraction efficiency. Another object of the present invention is to provide an optical head device that solves the above-mentioned drawbacks of the prior art by using the diffractive element of the present invention in an optical head device using a plurality of wavelengths and widens the degree of design freedom.
[0007]
[Means for Solving the Problems]
The present invention comprises a light source, an objective lens for condensing the light emitted from the light source onto an optical recording medium, and a photodetector for detecting the light emitted and reflected by the optical recording medium. In the optical head device, a diffractive element is disposed in an optical path between the light source and the objective lens, and the diffractive element includes a diffraction grating having a periodic uneven portion formed on a transparent substrate, and the diffractive element The convex part of the concavo-convex part of the grating includes an optical multilayer film arranged in parallel to the transparent substrate surface, the concave part is filled with a filler made of solid or liquid, and the diffraction grating is the transparent substrate. wavelength of light emitted from said light source incident perpendicular to the plane is substantially transmits without expression 1-order diffraction efficiency at 790 nm, Oite first-order diffraction efficiency on the wavelength is 6 4 0~670n m is about 6% Optical head device Subjected to.
[0008]
In addition, the above-mentioned optical head device is provided in which the optical multilayer film includes alternately stacked SiO ₂ films and TiO ₂ films.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
A conceptual cross-sectional view of an example of the diffraction element of the present invention is shown in FIG. In FIG. 1, reference numeral 1 denotes an optical multilayer film, which comprises a plurality of layers formed on a transparent substrate 3 in parallel with the substrate surface, and constitutes a convex portion. In FIG. 1, for simplicity, the film has five layers of two materials, but the material, type, and number of layers of the film may be designed so as to obtain a desired diffraction efficiency. 2 is a filler having a refractive index different from that of the transparent substrate, and may be filled between the convex portions by the optical multilayer film 1 on the transparent substrate 3, that is, filled with the concave portions. (See FIG. 2. The meaning of the reference numerals is the same as in FIG. 1.)
[0011]
The filler is made of solid or liquid, for example, metal materials such as Au and Al, oxide and fluoride materials such as SiO ₂ , ZrO ₂ and MgF ₂ , inorganic materials such as H ₂ O, acrylic resins, epoxy resins, etc. Organic materials, and birefringent materials such as liquid crystals can be used. As the transparent substrate, a substrate such as glass or plastic can be used.
[0012]
As the material of the optical multilayer film, inorganic materials such as SiO ₂ , SiON, ZrO ₂ , TiO ₂ , Al ₂ O ₃ , CeO ₂ , MgO, and MgF ₂ can be used. Moreover, an organic material may be sufficient. Examples of the method for forming the multilayer film include a vacuum deposition method and a sputtering method.
[0013]
In order to improve the transmitted wavefront aberration, it is preferable to provide a cover 4 having a flat surface made of glass or resin on the filler side of the diffraction element as shown in FIGS.
[0014]
FIG. 5 shows an example in which the diffraction element of the present invention is formed on both the front surface and the back surface of the transparent substrate 3. 11 is a 2nd optical multilayer film, 12 is a 2nd filler. Here, it is preferable because the degree of freedom in design can be increased, such as the wavelength dependency of the diffraction efficiency of the diffraction elements formed on both sides, and adjustment of the diffraction direction by changing the grating pitch.
[0015]
Moreover, this diffraction element and another optical element may be laminated and integrated. Examples of other optical elements include a wave plate, a normal diffraction grating, a polarizing diffraction grating, an optical element using liquid crystal, a phase control element, and the like. By integrating these optical elements and the diffractive element of the present invention into an integrated diffractive element, a single integrated diffractive element has multiple functions and the number of optical components when incorporated in an optical head device. This is preferable.
[0016]
A first design example of the constituent material of the light Gakuo layer film in the diffraction element of the present invention shown in Table 1.
[0017]
[Table 1]

[0018]
For example, 13 layers of SiO ₂ films and TiO ₂ films are alternately stacked on a transparent substrate made of glass except for the lowermost layer and the uppermost Al ₂ O ₃ film, and when an Al ₂ O ₃ film is added, a total of 15 After the optical multilayer film is processed into a periodic uneven shape and a grating is produced, the concave portion of the grating is filled with a filler having a refractive index of 1.93 to form a diffraction element ( (See FIG. 1).
[0019]
An example of the wavelength dependence of the first-order diffraction efficiency in this diffraction element is shown in FIG. FIG. 6 also shows the first-order diffraction efficiency of a conventional surface uneven-type diffraction grating as a comparative example. Thus, it can be seen that the concavo-convex diffraction grating formed by the optical multilayer film of the present invention exhibits a diffraction efficiency close to the theoretical limit of 40% for light with a wavelength of 350 nm to 900 nm. The refractive indexes of the SiO ₂ film, the TiO ₂ film, and the Al ₂ O ₃ film used in Table 1 were 1.47, 2.27, and 1.56, respectively, in order. However, if the refractive index is slightly different from the designed value depending on the film forming conditions, the film thickness is adjusted.
[0020]
The second design example of the constituent material of the light Gakuo layer film in the diffraction element of the present invention shown in Table 2.
[0021]
[Table 2]

[0022]
As described above, SiO ₂ films and TiO ₂ films are alternately stacked on a transparent substrate made of glass, for example, to form an 29-layer optical multilayer film. The optical multilayer film is processed into a periodic concavo-convex shape to form a lattice . After fabrication, the grating recesses were filled with a filler having a refractive index of 1.51 to form a diffraction element (see FIG. 2).
[0023]
An example of the wavelength dependence of the first-order diffraction efficiency in this diffraction element is shown in FIG. FIG. 7 also shows the diffraction efficiency of a conventional surface uneven diffraction grating as a comparative example. Thus, it can be seen that when the concave and convex diffraction grating is formed on the optical multilayer film of the present invention, light with a wavelength of 790 nm is transmitted without being diffracted, and light with a wavelength of 650 nm is diffracted by about 6%. Furthermore, it can be seen that there is almost no change in diffraction efficiency over a wide wavelength band from 600 nm to 670 nm. Note that the refractive indexes of the SiO ₂ film and the TiO ₂ film used in Table 2 were 1.48 and 2.32, respectively, in different order. However, similarly to the above, when the refractive index is slightly different from the design depending on the film forming conditions, the film thickness is adjusted.
[0024]
The case where the above-described diffraction element is mounted on an optical head device having a light source for DVD (wavelength 650 nm) and CD (wavelength 790 nm) optical disks, for example, will be described. For example, as shown in FIG. 8, the diffraction element 23 of the present invention is disposed between the collimating lens 24 and the

light sources

21 and 22.
[0025]
This diffraction element will be described with reference to FIG. For example, the first diffractive portion (upper side of the transparent substrate 3) made of the first optical multilayer film 1 and the first filler 2 diffracts light having a wavelength of 650 nm by about 6%, and light having a wavelength of 790 nm is almost 100%. The second diffractive portion (the lower side of the transparent substrate 3) that is designed to transmit light and includes the second multilayer film 11 and the second filler 12 diffracts light having a wavelength of 790 nm by about 6%, and has a wavelength of 650 nm. Is designed to transmit almost 100% of the light. Thereby, the diffraction element 23 can generate | occur | produce three beams of each wavelength efficiently.
[0026]
The light from each light source divided into three beams by the diffraction element 23 passes through the collimating lens 24 and the beam splitter 25 and is condensed on the optical recording medium 27 by the objective lens 26. The light reflected by the optical recording medium 27 returns in the order of the objective lens 26 and the beam splitter 25 and is detected by the light detection optical system 28. Three beams of light of each wavelength obtained by the diffraction element 23 can be used for tracking.
[0027]
Further, the diffraction element of the present invention can also be used for near infrared light with a wavelength of 950 nm to 1700 nm used for optical communication, and in particular, it can be used as a component for wavelength division multiplex communication using light with a wavelength of 1550 nm band. For example, a beam splitter using a diffraction element having a constant diffraction efficiency in a wide wavelength band, a gain equalizer that adjusts the wavelength dispersion of the diffraction efficiency, and the like can be given.
[0028]
【Example】
This embodiment will be described with reference to FIG. To prepare a first diffraction grating 32 of the relief by one surface to the photolithography and edge switch ing technology quartz glass substrate 31 is a transparent substrate. The grating pitch was 20 μm, the duty ratio was ½, the depth was about 1.4 μm, light with a wavelength of 650 nm was transmitted without being diffracted, and light with a wavelength of 780 nm was diffracted by about 10%.
[0029]
Further, an antireflection film for light having a wavelength of 650 nm and a wavelength of 790 nm was formed on the concavo-convex portion of the relief type diffraction grating by a vacuum deposition method. This relief type diffraction grating hardly diffracts light having a wavelength of 650 nm and has a diffraction efficiency (that is, transmittance) of 0th order light of about 90%, while light having a wavelength of 790 nm is diffracted by about 10% as first order diffracted light. The diffraction efficiency of the next light was about 55%.
[0030]
A dielectric multilayer film, which is an optical multilayer film shown in Table 2, was formed on the surface of the quartz glass substrate 31 opposite to the surface on which the relief-type diffraction grating 32 was produced by vacuum deposition. In the same manner as described above, the second diffraction grating having the dielectric multilayer film convex portion 33 was produced by photolithography and etching techniques. The lattice pitch was 20 μm, the duty ratio was 1/2, and the etching was stopped when the quartz glass of the substrate was exposed, so that only the dielectric multilayer film was uneven.
[0031]
On one surface of a glass substrate, a cover glass 35 was created made by forming an antireflection film with respect to light having a wavelength of 650nm and wavelength 790nm by vacuum evaporation. The concave portion of the second diffraction grating is filled with an acrylic adhesive 34 having a refractive index of 1.51 as a filler, the cover glass 35 is overlaid thereon, and the cover glass 35 is adhered by the adhesive 34. A diffraction element as shown in FIG.
[0032]
When the transmittance and diffraction efficiency of this diffractive element were examined, for light with a wavelength of 650 nm, the diffraction efficiency of the 0th order light was about 70%, the diffraction efficiency of the first order light was about 6%, while the light with a wavelength of 790 nm In contrast, the diffraction efficiency of the zero-order light was about 54%, and the diffraction efficiency of the first-order light was about 9%. And it was possible to obtain three beams efficiently for light of any wavelength.
[0033]
This diffractive element was mounted at the position of the diffractive element 23 in the optical head device shown in FIG. When an LD with a wavelength of 650 nm, which is a light source for DVD, is used as the light source 21, and an LD with a wavelength of 790 nm, which is a light source for CD, is used as the light source 22, the light of 790 nm and the light of 650 nm are efficiently divided into three beams. In addition, the light utilization efficiency of the optical head device could be improved. These three beams were used for tracking of the optical head device, and good reproduction characteristics were obtained when reproducing information from the optical disk.
[0034]
【The invention's effect】
As described above, the diffractive element of the present invention has a diffractive diffraction grating formed by using an optical multilayer film such as an oxide periodically stacked on a transparent substrate. The diffraction efficiency can be controlled by making the wavelength dependence almost constant.
[0035]
Further, by mounting the diffraction element of the present invention on an optical head device using light of a plurality of wavelengths, an optical head device having a high degree of design freedom and high light utilization efficiency can be obtained.
[Brief description of the drawings]
FIG. 1 is a side sectional view showing an example of a diffraction element of the present invention.
FIG. 2 is a side sectional view showing another example of the diffraction element of the present invention.
3 is a side sectional view showing a diffraction grating of the present invention, in which a transparent substrate is superimposed on the diffraction element of FIG.
4 is a side sectional view showing the diffraction grating of the present invention, in which a transparent substrate is superimposed on the diffraction element of FIG.
FIG. 5 is a side sectional view showing another example of the diffraction element of the present invention.
6 is a graph showing the wavelength dependence of first-order diffraction efficiency in a diffraction element obtained by the design example of the optical multilayer film shown in Table 1. FIG.
7 is a graph showing the wavelength dependence of first-order diffraction efficiency in a diffraction element obtained by the design example of the optical multilayer film shown in Table 2. FIG.
FIG. 8 is a schematic side view showing an example of an optical head device equipped with the diffraction element of the present invention.
FIG. 9 is a schematic side cross-sectional view of a diffraction element in an example.
[Explanation of symbols]
1,11: Light

Gakuo

layer film 2, 12: filler 3: transparent substrate 4: Cover 21,22: light source 23: the diffraction element 24: collimating lens 25: beam splitter 26: objective lens 27: optical recording medium 28: Optical detection optical system 31: quartz glass substrate 32: diffraction grating 33: dielectric multilayer film convex portion 34: adhesive 35: cover glass

Claims

In an optical head device comprising a light source, an objective lens for condensing the light emitted from the light source onto an optical recording medium, and a photodetector for detecting the emitted light collected and reflected by the optical recording medium A diffractive element is disposed in the optical path between the light source and the objective lens;
The diffraction element includes a diffraction grating having periodic irregularities formed on a transparent substrate,
The convex part of the concave-convex part of the diffraction grating includes an optical multilayer film arranged in parallel to the transparent substrate surface, and the concave part is filled with a filler made of solid or liquid,
The diffraction grating, the wavelength of the light emitted from the light source incident perpendicularly to the transparent substrate surface is substantially transmitted without expression 1-order diffraction efficiency at 790 nm, Oite 1 wavelength 6 4 0~670n m An optical head device having a next diffraction efficiency of about 6%.

The optical head device according to claim 1, wherein the optical multilayer film includes SiO ₂ films and TiO ₂ films alternately stacked.