JP3545380B2 - Optical recording information reading device - Google Patents

Description

上記（６）式からわかるように、この場合は、通常の両レーザ光の強度に比例する成分以外に、両レーザ光の位相差に関連して電流成分が現れることになる。従って、両レーザ光の位相差関係が一定であれば、この成分は直流電流成分となり、交流成分は出力されない。
【００４４】
２つのレーザ光を上記のように光コヒーレント検波するためには、両レーザ光の波面が一致していることが重要である。以下にその理由を説明する。今、図３（ａ）に示すように、波面が角度δだけ傾いた２つのレーザ光３０１、３０２を合波した場合を仮定すると、この場合は図３（ｂ）に示すように、合波面内にν１−ν２成分の信号位相が正弦波状に反転する部分が存在する。このため、反転部分同士が互いに打ち消し合い、結果的にν１−ν２周波数成分の振幅が著しく低下する。
【００４５】
但し、実際の光コヒーレント検波では、合波の後にレンズで集光してＰＩＮフォトダイオード１１６、１１８で検波しているが、合波直後の位相関係がそのままＰＩＮフォトダイオード１１６、１１８上でも相似関係で再現されるため、平面波同士を合波した合波面で説明する場合と同様の現象を生ずる。
【００４６】
図４は、直径５ｍｍの２つの平面波レーザ光同士をコヒーレント検波した場合の、両レーザ光の波面のなす角度に対する周波数ν１−ν２の信号成分強度の変化を示す。図４より、波面が約０．０１゜傾くと、周波数ν１−ν２の信号成分の強度は−１０ｄＢに減衰することがわかる。この波面の一致精度は平面波状態での２つのレーザ光の重なり合う部分の面積が大きい程厳しくなる。
【００４７】
ここで、本実施例の記録情報読み出し装置では、反射レーザ光および第２レーザ光共に平面波であり、かつビーム径を３ｍｍとしたので、偏光ビーム・スプリッタ１１２において反射レーザ光の波面に対して０．０２゜以下の波面一致精度とするべく、半導体レーザ１１３素子およびコリメータ・レンズ１１４を配置する必要があった。
【００４８】
本実施例におけるＰＩＮフォトダイオード１１６では、上記のように３つのレーザ光をコヒーレント検波することになるが、上記の考察から容易にわかるように、交流電流成分として出力されるのは、ベクトルＯＬ（１）成分のレーザ光とベクトルＯＡ（１）（又はＯＢ（１））成分のレーザ光の検波出力、およびベクトルＯＬ（１）成分のレーザ光とベクトルＯＲ（１）成分のレーザ光の検波出力の２つとなる。
【００４９】
なお、以後添字（１）は偏光ビーム・スプリッタ１１２の偏光分離の２方向の光ベクトル成分の内の一方の方向のベクトル成分を示し、他方の方向のベクトル成分を添字（２）で示す。
【００５０】
ここで、ベクトルＯＬ（１）成分、ベクトルＯＡ（１）成分およびベクトルＯＲ（１）成分のレーザ光の光電界は、それぞれ下記（７）式、（８）式、（９）式で表される。
ＥＬ（１）＝ａＬ（１）ｃｏｓ（２πν２ｔ）＋φＬ（１） …（７）
ＥＡ（１）＝ａＡ（１）ｃｏｓ（２πν１ｔ）＋φＡ（１） …（８）
ＥＲ（１）＝ａＲ（１）ｃｏｓ（２πν１ｔ）＋φＲ（１） …（９）
従って、出力電流の交流成分は、
ＩＡＣ（１）＝ηａＡ（１）・ａＬ（１）ｃｏｓ［ ２π（ν１−ν２）ｔ＋φＡ（１）−φＬ（１）］ ＋ηａＲ（１）・ａＬ（１）ｃｏｓ［ ２π（ν１−ν２）ｔ＋φＲ（１）−φＬ（１）］ …（１０）
となる。
【００５１】
上記（１０）式中の２つの成分は同一周波数を有し、位相のみ異なる信号を表している。この位相の異なりは、ベクトルＯＡ成分のレーザ光とベクトルＯＲ成分のレーザ光の位相差に等しくなっている。これらの反射レーザ光を発生させるのはディスク１０６の表面又は記録媒体１０７の表面であるが、ディスク１０６で反射されるレーザ光は集光レンズ１０５により平行光に変換されず球面波発散光になるため、ＰＩＮフォトダイオード１１６に到達するまでにその感度が減衰する。また、半導体レーザ１１３から出射され、平行光である第２レーザ光と波面が一致しないため、光コヒーレント検波が不可能である。たとえ、ディスク１０６からの反射光があったとしても、ＰＩＮフォトダイオード１１６出力の直流成分にしか寄与しないからである。従って、ベクトルＯＡ成分のレーザ光とベクトルＯＲ成分のレーザ光の両方とも記録媒体１０７からの反射であり、両反射レーザ光の位相は一致することになる。即ち、φＡ（１）＝φＲ（１）となる。従って、この関係を導入すれば、上記（１０）式は下記（１１）式で表される。
【００５２】
ＩＡＣ（１）＝η（ａＡ（１）・ａＬ（１）＋ａＲ（１）・ａＬ（１））・ｃｏｓ［ ２π（ν１−ν２）ｔ＋φＲ（１）−φＬ（１）］ …（１１）
同様に、ＰＩＮフォトダイオード１１８においても、交流成分として出力されるのはベクトルＯＬ（２）成分のレーザ光とベクトルＯＡ（２）（又はＯＢ（２））成分のレーザ光の検波出力およびベクトルＯＬ（２）成分のレーザ光とベクトルＯＲ（２）成分のレーザ光の検波出力である。
【００５３】
よって、この場合の出力電流の交流成分は、下記（１２）式で表される。
【００５４】
ＩＡＣ（２）＝η（ａＡ（２）・ａＬ（２）＋ａＲ（２）・ａＬ（２））・ｃｏｓ［ ２π（ν１−ν２）ｔ＋φＲ（２）−φＬ（２）］ …（１２）
ここで、上記の（１１）式と（１２）式とを比較することにより、ＰＩＮフォトダイオード１１６、１１８からの電流出力の内の交流成分の振幅を比較する。図２に示すように、ベクトルＯＲとベクトルＯＬは偏光ビーム・スプリッタ１１２の偏光分離方向に対して４５゜の角度をなしている。
【００５５】
従って、
ａＲ（１）＝ａＲ（２） …（１３）
ａＬ（１）＝ａＬ（２） …（１４）
の関係が成立し、両式の振幅成分の差は、
△＝η（ａＡ（１）−ａＡ（２））・ａＬ（１） …（１５）
となる。ここで、ベクトルＯＡの４５゜からの回転角度をθとすると、振幅成分の差は下記（１６）式で表される。
【００５６】
△＝ηａＡａＬｓｉｎθ …（１６）
一方、記録媒体１０７でのカー効果を受けた反射レーザ光の回転角が反対方向の場合はベクトルＯＢ成分のレーザ光が信号光になるため、ベクトルＯＡの場合と同様にベクトルＯＢの４５゜からの回転角をθとすると、振幅成分の差は下記（１７）式で表される。
【００５７】
△＝−ηａＡａＬｓｉｎθ …（１７）
以上の理由により、ＰＩＮフォトダイオード１１６、１１８により光コヒーレント検波された出力電流の交流成分の振幅の差分の正負により記録情報の”１”、”０”を判定できることがわかる。
【００５８】
また、上記の説明より、従来の直接検波方式による”１”、”０”信号に対応する信号成分は±ηａＡ２ｓｉｎθであるのに対し、光コヒーレント検波を応用した本実施例の場合の”１”、”０”に対応する信号成分は±ηａＡａＬｓｉｎθとなることがわかる。
【００５９】
ここで、ａＬは、第１レーザ光の光強度には依存せず、第２の半導体レーザ素子１１３より発生させられたレーザ光の光強度で任意に設定可能な値である。従って、ディスク１０６における高密度化や読み出しの高速化によりビット当りのａＡが小さくなった場合にも、ａＡ・ａＬとして信号成分を大きく保つことができる。即ち、第２レーザ光の振幅ａＬにより信号成分を含む反射レーザ光の振幅ａＡが疑似的に増幅されることになる。このような光の領域における増幅では、電子回路における増幅につきものの雑音成分の増幅はない。従って、純粋に信号成分のみを増幅できるので、Ｃ／Ｎ値を大幅に改善できる。
【００６０】
次に、実際の信号検出について図面に基づき詳細に説明する。上記のように、直交する２つの偏光成分を別々に光コヒーレント検波した後、ＰＩＮフォトダイオード１１６、１１８からの電気信号は高域通過フィルタ１１９、１２０により直流成分が除去される。続いて、直流成分を除去された電気信号は前置増幅器１２１、１２２に入力され、ここで信号として重要な交流成分が増幅される。続いて、中心通過周波数がν１−ν２に設定された狭帯域通過フィルタ１２３、１２４が交流成分から雑音成分を除去し、上記（１１）、（１２）式に示したν１−ν２成分（以下ビート成分と称する）のみを抽出する。本実施例では両レーザ光の光周波数差が１ＧＨｚとなるように設定したため、ビート信号周波数も１ＧＨｚである。
【００６１】
図５は前置増幅器１２１、１２２の出力波形を示す。同図からわかるように、記録信号の”１”、”０”に対応して、前置増幅器１２１、１２２の出力ビート信号の振幅が相互に変化している。また、同図から、本実施例では、記録信号が”１”の場合に前置増幅器１２１からのビート信号の振幅が大きくなり、記録信号”０”の場合に前置増幅器１２２からのビート信号の振幅が大きくなっている。この信号形態は、ビート信号周波数をキャリアとした振幅シフトキーイング（ＡＳＫ）変調による信号伝送方式を表している。但し、２チャンネルによる差動型ＡＳＫである。
【００６２】
従って、信号を復調するためには、ビート信号周波数をデジタル情報情報読み出しレートＲ（本実施例ではＲ＝０．７Ｍｂｐｓ）よりも十分に高い周波数とする必要がある。但し、実際には、デジタル情報読み出しレートＲの２倍以上に設定することが最低限必要であり、好ましくは、１０倍以上に設定する。
【００６３】
また、このように電気信号においてＡＳＫ変調方式で変換することの利点としては、比較的キャリア波（ここではビート信号）の周波数や位相が不安定でも信号再生に問題を生じないという点が挙げられる。通常の無線通信の場合とは異なり、本実施例の場合には半導体レーザ自身のスペクトル線幅が１ＭＨｚであり、ν１−ν２が揺らぐこと、およびデジタル情報をもつディスク１０６が回転運動することにより発生するディスク振動によりφＡ（１）、φＡ（２）の値が変化することによりビート信号の周波数や位相が不安定となるのが普通である。従って、本実施例のようにＡＳＫ変調方式に一旦変換された信号からデジタル情報を再生する方式が適しているといえる。
【００６４】
上記のようにして、狭帯域通過フィルタ１２３、１２４を通過したビート成分の振幅値は、全波整流回路等の非同期型の振幅検波回路１２５、１２６により直流電圧として検出される。続いて、直流電圧成分に変換されたビート成分の振幅値を差動増幅回路１２７により差動増幅し、最後に、差動増幅回路１２７の出力の正負を信号レベル比較回路１２８により判定する。この判定結果により、上記（１６）式又は（１７）式に示した記録情報”１”、”０”に対応する信号成分の正負を判定でき、記録情報を読み出すことができる。
【００６５】
ここで、信号読み出しに十分なＣ／Ｎ値を確保するためには、光コヒーレント検波による信号成分の増幅と共に、雑音レベルの抑制が必要である。このため、本実施例では、以下に示す２点に重点を置いて検出システムを構成している。
【００６６】
前置増幅器１２１、１２２として低雑音指数特性を有する増幅器、例えば、ＧａＡｓ高電子移動トランジスタを使用する。
【００６７】
狭帯域通過フィルタ１２３、１２４の通過帯域幅をできるだけ狭く設定する。
【００６８】
なお、この通過帯域幅はビート信号周波数の短期／長期の揺らぎ幅と情報読み出し速度の和に律速される。本実施例では両方の半導体レーザ素子１０１、１１３から発生されるレーザ光のスペクトル線幅によりビート信号周波数の揺らぎ幅は決定されており、情報読み出し速度は０．７Ｍｂｐｓであったので、狭帯域通過フィルタ１２３、１２４の通過帯域幅としては３ＭＨｚとした。
【００６９】
次に、２つの半導体レーザ素子１０１、１１３からの第１レーザ光と第２レーザ光の光周波数差（ビート信号周波数）ν１−ν２を一定に保つ手段（自動周波数制御：ＡＦＣ）について説明する。ＰＩＮフォトダイオード１１８で光コヒーレント検波し、その電気出力から高帯域フィルタ１２０と前置増幅器１２２により抽出したビート信号成分を狭帯域フィルタ１２４に入力すると共に、その周波数弁別回路１２９入力し、交流成分の周波数を電圧値として検出する。この周波数弁別回路１２９としては検波帯域が広く同期に時間を要しない非同期検波が適しており、ここでは遅延検波回路を用いた。
【００７０】
続いて、ＰＩＤ制御回路１３０により周波数弁別回路１２９の出力を外部から入力する一定の設定電圧（図示せず）と一致するように制御信号を発生させ、この制御信号に従ってレーザ駆動回路１３１により第１の半導体レーザ素子１０１と第２の半導体レーザ素子１１３の駆動条件を変化、即ち制御する。レーザ駆動回路１３１は別途設けた自動パワー制御回路系（図示せず）からの制御信号に従い、両半導体レーザ１０１、１１３の光出力の制御（自動パワー制御：ＡＰＣ）も同時に行っている。なお、本実施例では、半導体レーザ素子として波長可変型レーザ素子を使用しており、そのレーザ光の光強度と光周波数をそれぞれ設けられた個別の制御端子により制御可能である。このような、帰還回路系を設けたことにより、光周波数差を一定に保ち、ビート信号周波数を一定に保つことができることが確認できた。
【００７１】
次に、初期状態における光周波数差の設定手段について説明する。初期状態では、両方の半導体レーザ素子１０１、１１３が無関係な光周波数を有するレーザ光を発するので、初期状態で上記の帰還回路による制御を実行することはできない。ディスク１０６をセットし、フォーカスサーボがかかった状態で初めて第１レーザ光のディスク１０６からの反射光がＰＩＮフォトダイオード１１８に到達し、光コヒーレント検波を行うことが可能になる。この状態では、普通、両レーザ光の光周波数が大きく異なっており、その差が高周波であるため、ＰＩＮフォトダイオード１１８、高域通過フィルタ１２０、前置増幅器１２２、周波数弁別回路１２９のいずれか（又は複数）の応答帯域外となるため、周波数弁別回路１２９の出力電圧は０Ｖである。
【００７２】
この状態で、まず、レーザ駆動回路１３１により第１の半導体レーザ素子１０１の駆動条件を一定に保持したまま、周波数弁別回路１２９の出力電圧をモニタし、第２の半導体レーザ素子１１３の光周波数を徐々に一定方向に走査する。この一連の動作は、周波数弁別回路１２９の出力電圧を監視するプロセッサ回路１３２により制御される。この時点では、ＰＩＤ制御回路１３０とレーザ駆動回路１３１との間を入切りするスイッチ回路１３３はＯＦＦ状態にあり、ＰＩＤ制御回路１３０はレーザ駆動回路１３１から切り離されている。従って、この状態では帰還回路は動作しない。
【００７３】
更に、第２の半導体レーザ素子１１３から出射される第２レーザ光の光周波数を走査し続けると、ある時点で第２レーザ光の光周波数が第１レーザ光の光周波数に接近する。この時、その光周波数差がＰＩＮフォトダイオード１１８、高域通過フィルタ１２０、前置増幅器１２２および周波数弁別回路１２９の全ての応答速度帯域に入り、周波数弁別回路１２９の出力端子に正の出力電圧が出力される。なお、第２レーザ光の光周波数走査のステップは上記の回路系１１８、１２０、１２２、１２９の総合応答帯域より小さいことが必要であるため、本実施例では応答帯域を１ＭＨｚとした。
【００７４】
そして、周波数弁別回路１２９からの出力電圧がこの帰還回路を機能させるのに十分な範囲となる一定の範囲になったとプロセッサ回路１３２が判定すると、プロセッサ回路１３２はスイッチ回路１３３をＯＮし、ＰＩＤ制御回路１３０を動作させる。これにより、制御状態が帰還回路制御に切り替えられる。
【００７５】
上記のような回路構成とすると、従来例の問題点として指摘したディスクメモリの高密度化・高速読み出し化に伴う記録媒体１０７からの信号情報を含む反射レーザ光（本実施例のベクトルＯＡ成分、ベクトルＯＢ成分に相当）のビット当りのパワーが小さくなり直接検波方式での再生では実用的なビット誤り率を実現するのに必要なＣ／Ｎ値を確保できない場合であっても、本実施例の光コヒーレント検波によれば十分なＣ／Ｎ値を確保できる。従って、ディスクメモリの高密度化・高速アクセス化に十分に対処可能である。なお、実用的なビット誤り率は記録データの重要度や冗長度により異なるが、一般には１０−４〜１０−５とされる。
【００７６】
本実施例の装置を使用し、ビット読み出し速度１Ｍビット／ｍｍ２の記録密度を有するディスクを用いて記録情報を読み出したところ、ビット誤り率を１０−６とすることができた。これは、第２レーザ光の発生手段を装置内部に有し、この第２レーザ光の電界振幅により雑音成分の増幅を含まない純粋な信号成分のみの増幅を可能とする光コヒーレント検波を実施し、かつビート信号周波数をサブキャリアとするＡＳＫ変調信号として記録情報を読み出し／再生した結果による。
（実施例２）
本発明の他の実施例について説明する。
【００７７】
図６に本実施例の光記録情報読み出し装置の構成図を示す。第１の半導体レーザ素子６０１から出射した第１のレーザ光は、ホログラム板６０２を通ってコリメータ・レンズ６０３により平面波に変換される。この第１の半導体レーザ素子６０１からのレーザ光の光周波数はν１とした。平面波に変換された第１のレーザ光は、ビーム・スプリッタ６０４を通過し、集光レンズ６０５により集光され、光ディスク６０６に形成された記録媒体６０７に照射される。照射された、第１のレーザ光は、前述の実施例１と同様のカー効果により、反射レーザ光の偏光面が記録デジタル情報の１、０に対応して、反対方向に回転させられる。
【００７８】
記録媒体６０７で反射された反射レーザ光は、集光レンズ６０５により再び平面波に変換され、ビーム・スプリッタ６０４で２分される。ビーム・スプリッタ６０４を直進した反射レーザ光は、コリメータ・レンズ６０３により集光され、ホログラム板６０２に形成されたホログラムにより回折される。この回折光は、第１の半導体レーザ素子６０１の近傍に配置された多分割フォトダイオード６０８に照射され、光ディスク６０６における照射レーザ光のフォーカス状況およびトラッキング状況を検出するのに使用される。多分割フォトダイオード６０８からの出力は、図示しないピックアップ位置制御回路系に入力され、ピックアップ位置制御回路系は図示した光学系全体から成る光ピックアップ６０９をアクチュエータにより動かすことにより、フォーカスとトラッキング状態が一定となるようにする。
【００７９】
ビーム・スプリッタ６０４で２分されたレーザ光のうち、光路を直角に曲げられた反射レーザ光は、偏光ビーム・スプリッタ６１０により偏光方向の直交する２つの成分に分離される。このときの状況を図７に示す。前実施例と同様に、ベクトルＯＡ（またはベクトルＯＢ）は、カー効果により偏光方向が回転された信号成分を含む反射レーザ光であり、ベクトルＯＲは、カー効果なしに、つまり記録媒体６０７との相互作用なしに、反射された反射レーザ光である。
【００８０】
偏光ビーム・スプリッタ６１０には、これらの２つの反射レーザ光と共に、第２の半導体レーザ素子６１１から発せられた光周波数ν２を有する第２のレーザ光も同時に入射される。この第２のレーザ光は、コリメータ・レンズ６１２により平面波に変換された後に偏光ビーム・スプリッタ６１０に入射される。第２のレーザ光の偏光面は、図７のベクトルＯＬで示すように、偏光ビーム・スプリッタ６１０の偏光分離方向に対して４５゜の角度をなすようにした。このときの偏光面制御は、第２の半導体レーザ素子６１１を、光軸を中心に４５゜回転したように配置することにより対応した。また、反射レーザ光と第２のレーザ光とは共に、平面波の状態で偏光ビーム・スプリッタ６１０により合波される訳であるが、この時の両平面波の光波面は図３〜５で説明したように０．０３゜以下の精度で合波させる必要があるため（ビーム径は２ｍｍ）、第２の半導体レーザ素子６１１およびコリメータ・レンズ６１５は０．０３゜の実装精度で固定した。
【００８１】
偏光ビーム・スプリッタ６１０により分離されたそれぞれの偏光成分の光群は、集光レンズ６１３、６１５によりフォトダイオード６１４、６１６に入射され、光コヒーレント検波される。
【００８２】
図７において、横軸がフォトダイオード６１６の方向に進行する光群を表し、縦軸がフォトダイオード６１４の方向に進行する光群を表す。フォトダイオード６１６では、光周波数ν１を有する第１のレーザ光の反射光であるベクトルＯＲとベクトルＯＡ（１）（またはベクトルＯＢ（１））で表される光、および光周波数ν２を有するベクトルＯＬ（１）で表される第２のレーザ光が、光コヒーレント検波されることとなる。交流成分（ビート信号）を発生させるのは、ベクトルＯＡ（１）（またはベクトルＯＢ（１）とベクトルＯＬ、およびベクトルＯＲとベクトルＯＬである。ここで、記録情報の１、０に拠って、ベクトルＯＡの光が反射されるか、ベクトルＯＢの光が反射されるかが決定されるわけであるが、図７の横軸に対してはいずれの場合においても同一のベクトル成分を有する光（ベクトルＯＡ（１）＝ベクトルＯＢ（１））が到来することとなる。言い換えれば、フォトダイオード６１６からの出力電流に含まれるビート信号成分は、
ＩＡＣ（１）＝η（ａＡ（１）＋ａＲ）ａＬ（１）ｃｏｓ｛２π（ν１−ν２）ｔ＋φＡ（１）−φＬ（１）｝ …（１８）
と表され、その振幅は記録情報の１、０に依存せず一定となる。ただし、前実施例と同様の理由によりベクトルＯＡ（またはＯＢ）成分光とベクトルＯＲ成分光との光位相は一致しているとした。
【００８３】
一方、フォトダイオード６１４側では、第１の光周波数ν１を有するベクトルＯＡ（２）（またはＯＢ（２））成分光と、光周波数ν２を有するベクトルＯＬ（２）成分で表される第２のレーザ光との２つの光が光コヒーレント検波されることとなる。この場合のフォトダイオード６１４の出力電流のビート信号成分は、
ＩＡＣ（２）＝ηａＡ（２）ａＬ（２）ｃｏｓ｛２π（ν１−ν２）ｔ＋φＡ（２）−φＬ（２）｝…（１９）
となる。この場合、ベクトル−ＯＡ（２）＝ベクトルＯＢ（２）なる関係があり、記録情報の１、０が切り替わるたびに、フォトダイオード６１４から出力されるビート信号の位相が１８０゜反転することとなる。これは、図８に示したような、１、０情報を２つの位相状態に対応させて信号変調する２位相シフトキーイング（２ＰＳＫ）に相当する。
【００８４】
以上のようにフォトダイオード６１４からは記録情報が１、０反転する対応して２ＳＰＫにより変調されたビート信号が出力され、フォトダイオード６１６からは記録情報の１、０に依存せず一定の振幅を有するビート信号が出力されることとなる。そのため、フォトダイオード６１４の出力は信号再生用として用い、フォトダイオード６１６の出力は第１のレーザ光と第２のレーザ光との光周波数差を一定に保つための帰還制御に用いる。以下にその詳細を述べる。
【００８５】
まず、光周波数差を一定に保つための帰還制御について述べる。フォトダイオード６１６の出力は、まず、高帯域通過フィルタ６１７により直流成分を除去した後、ビート信号成分のみをゲイン可変前置増幅器６１８により増幅する。前置増幅器６１８の出力は２分され、その片方が周波数弁別回路６１９に入力される。周波数弁別回路６１９はビート信号周波数に対応した直流電圧を発生させ、この弁別電圧を自動波長制御回路６２０により別途入力される設定電圧と一致するように制御信号を出力する。この制御信号に従って、レーザ駆動回路６２１で２つのレーザの駆動条件を変化させることにより、２つのレーザ光の光周波数差が一定に保持される。
【００８６】
また、初期状態における光周波数差の制御は、実施例１と同様にプロセッサ回路６２２とスイッチ回路６２３とにより実現した。ただし、本実施例では、第１及び第２の半導体レーザ素子６０１、６１１として単一端子しか有しない波長安定化レーザ素子（分布帰還型レーザ素子）を適用したので、それぞれのレーザ光の光周波数を変化させるために半導体レーザ素子６０１、６１１の温度を変化させた。温度変化には半導体レーザ素子６０１、６１１近傍に併設した抵抗体による加熱を利用した。本実施例では２つの半導体レーザ素子６０１、６１１を制御する必要があり、片方の半導体レーザ素子の光周波数を一定にしたまま、他方の光周波数を一定の方向に走査することにより所望の光周波数差を実現した。また、片方の半導体レーザ素子のみの光周波数走査で所望の光周波数差が実現されない場合には、最初に光周波数走査を行った半導体レーザ素子の駆動条件を走査終了状態に維持したまま、他方の半導体レーザ素子の光周波数を最初に実行した光周波数走査とは逆方向に走査し、所望の光周波数差（ビート信号周波数）が得られるように制御した。このような温度による光周波数の制御については類似技術が特開平４−１３４９３７〜８に示されており、詳細はこれらを参照されたい。
【００８７】
次に、記録情報の読み出し／再生について説明する。フォトダイオード６１４からの出力を高帯域通過フィルタ６２４に与えて、ここで直流成分を除去し、ゲイン可変前置増幅器６２５によりビート信号成分のみを増幅する。増幅されたビート信号は、狭帯域通過フィルタ６２６に与えられ、ビート信号周波数成分以外の雑音が除去される。狭帯域通過フィルタ６２６の通過帯域の中心周波数は、ビート信号周波数ν１−ν２に一致させてあり、本実施例では２ＧＨｚとした。つまり、実施例１で説明したように、情報読み出しレート、ここでは５Ｍｂｐｓよりビート信号周波数が十分に高周波である必要があるからである。
【００８８】
また、狭帯域通過フィルタ６２６の通過帯域幅は、ビート信号の周波数揺らぎ幅と読み出し情報の転送レートとの和と同等に選択する必要があり、８ＭＨｚとした。この値は、半導体レーザ素子６０１、６１１における出力レーザ光のスペクトル線幅が０．４ＭＨｚであるからである。なお、情報読み出しレートは５Ｍｂｐｓとした。この狭帯域通過フィルタ６２６の出力信号波形は、図８に示したようなビート信号をキャリアとした２ＰＳＫ変調波形となっている。
【００８９】
狭帯域通過フィルタ６２６を通過したビート信号は、位相検波回路６２７により１８０゜ずつ反転する位相状態を正負の電圧として出力される。この場合の位相検波回路６２７としては、非同期検波方式である遅延検波回路を適用した。この回路としては、通常、無線通信用の回路として従来から適用されてきたものを利用することが可能であり、その詳細については畔柳編の「ディジタル通信回路」（産業図書、１９９０年）における第５章“無線伝送回路”、等を参考にされたい。
【００９０】
ところで、実際の光ディスク６０６は高速で回転運動をしており、記録媒体６０７の光照射面は振動するため、この振動に応じて（１９）式のφＡ（２）は変化する。一般に、光ディスク面のピックアップとの相対振動振幅は、数μｍであり光の波長（７８５ｎｍ）より大きいために、φＡ（２）は１０００゜以上にわたって変化する。このディスク６０６の振動により、ビート信号位相が１８０゜分変化する速度が情報読み出しレート（ここでは５Ｍｂｐｓ）と同等か、それ以上の場合には、信号の再生は不可能となる。すなわち、ディスク６０６の振動による位相変化と、記録情報の１、０反転による位相変化とが区別できなくなる。本実施例では、このような状況にならないように、以下のように対処する。
【００９１】
集光レンズ６０５としてＮＡの大きな、つまり焦点深度の浅いレンズを用いて、第１のレーザ光の波長の１／４程度のフォーカスエラーまでを検出可能とすること、および、光ピックアップ６０９のフォーカス制御のためのアクチュエータとして高速応答型アクチュエータを適用すること等によりフォーカスサーボ系を充実させ、ディスクとピックアップとの相対振動周波数の低周波化（２ＭＨｚ以下）を図るようにした。
【００９２】
位相検波回路６２７において、通常のＰＬＬ検波回路の出力を通過帯域を情報読み出しレート相当（５ＭＨｚ）からその２倍（１０ＭＨｚ）の範囲とした狭帯域通過フィルタ（図示せず）を通し、この帯域内に存在する位相変化分に相当する電圧変換のみを検出の対策を講じた。
【００９３】
このようにすることにより、位相検波回路６２７の出力として、読み出し情報の１、０反転時に対応して電圧が変化するような信号が出力される。最後に、この信号に基づいて信号再生回路６２８は記録信号を再生した。
【００９４】
本実施例では、位相検波回路６２７への入力信号の振幅が一定となるように、第２の半導体レーザ素子６１１のレーザ出力強度と前置増幅器６１８、６２５の増幅率とを、帰還回路系により制御した。この場合、電子回路（前置増幅器６１８、６２５）による増幅では、フォトダイオード６１４、６１６の周辺回路や高域通過フィルタ６１７、６２４で発生する各種雑音も増幅してしまう上、実際上必ず前置増幅器６１８、６２５において過剰な雑音が発生する。一方、光コヒーレント検波では疑似的に信号振幅を雑音の増加なしに増幅することができる。従って、光コヒーレント検波機能を最大限に発揮させることが望ましい。
【００９５】
具体的構成としては、本実施例では次のようにした。前述した前置増幅器６１８の出力を周波数弁別回路６１９に入力すると共に振幅検波回路６２９に与え、振幅検波回路６２９によりビート信号振幅を直流電圧として検出し、この電圧値を自動ゲイン制御回路６３０に入力する。自動ゲイン制御回路６３０は、入力した電圧値と、別途外部より与えられる設定電圧とを比較し、これらが一致するように制御信号を出力する。この制御信号は、レーザ駆動回路６２１と前置増幅器６１８、６２５とに入力され、これに従って半導体レーザ素子６１１の第２のレーザ光強度と２つの前置増幅器６１８、６２５のゲインが変化させられる構成とした。
【００９６】
この帰還回路系により、前置増幅器６１８の出力ビート信号振幅が一定となり、等価的に前置増幅器６２５の出力ビート信号増幅も一定となる。この場合の制御の指針としては、出来る限り半導体レーザ素子６１１の出力レーザ光強度を大きくすることにより光コヒーレント検波による疑似的ビート信号振幅の増幅を優先することが重要であり、半導体レーザ素子６１１の構造上の出力限界やフォトダイオード６１４、６１６の光入力強度に対する制限より、半導体レーザ素子６１１のレーザ光出力強度を増加させられない場合に限り、前置増幅器６１８、６２５による増幅を利用するようにした。
【００９７】
このように光コヒーレント検波を実行することにより、（１９）式に示したようにフォトダイオード６１４からのビート信号振幅は、第２のレーザ光の電界振幅により疑似的に増幅された形となっており、ビット当たりの信号光パワーが低下した状態でも、高いＣ／Ｎ値を得ることが可能となる。また、上記の光コヒーレント検波による疑似的増幅に加えて、本実施例では、図８に示すように、光コヒーレント検波にしたビート信号がビート信号周波数をキャリア周波数とする２ＰＳＫ変調信号となっているので、Ｃ／Ｎの値がかなり低い状態でも相対的に高いビット誤り率で信号を再生することが可能である利点も合わせ有している。例えば、１０−４のビット誤り率を実現するに必要なＣ／Ｎ値を、実施例１で用いたＡＳＫ方式と本実施例における２ＰＳＫとで比較すると、
ＡＳＫ（振幅検波）の場合 １５ｄＢ
２ＰＳＫ（遅延検波）の場合 ９．４ｄＢ
となる。より詳細には前述の「ディジタル通信回路」ｐ．２３５を参照して下さい。
【００９８】
このことからも分かるように、ＡＳＫ方式に対して５．６ｄＢ低いＣ／Ｎ値の信号でもビット誤り率１０−４での信号再生が可能である。ただし、ＡＳＫ変調方式と比較してキャリアとなるビート信号の周波数や位相が安定していることが必要であり、本実施例では半導体レーザ素子６０１、６１１に狭スペクトル線幅（０．４ＭＨｚ）を有する素子を適用することや、上記のようなφＡ（２）の揺らぎ成分を抑制することなどの対策が必要であった。実際に、１Ｍ／ｍｍ２の記録密度のディスクから本実施例装置により情報を読み出したところ、５Ｍｂｐｓと情報読み出し速度を高速化したにも拘らず、１０−６のビット誤り率を実現できた。
（実施例３）
本発明の更に他の実施例について説明する。
【００９９】
上述した実施例１、２では記録媒体として光磁気効果を用いたものを説明したが、本実施例３では記録媒体として光の散乱効果を利用したもの（通常のコンパクトディスクにおける光情報読み出し方法と同じ）を使用する。
【０１００】
図９に本実施例３の光記録情報読み出し装置の構成図を示す。半導体レーザ素子９０１から発せられた、光周波数：ν１を有する第１のレーザ光は、コリメータ・レンズ９０２により平面波に変換され、ビーム・スプリッタ９０３を通過して集光レンズ９０４により光ディスク９０５に照射される。光ディスク９０５にはピットの有無として情報が記録された記録媒体９０６が配置されており、この記録媒体９０６に第１のレーザ光が最小のスポットサイズとなる点で照射されることとなる。記録媒体９０６で反射された第１のレーザ光（以後、信号光と呼ぶ。）は、再び集光レンズ９０４により平面波に変換され、ビーム・スプリッタ９０３により光路を直角に変換された後、ビーム・スプリッタ９０７に入射される。
【０１０１】
一方、第２の半導体レーザ素子９１０から発せられた、光周波数ν２を有する第２のレーザ光は、コリメータ・レンズ９１１により平面波に変換された後、ビーム・スプリッタ９０７に入射される信号光と合波される。ビーム・スプリッタ９０７で合波された信号光と第２のレーザ光とは共に平面波であり、その波面は一致するように第２の半導体レーザ素子９１０およびコリメータ・レンズ９１１の位置を調整した。なお、この場合、レーザビーム径は１ｍｍであったので、合波において要求される波面一致精度は約０．０５°であった。
【０１０２】
ビーム・スプリッタ９０７で合波された両レーザ光は、集光レンズ９０８により集光され、フォトダイオード９０９に至る。フォトダイオード９０９の出力は、高域通過フィルタ９１２により直流成分を除去された後、増幅器９１３によりビート信号のみが増幅される。合波後の両光の偏光方向の関係を図１０に示す。第２のレーザ光をベクトルＯＬで、信号光をベクトルＯＲ（またはＯＲ’）で示している。
【０１０３】
情報は、記録媒体９０６上のピットの有無として記録されている。このため、第１のレーザ光が集光されてディスク９０５に照射された場所にピットが無い場合、つまり記録媒体９０６表面が鏡面の場合には、照射レーザ光の大部分が同一の光路を戻りレンズ９０４により平面波に変換され、フォトダイオード９０９まで到達する。一方、ピットが有る場合には、照射レーザ光の半分以上がレンズ９０４の有効ＮＡより大きな角度をもって散乱されるため、平面波としてフォトダイオード９０９まで達する成分が減少する。従って、図１０において、ベクトルＯＲはピットが無い場合の信号光を、ベクトルＯＲ’はピットが有る場合の信号光を示している。
【０１０４】
また、本実施例では第２のレーザ光と第１のレーザ光の偏光面が一致するように配置してある。このような条件下でフォトダイオード９０９で光コヒーレント検波した場合に出力されるν１−ν２の周波数を有するビート信号成分は、
ＩＡＣ＝ηａＲａＬｃｏｓ｛２π（ν１−ν２）ｔ＋φＲ−φＬ｝ …（２０）
となる。ここで、記録情報の１、０によって変化するパラメータはａＲであり、ａＲとａＲ’の間で変化する。
【０１０５】
増幅器９１３の出力波形例を図１１に示す。この図から理解されるように、増幅器９１３の出力は、記録情報の１、０に対応してビート信号の振幅が変化するＡＳＫ変調方式となる。
【０１０６】
このＡＳＫ変調信号は、狭帯域通過フィルタ９１４を通してビート信号周波数以外の不要な雑音成分が除去された後、振幅検波回路９１５により振幅が検出される。ここでの振幅検波回路９１５としては、通常の包落線検波回路を用いた。これにより、図１１に示した信号振幅に対応した電圧が出力される。最後に、この出力が電圧比較回路９１６に入力され、信号が再生される。
【０１０７】
本実施例においても、（２０）式から分かるように記録情報に関連した信号振幅は、第２のレーザ光により疑似的に増幅された形となっているため、実施例１、２と同様の理由により、通常の直接検波と比較して高いＣ／Ｎ値を得ることができる。
【０１０８】
次に、本実施例３における２つの半導体レーザ素子９０１、９１０相互の光周波数差保持の機構について説明する。第１および第２の半導体レーザ素子９０１、９１０の各々には、単一モードの光ファイバ９１７が結合されている。第１の半導体レーザ素子９０１から裏面に出射された光は、光ファイバ９１７に入射され、分岐部９１８により２分された後、片方は出力モニタ用のフォトダイオード９２０に入射される。これにより、第１の半導体レーザ素子９０１の出力レーザ強度が、フォトダイオード９２０により検出される。一方、２分された他方の光は、方向性結合器から構成される合波部９２２に導かれる。
【０１０９】
第２の半導体レーザ素子９１０から裏面に出射された光も同様に、光ファイバ９１７に入射され、分岐部９１９により２分された後、片方はフォトダイオード９２１へ、他方は合波部９２２に導波される。
【０１１０】
上記合波部９２２において、第１のレーザ光と第２のレーザ光とは、それぞれ１：１に混合されて、２つのフォトダイオード等から成るバランス型受光回路９２３に入射され、光コヒーレント検波される。バランス型受光回路９２３からの出力は、高域通過フィルタ９２４により直流成分を除去され、増幅器９２５で適宜増幅された後、周波数弁別回路９２６にて周波数弁別される。この周波数弁別出力は、比例（Ｐ）制御回路９２７に入力される。Ｐ制御回路９２７は、この周波数弁別出力と別途外部より与えられる直流設定電圧とを比較し、この両者が一致するように制御信号を出力する。この制御信号に従いレーザ駆動回路９２８は第１および第２の半導体レーザ素子９０１、９１０の駆動条件を制御する。これにより、両者の悲哀周波数差ν１−ν２が一定に保持されることとなる。これらの制御の方法や初期状態での設定等は、実施例１で説明した内容と同等であるため、ここでの詳細説明は省略する。
【０１１１】
前述した実施例１、２においては２つのレーザ光の光周波数差の制御にディスクでの第１のレーザ光の反射が必須であったが、本実施例では、このような構成としているので、光ディスク９０５が装置に装着されない状態でも光周波数差の制御を実行することが可能となる。従って、前述の実施例構成ではディスク交換後ごとに実行が必要であった光周波数制御の初期動作を不要とすることが可能となった。但し、電源投入時のみ必要である。
【０１１２】
また、半導体レーザ素子９０１、９１０の出力レーザ光強度の自動制御は、フォトダイオード９２０、９２１からの出力と設定電圧とをＰ制御回路９２９、９３０により比較制御することにより行った。
【０１１３】
実際に図９に示した構成の光記録情報読み出し装置を用いて、２Ｍ／ｍｍ２密度で記録されたディスクから情報読み出し速度３Ｍｂｐｓで情報を読み出したところ、１０−７のビット誤り率が達成され、本発明を適用したことによる効果を確認できた。
（実施例４）
本発明の更に他の実施例について説明する。
【０１１４】
上記の３つの実施例１〜３では、従来技術の問題である記録情報の高密度化や高速読み出し化に伴う情報読み出し時のビット誤り率の低下を防止するために、単層記録媒体の光ディスクおいて低ビット誤り率を確保する方法を示した。
【０１１５】
ところで、高密度化に伴う情報読み出し時のビット誤り率低下の防止対策としては、上記方法の他に、記録媒体を多層に形成したディスクから情報読み出す方法も考えられる。即ち、ビット当りの情報記録面積を一定とし、記録媒体を厚み方向に多重化する方法である。このような、多重記録媒体ディスクからの情報読み出し方法は、従来より検討されているが、多層記録媒体の内のどの層の情報を読み出すかの選択には、レンズにより集光されたレーザビームの焦点深度を利用しているため、他層の状態が読み出すべき層からの情報を有する反射レーザ光に雑音を重畳することとなり、実用上十分なビット誤り率を得ることが困難となっていた。
【０１１６】
本発明は、上記のような多層記録媒体光ディスクの情報読み出しにも適用でき、その効果を発揮することが可能である。
【０１１７】
以下に、本実施例の光記録情報読み出し装置を図１２に基づいて説明する。図１２は、記録媒体を２層にした多層光ディスク装置に適用させた場合における、本実施例の光記録情報読み出し装置の構成図を示す。光ディスク１２００は、第１記録媒体層１２０１と第２記録媒体層１２０２とが透明な分離層１２０３を挟み平行に配置された状態とした。この記録媒体１２０１、１２０２には、デジタル情報が散乱／非散乱の２状態として記録されている。
【０１１８】
２波長半導体レーザ素子１２０４の第１レーザストライプ１２０５および第２レーザストライプ１２０６から発せられた第１および第２のレーザ光は、レンズアレイ１２０７によりそれぞれ平面波に変換される。２波長半導体レーザ素子１２０４における２つのレーザストライプ１２０５、１２０６の間隔と、レンズアレイ１２０７の２つの光学軸との間隔は一致させる。ここでは２０００μｍとした。また、図示したような平面波に変換された両レーザ光ビームが空間的に重ならないように、半導体レーザ素子１２０４のビーム放射角度およびレンズアレイ１２０７のレンズの焦点距離を設計した。
【０１１９】
ここで、第１のレーザ光と第２のレーザ光の光周波数はそれぞれν１、ν２であり、その差ν１−ν２は一定に保たれている。また、第１レーザ光と第２レーザ光はそれぞれ直線偏光状態を有しており、両者の偏光方向は一致している。
【０１２０】
このような条件下において、平面波に変換された２つのレーザ光は、ビーム・スプリッタ１２０８によりそれぞれ２分される。ビーム・スプリッタ１２０８を直進通過した両レーザ光は、レンズ１２０９により同一焦点位置Ｆに集光される。焦点位置Ｆに集光された２つのレーザ光は、この焦点位置Ｆ付近のみで同一空間を共有することとなる。より詳細には、光ディスク１２００の記録媒体付近の拡大図である図１３に示すように、２つの平面波レーザ光１３０１、１３０２がレンズ１２０９により同一焦点位置Ｆに集光されている。
【０１２１】
焦点位置Ｆに光散乱体が存在する場合（記録情報が“１”の場合に相当）には、第１レーザ光および第２光レーザ光ともこの光散乱体により散乱され、焦点位置Ｆを中心とする同一の波面を有する球面波となって反射される。この同一波面を有する第１および第２の球面波レーザ光は、レンズ１２０９により同一波面の平面波１３０３（以後、信号レーザ光とも呼ぶ。）に変換され、ビーム・スプリッタ１２０８へと入射することとなる。ビーム・スプリッタ１２０８により直角に進行方向を変換された信号レーザ光１３０３は、ビーム・スプリッタ１２１９により２分され、片方は光ピックアップ１２２９の位置制御のための多分割フォトダイオード１２２０へレンズ１２２１を介して入射される。一方、ビーム・スプリッタ１２１９により分岐された他方の信号レーザ光１３０３は、レンズ１２２２を透過してフォトダイオード１２２３に入射される。レンズ１２０９の焦点位置Ｆにて散乱反射された第１及び第２のレーザ光は、上述のようにレンズ１２０９により同一波面を有す平面波１３０３に変換されているため、フォトダイオード１２２３の受光面上での両レーザ光の波面はおのずと一致しており、光コヒーレント検波することが可能となる。従って、両レーザ光の光周波数差に相当する周波数を有するビート信号が発生させられることとなる。
【０１２２】
一方、焦点位置Ｆに光散乱体が存在しない場合には、両レーザ光は第１記録媒体層１２０１を透過または境面反射することとなる。第１記録媒体層１２０１を透過したレーザ光１３０４、１３０５は分離層１２０３を通過して第２記録媒体層１２０２へ達する。第２記録媒体層１２０２にも情報の１、０に対して、光散乱体が分布配置されているので、それぞれのレーザ光は散乱を受けることとなる。
【０１２３】
例えば、第１のレーザ光は点Ｑで、第２のレーザ光は点Ｐで散乱されたとする。すると、点Ｐで散乱された第１のレーザ光と点Ｑで散乱された第２のレーザ光は、それぞれ散乱点Ｐ、Ｑを中心とする球面波１３０６、１３０７として反射されることとなる。点Ｐ、Ｑはレンズ１２０９の焦点位置Ｆとは異なっているため、散乱球面波１３０６、１３０７はレンズ１２０９で平面波には変換されない。この場合、第２記録媒体層１２０２がレンズ１２０９の焦点位置Ｆより遠方に存在するため、収束球面波１３０６′、１３０７′へと変換されることとなる。更には、第１と第２のレーザ光は、レンズ１２０９の焦点位置Ｆでのみ同一空間を共有するように光学系を配置しているので、点Ｐと点Ｑは同一点となることはあり得ない。
【０１２４】
従って、点Ｐ、点Ｑで散乱反射された球面波１３０６、１３０７の波面は必ず異なることとなり、フォトダイオード１２２３で光コヒーレント検波することは不可能である。この場合は、波面の異なる２つのレーザ光の合波検波になるが、図３および図４を用いて説明した平面波の光コヒーレント検波の場合と同様の議論が可能であり、波面の傾き角度が大きい場合にはビート信号成分を出力することは不可能となる。以下に、その理由について詳細に説明する。ここで、レンズ１２０９の焦点距離をｆ、透明分離層１２０３の厚さをｄ（簡単化のために記録媒体層１２０１、１２０２の厚さは０とする）、レンズ１２０９の光軸を中心として点Ｐと点Ｑが距離ａだけ隔てた位置に存在すると仮定する。ただし、
ｆ＋ｄ》ａ …（２１）
である。
【０１２５】
レンズ１２０９の中心位置を原点とする図１３に示したようなｘ−ｚ座標を取ると、点Ｐと点Ｑとで散乱されたレーザ光がレンズ１２０９を通過して再び集光される位置Ｐ′（ｘｐ′，ｚｐ′）、（ｘＱ′，ｚＱ′）は一般のレンズの結像関係の式より、
ｘｐ′＝−ａｆ／ｄ …（２２）
ｘＱ′＝ａｆ／ｄ …（２３）
ｚｐ′＝ｚＱ′＝ｆ（ｄ＋ｆ）／ｄ≒ｆ２／ｄ …（２４）
と算出される。本実施例ではｆ＝５ｍｍ、ｄ＝５０μｍとしたので、（２４）式よりｚｐ´＝ｚＱ´＝５００ｍｍとなる。このｚｐ´の値は、レンズ１２０９とレンズ１２２２との間隔（本実施例では５０ｍｍ）より大きいために、実際にはこれら２つの散乱レーザ光は集光されることなく、レンズ１２２２とフォトダイオード１２２３によって光コヒーレント検波されることとなる。
【０１２６】
この関係を図１４に示す。ここで、２枚のレンズ１２０９と１２２２との間隔をＡ、レンズ１２０９の有効開口径を２Ｄとした。レンズ１２２２の位置での有効面積内での両レーザ光１３０６´、１３０７´の波面の傾きに起因する光の波面遅れξは、近似的に、
ξ≒ａｆＤ／ｄ（ｚｐ´−Ａ） …（２５）
と表せる。このため、この値がレーザ光波長λ０以上となると、光コヒーレント検波時のビート信号の強度は顕著に減衰することとなる。従って、点Ｐと点Ｑからの散乱レーザ光が光コヒーレント検波され相当の強度を有するビート信号を発生せない状態における両点の最小間隔２ａｍｉｎは、
２ａｍｉｎ≒２ｄλ０（ｚｐ′−Ａ）／ｆＤ …（２６）
となる。
【０１２７】
本実施例におけるレーザ波長は、第１レーザ光及び第２レーザ光ともほぼ７８５ｎｍであり、レンズ１２０９の有効径は１０ｎｍとしたので、（２６）式より点Ｐと点Ｑからの２つの散乱反射光がフォトダイオード１２２３でビート信号を誘起させないためには、２ａ≧２ａｍｉｎ＝１μｍなる条件が必要となることが分かる。実際の装置では、レンズ１２０９に入射する前の第１のレーザ光と第２の光レーザ光の平面波同士の間隔２Ｓを２００μｍとしたため、第２記録媒体層１２０２位置での両光の透過光の隔たり２σは約２０μｍとなり、本実施例で上記の条件（２６）式が満たされることはあり得ない。つまり、第２記録媒体層１２０２の位置において１μｍしか離れていない２点に、第１レーザ光と第２光レーザ光とのそれぞれが照射される状況はない。この条件を式に表すと、下記（２７）式となる。
【０１２８】
２σ≧２ａｍｉｎ …（２７）
従って、第１記録媒体層１２０１を透過し、第２記録媒体層１２０２に存在する光散乱体により散乱された光は、フォトダイオード１２２３に入射した場合にも、第１レーザ光と第２レーザ光の光周波数差ν１−ν２に相当する周波数を有する強度の大きなビート信号を発生させることはない。
【０１２９】
このようにして、第１記録媒体層１２０１における焦点位置Ｆに光散乱体が存在する場合にのみ、光コヒーレント検波された第１及び第２レーザ光からなる信号レーザ光はフォトダイオード１２２３の出力電流に両光の光周波数差ν１−ν２を周波数とするビート信号成分が含まれることとなる。
【０１３０】
したがって、フォトダイオード１２２３の出力を高帯域通過フィルタ１２２４を通し、このビート信号成分を抽出し、前置増幅器１２２５により増幅した。前置増幅器１２２５の出力波形例を図１５に示す。その波形は、第１と第２のレーザ光の光周波数差ν１−ν２を有するビート信号が第１記録媒体に記録されたデジタル信号の１、０に従ってその振幅が変調された形、すなわちＡＳＫ変調された形となっている。次に、このような信号を狭帯域通過フィルタ１２２６に与えて、ビート周波数成分以外の周波数成分を有する雑音を除去した後、振幅検波回路１２２７により包落線検波した。振幅検波回路１２２７の出力電圧を電圧比較器１２２８により比較し、記録情報を再生した。
【０１３１】
以上のように、第１記録媒体層１２０１に光散乱体の有無として記録された情報を読み出すことが可能となった。
【０１３２】
上記のように本発明を実施し、ビート信号より情報を再生することにより、レンズ１２０９の焦点位置Ｆに存在する記録媒体層（この場合は１２０１）からの情報のみを選択的に読み出すことが可能となり、従来困難であった多層からの反射情報と確実に分離することができるようになった。
【０１３３】
以上の説明では、第１記録媒体層１２０１を情報読み出し対象として説明したが、逆に第２記録媒体層１２０２から情報を読み出す場合にも、同様の方法により第１記録媒体層１２０１での光散乱の影響なしに信号再生を実行することが可能であった。ただし、この場合には、レンズ１２０９の焦点位置Ｆを第２記録媒体層１２０２位置に一致させるように光ピックアップ１２２９の位置を制御する必要がある。この光ピックアップ１２２９の位置制御には、多分割フォトダイオード１２２０からの出力を第１記録媒体層１２０１に焦点位置Ｆが一致する状態からわざとずらすことにより実行した。なお、トラッキングは第１記録媒体層と第２記録媒体層は同じであるため、記録媒体を多層にしたことによる新たな機構は必要なかった。
【０１３４】
また、より精度の良いフォーカスサーボをかけることを目的として多分割フォトダイオード１２２０での受光に光コヒーレント検波を導入し、ビート信号振幅としてそれぞれのフォトダイオードからの出力を検出する方法も実施した。このようにビート信号を光ピックアップ１２２９位置制御に応用することにより、フォーカスサーボがかかった状態での制御の安定性は非常に高いことが分かった。ただし、本方法の場合、多分割フォトダイオード１２２０の各エレメントビート信号周波数より高い応答周波数を有するものを選択すること、および各フォトダイオード個々に高域通過フィルタ、前置増幅器、狭帯域通過フィルタ、振幅検波の各回路（信号検出系における１２２４〜１２２７と同等の回路）を併設する必要がある。
【０１３５】
次に、２波長半導体レーザ素子１２０４に含まれる２つのレーザ１２０５、１２０６の光周波数差を一定に保つための機構について説明する。ビーム・スプリッタ１２０８により２分された他方の２つの平面波レーザ光はミラー１２１０、ビーム・スプリッタ１２１１により波面が一致する様に合波され、レンズ１２１２によりフォトダイオード１２１３の受光面上に集光される。フォトダイオード１２１３ではこれら２つのレーザ光が光コヒーレント検波させることとなり、実施例１〜３で説明したのと同様に、高域通過フィルタ１２１４、前置増幅器１２１５、周波数弁別回路１２１６、比例積分（ＰＩ）制御回路１２１７、レーザ駆動回路１２１８により構成される帰還制御系により２つのレーザ光の光周波数差ν１−ν２を一定に保持した。詳細な回路構成や動作に関しては、実施例１〜３で詳細に説明したのでここでは省略する。
【０１３６】
本実施例装置を用いて実際に記録密度０．７Ｍ／ｍｍ２の光ディスクより情報を読み出したところ、情報読み出しレート１Ｍｂｐｓにおいて１０−６のビット誤り率を達成することができ、さらに多層に記録された情報を互いに影響なしに読み出すことが可能となった。
【０１３７】
（実施例５）
次に、本発明の第５の実施例について説明する。
【０１３８】
図１６に本実施例の光記録情報読み出し装置の構成図を示す。本実施例における光メモリディスク１６００は、第１記録媒体層１６０１と第２記録媒体層１６０２とを有しており、それらが厚さｄの透明分離層により分離された構造となっている。情報は、記録媒体層１６０１、１６０２における光散乱体の有無により記録されている。実際の記録媒体層１６０１、１６０２には液晶材料を溶媒中に分散させたものを適用している。この場合、記録情報の書き込みは、レーザ光により液晶分子または液晶層１６０１、１６０２の上下に位置する光吸収膜で光吸収させることにより液晶層１６０１、１６０２を過熱し、その後、自然急冷することにより、液晶分子の配列がランダム化される現象を利用した。ただし、前実施例共、記録媒体層１６０１、１６０２は液晶材料に限られる訳ではなく、情報を光の散乱状態として記録できるものであればよく、本発明の効果には何ら支障はない。例えば、書き換え不可能なピットをスタンプ方式でプラスチック材料に印刻したディスク等が考えられる。
【０１３９】
半導体レーザ素子１６０３から出射されたレーザ光は、レンズ１６０４によりビーム・スプリッタ１６０５に集光される。ビーム・スプリッタ１６０５でレーザ光は２分された後、それぞれのレーザ光は、同一のレンズ１６０６により２つの平面波に変換される。この２つの平面波は、集光レンズ１６０７により光メモリディスク１６００に集光照射される。この状態を図１７に拡大して表す。この図から分かるように、まったく同一の光周波数を有する２つのレーザ光１７０１、１７０２が角度：２αをなして、第２記録媒体層１６０２上の共焦点位置Ｆに集光照射されている。
【０１４０】
第２記録媒体層１６０２の光照射（共焦点）位置Ｆに光散乱体が存在する場合には、両レーザ光１７０１、１７０２の散乱反射光は、レンズ１６０７により再び平面波（以下レーザ光ともいう）１７０３に変換されることとなる。すなわち、２つのレーザ光１７０１、１７０２の反射光の波面は一致することとなる。
【０１４１】
このとき、光メモリディスク１６００はモータ（図示せず）により回転運動しているため、光照射位置Ｆにおける第２記録媒体層１６０２は、速度ｖで（図中では左方向へ）移動していることとなる。このように、２つの同一光周波数のレーザ光が移動物体に移動方向と平行にある角度をなして照射された場合、その移動物体で反射されるレーザ光１７０３はドップラーシフトを受けることとなる（レーザドップラー効果の詳細は、例えば「Ｏ ｐｌｕｓ Ｅ」Ｎｏ．１５０（５月）ｐ．１０９を参照されたい）。
【０１４２】
ここで、レーザ光１７０１が第２記録媒体層１６０２により散乱反射されたレーザ光（第１のレーザ光）の光周波数ν１ と、レーザ光１７０２が同様にして散乱反射されたレーザ光（第２のレーザ光）の光周波数ν２ とは、それぞれ（２８）、（２９）式となる。
ν１＝ν０（１＋２ｖｓｉｎα／ｃ） …（２８）
ν２＝ν０（１−２ｖｓｉｎα／ｃ） …（２９）
ただし、ν０ は記録媒体１７０２照射前のレーザ光１７０１、１７０２の光周波数を表している。
【０１４３】
上記（２８）式、（２９）式より、両レーザ光の光周波数差：ν１−ν２は、
ν１−ν２＝４ν０ｖｓｉｎα／ｃ …（３０）
となり、照射するレーザ光の周波数ν０と速度ｖはほぼ一定であるので一定の値となる。ここでは、光メモリディスク１６００の上下方向の移動運動は無視した。実際には、光メモリディスク１６００は回転運動しながら上下に振動し、その移動速度成分により第１の第２のレーザ光１７０３の光周波数差は（３０）式よりずれることとなるが、実際の装置では光メモリディスク１６００の回転による記録媒体１６０１、１６０２の移動速度ｖは２０ｍ／ｓ以上と上下方向の振動に起因する光メモリディスク１６００の移動速度より充分に大きいので、この影響は無視できるからである。
【０１４４】
このようにして、第２記録媒体層１６０２の光散乱体により反射された２つの異なる光周波数ν１、ν２を有するレーザ光１７０３が波面が一致した状態でミラー１６０８に入射することとなる（以後、この波面が一致した第１および第２レーザ光１７０３を信号光と呼ぶ）。
【０１４５】
その後、ミラー１６０８により光路を９０゜ 変換された信号光１７０３は、ビーム・スプリッタ１６０９により２分され、ビーム・スプリッタ１６０９を直進する信号光１７０３はレンズ１６１０により集光され多分割フォトダイオード１６１１に入射される。この多分割フォトダイオード１６１１からの出力に従って、ピックアップ１６２２の位置を制御し、集光照射位置Ｆが第２記録媒体層１６０２表面になるようにした。
【０１４６】
一方、ビーム・スプリッタ１６０９で２分された他方の信号光１７０３は、レンズ１６１２で集光されてフォトダイオード１６１３に入射される。信号光１７０３は、上述のように共焦点Ｆからの散乱反射光として同一の波面を有しているため、フォトダイオード１６１３で両レーザ光１７０３は光コヒーレント検波されることとなる。第１のレーザ光と第２のレーザ光の光周波数差は、（３０）式で表されるようにほぼ一定であるため、フォトダイオード１６１３の出力にはこの光周波数差に一致した周波数を有するビート信号が含まれることとなる。
【０１４７】
以上のように、第２記録媒体層１６０２の光照射位置Ｆに光散乱体が存在する場合には、フォトダイオード１６１３からの出力にビート信号成分が含まれることとなる。一方、光照射位置Ｆに光散乱媒体がなく反射がほとんど無い場合には、信号光１７０３の強度は極度に小さくなり、このためフォトダイオード１６１１の出力に含まれるビート信号成分の振幅も減衰する。
【０１４８】
次に、第２記録媒体層１６０２に焦点位置Ｆを設定した状態における第１記録媒体層１６０１からの散乱反射光の影響について説明する。この場合、図１７における点Ｐ、Ｑからレーザ光が散乱反射されることとなるが、レーザ光照射光学系の性質上これら２つの点が一致することは有り得ない。また、第２記録媒体層１６０２に照射される２つの平面波レーザ光１７０１、１７０２の間隔２Ｓは２ｍｍとし、使用したレンズ１６０７の焦点距離ｆは５．５ｍｍ、透明分離層１６０３の厚さは１０μｍとしたため、第１記録媒体層１６０１における点ＰとＱとの間の距離２σが取り得る最小の値は２８μｍとなる。
【０１４９】
ここで、実施例４での（２１）式〜（２７）式における議論と同様の考察により、第１記録媒体層１６０１における異なる２点から散乱反射される光がフォトダイオード１６１３においてビート信号を生成するためには、実際的な光学構成においてはその点間の距離が１０μｍ未満となる必要があることが分かる。従って、上記の点ＰとＱとの間隔２σ（最低でも２８μｍ）では、フォトダイオード１６１３上での波面がずれ、ビート信号は生成されない。
【０１５０】
以上のことより、フォトダイオード１６１３の出力にビート信号が含まれるのは、第２記録媒体層１６０２のレーザ照射位置Ｆに光散乱体が存在する場合のみであることが分かる。この場合において、フォトダイオード１６１３の出力は高域通過フィルタ１６１４により直流成分を除去した後、前置増幅器１６１５によりビート信号成分を増幅した。
【０１５１】
この前置増幅器１６１５の出力波形例を図１８に示す。この出力波形は、第２記録媒体層１６０２における記録情報の１、０に対応して、ビート信号振幅が変化するＡＳＫ変調信号となっている。このＡＳＫ変調信号からの信号再生には、前述の実施例と同様に、狭帯域通過フィルタ１６１６、包落線検波回路１６１７、電圧比較回路１６１８を用いた。また、本実施例ではｖ＝２０ｍ／ｓ、２α＝４０゜ としたため、ビート信号周波数ν１−ν２は６６ＭＨｚであった。このビート信号周波数ν１−ν２は記録媒体からの情報の読み出しレートＲ＝１．５Ｍｂｐｓよりも充分に高くなっており、ＡＳＫ信号からのビット信号再生が可能であった。
【０１５２】
また、光メモリディスク１６００を回転させる方式としては、角速度一定方式と速度一定方式があるが、ここでは狭帯域通過フィルタ１６１６の通過帯域幅をできるだけ狭くするために、速度一定方式を選択した。この速度一定方式を制御は以下のように行った。
【０１５３】
まず、前置増幅器１６１５からのビート信号の周波数を周波数弁別回路１６１９により検出した。（３０）式に示すように、ビート信号周波数ν１−ν２には速度ｖの情報が含まれているため、この検出値と別途外部より与えられる設定電圧値とを比例（Ｐ）制御回路１６２０により比較し、これらが一致するようにモータ駆動回路１６２１によりモータの回転速度を変化させることにより、速度を一定に保つことができた。
【０１５４】
また、（３０）式から分かるようにビート信号周波数は速度ｖと照射レーザ光の光周波数ν０とにより決定されるため、速度を制御するためにはレーザ光周波数ν０を一定にする必要がある。このため、本実施例では、半導体レーザ素子として波長変動の少ない分布帰還型レーザ素子を適用した。なお、速度を一定に保つ他の方法として、再生されたビット情報のレートを検出し、その値が一定となるようにモータの回転数を制御する方法も可能あり、同様の効果が得られた。
【０１５５】
以上のような構成とすることにより、２つの半導体レーザ素子からのレーザ出力の光周波数差を一定にする為の複雑な帰還回路系を必要とせずに、光コヒーレント検波により生成されるビート信号の振幅変化を介して記録情報を読み出すことが可能となり、多層記録媒体ディスク１６００からも読み出し対象層以外の層での光の散乱の影響を全く受けることなく、記録情報を読み出すことが可能となった。このため、本実施例装置において、記録密度１Ｍ／ｍｍ２の２層ディスクより情報を読み出したところ、ビット誤り率１０−７が達成された。
【０１５６】
【発明の効果】
以上詳述したように、本発明においては以下のような効果を有する。
【０１５７】
（１）光記録情報読み出しにおいて、情報読み出しレートより大きな周波数差を有する２つのレーザ光を用いて光コヒーレント検波することにより、１ビット当たりの光パワーが微弱となった場合にでも記録情報再生に十分なＣ／Ｎ値を確保できる。
【０１５８】
（２）光周波数が異なる２つの光を共焦点状態で光記録媒体上に照射することにより、多層光記録媒体の特定の媒体からの記録情報再生において十分なＣ／Ｎ値を確保できる。
【０１５９】
（３）光コヒーレント検波出力のビート信号成分の周波数を検知し、その値を一定に保つ制御を実行して２つの光の周波数差を一定に保つことにより、情報再生時のキャリア周波数が安定化され、記録情報再生におけるビット誤り率を小さく抑制することができる。
【０１６０】
（４）共焦点状態で記録媒体上に照射する２つの光を同一光源からの同一周波数を有する光とすると共に、記録媒体の移動によりそれらの反射光の光周波数にドップラーシフトを与えることにより、単一のレーザ光源でかつ光周波数シフトのための余分な素子を必要とすることなく、光周波数差が一定の値を有する２つのレーザ光を得ることが可能となり、光コヒーレント検波を利用する光記録情報読み出しシステムの簡素化・装置の小型化ができるようになる。
【０１６１】
また、本発明は、上記実施例に限られるものではなく、具体的な光学構成や電気回路構成が異なる場合、信号再生における信号処理方法が異なる場合、ビート信号周波数や情報読み出しレート、情報記録密度、使用した半導体レーザ素子のスペクトル線幅、等の各種数値が異なる場合、あるいは、多層記録媒体の層数が２層以上の場合にも、効果を発揮することは言うまでもない。更には、受光素子に光コヒーレント検波するための２つの異なる波長を生成する手段が実施例（自動周波数制御回路の利用、ディスクの移動を利用したドップラーシフト効果の利用）と異なる場合にも効果を発揮することは言うまでもない。その場合としては、例えば、図１９に示すように、１つの半導体レーザ素子からの光をビーム・スプリッタ２１０１にて２分し、片方を音響光学素子２１０３にて光周波数をシフトさせる方法を適用した場合等が考えられる。
【０１６２】
この場合、ビーム・スプリッタ２１０１にて２分された他方のレーザ光はビーム・スプリッタ２１０２を介して光ディスクに照射される。光ディスクにより反射された光はビーム・スプリッタ２１０２により方向を変換された後に、ビーム・スプリッタ２１０５に入射され、音響光学素子２１０３により光周波数を変換された光はミラー２１０４にて方向を変換された後ビーム・スプリッタ２１０５に入射される。ビーム・スプリッタ２１０５ではこの２つの異なる光周波数を有するレーザ光は波面が一致した状態で合波され、受光素子へと導かれることとなる。
【０１６３】
また、本発明によれば、２つの半導体レーザ素子からのレーザ出力の光周波数差を一定にする為の複雑な帰還回路系を必要とせずに、光コヒーレント検波により生成されるビート信号の振幅変化を介して記録情報を読み出すことが可能となり、多層記録媒体ディスク１６００からも読み出し対象層以外の層での光の散乱の影響を全く受けることなく、記録情報を読み出すことが可能となった。このため、本実施例装置において、記録密度１Ｍ／ｍｍ２の２層ディスクより情報を読み出したところ、ビット誤り率１０−７が達成された。
【図面の簡単な説明】
【図１】本発明の実施例１に係る光記録情報読み出し装置の構成図である。
【図２】実施例１の光記録情報読み出し装置における、偏光ビーム・スプリッタで合波される第１のレーザ光と第２のレーザ光との偏光関係を示す図である。
【図３】実施例１の説明図であり、（ａ）は２つの平面波が微小に傾いて合波された場合の波面の模式図であり、（ｂ）はその場合の合波面上でのν１−ν２信号成分の位相関係を示す図である。
【図４】実施例１の説明図であり、２つの平面波を光コヒーレント検波した場合の波面のずれとビート信号強度の関係を示す図である。
【図５】実施例１の光記録情報読み出し装置における前置増幅器の出力信号波形例を示す図である。
【図６】本発明の実施例２に係る光記録情報読み出し装置の構成図である。
【図７】実施例２の光記録情報読み出し装置において偏光ビーム・スプリッタで合波される第１のレーザ光と第２のレーザ光の偏光関係を示す図である。
【図８】実施例２の光記録情報読み出し装置における前置増幅器の出力波形例を示す図である。
【図９】本発明の実施例３に係る光記録情報読み出し装置の構成図である。
【図１０】実施例３の光記録情報読み出し装置においてビーム・スプリッタで合波される第１のレーザ光と第２のレーザ光の偏光関係を示す図である。
【図１１】実施例３の光記録情報読み出し装置における増幅器の出力波形例を示す図である。
【図１２】本発明の実施例４に係る光記録情報読み出し装置の構成図である。
【図１３】実施例４の光記録情報読み出し装置における２つの記録媒体層からの散乱反射光の状態を示す図である。
【図１４】実施例４の光記録情報読み出し装置における読み出し対象でない記録媒体層において散乱反射されたレーザ光の信号受光用レンズ開口における波面状態の説明図である。
【図１５】実施例４の光記録情報読み出し装置における前置増幅器の出力電圧波形例を示す図である。
【図１６】本発明の実施例５に係る光記録情報読み出し装置の構成図である。
【図１７】実施例５の光記録情報読み出し装置における２つの記録媒体層からの散乱反射光の状態を示す図である。
【図１８】実施例５の光記録情報読み出し装置における前置増幅器の出力電圧波形例を示す図である。
【図１９】本発明の適用が可能であり、異なる２つの光周波数を得るために音響光学素子を用いた光学系の構成を示す図である。
【図２０】従来の光記録情報読み出し装置の構成図である。
【図２１】従来の光記録情報読み出し装置において偏光ビーム・スプリッタで分波される信号光の偏光状態を示す図である。
【符号の説明】
１０１ 半導体レーザ素子
１０２ コリメータ・レンズ
１０３ ビーム整形用プリズム
１０４ ビーム・スプリッタ
１０５ 集光レンズ
１０６ ディスク
１０７ 記録媒体
１０８ ビーム・スプリッタ
１０９ 集光レンズ
１１０ 多分割フォトダイオード
１１１ λ／２板
１１２ 偏光ビーム・スプリッタ
１１３ 半導体レーザ素子
１１４ コリメータ・レンズ
１１５、１１７ 集光レンズ
１１６、１１８ ＰＩＮフォトダイオード
１１９、１２０ 高域通過フィルタ
１２１、１２２ 前置増幅器
１２３、１２４ 狭帯域通過フィルタ
１２５、１２６ 振幅検波回路
１２７ 差動増幅回路
１２８ 比較回路
１２９ 周波数弁別回路
１３０ ＰＩＤ制御回路
１３１ レーザ駆動回路
１３２ プロセッサ回路
１３３ スイッチ回路
６０１ 半導体レーザ素子
６０２ ホログラム板
６０３ コリメータ・レンズ
６０４ ビーム・スプリッタ
６０５ 集光レンズ
６０６ 光ディスク
６０７ 記録媒体
６０８ 多分割フォトダイオード
６０９ 光ピックアップ
６１０ 偏光ビーム・スプリッタ
６１１ 半導体レーザ素子
６１２ コリメータ・レンズ
６１５ コリメータ・レンズ
６１４、６１６ フォトダイオード
６１７ 高帯域通過フィルタ
６１８ 前置増幅器
６１９ 周波数弁別回路
６２０ 自動波長制御回路
６２１ レーザ駆動回路
６２２ プロセッサ回路
６２３ スイッチ回路
６２４ 高帯域通過フィルタ
６２５ 前置増幅器
６２６ 狭帯域通過フィルタ
６２７ 位相検波回路
６２８ 信号再生回路
６２９ 振幅検波回路
６３０ 自動ゲイン制御回路
９０１ 半導体レーザ素子
９０２ コリメータ・レンズ
９０３ ビーム・スプリッタ
９０４ 集光レンズ
９０５ 光ディスク
９０６ 記録媒体
９０７ ビーム・スプリッタ
９０８ 集光レンズ
９０９ フォトダイオード
９１０ 半導体レーザ素子
９１１ コリメータ・レンズ
９１２ 高域通過フィルタ
９１３ 増幅器
９１４ 狭帯域通過フィルタ
９１５ 振幅検波回路
９１６ 電圧比較回路
９１７ 光ファイバ
９１８、９１９ 分岐部
９２０、９２１ フォトダイオード
９２２ 合波部
９２３ 受光回路
９２４ 高域通過フィルタ
９２５ 増幅器
９２６ 周波数弁別回路
９２７、９２９、９３０ 比例制御回路
９２８ レーザ駆動回路
１２００ 光ディスク
１２０１ 第１記録媒体層
１２０２ 第２記録媒体層
１２０３ 分離層
１２０４ ２波長半導体レーザ素子
１２０５ 第１レーザストライプ
１２０６ 第２レーザストライプ
１２０７ レンズアレイ
１２０８ ビーム・スプリッタ
１２０９ レンズ
１２１０ ミラー
１２１１ ビーム・スプリッタ
１２１２ レンズ
１２１３ フォトダイオード
１２１９ ビーム・スプリッタ
１２２０ 多分割フォトダイオード
１２２１、１２２２ レンズ
１２２３ フォトダイオード
１２２４ 高帯域通過フィルタ
１２２５ 前置増幅器
１２２６ 狭帯域通過フィルタ
１２２７ 振幅検波回路
１２２８ 電圧比較器
１２２９ 光ピックアップ
１３０１、１３０２ 平面波レーザ光
１３０３ 信号レーザ光
１３０４、１３０５ レーザ光
１３０６、１３０７ （散乱）球面波
１３０６′、１３０７′ 収束球面波
１６００ 光メモリディスク
１６０１ 第１記録媒体層
１６０２ 第２記録媒体層
１６０３ 半導体レーザ素子
１６０４ レンズ
１６０５ ビーム・スプリッタ
１６０６ レンズ
１６０７ 集光レンズ
１７０１、１７０２ レーザ光
１７０３ 平面波[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical recording information reading method using a reading device for an optical memory (hereinafter referred to as an optical pickup), and particularly to an optical recording information reading device suitable for a high-density optical recording system.
[0002]
[Prior art]
An optical memory system using light for recording and reproducing information has advantages such as large capacity, low cost per bit, and portability of a medium. For this reason, it has been practically used as a read-only optical memory such as a compact disk or a video disk, or a rewritable optical memory represented by a magneto-optical disk. With the progress of the information society in the future, further increase in the capacity and size of this type of optical memory is desired.
[0003]
FIG. 20 shows a conventional example of an optical pickup for a magneto-optical disk. This optical pickup includes a semiconductor laser element 1901 as a light source, a collimator lens 1902, a beam shaping prism 1903, a first beam splitter 1904, an objective lens 1905, a magneto-optical recording medium 1906, a second beam splitter 1907, λ / 2 plate 1908, polarization beam splitter 1909, signal light focusing lenses 1910, 1911, PIN light receiving elements 1912, 1913, light focusing lens 1915 for tracking error / focus error detection, light reception for tracking error / focus error detection A magnet 1917 for applying a magnetic field to the element 1916 and a magneto-optical recording medium (hereinafter, referred to as a disk) 1906 is provided. The operation of the optical pickup will be described below.
[0004]
The laser light emitted obliquely upward from the semiconductor laser element 1901 is converted into parallel light by the collimator lens 1902, and enters the beam shaping prism 1903. The laser light incident on the beam shaping prism 1903 is shaped into a circular beam. Here, the outgoing light from the semiconductor laser element 1901 to the output light from the beam shaping prism 1903 are all linearly polarized light.
[0005]
Subsequently, the linearly polarized laser light is condensed by the objective lens 1905 and applied to the disk 1906 located thereon. Digital information is recorded on the disk 1906 as perpendicular magnetization, and when the laser light is reflected, the polarization plane of the laser light is rotated by the Kerr effect according to the recorded information. At this time, the rotation direction of the polarization plane is opposite to the recorded digital information (digital signal) "1", "0". Therefore, by detecting the rotation direction of the plane of polarization, digital information "1" and "0" can be read.
[0006]
The signal light reflected by the disc 1906 has its optical path changed by 90 ° by the first beam splitter 1904, and subsequently enters the second beam splitter 1907, where it is split into two lights in the horizontal and vertical directions. Is done. Of the split light, the light in the horizontal direction becomes a tracking error / focus error detection beam. After being collected by a condenser lens 1915, it is guided to a light receiving element 1916 and photoelectrically converted. Thereby, a tracking error / focus error signal, which is an electric signal, is obtained.
[0007]
On the other hand, the light branched in the vertical direction becomes light for detecting a recording signal, and the polarization plane is rotated by 45 ° by the λ / 2 plate 1908. FIG. 21 shows the polarization components of the light reflected from the disk 1906 in this state. Here, the reflected light from the disk 1906 overlaps the light 2001 whose polarization plane has been rotated by the disk 1906 and the non-rotational component 2002 caused by reflection from the surface of various optical components existing in the path. It is in a state of being left. Then, the reflected light in the overlapped state is converted into two components orthogonal to each other by a polarizing beam splitter 1909 provided on the emission side of the λ / 2 plate 1908, that is, light of the s-wave component and light of the p-wave component shown in FIG. To separate.
[0008]
The light of the p-wave component travels straight through the polarizing beam splitter 1909 as it is. That is, the vehicle goes straight down vertically. On the other hand, the light of the s-wave component has its optical path changed by 90 ° by the polarizing beam splitter 1909 and travels in the horizontal direction. Subsequently, the light of the p-wave component is condensed by the condenser lens 1911 and then guided to the high-speed PIN light receiving element 1913, where the light is photoelectrically converted. Similarly, the s-wave component light is condensed by the condenser lens 1912, and then guided to the high-speed PIN light receiving element 1912, where it is photoelectrically converted. The electric signals photoelectrically converted by the two high-speed PIN light receiving elements 1913 and 1912 are amplified to a predetermined level by a differential amplifier 1914, and the difference between them is detected. Therefore, the non-rotationally reflected light 2002 shown in FIG. 21 is canceled, and only the signal light 2001 which is a component of the light rotated by the recording magnetization is eventually detected as an electric signal. Then, the recorded information is restored by the detection output of the differential amplifier 1914.
[0009]
In the signal light 2001 shown in FIG. 21, since the p-wave component is larger than the s-wave component, the detection output of the differential amplifier 1914, that is, the p-wave component−s-wave component is an electric signal having a positive value. . On the other hand, when the plane of polarization rotates in the direction opposite to that of the signal light 2001, that is, when information is recorded on the disk 1906 as magnetization in the opposite direction, the s-wave component becomes larger. Becomes a negative electric signal.
[0010]
Although the above optical pickup includes a differential amplifier 1914 of a direct detection type of light intensity as a unit for detecting recorded information, an optical pickup including another detection unit of a direct detection type is also known. In any case, in the conventional optical pickup, not only the magneto-optical disk but also all optical pickups employ a direct detection method of light intensity.
[0011]
[Problems to be solved by the invention]
By the way, optical disks and memories tend to be required to increase in capacity and read out at high speed (high speed access) in the future. In order to meet such demands, it is necessary to realize a higher density of the recording medium and a higher speed rotation of the disk. However, when the density and the rotation speed are increased, the occupied area per bit is further reduced, the reflection intensity is reduced, and the signal pulse width per bit is also reduced. That is, the signal light energy per bit is reduced.
[0012]
Under such circumstances, in an optical pickup having the above-described light intensity detection mechanism, the signal light power level approaches the level of shot noise or thermal noise in the light receiving circuit. For this reason, in the current system using light having a wavelength of 780 nm, the bit error rate becomes 10-5 or more when reading information from a high-density disc of 1 Mbit / mm2 or more, which is practically inconvenient. The reason is that in the direct detection method, the C / N value deteriorates in proportion to the fourth power of the attenuation of the reflected light intensity and inversely proportional to the first power of the increase in the information reading rate.
[0013]
As one method for solving the problems of the conventional optical pickup, the use of a shorter wavelength laser beam as a light source has been actively studied. However, the timing of the practical use of short-wavelength laser elements does not always match the need for large-capacity optical disk memory, and even if short-wavelength light is used, optical disk memory with a larger capacity using that wavelength will not be available. In consideration of the necessity, it is necessary to develop a highly sensitive detection system.
[0014]
Regarding the increase in the recording information density, a method of improving the substantial recording density by using multiple recording media has been studied. As means for selecting a specific recording medium layer, a method using only the focal depth of a lens for condensing and irradiating a laser beam has been used. For this reason, there is a possibility that information from a layer other than the read target may be superimposed on the reflected light, and these become noise, so that at present, it is impossible to read information at a sufficient bit error rate.
[0015]
For the above reasons, the conventional optical pickup cannot cope with further increase in capacity, density, and high-speed access of the optical disk memory in the future.
[0016]
The present invention has been made to solve such problems of the prior art, and has as its object the purpose of reducing the recording area per bit and reducing the intensity of the read signal light, or the bit size. An object of the present invention is to provide an optically recorded information reading method capable of reading recorded information with high accuracy even when the reading rate is increased to 1 Mbps or more, and an optically recorded information reading apparatus used for implementing the method.
[0017]
Another object of the present invention is to provide an optical recording information reading method capable of accurately reading a signal from only a specific recording medium layer without being affected by another layer in an optical system having a multilayer recording medium, and an implementation thereof. An object of the present invention is to provide an optically recorded information reading device used for the above.
[0018]
[Means for Solving the Problems]
The optical recording information reading device of the present invention comprises:
A recording medium having a plurality of recording layers,
First and second laser beams,
One condensing lens for converging and irradiating the first laser light and the second laser light to the same position on any one of the recording layers;
Photoelectric conversion means for performing photoelectric conversion by combining the first laser light and the second laser light reflected from one of the recording layers;
Means for detecting a signal having a beat frequency component of the first laser light and the second laser light based on the output of the photoelectric conversion means, thereby achieving the above object.
[0019]
Further, the optical recording information reading device of the present invention,
A recording medium having a recording layer,
First and second laser beams having different angle components in a direction in which the recording medium moves, and one condenser lens for condensing the first and second laser beams at the same position on the recording medium;
Photoelectric conversion means for performing photoelectric conversion by combining the first laser light and the second laser light reflected by the recording medium;
Frequency detecting means for detecting a beat frequency component of the first laser light and the second laser light based on an output of the photoelectric conversion means;
Rotation speed control means for controlling the rotation speed of the recording medium in accordance with the output of the frequency detection means, whereby the object is achieved.
[0020]
Further, the recording medium may have a plurality of recording layers.
[0021]
According to the above-described method, two lights having two different optical frequencies are optically coherently detected to read out the recorded information recorded on the recording medium. Pseudo signal amplification becomes possible without generating extra noise components which cannot be avoided by amplification. Therefore, according to this method, it is possible to read out recorded information without deteriorating the C / N ratio even when the optical power of the signal light per bit is weak.
[0022]
In addition, if control is performed so that the optical frequency difference between the two lights is constant as described above, it is possible to accurately reproduce recorded information. The reason for this will also be described in detail in the embodiments described later.
[0023]
Furthermore, by irradiating the recording medium with two lights having different optical frequencies in a confocal state, the influence of noise light from layers other than the layer to be read when reading from the recording medium having a multilayer structure is reduced. Can be eliminated.
[0024]
According to the present invention, there is no need for a complicated feedback circuit for keeping the optical frequency difference between the laser outputs from the two semiconductor laser elements constant, and the change in the amplitude of the beat signal generated by optical coherent detection is achieved. Thus, it is possible to read recorded information from the multilayer recording medium disk 1600 without being affected by light scattering in layers other than the read target layer. Therefore, when information is read from the dual-layer disc having a recording density of 1 M / mm2 in the apparatus of the present embodiment, a bit error rate of 10-7 is achieved.
[0025]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described.
(Example 1)
FIG. 1 shows a system configuration of an optically recorded information reading apparatus used for carrying out the method of the present invention. The configuration will be described below together with the operation.
[0026]
The first laser light having the optical frequency ν1, which is linearly polarized light emitted obliquely upward from the first semiconductor laser element 101, is converted into parallel light by the collimator lens 102, and then the beam shaping prism 103 and the beam The light enters the condenser lens 105 via the splitter 104. Subsequently, the light is condensed by the condensing lens 105 and irradiated on the disk 106 located above the light. More specifically, the light is focused and irradiated on a recording medium 107 having a magneto-optical effect provided on the upper surface side of the disk 106. Digital information is recorded on this recording medium 107 as perpendicular magnetization, as described in the above-mentioned prior art. The laser light applied to the recording medium 107 is reflected with the polarization plane rotated by a certain angle due to the Kerr effect in the recording medium 107. The rotation direction of the polarization plane at this time is determined by the direction of the magnetization recorded on the recording medium 107. Therefore, the recorded information is reproduced by detecting the rotation direction of the polarization plane of the reflected laser light.
[0027]
The reflected light from the recording medium 107 is collimated by the condenser lens 105, and then enters the beam splitter 104 again, where it is split into two optical paths. One of the split lights follows the original path, and the other split light becomes horizontal light and enters the beam splitter 108, where it is again converted into horizontal light and vertical light. Branched.
[0028]
The reflected laser light branched in the horizontal direction is guided to the multi-segment photodiode 110 via the condenser lens 109, where it is photoelectrically converted. Thereby, a tracking error / focus error signal, which is an electric signal, is obtained. This electric signal is input to an optical pickup position control circuit (not shown). The position control circuit controls the laser beam based on the input signal so that the laser beam incident on the disk 106 is accurately focused on the recording medium 107 and is tracked without deviating from the track engraved on the disk 106. The entire pickup 100 is controlled. That is, focus servo and tracking servo are performed.
[0029]
Although FIG. 1 shows only the main beam of the laser light, the tracking is actually performed with the irradiation beam to the disk 106 as three beams of the main beam and two sub-beams.
[0030]
On the other hand, the other reflected laser beam split by the beam splitter 108 has its polarization plane rotated by 45 ° by the λ / 2 plate 111, and is then separated by the polarization beam splitter 112 into two components having orthogonal polarization directions. You. The second laser light having the optical frequency ν2 emitted from the second semiconductor laser element 113 is applied to the collimator lens 114 from the surface orthogonal to the surface on which the reflected laser light is incident. After being converted into parallel light, the light is incident. This second laser light is also separated into two components whose polarization directions are orthogonal to each other, similarly to the above-mentioned reflected laser light.
[0031]
Here, the arrangement positions of the second semiconductor laser element 113 and the collimator lens 114 are set so that the wavefronts of the second laser light and the reflected laser light at this time coincide. The necessity of matching the wavefronts of the two lights and the required wavefront matching accuracy will be described later.
[0032]
In addition, the optical frequency ν2 of the second laser light is controlled so that the difference from the optical frequency ν1 of the reflected light, ν1-ν2, is constant. However, the details will be described later.
[0033]
The two polarized light components separated by the polarization beam splitter 112 as described above are guided to PIN photodiodes 116 and 118 via condensing lenses 115 and 117, respectively, and are photoelectrically converted.
[0034]
Next, the relationship between the polarization directions of the reflected laser light and the second laser light incident on the polarization beam splitter 112 and details of the polarization separation by the polarization beam splitter 112 will be described with reference to FIG.
[0035]
Here, in FIG. 2, the horizontal axis indicates the polarization component passing through the polarization beam splitter 112 toward the PIN photodiode 116 (downward), and the vertical axis indicates the polarization component passing through the PIN photodiode 118 (leftward). Is shown. The vectors OR, OA, OB, and OL indicate the following, respectively.
[0036]
Vector OR: reflected beam reflected from the first laser beam irradiated on the disk 106 without interaction with the recording medium 107, that is, without rotation of the polarization direction
Vector OA: Reflected laser light whose polarization direction has been rotated clockwise by the recording medium 107 Vector OB: Reflected laser light whose polarization direction has been rotated in the opposite direction to the vector OA due to the inversion of recorded information
Vector OL: second laser light emitted from second semiconductor laser element 113
However, the vector OA component and the vector OB component are switched by digital information “1” and “0” recorded on the recording medium 107, and both lights do not exist at the same time.
[0037]
As can be seen from FIG. 2, the arrangement is adjusted so that the two polarization separation directions of the polarization beam splitter 112 are basically at 45 ° to the polarization directions of the reflected laser light and the second laser light. That is, the light having the polarization deviated from 45 ° is adjusted so as to be only the component whose polarization direction is rotated by the Kerr effect with the recording medium 107. Accordingly, the light received by the PIN photodiode 116 is a vector component on the horizontal axis in FIG. 2, that is, three vectors of OA (1) (or OB (1)), OR (1), and OL (1). Among them, the laser light having two vector components of OA (1) (or OB (1)) and OR (1) is the reflected light of the first laser light, and the optical frequency is ν1. On the other hand, the vector OL (1) component is due to the second laser beam, and its optical frequency is ν2.
[0038]
Here, when the wavefronts of two laser lights having different optical frequencies are converted into an electric signal by a light receiving element, which is generally called coherent detection, square detection is performed after the two lights are added as an electric field. Will be.
[0039]
Therefore, the optical electric fields E1 and E2 of the two laser beams are
E1 = a1cos (2πν1t + φ1) (1)
E2 = a2cos (2πν2t + φ2) (2)
Then, the current I photoelectrically converted and output by the PIN photodiode 116 is expressed by the following equation (3).
[0040]
I = η (E1 + E2) 2 = η / 2 · {a12 + a22 + 2a1a2cos [2π (ν1−ν2) t + φ1−φ2]} (3)
However,
η: Proportional constant including the photoelectric conversion efficiency of PIN photodiode 116
a1, a2: Electric field amplitude of two laser beams
φ1, φ2: Optical phase of two laser beams
t: Time.
[0041]
Here, the high frequency components to which the PIN photodiode 116 does not substantially respond are the averaged 2ν1 component, 2ν2 component, and ν1 + ν2 component. The first and second terms in the above equation (3) represent the sum of DC electric components proportional to the intensities of the two laser beams as in the case of ordinary direct detection. This indicates that an alternating current component having the optical frequency ν1-ν2 is included. This alternating current component is unique to optical coherent detection, and the traveling directions of the two laser lights match, that is, the wavefronts match, and the frequency difference ν1-ν2 of the two laser lights is the frequency at which the PIN photodiode 116 can respond. It is detected only when it is within the range.
[0042]
Next, a case where two laser beams having the same optical frequency but different optical phases are subjected to optical coherent detection will be described. The optical electric field of the two laser beams
E1 = a1cos (2πν1t + φ1) (4)
E2 = a2cos (2πν1t + φ2) (5)
Then, the current I photoelectrically converted and output by the PIN photodiode 116 is expressed by the following equation (6).
[0043]

As can be seen from the above equation (6), in this case, in addition to the component proportional to the intensity of the two laser beams, a current component appears in relation to the phase difference between the two laser beams. Therefore, if the phase difference relationship between the two laser beams is constant, this component becomes a DC current component and no AC component is output.
[0044]
In order to optically coherently detect two laser lights as described above, it is important that the wavefronts of the two laser lights match. The reason will be described below. Now, assuming that two laser beams 301 and 302 whose wavefronts are inclined by an angle δ are multiplexed as shown in FIG. 3A, in this case, as shown in FIG. There is a portion where the signal phase of the ν1-ν2 component is inverted in a sine wave shape. For this reason, the inverted portions cancel each other, and as a result, the amplitude of the ν1-ν2 frequency component is significantly reduced.
[0045]
However, in actual optical coherent detection, although the light is condensed by a lens and detected by the PIN photodiodes 116 and 118 after the multiplexing, the phase relationship immediately after the multiplexing is similar on the PIN photodiodes 116 and 118 as it is. Therefore, a phenomenon similar to that described in the case of a combined plane where plane waves are combined occurs.
[0046]
FIG. 4 shows a change in the signal component intensity of the frequency ν1-ν2 with respect to the angle formed by the wavefronts of the two plane-wave laser beams when two plane-wave laser beams having a diameter of 5 mm are coherently detected. FIG. 4 shows that when the wavefront is inclined by about 0.01 °, the intensity of the signal component of the frequency ν1−ν2 is attenuated to −10 dB. The coincidence accuracy of the wavefront becomes stricter as the area of the overlapping portion of the two laser beams in the plane wave state is larger.
[0047]
Here, in the recording information reading apparatus of the present embodiment, both the reflected laser light and the second laser light are plane waves and the beam diameter is 3 mm. It was necessary to arrange the semiconductor laser 113 element and the collimator lens 114 in order to achieve a wavefront matching accuracy of .02 ° or less.
[0048]
In the present embodiment, the PIN photodiode 116 coherently detects the three laser beams as described above. However, as can be easily understood from the above consideration, the vector OL ( Detection output of laser light of component 1) and laser light of vector OA (1) (or OB (1)) component, and detection output of laser light of vector OL (1) component and laser light of vector OR (1) component It becomes two of.
[0049]
Hereinafter, the suffix (1) indicates the vector component in one direction of the light vector components in the two directions of polarization separation of the polarization beam splitter 112, and the suffix (2) indicates the vector component in the other direction.
[0050]
Here, the optical electric fields of the laser light of the vector OL (1) component, the vector OA (1) component, and the vector OR (1) component are expressed by the following equations (7), (8), and (9), respectively. You.
EL (1) = aL (1) cos (2πν2t) + φL (1) (7)
EA (1) = aA (1) cos (2πν1t) + φA (1) (8)
ER (1) = aR (1) cos (2πν1t) + φR (1) (9)
Therefore, the AC component of the output current is
IAC (1) = ηaA (1) · aL (1) cos [2π (ν1−ν2) t + φA (1) −φL (1)] + ηaR (1) · aL (1) cos [2π (ν1−ν2) t + φR (1) -φL (1)] (10)
It becomes.
[0051]
The two components in the above equation (10) have the same frequency and represent signals that differ only in phase. This phase difference is equal to the phase difference between the laser light of the vector OA component and the laser light of the vector OR component. It is the surface of the disk 106 or the surface of the recording medium 107 that generates the reflected laser light. The laser light reflected by the disk 106 is not converted into parallel light by the condenser lens 105 but becomes divergent spherical wave light. Therefore, the sensitivity is attenuated before reaching the PIN photodiode 116. In addition, since the wavefront does not coincide with the second laser light that is emitted from the semiconductor laser 113 and is parallel light, optical coherent detection is impossible. This is because even if there is reflected light from the disk 106, it only contributes to the DC component of the PIN photodiode 116 output. Therefore, both the laser light of the vector OA component and the laser light of the vector OR component are reflections from the recording medium 107, and the phases of the two reflected laser lights match. That is, φA (1) = φR (1). Therefore, if this relationship is introduced, the above equation (10) is expressed by the following equation (11).
[0052]
IAC (1) = η (aA (1) · aL (1) + aR (1) · aL (1)) · cos [2π (ν1−ν2) t + φR (1) −φL (1)] (11)
Similarly, also in the PIN photodiode 118, the detection output of the laser beam of the vector OL (2) component, the laser beam of the vector OA (2) (or OB (2)) component, and the output of the vector OL are output as the AC components. It is a detection output of the laser light of the component (2) and the laser light of the vector OR (2) component.
[0053]
Therefore, the AC component of the output current in this case is expressed by the following equation (12).
[0054]
IAC (2) = η (aA (2) · aL (2) + aR (2) · aL (2)) · cos [2π (ν1−ν2) t + φR (2) −φL (2)] (12)
Here, the amplitude of the AC component of the current output from the PIN photodiodes 116 and 118 is compared by comparing the above equations (11) and (12). As shown in FIG. 2, the vector OR and the vector OL form an angle of 45 ° with respect to the polarization separation direction of the polarization beam splitter 112.
[0055]
Therefore,
aR (1) = aR (2) (13)
aL (1) = aL (2) (14)
Holds, and the difference between the amplitude components of both equations is
Δ = η (aA (1) −aA (2)) · aL (1) (15)
It becomes. Here, assuming that the rotation angle of the vector OA from 45 ° is θ, the difference between the amplitude components is expressed by the following equation (16).
[0056]
Δ = ηaAaLsinθ (16)
On the other hand, when the rotation angle of the reflected laser light subjected to the Kerr effect on the recording medium 107 is in the opposite direction, the laser light of the vector OB component becomes the signal light. Is the rotation angle of θ, the difference between the amplitude components is expressed by the following equation (17).
[0057]
Δ = −ηaAaLsinθ (17)
For the above reasons, it is understood that "1" and "0" of the recorded information can be determined based on the sign of the difference between the amplitudes of the AC components of the output current optically coherently detected by the PIN photodiodes 116 and 118.
[0058]
Also, from the above description, the signal components corresponding to the “1” and “0” signals by the conventional direct detection method are ± ηaA2sin θ, whereas “1” in the present embodiment to which the optical coherent detection is applied. , "0" are ± ηaAaLsinθ.
[0059]
Here, aL is a value that can be arbitrarily set based on the light intensity of the laser light generated by the second semiconductor laser element 113 without depending on the light intensity of the first laser light. Therefore, even when the aA per bit is reduced due to the high density and the high speed reading of the disk 106, the signal component can be kept large as aA · aL. That is, the amplitude aA of the reflected laser light including the signal component is artificially amplified by the amplitude aL of the second laser light. In the amplification in such a light region, there is no amplification of a noise component associated with amplification in an electronic circuit. Therefore, since only the signal component can be purely amplified, the C / N value can be greatly improved.
[0060]
Next, actual signal detection will be described in detail with reference to the drawings. As described above, after the two orthogonal polarization components are separately optically coherently detected, the DC signals of the electrical signals from the PIN photodiodes 116 and 118 are removed by the high-pass filters 119 and 120. Subsequently, the electric signal from which the DC component has been removed is input to preamplifiers 121 and 122, where an AC component important as a signal is amplified. Subsequently, the narrow band-pass filters 123 and 124 whose center pass frequencies are set to ν1-ν2 remove noise components from the AC components, and the ν1-ν2 components (hereinafter referred to as beats) shown in the above equations (11) and (12). Components). In this embodiment, since the optical frequency difference between the two laser beams is set to 1 GHz, the beat signal frequency is also 1 GHz.
[0061]
FIG. 5 shows output waveforms of the preamplifiers 121 and 122. As can be seen from the figure, the amplitudes of the output beat signals of the preamplifiers 121 and 122 are mutually changed corresponding to the recording signals “1” and “0”. In addition, from the drawing, in this embodiment, when the recording signal is “1”, the amplitude of the beat signal from the preamplifier 121 increases, and when the recording signal is “0”, the beat signal from the preamplifier 122 increases. Are increasing in amplitude. This signal form represents a signal transmission method based on amplitude shift keying (ASK) modulation using a beat signal frequency as a carrier. However, it is a differential ASK with two channels.
[0062]
Therefore, in order to demodulate the signal, it is necessary to set the beat signal frequency to a frequency sufficiently higher than the digital information information read rate R (R = 0.7 Mbps in this embodiment). However, in practice, it is necessary to set the digital information read rate R at least twice or more, preferably at least 10 times.
[0063]
An advantage of the ASK modulation method for converting an electric signal in this way is that there is no problem in signal reproduction even when the frequency and phase of a carrier wave (beat signal in this case) are relatively unstable. . Unlike the case of ordinary wireless communication, in the case of this embodiment, the semiconductor laser itself has a spectral line width of 1 MHz, which is caused by fluctuation of ν1−ν2 and rotation of the disk 106 having digital information. Usually, when the values of φA (1) and φA (2) change due to the disk vibration, the frequency and phase of the beat signal become unstable. Therefore, it can be said that a method of reproducing digital information from a signal once converted to the ASK modulation method as in the present embodiment is suitable.
[0064]
As described above, the amplitude values of the beat components that have passed through the narrow band pass filters 123 and 124 are detected as DC voltages by asynchronous type amplitude detectors 125 and 126 such as full-wave rectifiers. Subsequently, the amplitude value of the beat component converted into the DC voltage component is differentially amplified by the differential amplifier circuit 127, and finally, the sign of the output of the differential amplifier circuit 127 is determined by the signal level comparison circuit 128. Based on the determination result, the sign of the signal component corresponding to the recording information “1” or “0” shown in the above equation (16) or (17) can be determined, and the recording information can be read.
[0065]
Here, in order to secure a sufficient C / N value for signal reading, it is necessary to amplify signal components by optical coherent detection and to suppress noise levels. For this reason, in this embodiment, the detection system is configured with emphasis on the following two points.
[0066]
As the preamplifiers 121 and 122, amplifiers having low noise figure characteristics, for example, GaAs high electron transfer transistors are used.
[0067]
The pass band widths of the narrow band pass filters 123 and 124 are set as narrow as possible.
[0068]
The pass bandwidth is determined by the sum of the short-term / long-term fluctuation width of the beat signal frequency and the information reading speed. In this embodiment, the fluctuation width of the beat signal frequency is determined by the spectral line width of the laser light generated from both the semiconductor laser elements 101 and 113, and the information reading speed was 0.7 Mbps. The pass band width of the filters 123 and 124 was 3 MHz.
[0069]
Next, a description will be given of a means (automatic frequency control: AFC) for keeping the optical frequency difference (beat signal frequency) ν1-ν2 between the first laser light and the second laser light from the two semiconductor laser elements 101 and 113 constant. Optical coherent detection is performed by a PIN photodiode 118, and a beat signal component extracted from the electrical output by a high-band filter 120 and a preamplifier 122 is input to a narrow-band filter 124, and is input to a frequency discriminating circuit 129 to input an AC component. The frequency is detected as a voltage value. As the frequency discrimination circuit 129, asynchronous detection that has a wide detection band and does not require time for synchronization is suitable, and a delay detection circuit is used here.
[0070]
Subsequently, a control signal is generated by the PID control circuit 130 so that the output of the frequency discrimination circuit 129 matches a constant set voltage (not shown) input from the outside, and the laser drive circuit 131 generates the first control signal in accordance with the control signal. The driving conditions of the semiconductor laser element 101 and the second semiconductor laser element 113 are changed, that is, controlled. The laser drive circuit 131 simultaneously controls the optical output of both the semiconductor lasers 101 and 113 (automatic power control: APC) in accordance with a control signal from a separately provided automatic power control circuit system (not shown). In this embodiment, a tunable laser device is used as a semiconductor laser device, and the light intensity and the optical frequency of the laser light can be controlled by individual control terminals provided respectively. By providing such a feedback circuit system, it was confirmed that the optical frequency difference could be kept constant and the beat signal frequency could be kept constant.
[0071]
Next, the means for setting the optical frequency difference in the initial state will be described. In the initial state, since both semiconductor laser elements 101 and 113 emit laser light having an irrelevant optical frequency, the control by the feedback circuit cannot be executed in the initial state. Only when the disk 106 is set and the focus servo is applied, the reflected light of the first laser light from the disk 106 reaches the PIN photodiode 118 and the optical coherent detection can be performed. In this state, the optical frequencies of the two laser beams are usually greatly different, and the difference is a high frequency. Therefore, any one of the PIN photodiode 118, the high-pass filter 120, the preamplifier 122, and the frequency discrimination circuit 129 ( Or a plurality of response bands, the output voltage of the frequency discrimination circuit 129 is 0V.
[0072]
In this state, first, the output voltage of the frequency discriminating circuit 129 is monitored while the driving condition of the first semiconductor laser element 101 is kept constant by the laser driving circuit 131, and the optical frequency of the second semiconductor laser element 113 is changed. Scan in a certain direction gradually. This series of operations is controlled by the processor circuit 132 that monitors the output voltage of the frequency discrimination circuit 129. At this point, the switch circuit 133 that switches between the PID control circuit 130 and the laser drive circuit 131 is in the OFF state, and the PID control circuit 130 is disconnected from the laser drive circuit 131. Therefore, the feedback circuit does not operate in this state.
[0073]
Further, when the optical frequency of the second laser light emitted from the second semiconductor laser element 113 is continuously scanned, the optical frequency of the second laser light approaches the optical frequency of the first laser light at a certain point in time. At this time, the optical frequency difference enters all response speed bands of the PIN photodiode 118, the high-pass filter 120, the preamplifier 122, and the frequency discrimination circuit 129, and a positive output voltage is output to the output terminal of the frequency discrimination circuit 129. Is output. In addition, since the step of scanning the optical frequency of the second laser beam needs to be smaller than the total response band of the circuit systems 118, 120, 122, and 129, the response band is set to 1 MHz in this embodiment.
[0074]
Then, when the processor circuit 132 determines that the output voltage from the frequency discriminating circuit 129 has reached a certain range that is sufficient to allow the feedback circuit to function, the processor circuit 132 turns on the switch circuit 133 and turns on the PID control. The circuit 130 is operated. Thereby, the control state is switched to the feedback circuit control.
[0075]
With the circuit configuration as described above, the reflected laser light containing the signal information from the recording medium 107 (the vector OA component of the present embodiment, Even if the power per bit (corresponding to the vector OB component) becomes small and the C / N value necessary for realizing a practical bit error rate cannot be secured by the direct detection method in the reproduction, According to the optical coherent detection, a sufficient C / N value can be secured. Therefore, it is possible to sufficiently cope with high density and high speed access of the disk memory. Although the practical bit error rate varies depending on the importance and the redundancy of the recording data, it is generally 10-4 to 10-5.
[0076]
The recorded information was read using a disk having a recording density of 1 Mbit / mm 2 at a bit read speed of 1 using the apparatus of the present example. As a result, the bit error rate could be set to 10 −6. In this method, a means for generating a second laser light is provided inside the apparatus, and optical coherent detection is performed which enables amplification of only a pure signal component without amplification of a noise component by the electric field amplitude of the second laser light. And the result of reading / reproducing recorded information as an ASK modulated signal having a beat signal frequency as a subcarrier.
(Example 2)
Another embodiment of the present invention will be described.
[0077]
FIG. 6 shows a configuration diagram of the optical recording information reading device of the present embodiment. The first laser light emitted from the first semiconductor laser element 601 passes through the hologram plate 602 and is converted into a plane wave by the collimator lens 603. The optical frequency of the laser light from the first semiconductor laser element 601 was ν1. The first laser light converted into a plane wave passes through the beam splitter 604, is condensed by a condenser lens 605, and is irradiated on a recording medium 607 formed on an optical disk 606. The irradiated first laser beam is rotated in the opposite direction in accordance with the Kerr effect as in the first embodiment, so that the polarization plane of the reflected laser beam corresponds to 1, 0 of the recorded digital information.
[0078]
The reflected laser light reflected by the recording medium 607 is converted into a plane wave again by the condenser lens 605 and is split into two by the beam splitter 604. The reflected laser light that has traveled straight through the beam splitter 604 is condensed by a collimator lens 603 and diffracted by a hologram formed on a hologram plate 602. This diffracted light is applied to a multi-segmented photodiode 608 disposed near the first semiconductor laser element 601 and used to detect the focus state and tracking state of the irradiated laser light on the optical disk 606. The output from the multi-segment photodiode 608 is input to a pickup position control circuit (not shown), and the pickup position control circuit moves the optical pickup 609 composed of the entire optical system shown in FIG. So that
[0079]
Of the laser light split into two by the beam splitter 604, the reflected laser light whose optical path is bent at a right angle is separated by the polarizing beam splitter 610 into two components whose polarization directions are orthogonal to each other. FIG. 7 shows the situation at this time. As in the previous embodiment, the vector OA (or vector OB) is a reflected laser beam containing a signal component whose polarization direction has been rotated by the Kerr effect, and the vector OR has no Kerr effect, that is, The reflected laser light is reflected without interaction.
[0080]
Along with these two reflected laser beams, the second laser beam having the optical frequency ν2 emitted from the second semiconductor laser element 611 is simultaneously incident on the polarization beam splitter 610. The second laser light is converted into a plane wave by the collimator lens 612 and then enters the polarization beam splitter 610. The polarization plane of the second laser light is set at an angle of 45 ° with respect to the polarization separation direction of the polarization beam splitter 610 as shown by a vector OL in FIG. At this time, the polarization plane was controlled by arranging the second semiconductor laser element 611 so as to be rotated by 45 ° about the optical axis. In addition, both the reflected laser light and the second laser light are combined by the polarization beam splitter 610 in the state of a plane wave, and the light wavefronts of both plane waves at this time have been described with reference to FIGS. As described above, since it is necessary to combine the beams with an accuracy of 0.03 ° or less (the beam diameter is 2 mm), the second semiconductor laser element 611 and the collimator lens 615 are fixed with a mounting accuracy of 0.03 °.
[0081]
Light groups of the respective polarization components separated by the polarization beam splitter 610 are incident on the photodiodes 614 and 616 by the condenser lenses 613 and 615, and are subjected to optical coherent detection.
[0082]
7, the horizontal axis represents a light group traveling in the direction of the photodiode 616, and the vertical axis represents a light group traveling in the direction of the photodiode 614. In the photodiode 616, the light represented by the vector OR and the vector OA (1) (or the vector OB (1)), which is the reflected light of the first laser light having the optical frequency ν1, and the vector OL having the optical frequency ν2 The second laser light represented by (1) is subjected to optical coherent detection. An AC component (beat signal) is generated by the vector OA (1) (or the vector OB (1) and the vector OL, and the vector OR and the vector OL. Here, based on the recording information 1, 0, It is determined whether the light of the vector OA is reflected or the light of the vector OB is reflected. In each case, the light having the same vector component ( Vector OA (1) = vector OB (1)) In other words, the beat signal component included in the output current from the photodiode 616 is:
IAC (1) = η (aA (1) + aR) aL (1) cos {2π (ν1−ν2) t + φA (1) −φL (1)} (18)
And the amplitude is constant without depending on 1 or 0 of the recording information. However, for the same reason as in the previous embodiment, the optical phases of the vector OA (or OB) component light and the vector OR component light are assumed to match.
[0083]
On the other hand, on the photodiode 614 side, a vector OA (2) (or OB (2)) component light having a first optical frequency ν1 and a second OL (2) component represented by a vector OL (2) component having an optical frequency ν2. Two light beams including the laser beam are subjected to optical coherent detection. The beat signal component of the output current of the photodiode 614 in this case is
IAC (2) = ηaA (2) aL (2) cos {2π (ν1-ν2) t + φA (2) −φL (2)} (19)
It becomes. In this case, there is a relationship of vector−OA (2) = vector OB (2), and the phase of the beat signal output from the photodiode 614 is inverted by 180 ° every time recording information is switched between 1 and 0. . This corresponds to two-phase shift keying (2PSK) in which 1,0 information is signal-modulated in accordance with two phase states as shown in FIG.
[0084]
As described above, the photodiode 614 outputs a beat signal modulated by 2SPK corresponding to the recording information being inverted by 1 or 0, and the photodiode 616 outputs a constant amplitude independent of the recording information of 1 or 0. The resulting beat signal is output. Therefore, the output of the photodiode 614 is used for signal reproduction, and the output of the photodiode 616 is used for feedback control for keeping the optical frequency difference between the first laser light and the second laser light constant. The details are described below.
[0085]
First, feedback control for keeping the optical frequency difference constant will be described. From the output of the photodiode 616, first, a DC component is removed by a high band pass filter 617, and then only a beat signal component is amplified by a variable gain preamplifier 618. The output of the preamplifier 618 is divided into two, one of which is input to the frequency discriminating circuit 619. The frequency discrimination circuit 619 generates a DC voltage corresponding to the beat signal frequency, and outputs a control signal such that the discrimination voltage matches a set voltage separately input by the automatic wavelength control circuit 620. By changing the driving conditions of the two lasers in the laser driving circuit 621 according to this control signal, the optical frequency difference between the two laser lights is kept constant.
[0086]
The control of the optical frequency difference in the initial state is realized by the processor circuit 622 and the switch circuit 623 as in the first embodiment. However, in this embodiment, a wavelength-stabilized laser device (distributed feedback laser device) having only a single terminal is applied as the first and second semiconductor laser devices 601 and 611, so that the optical frequency of each laser beam is increased. The temperature of the semiconductor laser elements 601 and 611 was changed to change the temperature. For the temperature change, heating by a resistor provided in the vicinity of the semiconductor laser elements 601 and 611 was used. In this embodiment, it is necessary to control the two semiconductor laser elements 601 and 611, and while keeping the optical frequency of one of the semiconductor laser elements constant, the other optical frequency is scanned in a fixed direction to obtain a desired optical frequency. Realized the difference. If a desired optical frequency difference is not realized by optical frequency scanning of only one of the semiconductor laser elements, while maintaining the driving conditions of the semiconductor laser element that first performed optical frequency scanning in the scanning completed state, the other is performed. The optical frequency of the semiconductor laser element was scanned in the direction opposite to the optical frequency scanning performed first, and control was performed so that a desired optical frequency difference (beat signal frequency) was obtained. A similar technique for controlling the optical frequency by such a temperature is disclosed in Japanese Patent Application Laid-Open Nos. 4-134937-8, to which reference should be made.
[0087]
Next, reading / reproducing of recorded information will be described. The output from the photodiode 614 is supplied to a high-bandpass filter 624 where the DC component is removed, and only the beat signal component is amplified by a variable gain preamplifier 625. The amplified beat signal is supplied to the narrow band pass filter 626, and noise other than the beat signal frequency component is removed. The center frequency of the pass band of the narrow band pass filter 626 is matched with the beat signal frequency ν1-ν2, and is set to 2 GHz in this embodiment. That is, as described in the first embodiment, the information read rate, here, the beat signal frequency needs to be sufficiently higher than 5 Mbps.
[0088]
Further, the pass band width of the narrow band pass filter 626 needs to be selected to be equal to the sum of the frequency fluctuation width of the beat signal and the transfer rate of the read information, and is set to 8 MHz. This is because the spectral line width of the output laser light from the semiconductor laser elements 601 and 611 is 0.4 MHz. The information reading rate was 5 Mbps. The output signal waveform of the narrow band pass filter 626 is a 2PSK modulation waveform using a beat signal as a carrier as shown in FIG.
[0089]
The beat signal that has passed through the narrow band pass filter 626 is output as a positive / negative voltage by the phase detection circuit 627 with the phase state inverted by 180 ° each time. As the phase detection circuit 627 in this case, a delay detection circuit of an asynchronous detection system is applied. As this circuit, a circuit that has been conventionally applied as a circuit for wireless communication can be used. For details, see "Digital Communication Circuit" edited by Kuroyanagi (Sangyo Tosho, 1990). Please refer to Chapter 5, “Wireless Transmission Circuits” and so on.
[0090]
By the way, the actual optical disk 606 is rotating at a high speed, and the light irradiation surface of the recording medium 607 vibrates, so that φA (2) in the expression (19) changes according to the vibration. Generally, since the relative vibration amplitude of the optical disk surface with respect to the pickup is several μm and is larger than the wavelength of light (785 nm), φA (2) changes over 1000 ° or more. If the speed at which the phase of the beat signal changes by 180 ° due to the vibration of the disk 606 is equal to or higher than the information reading rate (here, 5 Mbps), the signal cannot be reproduced. That is, the phase change due to the vibration of the disk 606 cannot be distinguished from the phase change due to the 1/0 inversion of the recorded information. In the present embodiment, the following measures are taken to prevent such a situation.
[0091]
A lens having a large NA, that is, a lens having a small depth of focus is used as the condenser lens 605 so that it is possible to detect a focus error of about 1/4 of the wavelength of the first laser light, and focus control of the optical pickup 609. The focus servo system has been enhanced by applying a high-speed response type actuator as an actuator for the purpose, and the relative vibration frequency between the disk and the pickup has been reduced (2 MHz or less).
[0092]
In the phase detection circuit 627, the output of the ordinary PLL detection circuit is passed through a narrow band-pass filter (not shown) whose pass band is in the range of (5 MHz) corresponding to the information reading rate to twice (10 MHz). A measure was taken to detect only the voltage conversion corresponding to the phase change existing in.
[0093]
Thus, a signal whose voltage changes in response to the inversion of the read information by 1 or 0 is output as the output of the phase detection circuit 627. Finally, based on this signal, the signal reproducing circuit 628 reproduced the recorded signal.
[0094]
In the present embodiment, the laser output intensity of the second semiconductor laser element 611 and the amplification factors of the preamplifiers 618 and 625 are determined by a feedback circuit so that the amplitude of the input signal to the phase detection circuit 627 is constant. Controlled. In this case, the amplification by the electronic circuits (the preamplifiers 618 and 625) amplifies various noises generated in the peripheral circuits of the photodiodes 614 and 616 and the high-pass filters 617 and 624. Excessive noise is generated in the amplifiers 618 and 625. On the other hand, in optical coherent detection, the signal amplitude can be pseudo-amplified without increasing noise. Therefore, it is desirable to maximize the optical coherent detection function.
[0095]
The specific configuration in the present embodiment is as follows. The output of the preamplifier 618 is input to the frequency discrimination circuit 619 and is also applied to the amplitude detection circuit 629. The amplitude detection circuit 629 detects the beat signal amplitude as a DC voltage, and inputs this voltage value to the automatic gain control circuit 630. I do. Automatic gain control circuit 630 compares the input voltage value with a set voltage separately provided from the outside, and outputs a control signal so that they match. This control signal is input to the laser drive circuit 621 and the preamplifiers 618 and 625, and the second laser light intensity of the semiconductor laser element 611 and the gains of the two preamplifiers 618 and 625 are changed accordingly. And
[0096]
With this feedback circuit system, the output beat signal amplitude of the preamplifier 618 becomes constant, and equivalently, the output beat signal amplification of the preamplifier 625 also becomes constant. As a guide for control in this case, it is important to give priority to amplification of the pseudo beat signal amplitude by optical coherent detection by increasing the output laser light intensity of the semiconductor laser element 611 as much as possible. The amplification by the preamplifiers 618 and 625 is used only when the laser light output intensity of the semiconductor laser element 611 cannot be increased due to a structural output limit or a limit on the light input intensity of the photodiodes 614 and 616. did.
[0097]
By executing the optical coherent detection in this manner, the beat signal amplitude from the photodiode 614 is pseudo-amplified by the electric field amplitude of the second laser light as shown in Expression (19). Thus, a high C / N value can be obtained even when the signal light power per bit is reduced. In addition, in addition to the pseudo amplification by the optical coherent detection, in the present embodiment, as shown in FIG. 8, the beat signal subjected to the optical coherent detection is a 2PSK modulation signal having a beat signal frequency as a carrier frequency. Therefore, there is an advantage that a signal can be reproduced with a relatively high bit error rate even when the value of C / N is considerably low. For example, when the C / N value required to realize a bit error rate of 10-4 is compared between the ASK method used in the first embodiment and 2PSK in the present embodiment,
15dB for ASK (amplitude detection)
9.4 dB for 2PSK (delay detection)
It becomes. More specifically, the above-mentioned “digital communication circuit” p. See 235.
[0098]
As can be seen from this, even a signal having a C / N value 5.6 dB lower than that of the ASK system can reproduce a signal at a bit error rate of 10-4. However, it is necessary that the frequency and phase of the beat signal serving as a carrier are stable as compared with the ASK modulation method. In this embodiment, the semiconductor laser elements 601 and 611 are provided with a narrow spectral line width (0.4 MHz). It is necessary to take measures such as applying an element having the above and suppressing the fluctuation component of φA (2) as described above. Actually, when information was read from the disk having a recording density of 1 M / mm 2 by the apparatus of this embodiment, a bit error rate of 10 −6 was realized despite the fact that the information reading speed was increased to 5 Mbps.
(Example 3)
Another embodiment of the present invention will be described.
[0099]
In the first and second embodiments described above, the recording medium using the magneto-optical effect is described. In the third embodiment, the recording medium using the light scattering effect (the optical information reading method in a normal compact disc and Use the same).
[0100]
FIG. 9 shows a configuration diagram of the optical recording information reading device of the third embodiment. The first laser light having an optical frequency: ν1 emitted from the semiconductor laser element 901 is converted into a plane wave by the collimator lens 902, passes through the beam splitter 903, and is irradiated on the optical disc 905 by the condenser lens 904. You. A recording medium 906 on which information is recorded as the presence or absence of pits is disposed on the optical disk 905, and the recording medium 906 is irradiated with the first laser beam at a point where the spot size becomes the minimum. The first laser light (hereinafter, referred to as signal light) reflected by the recording medium 906 is again converted into a plane wave by the condenser lens 904, and the beam path is converted to a right angle by the beam splitter 903, and then the beam The light enters the splitter 907.
[0101]
On the other hand, the second laser light having the optical frequency ν2 emitted from the second semiconductor laser element 910 is converted into a plane wave by the collimator lens 911 and then combined with the signal light incident on the beam splitter 907. Waved. The signal light multiplexed by the beam splitter 907 and the second laser light are both plane waves, and the positions of the second semiconductor laser element 910 and the collimator lens 911 were adjusted such that the wavefronts coincided with each other. In this case, since the laser beam diameter was 1 mm, the wavefront matching accuracy required for multiplexing was about 0.05 °.
[0102]
The two laser lights combined by the beam splitter 907 are condensed by the condenser lens 908 and reach the photodiode 909. From the output of the photodiode 909, the DC component is removed by the high-pass filter 912, and then only the beat signal is amplified by the amplifier 913. FIG. 10 shows the relationship between the polarization directions of both lights after the multiplexing. The second laser light is indicated by a vector OL, and the signal light is indicated by a vector OR (or OR ').
[0103]
The information is recorded as the presence or absence of a pit on the recording medium 906. For this reason, when there is no pit at the position where the first laser light is focused and irradiated on the disk 905, that is, when the surface of the recording medium 906 is a mirror surface, most of the irradiated laser light returns along the same optical path. The light is converted into a plane wave by the lens 904 and reaches the photodiode 909. On the other hand, when there is a pit, more than half of the irradiation laser light is scattered at an angle larger than the effective NA of the lens 904, so that the component reaching the photodiode 909 as a plane wave decreases. Therefore, in FIG. 10, the vector OR indicates the signal light when there is no pit, and the vector OR 'indicates the signal light when there is a pit.
[0104]
In this embodiment, the second laser light and the first laser light are arranged so that their polarization planes coincide with each other. A beat signal component having a frequency of ν1-ν2 output when optical coherent detection is performed by the photodiode 909 under such a condition,
IAC = ηaRaLcos {2π (ν1−ν2) t + φR−φL} (20)
It becomes. Here, the parameter that changes according to 1 or 0 of the recording information is aR, and changes between aR and aR ′.
[0105]
FIG. 11 shows an example of the output waveform of the amplifier 913. As can be understood from this figure, the output of the amplifier 913 is in the ASK modulation system in which the amplitude of the beat signal changes in accordance with 1 and 0 of the recording information.
[0106]
After removing unnecessary noise components other than the beat signal frequency through the narrow band pass filter 914, the amplitude of the ASK modulation signal is detected by the amplitude detection circuit 915. As the amplitude detection circuit 915, a normal envelope detection circuit was used. As a result, a voltage corresponding to the signal amplitude shown in FIG. 11 is output. Finally, this output is input to the voltage comparison circuit 916, and the signal is reproduced.
[0107]
Also in the present embodiment, as can be seen from Expression (20), the signal amplitude related to the recorded information is in a form that is pseudo-amplified by the second laser light, and thus is the same as in Embodiments 1 and 2. For this reason, a higher C / N value can be obtained as compared with normal direct detection.
[0108]
Next, a mechanism for maintaining the optical frequency difference between the two semiconductor laser elements 901 and 910 in the third embodiment will be described. A single mode optical fiber 917 is coupled to each of the first and second semiconductor laser elements 901 and 910. Light emitted from the first semiconductor laser element 901 to the back surface is incident on an optical fiber 917, is split into two by a branching portion 918, and is incident on a photodiode 920 for output monitoring. As a result, the output laser intensity of the first semiconductor laser element 901 is detected by the photodiode 920. On the other hand, the other light split into two is guided to a multiplexing unit 922 including a directional coupler.
[0109]
Similarly, the light emitted from the second semiconductor laser element 910 to the back surface is also incident on the optical fiber 917 and is divided into two by the branching portion 919. Then, one is guided to the photodiode 921 and the other is guided to the multiplexing portion 922. Waved.
[0110]
In the multiplexing section 922, the first laser light and the second laser light are mixed at a ratio of 1: 1 and input to a balanced light receiving circuit 923 including two photodiodes and the like, and subjected to optical coherent detection. You. The output from the balanced light receiving circuit 923 is filtered by a high-pass filter 924 to remove the DC component, and is appropriately amplified by an amplifier 925, and then frequency-discriminated by a frequency discrimination circuit 926. This frequency discrimination output is input to the proportional (P) control circuit 927. The P control circuit 927 compares the frequency discrimination output with a DC setting voltage separately supplied from the outside, and outputs a control signal so that the two coincide. The laser driving circuit 928 controls the driving conditions of the first and second semiconductor laser elements 901 and 910 according to the control signal. As a result, the sadness frequency difference ν1-ν2 between them is kept constant. Since the control method and the settings in the initial state are the same as those described in the first embodiment, detailed description thereof will be omitted.
[0111]
In Embodiments 1 and 2 described above, the reflection of the first laser light on the disk was essential for controlling the optical frequency difference between the two laser lights. However, in this embodiment, such a configuration is adopted. Even when the optical disk 905 is not mounted on the device, it is possible to execute the control of the optical frequency difference. Therefore, in the configuration of the above-described embodiment, the initial operation of the optical frequency control that has to be executed every time the disk is replaced can be eliminated. However, it is necessary only when the power is turned on.
[0112]
The automatic control of the output laser light intensity of the semiconductor laser elements 901 and 910 was performed by comparing and controlling the outputs from the photodiodes 920 and 921 and the set voltage by the P control circuits 929 and 930.
[0113]
When information was actually read at a data read speed of 3 Mbps from a disk recorded at a density of 2 M / mm2 using the optically recorded information readout device having the configuration shown in FIG. 9, a bit error rate of 10-7 was achieved. The effect of applying the present invention was confirmed.
(Example 4)
Another embodiment of the present invention will be described.
[0114]
In the first to third embodiments, an optical disk of a single-layer recording medium is used in order to prevent the bit error rate at the time of information reading from being reduced due to the high density and high speed reading of recorded information, which is a problem of the prior art. A method for ensuring a low bit error rate has been described.
[0115]
By the way, as a countermeasure for preventing the bit error rate from lowering at the time of reading information due to the increase in density, a method of reading information from a disk having a multi-layered recording medium can be considered in addition to the above method. That is, this is a method in which the information recording area per bit is fixed and the recording medium is multiplexed in the thickness direction. Such a method of reading information from a multiplex recording medium disk has been studied in the past.However, to select which layer of the multilayer recording medium to read information from, a laser beam focused by a lens must be selected. Since the depth of focus is used, the state of the other layer superimposes noise on the reflected laser light having information from the layer to be read, making it difficult to obtain a practically sufficient bit error rate.
[0116]
The present invention can also be applied to the information reading of the above-described multi-layer recording medium optical disk, and can exert its effects.
[0117]
Hereinafter, an optical recording information reading apparatus according to the present embodiment will be described with reference to FIG. FIG. 12 is a configuration diagram of an optically recorded information reading device of the present embodiment when applied to a multilayer optical disc device having two recording media. The optical disc 1200 has a state in which a first recording medium layer 1201 and a second recording medium layer 1202 are arranged in parallel with a transparent separation layer 1203 interposed therebetween. Digital information is recorded on the recording media 1201 and 1202 as two states of scattering and non-scattering.
[0118]
The first and second laser beams emitted from the first laser stripe 1205 and the second laser stripe 1206 of the two-wavelength semiconductor laser device 1204 are converted into plane waves by the lens array 1207, respectively. The interval between the two laser stripes 1205 and 1206 in the two-wavelength semiconductor laser device 1204 and the interval between the two optical axes of the lens array 1207 are made to match. Here, the thickness was 2000 μm. Further, the beam emission angle of the semiconductor laser element 1204 and the focal length of the lens of the lens array 1207 were designed so that the two laser light beams converted into the plane waves as shown in the drawing do not spatially overlap.
[0119]
Here, the optical frequencies of the first laser light and the second laser light are ν1 and ν2, respectively, and the difference ν1-ν2 is kept constant. Further, the first laser light and the second laser light each have a linear polarization state, and the polarization directions of the two laser lights are the same.
[0120]
Under such conditions, the two laser beams converted into the plane waves are each split into two by the beam splitter 1208. Both laser beams that have passed straight through the beam splitter 1208 are condensed by the lens 1209 at the same focal position F. The two laser beams focused on the focal position F share the same space only near the focal position F. More specifically, as shown in FIG. 13 which is an enlarged view of the vicinity of the recording medium of the optical disc 1200, two plane wave laser beams 1301 and 1302 are condensed at the same focal position F by a lens 1209.
[0121]
When the light scatterer exists at the focal position F (corresponding to the case where the recording information is “1”), the first laser light and the second light laser light are also scattered by the light scatterer, and the focal position F is centered. And is reflected as a spherical wave having the same wavefront. The first and second spherical wave laser lights having the same wavefront are converted into a plane wave 1303 (hereinafter also referred to as signal laser light) having the same wavefront by the lens 1209, and enter the beam splitter 1208. . The signal laser beam 1303 whose traveling direction has been changed at right angles by the beam splitter 1208 is split into two by a beam splitter 1219, and one of the two is passed through a lens 1221 to a multi-segment photodiode 1220 for position control of an optical pickup 1229. Incident. On the other hand, the other signal laser light 1303 split by the beam splitter 1219 passes through the lens 1222 and enters the photodiode 1223. Since the first and second laser beams scattered and reflected at the focal position F of the lens 1209 are converted into the plane waves 1303 having the same wavefront by the lens 1209 as described above, the first and second laser beams are reflected on the light receiving surface of the photodiode 1223. In this case, the wavefronts of both laser beams coincide with each other, and optical coherent detection can be performed. Therefore, a beat signal having a frequency corresponding to the optical frequency difference between the two laser beams is generated.
[0122]
On the other hand, when the light scatterer does not exist at the focal position F, both laser beams pass through the first recording medium layer 1201 or are reflected at the boundary. The laser beams 1304 and 1305 transmitted through the first recording medium layer 1201 pass through the separation layer 1203 and reach the second recording medium layer 1202. In the second recording medium layer 1202, the light scatterers are also distributed for the information 1 and 0, so that each laser beam is scattered.
[0123]
For example, assume that the first laser light is scattered at point Q and the second laser light is scattered at point P. Then, the first laser light scattered at the point P and the second laser light scattered at the point Q are reflected as spherical waves 1306 and 1307 around the scattering points P and Q, respectively. Since the points P and Q are different from the focal position F of the lens 1209, the scattered spherical waves 1306 and 1307 are not converted into plane waves by the lens 1209. In this case, since the second recording medium layer 1202 is located farther than the focal position F of the lens 1209, it is converted into convergent spherical waves 1306 'and 1307'. Furthermore, since the first and second laser beams are arranged in an optical system so as to share the same space only at the focal position F of the lens 1209, the point P and the point Q may be the same point. I can't get it.
[0124]
Therefore, the wavefronts of the spherical waves 1306 and 1307 scattered and reflected at the points P and Q are always different, and it is impossible to perform optical coherent detection by the photodiode 1223. In this case, the combined detection of two laser lights having different wavefronts is performed. However, the same discussion as in the case of the optical coherent detection of the plane wave described with reference to FIGS. If it is larger, it becomes impossible to output a beat signal component. Hereinafter, the reason will be described in detail. Here, the focal length of the lens 1209 is f, the thickness of the transparent separation layer 1203 is d (the thickness of the recording medium layers 1201 and 1202 is 0 for simplicity), and the optical axis of the lens 1209 is the center. It is assumed that P and point Q exist at a position separated by a distance a. However,
f + d >> a (21)
It is.
[0125]
Taking the xz coordinates as shown in FIG. 13 with the center position of the lens 1209 as the origin, the position P at which the laser light scattered at the points P and Q passes through the lens 1209 and is collected again '(Xp', zp ') and (xQ', zQ ') are given by the following formulas for image formation of a general lens
xp '=-af / d (22)
xQ '= af / d (23)
zp ′ = zQ ′ = f (d + f) / d ≒ f2 / d (24)
Is calculated. In this embodiment, since f = 5 mm and d = 50 μm, zp ′ = zQ ′ = 500 mm from equation (24). Since the value of zp 'is larger than the distance between the lens 1209 and the lens 1222 (50 mm in this embodiment), these two scattered laser beams are not actually collected, and the lens 1222 and the photodiode 1223 are not collected. Optical coherent detection.
[0126]
FIG. 14 shows this relationship. Here, the distance between the two lenses 1209 and 1222 is A, and the effective aperture diameter of the lens 1209 is 2D. The wavefront delay ξ of the light caused by the inclination of the wavefront of both laser beams 1306 ′ and 1307 ′ within the effective area at the position of the lens 1222 is approximately:
ξ ≒ afD / d (zp′-A) (25)
Can be expressed as Therefore, when this value is equal to or longer than the laser light wavelength λ0, the intensity of the beat signal at the time of optical coherent detection is significantly attenuated. Therefore, the minimum distance 2amin between the points P and Q in a state where the scattered laser light is optically coherently detected and a beat signal having a considerable intensity is not generated is:
2 amin≒2dλ0 (zp′-A) / fD (26)
It becomes.
[0127]
The laser wavelength in this embodiment is approximately 785 nm for both the first laser light and the second laser light, and the effective diameter of the lens 1209 is 10 nm. It is understood that the condition that 2a ≧ 2amin = 1 μm is necessary so that light does not induce a beat signal in the photodiode 1223. In an actual apparatus, since the interval 2S between the plane waves of the first laser light and the second optical laser light before being incident on the lens 1209 was set to 200 μm, the transmitted light of both lights at the position of the second recording medium layer 1202 was The gap 2σ is about 20 μm, and in this embodiment, it is impossible that the above-mentioned condition (26) is satisfied. In other words, there is no situation in which the first laser light and the second light laser light are respectively applied to two points separated by only 1 μm at the position of the second recording medium layer 1202. This condition is expressed by the following equation (27).
[0128]
2σ ≧ 2 amin (27)
Therefore, the light transmitted through the first recording medium layer 1201 and scattered by the light scatterers existing in the second recording medium layer 1202 is incident on the photodiode 1223 even when the first laser light and the second laser light A strong intensity beat signal having a frequency corresponding to the optical frequency difference ν1-ν2 is not generated.
[0129]
In this way, only when the light scatterer exists at the focal position F in the first recording medium layer 1201, the signal laser light composed of the first and second laser lights subjected to the optical coherent detection outputs the output current of the photodiode 1223. Includes a beat signal component having a frequency of the optical frequency difference ν1−ν2 of the two lights.
[0130]
Therefore, the output of the photodiode 1223 was passed through a high-pass filter 1224 to extract the beat signal component, and was amplified by the preamplifier 1225. FIG. 15 shows an example of the output waveform of the preamplifier 1225. The waveform has a form in which a beat signal having an optical frequency difference ν1-ν2 between the first and second laser beams is modulated in accordance with 1, 0 of a digital signal recorded on the first recording medium, that is, ASK modulation. It has been shaped. Next, such a signal was applied to a narrow band pass filter 1226 to remove noise having a frequency component other than the beat frequency component, and then the envelope detection was performed by an amplitude detection circuit 1227. The output voltage of the amplitude detection circuit 1227 was compared by the voltage comparator 1228, and the recorded information was reproduced.
[0131]
As described above, it is possible to read information recorded as the presence or absence of the light scatterer on the first recording medium layer 1201.
[0132]
By implementing the present invention as described above and reproducing information from the beat signal, it is possible to selectively read out only information from the recording medium layer (1201 in this case) existing at the focal position F of the lens 1209. Thus, it is possible to reliably separate the information from the reflection information from the multilayer, which has been difficult in the past.
[0133]
In the above description, the first recording medium layer 1201 has been described as an information reading target. Conversely, when information is read from the second recording medium layer 1202, light scattering on the first recording medium layer 1201 is performed in the same manner. It was possible to execute signal reproduction without the influence of. However, in this case, it is necessary to control the position of the optical pickup 1229 so that the focal position F of the lens 1209 matches the position of the second recording medium layer 1202. The position of the optical pickup 1229 was controlled by intentionally shifting the output from the multi-segment photodiode 1220 from the state where the focal position F coincides with the first recording medium layer 1201. Note that the tracking is the same for the first recording medium layer and the second recording medium layer, so that a new mechanism is not required due to the multi-layered recording medium.
[0134]
In addition, for the purpose of applying a more accurate focus servo, a method of introducing optical coherent detection into the light received by the multi-division photodiode 1220 and detecting the output from each photodiode as a beat signal amplitude was also implemented. By applying the beat signal to the position control of the optical pickup 1229 in this manner, it has been found that the stability of the control in a state where the focus servo is applied is very high. However, in the case of this method, it is necessary to select one having a response frequency higher than each element beat signal frequency of the multi-division photodiode 1220, and to individually select a high-pass filter, a preamplifier, a narrow band-pass filter, Each circuit for amplitude detection (a circuit equivalent to 1224 to 1227 in the signal detection system) must be provided.
[0135]
Next, a mechanism for maintaining a constant optical frequency difference between the two lasers 1205 and 1206 included in the two-wavelength semiconductor laser device 1204 will be described. The other two plane-wave laser beams split into two by the beam splitter 1208 are multiplexed by the mirror 1210 and the beam splitter 1211 so that the wavefronts coincide with each other, and are condensed on the light receiving surface of the photodiode 1213 by the lens 1212. . In the photodiode 1213, these two laser lights are subjected to optical coherent detection, and as described in the first to third embodiments, the high-pass filter 1214, the preamplifier 1215, the frequency discrimination circuit 1216, the proportional integration (PI 2) The optical frequency difference ν1-ν2 of the two laser beams was kept constant by a feedback control system composed of a control circuit 1217 and a laser drive circuit 1218. Detailed circuit configurations and operations have been described in detail in Embodiments 1 to 3, and will not be described here.
[0136]
When information was actually read from an optical disk having a recording density of 0.7 M / mm2 using the apparatus of this embodiment, a bit error rate of 10-6 could be achieved at an information read rate of 1 Mbps, and the information was recorded in multiple layers. Information can be read without affecting each other.
[0137]
(Example 5)
Next, a fifth embodiment of the present invention will be described.
[0138]
FIG. 16 shows a configuration diagram of the optical recording information reading apparatus of the present embodiment. The optical memory disk 1600 in this embodiment has a first recording medium layer 1601 and a second recording medium layer 1602, and has a structure in which they are separated by a transparent separation layer having a thickness d. Information is recorded by the presence or absence of a light scatterer in the recording medium layers 1601 and 1602. As the actual recording medium layers 1601 and 1602, a material in which a liquid crystal material is dispersed in a solvent is used. In this case, the recording information is written by overheating the liquid crystal layers 1601 and 1602 by absorbing light with a liquid crystal molecule or a light absorbing film located above and below the liquid crystal layers 1601 and 1602 by laser light, and then by natural quenching. Utilizing a phenomenon in which the arrangement of liquid crystal molecules is randomized. However, in both embodiments, the recording medium layers 1601 and 1602 are not limited to liquid crystal materials, but may be any as long as information can be recorded in a light scattering state, and the effects of the present invention are not affected at all. For example, a disk or the like in which non-rewritable pits are stamped on a plastic material by a stamp method is conceivable.
[0139]
Laser light emitted from the semiconductor laser element 1603 is focused on the beam splitter 1605 by the lens 1604. After the laser light is split into two by the beam splitter 1605, each laser light is converted into two plane waves by the same lens 1606. The two plane waves are focused and irradiated on the optical memory disk 1600 by the focusing lens 1607. FIG. 17 shows this state in an enlarged manner. As can be seen from this figure, two laser beams 1701 and 1702 having exactly the same optical frequency are converged and irradiated at a confocal position F on the second recording medium layer 1602 at an angle of 2α.
[0140]
When a light scatterer exists at the light irradiation (confocal) position F of the second recording medium layer 1602, the scattered reflected light of both laser beams 1701 and 1702 is again plane-wave (hereinafter also referred to as laser beam) by the lens 1607. 1703. That is, the wavefronts of the reflected lights of the two laser beams 1701 and 1702 coincide.
[0141]
At this time, since the optical memory disk 1600 is rotating by a motor (not shown), the second recording medium layer 1602 at the light irradiation position F is moving at a speed v (to the left in the figure). It becomes. As described above, when two laser beams having the same optical frequency are irradiated on the moving object at an angle parallel to the moving direction, the laser light 1703 reflected by the moving object undergoes the Doppler shift ( For details of the laser Doppler effect, see, for example, “O plus E” No. 150 (May), page 109).
[0142]
Here, the laser beam 1701 is scattered and reflected by the second recording medium layer 1602, and the optical frequency ν1 of the laser beam (first laser beam) is the same as the laser beam 1702. The optical frequency ν2 of the laser light is given by the equations (28) and (29), respectively.
ν1 = ν0 (1 + 2 vsin α / c) (28)
ν2 = ν0 (1-2 vsin α / c) (29)
Here, ν0 represents the optical frequency of the laser beams 1701 and 1702 before irradiation of the recording medium 1702.
[0143]
From the above equations (28) and (29), the optical frequency difference between the two laser beams: ν1-ν2 is:
ν1−ν2 = 4ν0vsinα / c (30)
Since the frequency ν0 and the speed v of the laser beam to be applied are almost constant, they have constant values. Here, the vertical movement of the optical memory disk 1600 is ignored. Actually, the optical memory disk 1600 vibrates up and down while rotating, and the optical frequency difference of the first and second laser beams 1703 is shifted from the equation (30) due to the moving speed component. In this case, since the moving speed v of the recording media 1601 and 1602 due to the rotation of the optical memory disk 1600 is 20 m / s or more, which is sufficiently higher than the moving speed of the optical memory disk 1600 caused by the vertical vibration, this effect can be ignored.
[0144]
In this way, the laser light 1703 having the two different optical frequencies ν1 and ν2 reflected by the light scatterer of the second recording medium layer 1602 enters the mirror 1608 with the wavefronts being coincident (hereinafter, referred to as the laser light 1703). The first and second laser beams 1703 having the same wavefront are referred to as signal light).
[0145]
After that, the signal light 1703 whose optical path is changed by 90 ° by the mirror 1608 is split into two by the beam splitter 1609, and the signal light 1703 traveling straight through the beam splitter 1609 is condensed by the lens 1610 and is incident on the multi-division photodiode 1611. Is done. The position of the pickup 1622 was controlled in accordance with the output from the multi-segment photodiode 1611 so that the focused irradiation position F was on the surface of the second recording medium layer 1602.
[0146]
On the other hand, the other signal light 1703 divided by the beam splitter 1609 is condensed by the lens 1612 and is incident on the photodiode 1613. Since the signal light 1703 has the same wavefront as the scattered and reflected light from the confocal point F as described above, the two laser lights 1703 are optically coherently detected by the photodiode 1613. Since the optical frequency difference between the first laser light and the second laser light is substantially constant as represented by Expression (30), the output of the photodiode 1613 has a frequency that matches the optical frequency difference. A beat signal will be included.
[0147]
As described above, when the light scatterer exists at the light irradiation position F of the second recording medium layer 1602, the output from the photodiode 1613 includes the beat signal component. On the other hand, when there is no light scattering medium at the light irradiation position F and there is almost no reflection, the intensity of the signal light 1703 becomes extremely small, and therefore, the amplitude of the beat signal component included in the output of the photodiode 1611 is also attenuated.
[0148]
Next, the influence of the scattered and reflected light from the first recording medium layer 1601 when the focal position F is set on the second recording medium layer 1602 will be described. In this case, the laser light is scattered and reflected from the points P and Q in FIG. The interval 2S between the two plane wave laser beams 1701 and 1702 irradiated on the second recording medium layer 1602 is 2 mm, the focal length f of the lens 1607 used is 5.5 mm, and the thickness of the transparent separation layer 1603 is 10 μm. Therefore, the minimum possible value of the distance 2σ between the points P and Q in the first recording medium layer 1601 is 28 μm.
[0149]
Here, based on the same considerations as in the expressions (21) to (27) in the fourth embodiment, light scattered and reflected from two different points in the first recording medium layer 1601 generates a beat signal in the photodiode 1613. In order to achieve this, it is understood that the distance between the points needs to be less than 10 μm in a practical optical configuration. Therefore, at the interval 2σ (at least 28 μm) between the points P and Q, the wavefront on the photodiode 1613 is shifted, and no beat signal is generated.
[0150]
From the above, it can be seen that the beat signal is included in the output of the photodiode 1613 only when the light scatterer exists at the laser irradiation position F of the second recording medium layer 1602. In this case, the output of the photodiode 1613 was obtained by removing a DC component by a high-pass filter 1614 and then amplifying a beat signal component by a preamplifier 1615.
[0151]
FIG. 18 shows an example of the output waveform of the preamplifier 1615. This output waveform is an ASK modulation signal in which the beat signal amplitude changes in accordance with recording information 1 and 0 in the second recording medium layer 1602. The narrow band pass filter 1616, envelope detection circuit 1617, and voltage comparison circuit 1618 were used for reproducing the signal from the ASK modulation signal, as in the above-described embodiment. In this embodiment, since v = 20 m / s and 2α = 40 °, the beat signal frequency ν1−ν2 was 66 MHz. The beat signal frequency ν1-ν2 was sufficiently higher than the information read rate R = 1.5 Mbps from the recording medium, and the bit signal could be reproduced from the ASK signal.
[0152]
As a method for rotating the optical memory disk 1600, there are a constant angular velocity method and a constant velocity method. Here, the constant velocity method is selected in order to make the pass bandwidth of the narrow band pass filter 1616 as narrow as possible. This constant speed control was performed as follows.
[0153]
First, the frequency of the beat signal from the preamplifier 1615 was detected by the frequency discrimination circuit 1619. As shown in equation (30), since the beat signal frequency ν1-ν2 contains information on the velocity v, the proportional (P) control circuit 1620 compares this detected value with a set voltage value separately given from the outside. In comparison, by changing the rotation speed of the motor by the motor drive circuit 1621 so that they match, the speed could be kept constant.
[0154]
Also, as can be seen from equation (30), since the beat signal frequency is determined by the speed v and the light frequency ν0 of the irradiating laser light, it is necessary to keep the laser light frequency ν0 constant in order to control the speed. For this reason, in the present embodiment, a distributed feedback laser device having a small wavelength variation is applied as the semiconductor laser device. As another method of keeping the speed constant, a method of detecting the rate of the reproduced bit information and controlling the number of rotations of the motor so that the value becomes constant is also possible, and a similar effect was obtained. .
[0155]
With the above configuration, a beat signal generated by optical coherent detection is not required without a complicated feedback circuit system for keeping the optical frequency difference between the laser outputs from the two semiconductor laser elements constant. The recorded information can be read through the amplitude change, and the recorded information can be read from the multilayer recording medium disk 1600 without being affected by the scattering of light in layers other than the read target layer at all. . For this reason, when information was read from the dual-layer disc having a recording density of 1 M / mm2 in the apparatus of the present embodiment, a bit error rate of 10-7 was achieved.
[0156]
【The invention's effect】
As described in detail above, the present invention has the following effects.
[0157]
(1) In optical recording information reading, optical coherent detection is performed using two laser beams having a frequency difference greater than the information reading rate, so that even if the optical power per bit is weak, recording information can be reproduced. A sufficient C / N value can be secured.
[0158]
(2) By irradiating the optical recording medium with two lights having different optical frequencies in a confocal state, it is possible to secure a sufficient C / N value in reproducing recorded information from a specific medium of the multilayer optical recording medium.
[0159]
(3) Carrier frequency at the time of information reproduction is stabilized by detecting the frequency of the beat signal component of the optical coherent detection output and performing control to keep the value constant to keep the frequency difference between the two lights constant. As a result, the bit error rate in reproducing the recorded information can be reduced.
[0160]
(4) By making the two lights irradiated on the recording medium in the confocal state into lights having the same frequency from the same light source, and giving the Doppler shift to the optical frequency of the reflected light by moving the recording medium, It is possible to obtain two laser beams having a constant optical frequency difference with a single laser light source and without requiring an extra element for optical frequency shift, and to use optical coherent detection. The recording information reading system can be simplified and the device can be downsized.
[0161]
In addition, the present invention is not limited to the above-described embodiments, but includes a specific optical configuration or an electrical circuit configuration, a different signal processing method in signal reproduction, a beat signal frequency, an information reading rate, and an information recording density. Needless to say, the effect is exhibited even when various numerical values such as the spectral line width of the used semiconductor laser element are different or when the number of layers of the multilayer recording medium is two or more. Furthermore, the effect is obtained even when the means for generating two different wavelengths for optically coherent detection in the light receiving element is different from the embodiment (use of the automatic frequency control circuit, use of the Doppler shift effect using the movement of the disk). Needless to say, it works. In that case, for example, as shown in FIG. 19, a method is used in which light from one semiconductor laser element is split into two by a beam splitter 2101 and one is shifted in optical frequency by an acousto-optic element 2103. Cases are conceivable.
[0162]
In this case, the other laser beam split by the beam splitter 2101 is applied to the optical disc via the beam splitter 2102. The light reflected by the optical disk has its direction changed by a beam splitter 2102 and then enters a beam splitter 2105, and the light whose optical frequency has been converted by an acousto-optic element 2103 has its direction changed by a mirror 2104. It is incident on a beam splitter 2105. In the beam splitter 2105, the laser beams having the two different optical frequencies are multiplexed in a state where the wavefronts coincide with each other, and are guided to the light receiving element.
[0163]
Further, according to the present invention, the amplitude change of the beat signal generated by the optical coherent detection can be achieved without requiring a complicated feedback circuit for keeping the optical frequency difference between the laser outputs from the two semiconductor laser elements constant. , The recorded information can be read from the multi-layer recording medium disk 1600 without any influence of light scattering in the layers other than the read target layer. For this reason, when information was read from the dual-layer disc having a recording density of 1 M / mm2 in the apparatus of the present embodiment, a bit error rate of 10-7 was achieved.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an optical recording information reading device according to a first embodiment of the present invention.
FIG. 2 is a diagram illustrating a polarization relationship between a first laser beam and a second laser beam that are combined by a polarization beam splitter in the optical recording information reading device according to the first embodiment.
FIGS. 3A and 3B are explanatory diagrams of the first embodiment, in which FIG. 3A is a schematic diagram of a wavefront when two plane waves are slightly inclined and combined, and FIG. 3B is a diagram on the combined surface in that case; FIG. 4 is a diagram illustrating a phase relationship between ν1 and ν2 signal components.
FIG. 4 is an explanatory diagram of the first embodiment, showing a relationship between a wavefront shift and a beat signal intensity when two plane waves are optically coherently detected.
FIG. 5 is a diagram illustrating an example of an output signal waveform of a preamplifier in the optical recording information reading device according to the first embodiment.
FIG. 6 is a configuration diagram of an optical recording information reading device according to a second embodiment of the present invention.
FIG. 7 is a diagram illustrating a polarization relationship between a first laser beam and a second laser beam that are combined by a polarization beam splitter in the optical recording information reading device according to the second embodiment.
FIG. 8 is a diagram illustrating an example of an output waveform of a preamplifier in the optical recording information reading device according to the second embodiment.
FIG. 9 is a configuration diagram of an optical recording information reading device according to a third embodiment of the present invention.
FIG. 10 is a diagram illustrating a polarization relationship between a first laser beam and a second laser beam that are combined by a beam splitter in the optical recording information reading device according to the third embodiment.
FIG. 11 is a diagram illustrating an example of an output waveform of an amplifier in the optical recording information reading device according to the third embodiment.
FIG. 12 is a configuration diagram of an optical recording information reading device according to a fourth embodiment of the present invention.
FIG. 13 is a diagram illustrating a state of scattered reflected light from two recording medium layers in the optically recorded information reading device according to the fourth embodiment.
FIG. 14 is an explanatory diagram of a wavefront state in a signal receiving lens aperture of laser light scattered and reflected on a recording medium layer that is not a reading target in the optical recording information reading device of the fourth embodiment.
FIG. 15 is a diagram illustrating an example of an output voltage waveform of a preamplifier in the optical recording information reading device according to the fourth embodiment.
FIG. 16 is a configuration diagram of an optical recording information reading device according to a fifth embodiment of the present invention.
FIG. 17 is a diagram illustrating a state of scattered reflected light from two recording medium layers in the optically recorded information reading apparatus of the fifth embodiment.
FIG. 18 is a diagram illustrating an example of an output voltage waveform of a preamplifier in the optical recording information reading device according to the fifth embodiment.
FIG. 19 is a diagram showing a configuration of an optical system to which the present invention can be applied and which uses an acousto-optic element to obtain two different optical frequencies.
FIG. 20 is a configuration diagram of a conventional optical recording information reading device.
FIG. 21 is a diagram illustrating a polarization state of signal light split by a polarization beam splitter in a conventional optical recording information reading device.
[Explanation of symbols]
101 Semiconductor laser device
102 Collimator lens
103 Beam Shaping Prism
104 beam splitter
105 Condensing lens
106 disks
107 Recording medium
108 beam splitter
109 Condenser lens
110 Multi-segment photodiode
111 λ / 2 plate
112 Polarized Beam Splitter
113 Semiconductor laser device
114 Collimator and Lens
115, 117 Condensing lens
116, 118 PIN photodiode
119,120 High pass filter
121, 122 Preamplifier
123, 124 Narrow band pass filter
125, 126 Amplitude detection circuit
127 differential amplifier circuit
128 comparison circuit
129 Frequency discrimination circuit
130 PID control circuit
131 Laser drive circuit
132 processor circuit
133 switch circuit
601 Semiconductor laser device
602 hologram plate
603 Collimator lens
604 beam splitter
605 Condensing lens
606 optical disk
607 Recording medium
608 Multi-segment photodiode
609 Optical pickup
610 Polarized Beam Splitter
611 Semiconductor laser device
612 Collimator lens
615 Collimator lens
614, 616 Photodiode
617 High bandpass filter
618 Preamplifier
619 Frequency discrimination circuit
620 Automatic wavelength control circuit
621 Laser drive circuit
622 Processor Circuit
623 switch circuit
624 high bandpass filter
625 preamplifier
626 Narrow bandpass filter
627 Phase detection circuit
628 signal reproduction circuit
629 Amplitude detection circuit
630 Automatic gain control circuit
901 Semiconductor laser device
902 Collimator lens
903 beam splitter
904 condenser lens
905 optical disk
906 Recording medium
907 Beam splitter
908 condenser lens
909 Photodiode
910 Semiconductor laser device
911 Collimator lens
912 High Pass Filter
913 amplifier
914 Narrow Bandpass Filter
915 Amplitude detection circuit
916 Voltage comparison circuit
917 Optical fiber
918, 919 Branch
920, 921 Photodiode
922 multiplexing section
923 Light receiving circuit
924 high-pass filter
925 amplifier
926 frequency discrimination circuit
927, 929, 930 Proportional control circuit
928 Laser drive circuit
1200 optical disk
1201 first recording medium layer
1202 second recording medium layer
1203 Separation layer
1204 two-wavelength semiconductor laser device
1205 First laser stripe
1206 Second laser stripe
1207 Lens array
1208 Beam splitter
1209 lens
1210 mirror
1211 Beam splitter
1212 lens
1213 Photodiode
1219 Beam splitter
1220 Multi-segment photodiode
1221, 1222 lenses
1223 Photodiode
1224 High bandpass filter
1225 Preamplifier
1226 Narrow bandpass filter
1227 Amplitude detection circuit
1228 Voltage comparator
1229 Optical pickup
1301, 1302 plane wave laser light
1303 Signal laser light
1304, 1305 laser light
1306, 1307 (scattered) spherical wave
1306 ', 1307' convergent spherical wave
1600 Optical memory disk
1601 First recording medium layer
1602 Second recording medium layer
1603 Semiconductor laser device
1604 lens
1605 Beam splitter
1606 lens
1607 Condenser lens
1701, 1702 laser light
1703 plane wave

Claims (4)

An optical information reading device for reading information recorded on a recording medium having a plurality of recording layers ,
First and second laser beams,
One condensing lens for converging and irradiating the first laser light and the second laser light to the same position on any one of the recording layers as an information reading target ;
Photoelectric conversion means for photoelectrically converting the first laser light and the second laser light, which are reflected and converged from one layer of the recording layer and have different optical frequencies from each other ,
Means for detecting a signal having a beat frequency component of the first laser light and the second laser light based on an output of the photoelectric conversion means, and reproducing information recorded on the recording layer ;
An optical information reading device, comprising: The condensing lens is arranged such that the first laser light and the second laser light are applied to one layer of the recording layer at a predetermined angle in a cross section including the direction of movement of the recording layer. 2. The optical information reading device according to claim 1, wherein: The optical frequency difference between the first laser light and the second laser light reflected from one layer of the recording layer is determined by the fact that the first laser light and the second laser light rotate one layer of the recording layer due to the rotation of the recording medium. 2. The optical information reading device according to claim 1, wherein the optical information reading device is generated by a Doppler shift received by the device.   The optical frequency difference between the first laser light and the second laser light reflected from one of the recording layers has a proportional relationship with the rotational movement speed of the recording layer due to the rotation of the recording medium. Item 2. The optical information reading device according to Item 1.

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