JP4725756B2

JP4725756B2 - Directional diffraction grating

Info

Publication number: JP4725756B2
Application number: JP17045798A
Authority: JP
Inventors: 英男前田
Original assignee: 前田資郎
Priority date: 1998-05-14
Filing date: 1998-05-14
Publication date: 2011-07-13
Anticipated expiration: 2018-05-14
Also published as: JPH11328713A

Description

【０００１】
【発明の属する技術分野】
本発明は、方向性のある方向性回折格子、それを用いた光ヘッド装置、光磁気ヘッド装置及び、アイソレータに関する。
【０００２】
【従来の技術】
従来、回折格子における方向性、すなわち、回折格子の回折格子がある側から入射された場合と、回折格子の裏側から入射させた場合に、回折効率や透過率に差異がでる回折格子（このような回折格子を方向性回折格子と称す）は提案されていなかった。光デイスクに用いられている回折格子には、方向性が必要とされ、偏光によって回折効率が変わる偏光性回折格子と波長板との組み合わせで方向性を持たせていた。その偏光性回折格子には種々のものが提案されている。例えば、特開昭６３−５５５０１号公報には、図３５に示すように、ｘ軸方向に結晶軸を有するニオブ酸リチウム結晶板７２に、ｚ軸方向に周期を有するイオン交換領域と非イオン交換領域とで構成される格子７３を形成したものが開示されている。このように、ニオブ酸リチウムにプロトンイオン交換を施すと、常光線に対する屈折率Ｎｏは変化せず、異常光線に対する屈折率Ｎｅは０．１３程度上昇して、ほぼＮｅ〜Ｎｏとなる。
【０００３】
したがって、入射光７４のｙ軸方向に振動する偏光成分すなわち常光成分に対しては、イオン交換による格子１２が形成されていても、面内においては屈折率が一様で、光学的回折格子の効果がないので、透過光７５として結晶板７２を直進透過することになる。これに対し、入射光７４のｚ軸方向に振動する偏光成分すなわち異常光成分に対しては、屈折率が、格子７３のプロトンイオン交換領域ではＮｅ＋０．１３、非交換領域ではＮｅと周期的に異なるので、回折光７６、７７となって結晶板７２から放射されることになる。
【０００４】
このように、この偏光性回折格子を用いれば、入射光７４を０次回折光および±１次回折光に分離することができ、これにより直交する偏光成分を分離することができる。また、この偏光板は、ニオブ酸リチウム結晶板７２に、イオン交換領域および非イオン交換領域よりなる格子７３を形成するようにしているので、薄型で、かつ小型にできると共に、ニオブ酸リチウム結晶ウエハーを素材として作成することができるので、バッチ処理により大量かつ安価にできる利点がある。
【０００５】
また、特開昭６３−２４７９４１号公報には、図３６に示すような偏光性回折格子が開示されている。この偏光性回折格子は、屈折率異方性を有する異方性板７８にグレーティング溝を形成し、この溝に、光学軸７９の方向の屈折率がＮｅまたはそれと直交する方向の屈折率がＮｏの充填材料８０を充填したもので、これにより光学軸７９と直交する偏光成分または平行な成分を回折光として分離するようにしている。この偏光性回折格子によれば、異方性板７８にグレーティング溝を形成し、その溝に充填材料８０を充填するようにしているので、小型にできる利点がある。
【０００６】
【発明が解決しようとする課題】
しかしながら、どの偏光性回折格子にあっても、入射光の方向性、即ち、回折格子側から入射させても、回折格子の裏側から入射させても回折光の効率や透過光の効率は、原理的に同じである。そのため、光路中に回折格子を置いて、入射させる方向によって特性が変わることが必要な用途においては、波長板を一方に設定して、入射方向によって偏光が変わるようにして用いる必要があり、コストの増大、大型化が避けられなかった。
【０００７】
そこで、本発明の目的は、方向性のある回折格子を提供することを目的とするものである。
【０００８】
【課題を解決するための手段】
本発明者は上記課題を解決するために鋭意検討、研究した結果、回折格子において、波長とピッチと屈折率などの諸元を、適切に設定する事によって、製造が容易で量産性に適した方向性回折格子を開発することに成功した。
【０００９】
すなわち、本発明は、屈折率Ｎ１と屈折率Ｎ２の互いに異なる屈折率を有する材質１及び材質２の間に、屈折率Ｎ１と屈折率Ｎ２の区画を交互にピッチΛで配設した回折格子（ただし、Ｎ１＞Ｎ２とする）を有し、ピッチを波長に対し、最適なる範囲に設定して形成されてなる方向性回折格子である。
【００１０】
本発明は、屈折率Ｎ１と屈折率Ｎ２の互いに異なる屈折率を有する材質１及び材質２の間に、屈折率Ｎ１と屈折率Ｎ２の区画を交互にピッチΛで配設した回折格子（ただし、Ｎ１＞Ｎ２とする）を有し、ピッチを波長に対し、上記範囲より大きめにピッチを形成されてなる方向性回折格子である。
【００１１】
本発明は、少なくとも１種類の層からなる材質（材質３）と、少なくとも１種類の層からなる材質（材質４）の間に、屈折率Ｎ１と屈折率Ｎ２の互いに異なる屈折率の区画を交互にピッチΛで配設した回折格子であって、少なくとも一方の材質中の前記区画に接した材質の屈折率Ｎ３とし、他方の材質中の最も低い屈折率をＮ４とし、用いる波長をλとする時に、各諸元を最適なる範囲に設定して形成されてなる方向性回折格子である。
【００１２】
本発明は、少なくとも１種類の層からなる材質（材質３）と、少なくとも１種類の層からなる材質（材質４）の間に、屈折率Ｎ１と屈折率Ｎ２の互いに異なる屈折率の区画を交互にピッチΛで配設した回折格子であって、少なくとも一方の材質中の前記区画に接した材質の屈折率Ｎ３とし、他方の材質中の最も低い屈折率をＮ４とし、用いる波長をλとする時に、上記範囲より大きめにピッチを形成されてなる方向性回折格子である。
【００１３】
本発明は、上記の回折格子において、回折格子を成す区画の少なくとも一方が複屈折性の物質で形成されてなる方向性回折格子である。
【００１４】
本発明は、光源からの光を上記の方向性回折格子に入射させ、透過光を記録媒体上にレンズを用いて絞り込み、反射光を方向性回折格子で回折させ、全反射面を設けずに受光素子へ光を導く光ヘッドである。
【００１５】
本発明は、光源からの光をプリズム上に配設した方向性回折格子に入射させ、透過光をレンズで記録媒体上に絞り込み、記録媒体からの反射光を方向性回折格子に戻し、回折した回折光のうち、プリズムの斜面を透過する光と、プリズムの一面で反射した後にプリズムの斜面を透過した光を受光し、信号を検出する光ヘッド装置である。
【００１６】
本発明は、光源からの光を上記の方向性回折格子に入射させ、透過光を記録媒体上にレンズを用いて絞り込み、反射光を方向性回折格子で回折させ、さらに別個の回折格子を設けて受光素子へ光を導く光ヘッドである。
【００１７】
本発明は、光源の光を方向性回折格子に対しＰ偏光に設定し、方向性回折格子と記録媒体の間に４分の１波長板を設け、方向性回折格子からの透過光を円偏光に変換し、記録媒体からの反射光をＳ偏光に変換して方向性回折格子で回折させる光ヘッドである。
【００１８】
本発明は、方向性回折格子の回折光発生部において、屈折率の小さい方から高い方へ光を入射させ、透過光と回折光に偏光を分離する偏光検出器である。
【００１９】
本発明は、上記偏光検出器を搭載した光磁気ヘッド装置である。
【００２０】
本発明は、方向性回折格子をプリズム上に設けた光ヘッド装置において、その斜面にさらに方向性回折格子を設けることで光磁気信号を検出する光磁気ヘッド装置である。
【００２１】
本発明は、方向性回折格子の回折光発生部において、屈折率の高い方から低い方へＰ偏光を入射させて光を通し、屈折率の低い方から高い方へＳ偏光を入射させて形成したアイソレータである。
【００２２】
本発明は、方向性回折格子の回折光発生部において、屈折率の高い方から低い方へＳ偏光を入射させて光を通し、屈折率の低い方から高い方へＰ偏光を入射させて形成したアイソレータである。
【００２３】
【発明の実施の形態】
以下、詳細に説明する。図１、図２は本発明に係る方向性回折格子１の要部断面図である。図１は、方向性回折格子１の両面から光が斜入射する際の回折光の光路図である。本発明では、斜入射の場合も垂直入射の場合も同様の原理であるので、説明の簡易化のために、以下は垂直入射の場合を図２を用いて説明する。図２において、方向性回折格子１は、透明な材料からなる基板の一面に形成されてなる回折格子である。方向性回折格子のピッチ２は回折格子の場所によって一定のピッチを成すいわゆる等ピッチ回折格子でもよいし、回折格子面の場所によって変わる回折格子でもよい。以下は光が入射する部分のピッチをΛとして説明する。方向性回折格子に回折格子の形成されていない面（以下、裏面とする）から光３を垂直に入射させるとする（以下、往路とする）。裏面は平らな面とし、回折格子のある面と平行とする。
【００２４】
図２に示すように、方向性回折格子１を取り巻く材質中（ここでは空気であるが、以下、外部材質と称す）から垂直に方向性回折格子１の裏面に入射した光は基板中でも方向を変えることなく真っ直ぐに進む。図２中の波形部分である屈折率が異なる二つの区画からなる周期構造の部分４（以下、回折光発生部とする）に到達すると、そこで回折が起こる。回折角５をθ１とすると、良く知られている回折条件式１が成立する。
【００２５】
【式１】

【００２６】
ここで、光の波長をλとし、外部材質の屈折率をＮ２とする。本式を用いることでθ１を求めることができる。次に、図２に示すように回折光発生部４のある面から光６を入射させるとする（以下、復路とする）。回折光７の回折角８をθ２とした時、回折光発生部４での回折条件式は式２のようになる。
【００２７】
【式２】

【００２８】
ここで、回折格子を成す材質の屈折率をＮ１とする。回折した光は裏面において外部材質中へ屈折する。屈折角９をθ３とした時の関係は式３のようになる。
【００２９】
【式３】

【００３０】
次に、式２と式３からθ２を消去することにより、屈折角９が式４のようになる。
【００３１】
【式４】

【００３２】
これにより、θ１とθ３が同じ値である事が分かる。これは、一般的な回折格子へは、回折格子面から入射するときと裏面から入射する時とでは、回折角に変わりはないことを示している。
次に、回折格子の基板中の回折角７であるが、式２にみるように、θ１やθ３と異なる値を示している。
【００３３】
具体的な数値を設定して回折格子ピッチΛをパラメーターとしてθ１とθ２を求める。λを６３５ｎｍ、Ｎ１を１．５、Ｎ２を１（空気中）とすると、式１、式２はそれぞれ式５、式６のようになる。
【００３４】
【式５】

【００３５】
【式６】

【００３６】
式５と式６にいて、ピッチΛをパラメーターとしてθ１とθ２の具体的な値を表１に例示する。ここで、回折しないとは、式５と式６の各右辺が１より大きくなり、回折条件が成立しないので回折光が発生しない場合である。
【００３７】
【表１】

【００３８】
これより、あるピッチではθ２しか値を持ち得ないという事が言える。すなわち、復路しか回折光が発生しない事が分かる。この具体例では、回折格子の屈折率が空気の屈折率より大きいので、往路にのみ回折する場合が生じるが、外部媒質の屈折率が回折格子の屈折率より大きい場合には、復路のみ回折する場合が発生する。要は、ピッチを適切に設定することで、回折格子での出射側（回折光の発生する側）の屈折率が入射側の屈折率より大きい方向（以下、高屈折率媒質への入射と称す）にのみ回折光を発生させることができる。この時、回折格子での出射側の屈折率が入射側の屈折率より小さい方向（以下、低屈折率媒質への入射と称す）は、透過光のみとすることができるので、方向によって透過率を異ならせる事ができる。
【００３９】
この方向性の機能を実現させる条件は、式５と式６において、式５の右辺が１より大きくなり、式６の右辺が１より小さくなるようなピッチの範囲であるので、遡って、式１の右辺が１より大きくなり、式６の右辺を１より小さくなるようなピッチの範囲であるので、容易に式７が求められる。
【００４０】
【式７】

【００４１】
これは、検討している回折格子は、一つの材質が空気層に対して区画が接しているということなので、屈折率の異なる区画から成る回折格子であって、それを取り巻く材質が、各々の区画の屈折率と同じ場合を示している。具体例としては、回折格子に表面レリーフタイプを用いたとし、Ｎ１をガラスを想定して１．５とし、外部媒質を空気としてＮ２を１とすると、式８が成立する。
【００４２】
【式８】

【００４３】
ピッチが、波長よりも小さく、波長の７割程度より大きい場合となる。方向性の機能を実現する条件の時には、前述のようにｓｉｎθ１が１を越える場合であるので、同様にｓｉｎθ３も１を越える事となり、高屈折率媒質への入射で発生した回折光は、回折格子の裏面から出射する事ができず、全反射する。これらの様子を図３に示す。
【００４４】
このように、本方向性回折格子の場合、復路の時の回折格子が接する外部媒質の層（ここでは空気）の屈折率と、裏面で回折光が出射する側の外部媒質の層（ここでは空気）の屈折率が同じ場合には、高屈折率媒質への回折光は裏面で臨界角を越えるので外部媒質中へ取り出すことができない。一般的なアイソレータの用途に使用する場合には、復路の光は不要であるので好都合である。
【００４５】
回折格子の構成は、図１に示したもの以外にも多く存在する。そこで、方向性回折格子１を一般化する。回折格子には幾つかの種類があり、屈折率の異なる区画を交互に配してなるものや、光を吸収する区画と透過させる区画が並んだものや、光を遮断する区画と透過させる区画とが並んだものがある。図４は一般的な回折格子を示すもので、屈折率が異なる区画が交互に並んだ回折格子、又は、光を吸収ないしは遮断する区画と、光を透過させる区画が交互に並んだ回折格子１０を挟んで、その上下に各々少なくとも一層の屈折率の異なる層（層１１、層１２、層１３、層１４、層１５）が存在している。光６は上方から垂直に入射する場合を考えるが、前述と同様に斜入射でも原理は同様である。
【００４６】
光６は方向性回折格子１に垂直に入射し、回折光発生部１０に到達し、回折光７が発生する。回折光は回折光発生部１０に接する層１３に入射し、さらに層１４を通って、空気に接する層１５に到達して外部媒質中へ出射する。層１３を第一層とし、回折光７の進行につれて、各層での光の角度をφｐ、屈折率をＮｐ（ｐは自然数の添え字）とし、用いる波長をλ、ピッチをΛとする。回折光発生部１０での回折条件式は式９のようになる。
【００４７】
【式９】

【００４８】
回折光発生部１０が屈折率の異なる区画が交互に並んだ回折格子の場合でも、光を吸収ないしは遮断する区画と、光を透過させる区画が交互に並んだ回折格子の場合でも、回折条件式の式９は同じ式である。回折させる場合には、式９において、φｐが存在しなければならないから、式１０が成り立つ。
【００４９】
【式１０】

【００５０】
次に各層での光の角度は層１３ではφ１、層１４ではφ２、層１５ではφ３であり、また、各層の屈折率がＮ１、Ｎ２、Ｎ３であるので、各界面では式１１が成立する。
【００５１】
【式１１】

式９と式１１から以下の式が成立する。
【００５２】
【式１２】

【００５３】
【式１３】

【００５４】
【式１４】

【００５５】
各層で全反射させないためには、式１２から式１４の右辺が１以下である必要があるので、Ｎ１、Ｎ２、Ｎ３のうち、もっとも小さい屈折率をＮｍとすると、式１５が成立する。
【００５６】
【式１５】

【００５７】
層１５が外部媒質である空気とすると、一般に、空気の屈折率は真空の屈折率と近く、非常に小さいので、Ｎｍは１としてよい。即ち、式１６となる。
【００５８】
【式１６】

【００５９】
式１０におけるλ／Ｎ１は、Ｎ１が空気や真空（以下、空気の場合のみ考える）でない場合、λより小さい値となる。これより、式１０と式１６から言えることは、Λがλ／Ｎ１未満なら回折光は発生せず、Λがλ／Ｎ１以上なら回折光は発生するが、Λがλより大きくならないと、全反射のために回折光は空気中へでないということである。
【００６０】
次に、逆方向の光１６が回折光を発生させない条件を説明する。光１６は図４中で方向性回折格子１の下から入射し、回折光発生部に到達する。回折光発生部を出射した直後の層１１の屈折率をＮ４とし、回折角をφ４とすると回折条件は式１７で表される。
【００６１】
【式１７】

【００６２】
回折光が発生しない条件は、右辺が１より大きいときであるから、式１８が成立する。
【００６３】
【式１８】

【００６４】
層１１へ透過した光は、次の層１２へも垂直入射し、透過していき、最後に外部媒質である空気中へ出射する。式１５と式１８をまとめ、Ｎ４をＮｎと標記を変えて、式１９を得る。
【００６５】
【式１９】

【００６６】
これが、本方向性回折格子１の性能を発揮させるための一般式である。
【００６７】
次に、外部へ取り出せない回折光を外部媒質へ（又は外部媒質中にある受光素子へ）取り出したい用途の場合の取り出し方法について以下に説明する。第一の方法として、かかる回折光を回折格子の裏面で全反射させないようにすることがあげられる。全反射させない方法は幾つか存在する。例えば、光の入射角を臨界角より小さくする方法がある。また、受光素子で受光する場合には、受光素子までの光路中に屈折率の最適化、又はマッチングなどを行う方法がある。光の入射角を臨界角より小さくする方法に、回折格子の裏面の角度を変える方法がある。
この構成を図５を用いて説明する。
【００６８】
回折格子をプリズム上に形成する。図５では予め平板状に作製した方向性回折格子１をプリズム１７に接着層１８を設けて接着した場合を示している。接着する場合には接着剤の屈折率に注意し、方向性回折格子１の接着面と接する部分の屈折率とプリズムの屈折率の中間の値のものが望ましい。これは接着材との界面での反射を少なく抑えることができるからである。
【００６９】
入射光１９を図６に示すようにプリズムの入射面２０からさせる場合を往路とする。図６では垂直入射の場合を示したが、斜入射でも本発明の原理は同様である。プリズム内に入射した光は斜面２１で全反射し、方向性回折格子の方向へ進む。方向性回折格子では前述の原理で諸元を設定し、透過光２２となって出射する。次に図７に復路の場合を示す。方向性回折格子のある面から光２３を入射させる。方向性回折格子で回折光が発生し、＋１次光２４と−１次光２５に分けられる。＋１次光は斜面に向かい、全反射角よりはるかに小さい角度で斜面に入射するので高い透過率で外部へ透過する。−１次光は入射面に向かい、全反射して斜面に向かい、＋１次光と同様に高い透過率で透過する。このように、回折光を外部に取り出すことができる。方向性回折格子では透過光２６も発生するので往路と同じ光路で戻っていく。
【００７０】
具体的な数字をあげて説明する。方向性回折格子のピッチが０．５３μｍ、波長が０．６３５μｍ、方向性回折格子とプリズムと接着剤の屈折率が１．５とすると、回折角θ２は５３度となる。プリズム形状は二等辺三角形とする。この時、−１次光の入射面２０への入射角は３７°となる。屈折率が１．５の物質と屈折率が１の物質の臨界角は４８度であるので−１次光は全反射する。さらに−１次光は斜面へ８度で入射するので非常に高い透過率で透過する。＋１次光の斜面への入射角も−１次光と同様に８であるので高い透過率で透過する。斜面に無反射コーテイングを施しておけばさらに高い透過率で回折光を取り出すことができる。プリズム内での回折光の斜面への入射角は全反射角を越えて無くとも、大きい角度で入射すれば高い反射率を得ることができることは言うまでもない。
【００７１】
次に、光の入射角を臨界角より小さくする方法に、方向性回折格子と空気との界面の一部を回折光発生部の面と傾けることで全反射角より小さくして透過させる方法がある。これを図８を用いて説明する。復路の光２３は方向性回折格子１に略垂直に入射させる。＋１次光２４と−１次光２５が発生し裏面の空気との界面に向かう。界面を一部加工し、回折光が全反射角より小さい入射角で界面に達するようにする。図では略垂直になるようにしているが、これは垂直の時が一番フレネル反射が小さいからであり、垂直でなくとも光は透過する。また、無反射コートを施すことでより透過率を大きくできることは言うまでもない。また、後述するが、受光素子２７ａ及び受光素子２７ｂとの間の屈折率をマッチさせることを併用してもよい。
【００７２】
回折光を外部媒質中への受光素子上へ取り出す他の方法としては、かかる回折光を回折格子の裏面で全反射させず、裏面に受光素子を配設して受光する方法がある。これを図９を用いて説明する。復路の光２３は方向性回折格子１に略垂直に入射させる。＋１次光２４と−１次光２５が発生し裏面の空気との界面２８に向かう。界面は回折光発生部と略平行であるとすると、回折光は前述の原理で全反射する。全反射するのは、前述のように空気の屈折率が小さいためであるので、屈折率をマッチングさせる材質（不図示）でうめることで受光素子２７ａ、２７ｂへの透過率を大きくする事ができる。例えばマッチングオイルが使用できるし、回折格子に受光素子を屈折率のマッチする接着剤で接着する方法もある。市場に出回っている受光素子を代表するフォトダイオードは受光面をプラスチックで保護されており、その屈折率がガラスの屈折率に極めて近いので、屈折率のマッチする接着剤を用いれば、極めて低反射率で接着できる。また、フォトダイオードの保護用のプラスチックと屈折率が近いプラスチックで作製した回折格子を用い、かつ屈折率のマッチする接着剤を用いれば、さらに低反射で接着できることは言うまでもない。
【００７３】
回折光を外部媒質へ取り出す他の方法としては、図１０のように回折格子の裏面に、さらに回折格子（以下、裏面回折格子と称す）２９ａ、２９ｂを設け、回折させることで外部に取り出す方法がある。こうすると、回折現象で外部媒質中に光をとりだす事ができる。加うるに、回折を２回起こすため、光源の波長が変化した時でも色消しの効果で光の方向が変化しにくいという長所を持たせることができる。
【００７４】
このように、式１９を満たす場合の方向性回折格子について説明してきたが、その条件を満たすピッチよりも大きいピッチにする場合を図１１を用いて説明する。この場合に、回折光発生部と裏面の空気との界面が平行であっても、空気との界面で全反射せずに外部に取り出すことができることは言うまでもない。ただ、その場合には、往路でも回折光３０が発生する訳であるが、本方向性回折格子の利用の形態上、往路でも回折光が発生することで、往路の透過光の利用効率をやや損なっても、復路の回折効率を高いまま保って、十分に使用できる場合がある。それは、往路である高屈折率媒質から低屈折率媒質での回折においては、回折角３１が９０°に近いために、位相整合条件が大きくはずれるので回折効率は低くなるが（回折角が９０°なら回折は殆ど無い）、復路である低屈折率媒質から高屈折率媒質への回折においては、回折角３２は９０°よりずっと小さいので回折効率は依然として大きく保たれるからである。具体的にピッチが波長に比してどれだけ大きな範囲まで効果があるかは、実施例とともに後述する。
【００７５】
つぎに、上記で示した方向性回折格子の各場合について、小型でかつ効率のよい光ヘッドを構成できることを示す。第一の構成として、かかる回折光を回折格子の裏面で全反射させないために、方向性回折格子と受光素子との間に屈折率のマッチングを行った構成を図１２に示す。方向性回折格子１は、平行基板の一面に形成する。光源である半導体レーザチップ３３からの光３を方向性回折格子の裏面から入射させる。基板中を透過し、さらに方向性回折格子を透過する。方向性回折格子を透過した光はレンズ３４で記録媒体（不図示）上に絞り込む。記録媒体からの反射光を方向性回折格子に戻し、受光素子２７ａ、２７ｂで受光する。各々の部品は小型の筐体３５中に接着等により組み込む。方向性回折格子は例えば図１３に示すような回折格子３６となっており、＋１次光２４と−１次光２５を生じさせる。回折格子表面はトラック検出領域３７ａ、３７ｂとフォーカス検出領域３８とに分かれている。トラック検出領域は記録媒体からの反射光中のトラックパターンからトラック信号を取り出すため、回折格子がトラックの溝に対象に二分割されており、各々で発生する−１次光が各々のフォトダイオード（以下、ＰＤ）３９ａと３９ｂに入射する。フォーカス用領域の回折格子で発生する−１次光は２分割ＰＤ４０に入射し、ナイフエッジ法によりフォーカス信号が検出される。＋１次光はＰＤ４１で受光する。各々のＰＤの出力は記録信号の検出にも利用する。他の信号検出法では、フォーカス検出法にはナイフエッジ法の他に、非点収差法、フーコー法、その他にもダブルナイフエッジ法、ダブル非点収差法、ダブルフーコー法など、その方式に合わせたパターンとすることができる。トラック検出法にはプッシュプル法、ウオブリング法、ＤＰＤ法、ＤＰＰ法など、その方式に合わせたパターンとすることができる。このように小型の光ヘッドを構成できる。記録媒体は、ＲＯＭ、ＲＡＭのどちらでも良い。方向性回折格子は効率が高いので、ＲＡＭでも十分使用できる。
【００７６】
第二の構成として、方向性回折格子の裏面を一部傾けた構成を図１４に示す。光源からの光３を方向性回折格子１の裏面から入射させる。基板中を透過し、さらに方向性回折格子を透過する。方向性回折格子を透過した光はレンズ３４で記録媒体（不図示）上に絞り込む。記録媒体からの反射光を方向性回折格子に戻し、±１次光を生じさせる。各々の回折光は前述のように、基板の裏面の形成された全反射角より小さい界面に入射し、ＰＤに到達する。回折格子のパターンは図１３で説明したものでもよいが、他の方式でもよい。信号検出法は上記と同様に自由度を持たせる事ができる。このように小型の光ヘッドを構成できる。記録媒体は、ＲＯＭ、ＲＡＭのどちらでも良い。方向性回折格子は効率が高いので、ＲＡＭでも十分使用できる。
【００７７】
第三の構成として、裏面回折格子を設けた方向性回折格子を用いた構成を図１５に示す。光源からの光３を方向性回折格子の対抗面から入射させる。基板中を透過し、さらに方向性回折格子１を透過する。方向性回折格子を透過した光はレンズで記録媒体（不図示）上に絞り込む。記録媒体からの反射光を方向性回折格子に戻し、±１次光を生じさせる。光は方向性回折格子に裏面に設けた裏面回折格子２９ａ、２９ｂに入射する。この裏面回折格子のピッチが方向性回折格子のピッチと同じであれば、裏面回折格子での回折光は、光源の光と平行することができる。光源の波長が変化した場合、方向性回折格子での回折光の回折角が変わって裏面回折格子上での位置が変化し、ＰＤ２７ａ、２７ｂ上ではスポット変化が生じる。しかし、方向性回折格子のピッチより裏面回折格子のピッチを小さくしておけば、裏面回折格子での回折角の変化が上回り、ＰＤ上でのスポット位置変化をも打ち消すことができる。このように光源の波長の変化に対しても、安定な小型の光ヘッドを構成できる。信号検出法は上記と同様に自由度を持たせる事ができる。記録媒体は、ＲＯＭ、ＲＡＭのどちらでも良い。方向性回折格子は効率が高いので、ＲＡＭでも十分使用できる。
【００７８】
第四の構成として、かかる回折光を回折格子の裏面で全反射させないように回折格子をプリズム上に形成した場合を図１６に示す。方向性回折格子は、直角二等辺三角形の斜面でない一面に形成する。光源からの光を斜面でない他の一面から入射させる。プリズム内での光の挙動は上記に説明した通りである。方向性回折格子は上記に説明したものである。半導体レーザ３３を出射した光３は方向性回折格子を透過し、レンズ３４で記録媒体（不図示）上に絞り込まれる。記録媒体からの反射光を方向性回折格子に戻し、±１次光を生じさせる。＋１次光２４はプリズムの斜面２１そのまま透過してＰＤ２７ａに入射し、−１次光２５は入射面で反射し、斜面を透過してＰＤ２７ｂに入射する。このように小型の光ヘッドを構成できる。信号検出法は上記と同様に自由度を持たせる事ができる。記録媒体は、ＲＯＭ、ＲＡＭのどちらでも良い。方向性回折格子は効率が高いので、ＲＡＭでも十分使用できる。
【００７９】
次に、方向性回折格子１の偏光特性について考える。図１７に一種類の材料で構成された表面レリーフ回折格子を示す。回折光発生部の裏側から光３を入射させる。方向性回折格子では、前述のように空気側へは回折光は出射されない。しかし、空気側へ出る前には＋１次光４３と−１次光４４の回折光が存在する。これらの回折光は空気側へは位相整合されないので出射しないが、屈折率が高い方向、即ち、反射する方向へは位相整合条件が満たされ、＋１次光４３は反射回折光としての＋１次光４５となり、−１次光４４は反射回折光としての−１次光４６となって存在する。反射回折光４５と４６は回折格子との相互作用で入射光３とは逆方向の反射光４７と反射光４８（以下、回折反射光と称す）に一部変換される。
【００８０】
これらの反射回折光や回折反射光の効率は主に回折格子の屈折率と相互作用長である格子深さに大きく依存する。方向性回折格子のように、ピッチが小さい回折格子では一般的に回折作用が異なる事が知られており、Ｓ偏光４９（偏光方向と格子ベクトルが垂直）とＰ偏光５０（偏光方向と格子ベクトルが平行）とでは、Ｓ偏光の方が回折作用が大きくなることが知られている。このため、入射光から反射回折光や回折反射光へ光のエネルギーが伝達される量は、Ｓ偏光よりＰ偏光の方が少ない。そのため、方向性回折格子における透過率はＰ偏光の方が大きくなる。さらに入射光中のＳ偏光の多くを回折反射光とし、Ｐ偏光の多くを透過光とすることができる。これは一種の偏光ビームスプリッター（ＰＢＳ）の作用を示しているが、従来の誘電体多層膜によるＰＢＳでは膜に対して斜入射させる必要があるが、方向性回折格子の場合、垂直入射でよい点に特徴がある。
【００８１】
上記と逆方向から光２３を入射させた場合は、やはり、Ｓ偏光４９の方がＰ偏光５０より回折作用が大きくなるので、＋１次光２４と−１次光２５の回折効率は、Ｓ偏光の方がＰ偏光より大きくなる。透過光５１はＰ偏光が支配的になる。このような偏光特性は、従来技術で説明した偏光性回折格子と同様であることから、本発明に係る方向性回折格子は方向性と共に偏光性回折格子の側面も持ち合わせると言える。
【００８２】
この偏光特性を利用し、方向性回折格子と記録媒体の間に４分の１波長板を設け、往路をＰ偏光、復路をＳ偏光として使用することで、さらに方向性を高める事ができる。具体的には、図１２から図１６までにおいて説明した光ヘッドにおいて、方向性回折格子と記録媒体の間に４分の１波長板を設定し、方向性回折格子に対し、光がＰ偏光になるように半導体レーザチップとの位置関係を設定し、方向性回折格子からの透過光が円偏光になるように４分の１波長板の軸方向を設定し、記録媒体から戻ってきた光をその４分の１波長板でＳ偏光に変換し、方向性回折格子で回折させてＰＤへ導く事によって実現できる（不図示）。
【００８３】
方向性回折格子を用いた光ヘッドにおいて、往路をＰ偏光とし、復路をＳ偏光として用い、往路の透過率と復路の回折効率を高くしたい場合には、Ｐ偏光にとって回折作用を小さくし、Ｓ偏光の回折作用を大きくすればよいが、これを実現する方法に、複屈折材料を用いる方法がある。図１７と図１８で説明した回折格子を例にとる。複屈折材料の例として３ｍ結晶であるニオブ酸リチウムやタンタル酸リチウムがある。常光の屈折率Ｎｏと異常光の屈折率Ｎｅという相直交する二つの直線偏光に対し、異なる屈折率を示す。ニオブ酸リチウムの場合を図１９で説明する。回折格子をＺ方向５３とＸ方向５２のなす面上に作製し、格子ベクトルの方向はＺ方向である。光はＹ方向５４に入射する。入射光３をＰ偏光とすると感じる屈折率はＮｅなので２．２０である。帰ってくる光２３をＳ偏光にすると、感じる屈折率はＮｏで２．２８６となる。こうすることで、往路のＰ偏光では屈折率の小さい方を選べるので透過率が高く、復路のＳ偏光では屈折率を大きく選べるので回折効率を高くできる。タンタル酸リチウムの場合も軸方向と格子ベクトル方向を適切に選ぶことで実現できる。
【００８４】
図１８で説明したように、回折格子の表側から光が入射した場合には透過光はＰ偏光、回折光はＳ偏光が支配的になる。これを利用すれば偏光の検出が可能となる。例えば、図７、図８、図９、図１０で各々示した構成を用いると、各々、図２０、図２１、図２２、図２３のように偏光分離せられ、±１次光を各々検出する受光素子出力の総和と透過光を検出する受光素子の出力の差をとることで、入射光の偏光状態を知ることができる。これを利用することで、小型の光磁気ヘッドに応用できる。図２０の構成を用いた例を図２４を用いて説明する。半導体レーザ３３からの光３を回折格子５５に入射させる。この回折格子は従来の回折格子でよい。出射光は光磁気記録媒体（付図示）に入射し、反射し、その反射光２３は再び回折格子５５に戻る。回折格子５５はフォーカス信号やトラック信号を検出するもので、回折格子パターンは前述のごとく様々なパターンが適応できる。ここでは図１３で示したものとする。回折光は＋１次光５６と−１次光５７に別れる。各々の回折光の偏光は光磁気信号によって回転するので、どちらの回折光の偏光状態を検出しても光磁気信号を検出できる。図では、説明の簡単化のために＋１次光にのみ方向性回折格子を適用する。＋１次光を図２０で説明した方向性回折格子５８に入射させる。方向性回折格子の格子ベクトルは図２５に示すように反射光の偏光状態が方向性回折格子の格子ベクトルと４５°を成すようにする。回折格子５５で３つの＋１次光に分離した光は、各々さらに透過光と±１次光に振り分けられるので、全部で九つの回折光が発生する。そのうち、回折光はＰＤ５９とＰＤ６０に入射し、透過光はＰＤ６１に入射し、前述の原理で偏光状態を検出することで光磁気信号を検出できる。回折格子５５での−１次光はフォーカス検出などに使用される。
【００８５】
回折格子５５をブレーズ化し、一方の回折光のみ高効率で発生させ、そちらに方向性回折格子を適用すれば効率よく光磁気信号を検出できることは言うまでもない。回折格子５５の−１次光の方にも方向性回折格子適用して光磁気信号を得ることは勿論可能である。、この場合には、透過光を従来通り分割受光素子６２を用いて各種信号を検出し、さらに方向性回折格子の回折光を受光するＰＤを設けて偏光状態を検出すればよい（不図示）。このように、回折格子５５の±１次光の両方に方向性回折格子を適用すればより大きな光磁気信号を得ることは言うまでもない。また、方向性回折格子には図１３のもを適用して説明したが、その他のものを使うことでも同様の機能が得られることは言うまでもない。
【００８６】
前述のような、方向性回折格子のＰＢＳの機能を利用することによっても、光磁気ヘッドを構成する事ができる。図２６を用いて説明する。図２６（ａ）が構成である。これは前述の図１６で説明した光ヘッドを基本的に利用したものである。光３をＰ偏光で入射させ、斜面２１で全反射させて方向性回折格子に入射させる。透過した光はレンズ等を経て（不図示）、光磁気記録媒体上に照射され（不図示）、記録信号によって信号成分であるＳ偏光の偏光成分が光に加えられ、反射して再び方向性回折格子に戻ってくる。前述のように方向性回折格子は回折効率に偏光依存性を示すが、記録信号であるＳ偏光は透過率を高くでき、信号成分でないＰ偏光は透過率を低くできる。これは、光磁気ヘッドにおいて、よく用いられている信号成分の増大の手法として知られる偏光角増大の手法と同じ効果を示している。次に回折光は斜面２１に到達する、斜面２１には前述のＰＢＳの機能を示す方向性回折格子を形成する。方向性回折格子は図２６（ｂ）に示すよう設定し、＋１次光の方向性回折格子６３と−１次光の方向性回折格子６４の格子ベクトルが垂直になり、かつ、Ｐ偏光４８とＳ偏光４９に４５°なす方向で設定する。図２６（ｃ）は、光磁気記録媒体からの反射光の偏光の様子を示す。入射光の偏光であるＰ偏光４８に対し、図では左に傾いた偏光６５が戻ってくる。方向性回折格子６４においては、Ｓ偏光成分が増えるので、反射光が増え、透過光量が下がる。一方、方向性回折格子６３においては、Ｐ偏光成分が増えるので、透過光量が上がる。このため、両方の光量の差分をとることで、光磁気信号を検出できることとなる。フォーカス信号やトラック信号は、方向性回折格子１自体を前述のパターンとすることで検出できる。よって、図１３で説明したＰＤ以外にＰＤを作る必要はなく、方向性回折格子１の＋１次光を検出するＰＤの出力と−１次光を検出するＰＤ出力の差をとればよい。
【００８７】
次に本発明に係る回折格子の作製法を説明する。回折格子には表面レリーフ回折格子が最も適している。これは、表面レリーフ回折格子が、回折格子の材料と空気の屈折率の変調に基づく構造なので、Ｎ１とＮ２の屈折率の差が大きくとれることになり、式７によりピッチの許容範囲が大きくなるからである。そこで、表面レリーフ回折格子の作成方法について記す。図２７はその工程図である。回折格子の材料をガラスとし、図２７（ａ）に示すようにガラス基板６６上にレジスト６７を塗布する。レジストはポジ型でもよいし、ネガ型でもよいが、分解能の優れているレジストが望ましい。塗布の方法はスピンコーティングでもよいしディップコーティングでもよい。次に図２７（ｂ）に示すように、回折格子の所望のピッチになるようにレジストをパターニングして回折格子６８を得る。パターニングの方法としては、マスクの密着露光や縮小投影露光を行った後に現像する方法が一般的な手法として採用できる（不図示）。簡単な露光法としては二光束干渉露光法を採用しても良い。これは図２８に示すように、二つの光７０と光７１をレジスト面上で干渉させることで干渉縞を記録する方法である。次に図２７（ｃ）に示すように、作製したレジストパターンをマスクとしてガラスのエッチングを行う。エッチングはドライでもウェットでもよい。次に図２７（ｄ）に示すように、レジストを溶解することで最終的に回折格子６９を得ることができる。ここで、図２７（ｂ）に示されている状態であっても回折格子として用いることができる。しかし、レジストは一般的には透過率がよいわけではないので、非常に高い透過率を望む場合には図２７（ｄ）までの工程を経て透過率の良いガラスの回折格子を作製することが望ましい。回折格子の格子の形状は問わない。
【００８８】
【実施例】
以下に本発明の実施例を説明する。回折格子の作製は、図２７に示した方法の中で、二光束干渉露光法を用いてパターニングを行い同図（ｂ）の状態の回折格子、すなわちレジストの回折格子を用いた。効率測定に用いる光の波長が６３５ｎｍの赤色であり、使用したレジストは赤色で透明である事から、光利用効率の点で透明なガラス基板に対して遜色がない。作製条件を表２に示す。式８からピッチΛは０．４２３μｍより大きく、０．６３５μｍより小さい事が求められるので、０．６μｍとした。
【００８９】
【表２】

【００９０】
この条件を基本とし、深さの異なる回折格子を作製した。露光量を変えることで深さを制御した。評価は図１６の設定で行った。直角二等辺三角形のプリズムを採用し、作製した回折格子は屈折率のマッチングオイルを用いてプリズムに接触させることで、回折格子の裏面とプリズムの面での反射光を抑えた。回折格子の裏面側から入射させる往路と、回折格子面から入射させる復路について測定を行った。６３５ｎｍの波長の半導体レーザを用い、Ｓ偏光、Ｐ偏光の各偏光について、透過光、＋１次光と−１次光の回折光、及び反射光の測定を行った。
【００９１】
測定結果を図２９から図３４に示す。図２９は往路のＰ偏光の往路の透過率、復路の各偏光の回折光の効率である。横軸は回折格子の相対的な深さである。Ｐ偏光については、深さ７の時に往路の透過率が９０％、復路の回折効率が＋１次光と−１次光の両方を併せて約２０％となっており、往復路の足し算で１００％を越えることが実証された。これにより方向性回折格子となっていることが実証された。
【００９２】
また復路のＳ偏光の回折効率はＰ偏光の回折効率の２倍以上の値を示しており、偏光性回折格子の側面が実証された。
【００９３】
次に、往路のＳ、Ｐ偏光の透過率と反射率を示したのが図３０である。Ｐ偏光は深さに従って透過率が下がり、反射率は１０％以上に上がった。透過率が下がるのは、主に反射回折光が多くなるためであるが、反射率はフレネル反射以上に大きいという特徴が示された。Ｓ偏光も反射回折の効率が上がるに従って透過率が減少するが、特徴的なのは反射率が４０％以上と非常に大きくなることである。これは前述のように、回折光がさらに回折反射光に結合を始めるからと推測できる。垂直入射のＰＢＳに相当することが示された。
【００９４】
次に、復路におけるＳ、Ｐ偏光の透過率を示すとともに、各々の偏光の±１次光同士を足し合わせた回折効率の総計を図３１に示す。深さが相対値７付近では、Ｓ偏光の回折効率の総計は８０％近くあり、その時に透過率は２０％まで落ちた。Ｐ偏光はそこまでの差は付かないが、回折効率の総計と透過率が同じ程度に近づいた。また、両偏光の透過率と回折効率の総計は、深さが相対値６付近で同等になった。その深さでは、Ｓ偏光は７０％が回折し、３０％が透過した。Ｐ偏光は逆に７０％が透過し、回折は３０％であった。両偏光ともに透過と回折で４０％もの差が生じた。
【００９５】
次に、往路と復路におけるＳ、Ｐ両偏光の透過率を示す。図３２は往路でのＰ偏光の透過率と復路でのＳ偏光の透過率を示す。図３３は往路でのＳ偏光の透過率と復路でのＰ偏光の透過率を示す。図３２の場合では、深さが相対値６の時に、往路のＰ偏光の透過率は８０％あるに対して、復路のＳ偏光の透過率は３０％と低くなった。また、図３３の場合では、深さが相対値６の時に、往路のＳ偏光の透過率は７３％あるに対して、往路のＰ偏光の透過率は３２％と低くなった。これから、往路と復路で偏光の状態を変えることで、透過率の方向性を持たせることができ、アイソレータを構成することができる事が分かった。
【００９６】
次にピッチが式８から逸脱した場合、即ち、ピッチが波長に対してどれだけ大きい場合に、高い回折効率を確保し続けるかの実験を行った。効率測定に用いる光の波長を６３５ｎｍとし、ピッチを６３５ｎｍから小刻みに大きくして作製した。作製方法は前述と同様であり、ピッチは二光束干渉の角度を変えることにより制御した。回折格子の深さを、各ピッチごとに様々に変えて作製し、往路のＰ偏光の透過率を、約７３％という高い数値になる深さを選び、その深さの回折格子に復路からＳ偏光を入射させて回折効率を測定した。結果を図３４に示す。ピッチが波長６３５ｎｍの１．２倍程度まで高い回折効率を示し、ピッチを大きくするに従い、振動しながら徐々に一定の回折効率に近づいていくことが示された。これから、少なくともピッチの１．２倍までは通常の回折格子より高効率であることが実証できた。これは、方向性回折格子の一般的な性質であり、この表面レリーフ回折格子に限定されるものではないことは言うまでもない。
【００９７】
【発明の効果】
以上のように、第一の発明によれば、屈折率Ｎ１と屈折率Ｎ２の互いに異なる屈折率を有する材質１及び材質２の間に、屈折率Ｎ１と屈折率Ｎ２の区画を交互にピッチΛで配設した回折格子を有し、ピッチを波長に対し、最適なる範囲に設定して形成するので、従来にない、入射光の方向によって回折特性を異ならしめることができる回折格子を提供する事ができる。
【００９８】
また、第二の発明によれば、屈折率Ｎ１と屈折率Ｎ２の互いに異なる屈折率を有する材質１及び材質２の間に、屈折率Ｎ１と屈折率Ｎ２の区画を交互にピッチΛで配設した回折格子を有し、ピッチを波長に対し、上記範囲よりやや大きめにピッチを形成しても、復路の回折効率を高めに設定できるので、ピッチが大きい分、作製が容易な回折格子を提供する事ができる。
【００９９】
また、第三の発明によれば、少なくとも１種類の層からなる材質（材質３）と、少なくとも１種類の層からなる材質（材質４）の間に、屈折率Ｎ１と屈折率Ｎ２の区画を交互にピッチΛで配設した回折格子においても同様に方向性回折格子の機能を実現することができるので、方向性回折格子を他の材料と積層する等して他の機能を持たせようとすることも可能であり。諸処の応用に使用できる。
【０１００】
また、第四の発明によれば、少なくとも１種類の層からなる材質（材質３）と、少なくとも１種類の層からなる材質（材質４）の間に、屈折率Ｎ１と屈折率Ｎ２の区画を交互にピッチΛで配設した回折格子を有し、ピッチを波長に対し、上記範囲よりやや大きめにピッチを形成しても、復路の回折効率を高めに設定できるので、ピッチが大きい分、作製が容易な回折格子を提供する事ができ、かつ、方向性回折格子を他の材料と積層する等して他の機能を持たせようとすることも可能であり、諸処の応用に使用できる。
【０１０１】
また、第五の発明によれば、方向性回折格子において、回折格子の区画を成す材質の少なくとも一方が複屈折性の物質で形成されてなるので、往路のある偏光の透過率と復路の直交する偏光の回折効率をどちらもさらに高くできる。
【０１０２】
また、第六の発明によれば、光源からの光を方向性回折格子に入射させ、透過光を記録媒体上にレンズを用いて絞り込み、反射光を方向性回折格子で回折させ、全反射面を設けずに受光素子へ光を導くようにすることで、高効率で小型な光ヘッドを実現することができる。
【０１０３】
また、第七の発明によれば、光源からの光をプリズム上に配設した方向性回折格子に入射させ、透過光をレンズで記録媒体上に絞り込み、記録媒体からの反射光を方向性回折格子に戻し、回折した回折光のうち、プリズムの斜面を透過する光と、プリズムの一面で反射した後にプリズムの斜面を透過した光を受光素子へ導くようにすることで、小型で高効率な光ヘッドを提供することができる。
【０１０４】
また、第八の発明によれば、光源からの光を方向性回折格子に入射させ、透過光を記録媒体上にレンズを用いて絞り込み、反射光を方向性回折格子で回折させ、さらに別個の回折格子を設けて受光素子へ光を導くようにすることで、小型で高効率かつ光源の波長変動に影響を受けにくい光ヘッドを提供することができる。
【０１０５】
また、第九の発明によれば、方向性回折格子と記録媒体の間に４分の１波長板を設けることで、方向性回折格子の偏光性回折格子としての特性を活かし、光利用効率の高い光ヘッドを提供することができる。
【０１０６】
また、第十の発明によれば、方向性回折格子の屈折率の小さい方から高い方へ光を入射させ、透過光と回折光に偏光を分離して受光するので、小型の偏光検出器を提供できる。
【０１０７】
また、第十一の発明によれば、上記の小型の偏光検出器を搭載するので小型の光磁気ヘッド装置を提供できる。
【０１０８】
また、第十二の発明によれば、方向性回折格子とプリズムを組み合わせた光ヘッドの斜面に方向性回折格子を設けるだけで小型の光磁気ヘッド装置を提供できる。
【０１０９】
また、第十三の発明によれば、方向性回折格子における入射の方向と偏光とを適切に設定することで、アイソレータの機能を実現することができる。
【０１１０】
また、第十四の発明によれば、上記のアイソレータと偏光の関係を逆にすることでもアイソレータの機能を実現することができる。
【図面の簡単な説明】
【図１】回折格子へ光が斜入射する場合の光路を示す図である。
【図２】回折格子へ光が垂直入射する場合の光路を示す図である。
【図３】方向性回折格子において、基板中に回折光が入射した場合に裏面から出射できないことをしめす図である。
【図４】多層の層に回折格子部が挟まれてなる場合の光路を示す図である。
【図５】方向性回折格子をプリズムと一体化した構成を示す図である。
【図６】方向性回折格子をプリズムと一体化した構成において、光を入射させた場合の光路を示す図である。
【図７】方向性回折格子をプリズムと一体化した構成において、光を方向性回折格子側から入射させた場合の光路を示す図である。
【図８】方向性回折格子において、空気との界面を全反射角より小さくして光を取り出す構成を示す図である。
【図９】方向性回折格子において、空気との界面を設けずに回折光を受光する構成を示す図である。
【図１０】方向性回折格子において、空気との界面に別途回折格子を設けて回折光を外部へ取り出して受光する構成を示す図である。
【図１１】方向性回折格子を定める諸元のうち、ピッチを範囲外に大きくした場合の光路を示す図である。
【図１２】空気との界面を設けずに回折光を受光する方向性回折格子を用いた場合の光ヘッドの構成を示す図である。
【図１３】光ヘッドの信号を得るための回折格子のパターンと光路を示す図である。
【図１４】空気との界面を全反射角より小さくして光を取り出す方向性回折格子を用いた場合の光ヘッドの構成を示す図である。
【図１５】空気との界面に別途回折格子を設けて回折光を外部へ取り出す方向性回折格子を用いた場合の光ヘッドの構成を示す図である。
【図１６】プリズムと一体化することで光を外部に取り出す方向性回折格子を用いた場合の光ヘッドの構成を示す図である。
【図１７】方向性回折格子の裏面側から光を入射させた場合の回折光や透過光の光路を示す図である。
【図１８】方向性回折格子の回折格子のある側から光を入射させた場合の回折光や透過光の光路を示す図である。
【図１９】複屈折媒質の軸方向と方向性回折格子の位置の関係を示す図である。
【図２０】空気との界面を設けずに回折光を受光する方向性回折格子において、復路から光を入射させた際の偏光の分かれ方を示す図である。
【図２１】空気との界面に別途回折格子を設けて回折光を外部へ取り出す方向性回折格子において、復路から光を入射させた際の偏光の分かれ方を示す図である。
【図２２】プリズムと一体化した方向性回折格子において、復路から光を入射させた際の偏光の分かれ方を示す図である。
【図２３】空気との界面を全反射角より小さくして光を取り出す方向性回折格子において、復路から光を入射させた際の偏光の分かれ方を示す図である。
【図２４】復路から光を入射させる場合の方向性回折格子を偏光検出器に用いた光磁気ヘッドの構成を示す図である。
【図２５】上記の光磁気ヘッドに用いる偏光検出器である方向性回折格子における偏光の分離の様子を示す図である。
【図２６】（ａ）偏光を反射光と透過光に分ける方向性回折格子を偏光検出器に用いた光磁気ヘッドの構成を示す図である。
【図２６】（ｂ）上記の偏光検出器である方向性回折格子における、偏光と格子ベクトルの関係を示す図である。
【図２６】（ｃ）上記の偏光検出器である方向性回折格子における、光磁気信号である偏光回転の関係を示す図である。
【図２７】方向性回折格子を作製する実施例を示す図である。
【図２８】上記の作製法における回折格子のパターニングの方法としての二光束干渉露光法の構成を示す図である。
【図２９】作製した方向性回折格子において、往路からＰ偏光を入射させ、復路からＰ偏光とＳ偏光の両方を入射させた場合の回折効率を示す図である。
【図３０】作製した方向性回折格子において、往路からＳ偏光とＰ偏光を入射させた場合の透過率と反射率を示す図である。
【図３１】作製した方向性回折格子において、復路からＳ偏光とＰ偏光を入射させた場合の透過率と回折効率を示す図である。
【図３２】作製した方向性回折格子において、往路のＰ偏光と復路のＳ偏光の透過率を示す図である。
【図３３】作製した方向性回折格子において、往路のＳ偏光と復路のＰ偏光の透過率を示す図である。
【図３４】往路のＰ偏光の透過率を約７０％という高い数値になる深さを選び、その深さの回折格子に復路からＳ偏光を入射させて測定した回折効率を示す図である。
【図３５】従来技術
【図３６】従来技術
【符号の説明】
１方向性回折格子
２回折格子のピッチ
３往路での入射光
４回折光発生部
５空気中への回折角
６復路からの入射光
７基板中での回折光
８基板中での回折角
９空気中への屈折角
１０回折格子部
１１、１２、１３、１４、１５層
１６一化した方向性回折格子への入射光
１７プリズム
１８接着層
１９プリズムへの入射光
２０プリズムへの入射面
２１プリズムの斜面
２２方向性回折格子からの透過光
２４＋１次光
２５ −１次光
２６復路の透過光
２７ａ、ｂ受光素子
２８空気との界面
２９ａ、ｂ裏面回折格子
３０空気中への回折光
３１空気中での回折角
３２基板中での回折角
３３半導体レーザ
３４レンズ
３５筐体
３６方向性回折格子のパターン
３７ａ、ｂトラックパターン
３８フォーカスパターン
３９ａ、ｂトラック信号検出用受光素子
４０フォーカス信号検出用受光素子
４１＋１次光検出用受光素子
４２方向性回折格子からの透過光
４３、４４方向性回折格子内での回折光
４５、４６方向性回折格子内での反射回折光
４７、４８方向性回折格子内での回折反射光
４９Ｓ偏光
５０Ｐ偏光
５１復路での方向性回折格子の透過光
５２Ｘ方向
５３Ｚ方向
５４Ｙ方向
５５従来の回折格子
５６従来の回折格子の＋１次光
５７従来の回折格子の−１次光
５８偏光検出器としての方向性回折格子
５９、６０方向性回折格子の回折光の受光素子
６１方向性回折格子の透過光の受光素子
６２従来の回折格子の−１次光の受光素子
６３、６４方向性回折格子
６５光磁気信号を示す偏光方向
６６ガラス基板
６７レジスト膜
６８レジスト状回折格子
６９ガラス状回折格子
７０、７１露光用の光
７２ニオブ酸リチウム
７３格子
７４入射光
７５透過光
７６、７７回折光
７８異方性板
７９光学軸
８０充填材料[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a directional diffraction grating having directionality, an optical head device using the same, a magneto-optical head device, and an isolator.
[0002]
[Prior art]
Conventionally, the directionality of the diffraction grating, that is, the incident light from the side where the diffraction grating is located. This Diffractive in diffraction efficiency and transmittance when entering from the back side of the diffraction grating (Such a diffraction grating is called a directional diffraction grating.) Was not proposed. The diffraction grating used in the optical disk requires directivity, and the directivity is provided by a combination of a polarizing diffraction grating and a wave plate whose diffraction efficiency changes depending on the polarization. Various polarizing diffraction gratings have been proposed. For example, in Japanese Patent Laid-Open No. 63-55501, as shown in FIG. 35, a lithium niobate crystal plate 72 having a crystal axis in the x-axis direction, an ion exchange region having a period in the z-axis direction and a non-ion exchange A structure in which a lattice 73 composed of regions is formed is disclosed. In this way, when proton ion exchange is performed on lithium niobate, the refractive index No for ordinary light does not change, and the refractive index Ne for extraordinary light increases by about 0.13 and becomes almost Ne to No.
[0003]
Therefore, for the polarization component that vibrates in the y-axis direction of the incident light 74, that is, the ordinary light component, even if the grating 12 is formed by ion exchange, the refractive index is uniform in the plane, and the optical diffraction grating Since there is no effect, the transmitted light 75 passes straight through the crystal plate 72. On the other hand, for the polarization component that vibrates in the z-axis direction of the incident light 74, that is, the extraordinary light component, the refractive index is periodically changed to Ne + 0.13 in the proton ion exchange region of the lattice 73 and Ne in the non-exchange region. Since they are different, the diffracted light 76 and 77 is emitted from the crystal plate 72.
[0004]
As described above, by using this polarizing diffraction grating, it is possible to separate the incident light 74 into the 0th-order diffracted light and the ± 1st-order diffracted light, thereby separating the orthogonal polarization components. Further, since this polarizing plate is formed with a lattice 73 composed of an ion exchange region and a non-ion exchange region on the lithium niobate crystal plate 72, it can be made thin and small, and a lithium niobate crystal wafer. Can be produced as a raw material, and there is an advantage that it can be made in large quantities and at low cost by batch processing.
[0005]
Japanese Unexamined Patent Publication No. 63-247941 discloses a polarizing diffraction grating as shown in FIG. In this polarizing diffraction grating, a grating groove is formed in an anisotropic plate 78 having refractive index anisotropy, and the refractive index in the direction of the optical axis 79 is Ne or the refractive index in the direction orthogonal thereto is No. In this way, a polarization component or a component parallel to the optical axis 79 is separated as diffracted light. According to this polarizing diffraction grating, the grating grooves are formed in the anisotropic plate 78 and the grooves are filled with the filling material 80, so that there is an advantage that the size can be reduced.
[0006]
[Problems to be solved by the invention]
However, in any polarizing diffraction grating, the direction of incident light, that is, the efficiency of diffracted light and the efficiency of transmitted light, regardless of whether it is incident from the diffraction grating side or the back side of the diffraction grating, is the principle. Are the same. Therefore, in applications where the diffraction grating is placed in the optical path and the characteristics need to change depending on the incident direction, it is necessary to set the wavelength plate to one side and use the polarization to change depending on the incident direction. An increase in size and an increase in size were inevitable.
[0007]
Therefore, an object of the present invention is to provide a diffractive grating having directionality.
[0008]
[Means for Solving the Problems]
As a result of intensive studies and studies to solve the above-mentioned problems, the present inventor is easy to manufacture and suitable for mass productivity by appropriately setting the specifications such as wavelength, pitch and refractive index in the diffraction grating. Succeeded in developing a directional diffraction grating.
[0009]
That is, according to the present invention, a diffraction grating in which sections of the refractive index N1 and the refractive index N2 are alternately arranged at a pitch Λ between the material 1 and the material 2 having different refractive indexes of the refractive index N1 and the refractive index N2. However, it is a directional diffraction grating formed by setting the pitch to an optimum range with respect to the wavelength.
[0010]
The present invention is a diffraction grating in which sections of refractive index N1 and refractive index N2 are alternately arranged at a pitch Λ between

materials

1 and 2 having different refractive indexes of refractive index N1 and refractive index N2. N1> N2), and the pitch is formed to be larger than the above range with respect to the wavelength.
[0011]
In the present invention, sections having different refractive indexes N1 and N2 are alternately arranged between a material (material 3) made of at least one layer and a material (material 4) made of at least one layer. And a refractive index N3 of a material in contact with the partition in at least one material, N4 being the lowest refractive index in the other material, and λ being a wavelength to be used. Sometimes, the directional diffraction grating is formed by setting each specification within an optimum range.
[0012]
In the present invention, sections having different refractive indexes N1 and N2 are alternately arranged between a material (material 3) made of at least one layer and a material (material 4) made of at least one layer. And a refractive index N3 of a material in contact with the partition in at least one material, N4 being the lowest refractive index in the other material, and λ being a wavelength to be used. Sometimes a directional diffraction grating is formed with a pitch larger than the above range.
[0013]
The present invention is a directional diffraction grating in which at least one of the sections constituting the diffraction grating is formed of a birefringent material.
[0014]
In the present invention, the light from the light source is incident on the directional diffraction grating, the transmitted light is narrowed down on the recording medium using a lens, the reflected light is diffracted by the directional diffraction grating, and the total reflection surface is not provided. It is an optical head that guides light to the light receiving element.
[0015]
In the present invention, light from a light source is incident on a directional diffraction grating disposed on a prism, transmitted light is narrowed down on a recording medium by a lens, and reflected light from the recording medium is returned to the directional diffraction grating and diffracted. Of the diffracted light, the optical head device detects light by transmitting light transmitted through the slope of the prism and light transmitted through the slope of the prism after being reflected by one surface of the prism.
[0016]
In the present invention, light from a light source is incident on the above directional diffraction grating, transmitted light is narrowed down on a recording medium using a lens, reflected light is diffracted by the directional diffraction grating, and a separate diffraction grating is provided. The optical head guides light to the light receiving element.
[0017]
In the present invention, the light from the light source is set to P-polarized light with respect to the directional diffraction grating, a quarter-wave plate is provided between the directional diffraction grating and the recording medium, and the transmitted light from the directional diffraction grating is circularly polarized. The optical head converts the reflected light from the recording medium into S-polarized light and diffracts it with a directional diffraction grating.
[0018]
The present invention is a polarization detector that allows light to be incident from a low refractive index to a high refractive index in a diffracted light generator of a directional diffraction grating, and separates polarized light into transmitted light and diffracted light.
[0019]
The present invention is a magneto-optical head device on which the polarization detector is mounted.
[0020]
The present invention is an optical head device in which a directional diffraction grating is provided on a prism, and a magneto-optical signal is detected by further providing a directional diffraction grating on the slope of the optical head device.
[0021]
In the diffracted light generating portion of the directional diffraction grating, the present invention is formed by allowing P-polarized light to enter from the higher refractive index to the lower and allowing S-polarized light to enter from the lower refractive index to the higher. This is an isolator.
[0022]
In the diffracted light generating portion of the directional diffraction grating, the present invention is formed by allowing S-polarized light to enter from the higher refractive index to lower light and allowing P-polarized light to enter from the lower refractive index to the higher refractive index. This is an isolator.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Details will be described below. 1 and 2 are cross-sectional views of the main part of a directional diffraction grating 1 according to the present invention. FIG. 1 is an optical path diagram of diffracted light when light is obliquely incident from both sides of the directional diffraction grating 1. In the present invention, the principle is the same in both cases of oblique incidence and perpendicular incidence. For the sake of simplicity of explanation, the case of perpendicular incidence will be described below with reference to FIG. In FIG. 2, a directional diffraction grating 1 is a diffraction grating formed on one surface of a substrate made of a transparent material. The pitch 2 of the directional diffraction grating may be a so-called equal pitch diffraction grating that forms a constant pitch depending on the location of the diffraction grating, or may be a diffraction grating that changes depending on the location of the diffraction grating surface. In the following description, the pitch of the light incident portion will be described as Λ. It is assumed that the light 3 is vertically incident on a directional diffraction grating from a surface where the diffraction grating is not formed (hereinafter referred to as a back surface) (hereinafter referred to as an outward path). The back side should be flat and parallel to the plane with the diffraction grating.
[0024]
As shown in FIG. 2, light incident on the back surface of the directional diffraction grating 1 from the material surrounding the directional diffraction grating 1 (here, air, but hereinafter referred to as an external material) vertically enters the direction of the substrate. Go straight without changing. When reaching the periodic structure portion 4 (hereinafter, referred to as a diffracted light generating portion) composed of two sections having different refractive indexes, which are waveform portions in FIG. 2, diffraction occurs there. If the diffraction angle 5 is θ1, the well-known diffraction conditional expression 1 is satisfied.
[0025]
[Formula 1]

[0026]
Here, the wavelength of light is λ, and the refractive index of the external material is N2. By using this equation, θ1 can be obtained. Next, as shown in FIG. 2, it is assumed that the light 6 is incident from a surface on which the diffracted light generator 4 is present (hereinafter referred to as a return path). When the diffraction angle 8 of the diffracted light 7 is θ2, the diffraction conditional expression in the diffracted light generating unit 4 is as shown in Expression 2.
[0027]
[Formula 2]

[0028]
Here, the refractive index of the material forming the diffraction grating is N1. The diffracted light is refracted into the external material on the back surface. The relationship when the refraction angle 9 is θ3 is as shown in Equation 3.
[0029]
[Formula 3]

[0030]
Next, by eliminating θ2 from Equation 2 and Equation 3, the refraction angle 9 becomes Equation 4.
[0031]
[Formula 4]

[0032]
Thereby, it can be seen that θ1 and θ3 have the same value. This indicates that the diffraction angle does not change for a general diffraction grating when it is incident from the diffraction grating surface and when it is incident from the back surface.
Next, the diffraction angle 7 in the substrate of the diffraction grating shows a value different from θ1 and θ3 as shown in Equation 2.
[0033]
Specific numerical values are set, and θ1 and θ2 are obtained using the diffraction grating pitch Λ as a parameter. Assuming that λ is 635 nm, N1 is 1.5, and N2 is 1 (in the air),

Equations

1 and 2 become

Equations

5 and 6, respectively.
[0034]
[Formula 5]

[0035]
[Formula 6]

[0036]
Table 1 exemplifies specific values of θ1 and θ2 in Equation 5 and Equation 6 with pitch Λ as a parameter. Here, “not diffracted” refers to a case in which each right side of Expression 5 and Expression 6 is greater than 1, and the diffraction condition is not satisfied, so that diffracted light is not generated.
[0037]
[Table 1]

[0038]
From this, it can be said that only θ2 can have a value at a certain pitch. That is, it can be seen that diffracted light is generated only in the return path. In this specific example, since the refractive index of the diffraction grating is larger than the refractive index of air, there are cases where diffraction occurs only in the forward path, but when the refractive index of the external medium is greater than the refractive index of the diffraction grating, only the return path is diffracted. A case occurs. In short, by appropriately setting the pitch, the refractive index on the exit side (the side where diffracted light is generated) in the diffraction grating is larger than the refractive index on the incident side (hereinafter referred to as incidence on a high refractive index medium). ) Can generate diffracted light only. At this time, the direction in which the refractive index on the exit side of the diffraction grating is smaller than the refractive index on the incident side (hereinafter referred to as incidence on the low refractive index medium) can be only transmitted light. Can be different.
[0039]
The condition for realizing this directional function is a pitch range in which the right side of Expression 5 is larger than 1 and the right side of Expression 6 is smaller than 1 in Expression 5 and Expression 6. Since the pitch range is such that the right side of 1 is larger than 1 and the right side of Equation 6 is smaller than 1, Equation 7 is easily obtained.
[0040]
[Formula 7]

[0041]
This is because the diffraction grating under consideration is that one material is in contact with the air layer, so the diffraction grating is composed of sections with different refractive indexes, and the surrounding material is different from each other. The case where it is the same as the refractive index of a division is shown. As a specific example, assuming that a surface relief type is used for the diffraction grating, N1 is 1.5 assuming glass, N2 is 1 when the external medium is air, and Equation 8 is established.
[0042]
[Formula 8]

[0043]
The pitch is smaller than the wavelength and larger than about 70% of the wavelength. When the condition for realizing the directional function is the case where sin θ1 exceeds 1, as described above, sin θ3 also exceeds 1, and the diffracted light generated by incidence on the high refractive index medium is diffracted. It cannot be emitted from the back of the grating and is totally reflected. These states are shown in FIG.
[0044]
Thus, in the case of the directional diffraction grating, the refractive index of the layer of the external medium (here, air) in contact with the diffraction grating during the return path and the layer of the external medium (here, the side where the diffracted light is emitted) If the refractive index of air) is the same, the diffracted light to the high refractive index medium exceeds the critical angle on the back surface and cannot be extracted into the external medium. When used for general isolator applications, light in the return path is unnecessary, which is convenient.
[0045]
There are many configurations of diffraction gratings other than those shown in FIG. Therefore, the directional diffraction grating 1 is generalized. There are several types of diffraction gratings, one in which sections having different refractive indexes are arranged alternately, one in which sections for absorbing and transmitting light are arranged, and one in which light is blocked and transmitted. There are things that are lined up. FIG. 4 shows a general diffraction grating. A diffraction grating in which sections having different refractive indexes are alternately arranged, or a section in which light is absorbed or blocked and sections in which light is transmitted are alternately arranged. There are at least one layer of different refractive indexes (layer 11, layer 12, layer 13, layer 14, and layer 15) above and below each other. Consider the case where the light 6 is incident vertically from above, but the principle is the same for oblique incidence as described above.
[0046]
The light 6 enters the directional diffraction grating 1 perpendicularly, reaches the diffracted light generation unit 10, and diffracted light 7 is generated. The diffracted light is incident on the layer 13 in contact with the diffracted light generator 10, further passes through the layer 14, reaches the layer 15 in contact with air, and exits into the external medium. The layer 13 is the first layer, and as the diffracted light 7 travels, the light angle in each layer is φp, the refractive index is Np (p is a subscript of a natural number), the wavelength used is λ, and the pitch is Λ. A diffraction conditional expression in the diffracted light generation unit 10 is as shown in Expression 9.
[0047]
[Formula 9]

[0048]
Even if the diffracted light generator 10 is a diffraction grating in which sections having different refractive indexes are alternately arranged, the diffraction conditional expression is used even in a case in which a section for absorbing or blocking light and a section for transmitting light are alternately arranged. Equation 9 is the same equation. In the case of diffracting, since φp must exist in Equation 9, Equation 10 holds.
[0049]
[Formula 10]

[0050]
Next, the angle of light in each layer is φ1 for the layer 13, φ2 for the layer 14, and φ3 for the layer 15, and the refractive indexes of the respective layers are N1, N2, and N3. .
[0051]
[Formula 11]

From the equations 9 and 11, the following equation is established.
[0052]
[Formula 12]

[0053]
[Formula 13]

[0054]
[Formula 14]

[0055]
In order to prevent total reflection in each layer, the right side of Expressions 12 to 14 needs to be 1 or less. Therefore, Expression 15 is established when Nm is the smallest refractive index among N1, N2, and N3.
[0056]
[Formula 15]

[0057]
If the layer 15 is air as an external medium, the refractive index of air is generally close to that of a vacuum and is very small, so Nm may be 1. That is, Expression 16 is obtained.
[0058]
[Formula 16]

[0059]
Λ / N1 in Equation 10 is smaller than λ when N1 is not air or vacuum (hereinafter, only considered in the case of air). Thus, what can be said from Equations 10 and 16 is that diffracted light is not generated if Λ is less than λ / N1, and diffracted light is generated if Λ is greater than λ / N1, but if Λ is not greater than λ, This means that diffracted light is not in the air due to reflection.
[0060]
Next, the conditions under which the reverse direction light 16 does not generate diffracted light will be described. The light 16 enters from below the directional diffraction grating 1 in FIG. 4 and reaches the diffracted light generating section. When the refractive index of the layer 11 immediately after exiting the diffracted light generating portion is N4 and the diffraction angle is φ4, the diffraction condition is expressed by Equation 17.
[0061]
[Formula 17]

[0062]
Since the condition that the diffracted light is not generated is when the right side is larger than 1, Expression 18 is satisfied.
[0063]
[Formula 18]

[0064]
The light transmitted to the layer 11 enters the next layer 12 perpendicularly, passes through, and finally exits into the air, which is an external medium. Formula 15 and Formula 18 are put together, N4 is changed to Nn, and the formula 19 is obtained.
[0065]
[Formula 19]

[0066]
This is a general formula for exerting the performance of the directional diffraction grating 1.
[0067]
Next, a description will be given of a method for taking out diffracted light that cannot be taken out to the outside medium (or to a light receiving element in the external medium) for use. As a first method, it is possible to prevent such diffracted light from being totally reflected on the back surface of the diffraction grating. There are several ways to avoid total reflection. For example, there is a method of making the incident angle of light smaller than the critical angle. When receiving light with a light receiving element, there is a method of optimizing or matching the refractive index in the optical path to the light receiving element. As a method of making the incident angle of light smaller than the critical angle, there is a method of changing the angle of the back surface of the diffraction grating.
This configuration will be described with reference to FIG.
[0068]
A diffraction grating is formed on the prism. FIG. 5 shows a case where the directional diffraction grating 1 produced in a flat plate shape is bonded to the prism 17 by providing an adhesive layer 18. In the case of bonding, pay attention to the refractive index of the adhesive, and the intermediate value between the refractive index of the portion in contact with the bonding surface of the directional diffraction grating 1 and the refractive index of the prism. of Is desirable. This is because reflection at the interface with the adhesive can be reduced.
[0069]
The case where the incident light 19 is emitted from the incident surface 20 of the prism as shown in FIG. Although FIG. 6 shows the case of normal incidence, the principle of the present invention is the same for oblique incidence. The light incident on the prism is totally reflected by the inclined surface 21 and travels in the direction of the directional diffraction grating. In the directional diffraction grating, specifications are set based on the above-described principle, and the transmitted light 22 is emitted. Next, FIG. 7 shows the case of the return path. The light 23 is incident from a surface having a directional diffraction grating. Diffracted light is generated by the directional diffraction grating and is divided into + 1st order light 24 and −1st order light 25. The + 1st order light travels toward the slope and is incident on the slope at an angle much smaller than the total reflection angle, so that it is transmitted to the outside with a high transmittance. The −1st order light is directed to the incident surface, totally reflected and directed to the inclined surface, and is transmitted with a high transmittance like the + 1st order light. In this way, diffracted light can be extracted to the outside. In the directional diffraction grating, the transmitted light 26 is also generated, so that it returns along the same optical path as the forward path.
[0070]
Explain with specific numbers. If the pitch of the directional diffraction grating is 0.53 μm, the wavelength is 0.635 μm, and the refractive index of the directional diffraction grating, the prism, and the adhesive is 1.5, the diffraction angle θ2 is 53 degrees. The prism shape is an isosceles triangle. At this time, the incident angle of the −1st-order light to the incident surface 20 is 37 °. Since the critical angle of a material having a refractive index of 1.5 and a material having a refractive index of 1 is 48 degrees, the −1st order light is totally reflected. Furthermore, since the −1st order light is incident on the inclined surface at 8 degrees, it is transmitted with a very high transmittance. Since the incident angle of the + 1st order light on the inclined surface is 8 as in the case of the −1st order light, it transmits with a high transmittance. If non-reflective coating is applied to the slope, diffracted light can be extracted with a higher transmittance. It goes without saying that even if the incident angle of the diffracted light in the prism with respect to the inclined surface does not exceed the total reflection angle, a high reflectance can be obtained if it is incident at a large angle.
[0071]
Next, there is a method in which the incident angle of light is made smaller than the critical angle, and a method in which a part of the interface between the directional diffraction grating and the air is inclined with respect to the surface of the diffracted light generating part so as to be transmitted with a smaller angle than the total reflection angle. is there. This will be described with reference to FIG. The light 23 on the return path enters the directional diffraction grating 1 substantially perpendicularly. + 1st order light 24 and −1st order light 25 are generated and travel toward the interface with the air on the back surface. A part of the interface is processed so that the diffracted light reaches the interface at an incident angle smaller than the total reflection angle. In the figure, it is set to be substantially vertical, but this is because the Fresnel reflection is the smallest when it is vertical, and light is transmitted even if it is not vertical. Needless to say, the transmittance can be increased by applying a non-reflective coating. Further, as will be described later, matching of the refractive index between the light receiving element 27a and the light receiving element 27b may be used in combination.
[0072]
As another method of extracting the diffracted light onto the light receiving element into the external medium, there is a method of receiving the light by arranging the light receiving element on the back surface without totally reflecting the diffracted light on the back surface of the diffraction grating. This will be described with reference to FIG. The light 23 on the return path enters the directional diffraction grating 1 substantially perpendicularly. + 1st order light 24 and −1st order light 25 are generated and travel toward the interface 28 with the air on the back surface. Assuming that the interface is substantially parallel to the diffracted light generator, the diffracted light is totally reflected by the above-described principle. The total reflection is because the refractive index of air is small as described above, and the transmittance to the light receiving elements 27a and 27b can be increased by filling with a material (not shown) that matches the refractive index. . For example, matching oil can be used, and there is a method in which the light receiving element is bonded to the diffraction grating with an adhesive having a refractive index matching. Photodiodes that represent light-receiving elements on the market have a light-receiving surface protected with plastic, and their refractive index is very close to the refractive index of glass. Can be glued at a rate. Also a plus for photodiode protection H Is close to refractive index H Needless to say, it is possible to bond with a lower reflection by using a diffraction grating manufactured by a sack and using an adhesive having a matching refractive index.
[0073]
As another method for extracting the diffracted light to the external medium, as shown in FIG. 10, diffraction gratings (hereinafter referred to as back surface diffraction gratings) 29a and 29b are further provided on the back surface of the diffraction grating, and the diffraction light is extracted outside by diffraction. There is. In this way, light can be extracted into the external medium by the diffraction phenomenon. In addition, since diffraction is caused twice, it is possible to provide an advantage that the direction of light is hardly changed due to the achromatic effect even when the wavelength of the light source is changed.
[0074]
As described above, the directional diffraction grating when Expression 19 is satisfied has been described. The case where the pitch is larger than the pitch satisfying the condition will be described with reference to FIG. In this case, it goes without saying that even if the interface between the diffracted light generator and the air on the back surface is parallel, it can be extracted outside without being totally reflected at the interface with air. However, in this case, the diffracted light 30 is generated even in the forward path. However, due to the form of use of the directional diffraction grating, the diffracted light is generated in the forward path, so that the efficiency of using the transmitted light in the forward path is slightly increased. Even if it is damaged, there are cases where the return path has a high diffraction efficiency and can be used sufficiently. In the diffraction from the high-refractive index medium to the low-refractive index medium, which is the forward path, the diffraction angle 31 is close to 90 °, so that the phase matching condition is largely off and the diffraction efficiency is low (the diffraction angle is 90 °). In this case, in the diffraction from the low refractive index medium to the high refractive index medium, which is the return path, the diffraction angle 32 is much smaller than 90 °, the diffraction efficiency is still kept large. Specifically, how much the pitch is effective as compared with the wavelength will be described later together with an example.
[0075]
Next, it will be shown that a compact and efficient optical head can be configured for each case of the directional diffraction grating described above. As a first configuration, FIG. 12 shows a configuration in which refractive index matching is performed between the directional diffraction grating and the light receiving element so that the diffracted light is not totally reflected by the back surface of the diffraction grating. The directional diffraction grating 1 is formed on one surface of a parallel substrate. Light 3 from a semiconductor laser chip 33 as a light source is incident from the back surface of the directional diffraction grating. It passes through the substrate and further passes through the directional diffraction grating. The light transmitted through the directional diffraction grating is narrowed down on a recording medium (not shown) by the lens 34. The reflected light from the recording medium is returned to the directional diffraction grating and received by the light receiving elements 27a and 27b. Each component is incorporated into a small casing 35 by bonding or the like. The directional diffraction grating is, for example, a diffraction grating 36 as shown in FIG. 13 and generates + 1st order light 24 and −1st order light 25. The surface of the diffraction grating is divided into track detection areas 37 a and 37 b and a focus detection area 38. In the track detection area, in order to extract a track signal from the track pattern in the reflected light from the recording medium, the diffraction grating is divided into two in the groove of the track, and the −1st order light generated in each of the photodiodes ( Hereinafter, the light is incident on PD) 39a and 39b. The −1st order light generated by the diffraction grating in the focusing region is incident on the two-divided PD 40, and the focus signal is detected by the knife edge method. The + 1st order light is received by the PD 41. The output of each PD is also used for recording signal detection. In other signal detection methods, in addition to the knife edge method for the focus detection method, the astigmatism method, the Foucault method, and other methods such as the double knife edge method, the double astigmatism method, and the double Foucault method are used. Pattern. The track detection method may be a pattern adapted to the method, such as a push-pull method, a wobbling method, a DPD method, or a DPP method. Thus, a small optical head can be configured. The recording medium may be either ROM or RAM. Since the directional diffraction grating is high in efficiency, it can be sufficiently used in a RAM.
[0076]
As a second configuration, FIG. 14 shows a configuration in which the back surface of the directional diffraction grating is partially inclined. Light 3 from the light source is incident from the back surface of the directional diffraction grating 1. It passes through the substrate and further passes through the directional diffraction grating. The light transmitted through the directional diffraction grating is narrowed down on a recording medium (not shown) by the lens 34. The reflected light from the recording medium is returned to the directional diffraction grating to generate ± first order light. As described above, each diffracted light enters the interface having a smaller total reflection angle formed on the back surface of the substrate and reaches the PD. The diffraction grating pattern may be the one described with reference to FIG. The signal detection method can have flexibility as described above. Thus, a small optical head can be configured. The recording medium may be either ROM or RAM. Since the directional diffraction grating is high in efficiency, it can be sufficiently used in a RAM.
[0077]
As a third configuration, FIG. 15 shows a configuration using a directional diffraction grating provided with a back surface diffraction grating. Light 3 from the light source is incident from the opposing surface of the directional diffraction grating. The light passes through the substrate and further passes through the directional diffraction grating 1. The light transmitted through the directional diffraction grating is narrowed onto a recording medium (not shown) by a lens. The reflected light from the recording medium is returned to the directional diffraction grating to generate ± first order light. The light is incident on the back surface diffraction gratings 29a and 29b provided on the back surface of the directional diffraction grating. If the pitch of the back surface diffraction grating is the same as the pitch of the directional diffraction grating, the diffracted light at the back surface diffraction grating can be parallel to the light of the light source. When the wavelength of the light source changes, the diffraction angle of the diffracted light at the directional diffraction grating changes, the position on the back surface diffraction grating changes, and spot changes occur on the PDs 27a and 27b. However, if the pitch of the back surface diffraction grating is made smaller than the pitch of the directional diffraction grating, the change in the diffraction angle at the back surface diffraction grating is increased, and the change in the spot position on the PD can be canceled out. In this manner, a stable and small optical head can be configured even when the wavelength of the light source is changed. The signal detection method can have flexibility as described above. The recording medium may be either ROM or RAM. Since the directional diffraction grating is high in efficiency, it can be sufficiently used in a RAM.
[0078]
As a fourth configuration, FIG. 16 shows a case where the diffraction grating is formed on the prism so that the diffracted light is not totally reflected by the back surface of the diffraction grating. The directional diffraction grating is formed on one surface which is not an inclined surface of a right isosceles triangle. The light from the light source is made incident from another surface that is not a slope. The behavior of light in the prism is as described above. The directional diffraction grating is as described above. The light 3 emitted from the semiconductor laser 33 passes through the directional diffraction grating, and is narrowed down onto a recording medium (not shown) by the lens 34. The reflected light from the recording medium is returned to the directional diffraction grating to generate ± first order light. The + 1st order light 24 passes through the prism slope 21 as it is and enters the PD 27a, and the −1st order light 25 is reflected by the incident surface, passes through the slope, and enters the PD 27b. Thus, a small optical head can be configured. The signal detection method can have flexibility as described above. The recording medium may be either ROM or RAM. Since the directional diffraction grating is high in efficiency, it can be sufficiently used in a RAM.
[0079]
Next, the polarization characteristics of the directional diffraction grating 1 will be considered. FIG. 17 shows a surface relief diffraction grating composed of one kind of material. The light 3 is incident from the back side of the diffracted light generator. In the directional diffraction grating, diffracted light is not emitted to the air side as described above. However, diffracted light of + 1st order light 43 and −1st order light 44 exists before exiting to the air side. These diffracted lights are not emitted because they are not phase-matched to the air side, but the phase matching condition is satisfied in the direction of high refractive index, that is, the direction of reflection, and the + 1st order light 43 is the + 1st order light as reflected diffracted light. 45, and the −1st order light 44 exists as −1st order light 46 as reflected diffracted light. The reflected diffracted lights 45 and 46 are partially converted into reflected light 47 and reflected light 48 (hereinafter referred to as diffracted reflected light) in the opposite direction to the incident light 3 due to the interaction with the diffraction grating.
[0080]
The efficiency of these reflected diffracted light and diffracted reflected light largely depends mainly on the refractive index of the diffraction grating and the grating depth which is the interaction length. It is known that a diffraction grating having a small pitch, such as a directional diffraction grating, generally has a different diffractive action. S-polarized light 49 (polarization direction and grating vector are perpendicular) and P-polarized light 50 (polarization direction and grating vector). Is parallel), it is known that s-polarized light has a larger diffraction effect. For this reason, the amount of light energy transferred from incident light to reflected diffracted light or diffracted reflected light is less for P-polarized light than for S-polarized light. Therefore, the transmittance in the directional diffraction grating is larger in the P-polarized light. Furthermore, most of the S-polarized light in the incident light can be diffracted and reflected light, and most of the P-polarized light can be transmitted light. This shows the action of a kind of polarizing beam splitter (PBS), but the conventional dielectric multilayered PBS is obliquely incident on the film. The In the case of a directional diffraction grating, it is characterized in that normal incidence is sufficient.
[0081]
When the light 23 is incident from the opposite direction, the S-polarized light 49 has a diffractive effect larger than that of the P-polarized light 50, so that the diffraction efficiency of the + 1st-order light 24 and the -1st-order light 25 is S-polarized. Becomes larger than P-polarized light. The transmitted light 51 is dominated by P-polarized light. Since such polarization characteristics are the same as those of the polarizing diffraction grating described in the prior art, it can be said that the directional diffraction grating according to the present invention has both the directionality and the side surface of the polarizing diffraction grating.
[0082]
By utilizing this polarization characteristic, a quarter wavelength plate is provided between the directional diffraction grating and the recording medium, and the forward path is used as P-polarized light and the return path is used as S-polarized light, whereby the directionality can be further improved. Specifically, in the optical head described in FIGS. 12 to 16, a quarter-wave plate is set between the directional diffraction grating and the recording medium, and the light is changed to P-polarized light with respect to the directional diffraction grating. The position of the quarter-wave plate is set so that the transmitted light from the directional diffraction grating is circularly polarized, and the light returned from the recording medium is This can be realized by converting the light into S-polarized light with the quarter-wave plate, diffracting it with a directional diffraction grating, and guiding it to the PD (not shown).
[0083]
In an optical head using a directional diffraction grating, when it is desired to use the forward path as P-polarized light and the return path as S-polarized light and to increase the forward transmittance and the diffraction efficiency of the return path, the diffraction effect for the P-polarized light is reduced. Although it is sufficient to increase the diffraction effect of polarized light, there is a method using a birefringent material as a method for realizing this. Take the diffraction grating described in FIGS. 17 and 18 as an example. Examples of the birefringent material include 3m crystal lithium niobate and lithium tantalate. Different refractive indexes are shown for two linearly polarized light beams, ie, ordinary light refractive index No and extraordinary light refractive index Ne. The case of lithium niobate will be described with reference to FIG. A diffraction grating is produced on the surface formed by the Z direction 53 and the X direction 52, and the direction of the grating vector is the Z direction. Light is incident in the Y direction 54. Since the refractive index felt that the incident light 3 is P-polarized light is Ne, it is 2.20. When the returning light 23 is S-polarized light, the refractive index felt is No. 2.286. By doing so, it is possible to select a smaller refractive index for the P-polarized light in the forward path, so that the transmittance is high, and for the S-polarized light in the backward path, a large refractive index can be selected, so that the diffraction efficiency can be increased. In the case of lithium tantalate, it can be realized by appropriately selecting the axial direction and the lattice vector direction.
[0084]
As described with reference to FIG. 18, when light is incident from the front side of the diffraction grating, the transmitted light is predominantly P-polarized light and the diffracted light is predominantly S-polarized light. If this is utilized, polarization can be detected. For example, when the configurations shown in FIGS. 7, 8, 9, and 10 are used, the polarizations are separated as shown in FIGS. 20, 21, 22, and 23, and ± 1st order lights are detected. The polarization state of the incident light can be known by taking the difference between the sum of the outputs of the light receiving elements to be detected and the output of the light receiving element for detecting the transmitted light. By utilizing this, it can be applied to a small magneto-optical head. An example using the configuration of FIG. 20 will be described with reference to FIG. Light 3 from the semiconductor laser 33 is incident on the diffraction grating 55. This diffraction grating may be a conventional diffraction grating. The emitted light is incident on and reflected by a magneto-optical recording medium (illustrated), and the reflected light 23 returns to the diffraction grating 55 again. The diffraction grating 55 detects a focus signal and a track signal, and various patterns can be applied to the diffraction grating pattern as described above. Here, it is assumed to be shown in FIG. Diffracted light is divided into + 1st order light 56 and −1st order light 57. Since the polarization of each diffracted light is rotated by the magneto-optical signal, the magneto-optical signal can be detected by detecting the polarization state of either diffracted light. In the figure, a directional diffraction grating is applied only to + 1st order light for simplification of explanation. The + 1st order light is incident on the directional diffraction grating 58 described with reference to FIG. The grating vector of the directional diffraction grating is set so that the polarization state of the reflected light forms 45 ° with the grating vector of the directional diffraction grating as shown in FIG. Since the light separated into the three first-order lights by the diffraction grating 55 is further divided into the transmitted light and the ± first-order light, nine diffracted lights are generated in total. Among them, the diffracted light enters the PD 59 and the PD 60, the transmitted light enters the PD 61, and the magneto-optical signal can be detected by detecting the polarization state based on the principle described above. The minus first-order light from the diffraction grating 55 is used for focus detection and the like.
[0085]
Needless to say, if the diffraction grating 55 is blazed, only one diffracted light is generated with high efficiency, and a directional diffraction grating is applied thereto, the magneto-optical signal can be detected efficiently. Of course, it is possible to obtain a magneto-optical signal by applying a directional diffraction grating to the minus first-order light of the diffraction grating 55. In this case, various signals may be detected using the divided light receiving element 62 as in the past, and the polarization state may be detected by providing a PD that receives the diffracted light of the directional diffraction grating (not shown). . In this way, it goes without saying that a larger magneto-optical signal can be obtained by applying a directional diffraction grating to both the ± first-order lights of the diffraction grating 55. In addition, the directional diffraction grating shown in FIG. 13 is applied, but it goes without saying that the same function can be obtained by using other directional diffraction gratings.
[0086]
The magneto-optical head can also be configured by utilizing the PBS function of the directional diffraction grating as described above. This will be described with reference to FIG. FIG. 26A shows the configuration. This basically uses the optical head described with reference to FIG. The light 3 is incident as P-polarized light, totally reflected by the inclined surface 21 and incident on the directional diffraction grating. The transmitted light passes through a lens or the like (not shown) and is irradiated onto a magneto-optical recording medium (not shown), and the S-polarized light component, which is a signal component, is added to the light by the recording signal, reflected, and directed again. Return to the diffraction grating. As described above, the directional diffraction grating exhibits polarization dependency in the diffraction efficiency, but the S-polarized light that is the recording signal can increase the transmittance, and the P-polarized light that is not the signal component can decrease the transmittance. This shows the same effect as a polarization angle increasing method known as a method for increasing a signal component which is often used in a magneto-optical head. Next, the diffracted light reaches the inclined surface 21, and the directional diffraction grating showing the function of the PBS is formed on the inclined surface 21. The directional diffraction grating is set as shown in FIG. 26B, the grating vectors of the + 1st-order directional diffraction grating 63 and the -1st-order directional diffraction grating 64 are perpendicular, and the P-polarized light 48 The direction is set to 45 ° with respect to the S-polarized light 49. FIG. 26C shows the state of polarization of the reflected light from the magneto-optical recording medium. In contrast to the P-polarized light 48 that is the polarization of the incident light, the polarized light 65 tilted to the left in the figure returns. In the directional diffraction grating 64, since the S-polarized light component increases, the reflected light increases and the transmitted light amount decreases. On the other hand, in the directional diffraction grating 63, the amount of transmitted light increases because the P-polarized component increases. For this reason, a magneto-optical signal can be detected by taking the difference between both light quantities. The focus signal and the track signal can be detected by setting the directional diffraction grating 1 itself to the aforementioned pattern. Therefore, there is no need to make a PD other than the PD described with reference to FIG. 13, and the difference between the PD output for detecting the + 1st order light of the directional diffraction grating 1 and the PD output for detecting the −1st order light may be taken.
[0087]
Next, a method for manufacturing a diffraction grating according to the present invention will be described. A surface relief diffraction grating is most suitable for the diffraction grating. This is because the surface relief diffraction grating is a structure based on modulation of the refractive index of the diffraction grating material and air, so that a large difference in refractive index between N1 and N2 can be obtained, and the allowable range of pitch is increased by Equation 7. Because. Therefore, a method for creating a surface relief diffraction grating will be described. FIG. 27 is a process diagram thereof. The material of the diffraction grating is glass, and a resist 67 is applied on the glass substrate 66 as shown in FIG. The resist may be a positive type or a negative type, but a resist having excellent resolution is desirable. The coating method is spin coating I Can be I Pukote I May be used. Next, as shown in FIG. 27B, the resist is patterned so as to have a desired pitch of the diffraction grating to obtain a diffraction grating 68. As a patterning method, a method of developing after performing mask contact exposure or reduced projection exposure can be adopted as a general method (not shown). As a simple exposure method, a two-beam interference exposure method may be adopted. As shown in FIG. 28, this is a method of recording interference fringes by causing two lights 70 and 71 to interfere on the resist surface. Next, as shown in FIG. 27C, the glass is etched using the produced resist pattern as a mask. Etching can be dry or Ye It may be. Next, as shown in FIG. 27D, the diffraction grating 69 can be finally obtained by dissolving the resist. Here, even the state shown in FIG. 27B can be used as a diffraction grating. However, resists generally do not have good transmittance, so if a very high transmittance is desired, a glass diffraction grating with good transmittance can be produced through the steps up to FIG. desirable. The shape of the diffraction grating is not limited.
[0088]
【Example】
Examples of the present invention will be described below. For the production of the diffraction grating, patterning was performed using the two-beam interference exposure method in the method shown in FIG. 27, and the diffraction grating in the state shown in FIG. Since the wavelength of light used for efficiency measurement is red at 635 nm and the resist used is red and transparent, there is no inferiority to a transparent glass substrate in terms of light utilization efficiency. The production conditions are shown in Table 2. From Equation 8, the pitch Λ is required to be larger than 0.423 μm and smaller than 0.635 μm, so it is set to 0.6 μm.
[0089]
[Table 2]

[0090]
Based on these conditions, diffraction gratings with different depths were produced. The depth was controlled by changing the exposure amount. The evaluation was performed with the setting shown in FIG. A prism with a right-angled isosceles triangle was adopted, and the produced diffraction grating was brought into contact with the prism using refractive index matching oil, thereby suppressing the reflected light on the back surface of the diffraction grating and the surface of the prism. Measurements were made on the forward path incident from the back side of the diffraction grating and the return path incident from the diffraction grating surface. Using a semiconductor laser with a wavelength of 635 nm, the transmitted light, the diffracted light of the + 1st order light and the −1st order light, and the reflected light were measured for each polarization of S polarization and P polarization.
[0091]
The measurement results are shown in FIGS. FIG. 29 shows the forward transmittance of the P-polarized light in the forward path and the efficiency of the diffracted light of each polarized light in the backward path. The horizontal axis is the relative depth of the diffraction grating. For P-polarized light, the forward transmission is 90% at a depth of 7, and the diffraction efficiency of the return path is about 20% for both the + 1st order light and the −1st order light. % Has been demonstrated. This proved to be a directional diffraction grating.
[0092]
Further, the diffraction efficiency of the S-polarized light in the return path is more than twice the diffraction efficiency of the P-polarized light, demonstrating the side surface of the polarizing diffraction grating.
[0093]
Next, FIG. 30 shows the transmittance and reflectance of forward S and P polarized light. The transmittance of P-polarized light decreased according to the depth, and the reflectance increased to 10% or more. The decrease in the transmittance is mainly due to an increase in the reflected diffracted light, but it is indicated that the reflectance is larger than that of Fresnel reflection. The transmittance of S-polarized light also decreases as the reflection diffraction efficiency increases, but the characteristic is that the reflectance is as large as 40% or more. As described above, this can be inferred from the fact that the diffracted light further starts to couple to the diffracted reflected light. It was shown to correspond to normal incidence PBS.
[0094]
Next, the transmittance of S and P polarizations in the return path is shown, and the total diffraction efficiency obtained by adding the ± primary lights of the respective polarizations is shown in FIG. When the depth is around a relative value of 7, the total diffraction efficiency of S-polarized light is close to 80%, and at that time, the transmittance has dropped to 20%. The P-polarized light is not so different, but the total diffraction efficiency and the transmittance approached the same level. Further, the sum of the transmittance and diffraction efficiency of both polarized lights became the same when the depth was around a relative value of 6. At that depth, 70% of S-polarized light was diffracted and 30% was transmitted. Conversely, 70% of the P-polarized light was transmitted, and the diffraction was 30%. A difference of as much as 40% in transmission and diffraction occurred for both polarized lights.
[0095]
Next, the transmittance of both S and P polarized light in the forward and return paths is shown. FIG. 32 shows the P-polarized light transmittance in the forward path and the S-polarized light transmittance in the return path. FIG. 33 shows the transmittance of S-polarized light in the forward path and the transmittance of P-polarized light in the backward path. In the case of FIG. 32, when the depth is a relative value 6, the transmittance of P-polarized light in the forward path is 80%, whereas the transmittance of S-polarized light in the backward path is as low as 30%. In the case of FIG. 33, when the depth is a relative value 6, the transmittance of the forward S-polarized light is 73%, whereas the transmittance of the forward P-polarized light is as low as 32%. From this, it was found that by changing the polarization state between the forward path and the return path, the directionality of the transmittance can be given, and an isolator can be configured.
[0096]
Next, when the pitch deviates from Equation 8, that is, how large the pitch is with respect to the wavelength, an experiment was conducted to keep high diffraction efficiency. Wavelength of light used for efficiency measurement The The thickness was 635 nm, and the pitch was increased from 635 nm in small increments. The manufacturing method was the same as described above, and the pitch was controlled by changing the angle of two-beam interference. The depth of the diffraction grating , Produced with various changes for each pitch, select a depth that makes the P-polarized light transmittance of the forward path as high as about 73%, and make the S-polarized light incident on the diffraction grating of that depth from the return path to make diffraction efficiency Was measured. The results are shown in FIG. It was shown that the diffraction efficiency is high up to about 1.2 times the wavelength of 635 nm, and gradually approaches a certain diffraction efficiency while vibrating as the pitch is increased. From this, it was proved that the efficiency was higher than that of a normal diffraction grating at least up to 1.2 times the pitch. Needless to say, this is a general property of a directional diffraction grating and is not limited to this surface relief diffraction grating.
[0097]
【The invention's effect】
As described above, according to the first invention, the sections of the refractive index N1 and the refractive index N2 are alternately arranged between the material 1 and the material 2 having different refractive indexes of the refractive index N1 and the refractive index N2. Since the diffraction grating is disposed in the above-described manner and the pitch is set in an optimum range with respect to the wavelength, a diffraction grating that can have different diffraction characteristics depending on the direction of incident light, which is not conventionally provided, is provided. Can do.
[0098]
Further, according to the second invention, the sections of the refractive index N1 and the refractive index N2 are alternately arranged at the pitch Λ between the material 1 and the material 2 having different refractive indexes of the refractive index N1 and the refractive index N2. Even if the pitch is slightly larger than the above range with respect to the wavelength, the diffraction efficiency of the return path can be set higher, so a diffraction grating that is easier to manufacture is provided by the larger pitch. I can do it.
[0099]
Further, according to the third invention, a partition having a refractive index N1 and a refractive index N2 is provided between a material (material 3) made of at least one layer and a material (material 4) made of at least one layer. The function of the directional diffraction grating can be realized in the same way even in the diffraction gratings alternately arranged at the pitch Λ, so that the directional diffraction grating is provided with other functions by stacking it with other materials. It is also possible to do. It can be used for various applications.
[0100]
Further, according to the fourth aspect of the invention, a partition having a refractive index N1 and a refractive index N2 is provided between a material (material 3) made of at least one layer and a material (material 4) made of at least one layer. Having diffraction gratings alternately arranged at a pitch Λ, even if the pitch is slightly larger than the above range with respect to the wavelength, the diffraction efficiency on the return path can be set higher, so the larger the pitch, the more In addition, it is possible to provide a diffraction grating that can be easily provided, and to provide other functions by, for example, laminating a directional diffraction grating with another material, which can be used for various applications.
[0101]
According to the fifth invention, in the directional diffraction grating, since at least one of the materials constituting the diffraction grating is formed of a birefringent material, the transmission of the polarized light in the forward path and the orthogonality of the return path Both of the diffraction efficiencies of polarized light can be further increased.
[0102]
According to the sixth invention, the light from the light source is incident on the directional diffraction grating, the transmitted light is narrowed down on the recording medium using a lens, the reflected light is diffracted by the directional diffraction grating, and the total reflection surface By guiding light to the light receiving element without providing a light source, a highly efficient and compact optical head can be realized.
[0103]
According to the seventh invention, the light from the light source is incident on the directional diffraction grating disposed on the prism, the transmitted light is focused on the recording medium by the lens, and the reflected light from the recording medium is directionally diffracted. Returning to the grating, among the diffracted diffracted light, light that passes through the slope of the prism and light that has been reflected by one side of the prism and then passed through the slope of the prism are guided to the light receiving element, making it compact and highly efficient. An optical head can be provided.
[0104]
According to the eighth invention, the light from the light source is incident on the directional diffraction grating, the transmitted light is narrowed down on the recording medium using a lens, and the reflected light is diffracted by the directional diffraction grating. By providing the diffraction grating to guide the light to the light receiving element, it is possible to provide an optical head that is small in size and highly efficient and hardly affected by wavelength fluctuations of the light source.
[0105]
According to the ninth aspect of the invention, by providing a quarter-wave plate between the directional diffraction grating and the recording medium, the characteristics of the directional diffraction grating as the polarizing diffraction grating can be utilized to improve the light utilization efficiency. A high optical head can be provided.
[0106]
According to the tenth aspect of the invention, light is incident on the directional diffraction grating from the lowest refractive index to the higher refractive index, and the polarized light is separated into the transmitted light and the diffracted light. Can be provided.
[0107]
According to the eleventh aspect of the invention, since the above-described small polarization detector is mounted, a small magneto-optical head device can be provided.
[0108]
According to the twelfth invention, the directional diffraction grating is simply provided on the inclined surface of the optical head in which the directional diffraction grating and the prism are combined. Small Type magneto-optical head device can be provided.
[0109]
According to the thirteenth invention, the function of the isolator can be realized by appropriately setting the incident direction and the polarization in the directional diffraction grating.
[0110]
According to the fourteenth invention, the function of the isolator can also be realized by reversing the relationship between the above-mentioned isolator and the polarization.
[Brief description of the drawings]
FIG. 1 is a diagram showing an optical path when light is obliquely incident on a diffraction grating.
FIG. 2 is a diagram showing an optical path when light is perpendicularly incident on a diffraction grating.
FIG. 3 is a diagram showing that a directional diffraction grating cannot emit light from the back surface when diffracted light enters the substrate.
FIG. 4 is a diagram showing an optical path when a diffraction grating portion is sandwiched between multiple layers.
FIG. 5 is a diagram showing a configuration in which a directional diffraction grating is integrated with a prism.
FIG. 6 is a diagram showing an optical path when light is incident in a configuration in which a directional diffraction grating is integrated with a prism.
FIG. 7 is a diagram showing an optical path when light is incident from the directional diffraction grating side in a configuration in which the directional diffraction grating is integrated with a prism.
FIG. 8 is a diagram showing a configuration in which light is extracted by making the interface with air smaller than the total reflection angle in a directional diffraction grating.
FIG. 9 is a diagram showing a configuration for receiving diffracted light without providing an interface with air in a directional diffraction grating.
FIG. 10 is a diagram showing a configuration of a directional diffraction grating in which a diffraction grating is separately provided at the interface with air and diffracted light is extracted and received.
FIG. 11 is a diagram showing an optical path when a pitch is increased out of a range among specifications for determining a directional diffraction grating.
FIG. 12 is a diagram showing a configuration of an optical head in the case of using a directional diffraction grating that receives diffracted light without providing an interface with air.
FIG. 13 is a diagram showing a diffraction grating pattern and an optical path for obtaining a signal of an optical head.
FIG. 14 is a diagram showing a configuration of an optical head in the case of using a directional diffraction grating that takes out light by making the interface with air smaller than the total reflection angle.
FIG. 15 is a diagram showing a configuration of an optical head in the case of using a directional diffraction grating in which a diffraction grating is separately provided at an interface with air to extract diffracted light to the outside.
FIG. 16 is a diagram showing a configuration of an optical head when a directional diffraction grating that extracts light to the outside by being integrated with a prism is used.
FIG. 17 is a diagram showing optical paths of diffracted light and transmitted light when light is incident from the back side of the directional diffraction grating.
FIG. 18 is a diagram showing optical paths of diffracted light and transmitted light when light is incident from a side of the directional diffraction grating having the diffraction grating.
FIG. 19 is a diagram showing the relationship between the axial direction of a birefringent medium and the position of a directional diffraction grating.
FIG. 20 is a diagram showing how to split polarized light when light is incident from the return path in a directional diffraction grating that receives diffracted light without providing an interface with air.
FIG. 21 is a diagram showing how polarized light is separated when light is incident from the return path in a directional diffraction grating in which a diffraction grating is separately provided at the interface with air to extract diffracted light to the outside.
FIG. 22 is a diagram showing how to split polarized light when light is incident from the return path in a directional diffraction grating integrated with a prism.
FIG. 23 is a diagram showing how the polarized light is separated when light is incident from the return path in a directional diffraction grating that extracts light by making the interface with air smaller than the total reflection angle.
FIG. 24 is a diagram showing a configuration of a magneto-optical head using a directional diffraction grating as a polarization detector when light is incident from the return path.
FIG. 25 is a diagram showing a state of polarization separation in a directional diffraction grating which is a polarization detector used in the magneto-optical head.
FIG. 26A is a diagram showing a configuration of a magneto-optical head using a directional diffraction grating that divides polarized light into reflected light and transmitted light in a polarization detector.
FIG. 26B is a diagram showing the relationship between the polarization and the grating vector in the directional diffraction grating which is the polarization detector described above.
FIG. 26C is a diagram showing the relationship of polarization rotation as a magneto-optical signal in the directional diffraction grating as the polarization detector.
FIG. 27 is a diagram showing an example of producing a directional diffraction grating.
FIG. 28 is a diagram showing a configuration of a two-beam interference exposure method as a method for patterning a diffraction grating in the above manufacturing method.
FIG. 29 is a diagram showing diffraction efficiency when P-polarized light is incident from the forward path and both P-polarized light and S-polarized light are incident from the backward path in the produced directional diffraction grating.
FIG. 30 is a diagram showing the transmittance and the reflectance when S-polarized light and P-polarized light are incident from the forward path in the produced directional diffraction grating.
FIG. 31 is a diagram showing transmittance and diffraction efficiency when S-polarized light and P-polarized light are incident from the return path in the produced directional diffraction grating.
FIG. 32 is a diagram showing the transmittance of forward P-polarized light and backward S-polarized light in the produced directional diffraction grating.
FIG. 33 is a diagram showing transmittances of forward S-polarized light and backward P-polarized light in the produced directional diffraction grating.
FIG. 34 is a diagram showing diffraction efficiency measured by selecting a depth at which the transmittance of P-polarized light in the forward path is a high value of about 70%, and allowing S-polarized light to enter the diffraction grating at that depth from the return path.
FIG. 35: Prior art
FIG. 36: Prior art
[Explanation of symbols]
1 Directional diffraction grating
2 Pitch of diffraction grating
3 Incident light in the outbound path
4 Diffracted light generator
5 Diffraction angle into the air
6 Incident light from the return path
7 Diffracted light in the substrate
8 Diffraction angle in substrate
9 Refraction angle into the air
10 Diffraction grating part
11, 12, 13, 14, 15 layers
16 Light incident on a unified directional diffraction grating
17 Prism
18 Adhesive layer
19 Light incident on the prism
20 Entrance surface to prism
21 The slope of the prism
22 Transmitted light from a directional diffraction grating
24 + 1st order light
25-1st order light
26 Transmitted light on the return path
27a, b Light receiving element
28 Interface with air
29a, b Back diffraction grating
30 Diffracted light into the air
31 Diffraction angle in air
32 Diffraction angle in substrate
33 Semiconductor laser
34 lenses
35 housing
36 Directional diffraction grating pattern
37a, b Track pattern
38 Focus pattern
39a, b Light receiving element for track signal detection
40 Light receiving element for focus signal detection
41 + 1 light receiving element for detecting primary light
42 Transmitted light from a directional diffraction grating
43, 44 Diffracted light in directional diffraction grating
45, 46 Reflected diffracted light in directional diffraction grating
47, 48 Diffracted reflected light in directional diffraction grating
49 S-polarized light
50 P polarized light
51 Light transmitted through the directional diffraction grating on the return path
52 X direction
53 Z direction
54 Y direction
55 Conventional diffraction grating
56 + 1st order light of conventional diffraction grating
57 First-order light of conventional diffraction grating
58 Directional diffraction grating as a polarization detector
59, 60 Directional diffraction grating diffracted light receiving element
61 Light-receiving element for light transmitted through a directional diffraction grating
62 Conventional light receiving element of diffraction grating minus first-order light
63, 64 Directional diffraction grating
65 Polarization direction indicating magneto-optical signal
66 Glass substrate
67 Resist film
68 resist diffraction grating
69 Glass-like diffraction grating
70, 71 Light for exposure
72 Lithium niobate
73 lattice
74 Incident light
75 Transmitted light
76, 77 Diffracted light
78 Anisotropic plate
79 Optical axis
80 Filling material

Claims

光を略垂直入射で用い、屈折率Ｎ１と屈折率Ｎ２の互いに異なる屈折率を有する材質１及び材質２の間に、屈折率Ｎ１と屈折率Ｎ２の区画を交互にピッチΛで配設した回折格子（ただし、Ｎ１＞Ｎ２とする）と、
該回折格子で発生する回折光と透過光とが出射する側に空気層を介さずに、前記回折光と前記透過光とを各々受光する受光素子と、
を有し、
用いる波長をλとするとき、以下の関係を満たすことを特徴とする偏光検出器。
λ／Ｎ１＜Λ＜λ／Ｎ２
６３５ｎｍ＜λ＜８３０ｎｍ
Ｎ２＝１．０
１．４５＜Ｎ１＜２．３Diffraction in which light is used at substantially normal incidence and sections of refractive index N1 and refractive index N2 are alternately arranged at a pitch Λ between materials 1 and 2 having different refractive indexes of refractive index N1 and refractive index N2. A lattice (where N1>N2);
A light receiving element that receives the diffracted light and the transmitted light without passing through an air layer on the side where the diffracted light and transmitted light generated by the diffraction grating are emitted;
Have
A polarization detector characterized by satisfying the following relationship when a wavelength used is λ.
λ / N1 <Λ <λ / N2
635 nm <λ <830 nm
N 2 = 1.0
1.45 <N 1 <2.3