JP5005850B2 - Optical head device - Google Patents

Description

【００３５】
したがって、透明電極の抵抗Ｒ₁、Ｒ₂および駆動電圧Ｖ₁、Ｖ₂が得られていればＲ_Sを適切に設定することで、従来、外部信号源から供給されていた電圧と等しい電圧を印加できる。
【００３６】
薄膜抵抗の抵抗Ｒ_Sは、薄膜抵抗を構成している導電性薄膜のシート抵抗ρ_L、薄膜抵抗の幅Ｗ、長さＬを用いてＲ_S＝ρ_L×Ｌ／Ｗと書くことができる。例えばＲ_S＝１０ｋΩにするには、ρ_L＝３００Ω／□、Ｌ＝１ｍｍ、Ｗ＝３０μｍにすればよい。薄膜抵抗の線幅Ｗは狭すぎると形状誤差による抵抗バラツキが大きくなるため１０μｍ以上にするのがよい。長さＬが長くなり基板上に設置が困難になった場合には、途中折り曲げてもよい。
【００３７】
上記の構成では信号１による電圧Ｖ₁を分圧して点４７の電圧を設定したが、同様な原理により信号２による電圧Ｖ₂を分圧してもよい。この場合、薄膜抵抗は図７で信号２側の配線に接続され、図８の等価回路では抵抗Ｒ₂と並列になるよう点４７と４８との間に設置される。
【００３８】
つぎに電極、給電部、薄膜抵抗の抵抗値、および材質について述べる。給電部を形成する給電部材料のシート抵抗ρ_Sと電極を形成する電極材料のシート抵抗ρ_Tの比ρ_T／ρ_Sを１０００以上にすることが好ましい。ρ_T／ρ_Sが小さい場合、電極にも比較的大きな電流が流れ、電極と接している給電部内で電圧降下が生じて、所望の電圧分布を得ることが困難となることがある。したがって、給電部材料に比べ電極材料のシート抵抗が高いほど、隣接する給電部間で電位を連続的に変化させやすく、所望の電位分布を得ることができる。ρ_T／ρ_Sを１０００以上にすることがこの条件を満たすための目安である。
【００３９】
しかし、ρ_Tが大きすぎると給電部の導電性がなくなり電位分布は発生しない。したがってρ_Sをできるだけ小さくする方が望ましく、ρ_Sは０．１〜１０Ω／□程度、ρ_Tは１００〜１００ｋΩ／□程度がよい。
以上の条件を満足し適切にρ_Sとρ_Tを設定すると、一方の電極のみに２つ以上の給電部を形成しこれら２つ以上の給電部にそれぞれ異なる電圧を供給した場合、給電部Ｓ₁、Ｓ₂、Ｓ₃・・・それぞれの給電部内では等電位となるが、電極面内の電位分布は給電部間で発生する電圧降下により連続的に変化する。この連続的に変化する状況は、２つ（２面）の電極に２つ以上の給電部を形成して異なる電圧を供給しても同じである。
【００４０】
給電部材料としては、銅、金、アルミニウム、クロムなどの金属材料が導電性・耐久性の点から好ましいが、比抵抗が室温で１０^-8〜１０^-7Ω・ｍ程度あれば金属以外の材料でもよい。例えばＩＴＯ膜などの透明導電膜を用いることもでき、金属材料を使用する場合に比べ遮光部がなくなるため光の透過率が高くなり好ましい。しかし透明導電膜は金属膜に比べ比抵抗が大きいため、シート抵抗を小さくするためには膜厚を厚くする必要がある。
【００４１】
給電部に外部の位相補正素子制御回路より電圧を印加するための電極引き出し部２７上の配線材料はＩＴＯ膜のような透明導電膜でもよく、クロムやニッケルのような金属膜でもよい。特にニッケルなどハンダで接続可能な金属の場合、外部の信号線を容易にハンダで接続でき好ましい。
【００４２】
一方、電極材料としては透明でありかつ給電部材料に比べシート抵抗が高い必要がある。透明導電膜であるＩＴＯ膜などがよく、ＩＴＯ膜はシート抵抗が高いほどよく１００Ω／□以上が好ましい。さらに、１ｋΩ／□以上にした方がρ_Sを１Ω／□程度にできるため、給電部の膜厚を薄くできるなど作製が容易になりより好ましい。
【００４３】
ρ_Tを大きくするために酸化亜鉛膜やガリウムを含む酸化亜鉛膜（ＧＺＯ膜）、またはガリウムとシリコンを含む酸化亜鉛膜（ＧＺＳ膜）を用いることはＩＴＯ膜に比べ容易に高抵抗膜を得られるため好ましい。特に、ＧＺＯ膜やＧＺＳ膜は高比抵抗でありながらエッチング性も良好であり、光の透過率、耐久性に優れている点で本発明の光ヘッド装置において好適な材料である。
【００４４】
酸化亜鉛膜へガリウムを添加する場合、膜の透過率が変化するためその添加量を１〜１０質量％にすることが好ましい。また、ガリウムとシリコンをともに添加する場合においても、膜の透過率が変化するためその合計した添加量を１〜２０質量％にすることが好ましい。
【００４５】
一方、薄膜抵抗の材料としては式（１）に示すＶ₃の値が所定値となるように、Ｒ₁、Ｒ₂、Ｒ₃を決める必要がある。光ヘッド装置に用いる場合、薄膜抵抗の抵抗値を１００Ω〜１０００ｋΩにすることは、電極および薄膜抵抗の作製が容易となり望ましく、ＩＴＯやＧＺＯ、ＧＺＳなどを使用できる。薄膜抵抗の材料を電極材料と同じにし電極形成時に同時に形成することは、材料の抵抗値がロットによりバラついても、その影響は式（１）に示すように分母・分子で相殺されるため、Ｖ₃には影響を与えず好ましい。
【００４６】
つぎに、給電部の形状や大きさに関して説明する。給電部の形状や大きさは、上述のように状況に応じて変化させることが好ましい。すなわち位相補正素子により発生する波面の変化は、給電部の形状や大きさなどに依存し、補正したい波面収差の種類や発生させたい波面形状に応じて変化させればよい。ここで、波面収差としてはコマ収差、球面収差、非点収差などがある。
【００４７】
コマ収差は、上述のように光ディスクのチルトにより発生する収差であり、位相補正素子上の入射光束の中心を通って素子面に平行で、かつ光ディスクの回転方向に平行な直線の回りに１８０°回転したとき重なる形状を有している。したがって、給電部は、上述の平行な直線に対して対称となるように配置されるのがよい。
【００４８】
具体的には、例えば連続する１枚の電極の中央部には通常長方形状または直線状の給電部を設け、周辺部には電極の周辺部での形状（円弧など）の給電部を設ける。そして、それらの給電部が上述の直線に対して対称となるよう給電部を配置する。このように給電部を配置することは、最も効果的にコマ収差を補正できるので好ましい。
【００４９】
球面収差を補正する場合は、球面収差が光軸を中心とする同心円形状であるため、給電部は、同心円状に形成された２つ以上の円環体を有していることが好ましい。さらに、位相補正素子を通過する光源からの出射光の光束半径を１とすると、１つの円環体の半径は０．６５から０．８５までの値を有し、かつ上記とは異なる１つの円環体の半径は０．２から０．４までの値を有することが好ましい。ここで、円環体はドーナツ状であって半径には幅があるので、円環体の半径とは内半径と外半径の平均値を意味する。
【００５０】
通常、半径０．６５の円と半径０．８５の円によって囲まれる領域（領域Ａ）に球面収差の最大値が存在するため、給電部である１つの円環体が、光軸と中心を合わせて領域Ａに形成されることが好ましい。さらに、半径が０．２と半径が０．４で囲まれる領域（領域Ｃ）に、精度を極めて高く球面収差を補正するため上記とは異なる１つの円環体状の給電部を光軸と中心を合わせて設けることが好ましい。
【００５１】
半径０．６５の円と半径０．８５の円のかわりに、半径０．７の円と０．８の円とすることは、領域Ａよりも領域は狭いが球面収差の最大値が存在する確率が高い領域に円環体が追加でき、さらに他の円環体が追加でき球面収差の微調整ができて好ましい。
【００５２】
電極が連続する１枚の電極の場合、光軸を含み半径が０．２より小さい光軸近傍領域（領域Ｂ）にさらに、円環体状の給電部を光軸と中心を合わせて設けることにより、さらに球面収差の微調整ができてより好ましい。電極が分割電極であって、領域Ｂが他の領域（領域Ａ、領域Ｃなど）と分離されている場合でも、上述のように精度を極めて高くして球面収差を補正できるので充分である。
【００５３】
非点収差の場合には、連続する１枚の電極に設ける、電極の中心部の１点を通る複数個の放射線状給電部が好ましく個数を増やすほど所望の電位分布が得られる。さらに、コマ収差と球面収差の両方を含む波面収差などを補正することもでき、この場合は上記の直線状の給電部と同心円状の給電部とを組み合わせるなどすればよい。
【００５４】
上述したように、一対の連続する電極のそれぞれに異なる形状の給電部を設けて、一方の電極がコマ収差を補正し、他方の電極が球面収差を補正するようにもできる。同様に、一対の電極の一方を給電部を有する連続する１枚の電極にして、他方を複数個に分割した分割電極とすることにより、連続した収差分布と階段状の収差分布の両方を発生させることもできる。波面収差であるコマ収差、球面収差、非点収差などはシステムとしての光ヘッド装置が発生させるものであり、したがって光ヘッド装置内に本発明における位相補正素子を組み込むことにより波面収差を有効に補正できる。
【００５５】
本発明における位相補正素子は透過する光の波面形状を変化させる機能を有しているため、波面収差を補正するだけでなく光の焦点位置を変えるなど他の目的においても同様な原理により使用できる。例えば、単に光学倍率を変化させて透過する光の焦点位置を変化させる目的に使用したり、また透過する波面を傾けて出射させることで光の進行方向を変える目的にも使用できる。
【００５６】
波面収差を補正する場合においても、上記のコマ収差、球面収差、非点収差などのより高次の波面収差も補正できる。これらの場合においても、所望する波面の変化に応じて給電部の形状や数、位置、または電極の分割方法などを適宜設定すればよい。
【００５７】
本発明における位相補正素子の、給電部が設けられた電極が分割されて複数の分割電極とされ、それぞれの分割電極には１つ以上の給電部が配置されており、給電部のうち２つ以上が薄膜抵抗により導電接続されていることが好ましい。導電接続される２つ以上の給電部が、同一の分割電極上の給電部であれば連続的な電圧分布が発生できて好ましく、異なる分割電極上の給電部であれば非連続的な電圧分布が必要な場合に使用できて好ましい。
【００５８】
いずれの場合においても、薄膜抵抗を使用して２つ以上の給電部を導電接続することにより、従来と比べより少ない外部信号源で光ヘッド装置を動作できる。また、薄膜抵抗の接続先の一方が給電部であり、他方が給電部を有しない分割電極である構成の場合においても同様の効果が得られる。
【００５９】
【実施例】
「例１」
本例の光ヘッド装置は、光ディスクの厚さムラにより生ずる球面収差を補正する位相補正素子を備えている。対物レンズは光ディスクの厚さが設計値からずれると球面収差を発生し信号の読み取り精度が低下する。この球面収差を補正する位相補正素子を図１の光ヘッド装置の位相補正素子４として組み込んだ。ただし、位相補正素子制御回路１０は本例の位相補正素子用に改良されている。
【００６０】
本例の位相補正素子の素子構造は図２に示したものと同じである。以下に本例の位相補正素子により球面収差を補正する原理を説明する。本例の光ヘッド装置における対物レンズのＮＡは０．６５、光源の波長は０．４μｍであった。光ディスクの厚さが、設計値の０．６ｍｍよりも０．０３ｍｍだけ厚い場合に発生する波面収差（球面収差）は上記のように図３に示される。本例における位相補正素子の電極パターンは図７に示すものであり、等価回路は図８に示したものである。
【００６１】
図７中の斜線部はＧＺＯ膜で形成された透明電極４０であり、太線部分（環状体）はクロム薄膜をエッチング技術により形成した給電部４１、４２、４３、４４である。給電部は、同一の基板面上に形成された同じクロム薄膜による配線によって、外部の信号源である信号１、２に接続されている。給電部４２は同一の基板面上に形成された薄膜抵抗４５により信号１に接続されている。
【００６２】
給電部４１〜４４の幅は１００μｍ、給電部４２、４３、４４の直径はそれぞれ０．５、１．５、２．２ｍｍであり、給電部４１は直径５０μｍの円形であった。以上の電極および給電部のパターンは以下のように形成した。まず、ガラス基板上にスパッタ法にてクロム膜を堆積させた後、エッチング技術により不用部分を取り除き給電部および配線を形成した。つぎに、ＩＴＯ膜をスパッタ法により堆積した後、エッチング技術により薄膜抵抗４５を形成した。その後、スパッタ法によりＧＺＯ膜を堆積しエッチング技術により透明電極４０を形成した。
【００６３】
各部のシート抵抗値として、給電部が１Ω／□、電極が１００ｋΩ／□、薄膜抵抗が３００Ω／□であった。図８の抵抗Ｒ₁、Ｒ₂、Ｒ₃に相当する給電部間の電極抵抗値はそれぞれ５０、２８、２０ｋΩであった。薄膜抵抗の抵抗値および形状は次のようにして決めた。給電部４１、４４の電位が信号１によりＶ₁＝２Ｖ、給電部４３の電位が信号２によりＶ₂＝３Ｖとした場合、図８の点４７に相当する給電部４２の電位を約２．１５Ｖにするために、式（１）よりＲ_S＝５．４８ｋΩとすればよい。したがって、シート抵抗ρ_Lは３００Ω／□であるので薄膜抵抗の線幅Ｗを３０μｍとすると、長さＬはＲ_S＝ρ_L×Ｌ／Ｗより約０．５５ｍｍになる。本例では幅３０μｍ、長さ０．５５ｍｍの線を３回折り曲げた形状の抵抗値５．４８ｋΩの薄膜抵抗（線状抵抗）を形成した。
【００６４】
０．０３ｍｍの光ディスクの厚さムラにより発生する球面収差を位相補正素子により補正するために、給電部４１、４４に２．３Ｖ、給電部４３に２．０Ｖの電圧を供給した。その結果、薄膜抵抗により給電部４２には約２．０５Ｖの電圧が供給された。ここで、図７に示した給電部を有する電極に対向する電極は、給電部を１つ有する一枚の透明電極で構成されており、常に０Ｖの電位になる。
【００６５】
図９に位相補正素子により発生した位相変化をｎｍ単位で示した。円の中心部と外周部の位相変化を０ｎｍとした場合、中間部領域の位相変化は−１００ｎｍ程度になる。ここで、複数個ある実線の円は等高線であって、−１００ｎｍの中間部領域の内側では１本の等高線が２０ｎｍを表し、外側では約３０ｎｍを表す。
透明電極４０（図７）には、給電部の電圧に応じて電圧分布が発生する。上述したように、図７の透明電極４０内の電圧分布により液晶の実質的な屈折率分布が生じる結果、位相補正素子は図９に示す同心円状の位相変化を発生できる。
【００６６】
一方、光ディスク厚さが０．０３ｍｍだけ薄い場合には、図７とは正負が逆転した球面収差を補正するために、給電部４１、４４に２．０Ｖ、給電部４３に２．３Ｖを供給すれば位相補正素子によって発生する位相変化が図９の正負を逆転した形になるため球面収差を相殺できる。以上のように所望の電圧が得られるよう薄膜抵抗４５を設け、給電部４１、４３、４４に適切な電圧を供給することにより図７の球面収差を補正できる。
【００６７】
「例２」
本例の光ヘッド装置は、光ディスクの厚さムラにより生ずる球面収差と光ディスクのチルトにより発生するコマ収差の両者を補正する位相補正素子を備えている。この位相補正素子を図１の光ヘッド装置の位相補正素子４として組み込んだ。ただし、位相補正素子制御回路１０は本例の位相補正素子用に改良されている。本例の位相補正素子の素子構造は図２に示したものと同じで、以下に述べる電極パターンおよび材料が異なる。
【００６８】
図２の電極２４ａは図１０に示すように分割電極５１、５２、５５と分割電極５５に形成した給電部５３、５４、および薄膜抵抗５６、５７により構成されている。図１０では薄膜抵抗５６、５７は模式的に表わされており、実際の形状は所望の抵抗値が得られるよう線状などにする。
【００６９】
分割電極５１、５２、５５の材質はＧＺＳ膜であり、給電部５３、５４、および薄膜抵抗５６、５７の材質はＩＴＯ膜であった。ＧＺＳ膜、ＩＴＯ膜のシート抵抗はそれぞれ１０００、１０ｋΩ／□であった。電極２４ａはまず、ガラス基板上にＩＴＯ膜をスパッタ法により成膜しフォトリソグラフィ技術およびエッチング技術を用い給電部５３、５４、および薄膜抵抗５６、５７を形成した。給電部５３、５４の幅は５０μｍとした。つぎにＧＺＳ膜をスパッタ法により成膜しフォトリソグラフィ技術およびエッチング技術を用いて分割電極５１、５２、５５を形成した。分割電極の分割間隔は５μｍとした。図１０の信号１および２は、それぞれ分割電極５１と給電部５４、および給電部５３に印加される信号であり、位相補正素子制御回路１０により発生する。
【００７０】
一方、電極２４ｂには図１１に示すように分割電極６１〜６５が形成されておりコマ収差を補正できる。図１１の分割電極は、ガラス基板にスパッタ法によりＩＴＯ膜を成膜し、フォトリソグラフィ技術およびエッチング技術によりパターンを形成した。図１０の太線は分割電極間ギャップを示しており、この部分はエッチングによりＩＴＯ膜が取り除かれているため電圧印加されない。分割電極間ギャップの幅は５μｍであった。
【００７１】
以下に本例の位相補正素子により球面収差とコマ収差を補正する原理を説明する。位相補正素子制御回路の出力波形は、周波数１ｋＨｚ、デューティ比１／２の矩形交流波信号であり、交流信号の位相は電極２４ａ内および電極２４ｂ内は同位相であり、電極２４ａと電極２４ｂ間は逆位相（位相差１８０°）になる。ここで、位相補正素子制御回路のコモン電圧（例えば０Ｖ）に対する電圧として、電極２４ａの分割電極、給電部の電圧をＶ_n（Ｓ）（ｎ＝１〜４）、電極２４ｂの分割電極の電圧をＶ_m（Ｄ）（ｍ＝１〜５）とすれば、位相は両者で１８０°ずれているから、ある瞬間ではＶ_n（Ｓ）＞０、Ｖ_m（Ｄ）＜０であり、またある瞬間ではＶ_n（Ｓ）＜０、Ｖ_m（Ｄ）＞０になる。したがって、液晶分子２８を駆動する実効電圧Ｖ_nm（Ｅ）は、［Ｖ_n（Ｓ）−Ｖ_m（Ｄ）］_rmsであり、Ｖ_n（Ｓ）とＶ_m（Ｄ）の差のｒｍｓ値（振幅の自乗の時間的平均の平方根）になる。
【００７２】
本例の場合、デューティ比が１／２で位相が１８０°ずれた矩形交流波であるので、単に差の絶対値｜Ｖ_n（Ｓ）−Ｖ_m（Ｄ）｜に一致する。印加する電圧Ｖ_n（Ｓ）、Ｖ_m（Ｄ）は補正する収差分布により異なる。
【００７３】
まず球面収差だけを補正する場合、コマ収差補正用である分割電極６１〜６５には固定電圧を印加する。本例ではＶ_m（Ｄ）＝１Ｖ（ｍ＝１〜５）とした。球面収差補正用である電極２４ａについては、信号１を固定電圧として分割電極５１、給電部５４に対してＶ_n（Ｓ）＝１Ｖ（ｎ＝１、４）とし、信号２には光ディスクの厚さムラに対応する電圧として分割電極５２、給電部５３にＶ_n（Ｓ）＝０．５〜１．５Ｖ（ｎ＝２、３）を印加した。よって実効電圧Ｖ_nm（Ｅ）は、分割電極５１、給電部５４では常に２Ｖ_rmsになり、分割電極５２、給電部５３では光ディスクの厚さムラに応じて１．５〜２．５Ｖ_rmsまで変化する。その結果、例１と同様にメタル電極間に発生する連続的な電位分布により、実効電圧も連続的に変化するため、電極パターンに応じた位相変化を得ることができる。
【００７４】
つぎにコマ収差だけを補正する場合、前述とは逆に球面収差補正用の電極２４ａに対しては、信号１、２とも固定電圧とし、分割電極、給電部ともＶ_n（Ｓ）（ｎ＝１〜４）＝１Ｖを印加した。一方、コマ収差補正用の電極２４ｂに対しては分割電極６３には固定電圧１Ｖを印加した。分割電極６１と６４、６２と６５に対しては、それぞれ等しい電圧を光ディスクのチルト量に応じて、０．５〜１．５Ｖの電圧を、（Ｖ₁（Ｄ）＋Ｖ₂（Ｄ））／２＝Ｖ₃（Ｄ）の関係を満足するよう印加した。
【００７５】
よって実効電圧Ｖ_nm（Ｅ）は、分割電極６３では常に２Ｖ_rmsになり、分割電極６１、６２、６４、６５では光ディスクのチルト量に応じて１．５〜２．５Ｖ_rmsの範囲で変化する。その結果、図１１に示した電極パタ−ンの形状に等しい電位分布が発生し、位相変化を得ることができた。
【００７６】
つぎに球面収差とコマ収差を同時に補正する場合について述べる。この場合、分割電極６３と分割電極５１、給電部５４には固定電圧１Ｖを印加し、分割電極６１、６２、６４、６５は光ディスクのチルト量に応じて０．５〜１．５Ｖを、メタル電極８２には光ディスクの厚さムラ量に応じて０．５〜１．５Ｖを印加する。これにより、上述の場合と同様にコマ収差、球面収差に対応した電位分布が発生する。したがって、位相変化が生じた結果、厚さムラのある光ディスクがチルトした場合においても球面収差とコマ収差を補正できるので、良好な再生信号を得ることができた。
【００７７】
以上は、コマ収差補正用の電極を分割電極としたが、例１のようにコマ収差用の電極を給電部であるメタル電極にして、球面収差用の電極を同心円に分割した分割電極としてもよい。また、ラジアルコマ収差補正とタンジェンシャルコマ補正の電極パターンを対にしてそれぞれの基板上に形成してもよく、球面収差と非点収差、コマ収差と非点収差を各々補正する電極パターンを対にしてもよい。いずれの場合にも２種類の収差や波面変化を同時に補正できる。
【００７８】
【発明の効果】
以上のように、本発明の光ヘッド装置においては、位相補正素子を構成する一対の基板のそれぞれに形成された電極の少なくとも一方の電極に２つ以上の給電部を設けることにより、この位相補正素子により光源からの出射光に連続的な位相（波面）変化を生じさせることができるので、光ディスクのチルトや光ディスク厚さムラなどにより発生する波面収差を効率よく補正でき、ノイズの少ない良好な信号光が得られる。また、複数の給電部を薄膜抵抗を介して導電接続することにより、従来よりも少ない信号源で同等の収差補正性能を発揮できる。
【図面の簡単な説明】
【図１】本発明の光ヘッド装置の原理構成の一例を示す概念的断面図。
【図２】本発明における位相補正素子の一例を示す断面図。
【図３】光ディスクが１゜チルトしたときの波面収差を示す図。
【図４】従来の位相補正素子の電極パターンを示す模式的平面図。
【図５】図４の位相補正素子の等価回路を示す回路図。
【図６】本発明における位相補正素子により発生した位相変化量の一例を示す図
【図７】本発明における位相補正素子の電極パターンの一例を示す模式的平面図。
【図８】図７の位相補正素子の等価回路の一例を示す回路図。
【図９】本発明の位相補正素子により発生した位相変化の一例を示す図。
【図１０】実施例２の位相補正素子の一方の電極パターンを示す模式的平面図。
【図１１】実施例２の位相補正素子の他方の電極パターンを示す模式的平面図。
【符号の説明】
１：半導体レーザ
２：偏光ビームスプリッタ
３：コリメートレンズ
４：位相補正素子
５：４分の１波長板
６：対物レンズ
７：アクチュエータ
８：光ディスク
９：光検出器
１０：位相補正素子制御回路
１１：立ち上げミラー
２１ａ、２１ｂ：ガラス基板
２２：シール材
２３：液晶
２４ａ、２４ｂ：電極
２５：絶縁膜
２６：配向膜
２７：電極引出部
２８：液晶分子
３０、４０：透明電極
３１〜３４、４１〜４４、５３、５４：給電部
４５、５６、５７：薄膜抵抗
５１、５２、５５、６１〜６５：分割電極[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical head device for recording / reproducing an optical recording medium such as an optical disk.
[0002]
[Prior art]
A DVD, which is an optical disk, records digital information at a higher density than a CD, which is also an optical disk, and an optical head device for reproducing a DVD has a light source wavelength of 650 nm or 635 nm, which is shorter than 780 nm of the CD. The numerical aperture (NA) of the objective lens is set to 0.6, which is larger than 0.45 of CD, so that the spot diameter focused on the optical disk surface is reduced.
[0003]
Further, in the next generation optical recording, it has been proposed to obtain a higher recording density by setting the wavelength of the light source to about 400 nm and the NA to 0.6 or more. However, due to the shortening of the wavelength of the light source and the increase of the NA of the objective lens, the allowable amount of tilt in which the optical disk surface is tilted from the right angle with respect to the optical axis and the allowable amount of uneven thickness of the optical disk are reduced.
[0004]
The reason why these allowances are small is that coma aberration occurs when the optical disk is tilted, and spherical aberration occurs when the optical disk is uneven in thickness. This is because it becomes difficult to read the signal. In high-density recording, several methods have been proposed in order to increase the allowable amount of the optical head device with respect to tilt and thickness unevenness of the optical disk.
[0005]
As one method, there is a method in which an axis for tilting is added to an objective lens actuator that normally moves in the biaxial direction of the tangential direction and the radial direction of the optical disc so that the objective lens is tilted according to the detected tilt angle. is there. However, this additional method has problems such that spherical aberration cannot be corrected and the structure of the actuator becomes complicated.
[0006]
As another method, there is a method in which wavefront aberration is corrected by a phase correction element provided between the objective lens and the light source. In this correction method, the allowable amount of tilt of the optical disk and the allowable amount of thickness unevenness can be increased only by incorporating an element into the optical head device without significantly modifying the actuator.
[0007]
For example, Japanese Patent Laid-Open No. 10-20263 discloses the above correction method for correcting the tilt of an optical disc using a phase correction element. This is because the voltage is applied to the divided electrodes formed by dividing the electrodes on each of the pair of substrates sandwiching the birefringent material such as liquid crystal constituting the phase correction element, and the birefringence is achieved. In this method, the substantial refractive index of the material is changed in accordance with the tilt angle of the optical disk, and the coma aberration generated by the tilt of the optical disk is corrected by the change of the phase (wavefront) of the transmitted light generated by the change of the refractive index. .
[0008]
[Problems to be solved by the invention]
In the conventional phase correction element, in order to correct the wavefront aberration by changing the wavefront of the light emitted from the light source, the electrodes included in the phase correction element are divided into a plurality of voltages and different control signals are applied. There is a need. Therefore, in order to obtain a desired wavefront shape, a large number of electrodes, wirings, and external signal sources (power supplies) are required, and problems such as complicated element configuration and complicated equipment due to the use of a large number of external signal sources (power supplies). Will occur. On the other hand, there has been a demand to reduce the number of electrodes, wirings, and external signal sources (power supplies) as much as possible.
[0009]
If attention is paid to one electrode, the change amount of the wavefront is the same, so it is difficult to change it continuously. In particular, it has been desired to continuously change a region where the amount of change in wavefront aberration, such as a peripheral portion of spherical aberration, is large. Furthermore, since an external signal cannot be applied to the region between the divided electrodes, it may cause a decrease in light transmittance due to light scattering or the like. Therefore, it has been desired to reduce the number of divided electrodes as much as possible to reduce the number of regions between the electrodes.
[0010]
[Means for Solving the Problems]
The present invention includes a light source, an objective lens for condensing the emitted light from the light source on the optical recording medium, a phase correction element for changing the wavefront of the emitted light provided between the light source and the objective lens, An optical head device comprising a control voltage generating means for outputting a voltage for changing the wavefront to the phase correction element, the phase correction element comprising an anisotropic optical medium sandwiched between a pair of substrates, Electrodes for applying a voltage to the anisotropic optical medium are formed on the opposing surfaces of the pair of substrates, and at least one of the electrodes has three or more power feeding portions different from each other. Of the three or more power supply units, two or more of the power supply units are conductively connected via a thin film resistor made of a conductive thin film, and one or more power supply units are connected to the thin film resistor. Not connected to The sheet resistance of the electrode material forming the electrode is all 1000 times or more the sheet resistance of the power feeding part material forming the power feeding part. An optical head device is provided.
[0011]
A light source; an objective lens for condensing the light emitted from the light source on the optical recording medium; a phase correction element for changing the wavefront of the emitted light provided between the light source and the objective lens; An optical head device comprising a control voltage generating means for outputting a voltage for changing to a phase correction element, the phase correction element comprising an anisotropic optical medium sandwiched between a pair of substrates, Electrodes for applying a voltage to the anisotropic optical medium are formed on the opposing surfaces of the substrate, and at least one of the electrodes has a plurality of power feeding portions formed at different positions, and the power feeding portion The electrode in which the electrode is formed is divided into a plurality of divided electrodes, and one or more of the divided electrodes are provided with two or more power feeding portions with respect to the one divided electrode. Through a thin film resistor consisting of And two or more of the number of feeding parts are conductively connected The sheet resistance of the electrode material forming the electrode is all 1000 times or more the sheet resistance of the power feeding part material forming the power feeding part. An optical head device is provided.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an example of the principle configuration of the optical head device of the present invention. The optical head device shown in FIG. 1 is for reproducing information recorded on an optical disk 8 such as a CD or a DVD. Light emitted from a semiconductor laser 1 as a light source is, for example, a hologram type polarization beam splitter 2. After being transmitted, it becomes parallel light by the collimating lens 3, and after passing through the phase correction element 4, Transmitted through the quarter wave plate 5, Reflected in the 90 ° direction by the rising mirror 11 A The light is condensed on the optical disk 8 by the objective lens 6 installed in the actuator 7. The pair of substrates constituting the phase correction element 4 are both transparent. Neither of the substrates may be transparent, but only one may be transparent, as will be described later.
[0013]
The condensed light is reflected by the optical disk 8 and the objective lens 6 , Standing Lifting mirror 11, Quarter wave plate 5, After passing through the phase correction element 4 and the collimating lens 3 in the reverse order, the light is diffracted by the polarization beam splitter 2 and incident on the photodetector 9. When the light emitted from the semiconductor laser 1 is reflected by the optical disk 8, the reflected light is amplitude-modulated by the information recorded on the surface of the optical disk, and the recorded information can be read as a light intensity signal by the photodetector 9. it can.
[0014]
The polarization beam splitter 2 includes, for example, a polarization hologram, and strongly diffracts light having a polarization component in the anisotropic direction (direction in which there is a difference in refractive index) and guides it to the photodetector 9. A voltage is output to the phase correction element 4 by the phase correction element control circuit 10 serving as a control voltage generation unit so that the intensity of, for example, a reproduction signal of the optical disk obtained from the photodetector 9 is optimized. The voltage output from the phase correction element control circuit 10 is a voltage corresponding to the tilt amount of the optical disk and the shift amount of the objective lens, and is a substantially changing voltage applied to the electrode of the phase correction element 4.
[0015]
The rising mirror 11 reflects the light emitted from the semiconductor laser 1 in the direction of approximately 90 ° and makes it incident on the optical disk, and reduces the thickness of the optical head device (direction perpendicular to the surface of the optical disk 8). Is a preferred optical component to use. Usually, a glass surface with a highly reflective film such as aluminum deposited thereon is used.
In FIG. 1, the rising mirror 11 is used and the optical path of the light emitted from the semiconductor laser 1 is changed. However, the direction of the emitted light from the semiconductor laser 1 is changed from the surface of the optical disk 8 without using the rising mirror 11 from the beginning. You may make it become perpendicular | vertical to.
[0016]
As the anisotropic optical medium, an optical crystal such as lithium niobate or a liquid crystal can be used. It is preferable to use a liquid crystal as the anisotropic optical medium because a substantial refractive index can be easily controlled by a voltage as low as about 6 V and continuously controlled according to the magnitude of the voltage. Furthermore, mass productivity is preferable as compared with optical crystals such as lithium niobate. Therefore, hereinafter, a case where a liquid crystal material is used as the anisotropic optical medium will be described.
[0017]
The liquid crystal material used is nematic liquid crystal used for display applications, and may be twisted by adding a chiral agent.
Further, as the material of the substrate to be used, glass, acrylic resin, epoxy resin, vinyl chloride resin, polycarbonate resin, and the like can be used, but a glass substrate is preferable from the viewpoint of durability. Therefore, the case where glass is used as the material of the substrate will be described below.
[0018]
Next, the configuration of the phase correction element used in the present invention will be described with reference to FIG. The glass substrates 21a and 21b are bonded by a sealing material 22 whose main component is, for example, an epoxy resin to form a liquid crystal cell. The sealing material 22 includes, for example, a glass spacer and, for example, a conductive spacer having a resin surface coated with gold or the like. On the inner surface of the glass substrate 21a, there are an electrode 24a from the inner surface, an insulating film 25 mainly composed of silica and the alignment film 26 in this order, and on the inner surface of the glass substrate 21b, the electrode 24b and silica from the inner surface. An insulating film 25 and an alignment film 26 mainly composed of, for example, are coated in this order. An antireflection film may be coated on the outer surface of the liquid crystal cell.
[0019]
The electrode 24a is wired in a pattern so that it can be connected to the phase correction element control circuit by a connection line at the electrode lead-out portion 27. The electrode 24b is conductively connected to the electrode 24a formed on the glass substrate 21a by the conductive spacer coated with gold or the like. Therefore, the electrode 24b is connected to the phase correction element control circuit by the connection line at the electrode lead portion 27. Can be connected. FIG. 2 does not show the state where the electrode 24b and the electrode 24a are in contact with the sealing material 22, but the electrode 24b and the electrode 24a are in contact with the sealing material parallel to the paper surface, and both electrodes are conductively connected through a conductive spacer. . Liquid crystal 23 is filled in the liquid crystal cell, and the liquid crystal molecules 28 shown in FIG. 2 are in a homogeneous alignment state aligned in one direction.
[0020]
In the phase correction element according to the present invention, a plurality (two or more) of power feeding portions for supplying different voltages are formed at different positions in the surface of at least one of the electrodes 24a and 24b. That is, in the case of one electrode, two or more power feeding parts are formed, and in the case of both electrodes, two or more power feeding parts (total of four or more) are formed.
[0021]
As a material for the alignment film, it is preferable that the pretilt angle of the liquid crystal molecules 28 is 2 to 10 °, and a polyimide film that is rubbed in the horizontal direction parallel to the paper surface of FIG. Good. In addition, it is preferable to increase the difference between the ordinary light refractive index and the extraordinary light refractive index of the liquid crystal so as to reduce the interval between the liquid crystal cells because the responsiveness can be improved. However, the smaller the interval between the liquid crystal cells, the more difficult it is to manufacture the liquid crystal cell. It is preferable to do.
[0022]
In the case of the optical head device shown in FIG. 1, since both of the pair of substrates are transparent and light is transmitted through the phase correction element 4, it is desirable that the materials of the electrodes 24a and 24b have high transmittance. A transparent conductive film such as a zinc oxide film may be used. In this case, the phase correction element 4 is used as a transmissive element.
[0023]
However, when only one of the pair of substrates is a transparent substrate, either one of the electrodes 24a and 24b is manufactured using a highly reflective material such as aluminum or chromium, and the phase correction element 4 is used as a reflective element. Can be used. At this time, the phase correction element 4 can be installed at this position instead of the rising mirror 11 of FIG. If the first electrode (for example, electrode 24a) on which light first enters is a transparent electrode having a high transmittance and the other electrode (for example, electrode 24b) is a highly reflective electrode, the light incident on the phase correction element 4 Passes through the transparent electrode 24a and the liquid crystal and is reflected by the electrode 24b, and then passes again through the liquid crystal and the transparent electrode 24a toward the optical disc 8.
[0024]
If a reflection type element is used as the phase correction element 4 as described above, that is, if one of the pair of substrates constituting the phase correction element is a transparent substrate, the rising mirror 11 in FIG. This is preferable because the number of parts can be reduced and the thickness of the optical head device can be reduced. In this case, since the light incident on the phase correction element 4 passes through the liquid crystal 23 at an angle of approximately 45 °, a different liquid crystal cell interval (the thickness of the liquid crystal layer in the liquid crystal cell) is set. You just have to.
[0025]
The configuration necessary for the function of changing the wavefront using the phase correction element has been described above. By laminating the wave plate and the polarization hologram on the phase correction element 4, the functions of the wave plate 5 and the polarization beam splitter 2 are phase-shifted. The correction element 4 can also be provided. In this case, the number of optical components constituting the optical head device is reduced, so that assembly and adjustment are simplified, and productivity is improved, which is preferable.
[0026]
The phase correction element 4 is laminated with a dichroic aperture limiting layer for changing the beam diameter according to the diffraction grating or the wavelength of the light source, or the dichroic aperture limiting layer is directly formed on the outer surfaces of the glass substrates 21a and 21b. In this case as well, productivity is improved as compared with the addition of individual parts. When laminating the wave plates, they may be directly bonded to the glass substrate on the optical disk side or the laminated glass substrate may be further laminated.
[0027]
Next, a thin film resistor that is conductively connected to a power feeding portion for supplying a voltage, which is formed on an electrode on a substrate constituting the phase correction element in the present invention, will be described. In the phase correction element according to the present invention, for example, two or more of the above-described feeding portions formed on electrodes on the same substrate are conductively connected on the substrate surface by a thin film resistor formed of a conductive thin film. The effect obtained by providing a thin film resistor will be described in detail below, taking as an example the case of correcting spherical aberration.
[0028]
FIG. 3 shows a wavefront aberration (spherical surface) that occurs when the optical disk thickness is 0.03 mm thicker than the designed value of 0.6 mm in an optical system with an objective lens NA of 0.65 and a light source wavelength of 0.4 μm. (Aberration). When the optical disc is thicker than the design value, the phase of the middle part sandwiched between the central part and the peripheral part of the effective pupil is advanced, and when the optical disk is thin, the phase is delayed. .
[0029]
In order to facilitate comparison with the phase correction element in the present invention, a conventional phase correction element will be described here. FIG. 4 is a conventional example of an electrode pattern of a phase correction element used for correcting the spherical aberration as described above, and has a configuration without a thin film resistor. The hatched portion in FIG. 4 is a transparent electrode 30 formed of a high-resistance transparent conductive film. Feed portions 32, 33, and 34 are formed concentrically around the feed portion 31. The power supply units 31 to 34 are connected to an external signal source by the wiring shown in FIG. 3, the power supply units 31 and 34 are supplied with the signal 1, the power supply unit 32 is supplied with the signal 3, and the power supply unit 33 is supplied with the signal 2. Voltage can be applied.
[0030]
Therefore, the conventional example requires an external signal source (three in FIG. 4) that can generate three or more signals. FIG. 5 is an equivalent circuit diagram of an electrode pattern of a conventional phase correction element. Points 35, 36, 37, and 38 in FIG. 5 correspond to the power feeding units 31, 32, 33, and 34 shown in FIG. Resistance R ₁ Is a resistance between the power feeding parts 31 and 32 caused by the transparent electrode 30, and similarly R ₂ , R _Three Is the resistance between the power supply units 32 and 33 and the power supply units 33 and 34. Here, the resistance of the power feeding unit and the resistance of the wiring between the power feeding unit and the external signal source are the resistance R caused by the transparent electrode 30. ₁ , R ₂ , R _Three Ignore it in the equivalent circuit because it is sufficiently small.
[0031]
The materials of the transparent electrode 30 and the power feeding part will be described in detail later. In order to correct the spherical aberration of FIG. ₁ , V ₂ , V _Three Is applied to generate a voltage drop in the plane of the transparent electrode 30 to develop a continuous potential distribution. FIG. 6 is a diagram comparing the relationship between the amount of spherical aberration and the magnitude of the potential at a cut surface passing through the center point, and the voltage V ₁ , V ₂ , V _Three By appropriately setting, the potential distribution shape can be matched with the aberration distribution shape.
[0032]
The orientation direction of the liquid crystal molecules inside the phase correction element is continuously changed depending on the location by applying a voltage. Therefore, in the voltage distribution that changes continuously as described above, the orientation direction changes continuously depending on the location, and thus the substantial refractive index difference δn of the liquid crystal birefringence changes continuously. Since the wavefront of the incident light is phase-shifted according to the magnitude of δn, the phase shift amount can be changed according to the magnitude of the applied voltage. Therefore, the wavefront aberration can be canceled and corrected by applying a voltage according to the amount of generated aberration.
[0033]
The above is the conventional phase correction element. Next, the phase correction element in the present invention will be described. FIG. 7 is a diagram showing an electrode pattern of the phase correction element in the present invention, in which a thin film resistor 45 is installed. The transparent electrode 40, the power feeding parts 41, 42, 43 and 44 and the signals 1 and 2 are the same as those in the conventional example of FIG. 4, and the power feeding part 42 is conductively connected to the power feeding parts 41 and 44 using a thin film resistor 45. The difference is that 1 can be applied.
In FIG. 8, the points 46, 47, 48, and 49 correspond to the power feeding portions 41, 42, 43, and 44 as in FIG. ₁ , R ₂ , R _Three Represents the resistance due to the transparent electrode 40 between the power feeding portions 41 and 42, 42 and 43, and 43 and 44, respectively. R _S Indicates the resistance of the thin film resistor 45, and the voltage V supplied from the signal 1 so that the point 47 becomes a desired voltage. ₁ Is divided. Therefore, in the conventional example, the voltage at the point 47 (corresponding to the point 36 in FIG. 5) is obtained from the signal 3 generated from another signal source. Therefore, it is possible to operate with fewer signal sources than in the past. Voltage V at point 47 _Three Is obtained from equation (1) calculated using Ohm's law.
[0034]
[Expression 1]

[0035]
Therefore, the resistance R of the transparent electrode ₁ , R ₂ And drive voltage V ₁ , V ₂ R is obtained if _S By appropriately setting, a voltage equal to the voltage conventionally supplied from the external signal source can be applied.
[0036]
Resistance R of thin film resistor _S Is the sheet resistance ρ of the conductive thin film constituting the thin film resistor. _L , R using the width W and length L of the thin film resistor _S = Ρ _L × L / W can be written. For example R _S = 10 kΩ for ρ _L = 300Ω / □, L = 1 mm, W = 30 μm. If the line width W of the thin film resistor is too narrow, the resistance variation due to the shape error increases. When the length L becomes long and it becomes difficult to install on the substrate, it may be bent halfway.
[0037]
In the above configuration, the voltage V by the signal 1 ₁ The voltage at point 47 is set by dividing the voltage V. ₂ May be divided. In this case, the thin film resistor is connected to the signal 2 side wiring in FIG. 7, and in the equivalent circuit of FIG. ₂ Are placed between points 47 and 48 so as to be in parallel.
[0038]
Next, the resistance value and material of the electrode, the power feeding unit, the thin film resistor will be described. Sheet resistance ρ of the power supply material forming the power supply _S And sheet resistance ρ of the electrode material forming the electrode _T Ratio of ρ _T / Ρ _S Is preferably 1000 or more. ρ _T / Ρ _S If the current is small, a relatively large current flows through the electrode, and a voltage drop may occur in the power supply unit in contact with the electrode, making it difficult to obtain a desired voltage distribution. Therefore, as the sheet resistance of the electrode material is higher than that of the power feeding part material, the potential can be easily changed continuously between the neighboring power feeding parts, and a desired potential distribution can be obtained. ρ _T / Ρ _S Is a guideline for satisfying this condition.
[0039]
However, ρ _T If is too large, the electrical conductivity of the power feeding section is lost and potential distribution does not occur. Therefore ρ _S Is preferably as small as possible, ρ _S Is about 0.1 to 10Ω / □, ρ _T Is preferably about 100 to 100 kΩ / □.
Satisfy the above conditions and properly ρ _S And ρ _T When two or more power feeding units are formed only on one electrode and different voltages are supplied to the two or more power feeding units, respectively, the power feeding unit S ₁ , S ₂ , S _Three ... Equipotentials in each power supply unit, but the potential distribution in the electrode surface changes continuously due to a voltage drop generated between the power supply units. This continuously changing situation is the same even when two or more power feeding portions are formed on two (two surfaces) electrodes and different voltages are supplied.
[0040]
As the power feeding part material, a metal material such as copper, gold, aluminum and chromium is preferable from the viewpoint of conductivity and durability, but the specific resistance is 10 at room temperature. ^-8 -10 ^-7 A material other than metal may be used as long as it is about Ω · m. For example, a transparent conductive film such as an ITO film can also be used, which is preferable because a light-shielding portion is eliminated as compared with the case where a metal material is used, and the light transmittance is increased. However, since the transparent conductive film has a larger specific resistance than the metal film, it is necessary to increase the film thickness in order to reduce the sheet resistance.
[0041]
The wiring material on the electrode lead portion 27 for applying a voltage to the power feeding portion from an external phase correction element control circuit may be a transparent conductive film such as an ITO film or a metal film such as chromium or nickel. In particular, a metal that can be connected by solder such as nickel is preferable because an external signal line can be easily connected by solder.
[0042]
On the other hand, the electrode material needs to be transparent and have a higher sheet resistance than the power supply material. An ITO film that is a transparent conductive film is preferable, and the ITO film is preferably as high as possible in sheet resistance. Furthermore, it is ρ more than 1kΩ / □ _S Can be reduced to about 1 Ω / □, so that the thickness of the power feeding portion can be reduced, which facilitates production and is more preferable.
[0043]
ρ _T Use of a zinc oxide film, a gallium-containing zinc oxide film (GZO film), or a gallium and silicon-containing zinc oxide film (GZS film) in order to increase the thickness of the film makes it easier to obtain a high resistance film than an ITO film. preferable. In particular, the GZO film and the GZS film are suitable materials in the optical head device of the present invention in that they have a high specific resistance but also have good etching properties and are excellent in light transmittance and durability.
[0044]
When gallium is added to the zinc oxide film, the amount of addition is preferably 1 to 10% by mass because the transmittance of the film changes. In addition, even when both gallium and silicon are added, since the transmittance of the film changes, the total addition amount is preferably 1 to 20% by mass.
[0045]
On the other hand, as a material of the thin film resistor, V shown in the formula (1) _Three R so that the value of ₁ , R ₂ , R _Three It is necessary to decide. When used in an optical head device, setting the resistance value of the thin film resistor to 100 Ω to 1000 kΩ is desirable because it facilitates the production of the electrode and the thin film resistor, and ITO, GZO, GZS, or the like can be used. Since the material of the thin film resistor is made the same as that of the electrode material and is formed at the same time as the electrode is formed, even if the resistance value of the material varies depending on the lot, the influence is offset by the denominator / numerator as shown in the equation (1) V _Three It is preferable without affecting.
[0046]
Next, the shape and size of the power feeding unit will be described. It is preferable to change the shape and size of the power feeding unit according to the situation as described above. That is, the change of the wavefront generated by the phase correction element depends on the shape and size of the power feeding unit, and may be changed according to the type of wavefront aberration to be corrected and the wavefront shape to be generated. Here, the wavefront aberration includes coma, spherical aberration, astigmatism and the like.
[0047]
As described above, coma is an aberration caused by the tilt of the optical disk, and is 180 ° around a straight line that passes through the center of the incident light beam on the phase correction element and is parallel to the element surface and parallel to the rotation direction of the optical disk. It has a shape that overlaps when rotated. Therefore, the power feeding unit is preferably arranged so as to be symmetric with respect to the above-described parallel straight lines.
[0048]
Specifically, for example, a rectangular or linear power supply unit is provided at the center of one continuous electrode, and a power supply unit having a shape (arc or the like) at the periphery of the electrode is provided at the periphery. And a power feeding part is arrange | positioned so that those power feeding parts may become symmetrical with respect to the above-mentioned straight line. Arranging the power feeding portion in this way is preferable because coma aberration can be corrected most effectively.
[0049]
When correcting spherical aberration, since the spherical aberration is concentric with the optical axis as the center, the power supply section preferably has two or more toric bodies formed concentrically. Further, when the luminous flux radius of the light emitted from the light source passing through the phase correction element is 1, the radius of one torus has a value from 0.65 to 0.85, and one different from the above The radius of the torus is preferably between 0.2 and 0.4. Here, since the torus is donut-shaped and the radius has a width, the radius of the torus means the average value of the inner radius and the outer radius.
[0050]
Usually, since there is a maximum value of spherical aberration in a region (region A) surrounded by a circle with a radius of 0.65 and a circle with a radius of 0.85, one toric body serving as a power feeding unit has an optical axis and a center. It is preferable to form the region A together. Further, in order to correct spherical aberration in a region (region C) surrounded by a radius of 0.2 and a radius of 0.4, one toroidal power feeding portion different from the above is used as the optical axis. It is preferable to provide the centers together.
[0051]
In place of the circle having the radius of 0.65 and the circle having the radius of 0.85, the circle having the radius of 0.7 and the circle having the radius of 0.8 are narrower than the region A but have the maximum value of spherical aberration. It is preferable because a torus can be added to a region with a high probability, and another torus can be added to finely adjust spherical aberration.
[0052]
In the case of a single electrode having a continuous electrode, a toric feed portion is provided in the vicinity of the optical axis (region B) including the optical axis and having a radius smaller than 0.2, with the optical axis and the center aligned. Therefore, it is more preferable that the spherical aberration can be finely adjusted. Even when the electrode is a divided electrode and the region B is separated from other regions (region A, region C, etc.), it is sufficient because the spherical aberration can be corrected with extremely high accuracy as described above.
[0053]
In the case of astigmatism, a desired potential distribution can be obtained as the number of the plurality of radial power supply portions provided on one continuous electrode and passing through one point at the center of the electrode is preferably increased. Furthermore, wavefront aberration including both coma and spherical aberration can be corrected. In this case, the above-described linear power supply unit and concentric power supply unit may be combined.
[0054]
As described above, a feeding portion having a different shape can be provided for each of a pair of continuous electrodes, and one electrode can correct coma aberration and the other electrode can correct spherical aberration. Similarly, by using one of the pair of electrodes as one continuous electrode having a power feeding part and the other as a divided electrode divided into a plurality of parts, both continuous aberration distribution and stepwise aberration distribution are generated. It can also be made. Wavefront aberrations such as coma, spherical aberration, and astigmatism are generated by the optical head device as a system. Therefore, the wavefront aberration is effectively corrected by incorporating the phase correction element of the present invention in the optical head device. it can.
[0055]
Since the phase correction element according to the present invention has a function of changing the wavefront shape of transmitted light, it can be used not only for correcting wavefront aberration but also for other purposes such as changing the focal position of the light based on the same principle. . For example, it can be used for the purpose of changing the focal position of the transmitted light simply by changing the optical magnification, or for changing the traveling direction of the light by inclining and emitting the transmitted wavefront.
[0056]
Even when wavefront aberration is corrected, higher-order wavefront aberrations such as the above-mentioned coma aberration, spherical aberration, and astigmatism can also be corrected. Even in these cases, the shape, number, position, or electrode dividing method of the power feeding unit may be set as appropriate in accordance with a desired change in the wavefront.
[0057]
In the phase correction element according to the present invention, an electrode provided with a power feeding unit is divided into a plurality of divided electrodes, and each divided electrode is provided with one or more power feeding units, two of the power feeding units. The above is preferably conductively connected by a thin film resistor. A continuous voltage distribution can be generated if two or more power supply parts that are conductively connected are power supply parts on the same divided electrode, and discontinuous voltage distribution is preferable if the power supply parts are on different divided electrodes. Can be used when necessary.
[0058]
In either case, the optical head device can be operated with fewer external signal sources than in the prior art by conductively connecting two or more power supply units using thin film resistors. In addition, the same effect can be obtained in the case where one of the connection destinations of the thin film resistor is a power supply unit and the other is a divided electrode having no power supply unit.
[0059]
【Example】
"Example 1"
The optical head device of this example includes a phase correction element that corrects spherical aberration caused by thickness unevenness of the optical disk. When the thickness of the optical disk deviates from the designed value, the objective lens generates spherical aberration and the signal reading accuracy is lowered. A phase correction element for correcting this spherical aberration was incorporated as the phase correction element 4 of the optical head device of FIG. However, the phase correction element control circuit 10 is improved for the phase correction element of this example.
[0060]
The element structure of the phase correction element of this example is the same as that shown in FIG. The principle of correcting spherical aberration by the phase correction element of this example will be described below. The NA of the objective lens in the optical head device of this example was 0.65, and the wavelength of the light source was 0.4 μm. The wavefront aberration (spherical aberration) generated when the thickness of the optical disk is 0.03 mm thicker than the designed value of 0.6 mm is shown in FIG. 3 as described above. The electrode pattern of the phase correction element in this example is as shown in FIG. 7, and the equivalent circuit is as shown in FIG.
[0061]
In FIG. 7, shaded portions are transparent electrodes 40 formed of a GZO film, and thick line portions (annular bodies) are power feeding portions 41, 42, 43, 44 in which a chromium thin film is formed by an etching technique. The power feeding unit is connected to signals 1 and 2 that are external signal sources by wiring of the same chrome thin film formed on the same substrate surface. The power feeding unit 42 is connected to the signal 1 by a thin film resistor 45 formed on the same substrate surface.
[0062]
The widths of the power feeding portions 41 to 44 were 100 μm, the diameters of the power feeding portions 42, 43, and 44 were 0.5, 1.5, and 2.2 mm, respectively, and the power feeding portion 41 was a circle with a diameter of 50 μm. The pattern of the above electrodes and the power feeding part was formed as follows. First, after a chromium film was deposited on a glass substrate by a sputtering method, unnecessary portions were removed by an etching technique to form a power feeding portion and a wiring. Next, after depositing an ITO film by a sputtering method, a thin film resistor 45 was formed by an etching technique. Thereafter, a GZO film was deposited by sputtering and a transparent electrode 40 was formed by etching technique.
[0063]
As the sheet resistance value of each part, the power feeding part was 1Ω / □, the electrode was 100 kΩ / □, and the thin film resistance was 300Ω / □. Resistance R in FIG. ₁ , R ₂ , R _Three The electrode resistance values between the power feeding portions corresponding to were 50, 28, and 20 kΩ, respectively. The resistance value and shape of the thin film resistor were determined as follows. The potentials of the power feeding units 41 and 44 are changed to V by the signal 1. ₁ = 2V, the potential of the power feeding unit 43 is V by the signal 2 ₂ = 3V, in order to set the potential of the power feeding section 42 corresponding to the point 47 in FIG. _S = 5.48 kΩ. Therefore, the sheet resistance ρ _L Is 300 Ω / □, and when the line width W of the thin film resistor is 30 μm, the length L is R _S = Ρ _L X It becomes about 0.55 mm from L / W. In this example, a thin film resistor (linear resistance) having a resistance value of 5.48 kΩ was formed by bending a line having a width of 30 μm and a length of 0.55 mm three times.
[0064]
A voltage of 2.3 V was supplied to the power feeding units 41 and 44 and 2.0 V was supplied to the power feeding unit 43 in order to correct the spherical aberration caused by the thickness unevenness of the 0.03 mm optical disk by the phase correction element. As a result, a voltage of about 2.05 V was supplied to the power feeding unit 42 by the thin film resistor. Here, the electrode opposed to the electrode having the power feeding portion shown in FIG. 7 is composed of one transparent electrode having one power feeding portion, and is always at a potential of 0V.
[0065]
FIG. 9 shows the phase change generated by the phase correction element in nm units. When the phase change between the central portion and the outer peripheral portion of the circle is 0 nm, the phase change in the intermediate region is about −100 nm. Here, a plurality of solid circles are contour lines, and one contour line represents 20 nm inside the intermediate region of −100 nm, and about 30 nm outside.
In the transparent electrode 40 (FIG. 7), a voltage distribution is generated according to the voltage of the power feeding unit. As described above, as a result of the substantial refractive index distribution of the liquid crystal generated by the voltage distribution in the transparent electrode 40 of FIG. 7, the phase correction element can generate the concentric phase change shown in FIG.
[0066]
On the other hand, when the thickness of the optical disk is as thin as 0.03 mm, 2.0 V is supplied to the power supply units 41 and 44 and 2.3 V is supplied to the power supply unit 43 in order to correct the spherical aberration whose polarity is reversed from that in FIG. By doing so, the phase change generated by the phase correction element has a shape in which the sign of FIG. 9 is reversed, so that the spherical aberration can be canceled. As described above, the thin film resistor 45 is provided so as to obtain a desired voltage, and the spherical aberration in FIG. 7 can be corrected by supplying an appropriate voltage to the power feeding units 41, 43, and 44.
[0067]
"Example 2"
The optical head device of this example includes a phase correction element that corrects both spherical aberration caused by thickness unevenness of the optical disc and coma aberration caused by tilt of the optical disc. This phase correction element was incorporated as the phase correction element 4 of the optical head device of FIG. However, the phase correction element control circuit 10 is improved for the phase correction element of this example. The element structure of the phase correction element of this example is the same as that shown in FIG. 2, and the electrode patterns and materials described below are different.
[0068]
As shown in FIG. 10, the electrode 24a in FIG. 2 includes divided electrodes 51, 52, and 55, power feeding portions 53 and 54 formed on the divided electrode 55, and thin film resistors 56 and 57. In FIG. 10, the thin film resistors 56 and 57 are schematically shown, and the actual shape is linear or the like so as to obtain a desired resistance value.
[0069]
The material of the divided electrodes 51, 52, and 55 is a GZS film, and the material of the power feeding portions 53 and 54 and the thin film resistors 56 and 57 is an ITO film. The sheet resistances of the GZS film and the ITO film were 1000 and 10 kΩ / □, respectively. As the electrode 24a, first, an ITO film was formed on a glass substrate by a sputtering method, and the power feeding parts 53 and 54 and the thin film resistors 56 and 57 were formed using a photolithography technique and an etching technique. The width | variety of the electric power feeding parts 53 and 54 was 50 micrometers. Next, a GZS film was formed by sputtering, and divided electrodes 51, 52, and 55 were formed using a photolithography technique and an etching technique. The division interval of the divided electrodes was 5 μm. Signals 1 and 2 in FIG. 10 are signals applied to the divided electrode 51, the power feeding unit 54, and the power feeding unit 53, respectively, and are generated by the phase correction element control circuit 10.
[0070]
On the other hand, the electrode 24b is formed with divided electrodes 61 to 65 as shown in FIG. In the divided electrode of FIG. 11, an ITO film was formed on a glass substrate by a sputtering method, and a pattern was formed by a photolithography technique and an etching technique. The thick line in FIG. 10 shows the gap between the divided electrodes, and no voltage is applied to this portion because the ITO film is removed by etching. The width of the gap between the divided electrodes was 5 μm.
[0071]
The principle of correcting spherical aberration and coma with the phase correction element of this example will be described below. The output waveform of the phase correction element control circuit is a rectangular AC wave signal having a frequency of 1 kHz and a duty ratio of 1/2. The phase of the AC signal is the same in the electrode 24a and the electrode 24b, and between the electrode 24a and the electrode 24b. Becomes an opposite phase (phase difference 180 °). Here, as the voltage with respect to the common voltage (for example, 0 V) of the phase correction element control circuit, the voltage of the divided electrode of the electrode 24a and the voltage of the power feeding unit is V _n (S) (n = 1 to 4), the voltage of the divided electrode of the electrode 24b is V _m If (D) (m = 1 to 5), the phase is shifted by 180 ° between the two. _n (S)> 0, V _m (D) <0, and at some moment V _n (S) <0, V _m (D)> 0. Therefore, the effective voltage V for driving the liquid crystal molecules 28. _nm (E) is [V _n (S) -V _m (D)] _rms And V _n (S) and V _m The rms value of the difference of (D) (the square root of the time average of the square of the amplitude).
[0072]
In this example, since the rectangular AC wave has a duty ratio of 1/2 and a phase of 180 °, the absolute value of the difference | V _n (S) -V _m (D) Matches |. Applied voltage V _n (S), V _m (D) differs depending on the aberration distribution to be corrected.
[0073]
First, when only spherical aberration is corrected, a fixed voltage is applied to the divided electrodes 61 to 65 for correcting coma aberration. In this example, V _m (D) = 1V (m = 1 to 5). With respect to the electrode 24 a for correcting spherical aberration, the signal 1 is set to a fixed voltage and the divided electrode 51 and the power feeding unit 54 have V _n (S) = 1V (n = 1, 4), and signal 2 has a voltage corresponding to the thickness unevenness of the optical disk, and V is applied to the divided electrode 52 and the power feeding unit 53. _n (S) = 0.5 to 1.5 V (n = 2, 3) was applied. Therefore, effective voltage V _nm (E) is always 2 V at the divided electrode 51 and the power feeding unit 54. _rms The split electrode 52 and the power feeding unit 53 are 1.5 to 2.5 V depending on the thickness unevenness of the optical disk. _rms Change to. As a result, the effective voltage also changes continuously due to the continuous potential distribution generated between the metal electrodes as in Example 1, so that a phase change corresponding to the electrode pattern can be obtained.
[0074]
Next, when only coma aberration is corrected, contrary to the above, for the spherical aberration correction electrode 24a, both the signals 1 and 2 are set to a fixed voltage, and both the divided electrode and the power feeding portion are set to V. _n (S) (n = 1-4) = 1V was applied. On the other hand, a fixed voltage of 1 V was applied to the divided electrode 63 for the coma aberration correcting electrode 24b. For the divided electrodes 61 and 64, 62 and 65, a voltage of 0.5 to 1.5 V is set to (V) according to the tilt amount of the optical disk. ₁ (D) + V ₂ (D)) / 2 = V _Three It applied so that the relationship of (D) might be satisfied.
[0075]
Therefore, effective voltage V _nm (E) is always 2 V in the divided electrode 63. _rms The split electrodes 61, 62, 64, 65 are 1.5 to 2.5 V depending on the tilt amount of the optical disk. _rms It varies in the range. As a result, a potential distribution equal to the shape of the electrode pattern shown in FIG. 11 was generated, and a phase change could be obtained.
[0076]
Next, a case where spherical aberration and coma are corrected simultaneously will be described. In this case, a fixed voltage of 1 V is applied to the divided electrode 63, the divided electrode 51, and the power feeding unit 54, and the divided electrodes 61, 62, 64, and 65 are set to 0.5 to 1.5 V depending on the tilt amount of the optical disk. A voltage of 0.5 to 1.5 V is applied to the electrode 82 according to the thickness unevenness of the optical disk. As a result, a potential distribution corresponding to coma and spherical aberration is generated in the same manner as described above. Therefore, as a result of the phase change, the spherical aberration and the coma aberration can be corrected even when the optical disk with thickness unevenness is tilted, so that a good reproduction signal can be obtained.
[0077]
As described above, the coma aberration correcting electrode is a divided electrode. However, as in Example 1, the coma aberration electrode is a metal electrode serving as a feeding portion, and the spherical aberration electrode is divided into concentric circles. Good. Alternatively, radial coma aberration correction and tangential coma correction electrode patterns may be formed on each substrate as a pair, and electrode patterns for correcting spherical aberration and astigmatism, and coma and astigmatism respectively. It may be. In either case, two types of aberrations and wavefront changes can be corrected simultaneously.
[0078]
【Effect of the invention】
As described above, in the optical head device of the present invention, this phase correction is achieved by providing two or more power feeding sections on at least one of the electrodes formed on each of the pair of substrates constituting the phase correction element. Since the element can cause a continuous phase (wavefront) change in the light emitted from the light source, it can efficiently correct the wavefront aberration caused by the tilt of the optical disk or uneven thickness of the optical disk, and a good signal with little noise Light is obtained. In addition, by electrically connecting a plurality of power feeding portions through thin film resistors, the same aberration correction performance can be exhibited with fewer signal sources than in the past.
[Brief description of the drawings]
FIG. 1 is a conceptual cross-sectional view showing an example of the principle configuration of an optical head device of the present invention.
FIG. 2 is a cross-sectional view showing an example of a phase correction element in the present invention.
FIG. 3 is a diagram showing wavefront aberration when the optical disc is tilted by 1 °.
FIG. 4 is a schematic plan view showing an electrode pattern of a conventional phase correction element.
5 is a circuit diagram showing an equivalent circuit of the phase correction element of FIG. 4;
FIG. 6 is a diagram showing an example of a phase change amount generated by the phase correction element in the present invention.
FIG. 7 is a schematic plan view showing an example of an electrode pattern of a phase correction element in the present invention.
8 is a circuit diagram showing an example of an equivalent circuit of the phase correction element of FIG. 7;
FIG. 9 is a diagram showing an example of a phase change generated by the phase correction element of the present invention.
10 is a schematic plan view showing one electrode pattern of the phase correction element of Example 2. FIG.
11 is a schematic plan view showing the other electrode pattern of the phase correction element of Example 2. FIG.
[Explanation of symbols]
1: Semiconductor laser
2: Polarizing beam splitter
3: Collimating lens
4: Phase correction element
5: Quarter wave plate
6: Objective lens
7: Actuator
8: Optical disc
9: Photodetector
10: Phase correction element control circuit
11: Launch mirror
21a, 21b: Glass substrate
22: Sealing material
23: Liquid crystal
24a, 24b: Electrodes
25: Insulating film
26: Alignment film
27: Electrode extraction part
28: Liquid crystal molecules
30, 40: Transparent electrode
31-34, 41-44, 53, 54: Feeding part
45, 56, 57: Thin film resistors
51, 52, 55, 61-65: Divided electrodes

Claims (5)

A light source, an objective lens for condensing the emitted light from the light source on the optical recording medium, a phase correction element for changing the wavefront of the emitted light provided between the light source and the objective lens, and the wavefront are changed. A control voltage generating means for outputting a voltage to the phase correction element,
The phase correction element includes an anisotropic optical medium sandwiched between a pair of substrates, and electrodes for applying a voltage to the anisotropic optical medium are formed on the opposing surfaces of the pair of substrates,
At least one of the electrodes has three or more feeding portions formed at different positions with respect to one of the electrodes,
Wherein the three or more feeding section, two or more power feeding portion via the thin-film resistor made of a conductive thin film are electrically conductive connection, in which one or more feeding unit is not connected to the thin film resistor There,
All of the sheet resistance of the electrode material which forms the said electrode is 1000 times or more of the sheet resistance of the power feeding part material which forms the said electric power feeding part, The optical head apparatus characterized by the above-mentioned . A light source, an objective lens for condensing the emitted light from the light source on the optical recording medium, a phase correction element for changing the wavefront of the emitted light provided between the light source and the objective lens, and the wavefront are changed. A control voltage generating means for outputting a voltage to the phase correction element,
The phase correction element includes an anisotropic optical medium sandwiched between a pair of substrates, and electrodes for applying a voltage to the anisotropic optical medium are formed on the opposing surfaces of the pair of substrates,
At least one of the electrodes has a plurality of power feeding portions formed at different positions,
The electrode on which the power feeding unit is formed is divided into a plurality of divided electrodes,
In one or more of the divided electrodes, two or more power feeding portions are arranged with respect to the one divided electrode,
Two or more of the plurality of power feeding units are conductively connected through a thin film resistor made of a conductive thin film ,
All of the sheet resistance of the electrode material which forms the said electrode is 1000 times or more of the sheet resistance of the power feeding part material which forms the said electric power feeding part, The optical head apparatus characterized by the above-mentioned .   3. The optical head device according to claim 1, wherein all of the thin film resistors have a resistance value ranging from 100Ω to 1000 kΩ. The electrode material, an optical head apparatus according zinc oxide film gallium is added or claims 1, gallium and silicon consists added zinc oxide film, to one of 3. The optical head device according to any one of 4 claims 1 formed by the same material as the thin film resistor and the electrode.

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