JP4599763B2

JP4599763B2 - Optical head device

Info

Publication number: JP4599763B2
Application number: JP2001171249A
Authority: JP
Inventors: 龍一郎後藤; 好晴大井; 弘昌佐藤; 真弘村川
Original assignee: Asahi Glass Co Ltd
Current assignee: AGC Inc
Priority date: 2001-06-06
Filing date: 2001-06-06
Publication date: 2010-12-15
Anticipated expiration: 2021-06-06
Also published as: JP2002365416A

Description

【０００１】
【発明の属する技術分野】
本発明は、偏光性回折素子および光ヘッド装置に関し、特に主に０次、±１次および±２次、または主に０次、±１次、±２次および±３次の回折光を発生する偏光性回折素子およびこの素子を搭載した光記録媒体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】
ＣＤ、ＤＶＤおよび光磁気ディスクなどの光記録媒体（以下、「光ディスク」いう）の情報の記録および再生を行う光ヘッド装置が用いられている。この光ヘッド装置には回折素子が種々の用途で使用されている。
例えば、特開平１１−６６６０９には、回折素子により半導体レーザからの出射光を複数の光ビームに分割し、この複数の光ビームを光ディスクの情報記録面の複数のトラック上に同時に集光させることで、複数のトラック上の記録情報を同時に読み取り、従来よりも高速に情報の再生を行うことが記載されている。
【０００３】
【発明が解決しようとする課題】
しかし、上記の光ヘッド装置においては、半導体レーザからの出射光を従来の回折素子により複数の光ビームに分割するため、分割された光ビームの光量が低下し、情報の記録が高速にできないなどの問題を有する。
本発明の目的は、上記の問題を解決するために、主に０次、±１次および±２次、または主に０次、±１次、±２次および±３次の高効率の回折光を発生する偏光性回折素子を得ること、そして、この偏光性回折素子を用いて光ディスクの再生時には複数光ビームで複数トラック上の情報を読み出すことにより高速に情報の再生ができるとともに、記録時には単一光ビームで高速に情報の記録を行うことができる光ヘッド装置を提供することである。
【０００４】
【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に集光する対物レンズと、集光され前記光記録媒体により反射された前記出射光を検出する光検出器とを備え、前記光源と前記対物レンズとの間の光路中に、前記光源側から偏光方向変換手段と偏光性回折素子が設置されている光ヘッド装置であって、前記偏光性回折素子は、透明基板上に形成された複屈折性材料に断面形状が凹凸状の格子が形成され、格子の凹凸部に光学的に等方な充填材が充填されてなり、前記充填材の屈折率が前記複屈折性材料の常光屈折率または異常光屈折率のいずれかに等しく、前記格子の２つの凸部と２つの凹部とからなる隣接する４領域がピッチＰのくり返し単位となっており、前記複屈折性材料の屈折率と前記充填材の屈折率とが異なる方向に偏光方向を有する入射直線偏光に対し発生する０次回折光の回折効率η0とｋ次回折光の回折効率ηkとの比が、−ｍ≦ｋ≦ｍ（ｍは２または３）となるｋに対し、１≦η0／ηk≦５の関係を満たすように、前記４領域それぞれの幅のピッチＰに対する比率が特定されており、前記偏光方向変換手段は、入射した直線偏光の偏光方向を変えずに出射する場合と、この偏光方向と直交する直線偏光に変えて出射する場合との２つの状態を切り替えることができ、前記偏光性回折素子は、前記偏光方向変換手段の前記２つの状態の切り替えに応じて、前記光記録媒体の再生時には０次、±１次および±２次、または、０次、±１次、±２次、±３次の回折光に分離し、前記光記録媒体の記録時には０次の回折光（透過光）のみとすることを特徴とする光ヘッド装置を提供する。
【０００６】
【発明の実施の形態】
本発明の偏光性回折素子の１例が示されている図１においては、偏光性回折素子１Ａは、第１の透光性基板１１の面上に、複屈折性材料が、断面形状が凹凸状の格子１３として形成されているとともに、格子１３を挟んで第１の透光性基板と対向して第２の透光性基板１２を配置する構成となっている。さらに第１の透光性基板１１、格子１３および第２の透光性基板１３で形成される空間を光学的等方性の充填材１４で隙間なく充填して構成される。充填材１４の屈折率は、複屈折性材料の常光屈折率または異常光屈折率のいずれかに等しく選ばれる。
【０００７】
格子は図２に拡大図を示すように、透明基板１１上において格子１３を形成する２つの凸部と２つの凹部とからなる隣接する４領域がピッチＰのくり返し単位となる。複屈折性材料の屈折率と充填材の屈折率とが異なる方向の偏光方向を有する入射直線偏光に対し凹凸部は回折格子となっている。
【０００８】
そして、回折格子により発生する０次回折光とｋ次回折光の回折効率の比が、−ｍ≦ｋ≦ｍ（ｍは２または３）となるｋに対し、１≦η₀／η_k≦５の関係を満たすように、４領域の幅ａ₁、ａ₂、ａ₃およびａ₄のそれぞれのＰに対する比率が特定されている。すなわち、ｍが２のとき、−２、−１、０、１、２のそれぞれのｋに対し、０次回折効率η₀とｋ次回折効率η_kとの比が１≦η₀／η_k≦５の関係を満たすように、ａ₁／Ｐ、ａ₂／Ｐ、ａ₃／Ｐおよびａ₄／Ｐの値が特定されている。ｍが３のとき、−３、−２、−１、０、１、２、３のそれぞれのｋに対しも同様である。
【０００９】
ここでη₀／η_k＞５のときには、０次以外の回折光量が低下し、複数光ビームで並列読み出しを行うための光量を得ることができない。また、η₀／η_k＜１のときには、０次回折光（メインビーム）の光量が他の回折光の光量より小さくなる。
【００１０】
透光性基板１１および透光性基板１２は、例えばガラス基板であれば、平坦度が高く波面収差の発生が低減でき、また耐久性に優れ好ましい。
複屈折性材料としては、例えば高分子液晶を使用できる。透光性基板の表面にポリイミドなどの薄膜を形成し、配向処理を施して配向膜とする。この配向膜上に液晶モノマの溶液を塗布し、液晶分子を配向させる。この状態で、液晶モノマ溶液に予め含有された光重合開始剤に、光重合用の光源から光を照射して液晶モノマを高分子化することにより、透光性基板上に複屈折性材料を形成できる。
【００１１】
上記のように透光性基板上に形成された複屈折性材料に、フォトリソグラフィとエッチングの技術を用いて、格子を形成する。充填される光学的等方性の充填材としては、アクリル系、エポキシ系、ポリエステル系の紫外線硬化型接着剤または熱硬化型の接着剤であれば作業性がよく好ましいが、これらに限定されない。また、充填材の屈折率は、例えば複屈折性材料の常光屈折率に等しくする。
【００１２】
上記のように構成された偏光性回折素子に、複屈折性材料に対し異常光となる直線偏光が入射したときには、複屈折性材料の屈折率と充填材の屈折率が異なるため、上記のように格子は回折格子として機能する。
【００１３】
また、偏光性回折素子に、複屈折性材料に対し常光となる直線偏光（直線偏光と偏光方向が直交）が入射したときには、複屈折性材料の屈折率と充填材の屈折率が等しいため、格子は回折格子として機能せず、入射光は回折されずに通過する。
【００１４】
すなわち、本発明の偏光性回折素子は、入射する直線偏光の偏光方向に応じて回折格子として機能する、または機能せず、さらに回折格子として機能する場合にはｍが２のときは主に±２次以下、またはｍが３のときは主に±３次以下の次数の回折光を発生するように、ａ₁／Ｐ、ａ₂／Ｐ、ａ₃／Ｐおよびａ₄／Ｐの値が特定されている（図２参照）。
【００１５】
本発明の光ヘッド装置には、本発明の偏光性回折素子が搭載される。図３に１例として示す本発明の光ヘッド装置は、光源である半導体レーザ２から出射した光が、偏光方向変換手段である偏光変換素子３を通過した後、偏光性回折素子１に入射する。半導体レーザ２からの出射光は、偏光性回折素子１により複数の光ビームに分割されるような直線偏光の偏光方向を有している。
【００１６】
また、偏光変換素子３は、半導体レーザ２から出射され、偏光変換素子３に入射した直線偏光の偏光方向を変えずに出射する場合と、この偏光方向と直交する直線偏光に変えて出射する場合との２つの状態を切り替えることができる。偏光変換素子３としては、具体的には、波長板、ツイステッドネマティック液晶セルを光路中に出し入れする、また波長板を光軸の回りに回転させる機構を備えるなどにより実現できる。
【００１７】
図３（ａ）では、偏光変換素子３により出射光の偏光方向が変化を受けずに偏光性回折素子１に入射し、偏光回折素子１によって、主な５つ、または７つの光ビーム（回折光）に分離された光は、ビームスプリッタ４を通過する。通過した光は、コリメートレンズ５によって平行光とされ、対物レンズ６により、光ディスク７上の複数のトラック上に集光され、反射される。反射された複数の光は、ビームスプリッタ４により反射され、複数の光検出器８で個別に検出されて、再生信号、トラッキングエラー信号、フォーカシングエラー信号などを与える。
【００１８】
図３（ｂ）では、偏光変換素子３によって、半導体レーザ２からの出射光が元の偏光方向と直交する偏光方向に変化を受けた後、偏光性回折素子１に入射し、回折されずに通過する。通過した光は、ビームスプリッタ４を透過し、コリメートレンズ５によって平行光とされ、対物レンズ６により、光ディスク上７の単一トラック上に集光され、一部は情報を記録するために使われ、残りは反射される。反射された光は、ビームスプリッタ４により反射され、複数の光検出器８のうちの１つで検出されて、トラッキングエラー信号やフォーカシングエラー信号を与える。
【００１９】
なお、偏光性回折素子１に、直線偏光が入射したときに３ビームを発生させる別の偏光性回折素子を積層して、再生時のトラッキングエラー信号を得るための３ビームを発生させる構成にしてもよいし、直線偏光が入射したときに３ビームを発生させる別の偏光回折素子を積層して、記録時のトラッキングエラー信号を得るための３ビームを発生させる構成にしてもよい。
【００２０】
【実施例】
「例１」
本例は、偏光性回折素子に関する。まず、ポリイミド樹脂をスピンコート法により第１のガラス基板（透光性基板）上に均一に塗布、成膜し、焼成した後、ラビング処理を施して、配向膜とした（図示せず）。配向膜の厚さは、６０ｎｍであった。その後、配向膜上に塗布した液晶モノマを光重合硬化し高分子液晶膜を形成した。高分子液晶の膜厚は３．８６μｍであり、屈折率は、波長λ＝７８５ｎｍで常光屈折率ｎ_o＝１．５５、異常光屈折率ｎ_e＝１．６０であった。
【００２１】
以上のようにして成膜された高分子液晶をフォトリソグラフィとエッチングの技術を用いて、格子を作製した。ここで、格子のピッチをＰ＝３０μｍとし、格子ピッチ内を幅ａ_１＝１．５μｍ、ａ_２＝９．６μｍ、ａ_３＝８．７μｍおよびａ_４＝１０．２μｍの順に４つの領域に分割し、高さｄ＝３．８６μｍの凹凸状の格子とした。この凹凸部の幅の比は、ピッチの端から順に１：６．４：５．８：６．８であった。
【００２２】
同時に、硬化後の屈折率が波長λで１．５５である光学的等方性の充填材として紫外線硬化型接着剤を格子間に充填し、さらに第２のガラス基板（透光性基板）で挟んだ。その後、紫外線を照射して接着剤を重合させて図１に示す構成の偏光性回折素子を作製した。
【００２３】
上記のように作製した偏光性回折素子に、波長λで、高分子液晶に対し異常光となる偏光方向の直線偏光を入射したところ、約４２％が０次回折光として透過し、約１０％の±１次および±２次回折光が発生する５ビーム回折素子が得られた。ここで、ｋ次回折光の回折効率η_kは、−２≦ｋ≦２の範囲において、η₀／η_k＝４．２＜５を満たしていた。また、この偏光性回折素子に、波長λで、高分子液晶に対し常光となる偏光方向の直線偏光を入射したところ、透過率が９８％であり、回折光は見られなかった。
【００２４】
「例２」
本例も、偏光性回折素子に関する。まず、例１と同様の方法で、例１と同様の高分子液晶膜を、厚さが５．２０μｍになるように形成した。以上のようにして成膜された高分子液晶をフォトリソグラフィとエッチングの技術を用いて、格子を作製した。ここで、格子のピッチをＰ＝３０μｍとし、格子ピッチ内を幅ａ_１＝６．９μｍ、ａ_２＝３．９μｍ、ａ_３＝６．３μｍおよびａ_４＝１２．９μｍの順に４つの領域に分割し、高さｄ＝５．２０μｍの凹凸状の格子とした。この凹凸部の幅の比は、ピッチの端から順に１．８：１：１．６：７．７であった。
【００２５】
また、硬化後の屈折率が波長λ＝７８５ｎｍで１．５５である光学的等方性の充填材として紫外線硬化型接着剤を格子間に充填し、さらに第２のガラス基板（透光性基板）で挟んだ。その後、紫外線を照射して接着剤を重合させて図１に示す構成の偏光性回折素子を作製した。
【００２６】
上記のように作製した偏光性回折素子に、波長λで、高分子液晶に対し異常光となる偏光方向の直線偏光を入射したところ、約２７％が０次回折光として透過し、約１０％の±１次、±２次および３次回折光が発生する７ビーム回折素子が得られた。ここで、ｋ次回折光の回折効率η_kは、−３≦ｋ≦３の範囲において、η₀／η_k＝２．７＜５を満たしていた。また、この偏光性回折素子に、波長λで、高分子液晶に対し常光となる偏光方向の直線偏光を入射したところ、透過率が９８％であり、回折光は見られなかった。
【００２７】
「例３」
本例は、光ヘッド装置に関する。図３に示すように、例１で得られた回折素子を、偏光方向変換手段である偏光変換素子３とビームスプリッタ４との間の光路中に偏光性回折素子１として配置した。偏光変換素子３として、１／２波長板を用いた。１／２波長板の板面は入射光の光軸に対して垂直に配置され、光軸の回りに回転できるようになっている。１／２波長板を回転することで出射光の偏光方向を変化させることができ、偏光性回折素子１へ入射する直線偏光を、高分子液晶に対して異常光となる偏光方向の場合と常光となる偏光方向の場合とに変化させることができた。
【００２８】
その結果、図３（ａ）に示すように、光ディスク７の再生時には５つの光ビームで５つのトラック上の情報を読み出すことにより高速に情報の再生ができるとともに、図２（ｂ）に示すように、記録時には単一光ビームで高速に情報の記録を行うことができた。
【００２９】
【発明の効果】
以上説明したように、本発明の偏光性回折素子は、素子に入射する直線偏光の偏光方向を選択することにより、入射光を主に０次、±１次および±２次の回折光に分離できる、または０次の回折光（透過光）のみとすることができる。また、入射光を主に０次、±１次、±２次および±３次の回折光に分離できる、または０次の回折光（透過光）のみとすることができる。
【００３０】
また、本発明の偏光性回折素子を搭載した本発明の光ヘッド装置を用いれば、光ディスクの再生時には主に５つ、または７つの複数の光ビームで同数のトラック上の情報を読み出すことにより高速に情報の再生ができるとともに、記録時には単一光ビームで高速に情報の記録を行うことができる。
【図面の簡単な説明】
【図１】本発明の偏光回折素子の１例を示す図で、（ａ）直線偏光Ｐ₁が入射したときの様子を示す側面図、（ｂ）直線偏光Ｐ₂が入射したときの様子を示す側面図。
【図２】図１の格子部分の拡大図。
【図３】本発明の光ヘッド装置の１例を示す図で、（ａ）光ディスクの再生時の様子を示す側面図、（ｂ）記録時の様子を示す側面図。
【符号の説明】
１：偏光性回折素子
２：半導体レーザ
３：偏光変換素子
４：ビームスプリッタ
５：コリメートレンズ
６：対物レンズ
７：光ディスク
８：光検出器
１１、１２：透光性基板
１３、１５：格子
１４：充填材[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a polarizing diffractive element and an optical head device, and generates mainly diffracted light of mainly 0th order, ± 1st order and ± 2nd order, or mainly 0th order, ± 1st order, ± 2nd order and ± 3rd order. The present invention relates to a polarizing diffraction element and an optical head device for recording and reproducing information on an optical recording medium equipped with the element.
[0002]
[Prior art]
2. Description of the Related Art Optical head devices that record and reproduce information on optical recording media (hereinafter referred to as “optical disks”) such as CDs, DVDs, and magneto-optical disks are used. In this optical head device, a diffraction element is used for various purposes.
For example, in Japanese Patent Laid-Open No. 11-66609, a light emitted from a semiconductor laser is divided into a plurality of light beams by a diffraction element, and the plurality of light beams are simultaneously condensed on a plurality of tracks on an information recording surface of an optical disk. Thus, it is described that the recorded information on a plurality of tracks is simultaneously read and the information is reproduced at a higher speed than in the past.
[0003]
[Problems to be solved by the invention]
However, in the above optical head device, the light emitted from the semiconductor laser is divided into a plurality of light beams by a conventional diffractive element, so that the amount of light of the divided light beams is reduced and information cannot be recorded at high speed. Have problems.
The object of the present invention is to solve the above problems mainly by high-efficiency diffraction of the 0th order, ± 1st order and ± 2nd order, or mainly 0th order, ± 1st order, ± 2nd order and ± 3rd order. It is possible to obtain a polarizing diffraction element that generates light, and to read out information on a plurality of tracks with a plurality of light beams at the time of reproducing an optical disk using the polarizing diffraction element, and at the time of recording An object of the present invention is to provide an optical head device capable of recording information at high speed with a single light beam.
[0004]
[Means for Solving the Problems]
The present invention includes a light source, an objective lens that condenses the light emitted from the light source on an optical recording medium, and a photodetector that detects the emitted light that is collected and reflected by the optical recording medium, In the optical path between the light source and the objective lens, an optical head device in which a polarization direction converting means and a polarizing diffraction element are installed from the light source side, and the polarizing diffraction element is formed on a transparent substrate is birefringence material in the cross-sectional shape uneven lattice formation, Ri Na isotropic filler optically is filled in the concavo-convex portion of the grating, the birefringent material the refractive index of the filler of ordinary equal to either the refractive index or extraordinary index, 4 adjacent areas consisting of two convex portions and two concave portions of the grating has a repeating unit of the pitch P, the birefringent material Henhikarikata in a direction different from the refractive index and the refractive index of the filler The ratio of the diffraction efficiency η0 of the 0th order diffracted light and the diffraction efficiency ηk of the kth order diffracted light generated with respect to the incident linearly polarized light having the following relationship is −m ≦ k ≦ m (m is 2 or 3). The ratio of the width of each of the four regions to the pitch P is specified so as to satisfy the relationship of η0 / ηk ≦ 5, and the polarization direction conversion means emits light without changing the polarization direction of the incident linearly polarized light And switching between the two states when changing to linearly polarized light orthogonal to the polarization direction and exiting, and the polarizing diffractive element according to the switching of the two states of the polarization direction conversion means, When reproducing the optical recording medium, it is separated into 0th order, ± 1st order and ± 2nd order, or 0th order, ± 1st order, ± 2nd order, ± 3rd order diffracted light, and when recording on the optical recording medium, 0th order an optical head device which is characterized in that the diffracted light (transmitted light) only To provide.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 1 in which an example of the polarizing diffraction element of the present invention is shown, the polarizing diffraction element 1A has a birefringent material on the surface of the first light-transmitting substrate 11, and the cross-sectional shape is uneven. The second light-transmitting substrate 12 is disposed so as to face the first light-transmitting substrate with the lattice 13 interposed therebetween. Further, the space formed by the first translucent substrate 11, the grating 13, and the second translucent substrate 13 is filled with an optically isotropic filler 14 without a gap. The refractive index of the filler 14 is selected equal to either the ordinary or extraordinary refractive index of the birefringent material.
[0007]
As shown in the enlarged view of FIG. 2, the adjacent four regions including two convex portions and two concave portions that form the lattice 13 on the transparent substrate 11 are repeated units of the pitch P. The concavo-convex portion is a diffraction grating for incident linearly polarized light having a polarization direction in which the refractive index of the birefringent material and the refractive index of the filler are different.
[0008]
The ratio of the diffraction efficiency of the 0th-order diffracted light and the kth-order diffracted light generated by the diffraction grating is 1 ≦ η ₀ / η _k ≦ 5 with respect to _k where −m ≦ k ≦ m (m is 2 or 3). In order to satisfy the relationship, the ratio of the widths a ₁ , a ₂ , a ₃ and a ₄ of the four regions to P is specified. That is, when m is 2, the ratio of the 0th-order diffraction efficiency η ₀ and the k-th order diffraction efficiency η _k is 1 ≦ η ₀ / η _{k with} respect to _{k of} −2, −1, 0, 1 and 2, respectively. The values of a ₁ / P, a ₂ / P, a ₃ / P and a ₄ / P are specified so as to satisfy the relationship of ≦ 5. The same applies to each of k of -3, -2, -1, 0, 1, 2, 3 when m is 3.
[0009]
Here, when η ₀ / η _k > 5, the amount of diffracted light other than the 0th order decreases, and the amount of light for performing parallel reading with a plurality of light beams cannot be obtained. When η ₀ / η _k <1, the light amount of the 0th-order diffracted light (main beam) is smaller than the light amounts of the other diffracted light.
[0010]
If the translucent board | substrate 11 and the translucent board | substrate 12 are glass substrates, for example, the flatness will be high and generation | occurrence | production of a wavefront aberration can be reduced, and it is excellent in durability, and preferable.
As the birefringent material, for example, a polymer liquid crystal can be used. A thin film such as polyimide is formed on the surface of the translucent substrate, and an alignment treatment is performed to obtain an alignment film. A liquid crystal monomer solution is applied onto the alignment film to align liquid crystal molecules. In this state, the photopolymerization initiator previously contained in the liquid crystal monomer solution is irradiated with light from a light source for photopolymerization to polymerize the liquid crystal monomer, thereby forming a birefringent material on the translucent substrate. Can be formed.
[0011]
A grating is formed on the birefringent material formed on the light-transmitting substrate as described above using photolithography and etching techniques. The optically isotropic filler to be filled is preferably an acrylic, epoxy, or polyester ultraviolet curable adhesive or thermosetting adhesive, which is good in workability, but is not limited thereto. Further, the refractive index of the filler is made equal to the ordinary refractive index of the birefringent material, for example.
[0012]
When the linearly polarized light that becomes extraordinary light is incident on the birefringent material, the refractive index of the birefringent material and the refractive index of the filler are different from each other. The grating functions as a diffraction grating.
[0013]
In addition, when linearly polarized light (normally polarized light and polarization direction is orthogonal) incident on the birefringent material is incident on the polarizing diffraction element, the refractive index of the birefringent material is equal to the refractive index of the filler. The grating does not function as a diffraction grating, and incident light passes through without being diffracted.
[0014]
That is, the polarizing diffraction element of the present invention functions or does not function as a diffraction grating depending on the polarization direction of incident linearly polarized light, and further functions as a diffraction grating. The values of a ₁ / P, a ₂ / P, a ₃ / P, and a ₄ / P are set so that diffracted light of the order of ± 3rd order or less is mainly generated when m is 3 or less. Has been identified (see FIG. 2).
[0015]
The polarizing diffraction element of the present invention is mounted on the optical head device of the present invention. In the optical head device of the present invention shown as an example in FIG. 3, the light emitted from the semiconductor laser 2 as the light source passes through the polarization conversion element 3 as the polarization direction converting means and then enters the polarizing diffraction element 1. . The light emitted from the semiconductor laser 2 has a linearly polarized polarization direction that is split into a plurality of light beams by the polarizing diffraction element 1.
[0016]
Further, the polarization conversion element 3 is emitted from the semiconductor laser 2 and emitted without changing the polarization direction of the linearly polarized light incident on the polarization conversion element 3, and when the polarization conversion element 3 emits the linearly polarized light orthogonal to the polarization direction. The two states can be switched. Specifically, the polarization conversion element 3 can be realized by providing a mechanism for rotating the wave plate around the optical axis, and putting the wave plate and the twisted nematic liquid crystal cell in and out of the optical path.
[0017]
In FIG. 3 (a), the polarization direction of the emitted light is not changed by the polarization conversion element 3 and enters the polarizing diffraction element 1, and the polarization diffraction element 1 causes the main five or seven light beams (diffraction). The light separated into (light) passes through the beam splitter 4. The light that has passed is converted into parallel light by the collimator lens 5, collected by the objective lens 6 onto a plurality of tracks on the optical disk 7, and reflected. The reflected plurality of lights are reflected by the beam splitter 4 and individually detected by the plurality of photodetectors 8 to give a reproduction signal, a tracking error signal, a focusing error signal, and the like.
[0018]
In FIG. 3B, after the outgoing light from the semiconductor laser 2 is changed in the polarization direction orthogonal to the original polarization direction by the polarization conversion element 3, it enters the polarizing diffraction element 1 and is not diffracted. pass. The light passing therethrough is transmitted through the beam splitter 4, converted into parallel light by the collimator lens 5, condensed by the objective lens 6 onto a single track on the optical disk 7, and a part thereof is used for recording information. The rest is reflected. The reflected light is reflected by the beam splitter 4 and detected by one of the plurality of photodetectors 8 to give a tracking error signal or a focusing error signal.
[0019]
The polarizing diffractive element 1 is stacked with another polarizing diffractive element that generates three beams when linearly polarized light is incident thereon, thereby generating three beams for obtaining a tracking error signal during reproduction. Alternatively, another polarization diffraction element that generates three beams when linearly polarized light is incident may be stacked to generate three beams for obtaining a tracking error signal during recording.
[0020]
【Example】
"Example 1"
This example relates to a polarizing diffraction element. First, polyimide resin was uniformly coated on a first glass substrate (translucent substrate) by spin coating, formed into a film, baked, and then rubbed to give an alignment film (not shown). The thickness of the alignment film was 60 nm. Thereafter, the liquid crystal monomer applied onto the alignment film was photopolymerized and cured to form a polymer liquid crystal film. Thickness of the polymer liquid crystal is 3.86Myuemu, the refractive index, the ordinary refractive index n _o = 1.55 at a wavelength lambda = 785 nm, was extraordinary refractive index n _e = 1.60.
[0021]
Using the above manner the formed polymer liquid crystal of photolithography and etching techniques were manufactured create a grid. Here, the pitch of the grating and P = 30 [mu] m, width in the grating pitch _{_{a 1 = 1.5μm, a 2 =}} 9.6μm, into four areas in order of a 3 = 8.7 .mu.m and _a 4 = 10.2 .mu.m Divided into a rugged lattice with a height d = 3.86 μm. The ratio of the width of the uneven portions was 1: 6.4: 5.8: 6.8 in order from the end of the pitch.
[0022]
At the same time, an ultraviolet curable adhesive is filled between the lattices as an optically isotropic filler having a refractive index after curing of 1.55 at a wavelength λ, and further a second glass substrate (translucent substrate). I caught it. Thereafter, ultraviolet rays are irradiated to polymerize the adhesive polarizing diffraction element having the structure shown in FIG. 1 was created made.
[0023]
When the linearly polarized light in the polarization direction that becomes an extraordinary light is incident on the polymer liquid crystal at the wavelength λ, about 42% is transmitted as the zero-order diffracted light and about 10% of the polarizing diffraction element produced as described above is incident. A five-beam diffractive element that generates ± first-order and ± second-order diffracted light was obtained. Here, the diffraction efficiency η _k of the _k-th order diffracted light satisfies η ₀ / η _k = 4.2 <5 in the range of −2 ≦ k ≦ 2. Further, when linearly polarized light having a wavelength λ and having a polarization direction which is normal light was incident on the polarizing diffraction element, the transmittance was 98%, and no diffracted light was observed.
[0024]
"Example 2"
This example also relates to a polarizing diffraction element. First, in the same manner as in Example 1, a polymer liquid crystal film similar to that in Example 1 was formed so as to have a thickness of 5.20 μm. Using the above manner the formed polymer liquid crystal of photolithography and etching techniques were manufactured create a grid. Here, the pitch of the grating is P = 30 μm, and the inside of the grating pitch is divided into four regions in the order of width a ₁ = 6.9 μm, a ₂ = 3.9 μm, a ₃ = 6.3 μm and a ₄ = 12.9 μm. Divided into a concavo-convex lattice with a height d = 5.20 μm. The ratio of the width of the uneven portions was 1.8: 1: 1.6: 7.7 in order from the end of the pitch.
[0025]
Further, an ultraviolet curable adhesive is filled between the lattices as an optically isotropic filler having a refractive index after curing of 1.55 at a wavelength λ = 785 nm, and further a second glass substrate (translucent substrate). ). Thereafter, ultraviolet rays are irradiated to polymerize the adhesive polarizing diffraction element having the structure shown in FIG. 1 was created made.
[0026]
When the linearly polarized light in the polarization direction that becomes an extraordinary light is incident on the polymer liquid crystal at the wavelength λ, about 27% is transmitted as the zero-order diffracted light and about 10% of the polarizing diffraction element manufactured as described above is incident. A seven-beam diffractive element that generates ± first-order, ± second-order, and third-order diffracted light was obtained. Here, the diffraction efficiency η _k of the _k-th order diffracted light satisfies η ₀ / η _k = 2.7 <5 in the range of −3 ≦ k ≦ 3. Further, when linearly polarized light having a wavelength λ and having a polarization direction which is normal light was incident on the polarizing diffraction element, the transmittance was 98%, and no diffracted light was observed.
[0027]
"Example 3"
This example relates to an optical head device. As shown in FIG. 3, the diffractive element obtained in Example 1 was arranged as the polarizing diffractive element 1 in the optical path between the polarization converting element 3 serving as the polarization direction converting means and the beam splitter 4. A ½ wavelength plate was used as the polarization conversion element 3. The plate surface of the half-wave plate is arranged perpendicular to the optical axis of the incident light, and can rotate around the optical axis. The polarization direction of the emitted light can be changed by rotating the half-wave plate, and the linearly polarized light incident on the polarizing diffraction element 1 is polarized in the polarization direction that becomes abnormal light with respect to the polymer liquid crystal and the ordinary light. It was possible to change in the case of the polarization direction.
[0028]
As a result, as shown in FIG. 3A, at the time of reproduction of the optical disc 7, information on five tracks can be read at a high speed by reading information on five tracks with five light beams, and as shown in FIG. In addition, it was possible to record information at a high speed with a single light beam during recording.
[0029]
【The invention's effect】
As described above, the polarizing diffraction element of the present invention mainly separates incident light into 0th order, ± 1st order, and ± 2nd order diffracted light by selecting the polarization direction of linearly polarized light incident on the element. Or zero-order diffracted light (transmitted light) only. Further, incident light can be mainly separated into 0th-order, ± 1st-order, ± 2nd-order, and ± 3rd-order diffracted light, or only 0th-order diffracted light (transmitted light) can be used.
[0030]
In addition, when the optical head device of the present invention equipped with the polarizing diffraction element of the present invention is used, information on the same number of tracks is read out mainly by using five or seven light beams when reproducing an optical disk. In addition, information can be reproduced and information can be recorded at high speed with a single light beam during recording.
[Brief description of the drawings]
In view showing an example of the polarization diffraction element of the present invention; FIG, side view showing a state in which the incident (a) linearly polarized light P _1, a state in which the incident (b) linearly polarized light P ₂ FIG.
FIG. 2 is an enlarged view of a lattice portion in FIG.
3A and 3B are diagrams showing an example of an optical head device according to the present invention, in which FIG. 3A is a side view showing a state during reproduction of an optical disc, and FIG.
[Explanation of symbols]
1: Polarization diffraction element 2: Semiconductor laser 3: Polarization conversion element 4: Beam splitter 5: Collimator lens 6: Objective lens 7: Optical disk 8: Optical detector 11, 12: Translucent substrate 13, 15: Grating 14: Filler

Claims

A light source; an objective lens that condenses the light emitted from the light source onto an optical recording medium; and a photodetector that detects the emitted light that is collected and reflected by the optical recording medium. An optical head device in which a polarization direction conversion means and a polarizing diffraction element are installed from the light source side in an optical path between the lens,
The polarization diffraction element, the cross-sectional shape is uneven grating formed birefringent material formed on a transparent substrate, Ri Na isotropic filler optically is filled in the concavo-convex portion of the grating, the refractive index of the filler is equal to either the ordinary refractive index or extraordinary index of the birefringent material, repeating the two protrusions and consisting of two recesses adjacent four regions the pitch P of the grating has become a unit, the diffraction of the birefringent material having a refractive index between the filler refractive index is different from the direction to the incident linearly polarized generated to light 0 next diffracted light diffraction efficiency η0 and k-order diffracted light having the polarization direction The k with respect to the efficiency ηk is −m ≦ k ≦ m (m is 2 or 3), so that the relationship of 1 ≦ η0 / ηk ≦ 5 is satisfied with respect to the pitch P of the width of each of the four regions. The ratio is specified ,
The polarization direction converting means can switch between two states: a case of emitting without changing the polarization direction of incident linearly polarized light and a case of emitting by changing to linearly polarized light orthogonal to the polarization direction,
The polarizing diffractive element is in the 0th order, ± 1st order and ± 2nd order, or 0th order, ± 1st order, when reproducing the optical recording medium, according to the switching of the two states of the polarization direction changing means. An optical head device, wherein the optical head device is separated into ± 2nd order and ± 3rd order diffracted light, and only 0th order diffracted light (transmitted light) is used for recording on the optical recording medium .