JP4174982B2 - Hologram recording / reproducing method, hologram recording / reproducing apparatus, hologram recording method, and hologram recording apparatus - Google Patents

Description

【００２９】
例えば、光記録装置と光再生装置とを別々に構成した場合には、光記録装置として、以下の「ホログラム記録方法」及び「ホログラム記録装置」を用いることができる。
【００３０】
基板と、該基板表面上に形成され、前記表面に対する入射角度が所定値以上の光を導波すると共に、所定波長の信号光及び参照光が同時に照射されることにより屈折率または吸収率が変化し、変化した屈折率または吸収率を保持してホログラムを記録可能な記録層と、を含む光記録媒体にホログラムを記録するホログラム記録方法であって、入射角度αが上記式（４）の関係を満たす信号光と、入射角度βが上記式（５）の関係を満たす参照光と、を同時に照射することによってホログラムを記録することを特徴とするホログラム記録方法。
【００３１】
基板と、該基板表面上に形成され、前記表面に対する入射角度が所定値以上の光を導波すると共に、所定波長の信号光及び参照光が同時に照射されることにより屈折率または吸収率が変化し、変化した屈折率または吸収率を保持してホログラムを記録可能な記録層と、を含む光記録媒体にホログラムを記録するホログラム記録装置であって、入射角度αが上記式（４）の関係を満たす信号光と、入射角度βが上記式（５）の関係を満たす参照光と、を同時に照射することによってホログラムを記録することを特徴とするホログラム記録装置。
【００３２】
【発明の実施の形態】
以下、図面を参照して本発明の実施の形態について説明する。
（ホログラム光記録媒体）
まず、本発明の光記録媒体について説明する。
【００３３】
図１（Ａ）に示すように、光記録媒体１０は、石英基板やプラスチック基板などの透明基板１１の一面側にホログラムを記録可能な記録層１２を形成して構成されている。ここで、再生時に読出光として照射するレーザ波長においては、記録層１２の屈折率ｎ₁は透明基板１１の屈折率ｎ₂よりも高く、記録層１２がスラブ型光導波路となる。また、記録層１２の屈折率ｎ₁は空気層の屈折率よりも高い。なお、記録時の信号光（物体光）１および参照光２は、図示するように透明基板１１側から照射する。
【００３４】
また、図１（Ｂ）に示すように、複数組の記録層１２及びクラッド層１３を交互に積層してスラブ型導波路を複数作製し、多層構成の光導波路型の光記録媒体とすることもできる。クラッド層１３の屈折率は、透明基板１１の屈折率と同じｎ₂とすることができる。
【００３５】
図１（Ａ）及び（Ｂ）のいずれの場合においても、記録層１２はシート状に、すなわち厚みに比べて十分大きな拡がりを有するように形成する。好ましくは、光記録媒体１０を全体としてシート状に形成する。また、光記録媒体１０はディスク形状あるいはカード形状とするのが好ましい。
【００３６】
記録層１２は、屈折率または吸収率が変化してホログラムを記録することが可能であり、変化した屈折率または吸収率が常温で保持される材料であれば、どのような材料で構成されていてもよい。好適な材料としては、光誘起複屈折性を示す光感応性の材料が挙げられる。光誘起複屈折性を示す材料は、入射する光の偏光状態に感応し、入射光の偏光方向を記録することができる。なお、偏光分布に対応した光誘起複屈折によるホログラムを記録することができる光記録媒体を、偏光感応型の光記録媒体と称する。
【００３７】
光誘起複屈折性を示す材料としては、側鎖に光異性化する基を有する高分子または高分子液晶、または光異性化する分子を分散させた高分子が特に好適である。また、光異性化する基または分子としては、例えば、アゾベンゼン骨格を含むものが好適である。
【００３８】
ここで、アゾベンゼンを例に光誘起複屈折の原理について説明する。アゾベンゼンは、下記化学式に示すように、光の照射によってトランス−シスの光異性化を示す。光記録層に光照射する前は、光記録層にはトランス体のアゾベンゼンが多く存在する。これらの分子はランダムに配向しており、マクロに見て等方的である。光記録層に矢印で示す所定方向から直線偏光を照射すると、その偏光方位と同じ方位に吸収軸を持つトランス１体は選択的にシス体に光異性化される。偏光方位と直交した吸収軸を持つトランス２体に緩和した分子は、もはや光を吸収せずその状態に固定される。結果として、マクロに見て吸収係数及び屈折率の異方性、つまり二色性と複屈折が誘起される。一般に、これらの性質は、光誘起複屈折性、光誘起２色性、または光誘起異方性と呼ばれている。また、円偏光または無偏光の光を照射することによって、これら励起された異方性を消去することができる。
【００３９】
【化１】

【００４０】
このような光異性化基を含む高分子は、光異性化により高分子自身の配向も変化し大きな複屈折を誘起することができる。このように誘起された複屈折は高分子のガラス転移温度以下で安定であり、ホログラムの記録に好適である。
【００４１】
記録層１２を構成する材料の好適な例として、下記の化学式で表される側鎖にシアノアゾベンゼンを有するポリエステルを挙げることができる。このポリエステルは、側鎖のシアノアゾベンゼンの光異性化による光誘起異方性に起因して、信号光の強度及び偏光方向をホログラムとして記録できる（"Holographic recording and retrieval of polarized light by use of polyester containing cyanoazobenzene units in the side chain", K.Kawano, T. Ishii, J. Minabe, T. Niitsu, Y. Nishikata and K. Baba, Opt. Lett. Vol. 24 (1999) pp. 1269-1271）。
【００４２】
【化２】

【００４３】
上記のポリエステル材料からなる記録層１２を備えた光記録媒体１０は、ポリエステルのクロロホルム溶液を洗浄したガラス基板上にキャストし乾燥させることによって作製することができる。膜厚２０μｍの記録層１２が形成された光記録媒体１０の吸収スペクトルを測定したところ、アゾベンゼンのπ−π^*遷移に相当する３６５ｎｍ付近にピークを有するスペクトルが得られた。
【００４４】
なお、光記録媒体の作製方法はこれに限られるものではなく、記録層の材料を基板上にスピンコートして光記録媒体を作製してもよく、記録層の材料を平行平板セルへ注入して光記録媒体を作製してもよい。また、フィルム状基板に記録層の材料をホットプレスにより接着して光記録媒体を作製してもよい。
【００４５】
（偏光ホログラム記録の原理）
このアゾベンゼンを側鎖に有する高分子または高分子液晶、またはアゾベンゼンを分散させた高分子からなる記録層１２を備える光記録媒体１０に、ホログラムを記録する場合、それぞれコヒーレントな信号光１および参照光２を、光記録媒体１０の同一領域に同時に照射する。
【００４６】
この場合、信号光１と参照光２の偏光方向が互いに平行なとき、例えば、図２（Ａ）に示すように、信号光１と参照光２がともにｓ偏光のときには、光記録媒体１０中に、信号光１と参照光２の２光波干渉により光強度分布を生じる。そして、光強度の強いところでは、前述したように、トランス−シス−トランスの異性化サイクルによって光学異方性が誘起される。従って、光強度分布に対応した吸収率または屈折率の格子がホログラムとして記録される。
【００４７】
これに対して、信号光１と参照光２の偏光方向を互いに直交させたとき、例えば、図２（Ｂ）に示すように、信号光１をｐ偏光とし、参照光２をｓ偏光としたときには、信号光１と参照光２の偏光方向が互いに平行なときのような光強度分布は生じない。その代わりに、偏光方向が空間的・周期的に変調され、直線偏光部分８と楕円偏光部分９が交互に周期的に現れる。
【００４８】
この場合、光強度分布は一様となるが、変調された偏光方向と同一の方向を向くアゾベンゼンが、前述したように、トランス−シス−トランスの異性化サイクルによって光学異方性が誘起され、ホログラムとして記録される。
【００４９】
以後、図２（Ａ）のように信号光１と参照光２の偏光方向が平行なときの光強度分布によるホログラムを光強度ホログラムと称し、図２（Ｂ）のように信号光１と参照光２の偏光方向が直交するときの偏光分布によるホログラムを偏光ホログラムと称する。
【００５０】
このように、アゾベンゼンを側鎖に有する高分子または高分子液晶、またはアゾベンゼンを分散させた高分子からなる記録層１２を備える光記録媒体１０によれば、信号光１と参照光２の偏光方向が平行であっても直交していても、アゾベンゼンの異方性が誘起される結果、ホログラムを記録することができる。
【００５１】
上記の各々の場合にホログラム読出光３の偏光方向を参照光２の偏光方向と同じ方向とすれば、信号光１と同じ偏光状態を持つ回折光４を得ることができる。また、記録されたホログラムは室温自然光のもとで数年以上緩和なく保持される。
【００５２】
（ホログラム記録再生装置）
図３に本発明のホログラム記録再生装置の一例を示す。このホログラム記録再生装置は、図３（Ａ）に示す記録装置、及び図３（Ｂ）に示す再生装置から構成されている。記録装置と再生装置とは一体に構成してもよく、各々別々に構成してもよい。
【００５３】
記録装置は、図３（Ａ）に示すように、光記録媒体１０の所定領域に信号光１及び参照光２を同時に照射してホログラムを記録する記録ヘッド２２を備えている。記録ヘッド２２は、コヒーレントな光を発する光源４０、光源４０からの光を信号光用及び参照光用の二光波に分けるビームスプリッタ４１、ビームスプリッタ４１を透過した光波を平行光化するレンズ４３及び４４、平行光化された光波を変調する空間光変調器３０、変調された信号光１を光記録媒体１０の所定領域に集光する集光レンズ４５、及びビームスプリッタ４１で反射された光波を参照光２として光記録媒体１０の所定領域に導くミラー４７、４８を備えている。なお、空間光変調器３０はコンピュータ２１に接続され、コンピュータ２１により制御されている。
【００５４】
記録ヘッド２２の光源４０としては、光記録媒体１０の記録層１２の材料に感度があり、且つコヒーレントな光を発するものを使用することができる。側鎖にシアノアゾベンゼンを有するポリエステルを記録層１２に用いる場合には、光記録媒体１０の吸収ピークの裾に対応する発振波長５１５ｎｍのアルゴンイオンレーザを光源に使用するのが好ましい。
【００５５】
空間光変調器３０としては、液晶等の電気光学変換材料の両面に透明電極を形成した透過型の空間光変調器を用いることができる。このタイプの空間光変調器としては、プロジェクタ用の液晶パネルを挙げることができる。
【００５６】
但し、偏光変調を可能にするためには、上記のプロジェクタ用の液晶パネルを用いる場合には、少なくとも出力側に配置された偏光板を取り除く必要がある。例えば、空間光変調器３０は、図４に示すように、電気光学変換部材の一つである液晶１２１の両面に電極１２２、１２３を形成した透過型の液晶セル１２４として構成することができる。この偏光変調を行う空間光変調器では、２次元的に複数の画素を形成して、それぞれの画素を１／２波長板として機能させ、それぞれの画素に２次元データの対応するビットの情報を電圧印加の有無として与えることにより、それぞれの画素に入射する光の偏光を変調する。
【００５７】
再生装置は、図３（Ｂ）に示すように、コヒーレントな光を光記録媒体１０の端面から入射させるための読出し光学系３１と、記録されたホログラムによる回折光４を読み取る読取り部２３と、を備えている。読取り部２３は、回折光４を平行光化するレンズ４６、及び入射される回折光を検出するＣＣＤ等の光検出器５０を備えている。
【００５８】
読出し光学系３１は、光記録媒体１０の記録層１２に記録されたホログラムが保持する屈折率または吸収率を変化させず（即ち、記録されたホログラムを破壊することなく）、且つコヒーレントな光を発する光源を含んで構成することができる。側鎖にシアノアゾベンゼンを有するポリエステルを記録層１２に用いる場合には、光記録媒体１０の記録層１２に感度の無い（吸収の無い）発振波長６３３ｎｍのヘリウムネオンレーザを光源として使用するのが好ましいが、それ以外のレーザダイオードを用いても良い。
【００５９】
（ホログラム記録再生方法）
次に、本発明のホログラム記録再生方法を説明する。
【００６０】
まず、記録時には、図３（Ａ）に示す記録装置において、記録ヘッド２２の光源４０から出力されたコヒーレント光は、ビームスプリッタ４１により信号光用及び参照光用の二光波に分けられる。ビームスプリッタ４１を透過した光波はレンズ４３および４４により光径の大きな平行光とされる。その後、平行光化された光波は、空間光変調器３０により変調されて信号光１が生成される。
【００６１】
変調された信号光１は、レンズ４５により縮小またはフーリエ変換されて、光記録媒体１０の所定領域に照射される。一方、ビームスプリッタ４１で反射された参照光２は、ミラー４７及び４８で反射されて光記録媒体１０の所定領域に導かれ、光記録媒体中で信号光１と交差するように光記録媒体１０に入射される。このように信号光１と参照光２とを同一領域に同時に照射することによりホログラム記録を行う。
【００６２】
上述した通り、信号光１及び参照光２としては、光記録媒体１０の記録層１２の材料に感度がある波長のレーザ光を用いる。また、信号光１及び参照光２の入射角度は以下のように求めることができる。
【００６３】
図６に示すように、記録時の信号光１及び参照光２の波数ベクトルをそれぞれk_object及びk_referenceとし、読出光３の波数ベクトルをk_readとすると、読出光３のベクトル方向を基準にして、k_object及びk_referenceはそれぞれ角度がα及びβであり、二光波の交差角はα−βである。k_object及びk_referenceによって形成される格子ベクトルは、k_g= k_object−k_referenceで与えられ、記録されたホログラムの格子間隔Λは、下記式１で与えられる。
【００６４】
【数６】

【００６５】
ここで、記録に用いた信号光及び参照光の波長をλとし、波長λ´の読出光（波数ベクトルk_read）が基板面に対し角度０で入射する場合を考える。このとき格子への入射角度は（α＋β）／２であり、下記式２の位相整合条件を満たす場合に強い回折光を得る。
【００６６】
【数７】

【００６７】
式２は多重反射による各層からの反射光が同位相になる条件であるから、式１と式２とにより式３の関係が導かれる。
【００６８】
【数８】

【００６９】
式３を満たすようなαとβであれば、読出光が導波路を導波したときに、ホログラムの位相整合条件を満たし、回折光を得ることができる。回折光が導波路に対して垂直に出射する条件は、格子への入射角度（α＋β）／２＝４５°の場合である。この条件を式３に代入すると、各々式４及び式５で表されたα及びβが求められる。即ち、信号光の入射角度α及び参照光の入射角度βは、信号光及び参照光の波長λ及び読出光の波長λ´が決まれば、その値に応じて算出することができる。
【００７０】
【数９】

【００７１】
例えば、λ＝５１５ｎｍ、λ´＝６３３ｎｍ、ｍ＝１として、αおよびβを求めると、信号光の入射角度α＝８０°、参照光の入射角度β＝１０°と算出できる。
【００７２】
次に、読み出し時に、図３（Ｂ）に示す再生装置において、読出し光学系３１から出力されたコヒーレント光は、読出光３として光記録媒体１０の記録層１２の端面から入射される。図８に示すように、入射された読出光３は記録層１２を導波し、回折光が記録層１２から回折される。回折された光波（回折光４）は、これをレンズ４６により光検出器５０に結像させる。
【００７３】
上述した通り、読出光３としては、光記録媒体１０の記録層１２に記録されたホログラムが保持する屈折率または吸収率を変化させない波長のレーザ光、例えば、光記録媒体１０の記録層１２の材料に感度が無い波長のレーザ光を用いる。このように、再生時に光記録媒体１０の記録層１２に記録されたホログラムが保持する屈折率または吸収率を変化させない波長のレーザ光を読出光３として用いることにより、読出光３を記録層１２に導波させて、記録されたホログラムを破壊すること無く再生することができる。
【００７４】
また、読出光３が記録層１２を導波するので、読出光３の導波路に沿って記録されたホログラムを一度に読み出すことができる。
【００７５】
なお、図５に示すように、空間変調器３０で変調された信号光１を、レンズを介さずに光記録媒体１０に入射させてホログラム記録を行った場合には、図９に示すように、記録に用いた参照光２の入射方向と反対の方向から読出光３を記録層１２に導波させてホログラムを読み出すこともできる。この場合、参照光２の波長と読出光３の波長は異なるので倍率及び光路は変化するが、回折光４として信号光１の位相共役光を再生することができる。位相共役光は、信号光と同一波面を備えており、信号光が入射した光路を逆進する。そのため、特別な結像光学系を用いることなく光検出器５０に結像することができる。この場合、光記録媒体１０の端面から読出光３を入射して、信号光１の入射側から回折光４を取り出すことになる。
【００７６】
また、参照光２を光記録層１２に例えば１０°といった狭い角度で入射させるためには、例えば、図７に示すように、透明基板１１と同じ屈折率の厚手のガラス基板１４を、光記録媒体１０の透明基板１１側に接触させ、このガラス基板１４の側面から参照光２を入射させればよい。ガラス基板１４の側面から入射された参照光２は、空気とガラス基板１４との界面で屈折し、ガラス基板１４への入射角度より狭い角度で光記録層１２に入射する。また、透明基板１１を十分厚くして、透明基板１１の側面から参照光２を入射させてもよい。
【００７７】
（偏光ホログラム記録）
上記のホログラム記録再生方法を用いて偏光ホログラム記録を行う例について説明する。前記した側鎖にシアノアゾベンゼンを有するポリエステルからなる記録層を備えた偏光感応型の光記録媒体を用いて、図３（Ａ）及び（Ｂ）に示すホログラム記録再生装置により記録及び再生を行った。
【００７８】
記録用の光源４０には、記録層の材料であるポリエステルに感度のある発振波長５１５ｎｍのアルゴンイオンレーザを用い、信号光と参照光との交差角φは７０°とした。偏光変調型の空間光変調器３０を用いて、図１０に示す偏光分布を持つ信号光を生成した。この信号光では、各画素毎に直線偏光の方位が変化しており偏光方位がデータ情報を表している。ここで、Ｎ値の方位を記録するようにすると、各画素毎にｌｏｇ₂Ｎビットのデータが蓄積できる。
【００７９】
また、読出光学系３０の光源には、記録層の材料であるポリエステルに感度の無い発振波長６３３ｎｍのヘリウムネオンレーザを用いた。但し、記録に用いた参照光の入射方向と対向する方向から読出光を記録層に導波させて、信号光の位相共役光を回折させた。回折光の光路に検光子を配置し、直交する偏光成分（０°偏光成分と９０°偏光成分）に分離した。その結果、図１１（Ａ）及び（Ｂ）に示すような０°偏光成分と９０°偏光成分の二画像を得た。これら二画像間の光強度分布比より、下記式６を用いて再生光の偏光角ρを算出した。
【００８０】
【数１０】

【００８１】
ここで、Ｉ₀とＩ₉₀は、それぞれ各画素の０°偏光成分及び９０°偏光成分の強度である。得られた再生像の偏光角を信号光の偏光角に対してプロットした結果を図１２に示す。図１２より、再生像の偏光角は信号光の偏光角に対して略直線的に変化しており、信号光の偏光方位と再生光の偏光方位とが同じであることが分かる。従って、本発明のホログラム記録再生方法によれば、信号光の偏光分布を記録することができると共に、記録された偏光分布を忠実に再生することができる。
【００８２】
なお、上記では偏光分布によりデータ情報を保持する信号光を用いて、ホログラムの記録及び再生を行う例について説明したが、振幅（強度）分布や位相分布によってデータ情報を保持する信号光についても、同様にホログラムの記録及び再生を行うことができる。例えば、３次元物体からの反射光を信号光として記録することにより、立体像の再生も可能である。
【００８３】
（偏光ホログラム多重記録）
次に、上記のホログラム記録再生方法を用いて、信号光と参照光の偏光方向が互いに平行である場合と直交する場合の２つの条件で２重にホログラム記録を行う例、即ち、第１段階で信号光と参照光の偏光方向を平行にしてホログラム記録を行い、第２段階で信号光（あるいは参照光）の偏光方向を１／２波長板によって９０°回転させて、信号光と参照光の偏光方向を互いに直交させた条件で、２枚目のホログラムを同一領域に多重記録する例について説明する。ただし、互いに直交する円偏光を用いても、同様の偏光多重記録・再生は可能である。
【００８４】
前記した側鎖にシアノアゾベンゼンを有するポリエステルからなる記録層１２を備えた偏光感応型の光記録媒体１０を用いて、図１３（Ａ）に示すように、空間変調器３０と光記録媒体１０との間に１／２波長板２０を配置した以外は、図７に示す記録装置と同じ構成とし、図１３（Ｂ）に示すように、光記録媒体１０と光検出器５０との間に、回折光４に含まれる所定の偏光成分を分離する偏光ビームスプリッタ等の偏光子４９を配置した以外は、図９に示す再生装置と同じ構成として、ホログラムの記録及び再生を行った。
【００８５】
図１３（Ａ）に示すように、後述するように、図示しないコンピュータによって空間光変調器３０に画像を表示して、空間光変調器３０を通過した光として、空間光変調器３０に表示された画像の情報を有する信号光を得、この信号光を１／２波長板２０を通過させて、所定の偏光方向を有する信号光１に変換する。この１／２波長板２０を通過した信号光１を上述した光記録媒体１０に照射する。同時に、参照光２を光記録媒体１０の信号光１が照射される領域に照射する。これによって、光記録媒体１０中で信号光１と参照光２とが干渉して、光記録媒体１０中にホログラムが記録される。
【００８６】
この場合、第１段階では、１枚目の画像として、図１４（Ａ）に示したような２値強度画像を空間光変調器３０に表示し、１／２波長板２０を、これを通過した信号光１の偏光方向が紙面に垂直（これを０°とする）となるように調整して、信号光１と参照光２を同時に光記録媒体１０に照射することによって、光記録媒体１０中に１枚目のホログラムを記録する。
【００８７】
次に、第２段階では、２枚目の画像として、図１４（Ｂ）に示したような２値強度画像を空間光変調器３０に表示し、１／２波長板２０を、これを通過した信号光１の偏光方向が紙面に平行（これを９０°とする）となるように調整して、信号光１と参照光２を同時に光記録媒体１０に照射することによって、光記録媒体１０中の１枚目のホログラムが記録された領域に２枚目のホログラムを記録する。ただし、参照光２の偏光方向は、１枚目のホログラムの記録時と２枚目のホログラムの記録時で同一にする。
【００８８】
読み出し時には、図１３（Ｂ）に示すように、読出し光学系３１から出力されたコヒーレント光は、読出光３として光記録媒体１０の記録層１２の端面から入射される。入射された読出光３は記録層１２を導波し、２次回折光が記録層１２から回折される。
【００８９】
その回折光４を、ＣＣＤなどの光検出器５０上に結像させて、信号光１のデータ画像を読み取る。即ち、第１段階および第２段階で記録された２枚のホログラムからの再生像を得ることができる。但し、２枚のホログラムからの回折光の各々は、偏光方向が互いに直交している。この場合、光記録媒体１０と光検出器５０との間に偏光子４９を配置し、図示しないコンピュータによって、この偏光子４９の透過軸を任意の方向に調整することによって、２つの回折像を分離して読み出すことができる。例えば、以下のように、１枚目の画像または２枚目の画像のみ、または１枚目の画像と２枚目の画像との間の任意の演算出力を、読み出すことができる。
【００９０】
図１５に示すように、１枚目の画像のみを読み出すときには、偏光子４９の透過軸を０°とし、２枚目の画像のみを読み出すときには、偏光子４９の透過軸を９０°とする。第１段階で記録された１枚目のホログラムからの回折光成分Ａ₁の振幅をＴ₁、第２段階で記録された２枚目のホログラムからの回折光成分Ａ₂の振幅をＴ₂とすると、偏光子４９の透過軸を０°としたときには、偏光子４９を透過する光強度は｜Ｔ₁｜²に比例し、偏光子４９の透過軸を９０°としたときには、偏光子４９を透過する光強度は｜Ｔ₂｜²に比例する。
【００９１】
但し、これは、信号光１の偏光が回折光４に忠実に再生された場合である。実際には、光学系や光記録媒体１０の偏光特性によって、回折光４の偏光方向は信号光１のそれと若干ずれる可能性がある。しかし、その場合でも、多重記録されている２つの画像の偏光方向は互いに直交した関係に保たれているので、偏光子４９の透過軸を適当に調整することによって、２つの画像をクロストークを生じることなく分離して取り出すことができる。
【００９２】
回折光成分Ａ₁と回折光成分Ａ₂の合成ベクトルと偏光子の方位とを平行にした場合、偏光子を透過する光強度は｜Ｔ₁＋Ｔ₂｜²に比例する。｜Ｔ₁｜＝｜Ｔ₂｜であれば、θ＝４５°で２つの回折光成分の和がとれる。即ち、偏光子４９の透過軸を４５°にすれば、偏光子４９を透過する光強度は｜Ｔ₁＋Ｔ₂｜²に比例するようになり、２つの画像の加算出力が得られる。また、回折光成分Ａ₁と回折光成分Ａ₂の合成ベクトルと偏光子の方位とを直交にした場合、偏光子を透過する光強度は｜Ｔ₁−Ｔ₂｜²に比例する。｜Ｔ₁｜＝｜Ｔ₂｜であれば、θ＝１３５°で２つの回折光成分の差がとれる。即ち、偏光子４９の透過軸を１３５°にすれば、偏光子４９を透過する光強度は｜Ｔ₁−Ｔ₂｜²に比例するようになり、２つの画像の減算出力が得られる。
【００９３】
振幅Ｔ₁およびＴ₂で表される２つの画像が、それぞれ、図１４（Ａ）及び（Ｂ）に示すように、光の「明」（データ“１”）と「暗」（データ“０”）で表される２値画像である場合には、２つの画像の減算出力｜Ｔ₁−Ｔ₂｜²も２値データとなり、図１４（Ｄ）に示したように２つの画像の排他的論理和（ＸＯＲ）と等価になる。一方、２つの画像の加算出力｜Ｔ₁＋Ｔ₂｜²は“１＋１”、“１”、“０”の３つの値（明るさ）を有するが、しきい値処理することによって、“１＋１”と“１”を「明」、“０”を「暗」とすれば、図１４（Ｃ）に示したように２つの画像の論理和（ＯＲ）が得られる。
【００９４】
以上の通り、本発明で用いる光記録媒体は、透明基板の一面側に光誘起複屈折性を示す光感応型の記録層を形成して構成されているが、再生時に読出光として照射するレーザ波長においては、透明基板の屈折率ｎ₂は記録層１２の屈折率ｎ₁よりも低いので、記録層がスラブ型光導波路となり、端面から入射された読出光が該記録層を導波することができる。
【００９５】
また、本発明のホログラム記録再生方法及びホログラム記録再生装置では、記録時には、光記録媒体の記録層に感度がある波長のレーザ光を、参照光として記録層に狭い角度で入射させて記録するので、導波損失が問題とならずホログラムを記録することができる。また、読み出し時には、光記録媒体の記録層が保持する屈折率または吸収率を変化ない波長のレーザ光を、読出光として記録層に導波させて回折光を得るので、記録されたホログラムを破壊すること無く再生することができる。
【００９６】
また、本発明のホログラム記録再生方法を用いて、偏光感応型の光記録媒体に信号光の偏光を偏光ホログラムとして記録し、再生することができる。この偏光ホログラムは、その回折光として信号光の偏光方向が保存された光を発生させるので、信号光の偏光角を所定角度ずつ回転させることにより、偏光角の違いによる情報の記録及び読み出しが可能になる。
【００９７】
更に、本発明のホログラム記録再生方法を用いて、信号光と参照光の偏光方向を互いに平行な方向と互いに直交する方向の２通りとすることにより、偏光感応型の光記録媒体の同一領域に、２つの信号光を２枚のホログラムとして多重に記録することができ、この光記録媒体の前記領域に、読出光を照射して、互いに直交する偏光方向を有する２つの回折光成分が合成された回折光を得、この回折光から任意の偏光成分を取り出すことによって、前記領域に２つの信号光として記録されている２つのデータ間の演算出力を得ることができる。即ち、光記録媒体に記録されている２つの画像などのデータ間で、和または差の演算や論理演算などの任意の演算を、簡便かつ高速に行うことができる。
【００９８】
【発明の効果】
本発明で用いる光記録媒体は、十分な回折効率が得られるホログラムを記録することができると共に、記録層がスラブ型光導波路となり、端面から入射された読出光が該記録層を導波することができる。本発明のホログラム記録再生方法及びホログラム記録再生装置によれば、十分な回折効率が得られるホログラムを記録することができると共に、記録層に端面から入射させた読出光を導波させて、記録されたホログラムを破壊することなく再生することができる、という効果を奏する。
【００９９】
本発明のホログラム記録方法及びホログラム記録装置によれば、記録層に端面から入射させた読出光を導波させて、記録されたホログラムを破壊することなく再生することができるように、十分な回折効率が得られるホログラムを記録することができる、という効果を奏する。
【図面の簡単な説明】
【図１】（Ａ）は、本発明の光記録媒体の構成を示す斜視図であり、（Ｂ）は、本発明の光記録媒体の他の構成を示す斜視図である。
【図２】（Ａ）及び（Ｂ）は、光強度分布によるホログラムと偏光分布によるホログラムとを説明するための説明図である。
【図３】（Ａ）及び（Ｂ）は、本発明のホログラム記録再生装置の構成の一例を示す断面図である。
【図４】本発明のホログラム記録再生装置に用いられる空間変調器の構成を示す断面図である。
【図５】本発明のホログラム記録再生装置の構成の他の例を示す断面図である。
【図６】信号光の波数ベクトル、参照光の波数ベクトル、読出光の波数ベクトル、及び形成される格子ベクトルの関係を示す線図である。
【図７】参照光の入射角度が小さい場合の入射方法を示す断面図である。
【図８】本発明のホログラム記録再生方法により回折光の読み出しを行う様子を示す断面図である。
【図９】読出光を図８とは逆方向に導波させて回折光として位相共役光を得る様子を示す断面図である。
【図１０】本発明のホログラム記録再生方法を偏光ホログラム記録に適用した場合における、信号光の偏光分布を表す図である。
【図１１】（Ａ）は、図１０に示す信号光を記録した偏光ホログラムに基づいて得られた回折光の０°偏光成分の画像を表す図であり、（Ｂ）は９０°偏光成分の画像を表す図である。
【図１２】図１０に示す信号光を記録した偏光ホログラムに基づいて得られた再生像の偏光角を、信号光の偏光角に対してプロットした線図である。
【図１３】（Ａ）及び（Ｂ）は、本発明の光記録装置の他の構成例を示す断面図である。
【図１４】本発明のホログラム記録再生方法を偏光ホログラム多重記録に適用した場合における、（Ａ）及び（Ｂ）は入力画像であり、（Ｃ）及び（Ｄ）は（Ａ）及び（Ｂ）に示す入力画像を処理して得られる出力画像である。
【図１５】偏光ホログラム多重記録における、信号光の偏光方向、回折光の偏光方向、及び偏光子の方位の関係を示す線図である。
【符号の説明】
１ 信号光
２ 参照光
３ 読出光
４ 回折光
１０ 光記録媒体
１１ 透明基板
１２ 記録層
１３ クラッド層
２０ １／２波長板
３０ 空間光変調器
２１ コンピュータ
４０ 光源
４１ ビームスプリッタ
４３、４４、４５、４６ レンズ
４７、４８ ミラー
４９ 偏光子
５０ 光検出器[0001]
BACKGROUND OF THE INVENTION
  The present inventionHologram recording / reproducing method, hologram recording / reproducing apparatus, hologram recording method, and hologram recording apparatusIn particular, it constitutes a holographic memory capable of recording a three-dimensional image, a two-dimensional image or a digital data pageHologram recording / reproducing method, hologram recording / reproducing apparatus, hologram recording method, and hologram recording apparatus.
[0002]
[Prior art]
  A two-dimensional optical memory represented by a digital versatile disk (DVD) or the like is used as a large-capacity and high-density recording medium. The higher density of these two-dimensional optical memories is achieved by shortening the recording laser wavelength and increasing the numerical aperture (NA) of the objective lens used for the pickup, thereby increasing the laser spot used for data recording / reproduction. It has been realized by making it smaller. Currently, research and development of a two-dimensional optical memory using a blue-violet laser as a light source is actively performed.
[0003]
  However, since there is no suitable optical material in the ultraviolet region, and there is no suitable optical material used for recording media, lenses, etc., the shortening of the recording laser wavelength is limited to the use of a blue-violet laser. However, it is considered that it is difficult to further shorten the wavelength. Further, as a method of increasing NA, a numerical aperture is increased by a refractive index multiple of the prism using a solid immersion lens (SIL) that uses a circular prism having a high refractive index to reduce a focused spot. A method has been proposed. In this method, a minute condensing spot is formed using evanescent light formed on the bottom surface of the prism. Since evanescent light is non-propagating light that is localized near the bottom surface (exit end) of the prism and exists only within the region below the wavelength of the light from the exit end of the SIL, the recording medium is arranged very close to the bottom surface of the prism. Recording and playback must be performed. For this reason, there are many problems to be solved such as distance control between the recording medium and the prism and establishment of portability of the recording medium. Further, the refractive index of the prism material is at most about 2, and the recording density is improved only up to about 4 times.
[0004]
  For these reasons, the improvement in recording density has reached the limit in the current two-dimensional optical memory. Therefore, in order to perform high density recording of 50 GB or more, it is necessary to record information (volume recording) in three dimensions including the depth direction of the recording medium.
[0005]
  A holographic memory that records information in the form of a hologram is a three-dimensional optical memory and can be recorded with a large capacity. The holographic memory is a page-type memory and has high speed by batch recording / reproduction of two-dimensional data in units of pages. For this reason, holographic memory is attracting attention as a next-generation recording medium.
[0006]
  Holography is a technique for recording and reproducing information on the amplitude (intensity) and phase of light waves on a medium. When the object is irradiated with coherent light such as laser light and the reflected light (object light) from the object is incident on the recording medium, another coherent light (reference light) is simultaneously incident on the recording medium. Interference fringes are formed on the recording medium. A hologram is a light intensity distribution resulting from this interference recorded in a medium as a change in refractive index or absorptance. When only the reference light is incident on the recording medium on which the hologram is recorded, the hologram functions as a diffraction grating and the object light is reproduced.
[0007]
  In a holographic memory, digital data (binary data of 0 or 1) is converted into an on / off (bright / dark) pattern using a spatial light modulator and incident on a recording medium as object light. Digital hologram recording is also possible. The original binary data can be reproduced from the obtained electrical signal by irradiating the recording medium with the reference light to reproduce the object light, and receiving the reproduced object light with a photodetector and performing photoelectric conversion. Recently, more engineering viewpoints such as S / N, bit error rate evaluation or two-dimensional encoding based on the specific optical system and volume multiplex recording system of this digital holographic memory, the effect of aberration of the optical system, etc. Research from is progressing.
[0008]
  As a hologram recording material, attention is paid to a polymer material that is inexpensive and can be easily formed into a disk shape. So-called photopolymers have been actively studied for ROM type media, and photosensitive polymers containing photoisomerizable groups such as azo groups are promising for rewritable media.
[0009]
  In order to realize a large capacity with the holographic memory, it is necessary to increase the thickness of the recording layer for recording the hologram and to multiplex-record a plurality of holograms in the same volume. For example, in order to store digital data of 100 GB or more on one disk, the recording layer needs to have a thickness of 1 mm or more. However, it is very difficult and costly to make the recording layer thick while maintaining the optical quality.
[0010]
  As a method of realizing a large capacity while avoiding this problem, there is an invention described in Japanese Patent Laid-Open No. 9-101735. Japanese Patent Application Laid-Open No. 9-101735 discloses a recording / reproducing method using a multi-layered optical waveguide type optical recording medium. In this optical recording medium, a plurality of optical waveguide layers and recording layers are laminated on a substrate via a cladding layer, and an optical waveguide layer sandwiched between adjacent cladding layers constitutes an optical waveguide. Using this optical recording medium, reference light is selectively incident on each optical waveguide from the end face of the optical waveguide layer, and object light (signal light) is incident on the interface of the optical waveguide layer to enter the recording layer. The hologram is recorded by causing the evanescent light and the object light that have soaked out to interfere with each other. In this case, the thickness of the recording layer required to record one hologram may be as thin as several μm, and the film can be formed without impairing optical quality by spin coating or casting. Multiple hologram recording is possible by laminating a plurality of such thin recording layers.
[0011]
[Problems to be solved by the invention]
  However, the recording / reproducing method using the optical waveguide type optical recording medium having the multi-layer structure described in JP-A-9-101735 has the following problems. In this optical recording medium, the reference light used for recording is guided to the waveguiding layer, and the hologram is recorded by interfering with the evanescent light that has leached into the recording layer and the object light. However, the evanescent light is recorded only in the wavelength order. Since it does not reach the layer, a hologram cannot be recorded at a sufficient depth in the medium thickness direction, and the evanescent light is very weak, so that a sufficient exposure intensity cannot be obtained. As a result, sufficient diffraction efficiency cannot be obtained with the recorded hologram. In addition, it is difficult to irradiate the readout light with sufficient intensity during reproduction.
[0012]
  In order to solve these problems, the recording layer is guided with the reference light and the reading light, and the hologram is recorded by causing the reference light guided through the recording layer instead of the evanescent light and the object light to interfere with each other. It is desired to reproduce the hologram by the reading light that is guided. However, the recording layer is made of a material that absorbs reference light for recording a hologram. For this reason, when the reference light is incident on the recording layer from the end face and guided, the hologram has a large waveguide loss and cannot be recorded. Normally, light having the same wavelength as that of the reference light is used as the reading light. However, if the reading light is guided during reproduction, the recorded hologram is destroyed.
[0013]
  The present invention has been made in view of the above circumstances, and the object of the present invention is toHologram recording / reproducing method capable of recording a hologram capable of obtaining a sufficient diffraction efficiency, and allowing reading light incident on the recording layer from the end face to be guided without destroying the recorded hologram An object of the present invention is to provide a hologram recording / reproducing apparatus.
[0014]
[Means for Solving the Problems]
  To achieve the above objective,The hologram recording / reproducing method of the present invention is formed on a substrate and the surface of the substrate, guides light having an incident angle with respect to the surface of a predetermined value or more, and simultaneously irradiates signal light and reference light of a predetermined wavelength. And a recording layer capable of recording a hologram while maintaining the changed refractive index or absorptance. During recording, a signal having a predetermined wavelength is recorded on the recording layer. When a hologram is recorded by simultaneously irradiating light and reference light, and at the time of reproduction, reading light having a wavelength that does not change the refractive index or absorption held in the recording layer is guided to the recording layer and guided. The hologram is reproduced by the diffracted light.
[0015]
  The above optical recording medium isThe refractive index or the absorptance is changed by simultaneously irradiating the substrate surface with the signal light and the reference light having a predetermined wavelength, and a recording layer capable of recording the hologram while maintaining the changed refractive index or the absorptivity is formed. However, since the recording layer guides light having an incident angle with respect to the substrate surface of a predetermined value or more, the reading light can be guided to the recording layer at the time of reproduction, and the hologram is generated by the diffracted light when guided. Can be played.
[0016]
  In the above optical recording mediumBy making the refractive index of the recording layer higher than the refractive index of the substrate, the recording layer becomes a slab type optical waveguide, and light incident on the recording layer at an incident angle of a predetermined value or more can be guided. A cladding layer having a lower refractive index than that of the recording layer can be further formed on the recording layer. Furthermore, a plurality of sets of the recording layer and the clad layer may be laminated to form an optical recording medium having a multilayer structure including a plurality of recording layers.
[0017]
  A material exhibiting light-induced birefringence (also referred to as light-induced dichroism or light-induced anisotropy) is sensitive to the polarization state of light incident thereon and can record the polarization direction of incident light. Among these, a polymer or a polymer liquid crystal having a photoisomerizable group in the side chain is excellent in recording characteristics. Accordingly, the optical recording medium preferably has a recording layer made of a polymer or a polymer liquid crystal having a photoinduced birefringence and having a photoisomerizable group in the side chain. As the group to be photoisomerized, those containing an azobenzene skeleton are preferable, and as the polymer or polymer liquid crystal, at least one monomer polymer selected from the polyester group is preferable.
[0018]
  The above optical recording medium isThe recording layer is provided, and a hologram capable of diffracting diffracted light when reading light guided through the recording layer is incident can be recorded on the recording layer. By recording a hologram on this optical recording medium, an optical recording medium can be obtained in which a hologram capable of diffracting diffracted light is recorded on the recording layer when reading light guided in the recording layer is incident. it can.
[0019]
  In the above optical recording mediumIn order to obtain diffracted light from the recorded hologram with sufficient diffraction efficiency when the read light guided in the recording layer is incident, read light that cannot be recorded on the recording layer can be used. For example, when a hologram is recorded by a change in the refractive index or absorption rate of the recording layer, the reading light can be light that does not induce a change in the refractive index or absorption rate of the recording layer.
[0020]
  Further, the reading light can be guided to the recording layer by making the reading light incident from the end face of the sheet-like recording layer.
[0021]
  The hologram recording / reproducing apparatus of the present invention comprises:A refractive index or an absorptance is changed by simultaneously irradiating a signal light having a predetermined wavelength and a reference light while guiding light having an incident angle with respect to the surface of the substrate and a predetermined value or more. A hologram recording and reproducing apparatus for recording a hologram on an optical recording medium including a recording layer capable of recording a hologram while maintaining a changed refractive index or absorption rate, and reproducing the recorded hologram, A signal light irradiating means for irradiating a predetermined region of the recording layer with signal light having a predetermined wavelength from a direction intersecting the waveguide direction of the recording layer, and changing the refractive index or absorptance of the recording layer by the interference action of the signal light As described above, the read light has the reference light irradiating means for irradiating the predetermined region with the reference light having the predetermined wavelength, and the read light having the wavelength that does not change the refractive index or the absorptivity held in the recording layer. And reading light projecting means for entering a recording layer on the recording layer so as to guide, and further comprising a.
[0022]
  In the hologram recording / reproducing method and hologram recording / reproducing apparatus of the present invention, during recording, a hologram is recorded by simultaneously irradiating the recording layer with signal light and reference light of a predetermined wavelength, but during reproduction, the refraction held in the recording layer is recorded. Since the hologram is reproduced by the diffracted light when the readout light having a wavelength that does not change the rate or the absorption rate is guided to the recording layer and guided, the recording loss is not an issue at the time of recording. The hologram can be reproduced without destroying it.
[0023]
  Applying the above hologram recording / reproducing method, the signal light holding the data information by the polarization distribution is irradiated onto the optical recording medium simultaneously with the reference light, the polarization distribution of the signal light is recorded as a hologram, and the reading light is A hologram can be reproduced by diffracted light when guided to the recording layer. Since this polarization hologram generates light in which the polarization direction of the signal light is preserved as its diffracted light, it is deflected by rotating the polarization angle of the signal light by a predetermined angle according to data information, for example, and distributing the polarization. It becomes possible to read information by the difference in corners.
[0024]
  Further, by applying the hologram recording / reproducing method described above, the signal light holding the first data information by the intensity distribution is irradiated to a predetermined area of the optical recording medium simultaneously with the reference light, and the intensity distribution of the signal light is Signal light that is recorded as one hologram, changes the polarization state of one of the signal light and the reference light, and retains the second data information by the intensity distribution, and a predetermined area of the optical recording medium simultaneously with the reference light And multiplex-recording the intensity distribution of the signal light as a second hologram and reproducing the first hologram and the second hologram by diffracted light when the readout light is guided to the recording layer. Can do.
[0025]
  Two diffracted light components having polarization directions orthogonal to each other by irradiating the region of the optical recording medium on which the two signal lights having different polarization directions are recorded in the same region by the same reference light. Is obtained, and an arbitrary polarization component is extracted from the diffracted light, whereby an operation output between two data recorded as two signal lights in the region can be obtained. That is, it is possible to easily and quickly perform any operation such as a sum or difference operation or a logical operation between data such as two images recorded on an optical recording medium.
[0026]
  The hologram recording / reproducing apparatus includes an optical recording apparatus that records a hologram on the recording layer of the optical recording medium, and an optical reproducing apparatus that diffracts diffracted light from the recording layer of the optical recording medium on which the hologram is recorded. Can do. The optical recording device and the optical reproducing device may be configured integrally or may be configured separately.
[0027]
  In this caseThe optical recording apparatus is an optical recording apparatus for recording a hologram on a recording layer of an optical recording medium, and a hologram capable of diffracting diffracted light when reading light guided in the recording layer is incident on the recording layer. It is characterized by comprising recording means for recording in Also,In this caseThe optical reproducing device is an optical reproducing device that diffracts diffracted light from a recording layer of an optical recording medium on which a hologram is recorded, and the reading light guided in the recording layer is incident to the diffraction layer from the recording layer. A reading means for diffracting light is provided. That is,Optical recording deviceCan record holograms with sufficient diffraction efficiency,Optical regeneratorThen, it is possible to guide the reading light incident from the end face to the recording layer and read the recorded hologram without destroying it.
[0028]
  In the above method and apparatus, the hologram is recorded by simultaneously irradiating the signal light having the incident angle α satisfying the relationship of the following formula (4) and the reference light having the incident angle β satisfying the relationship of the following formula (5). It is preferable to do.
[Equation 5]

[0029]
  For example, when the optical recording apparatus and the optical reproducing apparatus are configured separately, the following “hologram recording method” and “hologram recording apparatus” can be used as the optical recording apparatus.
[0030]
  A refractive index or an absorptance is changed by simultaneously irradiating a signal light having a predetermined wavelength and a reference light while guiding light having an incident angle with respect to the surface of the substrate and a predetermined value or more. And a recording layer capable of recording a hologram while maintaining a changed refractive index or absorptance, and a hologram recording method for recording a hologram on an optical recording medium, wherein the incident angle α is a relationship of the above formula (4) A hologram recording method comprising: recording a hologram by simultaneously irradiating a signal light satisfying the above and a reference light having an incident angle β satisfying the relationship of the above formula (5).
[0031]
  A refractive index or an absorptance is changed by simultaneously irradiating a signal light having a predetermined wavelength and a reference light while guiding light having an incident angle with respect to the surface of the substrate and a predetermined value or more. And a recording layer capable of recording a hologram while maintaining a changed refractive index or absorptance, and a hologram recording apparatus for recording a hologram on an optical recording medium, wherein the incident angle α is a relationship of the above formula (4) A hologram recording apparatus that records a hologram by simultaneously irradiating a signal light satisfying the above and a reference light having an incident angle β satisfying the relationship of the above expression (5).
[0032]
DETAILED DESCRIPTION OF THE INVENTION
  Embodiments of the present invention will be described below with reference to the drawings.
(Hologram optical recording medium)
  First, the optical recording medium of the present invention will be described.
[0033]
  As shown in FIG. 1A, an optical recording medium 10 is configured by forming a recording layer 12 capable of recording a hologram on one surface side of a transparent substrate 11 such as a quartz substrate or a plastic substrate. Here, at the laser wavelength irradiated as readout light during reproduction, the refractive index n of the recording layer 12 is obtained.₁Is the refractive index n of the transparent substrate 11₂The recording layer 12 becomes a slab type optical waveguide. Further, the refractive index n of the recording layer 12₁Is higher than the refractive index of the air layer. The signal light (object light) 1 and the reference light 2 at the time of recording are irradiated from the transparent substrate 11 side as shown in the figure.
[0034]
  As shown in FIG. 1B, a plurality of sets of recording layers 12 and clad layers 13 are alternately laminated to produce a plurality of slab type waveguides to obtain a multi-layered optical waveguide type optical recording medium. You can also. The refractive index of the cladding layer 13 is the same as the refractive index of the transparent substrate 11 n₂It can be.
[0035]
  In both cases of FIGS. 1A and 1B, the recording layer 12 is formed in a sheet shape, that is, having a sufficiently large spread as compared with the thickness. Preferably, the optical recording medium 10 is formed in a sheet shape as a whole. Further, the optical recording medium 10 is preferably disk-shaped or card-shaped.
[0036]
  The recording layer 12 is made of any material as long as the refractive index or the absorptance is changed and the hologram can be recorded, and the changed refractive index or the absorptance is maintained at room temperature. May be. Suitable materials include light sensitive materials that exhibit light induced birefringence. A material exhibiting light-induced birefringence is sensitive to the polarization state of incident light and can record the polarization direction of incident light. An optical recording medium capable of recording a hologram by light-induced birefringence corresponding to the polarization distribution is referred to as a polarization-sensitive optical recording medium.
[0037]
  As a material exhibiting photoinduced birefringence, a polymer or polymer liquid crystal having a photoisomerizable group in the side chain, or a polymer in which a photoisomerizable molecule is dispersed is particularly preferable. Moreover, as a group or molecule to be photoisomerized, for example, those containing an azobenzene skeleton are suitable.
[0038]
  Here, the principle of light-induced birefringence will be described using azobenzene as an example. As shown in the following chemical formula, azobenzene exhibits trans-cis photoisomerization upon irradiation with light. Before the optical recording layer is irradiated with light, a large amount of trans azobenzene exists in the optical recording layer. These molecules are randomly oriented and are isotropic in macro. When the optical recording layer is irradiated with linearly polarized light from a predetermined direction indicated by an arrow, one transformer having an absorption axis in the same direction as the polarization direction is selectively photoisomerized to a cis form. Molecules relaxed in the two transformers having an absorption axis perpendicular to the polarization direction no longer absorb light and are fixed in that state. As a result, anisotropy of absorption coefficient and refractive index, that is, dichroism and birefringence are induced macroscopically. In general, these properties are called light-induced birefringence, light-induced dichroism, or light-induced anisotropy. Moreover, these excited anisotropies can be eliminated by irradiating circularly polarized light or non-polarized light.
[0039]
[Chemical 1]

[0040]
  A polymer containing such a photoisomerization group can change the orientation of the polymer itself by photoisomerization and induce a large birefringence. The birefringence induced in this way is stable below the glass transition temperature of the polymer and is suitable for hologram recording.
[0041]
  Preferable examples of the material constituting the recording layer 12 include polyester having cyanoazobenzene in the side chain represented by the following chemical formula. This polyester can record the intensity and polarization direction of signal light as a hologram due to photo-induced anisotropy due to photoisomerization of side chain cyanoazobenzene ("Holographic recording and retrieval of polarized light by use of polyester containing" cyanoazobenzene units in the side chain ", K. Kawano, T. Ishii, J. Minabe, T. Niitsu, Y. Nishikata and K. Baba, Opt. Lett. Vol. 24 (1999) pp. 1269-1271).
[0042]
[Chemical 2]

[0043]
  The optical recording medium 10 provided with the recording layer 12 made of the above polyester material can be produced by casting and drying a polyester chloroform solution on a washed glass substrate. When the absorption spectrum of the optical recording medium 10 on which the recording layer 12 having a thickness of 20 μm was formed was measured, π-π of azobenzene was measured.^*A spectrum having a peak in the vicinity of 365 nm corresponding to the transition was obtained.
[0044]
  Note that the method of manufacturing the optical recording medium is not limited to this, and the optical recording medium may be manufactured by spin-coating the recording layer material on the substrate, and the recording layer material is injected into the parallel plate cell. Thus, an optical recording medium may be manufactured. Alternatively, the optical recording medium may be produced by bonding the recording layer material to the film substrate by hot pressing.
[0045]
(Principle of polarization hologram recording)
  When recording a hologram on the optical recording medium 10 including the recording layer 12 made of a polymer or polymer liquid crystal having azobenzene in the side chain, or a polymer in which azobenzene is dispersed, the coherent signal light 1 and reference light, respectively. 2 to the same area of the optical recording medium 10 at the same time.
[0046]
  In this case, when the polarization directions of the signal light 1 and the reference light 2 are parallel to each other, for example, as shown in FIG. 2A, both the signal light 1 and the reference light 2 are s-polarized light, the optical recording medium 10 In addition, a light intensity distribution is generated by two-wave interference between the signal light 1 and the reference light 2. In a place where the light intensity is strong, as described above, the optical anisotropy is induced by the trans-cis-trans isomerization cycle. Accordingly, an absorptive or refractive index grating corresponding to the light intensity distribution is recorded as a hologram.
[0047]
  On the other hand, when the polarization directions of the signal light 1 and the reference light 2 are orthogonal to each other, for example, as shown in FIG. 2B, the signal light 1 is p-polarized and the reference light 2 is s-polarized. Sometimes, the light intensity distribution does not occur as in the case where the polarization directions of the signal light 1 and the reference light 2 are parallel to each other. Instead, the polarization direction is spatially and periodically modulated, and linearly polarized portions 8 and elliptically polarized portions 9 appear alternately and periodically.
[0048]
  In this case, the light intensity distribution is uniform, but the azobenzene directed in the same direction as the modulated polarization direction is induced in the optical anisotropy by the trans-cis-trans isomerization cycle as described above, Recorded as a hologram.
[0049]
  Hereinafter, a hologram based on a light intensity distribution when the polarization directions of the signal light 1 and the reference light 2 are parallel as shown in FIG. 2A is referred to as a light intensity hologram, and is referred to as the signal light 1 as shown in FIG. A hologram based on the polarization distribution when the polarization directions of the light 2 are orthogonal is referred to as a polarization hologram.
[0050]
  Thus, according to the optical recording medium 10 including the recording layer 12 made of a polymer or polymer liquid crystal having azobenzene in the side chain, or a polymer in which azobenzene is dispersed, the polarization directions of the signal light 1 and the reference light 2 Regardless of whether they are parallel or orthogonal, holograms can be recorded as a result of the induction of anisotropy of azobenzene.
[0051]
  In each of the above cases, if the polarization direction of the hologram readout light 3 is the same as the polarization direction of the reference light 2, the diffracted light 4 having the same polarization state as the signal light 1 can be obtained. In addition, the recorded hologram is retained without relaxation for several years or more under room temperature natural light.
[0052]
(Hologram recording / reproducing device)
  FIG. 3 shows an example of the hologram recording / reproducing apparatus of the present invention. This hologram recording / reproducing apparatus includes a recording apparatus shown in FIG. 3A and a reproducing apparatus shown in FIG. The recording device and the playback device may be configured integrally or may be configured separately.
[0053]
  As shown in FIG. 3A, the recording apparatus includes a recording head 22 that records a hologram by simultaneously irradiating a predetermined area of the optical recording medium 10 with the signal light 1 and the reference light 2. The recording head 22 includes a light source 40 that emits coherent light, a beam splitter 41 that divides the light from the light source 40 into two light waves for signal light and reference light, a lens 43 that collimates the light wave that has passed through the beam splitter 41, and 44, a spatial light modulator 30 that modulates the collimated light wave, a condensing lens 45 that condenses the modulated signal light 1 on a predetermined region of the optical recording medium 10, and a light wave reflected by the beam splitter 41. Mirrors 47 and 48 for guiding the reference light 2 to a predetermined area of the optical recording medium 10 are provided. The spatial light modulator 30 is connected to the computer 21 and controlled by the computer 21.
[0054]
  As the light source 40 of the recording head 22, a material that is sensitive to the material of the recording layer 12 of the optical recording medium 10 and emits coherent light can be used. When polyester having cyanoazobenzene in the side chain is used for the recording layer 12, it is preferable to use an argon ion laser having an oscillation wavelength of 515 nm corresponding to the bottom of the absorption peak of the optical recording medium 10 as the light source.
[0055]
  As the spatial light modulator 30, a transmissive spatial light modulator in which transparent electrodes are formed on both surfaces of an electro-optic conversion material such as liquid crystal can be used. An example of this type of spatial light modulator is a liquid crystal panel for a projector.
[0056]
  However, in order to enable polarization modulation, when using the projector liquid crystal panel, it is necessary to remove at least the polarizing plate disposed on the output side. For example, the spatial light modulator 30 can be configured as a transmissive liquid crystal cell 124 in which electrodes 122 and 123 are formed on both surfaces of a liquid crystal 121 that is one of electro-optic conversion members, as shown in FIG. In the spatial light modulator that performs polarization modulation, a plurality of pixels are formed two-dimensionally, each pixel functions as a half-wave plate, and each pixel receives information on a bit corresponding to two-dimensional data. By giving the presence or absence of voltage application, the polarization of light incident on each pixel is modulated.
[0057]
  As shown in FIG. 3B, the reproducing apparatus includes a reading optical system 31 for causing coherent light to enter from the end face of the optical recording medium 10, a reading unit 23 for reading the diffracted light 4 from the recorded hologram, It has. The reading unit 23 includes a lens 46 that collimates the diffracted light 4 and a photodetector 50 such as a CCD that detects incident diffracted light.
[0058]
  The reading optical system 31 does not change the refractive index or the absorptivity held by the hologram recorded on the recording layer 12 of the optical recording medium 10 (that is, without destroying the recorded hologram) and emits coherent light. A light source that emits light can be included. When polyester having cyanoazobenzene in the side chain is used for the recording layer 12, it is preferable to use a helium neon laser having an oscillation wavelength of 633 nm that has no sensitivity (no absorption) in the recording layer 12 of the optical recording medium 10 as the light source. However, other laser diodes may be used.
[0059]
(Hologram recording and playback method)
  Next, the hologram recording / reproducing method of the present invention will be described.
[0060]
  First, at the time of recording, in the recording apparatus shown in FIG. 3A, the coherent light output from the light source 40 of the recording head 22 is divided into two light waves for signal light and reference light by the beam splitter 41. The light wave transmitted through the beam splitter 41 is converted into parallel light having a large light diameter by the lenses 43 and 44. Thereafter, the collimated light wave is modulated by the spatial light modulator 30 to generate the signal light 1.
[0061]
  The modulated signal light 1 is reduced or Fourier transformed by the lens 45 and irradiated to a predetermined area of the optical recording medium 10. On the other hand, the reference light 2 reflected by the beam splitter 41 is reflected by the mirrors 47 and 48 and guided to a predetermined region of the optical recording medium 10 and crosses the signal light 1 in the optical recording medium. Is incident on. In this way, hologram recording is performed by simultaneously irradiating the same region with the signal light 1 and the reference light 2.
[0062]
  As described above, as the signal light 1 and the reference light 2, laser light having a wavelength sensitive to the material of the recording layer 12 of the optical recording medium 10 is used. Further, the incident angles of the signal light 1 and the reference light 2 can be obtained as follows.
[0063]
  As shown in FIG. 6, the wave number vectors of the signal light 1 and the reference light 2 at the time of recording are respectively represented by k._objectAnd k_referenceAnd the wave number vector of the readout light 3 is k_readThen, with reference to the vector direction of the readout light 3, k_objectAnd k_referenceAre α and β, respectively, and the crossing angle of the two light waves is α-β. k_objectAnd k_referenceThe lattice vector formed by_g= k_object−k_referenceThe grating interval Λ of the recorded hologram is given by the following formula 1.
[0064]
[Formula 6]

[0065]
  Here, the wavelength of the signal light and the reference light used for recording is λ, and the readout light (wave vector k_read) Is incident on the substrate surface at an angle of 0. At this time, the incident angle to the grating is (α + β) / 2, and strong diffracted light is obtained when the phase matching condition of the following formula 2 is satisfied.
[0066]
[Expression 7]

[0067]
  Since Expression 2 is a condition that the reflected light from each layer due to multiple reflection is in phase, Expression 1 and Expression 2 lead to the relationship of Expression 3.
[0068]
[Equation 8]

[0069]
  If α and β satisfy Equation (3), the phase matching condition of the hologram is satisfied and the diffracted light can be obtained when the readout light is guided through the waveguide. The condition for the diffracted light to exit perpendicularly to the waveguide is when the incident angle to the grating (α + β) / 2 = 45 °. By substituting this condition into Equation 3, α and β represented by Equation 4 and Equation 5 are obtained. That is, the incident angle α of the signal light and the incident angle β of the reference light can be calculated according to the values of the wavelength λ of the signal light and the reference light and the wavelength λ ′ of the reading light.
[0070]
[Equation 9]

[0071]
  For example, when α and β are obtained with λ = 515 nm, λ ′ = 633 nm, and m = 1, the incident angle α of the signal light can be calculated as 80 ° and the incident angle β of the reference light can be calculated as 10 °.
[0072]
  Next, at the time of reading, in the reproducing apparatus shown in FIG. 3B, coherent light output from the reading optical system 31 is incident as reading light 3 from the end face of the recording layer 12 of the optical recording medium 10. As shown in FIG. 8, the incident readout light 3 is guided through the recording layer 12, and the diffracted light is diffracted from the recording layer 12. The diffracted light wave (diffracted light 4) is imaged on the photodetector 50 by the lens 46.
[0073]
  As described above, the readout light 3 is a laser beam having a wavelength that does not change the refractive index or the absorptivity held by the hologram recorded on the recording layer 12 of the optical recording medium 10, for example, the recording layer 12 of the optical recording medium 10. A laser beam having a wavelength with which the material is not sensitive is used. In this way, the laser beam having a wavelength that does not change the refractive index or the absorptivity held by the hologram recorded on the recording layer 12 of the optical recording medium 10 at the time of reproduction is used as the reading light 3. It is possible to reproduce the recorded hologram without destroying it.
[0074]
  Further, since the reading light 3 is guided through the recording layer 12, the hologram recorded along the waveguide of the reading light 3 can be read at a time.
[0075]
  As shown in FIG. 9, when hologram recording is performed by making the signal light 1 modulated by the spatial modulator 30 incident on the optical recording medium 10 without passing through a lens, as shown in FIG. The hologram can also be read by guiding the reading light 3 to the recording layer 12 from the direction opposite to the incident direction of the reference light 2 used for recording. In this case, since the wavelength of the reference light 2 and the wavelength of the readout light 3 are different, the magnification and the optical path change, but the phase conjugate light of the signal light 1 can be reproduced as the diffracted light 4. The phase conjugate light has the same wavefront as the signal light, and travels backward in the optical path on which the signal light is incident. Therefore, it is possible to form an image on the photodetector 50 without using a special imaging optical system. In this case, the reading light 3 is incident from the end face of the optical recording medium 10 and the diffracted light 4 is extracted from the incident side of the signal light 1.
[0076]
  Further, in order to make the reference light 2 enter the optical recording layer 12 at a narrow angle such as 10 °, for example, as shown in FIG. 7, a thick glass substrate 14 having the same refractive index as that of the transparent substrate 11 is optically recorded. The medium 10 may be brought into contact with the transparent substrate 11 and the reference light 2 may be incident from the side surface of the glass substrate 14. The reference light 2 incident from the side surface of the glass substrate 14 is refracted at the interface between the air and the glass substrate 14 and enters the optical recording layer 12 at an angle narrower than the incident angle to the glass substrate 14. Further, the transparent substrate 11 may be sufficiently thick and the reference light 2 may be incident from the side surface of the transparent substrate 11.
[0077]
(Polarized hologram recording)
  An example of performing polarization hologram recording using the above-described hologram recording / reproducing method will be described. Recording and reproduction were performed by the hologram recording / reproducing apparatus shown in FIGS. 3A and 3B using the polarization-sensitive optical recording medium provided with the recording layer made of polyester having cyanoazobenzene in the side chain. .
[0078]
  As the recording light source 40, an argon ion laser having an oscillation wavelength of 515 nm sensitive to polyester as a recording layer material was used, and the crossing angle φ between the signal light and the reference light was set to 70 °. Using the polarization modulation type spatial light modulator 30, signal light having a polarization distribution shown in FIG. 10 was generated. In this signal light, the direction of linearly polarized light changes for each pixel, and the direction of polarization represents data information. Here, if the direction of the N value is recorded, log for each pixel₂N-bit data can be stored.
[0079]
  Further, as the light source of the reading optical system 30, a helium neon laser having an oscillation wavelength of 633 nm, which is insensitive to polyester as a recording layer material, was used. However, the reading light was guided to the recording layer from the direction opposite to the incident direction of the reference light used for recording, and the phase conjugate light of the signal light was diffracted. An analyzer was placed in the optical path of the diffracted light and separated into orthogonal polarization components (0 ° polarization component and 90 ° polarization component). As a result, two images of a 0 ° polarization component and a 90 ° polarization component as shown in FIGS. 11A and 11B were obtained. From the light intensity distribution ratio between these two images, the polarization angle ρ of the reproduction light was calculated using the following formula 6.
[0080]
[Expression 10]

[0081]
  Where I₀And I₉₀Are the intensities of the 0 ° polarization component and the 90 ° polarization component of each pixel, respectively. FIG. 12 shows the result of plotting the polarization angle of the obtained reproduced image against the polarization angle of the signal light. From FIG. 12, it can be seen that the polarization angle of the reproduced image changes substantially linearly with respect to the polarization angle of the signal light, and the polarization direction of the signal light and the polarization direction of the reproduced light are the same. Therefore, according to the hologram recording / reproducing method of the present invention, the polarization distribution of the signal light can be recorded and the recorded polarization distribution can be reproduced faithfully.
[0082]
  In the above description, an example of performing recording and reproduction of a hologram using signal light that holds data information by polarization distribution has been described. However, for signal light that holds data information by amplitude (intensity) distribution and phase distribution, Similarly, hologram recording and reproduction can be performed. For example, a three-dimensional image can be reproduced by recording reflected light from a three-dimensional object as signal light.
[0083]
(Polarized hologram multiplex recording)
  Next, using the above-described hologram recording / reproducing method, an example of performing hologram recording twice under the two conditions of the case where the polarization directions of the signal light and the reference light are parallel to each other and orthogonal to each other, that is, the first stage The hologram recording is performed with the polarization directions of the signal light and the reference light in parallel with each other, and in the second stage, the polarization direction of the signal light (or the reference light) is rotated by 90 ° by the half-wave plate, so that the signal light and the reference light are An example will be described in which the second hologram is multiplexed and recorded in the same region under the condition that the polarization directions are orthogonal to each other. However, the same polarization multiplexed recording / reproduction is possible even if circularly polarized light orthogonal to each other is used.
[0084]
  As shown in FIG. 13A, using the polarization-sensitive optical recording medium 10 provided with the recording layer 12 made of polyester having cyanoazobenzene in the side chain, the spatial modulator 30 and the optical recording medium 10 7 except that the half-wave plate 20 is disposed between the optical recording medium 10 and the optical detector 50 as shown in FIG. 13B. Hologram recording and reproduction were performed with the same configuration as the reproduction apparatus shown in FIG. 9 except that a polarizer 49 such as a polarization beam splitter for separating a predetermined polarization component included in the diffracted light 4 was disposed.
[0085]
  As shown in FIG. 13A, as will be described later, an image is displayed on the spatial light modulator 30 by a computer (not shown), and is displayed on the spatial light modulator 30 as light passing through the spatial light modulator 30. Then, the signal light having the image information is obtained, and the signal light is passed through the half-wave plate 20 and converted into the signal light 1 having a predetermined polarization direction. The optical recording medium 10 is irradiated with the signal light 1 that has passed through the half-wave plate 20. At the same time, the reference light 2 is applied to the area of the optical recording medium 10 where the signal light 1 is applied. As a result, the signal light 1 and the reference light 2 interfere with each other in the optical recording medium 10, and a hologram is recorded in the optical recording medium 10.
[0086]
  In this case, in the first stage, a binary intensity image as shown in FIG. 14A is displayed on the spatial light modulator 30 as the first image and passes through the half-wave plate 20. The optical recording medium 10 is adjusted by irradiating the optical recording medium 10 with the signal light 1 and the reference light 2 at the same time by adjusting the polarization direction of the signal light 1 to be perpendicular to the paper surface (this is 0 °). Record the first hologram inside.
[0087]
  Next, in the second stage, a binary intensity image as shown in FIG. 14B is displayed on the spatial light modulator 30 as the second image, and passes through the half-wave plate 20. The optical recording medium 10 is adjusted such that the polarization direction of the signal light 1 is adjusted to be parallel to the plane of the paper (this is 90 °) and the optical recording medium 10 is irradiated with the signal light 1 and the reference light 2 simultaneously. The second hologram is recorded in the area where the first hologram is recorded. However, the polarization direction of the reference beam 2 is the same when the first hologram is recorded and when the second hologram is recorded.
[0088]
  At the time of reading, as shown in FIG. 13B, the coherent light output from the reading optical system 31 is incident as reading light 3 from the end face of the recording layer 12 of the optical recording medium 10. The incident readout light 3 is guided through the recording layer 12, and the second-order diffracted light is diffracted from the recording layer 12.
[0089]
  The diffracted light 4 is imaged on a photodetector 50 such as a CCD, and the data image of the signal light 1 is read. That is, it is possible to obtain reproduced images from the two holograms recorded in the first stage and the second stage. However, the polarization directions of the diffracted lights from the two holograms are orthogonal to each other. In this case, a polarizer 49 is arranged between the optical recording medium 10 and the photodetector 50, and two diffracted images are obtained by adjusting the transmission axis of the polarizer 49 in an arbitrary direction by a computer (not shown). Can be read out separately. For example, as described below, it is possible to read out only the first image, the second image, or an arbitrary calculation output between the first image and the second image.
[0090]
  As shown in FIG. 15, when only the first image is read, the transmission axis of the polarizer 49 is 0 °, and when only the second image is read, the transmission axis of the polarizer 49 is 90 °. Diffracted light component A from the first hologram recorded in the first stage₁The amplitude of T₁The diffracted light component A from the second hologram recorded in the second stage₂The amplitude of T₂Then, when the transmission axis of the polarizer 49 is set to 0 °, the light intensity transmitted through the polarizer 49 is | T₁｜²When the transmission axis of the polarizer 49 is 90 °, the light intensity transmitted through the polarizer 49 is | T₂｜²Is proportional to
[0091]
  However, this is a case where the polarization of the signal light 1 is reproduced faithfully to the diffracted light 4. Actually, the polarization direction of the diffracted light 4 may slightly deviate from that of the signal light 1 depending on the polarization characteristics of the optical system and the optical recording medium 10. However, even in that case, the polarization directions of the two images recorded in multiple recording are maintained in a relationship orthogonal to each other. Therefore, by appropriately adjusting the transmission axis of the polarizer 49, the two images are crosstalked. It can be separated and taken out without occurring.
[0092]
  Diffracted light component A₁And diffracted light component A₂When the resultant vector and the orientation of the polarizer are parallel, the light intensity transmitted through the polarizer is | T₁+ T₂｜²Is proportional to ｜ T₁| = | T₂If |, the sum of the two diffracted light components can be obtained at θ = 45 °. That is, if the transmission axis of the polarizer 49 is set to 45 °, the light intensity transmitted through the polarizer 49 is | T₁+ T₂｜²In this case, the addition output of the two images is obtained. Further, the diffracted light component A₁And diffracted light component A₂When the resultant vector and the orientation of the polarizer are orthogonal, the intensity of light transmitted through the polarizer is | T₁-T₂｜²Is proportional to ｜ T₁| = | T₂If |, the difference between the two diffracted light components can be obtained at θ = 135 °. That is, if the transmission axis of the polarizer 49 is set to 135 °, the light intensity transmitted through the polarizer 49 is | T₁-T₂｜²The subtraction output of two images is obtained.
[0093]
  Amplitude T₁And T₂Are represented by light “bright” (data “1”) and “dark” (data “0”) as shown in FIGS. 14A and 14B, respectively. In the case of a binary image, the subtraction output of the two images | T₁-T₂｜²Is also binary data, and is equivalent to an exclusive OR (XOR) of two images as shown in FIG. On the other hand, the addition output of two images | T₁+ T₂｜²Has three values (brightness) of “1 + 1”, “1”, and “0”, but “1 + 1” and “1” are “bright” and “0” is “dark” by threshold processing. ", The logical sum (OR) of the two images is obtained as shown in FIG.
[0094]
  As described above, the present inventionUsed inAn optical recording medium is configured by forming a light-sensitive recording layer exhibiting light-induced birefringence on one side of a transparent substrate, but at the laser wavelength irradiated as readout light during reproduction, the optical substrate is refracted. Rate n₂Is the refractive index n of the recording layer 12₁Therefore, the recording layer becomes a slab type optical waveguide, and the readout light incident from the end face can be guided through the recording layer.
[0095]
  In the hologram recording / reproducing method and hologram recording / reproducing apparatus of the present invention, at the time of recording, a laser beam having a wavelength sensitive to the recording layer of the optical recording medium is recorded as a reference light incident on the recording layer at a narrow angle. Waveguide loss is not a problem, and a hologram can be recorded. At the time of reading, laser light having a wavelength that does not change the refractive index or absorption held by the recording layer of the optical recording medium is guided to the recording layer as reading light to obtain diffracted light, so that the recorded hologram is destroyed. You can play without having to.
[0096]
  Further, by using the hologram recording / reproducing method of the present invention, the polarization of the signal light can be recorded and reproduced as a polarization hologram on a polarization-sensitive optical recording medium. This polarization hologram generates light that preserves the polarization direction of the signal light as its diffracted light. By rotating the polarization angle of the signal light by a predetermined angle, information can be recorded and read out depending on the difference in the polarization angle. become.
[0097]
  Further, by using the hologram recording / reproducing method of the present invention, the polarization direction of the signal light and the reference light is set to two directions, ie, a direction parallel to each other and a direction orthogonal to each other, so that the same region of the polarization-sensitive optical recording medium can be obtained. Two signal lights can be multiplexed and recorded as two holograms, and the region of this optical recording medium is irradiated with readout light, and two diffracted light components having polarization directions orthogonal to each other are synthesized. By obtaining the diffracted light and extracting an arbitrary polarization component from the diffracted light, it is possible to obtain a calculation output between two data recorded as two signal lights in the region. That is, it is possible to easily and quickly perform any operation such as a sum or difference operation or a logical operation between data such as two images recorded on an optical recording medium.
[0098]
【The invention's effect】
  The optical recording medium used in the present invention can record a hologram with sufficient diffraction efficiency, and the recording layer becomes a slab type optical waveguide, and the readout light incident from the end face is guided through the recording layer. Can do. According to the hologram recording / reproducing method and the hologram recording / reproducing apparatus of the present invention, it is possible to record a hologram with sufficient diffraction efficiency and to record the recording light by guiding the reading light incident on the recording layer from the end face. It is possible to reproduce the hologram without destroying it.
[0099]
  According to the hologram recording method and the hologram recording apparatus of the present invention, sufficient diffraction is performed so that the recorded hologram can be reproduced without being destroyed by guiding the reading light incident on the recording layer from the end face. There is an effect that a hologram capable of obtaining efficiency can be recorded.
[Brief description of the drawings]
FIG. 1A is a perspective view showing the configuration of an optical recording medium of the present invention, and FIG. 1B is a perspective view showing another configuration of the optical recording medium of the present invention.
FIGS. 2A and 2B are explanatory diagrams for explaining a hologram based on a light intensity distribution and a hologram based on a polarization distribution. FIGS.
3A and 3B are cross-sectional views showing an example of the configuration of the hologram recording / reproducing apparatus of the present invention.
FIG. 4 is a sectional view showing a configuration of a spatial modulator used in the hologram recording / reproducing apparatus of the present invention.
FIG. 5 is a cross-sectional view showing another example of the configuration of the hologram recording / reproducing apparatus of the present invention.
FIG. 6 is a diagram showing a relationship among a wave number vector of signal light, a wave number vector of reference light, a wave number vector of readout light, and a formed lattice vector.
FIG. 7 is a cross-sectional view showing an incident method when the incident angle of the reference light is small.
FIG. 8 is a cross-sectional view showing how diffracted light is read out by the hologram recording / reproducing method of the present invention.
9 is a cross-sectional view showing a state in which readout light is guided in a direction opposite to that in FIG. 8 to obtain phase conjugate light as diffracted light.
FIG. 10 is a diagram showing a polarization distribution of signal light when the hologram recording / reproducing method of the present invention is applied to polarization hologram recording.
11A is a diagram showing an image of a 0 ° polarization component of diffracted light obtained based on a polarization hologram recording the signal light shown in FIG. 10, and FIG. 11B is a diagram showing a 90 ° polarization component. It is a figure showing an image.
12 is a diagram in which the polarization angle of a reproduced image obtained based on the polarization hologram recording the signal light shown in FIG. 10 is plotted against the polarization angle of the signal light.
FIGS. 13A and 13B are cross-sectional views showing another configuration example of the optical recording apparatus of the present invention. FIGS.
14A and 14B are input images when the hologram recording / reproducing method of the present invention is applied to polarization hologram multiplex recording, and FIGS. 14C and 12D are input images. Is an output image obtained by processing the input image shown in FIG.
FIG. 15 is a diagram showing the relationship between the polarization direction of signal light, the polarization direction of diffracted light, and the orientation of a polarizer in polarization hologram multiplex recording.
[Explanation of symbols]
1 signal light
2 Reference light
3 Reading light
4 Diffracted light
10 Optical recording media
11 Transparent substrate
12 Recording layer
13 Cladding layer
20 1/2 wavelength plate
30 Spatial light modulator
21 computer
40 Light source
41 Beam splitter
43, 44, 45, 46 Lens
47, 48 mirror
49 Polarizer
50 photodetectors

Claims (9)

A refractive index or an absorptance is changed by simultaneously irradiating a signal light having a predetermined wavelength and a reference light while guiding light having an incident angle with respect to the surface of the substrate and a predetermined value or more. And an optical recording medium including a recording layer capable of recording a hologram while maintaining a changed refractive index or absorptance,
During recording, the hologram is recorded by simultaneously irradiating the recording layer with signal light and reference light of a predetermined wavelength,
At the time of reproduction, reading light having a wavelength that does not change the refractive index or absorption held in the recording layer is guided to the recording layer, and the hologram is reproduced by diffracted light when guided.
Hologram recording / reproducing method.   The signal light holding the data information by the polarization distribution is irradiated onto the optical recording medium simultaneously with the reference light, the polarization distribution of the signal light is recorded as a hologram, and the diffraction when the readout light is guided to the recording layer. The hologram recording / reproducing method according to claim 1, wherein the hologram is reproduced by light.   3. The hologram recording / reproducing method according to claim 2, wherein the polarization distribution is performed by rotating the polarization angle of the signal light by a predetermined angle according to the data information. Irradiating a predetermined area of the optical recording medium simultaneously with the reference light with the signal light holding the first data information by the intensity distribution, and recording the intensity distribution of the signal light as a first hologram;
By changing the polarization state of one of the signal light and the reference light and irradiating a predetermined area of the optical recording medium simultaneously with the reference light with the signal light holding the second data information by the intensity distribution, the signal light And multiple recording the intensity distribution as a second hologram,
The hologram recording / reproducing method according to claim 1, wherein the first hologram and the second hologram are reproduced by diffracted light when reading light is guided to the recording layer. A hologram is recorded by simultaneously irradiating a signal beam having an incident angle α satisfying the relationship of the following equation (5) and a reference beam having an incident angle β satisfying the relationship of the following equation (4). Item 5. The hologram recording / reproducing method according to any one of Items 1 to 4.

(In the above formulas (4) and (5), λ represents the wavelength of the signal light and the reference light, λ ′ represents the wavelength of the readout light, and m represents an integer of 1 or more.) A refractive index or an absorptance is changed by simultaneously irradiating a signal light having a predetermined wavelength and a reference light while guiding light having an incident angle with respect to the surface of the substrate and a predetermined value or more. A hologram recording / reproducing apparatus for recording a hologram on an optical recording medium including a recording layer capable of recording a hologram while maintaining a changed refractive index or absorption rate, and reproducing the recorded hologram,
Signal light irradiating means for irradiating a predetermined region of the recording layer with signal light having a predetermined wavelength from a direction intersecting the waveguide direction of the recording layer;
Reference light irradiating means for irradiating the predetermined region with the reference light of the predetermined wavelength so as to change the refractive index or the absorptance of the recording layer by the interference with the signal light;
Read light incident means for causing the read light having a wavelength that does not change the refractive index or the absorptivity held in the recording layer to enter the recording layer so that the read light is guided through the recording layer;
Hologram recording / reproducing apparatus. A hologram is recorded by simultaneously irradiating a signal beam having an incident angle α satisfying the relationship of the following equation (5) and a reference beam having an incident angle β satisfying the relationship of the following equation (4). Item 7. The hologram recording / reproducing apparatus according to Item 6.

(In the above formulas (4) and (5), λ represents the wavelength of the signal light and the reference light, λ ′ represents the wavelength of the readout light, and m represents an integer of 1 or more.) A refractive index or an absorptance is changed by simultaneously irradiating a signal light having a predetermined wavelength and a reference light while guiding light having an incident angle with respect to the surface of the substrate and a predetermined value or more. And a hologram recording method for recording a hologram on an optical recording medium including a recording layer capable of recording a hologram while maintaining a changed refractive index or absorption rate,
A hologram that records a hologram by simultaneously irradiating a signal beam having an incident angle α satisfying the relationship of the following equation (5) and a reference beam having an incident angle β satisfying the relationship of the following equation (4). Recording method.

(In the above formulas (4) and (5), λ represents the wavelength of the signal light and reference light that change the refractive index or absorption rate of the recording layer , and λ ′ is the refractive index or absorption held in the recording layer. (This represents the wavelength of the reading light that does not change the rate, and m represents an integer of 1 or more.) A refractive index or an absorptance is changed by simultaneously irradiating a signal light having a predetermined wavelength and a reference light while guiding light having an incident angle with respect to the surface of the substrate and a predetermined value or more. A hologram recording apparatus for recording a hologram on an optical recording medium including a recording layer capable of recording a hologram while maintaining a changed refractive index or absorption rate,
A hologram that records a hologram by simultaneously irradiating a signal beam having an incident angle α satisfying the relationship of the following equation (5) and a reference beam having an incident angle β satisfying the relationship of the following equation (4). Recording device.

(In the above formulas (4) and (5), λ represents the wavelength of the signal light and reference light that change the refractive index or absorption rate of the recording layer , and λ ′ is the refractive index or absorption held in the recording layer. (This represents the wavelength of the reading light that does not change the rate, and m represents an integer of 1 or more.)

Images