JP3780718B2 - Optical recording medium, optical recording / reproducing method and apparatus - Google Patents

Description

【００１５】
ここで、φ_x及びφ_yはそれぞれｘ軸とｙ軸に対する初期位相を表す。この電場ベクトルＥの先端は、ｘ軸とｙ軸の位相差（φ_x−φ_y）の値によっていろいろな軌道を描く。これを模式的に表したものが図１（ａ）〜（ｉ）であり、ｘ軸とｙ軸の位相差φ_x−φ_yが変化したときの光の偏光状態を表している。図１より、位相差（φ_x−φ_y）＝０＝π＝２πのときは、電場ベクトルＥとｘ−ｙ座標軸とのなす角は時間に関係なく常に一定であり、いわゆる直線偏光となる。いま図１（ａ）に示す位相差（φ_x−φ_y）＝０の直線偏光に対して、位相差（φ_x−φ_y）がπだけ増加するような作用が加えられると、位相差（φ_x−φ_y）がπとなり、図１（ｅ）のような直線偏光になる。このことは、位相差（φ_x−φ_y）がπだけ変化するような作用を加えれば、直線偏光した光の偏光角が回転したことに相当する。
【００１６】
実際にそのような作用をする光学素子としては、図２に示すような１／２波長板がある。この素子では進相軸と遅相軸での光路差Δが次の（数２）となるようにされている。
【００１７】
【数２】
Δ＝（ｍ＋１／２）＊λ （数２）
【００１８】
ここで、ｍは整数、λは波長を表す。よって光はこの素子を通過すると、前述の位相差（φ_x−φ_y）においてπの変化を生じる。図２のように進相軸に対して、角度θ（但し０度＜θ＜９０度）の偏光角を持つ直線偏光の光Ａを１／２波長板に照射し通過させると、その透過光Ｂは進相軸に対して反対側で角度θの偏光角を持つ直線偏光の光となる。このことから、１／２波長板の働きを利用すれば、進相軸と入射直線偏光光のなす角度θを制御することで、入射直線偏光光と２θの角度をなす透過光を得ることができる。
【００１９】
また透過光でなく反射光でも同様の効果が期待できる。このことを図３を使って説明する。反射光の場合には入射と反射の２回、光が素子を通過するので、１／２波長板の半分の効果すなわち１／４波長板を利用すればよい。この素子では進相軸と遅相軸での光路差Δが次の（数３）となるようにされている。
【００２０】
【数３】
Δ＝（ｍ＋１／４）＊λ （数３）
【００２１】
ここでｍは整数、λは波長を表す。よって光はこの素子を通過すると、前述の位相差（φ_x−φ_y）においてπ／２の変化を生じる。図１よりこのような位相差変化が起きると、以下のような変化を生じる。
（１）入射した光が例えば図１（ａ）のような直線偏光であれば、例えば図１（ｃ）のような円偏光の光になる。
（２）入射した光が例えば図１（ｃ）のような円偏光の光であれば、例えば図１（ｅ）のような直線偏光の光になる。
【００２２】
よって、まず図３のように進相軸に対して、角度θ（但し０度＜θ＜９０度）の偏光角を持つ直線偏光の光Ａを１／４波長板に照射し通過させると、その透過光Ｂは左回りの円偏光の光となる。このあと反射板でその透過光Ｂが反射されると、その反射光Ｃは、右回りの円偏光の光となる。次に反射光Ｃが逆側から１／４波長板に入射するとその透過光Ｄは、進相軸に対して最初の入射とは反対側で角度θの偏光角を持つ直線偏光の光となる。このことから、最初の入射光Ａと最終的に反射して得られる反射光Ｄは、進相軸を中心に２θの角度をなすことになる。このように反射板を利用して１／４波長板に２回光を透過させても、先に説明した１／２波長板と同様の効果を得ることができる。
【００２３】
以上のように、１／２波長板もしくは反射板と１／４波長板の組み合わせのどちらを利用しても、入射する直線偏光光の偏光角θを制御することで、透過もしくは反射してくる直線偏光光の偏光角を、入射光に対して２θの角度にすることができる。
【００２４】
次に、以上説明したような１／２波長板や１／４波長板の効果を発現する光記録材料について説明する。このような記録材料としては、光誘起複屈折性を示し、かつその複屈折性が記録・保持される材料であればどのようなものであってもよい。ここで、光誘起複屈折性とは、もともと等方的な媒体に光を照射することによって屈折率の異方性（複屈折性）が生じることを言う。
【００２５】
このような光誘起複屈折性を示す材料の一例として、光異性化する基を側鎖に持つ高分子化合物または高分子液晶、または、光異性化する分子を分散させた高分子材料がある。この材料はマクロに見て等方的であるが、直線偏光を照射すると光異性化が誘起されて、それにより屈折率の異方性が生じる。光異性化する基または分子としては、異性化により大きな複屈折性を示すものが望まれ、例えば、アゾベンゼン骨格を含むものが好適である。光異性化基あるいは分子を保持する高分子化合物または高分子液晶材料としては、光異性化基の誘起された異方性が高分子あるいは高分子液晶に伝達され、結果として高分子あるいは高分子液晶全体に大きな複屈折性が生じ、かつ、その複屈折性が記録されるものが望まれる。例えば、ポリエステル群から選ばれた少なくとも一種のモノマー重合体である高分子または高分子液晶、あるいは、ポリメチルメタクリレートやポリビニルアルコールが好適である。以下に、光異性化基としてアゾベンゼンを例に説明する。
【００２６】
アゾベンゼンは、光照射によりトランス−シスの光異性化を示す。トランス型になると分子構造が図４（ａ）のようになり、一方シス型になると、分子構造が図４（ｂ）のようになる。
【００２７】
アゾベンゼンは単体では異方性を示すが、図５（ａ）のように記録材料中でランダムに分散されている場合は、記録材料全体として等方性を示す。また記録材料中では、光励起される前はトランス型が多く存在する。これに対して光励起されることによりトランス型がシス型に変化し、材料中にはシス型が多く存在するようになる。特にこの材料にある偏光方向をもつ直線偏光のポンプ光を照射すると、図５（ｂ）のようにその偏光方向と同方向であるアゾベンゼンのみが光を吸収しシス型へと変化する。この場合、アゾベンゼンの異性化により生じるアゾベンゼン自身の複屈折と、アゾベンゼンの異性化によって誘起される高分子または高分子液晶の複屈折が組み合わさって、光記録媒体中にポンプ光の偏光方向を軸とした複屈折が生じる。この複屈折性を利用して、高分子膜を先に説明した波長板として機能させることができる。
【００２８】
この種の高分子膜を例えば１／４波長板として用いる場合を考える。高分子膜の厚さをｄとし、光誘起屈折率変化をΔｎとすれば、高分子膜中を波長λの光が通過する時に起こる光路差は、Δｎ・ｄとなる。よって、これがちょうどλ／４になれば、高分子膜は１／４波長板として機能する。すなわち、次の（数４）の条件を満たすように複屈折を誘起すればよい。
【００２９】
【数４】
Δｎ・ｄ＝λ／４ （数４）
【００３０】
また同様に１／２波長板として用いる場合には、光路差Δｎ・ｄがちょうどλ／２になれば良い。すなわち、次の（数５）の条件を満たすように複屈折を誘起すればよい。
【００３１】
【数５】
Δｎ・ｄ＝λ／２ （数５）
【００３２】
つぎに本発明で使用した光記録材料の記録特性について図面により説明する。光記録材料としては、図４（ｃ）に示す側鎖にシアノアゾベンゼンを持つポリエステルを用いる。この材料を使用した光記録媒体１１について図６を用いて説明する。図６（ａ）と図６（ｂ）は、光を透過させて記録再生を行う方式で利用する光記録媒体の一実施例を示している。
【００３３】
図６（ａ）で示される光記録媒体１１では、ガラス基板などの透明基板１３上に、上述した光記録材料からなる光記録層１２を一面に形成することで作製されている。また別の実施例として、図６（ｂ）のように光記録媒体１１を光記録層１２のみで形成しても良い。また図６（ｃ）と図６（ｄ）は、光を反射させて記録再生を行う方式で利用する光記録媒体の一実施例を示している。図６（ｃ）で示される光記録媒体１１では、ガラスや樹脂などの基板１３上に、まずアルミ蒸着などで反射層１４をあらかじめ一面に形成しておき、さらに上述した光記録層１２を一面に形成することで作製されている。また別の実施例として、図６（ｄ）のように光記録媒体１１を、反射層１４と光記録層１２の２層構造として形成しても良い。またいずれの実施例の場合においても、光記録媒体の形状をシート状もしくはディスク状にしてもよい。
【００３４】
ここでは図６（ａ）で示される形状の光記録媒体１１について、図７に示す光学系によって光誘起複屈折の記録特性を測定した。光記録層１２の厚みは２μｍのものを使用した。
【００３５】
光記録材料に異方性（複屈折性）を誘起するのに使用するポンプ光７５の光源７３は、側鎖にシアノアゾベンゼンを持つポリエステルに感度のある５１５ｎｍのアルゴンイオンレーザを用いた。光源７３から出た光の偏光はｓ偏光（紙面に垂直）であり、そのレーザ光を１／２波長板７２を通し、光記録媒体１１に照射する。この１／２波長板７２でポンプ光７５の偏光角を変化させることで、光記録媒体１１に誘起される異方性の方位が変化するので、その特性を測定する。その誘起される異方性の方位は、ポンプ光とは別の光源７１で発生されるプローブ光７６で測定する。この光源７１は、光記録媒体１１に誘起される異方性に影響を与えない波長をもつ６３３ｎｍのヘリウムネオンレーザを使用した。このレーザ光を偏光子７４に透過させ、ｓ偏光（紙面に垂直）の光とした。この光を光記録媒体１１に照射しその透過光を検光子７７に導く。ポンプ光７５の照射によって光記録媒体１１に複屈折が誘起されていれば、プローブ光の偏光方向は光記録媒体１１を透過することによって回転するはずである。検光子７７を回転することによって光記録媒体１１を透過したプローブ光７６の偏光方向を調べる。
【００３６】
複屈折性を光誘起するときには、プローブ光７６を照射せず、ポンプ光７５のみを光記録媒体１１に照射する。ポンプ光７５の光強度を１Ｗ／ｃｍ²とし１５分間記録を行った。そのあと、ポンプ光７５の照射をやめ、プローブ光７６のみで誘起された光誘起複屈折性を測定している。
【００３７】
図８（ａ）〜（ｄ）に、ポンプ光７５の偏光角を変え光記録媒体１１に複屈折を誘起し、そこにプローブ光７６を透過させて複屈折の記録特性を測定した結果を示す。横軸は検光子７７の偏光回転角を示し、９０度がｓ偏光に一致する。縦軸は検光子７７の透過光強度である。ポンプ光７５を照射していない時の透過したプローブ光７６の特性は、図８（ａ）に示してある。図より９０度で検光子透過強度が大きくなることから、プローブ光７６はｓ偏光である。ポンプ光７５が照射される前は光記録媒体１１は等方性であり、プローブ光７６が透過するときに偏光角には何の変化も与えていない。次に１／２波長板７２でポンプ光７５をｓ偏光として、光記録媒体１１に光誘起複屈折性を記録した結果を図８（ｂ）に示す。図より８０度、２６０度で検光子透過強度が大きくなることがわかる。このことからポンプ光７５によって光記録媒体１１に複屈折が記録されていることがわかる。次に１／２波長板７２でポンプ光７５をｓ偏光から３０度だけ偏光角を変化させ光記録媒体１１に光誘起複屈折性を記録した結果を図８（ｃ）に示す。図より４０度、２２０度で検光子透過強度が大きくなっていることがわかる。このことからポンプ光７５の偏光角を変えることによって光記録媒体１１に記録されている複屈折性の方位が４０度だけずれたことがわかる。さらに１／２波長板７２でポンプ光７５をｓ偏光から６０度だけ偏光角を変化させ光記録媒体１１に光誘起複屈折性を記録した結果を図８（ｄ）に示す。図より１３０度で検光子透過強度が大きくなることがわかる。このことからポンプ光７５の偏光角を変えることによって光記録媒体１１に記録されている複屈折性の方位がさらに９０度だけずれたことがわかる。
【００３８】
以上の結果から、側鎖にシアノアゾベンゼンを持つポリエステルでは、複屈折性を誘起して記録することができ、ポンプ光の偏光角を回転することで、それに合わせてその複屈折性の方位を回転できることがわかる。記録された光誘起複屈折性は室温で十分長期間保持されることも確認できた。さらにポンプ光７５の偏光角を変えた上述の各記録は同じ領域で行っており、特に前回の記録の消去等のプロセスは行っていない。このことから、この光記録媒体１１では、前回の記録の消去をすることなく次回のデータを上書き記録できることも確認できた。
【００３９】
次に図６（ａ）で示される形状の光記録媒体１１について、光誘起複屈折による１／２波長板の記録再生特性を測定した。測定光学系は先に図７に示した光学系を用い、光記録材料１２の厚みは２０μｍのものを使用した。
【００４０】
光記録材料に異方性（複屈折性）を誘起するのに使用するポンプ光７５の光源７３は、側鎖にシアノアゾベンゼンを持つポリエステルに感度のある５１５ｎｍのアルゴンイオンレーザを用いた。光源７３から出た光の偏光はｓ偏光（紙面に垂直）であり、そのレーザ光を１／２波長板７２を通し、光記録媒体１１に照射する。この１／２波長板７２でポンプ光７５の偏光角を変化させることで、光記録媒体１１に誘起される異方性の方位が変化するのでその特性を測定する。その誘起される異方性の方位は、ポンプ光とは別の光源７１で発生されるプローブ光７６で測定する。この光源７１は、光記録媒体１１に誘起される異方性に影響を与えない波長をもつ６３３ｎｍのヘリウムネオンレーザを使用した。このレーザ光を偏光子７４に透過させ、ｓ偏光（紙面に垂直）の光とした。この光を光記録媒体１１に照射しその透過光を検光子７７に導く。ポンプ光７５の照射によって光記録媒体１１に複屈折が誘起されていれば、プローブ光の偏光方向は光記録媒体１１を透過することによって回転するはずである。検光子７７を回転することによって光記録媒体１１を透過したプローブ光７６の偏光方向を調べる。
【００４１】
複屈折性を光誘起するときには、プローブ光７６を照射せず、ポンプ光７５のみを光記録媒体１１に照射する。ポンプ光７５の光強度を１．３４Ｗ／ｃｍ²とし２０秒間記録を行った。この記録条件でポンプ光７５を光記録媒体１１に照射すると、記録領域が１／２波長板として機能することになる。そのあと、ポンプ光７５の照射をやめ、プローブ光７６のみで誘起された光誘起複屈折性を測定している。
【００４２】
図９にポンプ光７５の偏光角を変え光記録媒体１１に複屈折を誘起し、そこにプローブ光７６を透過させて複屈折の記録特性を測定した結果を示す。横軸は検光子７７の偏光回転角を示し、９０度がｓ偏光に一致する。縦軸は検光子７７の透過光強度である。ポンプ光７５を照射していない時の透過したプローブ光７６の特性は、図９（ａ）に示してある。図より９０度で検光子透過強度が大きくなることから、プローブ光７６はｓ偏光である。ポンプ光７５を照射前は等方性でありプローブ光７６が透過する時に偏光角には何の変化も与えていない。次に１／２波長板７２でポンプ光７５をｓ偏光として、光記録媒体１１に光誘起複屈折性を記録した結果を図９（ｂ）に示す。図よりほぼ９０度、２７０度で検光子透過強度が大きくなることがわかる。このことからポンプ光７５によって光記録媒体１１に複屈折が記録されているが、プローブ光７６の偏光角が変化しないことがわかる。これはプローブ光７６とポンプ光７５の偏光方向が同じためである。次に１／２波長板７２でポンプ光７５をｓ偏光から２２．５度だけ偏光角を変化させ光記録媒体１１に光誘起複屈折性を記録した結果を図９（ｃ）に示す。図より１３０度で検光子透過強度が大きくなっていることがわかる。このことからプローブ光の偏光角が４０度だけずれたことがわかる。またこのずれた角度はポンプ光とプローブ光のなす角の約２倍である。さらに１／２波長板７２でポンプ光７５をｓ偏光から４５度だけ偏光角を変化させ光記録媒体１１に光誘起複屈折性を記録した結果を図９（ｄ）に示す。図よりプローブ光は１７５度付近で検光子透過強度が大きくなっていることがわかる。このことからポンプ光７５の偏光角をｓ偏光から４５度だけ変えることによって、プローブ光の偏光角がポンプ光とプローブ光のなす角の約２倍である８５度だけずれたことがわかる。さらに１／２波長板７２でポンプ光７５をｓ偏光から６７．５度だけ偏光角を変化させ光記録媒体１１に光誘起複屈折性を記録した結果を図９（ｅ）に示す。図よりプローブ光は２２０度付近で検光子透過強度が大きくなることがわかる。この場合にもプローブ光の偏光角がポンプ光とプローブ光のなす角の約２倍でずれていることがわかる。
【００４３】
以上のように、記録領域にプローブ光を照射すると、記録した方位とプローブ光のなす角度の約２倍だけ偏光角がずれて、プローブ光が透過してくることがわかる。このことから、側鎖にシアノアゾベンゼンを持つポリエステルでは、原理で説明したような１／２波長板が記録・再生できることがわかる。しかもポンプ光の偏光角を回転することで、１／２波長板の方位を任意の方向で記録できることもわかる。
【００４４】
今回使用した光記録材料の光誘起屈折率変化を、図１０に示す測定系で測定した結果を図１１に示す。この測定でも図６（ａ）で示される形状の光記録媒体１１について特性を測定した。光記録層１２の厚みは２μｍのものを使用した。
【００４５】
図１０において、光記録材料に異方性を誘起するのに使用するポンプ光７５の光源７３は、側鎖にシアノアゾベンゼンを持つポリエステルに感度のある５１５ｎｍのアルゴンイオンレーザを用いた。光源７３から出た光の偏光はｓ偏光（紙面に垂直）であり、そのレーザ光を１／２波長板７２を通し、光記録媒体１１に照射する。この１／２波長板７２でポンプ光７５の偏光角をｓ偏光から４５度だけ変化させた。このポンプ光７５で誘起される複屈折性の方位は、ポンプ光とは別の光源７１で発生されるプローブ光７６で測定する。この光源７１には、光記録媒体１１に誘起される異方性に影響を与えない波長をもつ６３３ｎｍのヘリウムネオンレーザを使用した。このレーザ光を偏光子７４に透過させ、ｓ偏光（紙面に垂直）の光とした。この光を光記録媒体１１に照射しその透過光を偏光ビームスプリッター（ＰＢＳ）９１に導く。この偏光ビームスプリッター９１で透過してきたプローブ光７６のｓ偏光成分とｐ偏光成分を分離する。その各成分を光パワーメータＩ１とＩ２で測定することによって、光記録媒体１１を透過したプローブ光７６の偏光方向を調べることができる。そしてこの測定された偏光角の大きさから屈折率変化Δｎを求める。
【００４６】
測定では、初期化した光記録媒体１１にポンプ光７５を照射し複屈折性を誘起していく。このポンプ光７５の光強度は１Ｗ／ｃｍ²とした。これと同時にプローブ光７６を照射しておき、光パワーメータＩ１とＩ２で光記録媒体１１を透過したプローブ光７６の偏光方向を測定した。
【００４７】
図１１に、測定されたプローブ光７６の偏光方向から換算した屈折率変化Δｎの結果を示す。ここでは光誘起二色性Δαは無視できるものと仮定した。横軸はポンプ光照射量（強度と時間の積）であり、屈折率変化Δｎの飽和時点を１．０で示している。縦軸は測定により求めた屈折率変化Δｎの大きさを示している。この図から、ポンプ光７５で誘起される複屈折による屈折率変化Δｎはポンプ光の照射量とともに増加し飽和することがわかる。この図から飽和屈折率変化Δｎｓを求めると、今回の材料ではその値が約０．０３３となる。
【００４８】
先に説明したように、１／２波長板もしくは１／４波長板としてこの光記録材料を利用するには、（数４）もしくは（数５）を満足するようにすればよい。また特性が安定した１／２波長板もしくは１／４波長板に作製するには、（数４）もしくは（数５）より、光記録材料の厚みと屈折率変化Δｎが一定であることが望ましいことが分かる。そこで本実施例の光記録媒体では、屈折率変化Δｎに関しては、ある光照射量以上で安定した値となる飽和屈折率変化Δｎｓを使用することとした。これにより飽和屈折率変化Δｎｓの値と（数４）もしくは（数５）より、１／２波長板もしくは１／４波長板の厚みが決まる。その厚みで光記録材料を作製すれば、飽和屈折率変化Δｎｓを与える光量以上の露光量であれば、一定の屈折率変化Δｎｓを誘起できるため、光強度の変動に依存せず安定した記録再生を行うことができる。また記録速度であるが、現在利用できるレーザの光強度は測定に利用したものにくらべ１０⁶〜１０⁷倍程度大きい。その点を考慮すれば記録速度はおおよそｍｓのオーダーまでは高速化が可能である。
【００４９】
図１２は本発明の光記録再生装置の一実施例を示す図である。この装置では図６（ｃ）に示したような構造の反射型の光記録媒体１１０６を、図１２に示すようなディスク形状として利用した。まず光記録媒体１１０６上の光記録材料の厚みｄであるが、反射型の光記録再生を行うので、前述したように１／４波長板として機能すればよい。そこで厚みｄは（数４）を変形することにより、次の（数６）で表される。
【００５０】
【数６】
ｄ＝λ／４／Δｎ （数６）
【００５１】
光記録材料には前述した側鎖にシアノアゾベンゼンを持つポリエステルを用い、また光源１１０１には材料に感度のあるアルゴンイオンレーザを用いた。（数６）に、測定により求めたΔｎ＝０．０３３とアルゴンレーザの波長０．５１５μｍを代入すると、厚みｄは３．９μｍとなる。そこで本発明の光記録媒体１１０６の光記録材料の厚みを３．９μｍとなるように作製した。またレーザ光の偏光がｓ偏光であるものを用いた。ここでｓ偏光は、光記録媒体１１０６のディスク面に平行なある方向とする。
【００５２】
図１２の装置において、まず情報の記録方法について説明する。光源１１０１からの記録レーザ光は、コリメーターレンズ１１０２によって平行光とされ、空間光変調器としての偏光回転素子１１０３に入射する。偏光回転素子１１０３は、λ／２板、液晶バルブ、ポッケルス素子、ファラデー素子などが使用可能である。この偏光回転素子１１０３として液晶バルブを例に取ると、液晶は１／２波長板として機能するので、電圧が印可されない状態では、入射光の偏光方向と１／２波長板の軸は平行としておく。これにより入射光がｓ偏光であるため、透過光はｓ偏光となる。一方、液晶に最大電圧が印加されたときに１／２波長板の軸が４５度回転し、入射光の偏光を９０度回転するようにしておく。回転は、光記録媒体１１０６のディスク面に平行な面方向で行う。さらに最小電圧と最大電圧の間の中間的な電圧では、その電圧の大きさに応じて１／２波長板の軸が４５度まで増加するようにしておく。これにより偏光回転素子１１０３を通過した光は、偏光回転素子１１０３に供給する電圧に応じて、偏光角を０度から９０度まで変化できる。ここでｓ偏光の偏光角を０度とした。この偏光回転素子１１０３を通過した記録光は、ビームスプリッター１１０４を透過し、対物レンズ１１０５に入射する。この対物レンズ１１０５では記録光を集光し光記録媒体１１０６に照射する。これにより先に説明したように、光誘起複屈折性を光記録媒体１１０６に記録することができる。
【００５３】
この時、偏光回転素子１１０３に加える電圧を制御することで、記録光の偏光角θを０度から９０度まで変化できる。結果として図１３に示すように、記録光１２０１で誘起された複屈折性で作製される微小１／４波長板の方位をθ（０≦θ＜９０）度にできる。この制御可能な角度θを用いて、記録する情報データを多値変調した値にすることにより、多値記録することができる。記録に利用できるダイナミックレンジは９０度と大きく、光変調素子では、角度に関係なく高いＳ／Ｎで光の偏光を回転できる。また先に説明したように記録媒体を飽和光量以上で利用するため、光強度の変動に依存せず安定して記録ができる。よってこの多値記録方法及び装置によれば、階調値によらず高いＳ／Ｎで安定した記録を行うことができる。
【００５４】
次に記録された情報の再生方法について図１２を用いて説明する。光源１１０１からの再生レーザ光は、レンズ１１０２によって平行光とされ、偏光回転素子１１０３に入射する。ただしこの時の光強度は、記録されている光誘起異方性を破壊しないように、記録時に比べ大きく減少させておく。この再生光を偏光回転素子１１０３に入射させるが、このとき偏光回転素子１１０３は電圧が印加されない状態にしておき、透過光はｓ偏光となるようにしておく。さらに偏光回転素子１１０３を通過した再生光は、ビームスプリッター１１０４を透過し、対物レンズ１１０５に入射する。この対物レンズ１１０５では再生光を集光し光記録媒体１１０６に照射する。このあと再生光は、記録された微小１／４波長板に入射し、そのあと記録媒体内の反射膜で反射されもう一度微小１／４波長板を透過して反射光となって光記録媒体１１０６から出てくる。このとき再生光１２０２は図１３に示すようにｓ偏光であり、また記録されている微小１／４波長板の方位は、記録光１２０１に示すようにｓ偏光の軸から角度θだけずれている。そのため反射光１２０３の偏光角は原理で説明したように、ｓ偏光の軸から角度２θだけずれてでてくる。この反射光はこのあとビームスプリッター１１０４で反射され、検光子１１０８を通過して検出器１１０９に送られる。この光検子１１０８と検出器１１０９で反射光の偏光角２θを検出することで、多値変調記録された角度θを再生できる。ここで検出器１１０９にはＣＣＤやフォトディテクタを用いることができる。
【００５５】
このように本発明の再生方法及び再生装置では、記録した角度θを２倍の角度２θで検出できるので、記録した多値変調値の復調が可能になる。記録でのダイナミックレンジは９０度なので、再生で利用できるダイナミックレンジは１８０度とさらに大きくなる。また反射光の強度は階調値によらず一定であり、光検出器はその光量に合わせてＳ／Ｎを高く設定して利用できる。また先に説明したように記録媒体を飽和光量以上で利用するため、光強度の変動に依存せず安定した角度再生が可能である。よってこの再生方法及び装置によれば、階調値によらず高いＳ／Ｎで安定した再生を行うことができる。
【００５６】
本発明の光記録媒体１１０６はディスク形状にしており、モータ１１０７を回転させることで、記録媒体１１０６の周方向に場所を変えて複数のデータを記録することができる。また記録再生ヘッド１１１０全体を光記録媒体１１０６の径方向に移動させることによって、図１２に示すように光記録媒体１１０６中に同心円状の記録トラックを形成するように記録することもできる。
【００５７】
このように本実施例に係る光記録再生装置では、ディスク形状の光記録媒体１１０６を回転駆動する媒体駆動機構と、光源、空間光変調器、結像光学系、再生光学系および検出系を含む光記録再生ヘッド１１１０を光記録媒体１１０６の径方向に移動するヘッド移動機構とを備える。光記録媒体１１０６は光記録再生装置に内蔵することもできる。
【００５８】
図１２で説明した本発明の実施例は、記録と再生を同時に行えるように構成したものであるが、一部構成を変更するだけで記録専用もしくは再生専用とすることも可能である。まず記録専用とするためには、反射光の検出部分関連が不要となるので、図１２から、ビームスプリッター１１０４、検光子１１０８、検出器１１０９を取り除いた構成となる。これにより記録ヘッドの小型軽量化を実現できる。また記録再生装置に比べ低価格な装置が製造可能になる。また再生専用とするには、図１２から記録に必要な偏光回転素子１１０３を取り除いた構成となる。これにより再生ヘッドの小型軽量化を実現できる。また記録再生装置に比べ低価格な装置が製造可能になる。
【００５９】
図１２で示した光学系は従来のＣＤＲＯＭや光磁気ディスクで使用されるものを応用することができる。そのため記録できる最小サイズは同じ大きさまで可能である。しかし本発明の記録再生装置では多値記録が行える分だけ容量や速度の増加ができる。記録に関しては偏光回転素子での角度分解能が、記録可能な多値レベルを規定するが現状では２５５値までは可能である。また再生に関しては、検出器での再生角度分解能が多値レベルを規定する。光磁気ディスクなどでは１度以下の検出をすることは可能になっており、また本発明では１８０度までの再生可能なダイナミックレンジがあるため、２００値以上は可能である。よって記録再生のバランスを考慮しても、２００値以上の記録再生が可能となる。よって本発明の記録再生装置では、２値型のＣＤＲＯＭに換算して１００枚程度の記憶容量を達成でき、読み出し速度も１００倍程度高速化が可能である。また書き込み速度に関しても１ビット当たりｍｓオーダーまで高速化が可能であり、この時間で２００値以上の記録が行えれば、記録速度に関しても１桁以上の高速化は可能である。
【００６０】
本発明の記録再生装置では、簡単な工夫をすることで従来のＣＤＲＯＭや書き込みをされた光磁気ディスクを再生することが可能である。ＣＤＲＯＭの場合は、反射光の偏光角の変化はなく、反射光の強度の変化だけである。本発明の装置の再生光検出部では強度も検出できるので、ＣＤＲＯＭで再生される強度変化を２値的に判定できるようにしておくだけでＣＤＲＯＭの再生は可能になる。また光磁気ディスクの場合は、反射光の偏光角の検出をすればよい。本発明の装置では前述したように、光磁気ディスクでの微小な偏光角変化を検出できるだけの分解能があるので、光磁気ディスクの再生も可能である。
【００６１】
また本発明の記録再生装置で記録する光記録媒体に関しては、光磁気ディスク再生装置で再生できるように記録することも可能である。光磁気ディスクでは、微小な偏光角変化を記録している。本発明の記録ではそのような微小な偏光角変化を記録するのが原理であり、また光磁気ディスクと同等の分解能を持っているので、光磁気ディスク再生装置で再生できるように記録することは可能である。
【００６２】
図１４は本発明の光記録再生装置の別の実施例を示す図である。この装置では図６（ａ）のような構造の透過型の光記録媒体１３０６を、図１４に示すようなディスク形状として利用した。本図において、まず光記録媒体１３０６の光記録材料の厚みｄであるが、透過型の光記録再生を行うので、前述したように１／２波長板として機能すればよい。そこで厚みｄは（数５）を変形することにより、次の（数７）で表される。
【００６３】
【数７】
ｄ＝λ／２／Δｎ （数７）
【００６４】
光記録材料には反射型と同様の材料を用い、また光源１１０１もアルゴンイオンレーザを用いた。（数７）に、測定により求めたΔｎ＝０．０３３とアルゴンレーザの波長０．５１５μｍを代入すると、厚みｄは７．８μｍとなる。そこで本発明の光記録媒体１３０６の光記録材料の厚みを７．８μｍとなるように作製した。またレーザ光の偏光がｓ偏光であるものを用いた。ここでｓ偏光は、光記録媒体１３０６のディスク面に平行なある方向とする。
【００６５】
図１４において、まず情報の記録方法から説明する。この装置で利用する素子は反射型と同じものである。光源１１０１からの記録レーザ光は、レンズ１１０２によって平行光とされ、偏光回転素子１１０３に入射する。偏光回転素子１１０３では、偏光回転素子１１０３に供給する電圧に応じて、通過光の偏光角を０度から９０度まで変化できる。ここでｓ偏光の偏光角を０度とし、回転は光記録媒体１３０６のディスク面に平行な面方向で行う。この偏光回転素子１１０３を通過した記録光は、対物レンズ１１０５に入射し、この対物レンズ１１０５で記録光を集光し光記録媒体１３０６に照射する。これにより光誘起異方性を光記録媒体１３０６に記録することができる。この時偏光回転素子に加える電圧を制御することで、記録光の偏光角θを再生光（ｓ偏光）を基準に０度から９０度まで変化できる。これにより図１３のように、記録光１２０１で誘起された複屈折性で作製される微小１／２波長板の方位をθ（０≦θ＜９０）度にできる。この制御可能な角度θを用いて、記録する情報データを多値変調した値にすることにより、多値記録することができる。
【００６６】
次に記録された情報の再生方法について図１４を用いて説明する。光源１１０１からの再生レーザ光は、レンズ１１０２によって平行光とされ、偏光回転素子１１０３に入射する。ただしこの時の光強度は、記録されている光誘起複屈折性を破壊しないように、記録時に比べ大きく減少させておく。この再生光を偏光回転素子１１０３に入射させるが、このとき偏光回転素子１１０３には電圧が印加されない状態にしておき、透過光はｓ偏光となるようにしておく。さらに偏光回転素子１１０３を通過した再生光は対物レンズ１１０５に入射し、再生光を集光して光記録媒体１３０６に照射する。このあと再生光は、記録された微小１／２波長板を透過して光記録媒体１３０６の下面から出てくる。このとき再生光１２０２は図１３に示すようにｓ偏光であり、また記録されている微小１／２波長板の方位は、記録光１２０１で示すようにｓ偏光の軸から角度θだけずれている。そのため透過光の偏光角は原理で説明したように、反射光１２０３と同様にｓ偏光の軸から角度２θだけずれてでてくる。この透過光は、検光子１１０８を通過して検出器１１０９に送られる。この光検子１１０８と検出器１１０９で透過光の偏光角２θを検出することで、記録された角度θを再生できる。このように透過型の再生方法及び再生装置においても、記録した角度θを２倍の角度２θで検出できる。
【００６７】
図１４で説明した透過型の記録再生装置においても、図１２で説明した本発明の実施例と同様、光記録媒体１３０６の周方向及び径方向で記録再生することが可能である。そのため本記録再生装置には、ディスク形状の光記録媒体１３０６を回転駆動する媒体駆動機構と、光源、空間光変調器、結像光学系、再生光学系および検出系を含む光記録再生ヘッド１３１０を光記録媒体１３０６の径方向に移動するヘッド移動機構とを備える。光記録媒体１３０６は光記録再生装置に内蔵することもできる。
【００６８】
さらに図１４で説明した本発明の実施例は、記録と再生を同時に行えるように構成したものであるが、一部構成を変更するだけで記録専用もしくは再生専用とすることも可能である。まず記録専用とするためには、透過光の検出部分関連が不要となるので、図１４から、検光子１１０８、検出器１１０９を取り除いた構成となる。また再生専用とするには、図１２から記録に必要な偏光回転素子１１０３を取り除いた構成となる。このような専用機にすると、ヘッドの小型軽量化を実現できる。また、記録再生装置に比べ低価格な装置の製造が可能となる。
【００６９】
以上説明したように、本発明の記録媒体では、記録光の偏光角を多値変調して記録することができる。その情報を再生するときには、その偏光角を２倍にして再生できるので、再生時に安定したＳ／Ｎが得られる。さらにＳ／Ｎに関しては、記録材料の屈折率変化を光強度で飽和させ一定値にすることにより、材料によるＳ／Ｎ低下や光強度の変動によるＳ／Ｎ低下も防止できる。さらに、書き換えに関しては消去プロセスがいらないので、高速な書き換えが実現できる。記録媒体の形状をディスク状にして記憶容量やデータ転送レートを向上させることも可能である。
【００７０】
また、本発明の光記録材料を多層に積層することにより記録密度の更なる向上が可能となる。また、反射膜の表裏にそれぞれ光記録材料を形成して両面記録媒体を形成することにより、記録密度を上げることもできる。
【００７１】
本発明の光記録再生方法及び光記録再生装置によれば、記録光の偏光角θを多値変調して記録することができ、そしてその記録した偏光角θを２倍の角度として多値再生することができる。再生光の強度は記録した偏光角θの値によらず一定なので、記録及び再生において高いＳ／Ｎが得られる。この多値記録により、高密度高速な記録再生が可能となる。光記録媒体をディスク形状にして、記録再生をディスクの周方向及び径方向にすることも可能である。
【００７２】
本発明の記録再生装置では、簡単な工夫をすることで従来のＣＤＲＯＭや書き込みをされた光磁気ディスクを再生することが可能である。また、本発明の記録再生装置で記録する光記録媒体に関しては、光磁気ディスク再生装置で再生できるように記録することも可能である。
【００７３】
【発明の効果】
本発明によれば、多値の記録再生を高密度かつ高Ｓ／Ｎで行うことのできる光記録媒体、光記録再生方法及び装置を得ることができる。
【図面の簡単な説明】
【図１】（ａ）〜（ｉ）はそれぞれ光の偏光状態を示す図である。
【図２】１／２波長板を説明する図である。
【図３】１／４波長板を説明する図である。
【図４】（ａ）はアゾベンゼンのトランス構造、（ｂ）はアゾベンゼンのシス構造、（ｃ）は側鎖にシアノアゾベンゼンを有するポリエステルの構造をそれぞれ示す図である。
【図５】（ａ）はトランス型のアゾベンゼンが分散している状態、（ｂ）は光励起によりトランス型のアゾベンゼンがシス型に変化した状態をそれぞれ示す図である。
【図６】（ａ）〜（ｄ）はそれぞれ本発明に係る光記録媒体の構造を示す図である。
【図７】記録特性を測定するのに用いた光学系を示す図である。
【図８】（ａ）〜（ｄ）はそれぞれ本発明に係る光記録媒体の記録特性を示す図である。
【図９】（ａ）〜（ｅ）はそれぞれ本発明に係る光記録媒体の記録特性を示す図である。
【図１０】屈折率変化を測定するのに用いた光学系を示す図である。
【図１１】屈折率変化の飽和特性を示す図である。
【図１２】本発明に係る光記録再生装置の一実施例を示す図である。
【図１３】本発明に係る光記録再生方法を説明する図である。
【図１４】本発明に係る光記録再生装置の別の実施例を示す図である。
【符号の説明】
１１０１ 光源
１１０２ コリメーターレンズ
１１０３ 偏光回転素子
１１０４ ビームスプリッター
１１０５ 対物レンズ
１１０６ 光記録媒体
１１０７ モータ
１１０８ 検光子
１１０９ 検出器
１１１０ 記録再生ヘッド[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical recording medium, an optical recording / reproducing method, and an apparatus capable of recording data at high density and capable of high-speed data transfer.
[0002]
[Prior art]
In optical recording, rewritable optical disk devices such as phase change type and magneto-optical type have been widely used to realize high-density recording. These optical disk devices have a recording density that is one digit higher than that of a general magnetic disk device, but are not yet sufficient for digital recording of image information. In order to further increase the recording density, it is necessary to reduce the beam spot diameter to shorten the distance between adjacent tracks or adjacent bits.
[0003]
A DVD-ROM is already being put into practical use by the development of such technology. The DVD-ROM accommodates 4.7 GB data on a 12 cm disk on one side. A writable / erasable DVD-RAM is capable of high-density recording of 5.2 GB on both sides of a disk having a diameter of 12 cm by a phase change method. This is capable of writing and reading information having a capacity equivalent to 7 times or more of a read-only CD-ROM and 3600 or more floppy disks. As described above, the density of optical discs is increasing year by year. However, on the other hand, the above-mentioned optical disk is limited to the light diffraction limit in order to record data in a plane, and the recording density is 5 Gbit / inch, which is said to be the physical limit of high-density recording. ² Is approaching.
[0004]
Therefore, in order to realize further higher density and higher speed, there is a method of recording multivalued data in one recording pit. Examples of such a technique include those described in Japanese Patent Application Laid-Open Nos. 64-17248 and 4-38720. The basic principle of magneto-optical recording is to read the change in the polarization angle of reflected light due to the magneto-optical Kerr effect that occurs in the recording area. However, the polarization angle change obtained by the Kerr effect has been devised in various ways, but it is a slight change of about 1 degree. For this reason, it is difficult to increase the number of values within this slight change amount from the viewpoint of S / N, and contrivances have been made to expand the dynamic range of signals. In the technique described in Japanese Patent Laid-Open No. 64-17248, a recording medium in which recording films having different Curie temperatures are multilayered is used as means for realizing multi-value recording. Further, in the technique described in Japanese Patent Laid-Open No. 4-38720, in order to perform multi-value recording / reproduction on an optical disc, the recording pit is an ellipse, and the multi-value is achieved by changing the inclination amount of the elliptical recording pit.
[0005]
[Problems to be solved by the invention]
However, in the method of multilayering the recording film, the number of layers that can be multilayered is limited in the manufacturing process, and the dynamic range cannot be broadened greatly, so that many multivalues cannot be expected. Further, when the film thickness of the recording portion increases due to the increase in the number of layers due to the increase in the number of layers, there is a problem that the recording density in the surface direction is lowered due to heat diffusion in the disk surface direction. On the other hand, in the method of making the recording pits oval, in order to maintain the same recording density as the conventional method, the recording pit area is reduced by an amount corresponding to the oval of the recording pits compared to the normal method. For this reason, a decrease in signal intensity in the recording pit alone is unavoidable, and S / N deterioration occurs. For detection, a difference in the reproduction light pattern due to the amount of inclination is detected by a plurality of detectors. When the multi-value number is increased, the pattern difference between the gradations becomes considerably small, and the signal at each detector becomes minute. For this reason, large multi-values cause insufficient detection accuracy, and large multi-values cannot be expected.
[0006]
The present invention has been made based on such a background, and an object thereof is to provide an optical recording medium, an optical recording / reproducing method, and an apparatus capable of performing multi-level recording / reproducing with high density and high S / N. And
[0007]
[Means for Solving the Problems]
The optical recording medium of the present invention is prepared so as to include an optical recording layer made of an optical recording material having at least one layer of light-induced birefringence, and the optical recording layer is a half-wave plate or a quarter-wave plate. To function as. Specifically, the optical recording layer is composed of a polymer compound or polymer liquid crystal having a photoisomerizable group in the side chain.
[0008]
In the optical recording method of the present invention, birefringence is induced by irradiating an optical recording medium with linearly polarized recording light to form a half-wave plate or a quarter-wave plate in the polarization plane direction. By rotating the orientation of the polarization plane of the recording light (referred to as the polarization angle), the orientation of the half-wave plate or quarter-wave plate is rotated. Therefore, multi-value recording can be performed using this optical recording medium by making the polarization angle of the recording light multi-valued.
[0009]
In the light reproduction method of the present invention, a half-wave plate or a quarter-wave plate recorded in a specific direction by recording light is irradiated with reproduction light having an arbitrary polarization angle, and the transmitted light or reflected light is irradiated. Changes in the polarization angle of are detected. This change in the polarization angle corresponds to twice the angle difference between the polarization angle of the reproduction light and the polarization angle of the recording light. Therefore, the multi-value recording can be reproduced by detecting this angle.
[0010]
In the optical recording apparatus of the present invention, a light source that generates coherent recording light, a spatial modulator that rotates polarization of light from the light source according to recording information, and an imaging optical system that irradiates the optical recording medium with the recording light Thus, the axis of the half-wave plate or the quarter-wave plate in the optical recording medium is recorded by multi-value modulation. As the spatial modulator, for example, a polarization rotation element such as a liquid crystal bulb can be used.
[0011]
In the optical reproducing apparatus of the present invention, the reproducing light that irradiates the optical recording medium on which the recording is performed by multi-value-modulating the direction of the half-wave plate or the quarter-wave plate in the optical recording medium with the recording light. An optical system and a light detecting means for detecting the polarization angle of the reproduction light from the optical recording medium are provided, thereby reproducing the recorded information recorded by multi-level modulation.
[0012]
According to the present invention, high-density recording can be performed by multi-level modulation of the polarization angle of recording light. Moreover, since the polarization angle is doubled during reproduction, a stable S / N can be obtained.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
First, the principle of polarization angle multilevel recording according to the present invention will be described. A method of rotating the polarization angle of linearly polarized light by an arbitrary angle is as follows. Since light is a kind of electromagnetic wave, it can be expressed by Maxwell's equation. If the z-axis (light traveling direction) is fixed, the electric field vector E of light can be expressed by the following (Equation 1).
[0014]
[Expression 1]

[0015]
Where φ _x And φ _y Represents initial phases with respect to the x-axis and the y-axis, respectively. The tip of the electric field vector E has a phase difference between the x-axis and the y-axis (φ _x −φ _y ) Draw various trajectories depending on the value of). This is schematically shown in FIGS. 1A to 1I, where the phase difference φ between the x-axis and the y-axis is φ. _x −φ _y Represents the polarization state of light when. From FIG. 1, the phase difference (φ _x −φ _y ) = 0 = π = 2π, the angle formed by the electric field vector E and the xy coordinate axes is always constant regardless of time, and is so-called linearly polarized light. Now, the phase difference (φ shown in FIG. _x −φ _y ) = 0 for linearly polarized light, the phase difference (φ _x −φ _y ) Increases by π, the phase difference (φ _x −φ _y ) Becomes π, and linearly polarized light as shown in FIG. This means that the phase difference (φ _x −φ _y ) Is changed by π, this corresponds to rotation of the polarization angle of linearly polarized light.
[0016]
As an optical element that actually performs such an action, there is a half-wave plate as shown in FIG. In this element, the optical path difference Δ between the fast axis and the slow axis is set to the following (Equation 2).
[0017]
[Expression 2]
Δ = (m + 1/2) * λ (Equation 2)
[0018]
Here, m represents an integer, and λ represents a wavelength. Therefore, when light passes through this element, the aforementioned phase difference (φ _x −φ _y ) Causes a change in π. As shown in FIG. 2, when a linearly polarized light A having a polarization angle of θ (where 0 ° <θ <90 °) with respect to the fast axis is applied to the half-wave plate and transmitted, the transmitted light is transmitted. B is linearly polarized light having a polarization angle of θ on the opposite side to the fast axis. From this, if the action of the half-wave plate is used, the transmitted light having an angle of 2θ with the incident linearly polarized light can be obtained by controlling the angle θ formed by the fast axis and the incident linearly polarized light. it can.
[0019]
The same effect can be expected with reflected light instead of transmitted light. This will be described with reference to FIG. In the case of reflected light, light passes through the element twice, that is, incident and reflected, and therefore, an effect that is half that of a half-wave plate, that is, a quarter-wave plate may be used. In this element, the optical path difference Δ between the fast axis and the slow axis is set to the following (Equation 3).
[0020]
[Equation 3]
Δ = (m + 1/4) * λ (Equation 3)
[0021]
Here, m represents an integer and λ represents a wavelength. Therefore, when light passes through this element, the aforementioned phase difference (φ _x −φ _y ) Causes a change of π / 2. As shown in FIG. 1, when such a phase difference change occurs, the following change occurs.
(1) If the incident light is linearly polarized light as shown in FIG. 1A, for example, it becomes circularly polarized light as shown in FIG. 1C.
(2) If the incident light is circularly polarized light as shown in FIG. 1C, for example, it becomes linearly polarized light as shown in FIG. 1E.
[0022]
Therefore, as shown in FIG. 3, when the linearly polarized light A having a polarization angle of an angle θ (where 0 ° <θ <90 °) is irradiated to the quarter wavelength plate as shown in FIG. The transmitted light B becomes counterclockwise circularly polarized light. Thereafter, when the transmitted light B is reflected by the reflecting plate, the reflected light C becomes clockwise circularly polarized light. Next, when the reflected light C enters the quarter-wave plate from the opposite side, the transmitted light D becomes linearly polarized light having a polarization angle of an angle θ on the side opposite to the first incidence with respect to the fast axis. . For this reason, the first incident light A and the reflected light D that is finally reflected are formed at an angle of 2θ around the fast axis. Thus, even if light is transmitted twice through the quarter wavelength plate using the reflection plate, the same effect as that of the half wavelength plate described above can be obtained.
[0023]
As described above, regardless of whether a half-wave plate or a combination of a reflector and a quarter-wave plate is used, transmission or reflection is achieved by controlling the polarization angle θ of incident linearly polarized light. The polarization angle of the linearly polarized light can be set to 2θ with respect to the incident light.
[0024]
Next, an optical recording material that exhibits the effects of the half-wave plate and the quarter-wave plate as described above will be described. Such a recording material may be any material as long as it exhibits light-induced birefringence and can record and maintain the birefringence. Here, the light-induced birefringence means that anisotropy (birefringence) of a refractive index is generated by irradiating light to an originally isotropic medium.
[0025]
As an example of a material exhibiting such photoinduced birefringence, there is a polymer compound or polymer liquid crystal having a photoisomerizable group in the side chain, or a polymer material in which a photoisomerizable molecule is dispersed. This material is macroscopically isotropic, but when irradiated with linearly polarized light, photoisomerization is induced, thereby causing anisotropy of the refractive index. As the group or molecule that undergoes photoisomerization, a group or molecule that exhibits large birefringence upon isomerization is desired. For example, a group or molecule containing an azobenzene skeleton is suitable. As a polymer compound or polymer liquid crystal material holding a photoisomerizable group or molecule, the anisotropy induced by the photoisomerization group is transmitted to the polymer or polymer liquid crystal, and as a result, the polymer or polymer liquid crystal It is desired that a large birefringence is generated in the whole and the birefringence is recorded. For example, a polymer or polymer liquid crystal that is at least one monomer polymer selected from the polyester group, or polymethyl methacrylate or polyvinyl alcohol is suitable. Hereinafter, azobenzene will be described as an example of the photoisomerization group.
[0026]
Azobenzene exhibits trans-cis photoisomerization upon light irradiation. In the trans type, the molecular structure is as shown in FIG. 4 (a), whereas in the cis type, the molecular structure is as shown in FIG. 4 (b).
[0027]
Although azobenzene alone exhibits anisotropy, when it is randomly dispersed in the recording material as shown in FIG. 5A, the recording material as a whole is isotropic. In the recording material, there are many transformer types before photoexcitation. On the other hand, the photo-excitation changes the trans type to the cis type, and there are many cis types in the material. In particular, when linearly polarized pump light having a polarization direction in this material is irradiated, only azobenzene having the same direction as the polarization direction absorbs light and changes to a cis type as shown in FIG. In this case, the birefringence of azobenzene itself generated by the isomerization of azobenzene and the birefringence of the polymer or polymer liquid crystal induced by the isomerization of azobenzene are combined, and the polarization direction of the pump light is axised in the optical recording medium. Birefringence occurs. Utilizing this birefringence, the polymer film can be made to function as the wave plate described above.
[0028]
Consider a case where this type of polymer film is used as, for example, a quarter-wave plate. If the thickness of the polymer film is d and the light-induced refractive index change is Δn, the optical path difference that occurs when light of wavelength λ passes through the polymer film is Δn · d. Therefore, if this is exactly λ / 4, the polymer film functions as a ¼ wavelength plate. That is, birefringence may be induced so as to satisfy the following condition (Equation 4).
[0029]
[Expression 4]
Δn · d = λ / 4 (Equation 4)
[0030]
Similarly, when used as a half-wave plate, the optical path difference Δn · d only needs to be λ / 2. That is, birefringence may be induced so as to satisfy the following condition (Equation 5).
[0031]
[Equation 5]
Δn · d = λ / 2 (Equation 5)
[0032]
Next, the recording characteristics of the optical recording material used in the present invention will be described with reference to the drawings. As the optical recording material, a polyester having cyanoazobenzene in the side chain shown in FIG. An optical recording medium 11 using this material will be described with reference to FIG. FIG. 6A and FIG. 6B show an embodiment of an optical recording medium used in a system for recording and reproducing by transmitting light.
[0033]
The optical recording medium 11 shown in FIG. 6A is manufactured by forming an optical recording layer 12 made of the above-described optical recording material on one surface on a transparent substrate 13 such as a glass substrate. As another example, the optical recording medium 11 may be formed of only the optical recording layer 12 as shown in FIG. FIGS. 6C and 6D show an embodiment of an optical recording medium used in a system for recording and reproducing by reflecting light. In the optical recording medium 11 shown in FIG. 6C, the reflective layer 14 is first formed on one surface of the substrate 13 such as glass or resin by vapor deposition of aluminum, and the optical recording layer 12 described above is further formed on the one surface. It is produced by forming in. As another example, the optical recording medium 11 may be formed as a two-layer structure of a reflective layer 14 and an optical recording layer 12 as shown in FIG. In any of the embodiments, the optical recording medium may have a sheet shape or a disk shape.
[0034]
Here, for the optical recording medium 11 having the shape shown in FIG. 6A, the recording characteristics of light-induced birefringence were measured by the optical system shown in FIG. The thickness of the optical recording layer 12 was 2 μm.
[0035]
As the light source 73 of the pump light 75 used for inducing anisotropy (birefringence) in the optical recording material, a 515 nm argon ion laser sensitive to polyester having cyanoazobenzene in the side chain was used. The polarized light of the light emitted from the light source 73 is s-polarized light (perpendicular to the paper surface), and the laser light is irradiated to the optical recording medium 11 through the half-wave plate 72. By changing the polarization angle of the pump light 75 with the half-wave plate 72, the anisotropy direction induced in the optical recording medium 11 changes, and its characteristics are measured. The induced anisotropy orientation is measured by a probe light 76 generated by a light source 71 different from the pump light. As the light source 71, a 633 nm helium neon laser having a wavelength that does not affect the anisotropy induced in the optical recording medium 11 was used. This laser light was transmitted through the polarizer 74 to obtain s-polarized light (perpendicular to the paper surface). This light is applied to the optical recording medium 11 and the transmitted light is guided to the analyzer 77. If birefringence is induced in the optical recording medium 11 by the irradiation of the pump light 75, the polarization direction of the probe light should be rotated by being transmitted through the optical recording medium 11. By rotating the analyzer 77, the polarization direction of the probe light 76 transmitted through the optical recording medium 11 is examined.
[0036]
When birefringence is photoinduced, the probe light 76 is not irradiated, and only the pump light 75 is irradiated to the optical recording medium 11. The light intensity of the pump light 75 is 1 W / cm ² And recorded for 15 minutes. Thereafter, the irradiation with the pump light 75 is stopped, and the light-induced birefringence induced only by the probe light 76 is measured.
[0037]
8A to 8D show the results of measuring the birefringence recording characteristics by changing the polarization angle of the pump light 75 to induce birefringence in the optical recording medium 11 and transmitting the probe light 76 therethrough. . The horizontal axis indicates the polarization rotation angle of the analyzer 77, and 90 degrees coincides with the s-polarized light. The vertical axis represents the transmitted light intensity of the analyzer 77. The characteristics of the transmitted probe light 76 when the pump light 75 is not irradiated are shown in FIG. Since the analyzer transmission intensity increases at 90 degrees from the figure, the probe light 76 is s-polarized light. Before the pump light 75 is irradiated, the optical recording medium 11 is isotropic, and the polarization angle is not changed when the probe light 76 is transmitted. Next, FIG. 8B shows the result of recording the light-induced birefringence on the optical recording medium 11 with the half-wave plate 72 using the pump light 75 as s-polarized light. From the figure, it can be seen that the analyzer transmission intensity increases at 80 degrees and 260 degrees. From this, it can be seen that the birefringence is recorded on the optical recording medium 11 by the pump light 75. Next, FIG. 8C shows the result of recording the light-induced birefringence on the optical recording medium 11 by changing the polarization angle of the pump light 75 by 30 degrees from the s-polarized light by the half-wave plate 72. From the figure, it can be seen that the analyzer transmission intensity increases at 40 degrees and 220 degrees. From this, it can be seen that the birefringence orientation recorded on the optical recording medium 11 is shifted by 40 degrees by changing the polarization angle of the pump light 75. Further, FIG. 8D shows the result of recording the light-induced birefringence on the optical recording medium 11 by changing the polarization angle of the pump light 75 by 60 degrees from the s-polarized light by the half-wave plate 72. From the figure, it can be seen that the analyzer transmission intensity increases at 130 degrees. From this, it can be seen that the birefringence orientation recorded on the optical recording medium 11 is further shifted by 90 degrees by changing the polarization angle of the pump light 75.
[0038]
From the above results, polyesters with cyanoazobenzene in the side chain can record by inducing birefringence, and by rotating the polarization angle of the pump light, the birefringence orientation can be rotated accordingly. I understand that I can do it. It was also confirmed that the recorded light-induced birefringence was maintained for a sufficiently long time at room temperature. Further, each of the above-described recordings with the polarization angle of the pump light 75 being changed is performed in the same region, and in particular, a process such as erasing the previous recording is not performed. From this, it was confirmed that the next data can be overwritten on the optical recording medium 11 without erasing the previous recording.
[0039]
Next, with respect to the optical recording medium 11 having the shape shown in FIG. 6A, the recording / reproducing characteristics of the half-wave plate due to light-induced birefringence were measured. As the measurement optical system, the optical system shown in FIG. 7 was used, and the optical recording material 12 having a thickness of 20 μm was used.
[0040]
As the light source 73 of the pump light 75 used for inducing anisotropy (birefringence) in the optical recording material, a 515 nm argon ion laser sensitive to polyester having cyanoazobenzene in the side chain was used. The polarized light of the light emitted from the light source 73 is s-polarized light (perpendicular to the paper surface), and the laser light is irradiated to the optical recording medium 11 through the half-wave plate 72. By changing the polarization angle of the pump light 75 with the half-wave plate 72, the anisotropy direction induced in the optical recording medium 11 changes, and its characteristics are measured. The induced anisotropy orientation is measured by a probe light 76 generated by a light source 71 different from the pump light. As the light source 71, a 633 nm helium neon laser having a wavelength that does not affect the anisotropy induced in the optical recording medium 11 was used. This laser light was transmitted through the polarizer 74 to obtain s-polarized light (perpendicular to the paper surface). This light is applied to the optical recording medium 11 and the transmitted light is guided to the analyzer 77. If birefringence is induced in the optical recording medium 11 by the irradiation of the pump light 75, the polarization direction of the probe light should be rotated by being transmitted through the optical recording medium 11. By rotating the analyzer 77, the polarization direction of the probe light 76 transmitted through the optical recording medium 11 is examined.
[0041]
When birefringence is photoinduced, the probe light 76 is not irradiated, and only the pump light 75 is irradiated to the optical recording medium 11. The light intensity of the pump light 75 is 1.34 W / cm. ² Recording was performed for 20 seconds. When the optical recording medium 11 is irradiated with the pump light 75 under this recording condition, the recording area functions as a half-wave plate. Thereafter, the irradiation with the pump light 75 is stopped, and the light-induced birefringence induced only by the probe light 76 is measured.
[0042]
FIG. 9 shows the result of measuring the birefringence recording characteristics by changing the polarization angle of the pump light 75 to induce birefringence in the optical recording medium 11 and transmitting the probe light 76 therethrough. The horizontal axis indicates the polarization rotation angle of the analyzer 77, and 90 degrees coincides with the s-polarized light. The vertical axis represents the transmitted light intensity of the analyzer 77. The characteristic of the transmitted probe light 76 when the pump light 75 is not irradiated is shown in FIG. Since the analyzer transmission intensity increases at 90 degrees from the figure, the probe light 76 is s-polarized light. It is isotropic before irradiation with the pump light 75 and does not change the polarization angle when the probe light 76 is transmitted. Next, FIG. 9B shows the result of recording the light-induced birefringence on the optical recording medium 11 with the half-wave plate 72 using the pump light 75 as s-polarized light. From the figure, it can be seen that the analyzer transmission intensity increases at approximately 90 degrees and 270 degrees. From this, it can be seen that although the birefringence is recorded on the optical recording medium 11 by the pump light 75, the polarization angle of the probe light 76 does not change. This is because the polarization directions of the probe light 76 and the pump light 75 are the same. Next, FIG. 9C shows the result of recording the light-induced birefringence on the optical recording medium 11 by changing the polarization angle of the pump light 75 by 22.5 degrees from the s-polarized light by the half-wave plate 72. From the figure, it can be seen that the analyzer transmission intensity increases at 130 degrees. This shows that the polarization angle of the probe light is shifted by 40 degrees. The shifted angle is about twice the angle formed by the pump light and the probe light. Further, FIG. 9D shows the result of recording the light-induced birefringence on the optical recording medium 11 by changing the polarization angle of the pump light 75 by 45 degrees from the s-polarized light by the half-wave plate 72. From the figure, it can be seen that the probe light intensity increases at around 175 degrees. From this, it can be seen that by changing the polarization angle of the pump light 75 by 45 degrees from the s-polarized light, the polarization angle of the probe light is shifted by 85 degrees, which is approximately twice the angle formed by the pump light and the probe light. Further, FIG. 9E shows a result of recording the light-induced birefringence on the optical recording medium 11 by changing the polarization angle of the pump light 75 by 67.5 degrees from the s-polarized light by the half-wave plate 72. From the figure, it can be seen that the probe light intensity increases at around 220 degrees. Also in this case, it can be seen that the polarization angle of the probe light is shifted by about twice the angle formed by the pump light and the probe light.
[0043]
As described above, it is understood that when the recording area is irradiated with the probe light, the polarization angle is shifted by about twice the angle formed by the recorded orientation and the probe light, and the probe light is transmitted. From this, it can be seen that a polyester having cyanoazobenzene in the side chain can record and reproduce a half-wave plate as described in the principle. Moreover, it can be seen that the direction of the half-wave plate can be recorded in an arbitrary direction by rotating the polarization angle of the pump light.
[0044]
FIG. 11 shows the result of measuring the light-induced refractive index change of the optical recording material used this time with the measurement system shown in FIG. Also in this measurement, the characteristics of the optical recording medium 11 having the shape shown in FIG. The thickness of the optical recording layer 12 was 2 μm.
[0045]
In FIG. 10, a light source 73 of pump light 75 used for inducing anisotropy in the optical recording material is an argon ion laser of 515 nm sensitive to polyester having cyanoazobenzene in the side chain. The polarized light of the light emitted from the light source 73 is s-polarized light (perpendicular to the paper surface), and the laser light is irradiated to the optical recording medium 11 through the half-wave plate 72. With this half-wave plate 72, the polarization angle of the pump light 75 was changed by 45 degrees from the s-polarized light. The birefringent orientation induced by the pump light 75 is measured by a probe light 76 generated by a light source 71 different from the pump light. As the light source 71, a 633 nm helium neon laser having a wavelength that does not affect the anisotropy induced in the optical recording medium 11 was used. This laser light was transmitted through the polarizer 74 to obtain s-polarized light (perpendicular to the paper surface). The light is irradiated onto the optical recording medium 11 and the transmitted light is guided to a polarization beam splitter (PBS) 91. The s-polarized component and the p-polarized component of the probe light 76 transmitted through the polarization beam splitter 91 are separated. By measuring the respective components with the optical power meters I1 and I2, the polarization direction of the probe light 76 transmitted through the optical recording medium 11 can be examined. Then, the refractive index change Δn is obtained from the measured polarization angle.
[0046]
In the measurement, the initialized optical recording medium 11 is irradiated with pump light 75 to induce birefringence. The light intensity of the pump light 75 is 1 W / cm. ² It was. At the same time, the probe beam 76 was irradiated, and the polarization direction of the probe beam 76 transmitted through the optical recording medium 11 was measured by the optical power meters I1 and I2.
[0047]
FIG. 11 shows the result of the refractive index change Δn converted from the polarization direction of the measured probe light 76. Here, it is assumed that the light-induced dichroism Δα is negligible. The horizontal axis represents the pump light irradiation amount (product of intensity and time), and the saturation point of the refractive index change Δn is indicated by 1.0. The vertical axis represents the magnitude of the refractive index change Δn obtained by measurement. From this figure, it can be seen that the refractive index change Δn due to birefringence induced by the pump light 75 increases and saturates with the irradiation amount of the pump light. When the saturation refractive index change Δns is obtained from this figure, the value is about 0.033 in the present material.
[0048]
As described above, in order to use this optical recording material as a half-wave plate or a quarter-wave plate, it is only necessary to satisfy (Equation 4) or (Equation 5). In order to produce a ½ wavelength plate or a ¼ wavelength plate with stable characteristics, it is desirable that the thickness of the optical recording material and the refractive index change Δn are constant from (Equation 4) or (Equation 5). I understand that. Therefore, in the optical recording medium of this example, for the refractive index change Δn, the saturated refractive index change Δns that becomes a stable value at a certain light irradiation amount or more is used. Thus, the thickness of the half-wave plate or the quarter-wave plate is determined from the value of the saturated refractive index change Δns and (Equation 4) or (Equation 5). If an optical recording material is produced with such a thickness, a constant refractive index change Δns can be induced if the exposure amount is equal to or greater than the amount of light that gives a saturated refractive index change Δns. It can be performed. In addition, although the recording speed is, the light intensity of the laser that can be used at present is 10% higher than that used for the measurement. ⁶ -10 ⁷ About twice as large. Considering this point, the recording speed can be increased to the order of about ms.
[0049]
FIG. 12 shows an embodiment of the optical recording / reproducing apparatus of the present invention. In this apparatus, a reflection type optical recording medium 1106 having a structure as shown in FIG. 6C is used as a disk shape as shown in FIG. First, regarding the thickness d of the optical recording material on the optical recording medium 1106, since reflection type optical recording / reproduction is performed, it may function as a quarter-wave plate as described above. Therefore, the thickness d is expressed by the following (Equation 6) by modifying (Equation 4).
[0050]
[Formula 6]
d = λ / 4 / Δn (Equation 6)
[0051]
As the optical recording material, the polyester having cyanoazobenzene in the side chain described above was used, and as the light source 1101, an argon ion laser sensitive to the material was used. If Δn = 0.033 obtained by measurement and the wavelength of the argon laser of 0.515 μm are substituted into (Equation 6), the thickness d becomes 3.9 μm. Therefore, the thickness of the optical recording material of the optical recording medium 1106 of the present invention was prepared to be 3.9 μm. Also, the laser beam used was s-polarized light. Here, the s-polarized light is in a certain direction parallel to the disk surface of the optical recording medium 1106.
[0052]
In the apparatus of FIG. 12, a method for recording information will be described first. The recording laser light from the light source 1101 is collimated by the collimator lens 1102 and is incident on the polarization rotation element 1103 as a spatial light modulator. As the polarization rotation element 1103, a λ / 2 plate, a liquid crystal bulb, a Pockels element, a Faraday element, or the like can be used. Taking a liquid crystal bulb as an example of the polarization rotation element 1103, the liquid crystal functions as a half-wave plate, so that the polarization direction of incident light and the axis of the half-wave plate are parallel when no voltage is applied. . Accordingly, since the incident light is s-polarized light, the transmitted light is s-polarized light. On the other hand, when the maximum voltage is applied to the liquid crystal, the axis of the half-wave plate is rotated 45 degrees, and the polarization of incident light is rotated 90 degrees. The rotation is performed in a plane direction parallel to the disk surface of the optical recording medium 1106. Furthermore, at an intermediate voltage between the minimum voltage and the maximum voltage, the axis of the half-wave plate is increased to 45 degrees according to the magnitude of the voltage. Thus, the light passing through the polarization rotation element 1103 can change the polarization angle from 0 degree to 90 degrees according to the voltage supplied to the polarization rotation element 1103. Here, the polarization angle of s-polarized light was set to 0 degree. The recording light that has passed through the polarization rotation element 1103 passes through the beam splitter 1104 and enters the objective lens 1105. The objective lens 1105 collects the recording light and irradiates the optical recording medium 1106. As a result, the light-induced birefringence can be recorded on the optical recording medium 1106 as described above.
[0053]
At this time, the polarization angle θ of the recording light can be changed from 0 degree to 90 degrees by controlling the voltage applied to the polarization rotation element 1103. As a result, as shown in FIG. 13, the orientation of a minute quarter-wave plate manufactured with birefringence induced by recording light 1201 can be set to θ (0 ≦ θ <90) degrees. Using this controllable angle θ, multi-value recording can be performed by making the information data to be recorded into a multi-value modulated value. The dynamic range that can be used for recording is as large as 90 degrees, and the light modulation element can rotate the polarization of light with a high S / N regardless of the angle. Further, as described above, since the recording medium is used with a saturation light quantity or more, stable recording can be performed without depending on fluctuations in light intensity. Therefore, according to this multi-value recording method and apparatus, stable recording can be performed with a high S / N irrespective of the gradation value.
[0054]
Next, a method for reproducing recorded information will be described with reference to FIG. The reproduction laser light from the light source 1101 is converted into parallel light by the lens 1102 and is incident on the polarization rotation element 1103. However, the light intensity at this time is greatly reduced as compared with the recording so as not to destroy the recorded light-induced anisotropy. The reproduction light is incident on the polarization rotator 1103. At this time, the polarization rotator 1103 is not applied with a voltage, and the transmitted light is s-polarized. Further, the reproduction light that has passed through the polarization rotation element 1103 passes through the beam splitter 1104 and enters the objective lens 1105. The objective lens 1105 collects the reproduction light and irradiates the optical recording medium 1106. Thereafter, the reproduction light is incident on the recorded minute quarter-wave plate, then reflected by the reflection film in the recording medium, and once again transmitted through the minute quarter-wave plate to become reflected light. Come out from. At this time, the reproduction light 1202 is s-polarized light as shown in FIG. 13, and the orientation of the recorded minute quarter-wave plate is shifted from the axis of s-polarized light by an angle θ as shown in the recording light 1201. . Therefore, as described in the principle, the polarization angle of the reflected light 1203 is shifted from the axis of s-polarized light by an angle 2θ. The reflected light is then reflected by the beam splitter 1104, passes through the analyzer 1108, and is sent to the detector 1109. By detecting the polarization angle 2θ of the reflected light with the optical probe 1108 and the detector 1109, the angle θ recorded in the multilevel modulation can be reproduced. Here, a CCD or a photodetector can be used as the detector 1109.
[0055]
As described above, in the reproducing method and apparatus of the present invention, the recorded angle θ can be detected at the double angle 2θ, so that the recorded multilevel modulation value can be demodulated. Since the dynamic range for recording is 90 degrees, the dynamic range that can be used for reproduction is further increased to 180 degrees. The intensity of the reflected light is constant regardless of the gradation value, and the photodetector can be used with a high S / N according to the amount of light. Further, as described above, since the recording medium is used with a saturation light amount or more, stable angle reproduction is possible without depending on the fluctuation of the light intensity. Therefore, according to this reproduction method and apparatus, stable reproduction can be performed with a high S / N regardless of the gradation value.
[0056]
The optical recording medium 1106 of the present invention has a disk shape, and a plurality of data can be recorded at different locations in the circumferential direction of the recording medium 1106 by rotating the motor 1107. Further, by moving the entire recording / reproducing head 1110 in the radial direction of the optical recording medium 1106, recording can be performed so as to form concentric recording tracks in the optical recording medium 1106 as shown in FIG.
[0057]
As described above, the optical recording / reproducing apparatus according to the present embodiment includes the medium driving mechanism for rotating the disk-shaped optical recording medium 1106, the light source, the spatial light modulator, the imaging optical system, the reproducing optical system, and the detection system. A head moving mechanism for moving the optical recording / reproducing head 1110 in the radial direction of the optical recording medium 1106. The optical recording medium 1106 can be built in the optical recording / reproducing apparatus.
[0058]
The embodiment of the present invention described with reference to FIG. 12 is configured so that recording and reproduction can be performed at the same time. However, it is also possible to make recording only or reproduction only by changing a part of the configuration. First, in order to make it dedicated to recording, the detection portion related to the reflected light is not necessary, and thus the configuration is obtained by removing the beam splitter 1104, the analyzer 1108, and the detector 1109 from FIG. As a result, the recording head can be reduced in size and weight. In addition, a low-cost device can be manufactured as compared with the recording / reproducing device. Also, in order to make it exclusively for reproduction, the configuration is such that the polarization rotation element 1103 necessary for recording is removed from FIG. As a result, the reproducing head can be reduced in size and weight. In addition, a low-cost device can be manufactured as compared with the recording / reproducing device.
[0059]
The optical system shown in FIG. 12 can be applied to that used in a conventional CD ROM or magneto-optical disk. Therefore, the minimum recordable size can be the same size. However, in the recording / reproducing apparatus of the present invention, the capacity and speed can be increased as much as multi-value recording can be performed. Regarding recording, the angular resolution at the polarization rotation element defines a multi-value level that can be recorded, but at present it is possible up to 255 values. As for reproduction, the reproduction angle resolution at the detector defines a multilevel level. In a magneto-optical disk or the like, it is possible to detect 1 degree or less, and in the present invention, since there is a reproducible dynamic range up to 180 degrees, 200 values or more are possible. Therefore, even when recording / reproducing balance is taken into consideration, recording / reproducing of 200 values or more is possible. Therefore, in the recording / reproducing apparatus of the present invention, a storage capacity of about 100 sheets can be achieved in terms of a binary CDROM, and the reading speed can be increased about 100 times. The writing speed can be increased to the order of ms per bit, and if recording of 200 values or more can be performed in this time, the recording speed can be increased by one digit or more.
[0060]
In the recording / reproducing apparatus of the present invention, it is possible to reproduce a conventional CD-ROM or a written magneto-optical disk by a simple device. In the case of a CDROM, there is no change in the polarization angle of the reflected light, only a change in the intensity of the reflected light. Since the reproduction light detection unit of the apparatus of the present invention can also detect the intensity, it is possible to reproduce the CDROM only by making it possible to binaryly determine the intensity change reproduced by the CDROM. In the case of a magneto-optical disk, the polarization angle of reflected light may be detected. As described above, since the apparatus of the present invention has a resolution that can detect a minute change in the polarization angle on the magneto-optical disk, the magneto-optical disk can also be reproduced.
[0061]
The optical recording medium recorded by the recording / reproducing apparatus of the present invention can be recorded so that it can be reproduced by the magneto-optical disk reproducing apparatus. A magneto-optical disk records a minute change in polarization angle. In the recording of the present invention, it is a principle to record such a minute change in the polarization angle, and since it has a resolution equivalent to that of the magneto-optical disk, it is possible to record so that it can be reproduced by the magneto-optical disk reproducing apparatus. Is possible.
[0062]
FIG. 14 is a diagram showing another embodiment of the optical recording / reproducing apparatus of the present invention. In this apparatus, a transmission type optical recording medium 1306 having a structure as shown in FIG. 6A is used as a disk shape as shown in FIG. In this figure, the thickness d of the optical recording material of the optical recording medium 1306 is first used. However, since transmissive optical recording / reproduction is performed, it may function as a half-wave plate as described above. Therefore, the thickness d is expressed by the following (Equation 7) by modifying (Equation 5).
[0063]
[Expression 7]
d = λ / 2 / Δn (Equation 7)
[0064]
A material similar to the reflection type was used for the optical recording material, and an argon ion laser was also used for the light source 1101. If Δn = 0.033 obtained by measurement and the wavelength of argon laser of 0.515 μm are substituted into (Equation 7), the thickness d becomes 7.8 μm. Therefore, the thickness of the optical recording material of the optical recording medium 1306 of the present invention was prepared to be 7.8 μm. Also, the laser beam used was s-polarized light. Here, the s-polarized light is in a certain direction parallel to the disk surface of the optical recording medium 1306.
[0065]
In FIG. 14, the information recording method will be described first. The elements used in this apparatus are the same as those of the reflection type. The recording laser light from the light source 1101 is converted into parallel light by the lens 1102 and enters the polarization rotation element 1103. In the polarization rotation element 1103, the polarization angle of the passing light can be changed from 0 degree to 90 degrees according to the voltage supplied to the polarization rotation element 1103. Here, the polarization angle of the s-polarized light is 0 degree, and the rotation is performed in a plane direction parallel to the disk surface of the optical recording medium 1306. The recording light that has passed through the polarization rotating element 1103 enters the objective lens 1105, the recording light is collected by the objective lens 1105, and is applied to the optical recording medium 1306. Thereby, the light-induced anisotropy can be recorded on the optical recording medium 1306. At this time, by controlling the voltage applied to the polarization rotation element, the polarization angle θ of the recording light can be changed from 0 degree to 90 degrees with reference to the reproduction light (s-polarized light). As a result, as shown in FIG. 13, the orientation of the minute half-wave plate produced with the birefringence induced by the recording light 1201 can be set to θ (0 ≦ θ <90) degrees. Using this controllable angle θ, multi-value recording can be performed by making the information data to be recorded into a multi-value modulated value.
[0066]
Next, a method for reproducing recorded information will be described with reference to FIG. The reproduction laser light from the light source 1101 is converted into parallel light by the lens 1102 and is incident on the polarization rotation element 1103. However, the light intensity at this time is greatly reduced as compared with the time of recording so as not to destroy the recorded light-induced birefringence. The reproduction light is incident on the polarization rotator 1103. At this time, no voltage is applied to the polarization rotator 1103, and the transmitted light is s-polarized light. Further, the reproduction light that has passed through the polarization rotation element 1103 enters the objective lens 1105, collects the reproduction light, and irradiates the optical recording medium 1306. Thereafter, the reproduction light passes through the recorded minute half-wave plate and emerges from the lower surface of the optical recording medium 1306. At this time, the reproduction light 1202 is s-polarized light as shown in FIG. 13, and the orientation of the recorded minute half-wave plate is shifted from the axis of s-polarized light by an angle θ as shown by the recording light 1201. . Therefore, as described in the principle, the polarization angle of the transmitted light is shifted from the axis of s-polarized light by an angle 2θ as in the case of the reflected light 1203. This transmitted light passes through the analyzer 1108 and is sent to the detector 1109. The recorded angle θ can be reproduced by detecting the polarization angle 2θ of the transmitted light with the optical probe 1108 and the detector 1109. Thus, also in the transmission type reproducing method and reproducing apparatus, the recorded angle θ can be detected at a double angle 2θ.
[0067]
The transmission type recording / reproducing apparatus described with reference to FIG. 14 can also perform recording / reproducing in the circumferential direction and radial direction of the optical recording medium 1306 as in the embodiment of the present invention described with reference to FIG. Therefore, the present recording / reproducing apparatus includes a medium driving mechanism for rotationally driving a disk-shaped optical recording medium 1306, and an optical recording / reproducing head 1310 including a light source, a spatial light modulator, an imaging optical system, a reproducing optical system, and a detecting system. A head moving mechanism that moves in the radial direction of the optical recording medium 1306. The optical recording medium 1306 can also be built in the optical recording / reproducing apparatus.
[0068]
Furthermore, although the embodiment of the present invention described with reference to FIG. 14 is configured to be able to perform recording and reproduction at the same time, it can also be dedicated to recording or reproduction only by changing a part of the configuration. First, in order to make it dedicated to recording, the relationship with the transmitted light detection portion is not required, and thus the analyzer 1108 and the detector 1109 are removed from FIG. Also, in order to make it exclusively for reproduction, the configuration is such that the polarization rotation element 1103 necessary for recording is removed from FIG. With such a dedicated machine, the head can be reduced in size and weight. Also, it is possible to manufacture a device that is less expensive than the recording / reproducing device.
[0069]
As described above, with the recording medium of the present invention, recording can be performed with multi-value modulation of the polarization angle of the recording light. When the information is reproduced, the information can be reproduced by doubling the polarization angle, so that a stable S / N can be obtained during reproduction. Further, regarding the S / N, by saturating the refractive index change of the recording material with the light intensity to be a constant value, it is possible to prevent the S / N decrease due to the material and the S / N decrease due to the fluctuation of the light intensity. Furthermore, since no erasing process is required for rewriting, high-speed rewriting can be realized. It is also possible to improve the storage capacity and data transfer rate by making the shape of the recording medium a disk.
[0070]
Further, the recording density can be further improved by laminating the optical recording material of the present invention in multiple layers. Also, the recording density can be increased by forming optical recording materials on both sides of the reflective film to form a double-sided recording medium.
[0071]
According to the optical recording / reproducing method and the optical recording / reproducing apparatus of the present invention, the recording light can be recorded by performing multi-value modulation on the polarization angle θ of the recording light, and the recorded polarization angle θ is doubled. can do. Since the intensity of the reproduction light is constant regardless of the value of the recorded polarization angle θ, a high S / N can be obtained in recording and reproduction. This multi-level recording enables high-density and high-speed recording / reproduction. It is also possible to make the optical recording medium into a disk shape and perform recording and reproduction in the circumferential direction and radial direction of the disk.
[0072]
In the recording / reproducing apparatus of the present invention, it is possible to reproduce a conventional CD-ROM or a written magneto-optical disk by a simple device. The optical recording medium recorded by the recording / reproducing apparatus of the present invention can be recorded so that it can be reproduced by the magneto-optical disk reproducing apparatus.
[0073]
【The invention's effect】
According to the present invention, it is possible to obtain an optical recording medium, an optical recording / reproducing method, and an apparatus capable of performing multi-level recording / reproducing with high density and high S / N.
[Brief description of the drawings]
FIGS. 1A to 1I are diagrams showing polarization states of light, respectively.
FIG. 2 is a diagram illustrating a half-wave plate.
FIG. 3 is a diagram for explaining a quarter-wave plate.
4A is a diagram showing the trans structure of azobenzene, FIG. 4B is a cis structure of azobenzene, and FIG. 4C is a diagram showing the structure of a polyester having cyanoazobenzene in the side chain.
5A is a diagram showing a state in which trans-type azobenzene is dispersed, and FIG. 5B is a diagram showing a state in which trans-type azobenzene is changed to a cis-type by photoexcitation.
FIGS. 6A to 6D are diagrams showing the structure of an optical recording medium according to the present invention.
FIG. 7 is a diagram showing an optical system used for measuring recording characteristics.
8A to 8D are diagrams showing recording characteristics of an optical recording medium according to the present invention.
9A to 9E are diagrams showing recording characteristics of the optical recording medium according to the present invention.
FIG. 10 is a diagram showing an optical system used to measure a change in refractive index.
FIG. 11 is a diagram showing saturation characteristics of refractive index change.
FIG. 12 is a diagram showing an example of an optical recording / reproducing apparatus according to the present invention.
FIG. 13 is a diagram for explaining an optical recording / reproducing method according to the present invention.
FIG. 14 is a diagram showing another embodiment of the optical recording / reproducing apparatus according to the present invention.
[Explanation of symbols]
1101 Light source
1102 Collimator lens
1103 Polarization Rotating Element
1104 Beam splitter
1105 Objective lens
1106 Optical recording medium
1107 motor
1108 Analyzer
1109 Detector
1110 Recording / reproducing head

Claims (34)

An optical recording medium comprising at least one optical recording layer configured so that an optical recording material having light-induced birefringence functions as a half-wave plate. An optical recording material having light-induced birefringence includes at least one optical recording layer configured to function as a quarter-wave plate, and a light reflecting layer formed on one surface of the optical recording layer An optical recording medium characterized by the above. When the thickness d of the optical recording layer is Δn when the refractive index change induced by light is λ, the wavelength of irradiation light is λ, and m is an integer, Δn · d = (m + 1/2) · λ
The optical recording medium according to claim 1, wherein the optical recording medium is formed so as to satisfy the above. When the thickness d of the optical recording layer is Δn for the refractive index change induced by light, λ for the irradiation light wavelength, and m as an integer, Δn · d = (m + 1/4) · λ
The optical recording medium according to claim 2, wherein the optical recording medium is formed so as to satisfy the above. 5. The optical recording medium according to claim 3, wherein the light-induced refractive index change Δn is a saturated refractive index change value that saturates at a certain light irradiation amount or more. 6. The optical recording medium according to claim 1, wherein the optical recording layer comprises a polymer compound or a polymer liquid crystal having a photoisomerizable group in a side chain. 6. The optical recording medium according to claim 1, wherein the optical recording layer is made of a polymer compound in which molecules to be photoisomerized are dispersed. The optical recording medium according to claim 6 or 7, wherein the polymer compound or polymer liquid crystal contains an azobenzene skeleton. 9. The optical recording medium according to claim 6, wherein the polymer compound or polymer liquid crystal is at least one monomer polymer selected from the group of polyesters. The optical recording medium according to claim 1, wherein the optical recording layer is formed in a disk shape. An optical recording medium comprising an optical recording layer made of a material whose birefringence azimuth induced by irradiation light changes according to the rotation of the polarization angle of the irradiation light. A step of controlling a polarization angle of recording light emitted from a light source, and irradiating the optical recording medium according to claim 1 with a ½ wavelength plate having an orientation corresponding to the polarization angle as the optical recording medium And an optical recording method. A step of controlling a polarization angle of recording light emitted from a light source; and irradiating the optical recording medium with the recording light according to claim 2 to form a quarter-wave plate with an orientation corresponding to the polarization angle. And an optical recording method. 14. The optical recording method according to claim 12, wherein the step of controlling the polarization angle includes a step of passing recording light emitted from the light source through a polarization rotation element. The step of forming the wave plate includes a step of rotating the disc-shaped optical recording medium and a step of moving the irradiation of the recording light in a radial direction of the optical recording medium. Item 15. The optical recording method according to any one of Items 12 to 14. 16. The optical recording method according to claim 12, wherein the wave plate is formed by overwriting an existing wave plate in the optical recording medium. 12. An image forming unit configured to irradiate an optical recording medium according to claim 11 with a light source that generates recording light, a spatial light modulator that controls a polarization angle of the recording light, and recording light obtained through the spatial light modulator. An optical recording apparatus comprising: an optical system. The optical recording apparatus according to claim 17, wherein the spatial light modulator controls a polarization angle of the recording light in accordance with recording information. 19. The optical recording apparatus according to claim 17, wherein the spatial light modulator is a polarization rotation element. A medium driving mechanism for rotating and driving the disk-shaped optical recording medium; and a head moving mechanism for moving an optical recording head including the light source, a spatial light modulator, and an imaging optical system in a radial direction of the optical recording medium. The optical recording apparatus according to claim 17, wherein the optical recording apparatus is an optical recording apparatus. 21. The optical recording apparatus according to claim 17, wherein the optical recording medium is built therein. An optical recording material having a light-induced birefringence is recorded in an optical recording layer configured to function as a half-wave plate by multilevel modulation of the orientation of the half-wave plate induced by the recording light. An optical recording medium. The direction of the quarter-wave plate induced by the recording light is recorded by multi-level modulation on the optical recording layer configured so that the optical recording material having light-induced birefringence functions as a quarter-wave plate. An optical recording medium. The optical recording medium according to claim 22 or 23, wherein the optical recording layer is formed in a disk shape. An optical recording medium comprising an optical recording layer recorded with multi-value modulation of the birefringent orientation induced by irradiation light according to the rotation of the polarization angle of the irradiation light. The step of irradiating the optical recording medium with the reproducing light according to claim 1 , wherein the recording layer is formed by multilevel modulation of the orientation of the half-wave plate according to the polarization angle of the recording light, and the 1 / of the reproducing light. And a step of reading a polarization angle of transmitted light from the two-wavelength plate. The step of irradiating the optical recording medium with the reproducing light according to claim 2 formed by multi-level modulation of the direction of the quarter-wave plate on the recording layer in accordance with the polarization angle of the recording light; And a step of reading the polarization angle of the reflected light from the four-wave plate. 28. The optical reproduction method according to claim 26 or 27, wherein the light intensity of the reproduction light is smaller than the light intensity of the recording light. The step of reading the polarization angle includes a step of rotating the optical recording medium having a disk shape and a step of performing irradiation of the reproduction light by moving in a radial direction of the optical recording medium. The optical regeneration method according to any one of 26 to 28. 2. A reproducing light optical system for irradiating reproducing light to an optical recording medium according to claim 1 , wherein the reproducing light is formed by multi-value modulating the direction of a half-wave plate in the optical recording layer in accordance with the polarization angle of the recording light, and the reproducing light And a light detecting means for detecting the polarization angle of the transmitted light from the half-wave plate. The reproduction light optical system for irradiating the optical recording medium with the reproduction light according to claim 2 formed by multi-level modulation of the direction of the quarter-wave plate in the optical recording layer according to the polarization angle of the recording light, and the reproduction light An optical reproducing apparatus comprising: an analyzing means for detecting a polarization angle of reflected light from the quarter wavelength plate. A medium drive mechanism for rotating the disk-shaped optical recording medium, and a head moving mechanism for moving an optical reproducing head including the reproducing light optical system and a light detecting means in a radial direction of the optical recording medium. 32. The optical regenerator according to claim 30 or 31. The optical reproducing apparatus according to any one of claims 30 to 32, wherein the optical recording medium is built therein. 12. An imaging optical system for irradiating an optical recording medium according to claim 11 with a light source that generates recording light, a polarization rotation element that controls a polarization angle of the recording light, and recording light obtained through the polarization rotation element When the optical recording, characterized in that a test light means for detecting a reproducing optical system for irradiating a reproducing light to the optical recording medium, the polarization angle of the reproduction light obtained through the optical recording medium Playback device.

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