JP2004213699A

JP2004213699A - Double refraction amount estimating device of optical disk, and optical disk drive

Info

Publication number: JP2004213699A
Application number: JP2002344962A
Authority: JP
Inventors: Kenichi Furukawa; 憲一古河; Yuichi Maekawa; 雄一前川; Yoshihide Kawahara; 義英川原
Original assignee: Mitsumi Electric Co Ltd
Current assignee: Mitsumi Electric Co Ltd
Priority date: 2002-11-14
Filing date: 2002-11-28
Publication date: 2004-07-29

Abstract

<P>PROBLEM TO BE SOLVED: To estimate the double refraction amount of an optical disk in the rotated state of the optical disk. <P>SOLUTION: In a CPU 47, a signal which is obtained by an optical pickup OPU of polarizing optical system by the reflection from the optical disk DISC to be measured, is measured in the manner of rotating the optical disk DISC to be measured at a plurality of different rotation frequencies and also moving the optical pickup OPU of polarizing optical system toward an outer periphery from an inner periphery of the optical disk DISC to be measured. Then, the double refraction amount of the optical disk DISC to be measured is estimated by the CPU 47 from the above measurement result. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、光ディスクの複屈折量を推定する装置および光ディスクドライブに関する。
【０００２】
【従来の技術】
最近では、パーソナルコンピュータ等の電子機器には光ディスクドライブ（光ディスク装置）が搭載されることが多い。光ディスクドライブに使用可能な記録媒体として、ＣＤ−Ｒ（ｃｏｍｐａｃｔｄｉｓｃ−ｒｅｃｏｒｄａｂｌｅ）、ＣＤ−ＲＷ（ｃｏｍｐａｃｔｄｉｓｃ−ｒｅｗｒｉｔａｂｌｅ）が知られている。
【０００３】
ＣＤ−Ｒは追記が可能な記録媒体である。ＣＤ−Ｒでは、データを一度だけ書き込むことができ、書いたものは消去したり書換えることはできない。
【０００４】
ＣＤ−ＲＷは書き換え可能な記録媒体であるが、ＣＤ−ＲＯＭやオーディオＣＤ（ＣＤ−ＤＡ）と互換性がある。ＣＤ−ＲＷはＣＤ−Ｒとは異なり記録層に相変化材料を用いている。ＣＤ−ＲＷにおいて、レーザ光の照射で消去状態（結晶相）と記録状態（アモルファス相）を記録し、その反射率の違いによってデータを読み取る。ＣＤ−ＲＷは、プレス版のＣＤ−ＲＯＭや色素を使ったＣＤ−Ｒに比べて、メディアからの光の反射率が低い。
【０００５】
ＣＤ−Ｒ、ＣＤ−ＲＷへの情報（データ）の書込みには専用の装置と書込み用アプリケーションとが必要である。一方、ＣＤ−Ｒ、ＣＤ−ＲＷからの情報（データ）の読出しは通常のＣＤ−ＲＯＭドライブで実行できる。ＣＤ−Ｒ、ＣＤ−ＲＷ、ＣＤ−ＲＯＭ、およびオーディオＣＤ、ならびにＤＶＤ−ＲＯＭ、ＤＶＤ−Ｒ、ＤＶＤ−ＲＡＭ、ＤＶＤ＋ＲＷ、ＤＶＤ−ＲＷ等を、ここでは「光ディスク」と総称することにする。
【０００６】
さて、このような光ディスクに対して情報（データ）を書き込んだり、それから情報（データ）を読み出すために、光ディスクドライブは、光ディスク上にレーザビームを照射するための記録再生用光ピックアップを備えている。
【０００７】
一般に、この種の光ピックアップは、レーザビームを出射するレーザ光源と、この出射されたレーザビームを光ディスクへ導く光学系とを備えている。前述したように、ＣＤ−Ｒでは情報の読出しばかりでなく、情報の書込みをも行うことができる。ＣＤ−Ｒ用の光ピックアップでは、レーザ光源から出射されるレーザビームの出力を、情報の読出し時と情報の書込み時とで切り替える必要がある。その理由は、情報の書込みを、レーザビームの照射により光ディスクの記録層にピットを形成することで行うからである。情報書込み時におけるレーザ光源から出射されるレーザビームの出力は、情報読出し時における出力に比較して大きく、例えば、１０〜２０倍程度である。
【０００８】
さて、このような光ピックアップにおいて、上記レーザ光源から出射されたレーザビームは光学系を通り、その光学系を構成する対物レンズによって光ディスクの信号記録面上に集光させることによって、情報の記録（書込み）や消去を行う。一方、光ピックアップは、その信号記録面からの反射光（戻り光）を光検出手段である光検出器（フォトディテクタ）で検出することによって、情報の再生を行う。尚、光ピックアップ用の光学系には、偏光光学系と無偏光光学系との２種類がある。ここで、「偏光光学系」とは、レーザビームの偏光方向を変更することが出来る光学系のことをいい、「無偏光光学系」とは、レーザビームの偏光方向が変更しない光学系のことをいう。
【０００９】
このように、光ディスクドライブでは、光ピックアップから出射されるレーザビームを使用して光ディスクの記録・再生を行うので、フォーカシング制御とトラッキング制御とが不可欠である。ここで、「フォーカシング制御」とは、光ディスクと対物レンズとの間の距離を一定に保つように制御することをいい、「トラッキング制御」とは、光ディスクのトラック上にレーザビームのビームスポットを追従させるように制御することをいう。このフォーカシング制御とトラッキング制御とを行うために、光ピックアップは、上記対物レンズを上下方向（フォーカス方向）と左右方向（トラッキング方向）に変位させるための光ピックアップアクチュエータを備えている。
【００１０】
ところで、光ディスクには「複屈折」と呼ばれる光学的な欠陥が存在する。ここで、「複屈折」とは、光が境界面で屈折したときに２つの屈折光が現れる現象をいう。換言すれば、複屈折は、物質中を光が通過するとき、光の振動面の向きによってその進む速度が異なることをいい、光の振動面の向きによって屈折率が異なることから「複屈折」と呼ばれている。光の進む速度が速い（位相が進む）方位をその位相子の「進相軸」と呼び、反対に遅い（位相が遅れる）方位を「遅相軸」と呼ぶ。進相軸と遅相軸とを総称して、複屈折の「主軸」とも呼ばれる。
【００１１】
高分子配向膜、液晶高分子、光学結晶などは、複屈折性を示す。また、等方性の物質（媒質）でも外部から応力・電場・磁場などを加えると、一時的に異方性を示し、複屈折を生じることが知られている（光弾性効果、カー効果、磁気複屈折）。「光弾性効果」とは、光学的に等方な弾性体に機械的な力を加えた時、ひずみや応力により光学的ひずみ、すなわち光学的に異方性を生じ複屈折などを起こす現象をいう。「カー効果」とは、電気光学効果のひとつであって、電場によって物質の屈折率が変わる現象のうち、電場Ｅの２乗で誘起される複屈折をいう。「磁気複屈折」とは、磁場中にある光学的に透明な物質または透明な磁性体が光学的複屈折を起こす現象で、「コットン−ムートン効果」とも呼ばれる。
【００１２】
【発明が解決しようとする課題】
偏光光学系の光ピックアップを使用した光ディスクドライブでは、この複屈折現象により光ディスクからの反射信号の減少が起きる。
【００１３】
また、書き込み型の光ディスクの場合、書き込み特性の悪化という現象も発生する。これは、複屈折により光学的な歪みが光ディスク中に発生するために起こる。そのため、光ディスクに書き込んだ信号の品質劣化などは、スポットがどの程度歪んでいるか不明なため、予測がつかない。
【００１４】
さらに、光ディスクの持つ複屈折の値は、光ディスク毎（すなわち、成形条件や材料などよって）に異なる。また、この複屈折の値は、光ディスクを回転する回転数が上がることに起因して光ディスクにかかる応力によって、光ディスクの位置（場所）によっても異なる。
【００１５】
従来、静止した状態の光ディスクの複屈折量を測定する装置は存在する。しかしながら、高速回転中の光ディスクにかかる応力の影響による複屈折を測定する装置は存在しない。
【００１６】
また、光ディスクの複屈折は、光ディスクの回転数を上げたときの遠心力による応力で顕著に発生する。そのため、光ディスクを回転させながら光ディスクの複屈折を測定する必要がある。
【００１７】
それ故に本発明の課題は、回転中の光ディスクにかかる応力の影響に起因する複屈折を推定することができる、光ディスクの複屈折量推定装置および光ディスクドライブを提供することにある。
【００１８】
【課題を解決するための手段】
本発明によれば、回転している状態の被測定光ディスク（ＤＩＳＣ）の複屈折量を推定することを特徴とする光ディスクの複屈折量推定装置が得られる。
【００１９】
上記光ディスクの複屈折量推定装置に於いて、回転している状態の被測定光ディスク（ＤＩＳＣ）の複屈折量を、偏光光学系光ピックアップ（ＯＰＵ）を搭載した光ディスクドライブを用いて、推定することことが好ましい。
【００２０】
上記光ディスクの複屈折量推定装置は、好ましくは、被測定光ディスク（ＤＩＳＣ）を異なる複数の回転数で回転させ、かつ、偏光光学系光ピックアップ（ＯＰＵ）を被測定光ディスクの内周から外周へ向かって移動させることにより、偏光光学系光ピックアップ（ＯＰＵ）により被測定光ディスク（ＤＩＳＣ）から反射して得られる信号を測定する測定手段（４７）と、この測定結果より被測定光ディスク（ＤＩＳＣ）の複屈折量を推定する推定手段（４７）とを有する。
【００２１】
本発明の第１の態様によれば、上記推定手段（４７）は、偏光光学系光ピックアップ（ＯＰＵ）の同一位置において、被測定光ディスク（ＤＩＳＣ）の回転数を異ならせた場合に、偏光光学系光ピックアップ（ＯＰＵ）により被測定光ディスク（ＤＩＳＣ）から反射して得られる信号の測定結果の差異から被測定光ディスク（ＤＩＳＣ）の複屈折量を推定する。
【００２２】
上記本発明の第１の態様による複屈折量推定装置に於いて、測定手段（４７）は、被測定光ディスク（ＤＩＳＣ）を回転による遠心力で応力が発生して複屈折が顕著に発生しない十分に低い回転数で回転させた状態で、偏光光学系光ピックアップ（ＰＯＵ）を被測定光ディスク（ＤＩＳＣ）の内周から外周へ向かって移動させながら、偏光光学系光ピックアップ（ＯＰＵ）により被測定光ディスク（ＤＩＳＣ）から反射して得られる信号を測定し、基準信号として記憶する手段（４７５）と、被測定光ディスク（ＤＩＳＣ）を測定したい回転数で回転させた状態で、偏光光学系光ピックアップ（ＯＰＵ）を被測定光ディスク（ＤＩＳＣ）の内周から外周へ向かって移動させながら、偏光光学系光ピックアップ（ＯＰＵ）により被測定光ディスク（ＤＩＳＣ）から反射して得られる信号を測定する手段（４７）とを有し、推定手段（４７）は、基準信号に対する当該測定により得られた信号の減少度合から被測定光ディスクの複屈折量を推定するようにして良い。
【００２３】
上記本発明の第１の態様による複屈折量推定装置の変形例として、測定手段（４７）は、被測定光ディスク（ＤＩＳＣ）を回転による遠心力で応力が発生して複屈折が顕著に発生しない十分に低い回転数で回転させた状態で、偏光光学系光ピックアップ（ＯＰＵ）を被測定光ディスク（ＤＩＳＣ）に応力がかかっていない内側に移動して、当該位置で偏光光学系光ピックアップ（ＯＰＵ）により被測定光ディスク（ＤＩＳＣ）から反射して得られる信号を測定して、基準信号として記憶する手段（４７５）と、被測定光ディスク（ＤＩＳＣ）を測定したい回転数で回転させた状態で、最も回転による複屈折が被測定光ディスク（ＤＩＳＣ）に発生している場所に偏光光学系光ピックアップ（ＯＰＵ）を移動して、当該位置で偏光光学系光ピックアップ（ＯＰＵ）により被測定光ディスク（ＤＩＳＣ）から反射して得られる信号を測定する手段（４７）とを有し、推定手段（４７）は、基準信号に対する当該測定により得られた信号の減少度合から被測定光ディスクの複屈折量を推定するようにして良い。
【００２４】
上記本発明の第１の態様による複屈折量推定装置の別の変形例として、測定手段（４７）は、被測定光ディスク（ＤＩＳＣ）を、回転による遠心力で応力が発生して複屈折が顕著に発生しない十分に低い回転数で回転させた状態で、偏光光学系光ピックアップ（ＯＰＵ）を被測定光ディスク（ＤＩＳＣ）に応力がかかっていない内側に移動して、当該位置で偏光光学系光ピックアップ（ＯＰＵ）により被測定光ディスク（ＤＩＳＣ）から反射して得られる信号を測定して、基準信号として記憶する手段（４７５）と、被測定光ディスク（ＤＩＳＣ）を、測定したい回転数で回転させた状態で、偏光光学系光ピックアップ（ＯＰＵ）を被測定光ディスク（ＤＩＳＣ）の内周から外周へ向かって移動させながら、偏光光学系光ピックアップ（ＯＰＵ）により被測定光ディスク（ＤＩＳＣ）から反射して得られる信号を測定する手段（４７）とを有し、推定手段（４７）は、基準信号に対する当該測定により得られた信号の減少度合から被測定光ディスク（ＤＩＳＣ）の複屈折量を推定するようにして良い。
【００２５】
尚、複屈折が顕著に発生しない十分に低い回転数は、例えば、１０００ｒｐｍ以下である。
【００２６】
本発明の第２の態様によれば、上記推定手段（４７）は、被測定光ディスク（ＤＩＳＣ）を測定したい回転数で回転させた状態で、偏光光学系光ピックアップ（ＯＰＵ）の位置を異ならせた場合において、偏光光学系光ピックアップ（ＯＰＵ）により被測定光ディスク（ＤＩＳＣ）から反射して得られる信号の測定結果の差異から被測定光ディスクの複屈折量を推定する。
【００２７】
上記本発明の第２の態様による複屈折量推定装置に於いて、測定手段（４７）は、被測定光ディスク（ＤＩＳＣ）を測定したい回転数で回転させた状態で、偏光光学系光ピックアップ（ＯＰＵ）を被測定光ディスク（ＤＩＳＣ）に応力がかかっていない内側に移動して、当該位置で偏光光学系光ピックアップ（ＯＰＵ）により被測定光ディスク（ＤＩＳＣ）から反射して得られる信号を測定して、基準信号として記憶する手段（４７５）と、被測定光ディスク（ＤＩＳＣ）を測定したい回転数で回転させた状態で、最も回転による複屈折が被測定光ディスク（ＤＩＳＣ）に発生している場所に偏光光学系光ピックアップ（ＯＰＵ）を移動して、当該位置で偏光光学系光ピックアップ（ＯＰＵ）により被測定光ディスク（ＤＩＳＣ）から反射して得られる信号を測定する手段（４７）とを有し、推定手段（４７）は、基準信号に対する当該測定により得られた信号の減少度合から被測定光ディスクの複屈折量を推定するようにして良い。
【００２８】
尚、最も回転による複屈折が被測定光ディスク（ＤＩＳＣ）に発生している場所は、被測定光ディスク（ＤＩＳＣ）が直径１２ｃｍの光ディスクの場合、例えば、半径で５０〜５５ｍｍの範囲にある。
【００２９】
上記本発明の第２の態様による複屈折量推定装置に於いて、測定手段（４７）は、被測定光ディスク（ＤＩＳＣ）を測定したい回転数で回転させた状態で、偏光光学系光ピックアップ（ＯＰＵ）を被測定光ディスク（ＤＩＳＣ）に応力がかかっていない内側に移動して、当該位置で偏光光学系光ピックアップ（ＯＰＵ）により被測定光ディスク（ＤＩＳＣ）から反射して得られる信号を測定して、基準信号として記憶する手段（４７５）と、被測定光ディスク（ＤＩＳＣ）を測定したい回転数で回転させた状態で、偏光光学系光ピックアップ（ＯＰＵ）を被測定光ディスク（ＤＩＳＣ）の内周から外周へ向かって移動させながら、偏光光学系光ピックアップ（ＯＰＵ）により被測定光ディスク（ＤＩＳＣ）から反射して得られる信号を測定する手段（４７）とを有し、推定手段（４７）は、基準信号に対する当該測定により得られた信号の減少度合から被測定光ディスクの複屈折量を推定するようにしても良い。
【００３０】
また、本発明によれば、上述した複屈折量推定装置を備えた光ディスクドライブが得られる。
【００３１】
尚、上記括弧内の参照符号は、理解を容易にするために付したものであり、一例にすぎず、これらに限定されないのは勿論である。
【００３２】
【発明の実施の形態】
以下、本発明の実施の形態について図面を参照して詳細に説明する。
【００３３】
最初に図１及び図２を参照して、本発明の一実施の形態に係る光ディスクの複屈折量推定装置が適用される光ディスクドライブについて説明する。図１は光ピックアップＯＰＵが内周に移動したときの状態を示し、図２は光ピックアップＯＰＵが外周に移動したときの状態を示している。図１（ａ）および図２（ａ）は平面図であり、図１（ｂ）および図２（ｂ）は左側面図である。
【００３４】
シャーシ１１上には、スピンドルモータ１３と送りモータ１５とが搭載されている。スピンドルモータ１３はその上に取り付けられたターンテーブル１７を回転する。ターンテーブル１７上には図示しない光ディスクが搭載される。したがって、スピンドルモータ１３が回転することにより、ターンテーブル１７上に搭載された光ディスクも回転する。
【００３５】
送りモータ１５の駆動軸には、駆動減速ギヤ１９が係合し、この駆動減速ギヤ１９は、光ピックアップＯＰＵの一側に形成されたラック２１と係合する。光ピックアップＯＰＵは一対のガイドシャフト２３ａ、２３ｂによって案内される。したがって、送りモータ１５が回転すると、光ピックアップＯＰＵは一対のガイドシャフト２３ａ，２３ｂに沿って移送される。
【００３６】
図３を参照すると、光ピックアップＯＰＵは、半導体レーザ（レーザダイオード）ＬＤと、回折格子ＧＲＴと、コリメータレンズＣＬと、偏光ビームスプリッタＰＢＳと、１／４波長板ＱＷＰと、対物レンズＯＬと、センサレンズＳＬと、光検出器ＰＤとを有する。また、図示の光ピックアップＯＰＵは、半導体レーザＬＤから出射されたレーザビームの一部をモニタするためのフロントモニタＦＭと、半導体レーザＬＤを駆動するためのレーザドライバ２５とを備えている。
【００３７】
図示の光ピックアップＯＰＵは、偏光ビームスプリッタＰＢＳと１／４波長板ＱＷＰとを備えているので、偏光光学系光ピックアップと呼ばれる。
【００３８】
尚、半導体レーザＬＤから出射された１本のレーザビームは回折格子ＧＲＴで３本のレーザビームに分離される。これら３本のレーザビームは、中央にあるメインビームと、その両側にあるサブビームとから成る。また、半導体レーザＬＤから出射されるレーザビームは直線偏光である。
【００３９】
とにかく、半導体レーザＬＤから出射され、回折格子ＧＲＴで分離された３本のレーザビームは、コリメータレンズＣＬで平行光にされた後、偏光ビームスプリッタＰＢＳで直角に反射される。この偏光ビームスプリッタＰＢＳで反射されたレーザビームは、１／４波長板ＱＷＰで円偏光にされた後、対物レンズＯＬを介して光ディスクＤＩＳＣの信号記録面（反射面）へ集光（照射）される。
【００４０】
図４に光ディスクＤＩＳＣへ照射されたレーザビームのスポットを示す。上述したように回折格子ＧＲＴで分けられた３本のレーザビームは、光ディスクＤＩＳＣのピット面上のトラックに、図４（ａ）に示されるように、３個のスポットを結ぶ。
【００４１】
図３に戻って、光ディスクＤＩＳＣの信号記録面からの反射光（戻り光）は、対物レンズＯＬを通過し、１／４波長板ＱＷＰで往路の偏光方向と９０°曲げられ、偏光ビームスプリッタＰＢＳを通過し、センサレンズＳＬを通して光検出器ＰＤで検出される。
【００４２】
図示の光ピックアップＯＰＵでは、トラッキングエラー検出方法として、回折格子を用いて形成された３ビームを用いる方法を採用している。そして、３ビームを用いる方法の中でも、特に、ディファレンシャルプッシュプル法を用いている。
【００４３】
詳述すると、前述したように、光源であるレーザダイオードＬＤから出射された１本のレーザビームは、回折格子ＧＲＴによって３本のレーザビームに分離される。従って、光ディスクＤＩＳＣからの反射光（戻り光）も３本のレーザビームからなる。この３本のレーザビームのうち、中央のメインビームが読取り信号とフォーカスエラー信号を生成するために使用され、両側の２本のサブビームがトラッキングエラー信号を生成するために使用される。
【００４４】
図５に反射光（戻り光）を受光するための光検出器ＰＤの構成を示す。図５において、（Ａ）は正面図、（Ｂ）は右側面図である。光検出器ＰＤはメインビームを受光するためのメイン受光素子３１と、両側の２本のサブビームを受光するための一対のサブ受光素子３２、３３とを有する。メイン受光素子３１は４分割フォトダイオードから構成され、サブ受光素子３２、３３の各々は２分割フォトダイオードから構成されている。
【００４５】
したがって、図４（ａ）で図示された３個のスポットのうち、中央のスポット（図４（ａ）でＡ，Ｂ，Ｃ，Ｄの符号が付ってある部分）からの反射光（メインビーム）は、図５に示されるメイン受光素子３１によって、図３においてＡ、Ｂ、Ｃ、Ｄの符号で示される４つのメイン電気信号として受光される。また、一方の側のスポット（図４（ａ）でＥ，Ｆの符号が付ってある部分）からの反射光（サブビーム）は、図５に示される一方のサブ受光素子３２によって、図３においてＥ、Ｆの符号で示される２つのサブ電気信号として受光される。そして、他方の側のスポット（図４（ａ）でＧ，Ｈの符号が付ってある部分）からの反射光（サブビーム）は、図５に示される他方のサブ受光素子３３によって、図３においてＧ、Ｈの符号で示される２つのサブ電気信号として受光される。
【００４６】
次に、図６を参照して、ＨＦ信号の反射側信号（以下「Ｉ−ＴＯＰ」と呼ぶ）のレベルを検出する回路について説明する。
【００４７】
図示のＩ−ＴＯＰレベル検出回路は、加算回路４１と、ピークホールド回路４３と、Ａ／Ｄ変換回路４５とから構成されている。Ａ／Ｄ変換回路４５は中央処理装置（ＣＰＵ）４７に内蔵されている。
【００４８】
加算回路４１は、演算増幅器４１１から構成され、その非反転入力端子＋には上述した４つのメイン電気信号が抵抗器４２２〜４２５を介して供給され、その反転入力端子−にはＨＦ基準電圧が抵抗器４２６を介して供給され、その出力端子と反転入力端子−との間には抵抗器４２７が接続されている。加算回路４１の出力端子はピークホールド回路４３の入力端子に接続されている。ピークホールド回路４３は、加算回路４１で加算された信号のピークをホールドし、ピークホールド信号を出力する。ピークホールド回路４３の出力端子はＡ／Ｄ変換回路４５の入力端子に接続される。Ａ／Ｄ変換回路４５にはＨＦ基準信号も供給されている。Ａ／Ｄ変換回路４５は、ピークホールド回路４３から出力されたピークホールド信号をディジタル信号に変換する。このディジタル信号はＩ−ＴＯＰのレベルを示すものである。
【００４９】
とにかく、Ｉ−ＴＯＰレベル検出回路は、上述のようにして、Ｉ−ＴＯＰのレベルを検出する。
【００５０】
本発明は、このＩ−ＴＯＰのレベルと被測定光ディスクの複屈折量とが相関関係にあることを利用して、被測定光ディスクの複屈折量（実際には、「相対的な複屈折量」）を推定するものである。すなわち、Ｉ−ＴＯＰのレベルが高ければ複屈折量が小さく、Ｉ−ＴＯＰのレベルが低ければ複屈折量が大きくなる。前述したように、被測定光ディスクの回転数が十分に低ければ（例えば、１０００ｒｐｍ以下）、その被測定光ディスクには、回転による遠心力で応力が発生して複屈折が顕著に発生するということはない。また、たとえ被測定光ディスクの回転数が高くても、被測定光ディスクの内周側には応力が発生しないので、被測定光ディスクの内周側には複屈折が発生することはない。したがって、本発明では、この応力の影響を受けない、被測定光ディスクの回転数および／または位置における、偏光光学系光ピックアップで測定されたＩ−ＴＯＰのレベルを基準レベルにして、この基準レベルからのＩ−ＴＯＰのレベルの減少度合い（比率）から相対的な複屈折量を推定するものである。
【００５１】
前述したように、従来の光ディスクの複屈折量測定装置は、静止した状態の被測定光ディスクの複屈折量を測定するものである。これに対して、本発明による光ディスクの複屈折量推定装置は、回転している状態の被測定光ディスクの複屈折量を推定するものである。本発明では、回転している状態の被測定光ディスクの複屈折量を、偏光光学系光ピックアップＯＰＵ（図３）を搭載した光ディスクドライブ（図１、図２）を用いて、推定する。すなわち、本発明による光ディスクの複屈折量推定装置は、被測定光ディスクＤＩＳＣを異なる複数の回転数で回転させ、かつ、偏光光学系光ピックアップＯＰＵを被測定光ディスクＤＩＳＣの内周から外周へ向かって移動させることにより、偏光光学系光ピックアップＯＰＵにより被測定光ディスクＤＩＳＣから反射して得られる信号を測定する測定手段と、この測定結果より被測定光ディスクの複屈折量を推定する推定手段とを備えている。
【００５２】
図７に本発明の一実施の形態に係る複屈折量推定装置の構成の一例を示す。図示の複屈折量推定装置は、スピンドルモータ１３を駆動するためのスピンドルドライバ５１と、送りモータ１５の送り制御と偏光光学系光ピックアップＯＰＵのフォーカシング制御およびトラッキング制御とを行うＢＴＬドライバ５３と、上記加算回路４１とピークホールド回路４３を含むアナログシグナルプロセッサ（ＡＳＰ）５５とを有する。アナログシグナルプロセッサ５５は、レーザドライバ２５（図３）を制御して、半導体レーザＬＤから出射されるレーザビームの光量を制御できる。
【００５３】
アナログシグナルプロセッサ５５とＢＴＬドライバ５３とスピンドルドライバ５１とは、中央処理装置４７によって制御される。中央処理装置４７は、ＣＤ−ＤＳＰ４７１と、ＥＮＤＥＣ４７２と、Ｇ／Ａ４７３と、マイクロコントローラ４７４と、ダイナミックＲＡＭ４７５やフラッシュメモリ４７６などのメモリとを有する。中央処理装置４７が、上述した測定手段と推定手段として働く。
【００５４】
中央処理装置４７はＢＴＬドライバ５３へ送り指令を送出する。この送り指令に応答して、ＢＴＬドライバ５３は送りモータ１５を駆動して、偏光光学系光ピックアップＯＰＵを被測定光ディスクＤＩＳＣの内周から外周へ向かって移動させることができる。
【００５５】
また、中央処理装置４７はスピンドルドライバ５１へ回転数指令を送出する。この回転数指令に応答して、スピンドルドライバ５１は、その回転数指令で指定された回転数でスピンドルモータ１３を回転し、それにより、被測定光ディスクＤＩＳＣを異なる複数の回転数で回転させることができる。
【００５６】
偏光光学系光ピックアップＯＰＵにより被測定光ディスクＤＩＳＣから反射して得られる信号は、アナログシグナルプロセッサ５５を介して中央処理装置４７に取り込まれる。このとき、アナログシグナルプロセッサ５５内のピークホールド回路４３によって、Ｉ−ＴＯＰのレベルが中央処理装置４７に取り込まれる。
【００５７】
図８に、被測定光ディスクＤＩＳＣの回転数（速度）が１倍速、４倍速、１６倍速、３２倍速、４８倍速であるときに、偏光光学系光ピックアップＯＰＵを被測定光ディスクＤＩＳＣの内周から外周へ向かって移動させたときに得られたＩ−ＴＯＰのレベルを示す。このようにして得られたＩ−ＴＯＰのレベルのデータは、ダイナミックＲＡＭ４７５にログファイルとして保存され、図示しない表示装置上に画面として表示される。
【００５８】
ユーザは、これらデータの中から、回転数（速度）の違いや偏光光学系光ピックアップＯＰＵの位置（測定アドレス）の違いによるＩ−ＴＯＰのレベルを、中央処理装置４７によって比較させることが可能である。位置や速度により、Ｉ−ＴＯＰのレベルが減少していれば、中央処理装置４７は被測定光ディスクＤＩＳＣの回転応力による複屈折の影響であると判断（判定）出来る。
【００５９】
図８から明らかなように、被測定光ディスクＤＩＳＣの回転数（速度）が１倍速のときには、偏光光学系光ピックアップＯＰＵの位置に拘らず、偏光光学系光ピックアップＯＰＵから得られるＩ−ＴＯＰのレベルは変化しないことが分かる。換言すれば、このような低速では、被測定光ディスクＤＩＳＣには応力がかからず、被測定光ディスクＤＩＳＣの複屈折量はどの場所でも一定である、と推察される。そこで、本発明では、後述するように、このとき得られるＩ−ＴＯＰのレベルを基準信号として用いる。
【００６０】
また、被測定光ディスクＤＩＳＣの回転数（速度）を変えても、偏光光学系光ピックアップＯＰＵが被測定光ディスクＤＩＳＣの内周側にある場合、偏光光学系光ピックアップＯＰＵから得られるＩ−ＴＯＰのレベルは、速度が上がるにつれて僅かに低くなるに過ぎない。このことから、被測定光ディスクＤＩＳＣの内周側では、被測定光ディスクＤＩＳＣの回転による応力がほとんど発生しないことが分かる。すなわち、偏光光学系光ピックアップＯＰＵが被測定光ディスクＤＩＳＣの内周にいるときに、被測定光ディスクＤＩＳＣの回転数（速度）に拘らず、偏光光学系光ピックアップＯＰＵから得られるＩ−ＴＯＰのレベルを基準レベルとして用いることが可能である。
【００６１】
以上のことを考慮に入れて、本発明の実施の形態では、次に述べる２つのモードで、被測定光ディスクＤＩＳＣの複屈折量を測定、推定する。
【００６２】
本発明の第１のモードでは、中央処理装置４７は、偏光光学系光ピックアップＯＰＵの同一位置において、被測定光ディスクＤＩＳＣの回転数を異ならせた場合に、偏光光学系光ピックアップＯＰＵにより被測定光ディスクＤＩＳＣから反射して得られる信号の測定結果の差異から被測定光ディスクＤＩＳＣの複屈折量を推定する。
【００６３】
第１のモードの具体例は次の通りである。中央処理装置４７は、先ず、被測定光ディスクＤＩＳＣを回転による遠心力で応力が発生して複屈折が顕著に発生しない十分に低い回転数（例えば、１倍速）で回転させた状態で、偏光光学系光ピックアップＯＰＵを被測定光ディスクＤＩＳＣの内周から外周へ向かって移動させながら、偏光光学系光ピックアップＯＰＵにより被測定光ディスクＤＩＳＣから反射して得られる信号（Ｉ−ＴＯＰのレベル）を測定し、基準信号としてメモリに記憶する。次に、中央処理装置４７は、被測定光ディスクＤＩＳＣを測定したい回転数（例えば、４８倍速）で回転させた状態で、偏光光学系光ピックアップＯＰＵを被測定光ディスクＤＩＳＣの内周から外周へ向かって移動させながら、偏光光学系光ピックアップＯＰＵにより被測定光ディスクＤＩＳＣから反射して得られる信号（Ｉ−ＴＯＰのレベル）を測定する。最後に、中央処理装置４７は、基準信号に対する当該測定により得られた信号の減少度合から被測定光ディスクＤＩＳＣの複屈折量を推定する。
【００６４】
第１のモードでは、基準レベルとして、被測定光ディスクＤＩＳＣを１倍速で回転させ、偏光光学系光ピックアップＯＰＵを被測定光ディスクＤＩＳＣの内周から外周へ向かって移動させて得られるＩ−ＴＯＰのレベルを用いている。しかしながら、図８から明らかなように、この基準レベルは、偏光光学系光ピックアップＯＰＵの位置が異なってもほとんど変化せず、一定である。
【００６５】
そこで、第１のモードの変形例では、偏光光学系光ピックアップＯＰＵが被測定光ディスクＤＩＳＣの内周にあるときのＩ−ＴＯＰのレベルを基準信号として用いる。
【００６６】
第１のモードの第１の変形例について説明する。中央処理装置４７は、先ず、被測定光ディスクＤＩＳＣを回転による遠心力で応力が発生して複屈折が顕著に発生しない十分に低い回転数（例えば、１倍速）で回転させた状態で、偏光光学系光ピックアップＯＰＵを被測定光ディスクＤＩＳＣに応力がかかっていない内側に移動して、当該位置で偏光光学系光ピックアップＰＯＵにより被測定光ディスクＤＩＳＣから反射して得られる信号（Ｉ−ＴＯＰのレベル）を測定して、基準信号としてメモリに記憶する。次に、中央処理装置４７は、被測定光ディスクＤＩＳＣを測定したい回転数（例えば、４８倍速）で回転させた状態で、最も回転による複屈折が被測定光ディスクに発生している場所（図８で最もＩ−ＴＯＰのレベルが低い位置）に偏光光学系光ピックアップＯＰＵを移動して、当該位置で偏光光学系光ピックアップＯＰＵにより被測定光ディスクＤＩＳＣから反射して得られる信号（Ｉ−ＴＯＰのレベル）を測定する。最後に、中央処理装置４７は、基準信号に対する当該測定により得られた信号の減少度合から被測定光ディスクＤＩＳＣの複屈折量を推定する。
【００６７】
次に、第１のモードの第２の変形例について説明する。中央処理装置４７は、被測定光ディスクＤＩＳＣを、回転による遠心力で応力が発生して複屈折が顕著に発生しない十分に低い回転数（例えば、１倍速）で回転させた状態で、偏光光学系光ピックアップＯＰＵを被測定光ディスクＤＩＳＣに応力がかかっていない内側に移動して、当該位置で偏光光学系光ピックアップＯＰＵにより被測定光ディスクＤＩＳＣから反射して得られる信号（Ｉ−ＴＯＰのレベル）を測定して、基準信号としてメモリに記憶する。次に、中央処理装置４７は、被測定光ディスクＤＩＳＣを、測定したい回転数（例えば、４８倍速）で回転させた状態で、偏光光学系光ピックアップＯＰＵを被測定光ディスクＤＩＳＣの内周から外周へ向かって移動させながら、偏光光学系光ピックアップＯＰＵにより被測定光ディスクＤＩＳＣから反射して得られる信号（Ｉ−ＴＯＰのレベル）を測定する。最後に、中央処理装置４７は、基準信号に対する当該測定により得られた信号の減少度合から被測定光ディスクＤＩＳＣの複屈折量を推定する。
【００６８】
尚、複屈折が顕著に発生しない十分に低い回転数は、例えば、１０００ｒｐｍ以下である。
【００６９】
本発明の第２のモードでは、中央処理装置４７は、被測定光ディスクＤＩＳＣを測定したい回転数（例えば、４８倍速）で回転させた状態で、偏光光学系光ピックアップＯＰＵの位置を異ならせた場合において、偏光光学系光ピックアップＯＰＵにより被測定光ディスクＤＩＳＣから反射して得られる信号（Ｉ−ＴＯＰのレベル）の測定結果の差異から被測定光ディスクＤＩＳＣの複屈折量を推定する。
【００７０】
第２のモードの第１の具体例は次の通りである。中央処理装置４７は、被測定光ディスクＤＩＳＣを測定したい回転数（例えば、４８倍速）で回転させた状態で、偏光光学系光ピックアップＯＰＵを被測定光ディスクに応力がかかっていない内側に移動して、当該位置で偏光光学系光ピックアップＯＰＵにより被測定光ディスクＤＩＳＣから反射して得られる信号（Ｉ−ＴＯＰのレベル）を測定して、基準信号としてメモリに記憶する。次に、中央処理装置４７は、被測定光ディスクＤＩＳＣを測定したい回転数（例えば、４８倍速）で回転させた状態で、最も回転による複屈折が被測定光ディスクに発生している場所（図で最もＩ−ＴＯＰのレベルが低い位置）に偏光光学系光ピックアップＯＰＵを移動して、当該位置で偏光光学系光ピックアップＯＰＵにより被測定光ディスクＤＩＳＣから反射して得られる信号（Ｉ−ＴＯＰのレベル）を測定する。最後に、中央処理装置４７は、基準信号に対する当該測定により得られた信号の減少度合から被測定光ディスクＤＩＳＣの複屈折量を推定する。
【００７１】
尚、上記最も回転による複屈折が被測定光ディスクＤＩＳＣに発生している場所は、被測定光ディスクＤＩＳＣが直径１２ｃｍの光ディスクの場合、半径で５０〜５５ｍｍの範囲にある。
【００７２】
第２のモードの第２の具体例は次の通りである。中央処理装置４７は、先ず、被測定光ディスクＤＩＳＣを測定したい回転数（例えば、４８倍速）で回転させた状態で、偏光光学系光ピックアップＯＰＵを被測定光ディスクＤＩＳＣに応力がかかっていない内側に移動して、当該位置で偏光光学系光ピックアップＯＰＵにより被測定光ディスクＤＩＳＣから反射して得られる信号（Ｉ−ＴＯＰのレベル）を測定して、基準信号としてメモリに記憶する。次に、中央処理装置４７は、被測定光ディスクＤＩＳＣを測定したい回転数（例えば、４８倍速）で回転させた状態で、偏光光学系光ピックアップＯＰＵを被測定光ディスクＤＩＳＣの内周から外周へ向かって移動させながら、偏光光学系光ピックアップＯＰＵにより被測定光ディスクＤＩＳＣから反射して得られる信号（Ｉ−ＴＯＰのレベル）を測定する。最後に、中央処理装置４７は、基準信号に対する当該測定により得られた信号の減少度合から被測定光ディスクＤＩＳＣの複屈折量を推定する。
【００７３】
図９に光検出器ＰＤへの戻り光の光量と光ディスクＤＩＳＣの複屈折量の依存関係を図示する。図９において、縦軸は光検出器ＰＤへの戻り光の光量を最大値を１に規格化して示し、横軸は光ディスクＤＩＳＣの複屈折量［ｎｍ］を示している。ここでは、１／４波長板ＱＷＰから光ディスクＤＩＳＣへ向けて出射される出射光が完全円偏光であるとし、また、半導体レーザＬＤから出射されるレーザビームの波長が７８５ｎｍであると仮定する。
【００７４】
図９から明らかなように、光検出器ＰＤへの戻り光の光量が少なくなるにつれて、光ディスクＤＩＳＣの複屈折量が大きくなることが分かる。尚、光検出器ＰＤへの戻り光の光量が零のとき、光ディスクＤＩＳＣの複屈折量は３９２．５［ｎｍ］である。
【００７５】
次に、図１０を参照して、光検出器ＰＤへの戻り光の光ディスクＤＩＳＣの複屈折依存について説明する。１／４波長板ＱＷＰに対して光が垂直に入射し、直線偏光の方向がＸ（ラジアル）方向に平行で、光ディスクＤＩＳＣに複屈折がないという理想的な場合を想定する。この場合、１／４波長板ＱＷＰを透過した光は円偏光となる。ここでは、仮に円偏光が右回りとする。光ディスクＤＩＳＣで反射された光は左回りの円偏光となって１／４波長板ＱＷＰに入射する。この１／４波長板ＱＷＰの出射光はＹ（タンジェンシャル）方向に平行な直線偏光となって、偏光ビームスプリッタＰＢＳを１００％近く透過し、光検出器ＰＤに入射する。以降、この状態を１として規格化して考察を進める。
【００７６】
次に、光ディスクＤＩＳＣの複屈折により位相がδラジアン進んだ状態を想定する。以下では、▲１▼０＜δ＜π／２の時、▲２▼δ＝π／２の時、▲３▼π／２＜δ＜πの時、▲４▼δ＝πの時の４通りの場合に分けて説明する。
【００７７】
▲１▼０＜δ＜π／２の時
光ディスクＤＩＳＣで反射された光は左回りの楕円偏光となって１／４波長板ＱＷＰに入射し、１／４波長板ＱＷＰ内部のある位置で直線偏光となる。その後、１／４波長板ＱＷＰの出射光は右回りで位相が入射光からπ／２進んだ楕円偏光となって出射される。偏光ビームスプリッタＰＢＳを透過する光の光量は、楕円のＹ方向の成分で現されるので、光検出器ＰＤの入射光の光量はｃｏｓ（δ／２）となる。
【００７８】
▲２▼δ＝π／２の時
光ディスクＤＩＳＣで反射された光はＹ方向に平行な直線偏光となって１／４波長板ＱＷＰに入射し、１／４波長板ＱＷＰから右回りの円偏光となって出射される。従って、光検出器ＰＤの入射光の光量は同様にｃｏｓ（π／４）≒０．７０７となる。
【００７９】
▲３▼π／２＜δ＜πの時
光ディスクＤＩＳＣで反射された光は右回りの楕円偏光となって１／４波長板ＱＷＰに入射し、１／４波長板ＱＷＰ内部のある位置で円偏光となる。その後、１／４波長板ＱＷＰの出射光は右回りで位相が入射光からπ／２進んだ楕円偏光となって出射される。従って、光検出器ＰＤの入射光の光量はｃｏｓ（δ／２）となる。
【００８０】
▲４▼δ＝πの時
光ディスクＤＩＳＣで反射された光は右回りの円偏光となって１／４波長板ＱＷＰに入射し、１／４波長板ＱＷＰからＸ方向に平行な直線偏光となって出射される。従って、光検出器ＰＤの入射光の光量は同様にｃｏｓ（π／２）＝０となる。
【００８１】
以上をまとめて表およびグラフ化したものを、それぞれ、図１１および図１２に図示する。図１１は光ディスクの複屈折量と、位相ズレおよび光検出器の入射光量との関係を示す表である。図１２は光検出器の入射光量と光ディスクの複屈折量との関係を示す図である。
【００８２】
Ｙ方向が長径となる楕円偏光は、光ディスクＤＩＳＣの複屈折により進む位相δが０からπの範囲にある場合を示している。この場合は、図１２に示したグラフの光ディスクＤＩＳＣの複屈折量が０ｎｍから３９２．５ｎｍの部分に相当する。
【００８３】
一方、Ｘ方向が長径となる楕円偏光は、光ディスクＤＩＳＣの複屈折により進む位相δがπ以上又はマイナスになった場合を示している。この場合は、図１２に示したグラフの光ディスクＤＩＳＣの複屈折量が３９２．５ｎｍから７８５ｎｍの部分に相当する。
【００８４】
このようにして、本発明による複屈折量推定装置では、偏光光学系光ピックアップＯＰＵからみた回転中の被測定光ディスクＤＩＳＣの複屈折量を推定することができる。また、本発明による複屈折量推定装置において使用する光ディスクドライブは、上述したように、既存の光ディスクドライブをそのまま使用するか或いは僅かに改造したものを使用することができる。従って、非常に安価に複屈折量推定装置を製造することができる。換言すれば、既存の光ディスクドライブにＩ−ＴＯＰレベル検出回路（図６）を追加して、中央処理装置４７を動作させるためのプログラムさえ開発すれば、本発明による複屈折量推定装置を構成することができる。また、本発明では、このような複屈折量推定装置を備えた光ディスクドライブを提供できる。
【００８５】
本発明は上述した実施の形態に限定せず、本発明の要旨を逸脱しない範囲内で種々の変更・変形が可能なのは勿論である。例えば、上述した実施の形態では、Ｉ−ＴＯＰレベル検出回路は、加算回路４１とピークホールド回路４３とを備えているが、ピークホールド回路４３を省略して、加算回路４１の出力を直接ＣＰＵ４７へ供給するようにしても良い。この場合、ＣＰＵ４７内部において、Ａ／Ｄ変換回路４５で変換されたディジタル信号からピーク信号（Ｉ−ＴＯＰのレベル）を検出するようにすれば良い。また、上述した実施の形態では、偏光光学系光ピックアップにより被測定光ディスクから反射して得られる信号として、Ｉ−ＴＯＰのレベルを用いた場合についてのみ説明しているが、ＨＦ信号の振幅等を使用しても良いのは勿論である。
【００８６】
【発明の効果】
以上の説明から明らかなように、本発明によれば、静止している状態の光ディスクではなく、回転している状態の光ディスクの複屈折量を推定することができる。
【図面の簡単な説明】
【図１】本発明の一実施の形態に係る光ディスクの複屈折量推定装置が適用される光ディスクドライブの、光ピックアップが内周に移動したときの状態を示した図で、（ａ）は平面図、（ｂ）は左側面図である。
【図２】図１に図示した光ディスクドライブの、光ピックアップが外周に移動したときの状態を示した図で、（ａ）は平面図、（ｂ）は左側面図である。
【図３】図１および図２に示した光ディスクドライブに使用される、偏光光学系光ピックアップの構成を示すブロック図である。
【図４】光ディスクへ照射されたレーザビームのスポットを示す図で、（ａ）は平面図、（ｂ）は概略断面図である。
【図５】図３に示す偏光光学系光ピックアップに用いられる、反射光（戻り光）を受光するための光検出器の構成を示す図で、（Ａ）は正面図、（Ｂ）は右側面図である。
【図６】ＨＦ信号の反射側信号（Ｉ−ＴＯＰ）のレベルを検出するＩ−ＴＯＰレベル検出回路の構成を示すブロック図である。
【図７】本発明の一実施の形態に係る複屈折量推定装置の構成を示すブロック図である。
【図８】被測定光ディスクの回転数（速度）が１倍速、４倍速、１６倍速、３２倍速、４８倍速であるときに、偏光光学系光ピックアップを被測定光ディスクの内周から外周へ向かって移動させたときに得られたＩ−ＴＯＰのレベルを示す図である。
【図９】光検出器への戻り光の光量と光ディスクの複屈折量の依存関係を示す図である。
【図１０】光検出器への戻り光の光ディスクの複屈折依存を説明するための図である。
【図１１】光ディスクの複屈折量と、位相ズレおよび光検出器の入射光量との関係を示す表である。
【図１２】光検出器の入射光量と光ディスクの複屈折量との関係を示す図である。
【符号の説明】
ＯＰＵ偏光光学系光ピックアップ
ＤＩＳＣ被測定光ディスク
１３スピンドルモータ
１５送りモータ
４７中央処理装置（ＣＰＵ）
５１スピンドルドライバ
５３ＢＴＬドライバ
５５アナログシグナルプロセッサ（ＡＳＰ）[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an apparatus for estimating the birefringence of an optical disk and an optical disk drive.
[0002]
[Prior art]
Recently, electronic devices such as personal computers are often equipped with optical disk drives (optical disk devices). As recording media usable for the optical disk drive, a compact disc-recordable (CD-R) and a compact disc-rewritable (CD-RW) are known.
[0003]
CD-R is a recordable recording medium. With a CD-R, data can be written only once, and what has been written cannot be erased or rewritten.
[0004]
The CD-RW is a rewritable recording medium, but is compatible with a CD-ROM and an audio CD (CD-DA). Unlike CD-R, CD-RW uses a phase-change material for the recording layer. In a CD-RW, an erased state (crystalline phase) and a recorded state (amorphous phase) are recorded by laser light irradiation, and data is read based on the difference in reflectance. A CD-RW has a lower reflectance of light from a medium than a press-type CD-ROM or a CD-R using a dye.
[0005]
Writing information (data) to a CD-R or CD-RW requires a dedicated device and a writing application. On the other hand, reading of information (data) from a CD-R or CD-RW can be executed by a normal CD-ROM drive. The CD-R, CD-RW, CD-ROM, audio CD, DVD-ROM, DVD-R, DVD-RAM, DVD + RW, DVD-RW, and the like are collectively referred to herein as "optical disks".
[0006]
Now, in order to write information (data) to and read information (data) from or to such an optical disk, the optical disk drive is provided with a recording / reproducing optical pickup for irradiating a laser beam onto the optical disk. .
[0007]
Generally, this type of optical pickup includes a laser light source that emits a laser beam, and an optical system that guides the emitted laser beam to an optical disk. As described above, the CD-R can perform not only reading of information but also writing of information. In an optical pickup for a CD-R, the output of a laser beam emitted from a laser light source needs to be switched between when reading information and when writing information. The reason is that writing of information is performed by forming pits in the recording layer of the optical disk by irradiating a laser beam. The output of the laser beam emitted from the laser light source at the time of writing information is larger than the output at the time of reading information, for example, about 10 to 20 times.
[0008]
Now, in such an optical pickup, a laser beam emitted from the laser light source passes through an optical system, and is condensed on a signal recording surface of an optical disk by an objective lens constituting the optical system, thereby recording information ( Write) and erase. On the other hand, the optical pickup reproduces information by detecting reflected light (return light) from the signal recording surface with a photodetector (photodetector) as light detecting means. Note that there are two types of optical systems for optical pickups, a polarizing optical system and a non-polarizing optical system. Here, “polarizing optical system” refers to an optical system that can change the polarization direction of a laser beam, and “non-polarizing optical system” refers to an optical system that does not change the polarization direction of a laser beam. Say.
[0009]
As described above, in the optical disk drive, recording and reproduction of the optical disk are performed using the laser beam emitted from the optical pickup, so that focusing control and tracking control are indispensable. Here, “focusing control” refers to controlling the distance between the optical disc and the objective lens to be constant, and “tracking control” refers to following a beam spot of a laser beam on a track of the optical disc. It means to control to make it. In order to perform the focusing control and the tracking control, the optical pickup includes an optical pickup actuator for displacing the objective lens in a vertical direction (focus direction) and a horizontal direction (tracking direction).
[0010]
By the way, an optical disc has an optical defect called “birefringence”. Here, "birefringence" refers to a phenomenon in which two refracted lights appear when light is refracted at the boundary surface. In other words, birefringence means that when light passes through a substance, the speed at which the light travels varies depending on the direction of the vibrating surface of the light. is called. An azimuth in which light travels fast (phase advances) is called a “fast axis” of the retarder, and an azimuth that is slow (phase lags) is called a “slow axis”. The fast axis and the slow axis are collectively referred to as the "principal axis" of birefringence.
[0011]
A polymer alignment film, a liquid crystal polymer, an optical crystal and the like exhibit birefringence. Also, it is known that when an isotropic substance (medium) is applied with stress, an electric field, a magnetic field, or the like from the outside, it temporarily exhibits anisotropy and generates birefringence (photoelastic effect, Kerr effect, Magnetic birefringence). The "photoelastic effect" is a phenomenon in which, when a mechanical force is applied to an optically isotropic elastic body, strain or stress causes optical distortion, that is, optically anisotropic and birefringence. Say. The “Kerr effect” is one of the electro-optic effects, and refers to a birefringence induced by the square of the electric field E among phenomena in which the refractive index of a substance is changed by an electric field. “Magnetic birefringence” is a phenomenon in which an optically transparent substance or a transparent magnetic substance in a magnetic field causes optical birefringence, and is also called “Cotton-Mouton effect”.
[0012]
[Problems to be solved by the invention]
In an optical disk drive using an optical pickup of a polarization optical system, a reflection signal from an optical disk is reduced due to the birefringence phenomenon.
[0013]
Further, in the case of a writable optical disk, a phenomenon that writing characteristics are deteriorated also occurs. This occurs because optical distortion occurs in the optical disk due to birefringence. Therefore, it is unclear how much the spot is distorted, and the quality degradation of the signal written on the optical disc cannot be predicted.
[0014]
Further, the value of the birefringence of the optical disc differs for each optical disc (that is, depending on molding conditions, materials, and the like). The value of the birefringence also differs depending on the position (location) of the optical disk due to the stress applied to the optical disk due to the increase in the number of rotations of the optical disk.
[0015]
Conventionally, there is an apparatus for measuring a birefringence amount of a stationary optical disc. However, there is no device that measures birefringence due to the effect of stress applied to an optical disk rotating at high speed.
[0016]
Further, birefringence of the optical disk is remarkably generated by a stress caused by a centrifugal force when the rotation speed of the optical disk is increased. Therefore, it is necessary to measure the birefringence of the optical disk while rotating the optical disk.
[0017]
Therefore, an object of the present invention is to provide a device for estimating the amount of birefringence of an optical disk and an optical disk drive capable of estimating the birefringence caused by the influence of the stress applied to the rotating optical disk.
[0018]
[Means for Solving the Problems]
According to the present invention, an apparatus for estimating the amount of birefringence of an optical disk under measurement (DISC) in a rotating state is provided.
[0019]
Estimating the birefringence of the rotating optical disk under measurement (DISC) by using the optical disk drive equipped with the polarization optical system optical pickup (OPU) in the optical disk birefringence amount estimating apparatus. Is preferred.
[0020]
The apparatus for estimating the amount of birefringence of an optical disk preferably rotates the optical disk under measurement (DISC) at a plurality of different rotational speeds, and moves the polarization optical system optical pickup (OPU) from the inner circumference to the outer circumference of the optical disk under measurement. The measuring means (47) for measuring a signal obtained by being reflected from the optical disk (DISC) by the polarization optical system optical pickup (OPU) by moving the optical disk (DISC) based on the measurement result. Estimating means (47) for estimating the amount of refraction.
[0021]
According to the first aspect of the present invention, the estimating means (47) sets the polarization optical system (DISC) at the same position of the polarization optical system optical pickup (OPU) when the rotation speed of the optical disk under measurement (DISC) is different. The amount of birefringence of the optical disc (DISC) is estimated from the difference between the measurement results of the signals reflected from the optical disc (DISC) by the system optical pickup (OPU).
[0022]
In the apparatus for estimating the amount of birefringence according to the first aspect of the present invention, the measuring means (47) may be configured such that a centrifugal force caused by rotation of the measured optical disc (DISC) generates stress and birefringence does not significantly occur. The optical disk under test is moved by the polarization optical system optical pickup (OPU) while the polarization optical system optical pickup (POU) is moved from the inner circumference to the outer circumference of the optical disk under measurement (DISC) while being rotated at a low rotation speed. Means for measuring a signal obtained by reflection from the optical disc (DISC) and storing it as a reference signal (475), and a polarization optical system optical pickup (OPU) with the measured optical disc (DISC) rotated at the desired number of revolutions. ) Is moved from the inner circumference to the outer circumference of the optical disk under measurement (DISC), and the optical disk under measurement is read by the polarization optical system optical pickup (OPU). (47) means for measuring a signal obtained by reflection from the (DISC), and the estimating means (47) determines the amount of birefringence of the optical disc to be measured based on the degree of decrease of the signal obtained by the measurement with respect to the reference signal. May be estimated.
[0023]
As a modified example of the birefringence amount estimating apparatus according to the first aspect of the present invention, the measuring means (47) generates a stress due to a centrifugal force caused by rotation of the measured optical disc (DISC) and birefringence does not significantly occur. With the rotation at a sufficiently low rotational speed, the polarization optical system optical pickup (OPU) is moved inside the optical disk under test (DISC) where no stress is applied, and the polarization optical system optical pickup (OPU) is moved at the position. Means (475) for measuring a signal reflected from the optical disk under measurement (DISC) and storing it as a reference signal, and rotating the optical disk under measurement (DISC) at the number of rotations to be measured. Optical system optical pickup (OPU) is moved to a place where birefringence due to the light is generated on the optical disc under measurement (DISC), and the polarization optical system light pick-up is Means (47) for measuring a signal obtained by reflection from the optical disk to be measured (DISC) by up-up (OPU), and the estimating means (47) comprises: a degree of decrease of the signal obtained by the measurement with respect to the reference signal. The birefringence of the optical disk to be measured may be estimated from the above.
[0024]
As another modified example of the birefringence amount estimating apparatus according to the first aspect of the present invention, the measuring means (47) applies a stress to the measured optical disc (DISC) by centrifugal force due to rotation, and the birefringence is remarkable. In a state where the optical disk is rotated at a sufficiently low rotation speed which does not occur in the optical disk, the polarization optical system optical pickup (OPU) is moved inside the optical disk under test (DISC) where no stress is applied, and the polarization optical system optical pickup is moved at the position. Means (475) for measuring a signal reflected from the optical disk under measurement (DISC) by (OPU) and storing it as a reference signal, and a state in which the optical disk under measurement (DISC) is rotated at the desired number of revolutions Then, while moving the polarization optical system optical pickup (OPU) from the inner circumference to the outer circumference of the optical disk under measurement (DISC), the polarization optical system optical pickup (OP) is moved. ) And a means (47) for measuring a signal obtained by reflection from the optical disc under measurement (DISC), and the estimating means (47) measures the signal to be measured based on the degree of decrease of the signal obtained by the measurement with respect to the reference signal. The amount of birefringence of an optical disc (DISC) may be estimated.
[0025]
A sufficiently low rotation speed at which birefringence does not significantly occur is, for example, 1000 rpm or less.
[0026]
According to the second aspect of the present invention, the estimating means (47) changes the position of the polarization optical system optical pickup (OPU) while the measured optical disc (DISC) is rotated at the desired rotation speed. In this case, the birefringence of the optical disk to be measured is estimated from the difference between the measurement results of signals obtained by reflection from the optical disk to be measured (DISC) by the polarization optical system optical pickup (OPU).
[0027]
In the apparatus for estimating the amount of birefringence according to the second aspect of the present invention, the measuring means (47) rotates the optical disk under measurement (DISC) at the desired number of rotations while maintaining the polarization optical system optical pickup (OPU). ) Is moved inward without stress being applied to the optical disk under test (DISC), and a signal obtained by being reflected from the optical disk under test (DISC) by the polarization optical system optical pickup (OPU) at that position is measured. Means (475) for storing as a reference signal, and polarization optics at a place where birefringence due to rotation occurs most on the optical disc (DISC) while the optical disc (DISC) is rotated at the rotation speed to be measured. The system optical pickup (OPU) is moved and reflected from the optical disk (DISC) by the polarization optical system optical pickup (OPU) at that position. Means (47) for measuring the obtained signal, and the estimating means (47) may estimate the amount of birefringence of the optical disk to be measured from the degree of reduction of the signal obtained by the measurement with respect to the reference signal. .
[0028]
When the optical disc under measurement (DISC) is an optical disc having a diameter of 12 cm, the location where the birefringence due to rotation occurs most in the optical disc (DISC) is, for example, in the range of 50 to 55 mm in radius.
[0029]
In the apparatus for estimating the amount of birefringence according to the second aspect of the present invention, the measuring means (47) rotates the optical disk under measurement (DISC) at the desired number of rotations while maintaining the polarization optical system optical pickup (OPU). ) Is moved inward without stress being applied to the optical disk under test (DISC), and a signal obtained by being reflected from the optical disk under test (DISC) by the polarization optical system optical pickup (OPU) at that position is measured. Means (475) for storing as a reference signal, and a polarizing optical system optical pickup (OPU) from the inner circumference to the outer circumference of the optical disc (DISC) while the optical disc (DISC) is rotated at the desired number of revolutions. While moving the optical head, the signal obtained by reflection from the optical disk under measurement (DISC) by the polarization optical system optical pickup (OPU) is measured. And means (47), estimation means (47) from the degree of decrease of the signal obtained by the measurement with respect to the reference signal may be estimated amount of birefringence of the measured optical disc.
[0030]
Further, according to the present invention, an optical disc drive including the above-described birefringence amount estimating apparatus can be obtained.
[0031]
It should be noted that the reference numerals in the parentheses are provided for easy understanding, are merely examples, and are not limited to these.
[0032]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0033]
First, an optical disk drive to which an apparatus for estimating the amount of birefringence of an optical disk according to an embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows a state when the optical pickup OPU moves to the inner periphery, and FIG. 2 shows a state when the optical pickup OPU moves to the outer periphery. 1 (a) and 2 (a) are plan views, and FIGS. 1 (b) and 2 (b) are left side views.
[0034]
On the chassis 11, a spindle motor 13 and a feed motor 15 are mounted. The spindle motor 13 rotates a turntable 17 mounted thereon. An optical disk (not shown) is mounted on the turntable 17. Therefore, when the spindle motor 13 rotates, the optical disk mounted on the turntable 17 also rotates.
[0035]
A drive reduction gear 19 is engaged with the drive shaft of the feed motor 15, and the drive reduction gear 19 is engaged with a rack 21 formed on one side of the optical pickup OPU. The optical pickup OPU is guided by a pair of guide shafts 23a and 23b. Therefore, when the feed motor 15 rotates, the optical pickup OPU is transported along the pair of guide shafts 23a and 23b.
[0036]
Referring to FIG. 3, the optical pickup OPU includes a semiconductor laser (laser diode) LD, a diffraction grating GRT, a collimator lens CL, a polarizing beam splitter PBS, a quarter-wave plate QWP, an objective lens OL, and a sensor. It has a lens SL and a photodetector PD. The illustrated optical pickup OPU includes a front monitor FM for monitoring a part of the laser beam emitted from the semiconductor laser LD, and a laser driver 25 for driving the semiconductor laser LD.
[0037]
The illustrated optical pickup OPU includes a polarization beam splitter PBS and a quarter-wave plate QWP, and is therefore called a polarization optical system optical pickup.
[0038]
Incidentally, one laser beam emitted from the semiconductor laser LD is separated into three laser beams by the diffraction grating GRT. These three laser beams consist of a main beam at the center and sub-beams on both sides of the main beam. The laser beam emitted from the semiconductor laser LD is linearly polarized light.
[0039]
Anyway, the three laser beams emitted from the semiconductor laser LD and separated by the diffraction grating GRT are collimated by the collimator lens CL and then reflected at right angles by the polarization beam splitter PBS. The laser beam reflected by the polarizing beam splitter PBS is circularly polarized by the quarter-wave plate QWP, and then condensed (irradiated) on the signal recording surface (reflection surface) of the optical disc DISC via the objective lens OL. You.
[0040]
FIG. 4 shows a spot of the laser beam applied to the optical disc DISC. As described above, the three laser beams divided by the diffraction grating GRT connect three spots to the track on the pit surface of the optical disc DISC as shown in FIG.
[0041]
Returning to FIG. 3, the reflected light (return light) from the signal recording surface of the optical disc DISC passes through the objective lens OL, is bent 90 ° with respect to the polarization direction of the outward path by the quarter-wave plate QWP, and is polarized by the polarization beam splitter PBS. And is detected by the photodetector PD through the sensor lens SL.
[0042]
The illustrated optical pickup OPU employs a method using three beams formed using a diffraction grating as a tracking error detection method. Among the methods using three beams, the differential push-pull method is particularly used.
[0043]
More specifically, as described above, one laser beam emitted from the laser diode LD as a light source is separated into three laser beams by the diffraction grating GRT. Therefore, the reflected light (return light) from the optical disc DISC also includes three laser beams. Of the three laser beams, the central main beam is used to generate a read signal and a focus error signal, and the two sub beams on both sides are used to generate a tracking error signal.
[0044]
FIG. 5 shows a configuration of a photodetector PD for receiving reflected light (return light). 5A is a front view, and FIG. 5B is a right side view. The photodetector PD has a main light receiving element 31 for receiving a main beam, and a pair of sub light receiving elements 32 and 33 for receiving two sub beams on both sides. The main light receiving element 31 is constituted by a four-division photodiode, and each of the sub light receiving elements 32 and 33 is constituted by a two-division photodiode.
[0045]
Therefore, of the three spots shown in FIG. 4A, the reflected light (main part) from the center spot (the part marked with A, B, C, and D in FIG. 4A). 5) are received by the main light receiving element 31 shown in FIG. 5 as four main electric signals indicated by reference numerals A, B, C, and D in FIG. The reflected light (sub-beam) from the spot on one side (the portion denoted by reference signs E and F in FIG. 4A) is transmitted by one sub-light receiving element 32 shown in FIG. Are received as two sub-electrical signals indicated by symbols E and F. Then, the reflected light (sub-beam) from the other side spot (the part marked with G and H in FIG. 4A) is reflected by the other sub-light receiving element 33 shown in FIG. Are received as two sub-electric signals indicated by G and H symbols.
[0046]
Next, a circuit for detecting the level of a reflection-side signal (hereinafter, referred to as “I-TOP”) of the HF signal will be described with reference to FIG.
[0047]
The illustrated I-TOP level detection circuit includes an addition circuit 41, a peak hold circuit 43, and an A / D conversion circuit 45. The A / D conversion circuit 45 is built in a central processing unit (CPU) 47.
[0048]
The adder circuit 41 includes an operational amplifier 411. The four main electric signals described above are supplied to the non-inverting input terminal + thereof through the resistors 422 to 425, and the HF reference voltage is supplied to the inverting input terminal thereof. It is supplied via a resistor 426, and a resistor 427 is connected between its output terminal and the inverting input terminal-. The output terminal of the adding circuit 41 is connected to the input terminal of the peak hold circuit 43. The peak hold circuit 43 holds the peak of the signal added by the addition circuit 41 and outputs a peak hold signal. An output terminal of the peak hold circuit 43 is connected to an input terminal of the A / D conversion circuit 45. The A / D conversion circuit 45 is also supplied with an HF reference signal. The A / D conversion circuit 45 converts the peak hold signal output from the peak hold circuit 43 into a digital signal. This digital signal indicates the level of the I-TOP.
[0049]
Anyway, the I-TOP level detection circuit detects the level of I-TOP as described above.
[0050]
The present invention utilizes the fact that the level of I-TOP and the amount of birefringence of the optical disk to be measured are correlated, and the amount of birefringence of the optical disk to be measured (actually, “relative birefringence amount”). ). That is, the higher the level of I-TOP, the smaller the amount of birefringence, and the lower the level of I-TOP, the larger the amount of birefringence. As described above, if the rotation speed of the measured optical disk is sufficiently low (for example, 1000 rpm or less), it is possible that stress is generated on the measured optical disk by centrifugal force due to rotation and birefringence is significantly generated. Absent. Even if the measured optical disk has a high rotation speed, no stress is generated on the inner peripheral side of the measured optical disk, so that no birefringence occurs on the inner peripheral side of the measured optical disk. Therefore, in the present invention, the I-TOP level measured by the polarization optical system optical pickup at the rotation speed and / or the position of the optical disk to be measured, which is not affected by the stress, is set as a reference level. The relative birefringence is estimated from the degree of reduction (ratio) of the I-TOP level.
[0051]
As described above, the conventional optical disc birefringence measuring apparatus measures the birefringence of a measured optical disc in a stationary state. On the other hand, the apparatus for estimating the amount of birefringence of an optical disk according to the present invention estimates the amount of birefringence of an optical disk under measurement in a rotating state. In the present invention, the amount of birefringence of a measured optical disk in a rotating state is estimated using an optical disk drive (FIGS. 1 and 2) equipped with a polarization optical system optical pickup OPU (FIG. 3). That is, the apparatus for estimating the amount of birefringence of an optical disc according to the present invention rotates the measured optical disc DISC at a plurality of different rotation speeds and moves the polarization optical system optical pickup OPU from the inner circumference to the outer circumference of the measured optical disc DISC. By this means, there are provided measuring means for measuring a signal obtained by being reflected from the optical disc to be measured DISC by the polarization optical system optical pickup OPU, and estimating means for estimating the birefringence of the optical disc to be measured from the measurement result. .
[0052]
FIG. 7 shows an example of a configuration of a birefringence amount estimation device according to an embodiment of the present invention. The illustrated birefringence amount estimating apparatus includes a spindle driver 51 for driving the spindle motor 13, a BTL driver 53 for performing feed control of the feed motor 15 and focusing control and tracking control of the polarization optical system optical pickup OPU, An analog signal processor (ASP) 55 including an addition circuit 41 and a peak hold circuit 43 is provided. The analog signal processor 55 controls the laser driver 25 (FIG. 3) to control the amount of laser beam emitted from the semiconductor laser LD.
[0053]
The analog signal processor 55, the BTL driver 53, and the spindle driver 51 are controlled by the central processing unit 47. The central processing unit 47 has a CD-DSP 471, an ENDEC 472, a G / A 473, a microcontroller 474, and a memory such as a dynamic RAM 475 and a flash memory 476. The central processing unit 47 functions as the measuring means and the estimating means described above.
[0054]
The central processing unit 47 sends a sending command to the BTL driver 53. In response to the feed command, the BTL driver 53 drives the feed motor 15 to move the polarization optical system optical pickup OPU from the inner circumference to the outer circumference of the optical disc DISC to be measured.
[0055]
Further, the central processing unit 47 sends a rotation speed command to the spindle driver 51. In response to the rotation speed command, the spindle driver 51 rotates the spindle motor 13 at the rotation speed specified by the rotation speed command, thereby rotating the measured optical disc DISC at a plurality of different rotation speeds. it can.
[0056]
A signal obtained by being reflected from the optical disk to be measured DISC by the polarization optical system optical pickup OPU is taken into the central processing unit 47 via the analog signal processor 55. At this time, the level of the I-TOP is taken into the central processing unit 47 by the peak hold circuit 43 in the analog signal processor 55.
[0057]
FIG. 8 shows that when the rotation speed (speed) of the optical disk DISC to be measured is 1 ×, 4 ×, 16 ×, 32 ×, or 48 ×, the polarization optical system optical pickup OPU is moved from the inner circumference to the outer circumference of the optical disk DISC to be measured. 4 shows the level of I-TOP obtained when moving toward. The I-TOP level data obtained in this manner is stored as a log file in the dynamic RAM 475 and displayed as a screen on a display device (not shown).
[0058]
From these data, the user can use the central processing unit 47 to compare the I-TOP level due to the difference in the number of rotations (speed) or the position (measurement address) of the polarization optical system optical pickup OPU. is there. If the level of the I-TOP decreases due to the position or the speed, the central processing unit 47 can determine (determine) that it is the influence of the birefringence due to the rotational stress of the measured optical disc DISC.
[0059]
As is clear from FIG. 8, when the rotational speed (speed) of the optical disk under measurement DISC is 1 ×, the level of the I-TOP obtained from the polarization optical system optical pickup OPU regardless of the position of the polarization optical system optical pickup OPU. Does not change. In other words, at such a low speed, no stress is applied to the measured optical disc DISC, and it is assumed that the birefringence amount of the measured optical disc DISC is constant at any location. Therefore, in the present invention, as described later, the level of the I-TOP obtained at this time is used as a reference signal.
[0060]
Also, even if the rotation speed (speed) of the measured optical disc DISC is changed, when the polarization optical system optical pickup OPU is located on the inner peripheral side of the measured optical disc DISC, the level of the I-TOP obtained from the polarization optical system optical pickup OPU is obtained. Decreases only slightly with increasing speed. From this, it can be seen that almost no stress is generated due to the rotation of the measured optical disc DISC on the inner peripheral side of the measured optical disc DISC. That is, when the polarization optical system optical pickup OPU is on the inner circumference of the optical disk DISC to be measured, the level of I-TOP obtained from the polarization optical system optical pickup OPU is changed regardless of the rotation speed (speed) of the optical disk DISC to be measured. It can be used as a reference level.
[0061]
Taking the above into consideration, in the embodiment of the present invention, the birefringence of the optical disc DISC to be measured is measured and estimated in the following two modes.
[0062]
In the first mode of the present invention, the central processing unit 47 uses the polarization optical system optical pickup OPU to change the number of rotations of the measured optical disk DISC at the same position of the polarization optical system optical pickup OPU. The amount of birefringence of the measured optical disc DISC is estimated from the difference between the measurement results of the signals reflected from the DISC.
[0063]
A specific example of the first mode is as follows. The central processing unit 47 first rotates the optical disc under measurement in a state where the optical disc DISC is rotated at a sufficiently low rotation speed (for example, 1 × speed) at which stress is generated by centrifugal force due to rotation and birefringence does not significantly occur. While moving the system optical pickup OPU from the inner circumference to the outer circumference of the optical disk DISC to be measured, a signal (I-TOP level) obtained by reflection from the optical disk DISC to be measured by the polarization optical system optical pickup OPU is measured. It is stored in a memory as a reference signal. Next, the central processing unit 47 rotates the polarization optical system optical pickup OPU from the inner circumference to the outer circumference of the measured optical disc DISC while rotating the measured optical disc DISC at a desired rotation speed (for example, 48 × speed). While moving, a signal (I-TOP level) obtained by reflection from the optical disc DISC to be measured by the polarization optical system optical pickup OPU is measured. Finally, the central processing unit 47 estimates the amount of birefringence of the measured optical disc DISC from the degree of decrease of the signal obtained by the measurement with respect to the reference signal.
[0064]
In the first mode, the level of the I-TOP obtained by rotating the optical disk under test DISC at 1 × speed and moving the polarization optical system optical pickup OPU from the inner circumference to the outer circumference of the optical disk under test DISC as a reference level. Is used. However, as is clear from FIG. 8, this reference level hardly changes even when the position of the polarization optical system optical pickup OPU is different, and is constant.
[0065]
Therefore, in the modification of the first mode, the level of the I-TOP when the polarization optical system optical pickup OPU is located on the inner periphery of the optical disc DISC to be measured is used as the reference signal.
[0066]
A first modification of the first mode will be described. The central processing unit 47 first rotates the optical disc under measurement in a state where the optical disc DISC is rotated at a sufficiently low rotation speed (for example, 1 × speed) at which stress is generated by centrifugal force due to rotation and birefringence does not significantly occur. The system optical pickup OPU is moved to the inside where no stress is applied to the measured optical disc DISC, and the signal (I-TOP level) obtained by reflecting from the measured optical disc DISC by the polarization optical system optical pickup POU at that position is obtained. Measure and store in memory as reference signal. Next, the central processing unit 47 rotates the optical disk under measurement DISC at a desired rotation speed (for example, 48 times speed), and places the birefringence due to the rotation on the optical disk under measurement (see FIG. 8). The polarization optical system optical pickup OPU is moved to a position where the I-TOP level is the lowest, and a signal (I-TOP level) obtained by being reflected from the optical disc DISC by the polarization optical system optical pickup OPU at the position. Is measured. Finally, the central processing unit 47 estimates the amount of birefringence of the measured optical disc DISC from the degree of decrease of the signal obtained by the measurement with respect to the reference signal.
[0067]
Next, a second modification of the first mode will be described. The central processing unit 47 rotates the optical disc under measurement in a state where the optical disc DISC is rotated at a sufficiently low rotation speed (for example, 1 × speed) at which stress is generated by centrifugal force due to rotation and birefringence does not significantly occur, and the polarization optical system The optical pickup OPU is moved to the inside where no stress is applied to the measured optical disc DISC, and a signal (I-TOP level) obtained by reflecting from the measured optical disc DISC by the polarizing optical pickup OPU at the position is measured. Then, the reference signal is stored in the memory. Next, the central processing unit 47 moves the polarization optical system optical pickup OPU from the inner circumference to the outer circumference of the measured optical disc DISC while rotating the measured optical disc DISC at a rotation speed (for example, 48 × speed) to be measured. While moving, the signal (I-TOP level) obtained by reflecting from the optical disk under measurement DISC by the polarization optical system optical pickup OPU is measured. Finally, the central processing unit 47 estimates the amount of birefringence of the measured optical disc DISC from the degree of decrease of the signal obtained by the measurement with respect to the reference signal.
[0068]
A sufficiently low rotation speed at which birefringence does not significantly occur is, for example, 1000 rpm or less.
[0069]
In the second mode of the present invention, the central processing unit 47 changes the position of the polarization optical system optical pickup OPU in a state where the optical disk to be measured DISC is rotated at the desired rotation speed (for example, 48 times speed). In step (1), the amount of birefringence of the optical disc DISC to be measured is estimated from the difference between the measurement results of the signal (I-TOP level) obtained by reflecting the light from the optical disc DISC to be measured by the polarization optical system optical pickup OPU.
[0070]
A first specific example of the second mode is as follows. The central processing unit 47 moves the polarization optical system optical pickup OPU to the inside where no stress is applied to the measured optical disc while rotating the measured optical disc DISC at the rotation speed (for example, 48 × speed) to be measured. At this position, a signal (I-TOP level) obtained by reflection from the optical disk to be measured DISC by the polarization optical system optical pickup OPU is measured and stored in the memory as a reference signal. Next, the central processing unit 47 rotates the optical disk under measurement DISC at a rotation speed (for example, 48 times speed) at which measurement is to be performed (for example, at a 48-fold speed). The polarization optical system optical pickup OPU is moved to a position where the level of I-TOP is low, and the signal (I-TOP level) obtained by reflecting from the optical disc DISC to be measured by the polarization optical system optical pickup OPU at that position is obtained. Measure. Finally, the central processing unit 47 estimates the amount of birefringence of the measured optical disc DISC from the degree of decrease of the signal obtained by the measurement with respect to the reference signal.
[0071]
The location where the birefringence due to the most rotation occurs on the optical disc DISC to be measured is in the range of 50 to 55 mm in radius when the optical disc DISC to be measured is an optical disc having a diameter of 12 cm.
[0072]
A second specific example of the second mode is as follows. The central processing unit 47 first moves the polarization optical system optical pickup OPU to the inside where no stress is applied to the measured optical disc DISC, while rotating the measured optical disc DISC at the desired rotation speed (for example, 48 × speed). Then, a signal (I-TOP level) obtained by being reflected from the optical disk to be measured DISC by the polarization optical system optical pickup OPU at the position is measured and stored in the memory as a reference signal. Next, the central processing unit 47 rotates the polarization optical system optical pickup OPU from the inner circumference to the outer circumference of the measured optical disc DISC while rotating the measured optical disc DISC at a desired rotation speed (for example, 48 × speed). While moving, a signal (I-TOP level) obtained by reflection from the optical disc DISC to be measured by the polarization optical system optical pickup OPU is measured. Finally, the central processing unit 47 estimates the amount of birefringence of the measured optical disc DISC from the degree of decrease of the signal obtained by the measurement with respect to the reference signal.
[0073]
FIG. 9 illustrates the dependence of the amount of returning light to the photodetector PD and the amount of birefringence of the optical disc DISC. In FIG. 9, the vertical axis represents the amount of return light to the photodetector PD with the maximum value normalized to 1, and the horizontal axis represents the birefringence [nm] of the optical disc DISC. Here, it is assumed that the light emitted from the quarter-wave plate QWP toward the optical disc DISC is perfectly circularly polarized light, and that the wavelength of the laser beam emitted from the semiconductor laser LD is 785 nm.
[0074]
As is clear from FIG. 9, it is found that the birefringence of the optical disc DISC increases as the amount of return light to the photodetector PD decreases. When the amount of light returning to the photodetector PD is zero, the birefringence of the optical disc DISC is 392.5 [nm].
[0075]
Next, the dependence of the return light to the photodetector PD on the birefringence of the optical disc DISC will be described with reference to FIG. It is assumed that light is perpendicularly incident on the quarter-wave plate QWP, the direction of linearly polarized light is parallel to the X (radial) direction, and the optical disc DISC has no birefringence. In this case, light transmitted through the quarter-wave plate QWP becomes circularly polarized light. Here, it is assumed that the circularly polarized light is clockwise. The light reflected by the optical disc DISC is turned into counterclockwise circularly polarized light and enters the quarter-wave plate QWP. The light emitted from the quarter-wave plate QWP becomes linearly polarized light parallel to the Y (tangential) direction, transmits nearly 100% through the polarization beam splitter PBS, and enters the photodetector PD. Hereinafter, this state is set to 1 and standardized for consideration.
[0076]
Next, it is assumed that the phase is advanced by δ radians due to the birefringence of the optical disc DISC. In the following, 4) when (1) 0 <δ <π / 2, (2) when δ = π / 2, (3) when π / 2 <δ <π, and (4) when δ = π A description will be given separately for the following cases.
[0077]
{Circle around (1)} When 0 <δ <π / 2, the light reflected by the optical disk DISC is converted into left-handed elliptically polarized light, enters the quarter-wave plate QWP, and is straightened at a certain position inside the quarter-wave plate QWP. It becomes polarized light. Thereafter, the outgoing light from the quarter-wave plate QWP is emitted as elliptically polarized light in a clockwise direction with a phase advanced by π / 2 from the incident light. Since the amount of light transmitted through the polarization beam splitter PBS is represented by a component in the Y direction of the ellipse, the amount of light incident on the photodetector PD is cos (δ / 2).
[0078]
{Circle around (2)} When δ = π / 2, the light reflected by the optical disc DISC is converted into linearly polarized light parallel to the Y direction, enters the quarter-wave plate QWP, and rotates clockwise from the quarter-wave plate QWP. And emitted. Therefore, the light amount of the incident light on the photodetector PD is also cos (π / 4) ≒ 0.707.
[0079]
{Circle around (3)} When π / 2 <δ <π, the light reflected by the optical disc DISC becomes clockwise elliptically polarized light, enters the quarter-wave plate QWP, and forms a circle at a certain position inside the quarter-wave plate QWP. It becomes polarized light. Thereafter, the outgoing light from the quarter-wave plate QWP is emitted as elliptically polarized light in a clockwise direction with a phase advanced by π / 2 from the incident light. Therefore, the light quantity of the incident light on the photodetector PD is cos (δ / 2).
[0080]
{Circle around (4)} When δ = π, the light reflected by the optical disk DISC is converted into clockwise circularly polarized light, enters the quarter-wave plate QWP, and becomes linearly polarized light parallel to the X direction from the quarter-wave plate QWP. And is emitted. Therefore, the light quantity of the incident light on the photodetector PD is also cos (π / 2) = 0.
[0081]
Tables and graphs that summarize the above are shown in FIGS. 11 and 12, respectively. FIG. 11 is a table showing the relationship between the amount of birefringence of the optical disk, the phase shift, and the amount of incident light on the photodetector. FIG. 12 is a diagram showing the relationship between the amount of incident light on the photodetector and the amount of birefringence on the optical disk.
[0082]
The elliptically polarized light whose major axis is in the Y direction indicates a case where the phase δ advanced by the birefringence of the optical disc DISC is in the range of 0 to π. In this case, the birefringence of the optical disc DISC in the graph shown in FIG. 12 corresponds to the portion where the birefringence is from 0 nm to 392.5 nm.
[0083]
On the other hand, the elliptically polarized light whose major axis is in the X direction indicates a case where the phase δ advanced by the birefringence of the optical disc DISC becomes π or more or becomes negative. In this case, the birefringence of the optical disc DISC in the graph shown in FIG. 12 corresponds to the portion from 392.5 nm to 785 nm.
[0084]
In this manner, the birefringence amount estimation apparatus according to the present invention can estimate the birefringence amount of the rotating optical disc DISC as viewed from the polarization optical system optical pickup OPU. As described above, the optical disk drive used in the birefringence amount estimating apparatus according to the present invention can use an existing optical disk drive as it is or use a slightly modified one. Therefore, the birefringence amount estimation device can be manufactured at very low cost. In other words, if the I-TOP level detection circuit (FIG. 6) is added to the existing optical disk drive and a program for operating the central processing unit 47 is developed, the birefringence amount estimating apparatus according to the present invention is constituted. be able to. Further, the present invention can provide an optical disk drive including such a birefringence amount estimating apparatus.
[0085]
The present invention is not limited to the above-described embodiment, and it goes without saying that various changes and modifications can be made without departing from the spirit of the present invention. For example, in the above-described embodiment, the I-TOP level detection circuit includes the addition circuit 41 and the peak hold circuit 43, but the peak hold circuit 43 is omitted and the output of the addition circuit 41 is directly sent to the CPU 47. You may make it supply. In this case, the CPU 47 may detect a peak signal (I-TOP level) from the digital signal converted by the A / D conversion circuit 45. Further, in the above-described embodiment, only the case where the level of the I-TOP is used as the signal obtained by reflecting from the optical disk to be measured by the polarization optical system optical pickup is described. Of course, it may be used.
[0086]
【The invention's effect】
As apparent from the above description, according to the present invention, it is possible to estimate the birefringence amount of a rotating optical disc, not a stationary optical disc.
[Brief description of the drawings]
FIG. 1 is a diagram showing a state of an optical disk drive to which an apparatus for estimating the amount of birefringence of an optical disk according to an embodiment of the present invention is applied when an optical pickup moves to an inner periphery; FIG. 3B is a left side view.
FIGS. 2A and 2B are diagrams showing a state of the optical disk drive shown in FIG. 1 when an optical pickup moves to an outer periphery, where FIG. 2A is a plan view and FIG. 2B is a left side view.
FIG. 3 is a block diagram showing a configuration of a polarization optical system optical pickup used in the optical disk drive shown in FIGS. 1 and 2;
4A and 4B are diagrams showing spots of a laser beam applied to an optical disk, wherein FIG. 4A is a plan view and FIG. 4B is a schematic sectional view.
5A and 5B are diagrams illustrating a configuration of a photodetector for receiving reflected light (return light) used in the polarization optical system optical pickup illustrated in FIG. 3, wherein FIG. 5A is a front view and FIG. FIG.
FIG. 6 is a block diagram illustrating a configuration of an I-TOP level detection circuit that detects a level of a reflection side signal (I-TOP) of the HF signal.
FIG. 7 is a block diagram illustrating a configuration of a birefringence amount estimation device according to an embodiment of the present invention.
FIG. 8 shows a case where the rotation speed (speed) of the measured optical disc is 1 ×, 4 ×, 16 ×, 32 ×, or 48 ×, and the polarization optical system optical pickup is moved from the inner circumference to the outer circumference of the measured optical disc. It is a figure showing the level of I-TOP obtained when it moved.
FIG. 9 is a diagram showing a dependence relationship between the amount of returning light to the photodetector and the amount of birefringence of the optical disk.
FIG. 10 is a diagram for explaining dependence of return light to a photodetector on birefringence of an optical disk.
FIG. 11 is a table showing a relationship between a birefringence amount of an optical disc, a phase shift, and an incident light amount of a photodetector.
FIG. 12 is a diagram showing the relationship between the amount of incident light on a photodetector and the amount of birefringence on an optical disk.
[Explanation of symbols]
OPU Polarized optical system optical pickup DISC Optical disk under test 13 Spindle motor 15 Feed motor 47 Central processing unit (CPU)
51 Spindle driver 53 BTL driver 55 Analog signal processor (ASP)

Claims

An apparatus for estimating the amount of birefringence of an optical disc, wherein the apparatus estimates the amount of birefringence of the measured optical disc in a rotating state.

2. The apparatus for estimating the birefringence of an optical disc according to claim 1, wherein the birefringence of the optical disc to be measured in the rotating state is estimated using an optical disc drive equipped with a polarization optical system optical pickup. .

By rotating the optical disk under measurement at a plurality of different rotation speeds and moving the polarization optical system optical pickup from the inner circumference to the outer circumference of the optical disk under measurement, the measured light is measured by the polarization optical system optical pickup. Measuring means for measuring a signal obtained by reflection from the optical disc;
3. The optical disk birefringence amount estimating apparatus according to claim 2, further comprising estimating means for estimating a birefringence amount of the optical disk to be measured from the measurement result.

The estimating means measures a signal obtained by reflecting from the optical disk under measurement by the polarization optical system optical pickup when the rotation speed of the optical disk under measurement is changed at the same position of the optical pickup system. 4. The optical disc birefringence estimating apparatus according to claim 3, wherein the birefringence of the measured optical disc is estimated from a difference between the results.

The measuring means comprises:
In a state where the optical disc to be measured is rotated at a sufficiently low rotational speed at which stress is generated by centrifugal force due to rotation and birefringence does not significantly occur, the polarization optical system optical pickup is moved from the inner circumference to the outer circumference of the optical disc to be measured. Means for measuring a signal obtained by being reflected from the optical disk to be measured by the polarizing optical system optical pickup while moving toward the optical disk, and storing the signal as a reference signal,
While rotating the optical disc to be measured at the number of revolutions to be measured, while moving the polarization optical system optical pickup from the inner circumference to the outer circumference of the measurement optical disc, the polarization optical system optical pickup causes the optical disc to be measured. Means for measuring a signal obtained by reflection from
The optical disk birefringence amount estimating device according to claim 4, wherein the estimating unit estimates the amount of birefringence of the optical disk to be measured from a degree of decrease in a signal obtained by the measurement with respect to the reference signal.

The measuring means comprises:
In a state where the optical disc to be measured is rotated at a sufficiently low rotational speed at which stress is generated by centrifugal force due to rotation and birefringence does not significantly occur, stress is applied to the optical disc to be measured by the polarization optical system optical pickup. Moving inward, measuring a signal obtained by reflecting from the optical disc to be measured by the polarization optical system optical pickup at the position, and storing the signal as a reference signal;
With the measured optical disc rotated at the desired number of revolutions, move the polarization optical system optical pickup to a position where birefringence due to rotation occurs most on the measured optical disc, and at the position, the polarization optical Means for measuring a signal obtained by being reflected from the optical disk to be measured by a system optical pickup,
4. The optical disk birefringence amount estimating apparatus according to claim 3, wherein the estimating unit estimates the amount of birefringence of the measured optical disk from a degree of decrease of the signal obtained by the measurement with respect to the reference signal.

The measuring means comprises:
In a state where the optical disc to be measured is rotated at a sufficiently low rotational speed at which stress is generated by centrifugal force due to rotation and birefringence does not significantly occur, the polarization optical system optical pickup applies stress to the optical disc to be measured. Moving inward, measuring the signal obtained by reflection from the optical disk under measurement by the polarization optical system optical pickup at the position, and storing the signal as a reference signal;
While the optical disc to be measured is rotated at the rotation speed to be measured, the polarization optical system optical pickup is moved from the inner circumference to the outer circumference of the optical disk to be measured, and the measurement is performed by the polarization optical system optical pickup. Means for measuring a signal obtained by reflection from the optical disk,
4. The optical disk birefringence amount estimating apparatus according to claim 3, wherein the estimating unit estimates the amount of birefringence of the measured optical disk from a degree of decrease of the signal obtained by the measurement with respect to the reference signal.

8. The optical disc birefringence amount estimating apparatus according to claim 6, wherein the sufficiently low rotation speed at which the birefringence does not significantly occur is 1000 rpm or less. 9.

The estimating means reflects the reflected light from the measured optical disk by the polarizing optical system optical pickup when the position of the polarized optical system optical pickup is changed in a state where the measured optical disk is rotated at the rotation speed to be measured. 4. The optical disc birefringence amount estimating apparatus according to claim 3, wherein the birefringence amount of the optical disc to be measured is estimated from a difference between the measurement results of the signals obtained by the measurement.

The measuring means comprises:
In a state where the measured optical disk is rotated at the rotation number to be measured, the polarization optical system optical pickup is moved to the inside where no stress is applied to the measured optical disk, and the polarization optical system optical pickup is moved at the position. Means for measuring a signal obtained by reflection from the measured optical disk, and storing the signal as a reference signal;
With the optical disk under measurement rotated at the rotation number to be measured, move the polarization optical system optical pickup to a position where birefringence due to rotation occurs most on the optical disk under measurement, and the polarization optical system is moved at that position. Means for measuring a signal obtained by reflecting from the optical disk to be measured by an optical system optical pickup,
10. The optical disk birefringence amount estimating apparatus according to claim 9, wherein the estimating unit estimates the amount of birefringence of the optical disk to be measured from a degree of reduction of a signal obtained by the measurement with respect to the reference signal.

11. The optical disk according to claim 10, wherein the location where the birefringence due to the most rotation occurs on the optical disk under measurement is in a range of 50 to 55 mm in radius when the optical disk under measurement is an optical disk having a diameter of 12 cm. Refraction amount estimation device.

The measuring means comprises:
In a state where the measured optical disk is rotated at the rotation number to be measured, the polarization optical system optical pickup is moved to the inside where no stress is applied to the measured optical disk, and the polarization optical system optical pickup is moved at the position. Means for measuring a signal obtained by reflection from the measured optical disk, and storing the signal as a reference signal;
While the optical disk under measurement is rotated at the desired number of revolutions, the polarization optical system optical pickup moves the optical disk under measurement from the inner circumference to the outer circumference of the optical disk under measurement, and the measurement is performed by the polarization optical system optical pickup. Means for measuring a signal obtained by reflection from the optical disk,
10. The optical disk birefringence amount estimating apparatus according to claim 9, wherein the estimating unit estimates the amount of birefringence of the optical disk to be measured from a degree of reduction of a signal obtained by the measurement with respect to the reference signal.

An optical disk drive comprising means for estimating the amount of birefringence of a rotating optical disk.

14. The optical disk drive according to claim 13, wherein the birefringence amount estimating unit estimates a birefringence amount of the rotating optical disk using a polarization optical system optical pickup.

The birefringence amount estimation means,
By rotating the optical disk at a plurality of different rotation speeds and moving the polarization optical system optical pickup from the inner circumference to the outer circumference of the optical disk, the polarization optical system optical pickup reflects the optical disk from the optical disk. Measuring means for measuring a signal to be transmitted;
15. The optical disk drive according to claim 14, further comprising estimating means for estimating a birefringence amount of the optical disk from the measurement result.

The estimating means, when the rotation speed of the optical disc is changed at the same position of the polarization optical system optical pickup, from the difference in the measurement result of the signal reflected from the optical disc by the polarization optical system optical pickup. The optical disk drive according to claim 15, wherein a birefringence amount of the optical disk is estimated.

The measuring means comprises:
Move the polarization optical system optical pickup from the inner circumference to the outer circumference of the optical disk in a state where the optical disk is rotated at a sufficiently low rotational speed at which stress is generated by centrifugal force due to rotation and birefringence does not significantly occur. Means for measuring a signal obtained by reflecting from the optical disk by the polarization optical system optical pickup and storing the signal as a reference signal,
While the optical disc is rotated at the rotation speed to be measured, the polarization optical system optical pickup is reflected from the optical disk by the polarization optical system optical pickup while moving the optical system from the inner circumference to the outer circumference of the optical disk. Means for measuring a signal,
17. The optical disk drive according to claim 16, wherein the estimating unit estimates the amount of birefringence of the optical disk from a degree of decrease in a signal obtained by the measurement with respect to the reference signal.

The measuring means comprises:
When the optical disc is rotated at a sufficiently low rotational speed at which stress is generated by centrifugal force due to rotation and birefringence does not significantly occur, the polarization optical system optical pickup is moved inward without stress applied to the optical disc. A means for measuring a signal obtained by reflecting from the optical disc by the polarization optical system optical pickup at the position, and storing the signal as a reference signal;
With the optical disc rotated at the rotation speed desired to be measured, the polarization optical system optical pickup is moved to a position where birefringence due to rotation occurs most on the measured optical disc, and the polarization optical system light is moved at the position. Means for measuring a signal obtained by reflection from the optical disk by a pickup,
16. The optical disk drive according to claim 15, wherein the estimating unit estimates a birefringence amount of the optical disk from a degree of decrease of a signal obtained by the measurement with respect to the reference signal.

The measuring means comprises:
In a state where the optical disk is rotated at a sufficiently low rotational speed that stress is generated by centrifugal force due to rotation and birefringence does not significantly occur, the polarization optical system optical pickup is moved inside the optical disk where no stress is applied. Moving, measuring a signal obtained by reflecting from the optical disk by the polarizing optical system optical pickup at the position, and storing the signal as a reference signal;
While the optical disc is rotated at the rotation speed to be measured, while moving the polarization optical system optical pickup from the inner circumference to the outer circumference of the measured optical disc, the polarization optical system optical pickup reflects the optical disc from the optical disc. Means for measuring the signal obtained by
16. The optical disk drive according to claim 15, wherein the estimating unit estimates a birefringence amount of the optical disk from a degree of decrease of a signal obtained by the measurement with respect to the reference signal.

20. The optical disk drive according to claim 18, wherein the sufficiently low rotation speed at which the birefringence does not significantly occur is 1000 rpm or less.

The estimating means, when the optical disk is rotated at the rotation speed desired to be measured and the position of the polarization optical system optical pickup is changed, a signal obtained by reflecting from the optical disk by the polarization optical system optical pickup. The optical disk drive according to claim 15, wherein a birefringence amount of the optical disk is estimated from a difference between the measurement results.

The measuring means comprises:
While rotating the optical disk at the rotation speed to be measured, the polarization optical system optical pickup is moved inward without stress applied to the optical disk, and reflected from the optical disk by the polarization optical system optical pickup at the position. Means for measuring the signal obtained as a result and storing it as a reference signal;
With the optical disc rotated at the rotation number to be measured, the polarization optical system optical pickup is moved to a position where birefringence due to rotation occurs most in the optical disk, and the polarization optical system optical pickup is moved at the position. Means for measuring a signal obtained by reflection from the optical disk,
22. The optical disc drive according to claim 21, wherein the estimating unit estimates a birefringence amount of the optical disc from a degree of decrease of a signal obtained by the measurement with respect to the reference signal.

23. The optical disk drive according to claim 22, wherein the location where the birefringence due to the most rotation occurs on the optical disk is in a range of 50 to 55 mm in radius when the optical disk is an optical disk having a diameter of 12 cm.

The measuring means comprises:
While rotating the optical disk at the rotation speed to be measured, the polarization optical system optical pickup is moved inward without stress applied to the optical disk, and reflected from the optical disk by the polarization optical system optical pickup at the position. Means for measuring the signal obtained as a result and storing it as a reference signal;
While the optical disk is rotated at the rotation speed to be measured, the polarization optical system optical pickup is reflected from the optical disk by the polarization optical system optical pickup while moving the optical system from the inner circumference to the outer circumference of the optical disk. Means for measuring the signal to be transmitted,
22. The optical disc drive according to claim 21, wherein the estimating unit estimates a birefringence amount of the optical disc from a degree of decrease of a signal obtained by the measurement with respect to the reference signal.