JP4660915B2 - Optical pickup device - Google Patents

Description

【０１６６】
本実施の形態で用いられる回折面は、光路差関数として［数２］により表される。
【数２】

【０１６７】
以下、本発明の好適な実施の形態について、図面を参照しつつ説明する。
図１は、本実施の形態にかかる光ピックアップ装置の概略構成図である。図１において、第１の光情報記録媒体２３に対して記録および／または再生を行う第１光源１１と、第２の光情報記録媒体２４に対して記録および／または再生を行う第１光源１１とは波長の異なる第２光源１２とを備え、それぞれの光源から射出される発散光束の発散角を所望の発散角に変換するカップリングレンズ２１、２２と、上記それぞれの光源からの光束をほぼ同一の方向に進むようにする光路合成手段であるビームスプリッタ６２と、ビームスプリッタ６２からの光束を光情報記録媒体の情報記録面５に集光する対物レンズ３と、光情報記録媒体からの反射光を受光する光検出器４１、４２とを備えている。図中、８は絞り、９はシリンドリカルレンズ、７１、７２は１／４波長板、１５は光源１１からの発散光束の発散度を小さくするためのカップリングレンズ、１６は凹レンズ、１７は反射光束を分離するためのホログラムである。
【０１６８】
更に、本実施の形態においては、対物レンズ３の球面収差の変動を補正する手段及び発散度変更手段として、前記光源側から順に配置された負レンズ５と正レンズ４と、アクチュエータ７を備えている（以下、球面収差補正手段、及び発散度変更手段ともいう）。アクチュエータ７は、光学要素としての負レンズ５を光軸方向に移動させて光束の発散角度を変更する変移装置として機能する。また、本実施の形態に関連して、その具体的な一部の光学系を示す実施例１〜１４においては、この変移可能な負レンズ５と正レンズ４とから構成された、いわゆるビームエキスパンダーの一例のことを、球面収差補正手段と表現する場合がある。尚、６は、フォーカシングのため対物レンズ３を光軸方向に駆動するアクチュエータである。第１光源１１は波長λ１＝４０５ｎｍのレーザ光を射出し、第２光源１２は波長λ２＝６５５ｎｍのレーザ光を射出できるものとする。
【０１６９】
以下に述べる実施例において、実施例１、２、１１、１２は、対物レンズ３に回折面を設けて軸上色収差を補正しており、実施例３〜５は、負レンズ５と正レンズ４に特定の素材を用いて軸上色収差を補正しており、実施例６〜８、１３、１４は、負レンズ５と正レンズ４の少なくとも一方に回折面を設けて対物レンズ３の軸上色収差を補正しており、実施例９、１０は、負レンズ５と正レンズ４の特定の素材と、正レンズ４に設けた回折面の相乗効果で対物レンズ３の軸上色収差を補正している。また、実施例４、５、１２は、異なる光情報記録媒体に対し、同一の光学系を用いて情報の記録又は再生を行う例である。尚、以下の対物レンズの実施例では、吸水率０．０１％以下で、光源波長４００ｎｍの光束による透過率が９０．５％及び光源波長７００ｎｍの光束による透過率が９２％であるプラスチック材料を用いて形成した。また、以下の実施例において、図１に示した本実施の形態における第１光源１１のみを用いた実施例では、具体的な実施の形態の図面は省略したが、概して図１のピックアップ装置において、例えば、第２光源１２と、カップリングレンズ２２と、ビームスプリッタ６２と、光検出器４２と、１／４波長板７２と、ホログラム１７とを取り除いたような態様とすることができる。以下、各実施例について説明する。
【０１７０】
（実施例１）
表１に、実施例１における、負レンズ５、正レンズ４、対物レンズ３からなる光学系に関するデータを示す。尚、これより示すレンズデータ内において、１０のべき乗数（例えば、２．５×１０^-3）を、Ｅ（例えば、２．５×Ｅ−３）を用いて表している。また、回転対称な多項式によって表される回折面の回折による１次光は、回折後収束する方向に光線の角度が変化する光のことを意味する。
【表１】

【０１７１】
図２は、実施例１に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。図３は、対物レンズ３にかかる球面収差図である。実施例１においては、波長４０５ｎｍの第１光源１１と、対物レンズ３の像側開口数ＮＡ０．８５との組み合わせにより情報の記録又は再生を行うものとする。本実施例においては、前記球面収差の変動を補正する手段の負レンズ５及び正レンズ４の材料として、それぞれνｄＮ＝２３．８、νｄＰ＝８１．６の材料を選び、更に、対物レンズ３の光源側の面に回折面を設けることで、対物レンズ３で発生する軸上色収差を補正している。また、本実施例においては、ｆＮ＝−８．１３（ｍｍ）、ｆＰ＝９．４８（ｍｍ）であり、ｆ＝１．７６５（ｍｍ）、ｆＤ＝７１．４８３（ｍｍ）である。
【０１７２】
本実施例では、光源の発振波長の微小変動（以下、単に、波長変動ともいう）あるいは温度変化時の球面収差の変動の補正を、以下のように行うことができる。本実施例の場合は、波長が大きくなったとき、あるいは温度が上昇したときに対物レンズ３では、補正過剰の球面収差が発生する。かかる場合、発生した球面収差を、アクチュエータ７により負レンズ５を光軸に沿って動かすことで、負レンズ５と正レンズ４の間隔を小さくすれば、補正不足の球面収差を発生させることができる。適切な量だけ負レンズ５を動かせば、補正過剰の球面収差をキャンセルすることができ、球面収差の補正結果を示す表２から明らかなように、光学系全体の球面収差は良好となる。
【表２】

【０１７３】
（実施例２）
表３に、実施例２における、負レンズ５、正レンズ４、対物レンズ３からなる光学系に関するデータを示す。
【表３】

【０１７４】
図４は、実施例２に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。図５は、対物レンズ３にかかる球面収差図である。実施例２においては、波長４０５ｎｍの第１光源１１と、対物レンズ３の像側開口数ＮＡ０．８５との組み合わせにより情報の記録又は再生を行うものとする。実施例２では、前記球面収差の変動を補正する手段の負レンズ５及び正レンズ４の材料として、それぞれνｄＮ＝３０．０、νｄＰ＝５６．５の材料を選び、更に、対物レンズ３の光源側の面に回折面を設けることで、対物レンズ３で発生する軸上色収差を補正している。また、本実施例においては、ｆＮ＝−４．７５（ｍｍ）、ｆＰ＝６．４７（ｍｍ）であり、ｆ＝１．７６５（ｍｍ）、ｆＤ＝７１．４８３（ｍｍ）である。
【０１７５】
本実施例での波長変動あるいは温度変化時の球面収差の変動の補正については、実施例１と同様なので、説明は省略する。球面収差の補正結果を示す表４から明らかなように、波長変動時あるいは温度変化時の球面収差は良好なものとなっている。また、対物レンズ３及び球面収差の変動を補正する手段として、負レンズ５及び正レンズ４にプラスチック材料を用いることで、光学系の軽量化・可動機構への負担の軽減を図っている。
【表４】

【０１７６】
（実施例３）
表５に、実施例３における、負レンズ５、正レンズ４、対物レンズ３からなる光学系に関するデータを示す。
【表５】

【０１７７】
図６は、実施例３に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。図７は、対物レンズ３にかかる球面収差図である。実施例３においては、波長４０５ｎｍの第１光源１１と、対物レンズ３の像側開口数ＮＡ０．８５との組み合わせにより情報の記録又は再生を行うものとする。実施例３においては、負レンズ５及び正レンズ４の材料として、それぞれνｄＮ＝２３．８、νｄＰ＝８１．６の材料を選ぶことで、軸上色収差を補正している。また、本実施例においては、ｆＮ＝−９．２７（ｍｍ）、ｆＰ＝１１．０８（ｍｍ）であり、ｆ＝１．７６５（ｍｍ）である。
【０１７８】
本実施例での波長変動あるいは温度変化時の球面収差の補正は実施例１と同様なので、説明は省略する。球面収差の補正結果を示す表６から明らかなように、波長変動時あるいは温度変化時の球面収差は良好なものとなっている。また、対物レンズ３にプラスチック材料を用いることで、光学系の軽量化・可動機構への負担の軽減を図っている。
【表６】

【０１７９】
（実施例４）
表７に、実施例４における、負レンズ５、正レンズ４、対物レンズ３からなる光学系に関するデータを示す。
【表７】

【０１８０】
図８、９は、第４の実施例に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。図１０、１１は、異なる光情報記録媒体に対してそれぞれ、情報の記録又は再生を行う際における、対物レンズ３にかかる球面収差図である。実施例４においては、同一光学系を用いて、波長４０５ｎｍの第１光源１１と、透明基板厚０．１ｍｍの光情報記録媒体との組み合わせ、又は波長６５５ｎｍの第２光源１１と、透明基板厚０．６ｍｍの光情報記録媒体との組み合わせにより情報の記録又は再生を行う光ピックアップ装置の例である。実施例４においては、負レンズ５及び正レンズ４の材料として、それぞれνｄＮ＝３０．０、νｄＰ＝５６．５の材料を選ぶことで、軸上色収差を補正している。また、本実施例においては、ｆＮ＝−３．８２（ｍｍ）、ｆＰ＝６．８５（ｍｍ）であり、ｆ１＝１．７６５（ｍｍ）、ｆＤ１＝５００００００．０２（ｍｍ）である。なお、発振波長λ２＝６５５ｎｍにおける対物レンズの焦点距離は、ｆ２＝１．８０４である。
【０１８１】
実施例４においては、異なる光情報記録媒体における透明基板厚の違いに起因して発生する球面収差の変動を、光源側から順に１枚の負レンズ５、１枚の正レンズ４から構成される発散角度変更手段（本発明の球面収差の変動を補正する手段、或いは球面収差の変動と軸上色収差とを補正する手段に相当する）の間隔を変えることで補正している。また、対物レンズ３の光源側の面に回折面を設けることで、上記球面収差をより良好に補正している。更に、光源の波長変動時や温湿度変化時の対物レンズの球面収差劣化も、発散度変更手段の間隔を変えることで良好に補正している。すなわち、表８から明らかなように、負レンズ５と正レンズ４の間隔を適切な間隔に変更することで、基板厚変更時、波長変動時及び温湿度変化時の対物レンズ３の球面収差劣化を、良好に補正している。また、対物レンズ３及び、負レンズ５、正レンズ４にプラスチック材料を用いることで、光学系の軽量化・可動機構への負担の軽減を図っている。
【表８】

【０１８２】
（実施例５）
表９に、実施例５における、負レンズ５、正レンズ４、対物レンズ３からなる光学系に関するデータを示す。
【表９】

【０１８３】
図１２、１３は、実施例５に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。図１４、１５は、異なる光情報記録媒体に対してそれぞれ、情報の記録又は再生を行う際における、対物レンズ３にかかる球面収差図である。実施例５においては、同一光学系を用いて、波長４０５ｎｍの第１光源１１と、透明基板厚０．１ｍｍの光情報記録媒体との組み合わせ、又は波長６５５ｎｍの第２光源１１と、透明基板厚０．６ｍｍの光情報記録媒体との組み合わせにより情報の記録又は再生を行う光ピックアップ装置の例である。実施例５においては、負レンズ５及び正レンズ４の材料として、それぞれνｄＮ＝３０．０、νｄＰ＝５６．５の材料を選ぶことで、軸上色収差を補正している。また、本実施例においては、ｆＮ＝−６．５９（ｍｍ）、ｆＰ＝９．８５（ｍｍ）であり、ｆ１＝３．０１１（ｍｍ）、ｆＤ１＝８４９９６４．３３（ｍｍ）である。なお、発振波長λ２＝６５５ｎｍにおける対物レンズの焦点距離は、ｆ２＝３．０７６である。
【０１８４】
実施例４と同様に、表１０から明らかなように、負レンズ５と正レンズ４の間隔を適切な間隔に変更することで、透明基板厚変更時、波長変動時及び温湿度変化時の対物レンズの球面収差劣化を、良好に補正出来る。また、対物レンズ３、負レンズ５、正レンズ４にプラスチック材料を用いることで、光学系の軽量化・可動機構への負担の軽減を図っている。
【表１０】

【０１８５】
尚、球面収差の変動を補正する手段としての負レンズ５に入射する光束は、上述した実施例のように平行光だけでなく、発散光あるいは収斂光であっても、本発明の光学系を同様に適用することができ。また、本実施例では図示していないが、光源と球面収差補正手段の間に、光源からの光束の発散度を変えるカップリングレンズを設けることができる。かかるカップリングレンズに回折面を付加して、長波長側でバックフォーカスが短くなるような回折レンズとすることで、対物レンズで発生する軸上色収差を補正できる。
【０１８６】
本発明による光学系に用いるカップリングレンズは、上記の形態に限らず、同一出願人による特願２０００−０６０８４３号にあるようなものであれば、対物レンズ３で発生する軸上色収差をより良好に補正できる。
【０１８７】
また、上記カップリングレンズと球面収差の変動を補正する手段（負レンズ５、正レンズ４）の間に、光源からの光束の非点隔差を緩和し、球面収差補正手段にほぼ円形の光束を入射させることができるビーム整形素子を設ける場合、温湿度変化に起因するカップリングレンズの焦点移動により、カップリングレンズからの光束の発散度が変わって、上記ビーム整形素子により非点収差が発生してしまう。これを抑えるためには、同一出願人による特願２０００−０５３８５８号にあるようなカップリングレンズを用いることで、ビーム整形素子による非点収差の発生を抑えることができる。
【０１８８】
尚、実施例４、５において、光源波長６５５ｎｍ、透明基板厚０．６ｍｍの光情報記録媒体に対する収差図は、ＮＡ０．６５まで図示している。しかし、この時、対物レンズ３には光源波長４０５ｎｍ、ＮＡ０．８５で決まる絞りを全て通過する光束が入射している。結像に寄与しないＮＡ０．６５以上の光束は、対物レンズ３に設けた回折面の効果を利用してフレア成分とすることで、情報記録面上でスポット径がしぼられ過ぎず、光ピックアップ装置の受光素子での不要信号の検出を防止することが出来る。
【０１８９】
（実施例６）
表１１に、実施例６における、負レンズ５、正レンズ４、対物レンズ３からなる光学系に関するデータを示す。
【表１１】

【０１９０】
図１６は、実施例６に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。図１７は、対物レンズ３にかかる球面収差図である。実施例６においては、波長４０５ｎｍの第１光源１１と、対物レンズ３の像側開口数ＮＡ０．８５との組み合わせにより情報の記録又は再生を行うものとする。実施例６においては、正レンズ４の光情報記録媒体側の面に回折面を付加し、長波長側でバックフォーカスが短くなるような回折レンズとすることで、対物レンズ３の軸上色収差を補正している。また、本実施例においては、ｆＮ＝−５．０３（ｍｍ）、ｆＰ＝６．８１（ｍｍ）であり、ｆ＝１．７６５（ｍｍ）である。
【０１９１】
本実施例における光源波長変動あるいは温度変化時の球面収差の変動の補正については実施例１と同様なので、説明は省略する。表１２から明らかなように、波長変動時あるいは温度変化時の球面収差は良好なものとなっている。また、対物レンズ３、負レンズ５、正レンズ４にプラスチック材料を用いることで、光学系の軽量化・可動機構への負担の軽減を図っている。
【表１２】

【０１９２】
（実施例７）
表１３に、実施例７における、負レンズ５、正レンズ４、対物レンズ３からなる光学系に関するデータを示す。
【表１３】

【０１９３】
図１８は、実施例７に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。図１９は、対物レンズ３にかかる球面収差図である。実施例７においては、波長４０５ｎｍの第１光源１１と、対物レンズ３の像側開口数ＮＡ０．８５との組み合わせにより情報の記録又は再生を行うものとする。実施例７においては、正レンズ４の両面に回折面を付加し、長波長側でバックフォーカスが短くなるような回折レンズとすることで、対物レンズ３の軸上色収差を補正している。また、本実施例においては、ｆＮ＝−４．８９（ｍｍ）、ｆＰ＝５．８３（ｍｍ）であり、ｆ＝１．７６５（ｍｍ）である。
【０１９４】
本実施例における光源波長変動あるいは温度変化時の球面収差の変動の補正については実施例１と同様なので、説明は省略する。表１４から明らかなように、波長変動時あるいは温度変化時の球面収差は良好なものとなっている。また、対物レンズ３、負レンズ５、正レンズ４にプラスチック材料を用いることで、光学系の軽量化・可動機構への負担の軽減を図っている。
【表１４】

【０１９５】
（実施例８）
表１５に、実施例８における、負レンズ５、正レンズ４、対物レンズ３からなる光学系に関するデータを示す。
【表１５】

【０１９６】
図２０は、実施例８に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。図２１は、対物レンズ３にかかる球面収差図である。実施例８においては、波長４０５ｎｍの第１光源１１と、対物レンズ３の像側開口数ＮＡ０．８５との組み合わせにより情報の記録又は再生を行うものとする。実施例８においては、負レンズ５及び正レンズ４の両面に回折面を付加し、長波長側でバックフォーカスが短くなるような回折レンズとすることで、対物レンズ３の軸上色収差を補正している。また、本実施例においては、ｆＮ＝−５．５４（ｍｍ）、ｆＰ＝７．４２（ｍｍ）であり、ｆ＝１．７６５（ｍｍ）である。
【０１９７】
本実施例における光源長変動あるいは温度変化時の球面収差の補正については、実施例１と同様なので説明は省略する。表１６から明らかなように、波長変動時あるいは温度変化時の球面収差は良好なものとなっている。また、対物レンズ３、負レンズ５、正レンズ４にプラスチック材料を用いることで、光学系の軽量化・可動機構への負担の軽減を図っている。
【表１６】

【０１９８】
（実施例９）
表１７に、実施例９における、負レンズ５、正レンズ４、対物レンズ３からなる光学系に関するデータを示す。
【表１７】

【０１９９】
図２２は、実施例９に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。図２３は、対物レンズ３にかかる球面収差図である。実施例９においては、波長４０５ｎｍの第１光源１１と、対物レンズ３の像側開口数ＮＡ０．８５との組み合わせにより情報の記録又は再生を行うものとする。実施例９においては、正レンズ４の光情報記録媒体側の面に回折面を付加し、長波長側でバックフォーカスが短くなるような回折レンズとすることで、対物レンズ３の軸上色収差を補正している。また、上記球面収差補正手段の負レンズ５及び正レンズ４の材料として、それぞれＮ＝３０．０、Ｐ＝５６．５の材料を選ぶことで、より良好に対物レンズの軸上色収差を補正している。また、本実施例においては、ｆＮ＝−４．１５（ｍｍ）、ｆＰ＝５．９１（ｍｍ）であり、ｆ＝１．７６５（ｍｍ）である。
【０２００】
本実施例における光源波長変動あるいは温度変化時の球面収差の変動の補正については実施例１と同様なので、説明は省略する。表１８から明らかなように、波長変動時あるいは温度変化時の球面収差は良好なものとなっている。また、対物レンズ３、負レンズ５、正レンズ４にプラスチック材料を用いることで、光学系の軽量化・可動機構への負担の軽減を図っている。
【表１８】

【０２０１】
（実施例１０）
表１９に、実施例１０における、負レンズ５、正レンズ４、対物レンズ３からなる光学系に関するデータを示す。
【表１９】

【０２０２】
図２４は、実施例１０に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。図２５は、対物レンズ３にかかる球面収差図である。実施例１０においては、波長４０５ｎｍの第１光源１１と、対物レンズ３の像側開口数ＮＡ０．８５との組み合わせにより情報の記録又は再生を行うものとする。実施例１０においては、正レンズ４の両面に回折面を付加し、長波長側でバックフォーカスが短くなるような回折レンズとすることで、対物レンズ３の軸上色収差を補正している。このとき、対物レンズ３と、球面収差補正手段としての負レンズ５及び正レンズ４とを合わせた合成系の軸上色収差を補正過剰とすることで、図２５に示されているように、第１光源１１の発振波長（４０５ｎｍ）の球面収差カーブと長・短波長側の球面収差カーブとを交差させている。これにより、光源のモードホップ現象や高周波重畳時の波面収差の劣化が非常に小さく、例えば、光源の発振波長が微小変動した際でも、最適書き込み位置の移動を小さく抑えることができる。さらに、球面収差補正手段としての可動要素である負レンズ５を両面非球面レンズとしたことで、負レンズ５の偏芯やトラッキングエラー時の波面収差の劣化を小さく抑えている。また、負レンズ５及び正レンズ４の材料として、それぞれνｄＮ＝２４．３、νｄＰ＝５６．５の材料を選ぶことで、対物レンズ３の軸上色収差を補正しており、正レンズ４に付加した回折構造の負担を軽減している。また、本実施例においては、ｆＮ＝−７．７８（ｍｍ）、ｆＰ＝９．９５（ｍｍ）であり、ｆ＝１．７６５（ｍｍ）である。
【０２０３】
本実施例では、光束を規制する絞りを、対物レンズ３の光源側の面の頂点より光情報記録媒体側に配置しているので、発散光束が入射する場合に、対物レンズ３の最も光源側の面の光線通過高さを小さく抑えることができる。これは、対物レンズ３の小径化、あるいは収差補正上も好ましい。
【０２０４】
本実施例における光源の波長変動あるいは温度変化時の球面収差の変動の補正については実施例１と同様なので、説明は省略する。表２０から明らかなように、波長変動時あるいは温度変化時の球面収差は良好なものとなっている。また、対物レンズ３、負レンズ５、正レンズ４にプラスチック材料を用いることで、光学系の軽量化、可動機構への負担の軽減を図っている。また、短波長の光に対して透過率が高いプラスチック材料を用いているので、安価に大量生産でき、かつ光の利用効率の高い光学系を達成している。なお、可動機構は、本明細書中の実施例では、負レンズ５の変移装置及び対物レンズ３のフォーカシング機構である。
【表２０】

【０２０５】
なお、本実施例においては、図２５には図示していないが、図１の実施の形態に示したように、実際の光ピックアップ装置では、光源と球面収差補正手段との間にコリメータ等のカップリングレンズが設けられている。その場合に、カップリングレンズで発生する軸上色収差も本実施例の構成によって補正することができ、色収差が良好な集光光学系を得ることができる。
【０２０６】
更にまた、光情報記録媒体の片面に第１情報記録層と第２情報記録層との２つの相変化膜を設け、それぞれに情報の記録を行うことで、光情報記録媒体の記録容量を略２倍に高めた、いわゆる２層記録方式の光情報記録媒体が知られているが、本実施例のものは、そのような２層記録方式の光情報記録媒体に対して情報の記録又は再生を行うことにも適用することができ、各情報記録層の情報記録面まで厚さの違いによって発生する球面収差を補正することができる。たとえば、光情報記録媒体の表面側から順に第１情報記録層、第２情報記録層としたすると、図２６に示すように、球面収差補正手段としての負レンズ５と正レンズ４との間隔を小さくすることにより、第２情報記録層の情報記録面に対して情報の記録又は再生をすることができる。
【０２０７】
（実施例１１）
表２１に、実施例１１における、負レンズ５、正レンズ４、対物レンズ３からなる光学系に関するデータを示す。
【表２１】

【０２０８】
図２７は、実施例１１に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。図２８は、対物レンズ３にかかる球面収差図である。実施例１１においては、波長４０５ｎｍの第１光源１１と、対物レンズ３の像側開口数ＮＡ０．８５との組み合わせにより情報の記録又は再生を行うものとする。実施例１１においては、対物レンズ３の光源側の面に回折面を付加し、長波長側でバックフォーカスが短くなるような回折レンズとすることで、対物レンズ３の軸上色収差を補正している。さらに、球面収差補正手段としての可動要素である負レンズ５を両面非球面レンズとしたことで、負レンズ５の偏芯やトラッキングエラー時の波面収差の劣化を小さく抑えている。また、本実施例においては、ｆＮ＝−８．３２（ｍｍ）、ｆＰ＝１２．３０（ｍｍ）であり、ｆ＝１．７６５（ｍｍ）、ｆＤ＝２８．４１７（ｍｍ）である。
【０２０９】
本実施例における光源の波長変動あるいは温度変化時の球面収差の変動の補正については実施例１と同様なので、説明は省略する。表２２から明らかなように、波長変動時あるいは温度変化時の球面収差は良好なものとなっている。また、対物レンズ３、負レンズ５、正レンズ４にプラスチック材料を用いることで、光学系の軽量化、可動機構への負担の軽減を図っている。また、短波長の光に対して透過率が高いプラスチック材料を用いているので、安価に大量生産でき、かつ光の利用効率の高い光学系を達成している。
【表２２】

【０２１０】
（実施例１２）
表２３に、実施例１２における、負レンズ５、正レンズ４、対物レンズ３からなる光学系に関するデータを示す。
【表２３】

【０２１１】
図２９、３０は、実施例１２に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。図３１、３２は、異なる光情報記録媒体に対してそれぞれ、情報の記録又は再生を行う際における、対物レンズ３にかかる球面収差図である。実施例１２においては、同一光学系を用いて、波長４０５ｎｍの第１光源１１と、透明基板厚０．１ｍｍの光情報記録媒体との組み合わせ、又は波長６５５ｎｍの第２光源１１と、透明基板厚０．６ｍｍの光情報記録媒体との組み合わせにより情報の記録又は再生を行う光ピックアップ装置の例である。実施例１２においては、対物レンズ３の光源側の面に回折構造を設けることにより、透明基板厚の違いにより発生する球面収差および色の球面収差とを補正している。具体的には、球面収差補正手段としての負レンズ５が光軸方向に動かすことで、光情報記録媒体の透明基板厚に対応して対物レンズ３に入射される光束の発散角を変えることにより行う。また、本実施例においては、ｆＮ＝−６．３９（ｍｍ）、ｆＰ＝１０．５１（ｍｍ）であり、ｆ１＝１．７６５（ｍｍ）、ｆＤ１＝４５．４６（ｍｍ）である。なお、発振波長λ２＝６５５ｎｍにおける対物レンズの焦点距離は、ｆ２＝１．７９である。
【０２１２】
本実施例における光源の波長変動あるいは温度変化時の球面収差の変動の補正については実施例１と同様なので、説明は省略する。表２４から明らかなように、波長変動時あるいは温度変化時の球面収差は良好なものとなっている。また、対物レンズ３、負レンズ５、正レンズ４にプラスチック材料を用いることで、光学系の軽量化、可動機構への負担の軽減を図っている。また、短波長の光に対して透過率が高いプラスチック材料を用いているので、安価に大量生産でき、かつ光の利用効率の高い光学系を達成している。
【表２４】

【０２１３】
尚、実施例４、５と同様に、光源波長６５５ｎｍ、透明基板厚０．６ｍｍの光情報記録媒体に対するＮＡ０．６５以上の光束は、対物レンズ３に設けた回折面の効果を利用してフレア成分とすることで、情報記録面上でスポット径がしぼられ過ぎず、光ピックアップ装置の受光素子での不要信号の検出を防止することが出来る。
【０２１４】
（実施例１３）
表２５に、実施例１３における、カップリングレンズ２１又はカップリングレンズ１５、２１に対応するコリメータ、負レンズ５、正レンズ４、対物レンズ３からなる光学系に関するデータを示す。
【表２５】

【０２１５】
図３３は、実施例１３に係るコリメータ、負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。図３４は、対物レンズ３にかかる球面収差図である。実施例１３においては、波長４０５ｎｍの第１光源１１と、対物レンズ３の像側開口数ＮＡ０．８５との組み合わせにより情報の記録又は再生を行うものとする。実施例１３においては、球面収差補正手段中の負レンズ５を光軸方向に沿って変移することで、対物レンズ３に入射する光束の発散角を変化させ、集光光学系（コリメータ及び対物レンズ３）の各光学面で発生する球面収差の変動を補正するようにした。また、本実施例においては、ｆＮ＝−１０．７１（ｍｍ）、ｆＰ＝１３．１８（ｍｍ）であり、ｆ＝１．７６５（ｍｍ）である。
【０２１６】
また、正レンズ４の両面に回折面を付加し、球面収差補正手段自体では集光光学系の光学面で発生する軸上色収差とは逆の極性の軸上色収差を発生させることで、集光光学系の光学面で発生する軸上色収差を補正し、情報記録面上に焦点を結んだときの波面の軸上色収差を良好にした。本実施例の集光光学系において、その光学素子であるコリメータと対物レンズ３で発生する軸上色収差量をそれぞれΔｆＢ１、ΔｆＢ２として、その比を概略的に求めてみたところ、コリメータの焦点距離は１２ｍｍ、球面収差補正手段の倍率は１．２３倍、対物レンズの諸点距離は１．７６５ｍｍであるので、ΔｆＢ１／ΔｆＢ２＝１／３０となる。すなわち、球面収差補正手段で発生させる逆極性の軸上色収差の絶対値を対物レンズで発生する軸上色収差の絶対値とほぼ同じにすると、情報記録面上に焦点を結んだときの波面の軸上色収差を良好にできる。このとき、集光光学系と、球面収差補正手段としての負レンズ５及び正レンズ４とを合わせた合成系の軸上色収差を補正過剰とすることで、図３４に示されているように、第１光源１１の発振波長（４０５ｎｍ）の球面収差カーブと長・短波長側の球面収差カーブとを交差させている。これにより、光源のモードホップ現象や高周波重畳時の波面収差の劣化が非常に小さく、例えば、光源の発振波長が微小変動した際でも、最適書き込み位置の移動を小さく抑えることができる。さらに、球面収差補正手段中の可動要素である負レンズ５を両面非球面レンズとしたことで、負レンズ５の偏芯やトラッキングエラー時の波面収差の劣化を小さく抑えている。
【０２１７】
表２６から明らかなように、波長変動時あるいは温度変化時等、様々な要因により集光光学系の各光学面で発生する球面収差の変動を補正でき、良好な球面収差となっている。また、集光光学系を構成するコリメータ及び対物レンズ３、および球面収差補正手段を構成する負レンズ５及び正レンズ４の全てをプラスチック材料を用いることで、光学系の軽量化、可動機構への負担の軽減を図っている。また、短波長の光に対して透過率が高いプラスチック材料を用いているので、安価に大量生産でき、かつ光の利用効率の高い光学系を達成している。
【表２６】

【０２１８】
なお、本実施例においては、球面収差補正手段中の負レンズ５を変移可能としたが、正レンズ４を変移可動としても良く、また、両レンズを変移可能としても、同様に集光光学系の球面収差の変動を補正することができる。また、本実施例においては、球面収差補正手段中の正レンズ４に設けた回折構造により集光光学系と球面収差補正手段の軸上色収差を補正するようにしたが、回折構造を他のレンズの面に設けてもよく、他に回折構造を設けた面を有する光学素子を別途付加してもよい。
【０２１９】
（実施例１４）
表２７に、実施例１４における、カップリングレンズ１５、負レンズ５、正レンズ４、対物レンズ３からなる光学系に関するデータを示す。
【表２７】

【０２２０】
図３５は、実施例１４に係るカップリングレンズ１５、負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。本実施例のカップリングレンズ１５は、第１光源１１からの強い発散光束を弱い発散光束とする機能を有している。図３６は、対物レンズ３にかかる球面収差図である。実施例１４においては、波長４０５ｎｍの第１光源１１と、対物レンズ３の像側開口数ＮＡ０．８５との組み合わせにより情報の記録又は再生を行うものとする。実施例１４においては、球面収差補正手段中の負レンズ５を光軸方向に沿って変移することで、対物レンズ３に入射する光束の発散角を変化させ、集光光学系（カップリングレンズ１５及び対物レンズ３）の各光学面で発生する球面収差の変動を補正するようにした。また、本実施例においては、ｆＮ＝−１４．６７（ｍｍ）、ｆＰ＝１１．６６（ｍｍ）であり、ｆ＝１．７６５（ｍｍ）である。
【０２２１】
また、正レンズ４の両面に回折面を付加し、球面収差補正手段自体では集光光学系の光学面で発生する軸上色収差とは逆の極性の軸上色収差を発生させることで、集光光学系の光学面で発生する軸上色収差を補正し、情報記録面上に焦点を結んだときの波面の軸上色収差を良好にした。このとき、集光光学系と、球面収差補正手段としての負レンズ５及び正レンズ４とを合わせた合成系の軸上色収差を補正過剰とすることで、図３６に示されているように、第１光源１１の発振波長（４０５ｎｍ）の球面収差カーブと長・短波長側の球面収差カーブとを交差させている。これにより、光源のモードホップ現象や高周波重畳時の波面収差の劣化が非常に小さく、例えば、光源の発振波長が微小変動した際でも、最適書き込み位置の移動を小さく抑えることができる。
【０２２２】
表２８から明らかなように、波長変動時あるいは温度変化時等、様々な要因により集光光学系の各光学面で発生する球面収差の変動を補正でき、良好な球面収差となっている。また、集光光学系を構成するカップリングレンズ１５及び対物レンズ３、および球面収差補正手段を構成する負レンズ５及び正レンズ４の全てをプラスチック材料を用いることで、光学系の軽量化、可動機構への負担の軽減を図っている。また、短波長の光に対して透過率が高いプラスチック材料を用いているので、安価に大量生産でき、かつ光の利用効率の高い光学系を達成している。さらに、本実施例においては、球面収差補正手段への入射光を弱い発散光束としたので、カップリングレンズ１５のパワーおよび球面収差補正手段中の負レンズ５のパワーが小さくてすみ、それぞれのレンズの偏芯による波面収差の劣化を小さく抑えることができた。
【表２８】

【０２２３】
なお、本実施例においては、球面収差補正手段中の負レンズ５を変移可能としたが、正レンズ４を変移可動としても良く、また、両レンズを変移可能としても、同様に集光光学系の球面収差の変動を補正することができる。また、本実施例においては、球面収差補正手段中の正レンズ４に設けた回折構造により集光光学系と球面収差補正手段の軸上色収差を補正するようにしたが、回折構造を他のレンズの面に設けてもよく、他に回折構造を設けた面を有する光学素子を別途付加してもよい。
【０２２４】
以上に例示した各実施例は、球面収差補正手段として、ビームエキスパンダーを用いたものであり、そのビームエキスパンダーとしては、変移可能な単玉負レンズと、単玉正レンズとから構成した例を示したが、勿論、それに限定されるものではなく、複数のレンズからなる２群又はそれ以上のレンズ群からなる構成であってもよく、本発明を逸脱しない限り、種々の変更が可能である。
【０２２５】
図３７は、異なる実施の形態にかかる光学系を示す図である。カップリングレンズＣＬと、対物レンズＯＬとの間に、球面収差の変動を補正する素子ＳＥを挿入している。かかる光学系は、図１の負レンズ５，正レンズ４，対物レンズ３と置換されて用いられることが出来る。
【０２２６】
素子ＳＥは、４枚のガラス板ＳＥ４の間に、カップリングレンズＣＬ側からＸ方向液晶素子ＳＥ１、１／２波長板ＳＥ２、Ｙ方向液晶素子ＳＥ３をそれぞれ挟んでいる。両液晶素子ＳＥ１，ＳＥ２を電気的に駆動させることによって、球面収差の変動の補正が可能である。更に、カップリングレンズＣＬにおける対物レンズ側の面に、輪帯状の回折構造（不図示）を設けることで、対物レンズＯＬで発生する軸上色収差とは逆位相の色収差、すなわち短波町側ではオーバー、長波長側ではアンダーな軸上色収差を発生させることが出来る。その結果、軸上色収差がキャンセルされるので、球面収差の変動を補正する素子ＳＥと対物レンズＯＬとを透過して、光情報記録媒体（不図示）上に焦点を結んだときの波面は、軸受色収差が小さく抑えられた状態となる。
【０２２７】
図３８は、本実施の形態の変形例にかかる光学系を示す図である。図３８においては、対物レンズＯＬと、球面収差の変動を補正する素子ＳＥは、図３７に示す実施の形態と同一であるので説明を省略する。図３８においては、カップリングレンズＣＬが、負レンズＣＬ１と正レンズＣＬ２とを張り合わせた構成となっており、負レンズＣＬ１のアッベ数νｄＮと、正レンズＣＬ２のアッベ数νｄＰとは、νｄＮ＜νｄＰなる関係が成立している。
【０２２８】
このように負レンズＣＬ１と正レンズＣＬ２のアッベ数を調整することで、対物レンズＯＬで発生する軸上色収差とは逆位相の色収差、すなわち短波町側ではオーバー、長波長側ではアンダーな軸上色収差を発生させることが出来る。その結果、軸上色収差がキャンセルされるので、球面収差の変動を補正する素子ＳＥと対物レンズＯＬとを透過して、光情報記録媒体（不図示）上に焦点を結んだときの波面は、軸受色収差が小さく抑えられた状態となる。
【０２２９】
図３９は、本実施の形態の光ピックアップ装置に使用可能な対物レンズ３’を模式的に示した断面図（ａ）及び光源側から見た正面図（ｂ）である。〔一点鎖点は光軸を示している。〕
【０２３０】
この対物レンズ３’は、異なる光情報記録媒体の透明基板の厚さの違いによる球面収差変動の補正を行うことが出来るものである。図３６において、光源側の屈折面Ｓ１及び光ディスク側の屈折面Ｓ２は共に非球面形状を呈した正の屈折力を有する凸レンズである。また、対物レンズの光源側の屈折面Ｓ１は、光軸と同心状に４つの分割面ｂ１〜ｂ４から構成されている。分割面の境界は段差を設けて、それぞれの分割面を形成している。それに伴って、対物レンズの球面収差及び波面収差は上記境界部分に該当する箇所で段差を生じている。
【０２３１】
通常の対物レンズでは、異なる光情報記録媒体の透明基板厚さの違いによる球面収差発生は避けられない。しかしながら、本実施の形態に使用される対物レンズ３’では完全な球面収差補正は出来ないものの、次に説明するように、かかる収差をより緩和するように設計されている。
【０２３２】
まず、第１の光情報記録媒体に対して情報の再生及び／又は記録を行う場合、最良像面位置において波面収差の球面収差成分が０．０５λ1ｒｍｓ以内になるように屈折面Ｓ１及び屈折面Ｓ２を設計する。これにより設計された屈折面Ｓ１を第１分割面ｂ１及び第４分割面ｂ４に適用する。そして透明基板厚さｔ３（ｔ１≦ｔ３≦ｔ２）で最良像面位置において波面収差の球面収差成分が０．０５λ2ｒｍｓ以内になるように、前記屈折面Ｓ２を変数とせずに新たな屈折面Ｓ１’を設計する。
【０２３３】
この屈折面Ｓ１’を第２分割面ｂ２及び第３分割面ｂ３とするのであるが、透明基板厚さｔ３で最適化しているので、第１の光ディスク１０使用時において、第１分割面ｂ１と第４分割面ｂ４のつくる最良像面位置とは異なる位置に最良像面位置を見かけ上形成する。しかしながら、その波面収差は、分割面内での波面収差の傾きを変化させ、例えば第１の光情報記録媒体（例えばＤＶＤよりも高密度・大容量の次世代光ディスク）では右肩下がりの波面収差となり、第２の光情報記録媒体（例えばＤＶＤ）では逆に若干の右肩上がりとなる。このような分割面を２つ以上屈折面Ｓ１に一部設ける事で、異なる光情報記録媒体における波面収差両立が容易となる。
【０２３４】
これらの各分割面の境界位置や分割面の軸上厚を適宜設計することで、ＤＶＤよりも高密度・大容量の次世代光ディスクではビームスポット最小錯乱円位置及びＤＶＤでは前ピン位置それぞれにおいて波面収差補正が可能となる。すなわち、ＤＶＤよりも高密度・大容量の次世代光ディスクでは対物レンズによりビームスポット最小錯乱円位置に集光して第１〜４光束ＬＢ１〜ＬＢ４内の光線は、前記最小錯乱円位置においてほぼ波長λ1の整数倍、すなわち ｍiλ1（ ｍiは整数でｉ＝１、２、…、ｋ）の波面収差を有する。
【０２３５】
また、ＤＶＤでは必要開口数ＮＡ２がＮＡ１よりも小さいため、第１〜４光束ＬＢ１〜ＬＢ４をすべて有効活用しなくてもよく、本実施の形態の光ピックアップ装置では、第１〜３光束ＬＢ１〜ＬＢ３内の光線が、前記前ピン位置においてほぼ波長λ2の整数倍niλ1（niは整数でｉ＝１、２、…、ｋ）の波面収差を有する。第４光束ＬＢ４はＤＶＤの場合不要光であり、光ディスクの記録面上ではメインのスポット光から間隔をおいた場所にフレアーとして照射する。このフレアーはメインスポット光に対して十分に小さいので、絞り８をＤＶＤよりも高密度・大容量の次世代光ディスクの必要開口数相当にしておくだけで、絞り８の開口数を変える手段を必要とせすにＤＶＤ再生が可能となる。勿論、ＤＶＤ使用時に第４光束ＬＢ４を遮蔽する機能を持つ絞り８を用いてもよい。
【０２３６】
従って本実施例の光ピックアップ装置は、４つの分割面ｂ１〜ｂ４を設けてはいるが、従来技術の対物レンズと異なり、各ディスクにおいて焦点位置を複数持たないので、スポット光量損失を少なくできる。そして、各光ディスク使用時において必要開口数内の光線の波面収差をほぼ波長整数倍としており、必要開口数内を通った光束が互いに干渉して強め合うためスポット光の中心強度を高め、結果として光ディスクから充分な反射光量が得られ、互換性のある光ピックアップ装置として安定した動作が可能となる。
【０２３７】
なお、本実施例においては、対物レンズに４つの分割面を設けたが、基本的には３つの分割面となるような、入射光束を実質的に３つの光束に分割する３つの部分を有する面をもつ対物レンズを本発明の対物レンズに用いることもできる。たとえば、少なくとも１面に、光軸側からその外周に向かって順に、屈折作用により光源から出射された光束を複数の光束に分割する、少なくとも第１の部分、第２の部分及び第３の部分を有し、 その第１の部分及び第３の部分は、透明基板厚ｔ１の第１の光情報記録媒体の情報記録面に対して情報の記録または再生を行うことができるように、光源からの光束をその情報記録面上に集光可能であり、また、その第１の部分及び第２の部分は、透明基板厚ｔ２（ｔ１＜ｔ２）の第２の光情報記録媒体の情報記録面に対して情報の記録または再生を行うことができるように、光源からの光束を情報記録面上に集光可能であるように構成された、良く知られた対物レンズである。
【０２３８】
以上述べた本実施の形態によれば、半導体レーザのモードホップに起因する球面収差の変動を効果的に補正できる光ピックアップ装置及び光学系、温度・湿度変化等に起因する対物レンズの球面収差の変動を効果的に補正できる光ピックアップ装置及び光学系、短波長レーザと高ＮＡ対物レンズとを備え、異なる光情報記録媒体に対して情報の記録又は再生を行える光ピックアップ装置を提供することが出来る。なお、本発明は勿論、以上の実施の形態や種々の実施例に限定されるものではない。
【０２３９】
【発明の効果】
本発明によれば、光ピックアップ装置において、球面収差の変動を効果的に補正できる。
【図面の簡単な説明】
【図１】本実施の形態にかかる光ピックアップ装置の概略構成図である。
【図２】実施例１に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図３】実施例１の光学系に係る対物レンズ３にかかる球面収差図である。
【図４】実施例２に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図５】実施例２の光学系に係る対物レンズ３にかかる球面収差図である。
【図６】実施例３に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図７】実施例３の光学系に係る対物レンズ３にかかる球面収差図である。
【図８】実施例４に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図９】実施例４に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図１０】実施例４の光学系に係る対物レンズ３にかかる球面収差図である。
【図１１】実施例４の光学系に係る対物レンズ３にかかる球面収差図である。
【図１２】実施例５に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図１３】実施例５に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図１４】実施例５の光学系に係る対物レンズ３にかかる球面収差図である。
【図１５】実施例５の光学系に係る対物レンズ３にかかる球面収差図である。
【図１６】実施例６に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図１７】実施例６の光学系に係る対物レンズ３にかかる球面収差図である。
【図１８】実施例７に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図１９】実施例７の光学系に係る対物レンズ３にかかる球面収差図である。
【図２０】実施例８に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図２１】実施例８の光学系に係る対物レンズ３にかかる球面収差図である。
【図２２】実施例９に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図２３】実施例９の光学系に係る対物レンズ３にかかる球面収差図である。
【図２４】実施例１０に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図２５】実施例１０の光学系に係る対物レンズ３にかかる球面収差図である。
【図２６】実施例１０に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図２７】実施例１１に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図２８】実施例１１の光学系に係る対物レンズ３にかかる球面収差図である。
【図２９】実施例１２に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図３０】実施例１２に係る負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図３１】実施例１２の光学系に係る対物レンズ３にかかる球面収差図である。
【図３２】実施例１２の光学系に係る対物レンズ３にかかる球面収差図である。
【図３３】実施例１３に係るコリメータと、負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図３４】実施例１３の光学系に係る対物レンズ３にかかる球面収差図である。
【図３５】実施例１４に係るコリメータと、負レンズ５と、正レンズ４と、対物レンズ３の光学系構成図である。
【図３６】実施例１４の光学系に係る対物レンズ３にかかる球面収差図である。
【図３７】異なる実施の形態にかかる光学系を示す図である。
【図３８】本実施の形態の変形例にかかる光学系を示す図である。
【図３９】本実施の形態の光ピックアップ装置に使用可能な対物レンズ３’を模式的に示した断面図（ａ）及び光源側から見た正面図（ｂ）である。
【符号の説明】
３ 対物レンズ４ 正レンズ５ 負レンズ６ 対物レンズのアクチュエータ７
負レンズのアクチュエータ８ 絞り９ シリンドリカルレンズ１１ 第１光源１２ 第２光源１５ カップリングレンズ１６ 凹レンズ１７ ホログラム２１
カップリングレンズ４１、４２ 光検出器６２ ビームスプリッタ７１、７２ １／４波長板[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical pickup device, an objective lens, and a beam expander, and more particularly, to an optical pickup device, an objective lens, and a beam expander that can effectively correct variations in spherical aberration.
[0002]
[Prior art]
In recent years, along with the practical application of short-wavelength red semiconductor lasers, DVDs (digital versatile discs), which are high-density optical discs with the same size and capacity as conventional optical discs, ie, CDs (compact discs) that are optical information recording media ) Is under development, but it is expected that next-generation optical discs with higher density will appear in the near future. In an optical system of an optical information recording / reproducing apparatus using such an optical disc as a medium, the recording signal is condensed on the recording medium via an objective lens in order to increase the recording signal density or to reproduce the high-density recording signal. It is required to reduce the spot diameter. For this purpose, there is a fact that the wavelength of the laser as the light source is being shortened and the NA of the objective lens is being increased.
[0003]
[Problems to be solved by the invention]
By the way, when the laser wavelength is shortened and the objective lens has a high NA, a relatively long wavelength laser that records or reproduces information on a conventional optical disk such as a CD or a DVD. In an optical pickup device composed of a combination of a low NA of an objective lens, even a problem that can be almost ignored becomes more obvious.
[0004]
One of them is a problem of axial chromatic aberration that occurs in the objective lens due to minute fluctuations in the oscillation wavelength of the laser light source. The refractive index change due to the minute fluctuation of the wavelength of a general optical lens material increases as the short wavelength is handled. For this reason, the defocus amount of the focus caused by minute fluctuations in wavelength increases. However, the focal depth of the objective lens is k · λ / NA.²As can be seen from (k is a proportional constant, λ is a wavelength, and NA is the image-side numerical aperture of the objective lens), the shorter the oscillation wavelength of the light source used, the smaller the focal depth and the smaller the defocus amount. Is not allowed. Therefore, in an optical system using a short wavelength light source such as a blue-violet semiconductor laser (oscillation wavelength of about 400 nm) and an objective lens having a high image-side numerical aperture, wavelength fluctuation due to output change such as a mode hop phenomenon of the semiconductor laser, In order to prevent deterioration of wavefront aberration due to high frequency superposition, correction of axial chromatic aberration is important.
[0005]
Furthermore, another problem that becomes apparent in the combination of shortening the wavelength of the laser and increasing the NA of the objective lens is the fluctuation of the spherical aberration of the optical system due to temperature and humidity changes. That is, the plastic lens generally used in the optical pickup device is easily deformed by changes in temperature and humidity, and the refractive index changes accordingly. Even if the spherical aberration fluctuates due to a change in refractive index, which was not a problem with conventional optical systems used in pickup devices, the amount cannot be ignored when combining a laser light source with a shorter wavelength and a higher NA objective lens. This causes problems such as an increase in spot diameter.
[0006]
However, next-generation optical discs that require a combination of shorter laser wavelengths and higher NA objective lenses for information recording or reproduction, and conventional optical discs, as described above, include the light source wavelength and objective lens. NA is very different. In addition, it is effective to reduce the thickness of the transparent substrate in order to suppress coma aberration, which is largely caused by the inclination of the disk surface from the plane perpendicular to the optical axis expected in the next generation optical disk. However, the thickness of the transparent substrate is greatly different from that of a conventional optical disk such as a CD. Therefore, by using at least a common objective lens, the spherical aberration can be suppressed with respect to different optical information recording media including a next-generation optical disc by using a compact optical pickup device without significantly increasing the cost. The problem is whether to record or reproduce information.
[0007]
SUMMARY OF THE INVENTION An object of the present invention is to provide an optical pickup device, an objective lens, and a beam expander that can effectively correct fluctuations in spherical aberration of the objective lens caused by temperature and humidity changes.
[0008]
Another object of the present invention is to provide an optical pickup device capable of effectively correcting axial chromatic aberration caused by mode hopping and high-frequency superposition of a semiconductor laser, an objective lens thereof, and a beam expander.
[0009]
It is another object of the present invention to provide an optical pickup device that includes a short wavelength laser and a high NA objective lens and can record or reproduce information on different optical information recording media.
[0010]
[Means for Solving the Problems]
  The optical pickup device according to claim 1 includes a light source and a condensing optical system including an objective lens for condensing a light beam emitted from the light source onto an information recording surface via a transparent substrate of an optical information recording medium. And an optical pickup device having a photodetector for receiving reflected light from the optical information recording medium,The light source has an oscillation wavelength at a wavelength of at least 500 nm, the objective lens is a single objective lens having an aspheric surface on at least one surface, and an image-side numerical aperture NA of the objective lens is 0.75 or more The optical pickup device is capable of recording and / or reproducing information with respect to an optical information recording medium in which a plurality of transparent substrates and information recording layers are laminated in order from the surface side of the optical information recording medium. The divergence changing means for changing the divergence of the light beam incident on the objective lens according to the information recording layer when condensing each information recording layer is provided between the light source and the objective lens. The objective lens satisfies the following expression.
  1.1 ≦ d1 / f ≦ 3.0
However,
d1: On-axis lens thickness (mm)
f: Focal length (mm) at the oscillation wavelength of the light source (however, if the light source has a plurality of light sources having different oscillation wavelengths, the focal length at the oscillation wavelength with the shortest wavelength, and a diffraction surface on the objective lens) If equipped, the total focal length combining the refractive power and diffraction power)
[0012]
  The optical pickup device according to claim 2,In the invention according to claim 1, the divergence changing means isMeans for correcting variations in spherical aberrationAndThe means for correcting the fluctuation of the spherical aberration can correct the spherical aberration up to 0.2λ rms to 0.07λ rms or less, so that, for example, the temperature and humidity change of the environment where the optical pickup device is used and / or the light source Variations in spherical aberration of the objective lens caused by minute variations in oscillation wavelength can be effectively suppressed.
[0013]
  The optical pickup device according to claim 3 is:In the invention of claim 2,It is preferable that the means for correcting the fluctuation of the spherical aberration can correct the spherical aberration up to 0.5λrms to 0.07λrms or less.
[0015]
  Since the oscillation wavelength of a semiconductor laser has an individual variation of about ± 10 nm, in an optical system using a short wavelength light source and an objective lens having a high image side numerical aperture, a semiconductor laser shifted from a reference wavelength is used. If it is used, it causes deterioration of the performance of the apparatus and it may be necessary to select a semiconductor laser.
  Claim4The optical pickup device described inIn the invention according to any one of claims 1 to 3, the divergence changing means isSince the means for correcting the variation of spherical aberration generated in the objective lens due to the minute fluctuation of the oscillation wavelength of the light source is provided, the objective lens generated when the semiconductor laser deviated from the reference wavelength is used. Since the variation of the spherical aberration can be effectively suppressed, it is not necessary to select a semiconductor laser.
[0016]
  According to a fifth aspect of the present invention, in the invention according to any one of the first to fourth aspects, the objective lens is a plastic lens, and the divergence changing means is provided between the light source and the objective lens. It is preferable to correct a variation in spherical aberration that occurs in the condensing optical system due to a temperature change.
  The optical pickup device according to claim 6 is:In the invention according to any one of claims 1 to 5, the divergence changing means isBetween the light source and the objective lensProvided,HumidityIt is preferable to correct a variation in spherical aberration that occurs in the condensing optical system due to the change.
  According to claims 5 and 6,For example, it is possible to effectively suppress fluctuations in the spherical aberration of the objective lens that occur according to changes in the temperature and humidity of the environment in which the optical pickup device is used.
[0018]
  Claim7The optical pickup device described inIn the invention according to any one of claims 1 to 6, the divergence changing means is, Including at least one positive lens and at least one negative lens, at least one of which is a movable element capable of shifting in the optical axis direction.
[0019]
In the optical pickup device used for the short wavelength light source, as described above, the variation of the spherical aberration due to the wavelength variation of the light source and the temperature / humidity variation is large. In particular, when an objective lens having a high image side numerical aperture (high NA) or an objective lens made of a plastic material is used, the fluctuation is increased. Therefore, in an optical pickup device using a light source with a short wavelength, it is necessary to provide means for correcting the variation of these spherical aberrations. If the spherical aberration of the objective lens fluctuates due to minute fluctuations in the oscillation wavelength of the light source or changes in temperature and humidity, the movable element of the means for correcting the fluctuation of the spherical aberration is moved by an appropriate amount to By changing the divergence of the light beam incident on the lens so that the spherical aberration of the wavefront formed on the information recording surface is minimized, the variation of the spherical aberration can be corrected.
[0020]
  Claim8The optical pickup device described inThe invention according to claim 7, wherein the divergence change is performed.The means satisfies the following expression.
  νdP> νdN (1)
However,
  νdP: average of Abbe number of d-line of all positive lenses including the positive lens
  νdN: average of Abbe number of d-line of all negative lenses including the negative lens
[0021]
The above equation (1) relates to correction of axial chromatic aberration. When spherical aberration of the objective lens fluctuates due to minute fluctuations in the oscillation wavelength of the light source, changes in temperature and humidity, etc., means for correcting this, for example, using an optical element capable of shifting in the optical axis direction When configured, the optical element can be moved by an appropriate amount to change the divergence of the light beam incident on the objective lens so that the spherical aberration of the objective lens is minimized. The axial chromatic aberration of the objective lens, which is a problem when using a short wavelength light source, can be corrected by configuring the means for correcting the variation of the spherical aberration as described below.
[0022]
By selecting the materials of the positive lens and the negative lens in the means for correcting the variation of the spherical aberration so as to satisfy the above equation (1), chromatic aberration having a polarity opposite to that of the chromatic aberration generated in the objective lens can be generated. it can. Accordingly, since the axial chromatic aberration cancels out, the wavefront transmitted through the means for correcting the spherical aberration and the objective lens and focused on the optical information recording medium has a small axial chromatic aberration. It will be in the state. If a diffractive surface is added to the objective lens and / or means for correcting fluctuations of the spherical aberration, and the diffractive lens is such that the back focus of the objective lens is shortened on the long wavelength side, the aberration can be corrected better. Is possible. In this case, since the role of correcting the axial chromatic aberration can be shared between the means for correcting the variation of the spherical aberration and the diffractive surface, the means for correcting the variation of the spherical aberration is, for example, an optical that can be shifted in the optical axis direction. When configured using elements, the stroke of such optical elements can be small.
[0023]
Furthermore, the power of the diffractive surface can be reduced by sharing the role of axial chromatic aberration correction with the means for correcting the variation of the spherical aberration and the diffractive surface, thereby increasing the distance between the diffracting ring zones. This makes it easy to manufacture a diffractive lens having high diffraction efficiency. Accordingly, the spherical aberration of the entire optical system and the shaft can be obtained even when a wavelength variation or a temperature / humidity change occurs without separately providing a means for correcting the variation of the spherical aberration and a means for correcting the axial chromatic aberration. A compact optical pickup device in which the upper chromatic aberration is corrected well can be obtained.
[0024]
  Claim9The optical pickup device described inIn the invention of claim 8,The νdP and the νdN satisfy the following formula.
  νdP> 55 (2)
  νdN <35 (3)
[0025]
If the difference between the Abbe numbers of the positive lens and the negative lens is increased so as to satisfy the above expressions (2) and (3), chromatic aberration having a polarity opposite to that of the objective lens can be generated. The axial chromatic aberration of the optical pickup optical system can be corrected well.
[0026]
  Claim10The optical pickup device described inIn the invention according to any one of claims 7 to 9.The following equation holdsTossThe
  Δd · | fP / fN | /Δνd≦0.05 (4)
However,
  Δd: Amount of movement (mm) of the movable element when information is recorded or reproduced on one information recording surface of any one optical information recording medium capable of recording or reproducing information
  fP: the positive lensOfFocal length (mm) (however, the positive lensTo(If a diffractive surface is provided, the total focal length combining the refractive power and diffractive power)
  fN: the negative lensOfFocal length (mm) (however,negativeLenTo(If a diffractive surface is provided, the total focal length combining the refractive power and diffractive power)
  Δνd:SaidMaximum Abbe number of positive lens andSaidDifference from the minimum Abbe number of the negative lens
[0027]
The above equation (4) relates to the balance between the correction amount of the longitudinal chromatic aberration of the objective lens, the paraxial power of the means for correcting the variation of the spherical aberration, and the movement amount of the movable element of the means for correcting the variation of the spherical aberration. Here, even if the value of Δνd is small, if the value of | fP / fN | is increased, the axial chromatic aberration of the objective lens can be corrected satisfactorily, and the objective lens caused by the wavelength variation of the light source or the temperature and humidity change When the means for correcting the fluctuation of the spherical aberration is configured using an optical element that is displaceable in the optical axis direction, the stroke of the optical element can be reduced, but the effective diameter of the positive lens group is There is a possibility that it becomes too large or the effective diameter of the negative lens group becomes too small. Conversely, if the value of Δνd is increased, the axial chromatic aberration of the objective lens can be corrected satisfactorily even if the value of | fP / fN | is small, but the spherical aberration necessary for correcting the spherical aberration Since the amount of movement of the movable element of the means for correcting the fluctuation increases, the size of the optical system may increase. Therefore, the balance of these can be achieved by making the value of Δd · | fP / fN | / Δνd satisfy the above equation (5).
[0028]
  Claim11The optical pickup device described inIn the invention according to any one of claims 1 to 10,Information can be recorded and / or reproduced on at least two types of optical information recording media,The divergence changing meansSince the divergence of the light beam incident on the objective lens is changed according to the thickness of each transparent substrate for at least two types of optical information recording media having different transparent substrate thicknesses, the spherical surface due to the difference in the transparent substrate thickness Since the difference in aberration is corrected and the variation in spherical aberration that occurs when recording or reproduction is performed on each optical information recording medium is well corrected, a good wavefront can always be formed on the information recording surface.
[0030]
  Claim12The optical pickup device described inIn the invention of claim 11,When the transparent substrate thicknesses of the two types of optical information recording media are a and b (a <b), respectively, when information is recorded on or reproduced from the information recording surface of the optical information recording medium having the transparent substrate thickness a. Is characterized in that the distance between the negative lens and the positive lens is increased as compared with the case of recording or reproducing information on the information recording surface of the optical information recording medium having the transparent substrate thickness b.
[0031]
  Claim13The optical pickup device described inThe invention according to claim 11 or 12, wherein the divergence changing means includes at least one sheet.Positive lensAnd at least oneNegative lensWhenIncludingAt least one of them is a movable element that can be displaced in the optical axis direction.The following formula is satisfied.
  | FP / fN | ≧ 1.3 (5)
However,
  fP: the positive lensOfFocal length (however, the positive lensTo(If a diffractive surface is provided, the total focal length combining the refractive power and diffractive power)
  fN: the negative lensOfFocal length (however, the negative lensTo(If a diffractive surface is provided, the total focal length combining the refractive power and diffractive power)
[0032]
The above equation (5) relates to the relationship of the paraxial power of the means for correcting the variation of the spherical aberration. When the objective lens is corrected to minimize the aberration under a combination of transparent substrates having a certain thickness, when the thickness of the transparent substrate changes, the means for correcting the variation of the spherical aberration By moving the movable element, a light beam having a divergence that minimizes the spherical aberration of the objective lens with respect to its thickness must be incident on the objective lens. Therefore, by selecting the paraxial power of the means for correcting the variation of the spherical aberration so as to satisfy the above formula (5), the stroke of the movable element can be made small, so that an overall compact optical system is obtained. be able to.
[0035]
  Claim14The optical pickup device described inIn the invention according to any one of claims 7 to 13,At least one of the positive lens and the negative lens is made of a plastic material. In particular, by forming the movable element of the spherical aberration correction means from a plastic material, it is possible to reduce the burden on the shift device and to enable high-speed tracking. Furthermore, if the components providing the diffractive surface and the aspherical surface are formed from a plastic material, they can be easily added.
[0040]
  Claim15The optical pickup device described in15. The divergence change according to any one of claims 7 to 14.The means is composed of the one positive lens and the one negative lens.
  An optical pickup device according to a sixteenth aspect is the invention according to any one of the seventh to fifteenth aspects, wherein the divergence changing means has the one negative lens on the light source side, and the objective lens side. And a single positive lens.
  An optical pickup device according to a seventeenth aspect is characterized in that, in the invention according to any one of the seventh to sixteenth aspects, the negative lens is a movable element that is movable in the optical axis direction.
  An optical pickup device according to claim 18 corrects axial chromatic aberration of a combined system including the objective lens, the negative lens, and the positive lens in the invention according to any one of claims 7 to 17. It is characterized by being excessive.
[0080]
  Claim19The optical pickup device described in19. The divergence change according to any one of claims 1 to 18.MeansIt includes at least one positive lens and at least one negative lens, at least one of which is a movable element that can be displaced in the direction of the optical axis.When the paraxial power at the oscillation wavelength of the light source is P1, the paraxial power at a wavelength 10 nm shorter than the oscillation wavelength is P2, and the paraxial power at a wavelength 10 nm longer than the oscillation wavelength is P3, the following equation is satisfied. It is characterized by doing.
  P2 <P1 <P3 (8)
[0081]
  As a result, theDivergence changeMeans, objective lensInA role of correcting the generated axial chromatic aberration can be provided. That is,With the divergence changing meansCorrection of axial chromatic aberration generated in optical elements such as objective lenses and coupling lenses by generating axial chromatic aberration of the opposite polarity to axial chromatic aberration generated in optical elements such as objective lenses and coupling lenses can do.
[0105]
  Claim20The optical pickup device described inIn the invention according to any one of claims 1 to 19,Of the spherical aberrations of the objective lens, when the sum of third-order spherical aberration components is SA1, and the sum of fifth-order, seventh-order and ninth-order spherical aberration components is SA2, the following expression is satisfied.
  | SA1 / SA2 |> 1.0 (11)
However,
  SA1: Third-order spherical aberration component when the aberration function is expanded into a Zernike polynomial
  SA2: Square root of the sum of squares of the fifth-order spherical aberration component, the seventh-order spherical aberration component, and the ninth-order spherical aberration component when the aberration function is expanded into a Zernike polynomial
[0107]
  Claim21The optical pickup device described inIn the invention according to any one of claims 1 to 19,The stop for determining the numerical aperture of the objective lens is located on the side where the optical information recording medium is disposed from the surface vertex of the surface closest to the light source of the objective lens. Thus, when divergent light is incident on the objective lens, the light beam passing height on the surface closest to the light source of the objective lens can be kept small, which is preferable in terms of downsizing the objective lens or correcting aberrations.
  According to a twenty-second aspect of the present invention, in the invention according to any one of the first to twenty-first aspects, the divergence changing means is a beam expander.
[0156]
The diffractive surface used in this specification refers to a form (or surface) in which a relief is provided on the surface of an optical element, for example, the surface of a lens, so that the angle of the light beam is changed by diffraction. When there is a region where diffraction occurs on one optical surface and a region where it does not occur, the region where diffraction occurs. As the shape of the relief, for example, on the surface of the optical element, it is formed as a substantially concentric annular zone centered on the optical axis, and each annular zone is shaped like a sawtooth if the cross section is viewed in a plane including the optical axis. Are known, but include such shapes. In particular, such a sawtooth ring zone structure is preferable.
[0157]
In this specification, the objective lens is, in a narrow sense, a light collecting action that is arranged to face the optical information recording medium at the position closest to the optical information recording medium when the optical information recording medium is loaded in the optical pickup device. In a broad sense, it refers to a lens group that can be operated at least in the optical axis direction by an actuator together with the lens.
[0158]
In this specification, the condensing optical system includes at least an objective lens, and is disposed between the light source and the objective lens, and is a coupling lens (incident incident) for converting the incident light beam into a substantially parallel light beam. Including a collimator that makes a divergent light beam a parallel light beam. However, it is an assembly of at least optical elements that function integrally, such as a beam expander, which will be described later, and an assembly in which some of the optical elements constituting the assembly can be displaced along the optical axis direction, and Here, a part of the optical elements of the aggregate is not included in the condensing optical system. The coupling lens may be composed of a plurality of lenses, or may be configured such that these lenses are spaced apart and another optical element is interposed therebetween.
[0159]
In this specification, a beam expander can change an optical element such as at least one lens along the optical axis direction, thereby making it possible to vary the divergence angle (including divergence and convergence) of the emitted light beam. An assembly of optical elements such as a lens (an optical element group such as a lens group) capable of emitting a substantially parallel light beam when a substantially parallel light beam is incident thereon. It is preferable that a plurality of optical elements such as these lenses are integrated, and if at least one optical element such as a lens is configured to be capable of shifting along the optical axis direction, the transition is actually performed. The driving means such as the shifting device to be performed may not be included as a beam expander.
[0160]
In the present specification, the means for correcting the variation of spherical aberration and the axial chromatic aberration is a single means, for example, a single optical means, for correcting the variation of spherical aberration and for correcting the axial chromatic aberration. This means that the device and its aggregate (for example, a beam expander) have both of two correction functions, for example, a beam composed of a positive lens and a negative lens having a specific Abbe number. Examples thereof include an expander and a beam expander having a surface having a diffractive structure. In this specification, in the invention related to the optical pickup device, unless otherwise specified, the focal length is the focal length of the light source that emits the light having the shortest oscillation wavelength among the light sources used. Shall be pointed to.
[0161]
In this specification, the minute fluctuation of the oscillation wavelength of the light source refers to a wavelength fluctuation within a range of ± 10 nm with respect to the oscillation wavelength of the light source. Further, in this specification, correcting various aberrations (good) means that the wavefront aberration is 0.07λ rms or less, which is a so-called diffraction-limited performance (where λ is the oscillation wavelength of the light source used). Further, it is more preferable that it is 0.05λrms or less in consideration of mechanical accuracy on the apparatus. Accordingly, appropriate spot sizes can be obtained for various optical information recording media.
[0162]
In this specification, optical information recording media (optical disks) include, for example, various CDs such as CD-R, CD-RW, CD-Video, CD-ROM, DVD-ROM, DVD-RAM, DVD-R, Various DVDs such as DVD-RW and DVD-Video, and disk-like current optical information recording media such as MD and next-generation optical information recording media are also included. In addition, the transparent substrate used in this specification includes a case where the thickness is 0 mm, that is, there is no transparent substrate.
[0163]
In this specification, recording and reproducing information means recording information on the information recording surface of the optical information recording medium as described above and reproducing information recorded on the information recording surface. The optical pickup device of the present invention may be used only for recording or reproduction, or may be used for both recording and reproduction. Further, it may be used for recording with respect to a certain optical information recording medium and for reproducing with respect to another optical information recording medium, or with respect to a certain optical information recording medium. May be used for recording or reproduction, and recording and reproduction for another optical information recording medium. Note that reproduction here also includes simply reading information.
[0164]
The optical pickup device of the present invention can be mounted on audio and / or image recording and / or reproducing devices of various players or drives, or AV equipment, personal computers, and other information terminals in which they are incorporated.
[0165]
DETAILED DESCRIPTION OF THE INVENTION
The aspheric surface used in the present embodiment is expressed by the following [Equation 1]. Where x is an axis in the optical axis direction, h is an axis perpendicular to the optical axis, the traveling direction of light is positive, r is a paraxial radius of curvature, κ is a cone coefficient, A_2iIs the aspheric coefficient.
[Expression 1]

[0166]
The diffractive surface used in the present embodiment is expressed by [Equation 2] as an optical path difference function.
[Expression 2]

[0167]
Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram of an optical pickup device according to the present embodiment. In FIG. 1, a first light source 11 that performs recording and / or reproduction with respect to a first optical information recording medium 23, and a first light source 11 that performs recording and / or reproduction with respect to a second optical information recording medium 24. And a second light source 12 having different wavelengths, coupling lenses 21 and 22 for converting the divergence angles of divergent light beams emitted from the respective light sources into desired divergence angles, and substantially the light beams from the respective light sources. A beam splitter 62 which is an optical path combining means for traveling in the same direction, an objective lens 3 for condensing the light beam from the beam splitter 62 on the information recording surface 5 of the optical information recording medium, and reflection from the optical information recording medium Photodetectors 41 and 42 for receiving light are provided. In the figure, 8 is a stop, 9 is a cylindrical lens, 71 and 72 are quarter-wave plates, 15 is a coupling lens for reducing the divergence of a divergent light beam from the light source 11, 16 is a concave lens, and 17 is a reflected light beam. It is a hologram for separating.
[0168]
Further, in the present embodiment, a negative lens 5, a positive lens 4, and an actuator 7 which are arranged in order from the light source side are provided as means for correcting the variation of spherical aberration of the objective lens 3 and divergence changing means. (Hereinafter also referred to as spherical aberration correcting means and divergence changing means). The actuator 7 functions as a shift device that changes the divergence angle of the light beam by moving the negative lens 5 as an optical element in the optical axis direction. In addition, in Examples 1 to 14 showing a specific part of the optical system in relation to the present embodiment, a so-called beam expander composed of the shiftable negative lens 5 and positive lens 4 is used. One example may be expressed as spherical aberration correction means. Reference numeral 6 denotes an actuator for driving the objective lens 3 in the optical axis direction for focusing. It is assumed that the first light source 11 can emit laser light having a wavelength λ1 = 405 nm, and the second light source 12 can emit laser light having a wavelength λ2 = 655 nm.
[0169]
In Examples described below, Examples 1, 2, 11, and 12 are provided with a diffractive surface on the objective lens 3 to correct axial chromatic aberration, and Examples 3 to 5 are negative lens 5 and positive lens 4. The axial chromatic aberration of the objective lens 3 is corrected by providing a diffractive surface on at least one of the negative lens 5 and the positive lens 4 in Examples 6 to 8, 13, and 14. In Examples 9 and 10, the axial chromatic aberration of the objective lens 3 is corrected by the synergistic effect of the specific material of the negative lens 5 and the positive lens 4 and the diffraction surface provided on the positive lens 4. . Examples 4, 5, and 12 are examples in which information is recorded or reproduced on different optical information recording media using the same optical system. In the following objective lens embodiments, a plastic material having a water absorption of 0.01% or less, a transmittance of 90.5% by a light beam having a light source wavelength of 400 nm, and a transmittance of 92% by a light beam having a light source wavelength of 700 nm is used. Formed using. Further, in the following example, in the example using only the first light source 11 in the present embodiment shown in FIG. 1, the drawings of the specific embodiment are omitted, but generally in the pickup apparatus of FIG. For example, the second light source 12, the coupling lens 22, the beam splitter 62, the photodetector 42, the quarter wavelength plate 72, and the hologram 17 may be removed. Each example will be described below.
[0170]
Example 1
Table 1 shows data related to the optical system including the negative lens 5, the positive lens 4, and the objective lens 3 in Example 1. In the lens data shown here, a power of 10 (for example, 2.5 × 10^-3) Is expressed using E (for example, 2.5 × E-3). Further, the first-order light generated by diffraction on the diffraction surface represented by a rotationally symmetric polynomial means light whose angle of light changes in the direction of convergence after diffraction.
[Table 1]

[0171]
FIG. 2 is an optical system configuration diagram of the negative lens 5, the positive lens 4, and the objective lens 3 according to the first embodiment. FIG. 3 is a spherical aberration diagram concerning the objective lens 3. In the first embodiment, information is recorded or reproduced by a combination of the first light source 11 having a wavelength of 405 nm and the image-side numerical aperture NA 0.85 of the objective lens 3. In the present embodiment, materials of νdN = 23.8 and νdP = 81.6 are selected as materials of the negative lens 5 and the positive lens 4 of the means for correcting the variation of the spherical aberration, respectively. By providing a diffractive surface on the light source side surface, axial chromatic aberration generated in the objective lens 3 is corrected. In this embodiment, fN = −8.13 (mm), fP = 9.48 (mm), f = 1.765 (mm), and fD = 71.383 (mm).
[0172]
In this embodiment, correction of minute fluctuations in the oscillation wavelength of the light source (hereinafter also simply referred to as wavelength fluctuations) or fluctuations in spherical aberration at the time of temperature changes can be performed as follows. In this embodiment, overcorrected spherical aberration occurs in the objective lens 3 when the wavelength increases or when the temperature rises. In such a case, if the gap between the negative lens 5 and the positive lens 4 is reduced by moving the negative lens 5 along the optical axis by the actuator 7 with respect to the generated spherical aberration, an undercorrected spherical aberration can be generated. . If the negative lens 5 is moved by an appropriate amount, the overcorrected spherical aberration can be canceled, and the spherical aberration of the entire optical system becomes good as is apparent from Table 2 showing the correction result of the spherical aberration.
[Table 2]

[0173]
(Example 2)
Table 3 shows data related to the optical system including the negative lens 5, the positive lens 4, and the objective lens 3 in Example 2.
[Table 3]

[0174]
FIG. 4 is an optical system configuration diagram of the negative lens 5, the positive lens 4, and the objective lens 3 according to the second embodiment. FIG. 5 is a spherical aberration diagram concerning the objective lens 3. In the second embodiment, information is recorded or reproduced by a combination of the first light source 11 having a wavelength of 405 nm and the image-side numerical aperture NA 0.85 of the objective lens 3. In Example 2, materials of νdN = 30.0 and νdP = 56.5 are selected as materials of the negative lens 5 and the positive lens 4 of the means for correcting the variation of the spherical aberration, respectively, and the light source of the objective lens 3 is further selected. By providing a diffractive surface on the side surface, axial chromatic aberration generated in the objective lens 3 is corrected. In this embodiment, fN = −4.75 (mm), fP = 6.47 (mm), f = 1.765 (mm), and fD = 71.383 (mm).
[0175]
The correction of the variation of the spherical aberration at the time of the wavelength variation or the temperature variation in this embodiment is the same as that in the first embodiment, and thus the description thereof is omitted. As is apparent from Table 4 showing the correction result of the spherical aberration, the spherical aberration at the time of wavelength variation or temperature change is good. Further, by using plastic materials for the negative lens 5 and the positive lens 4 as means for correcting fluctuations in the objective lens 3 and spherical aberration, the weight of the optical system is reduced and the burden on the movable mechanism is reduced.
[Table 4]

[0176]
Example 3
Table 5 shows data related to the optical system including the negative lens 5, the positive lens 4, and the objective lens 3 in Example 3.
[Table 5]

[0177]
FIG. 6 is an optical system configuration diagram of the negative lens 5, the positive lens 4, and the objective lens 3 according to the third embodiment. FIG. 7 is a spherical aberration diagram concerning the objective lens 3. In the third embodiment, information is recorded or reproduced by a combination of the first light source 11 having a wavelength of 405 nm and the image-side numerical aperture NA 0.85 of the objective lens 3. In Example 3, axial chromatic aberration is corrected by selecting materials of νdN = 23.8 and νdP = 81.6 as materials of the negative lens 5 and the positive lens 4, respectively. In this embodiment, fN = −9.27 (mm), fP = 11.08 (mm), and f = 1.765 (mm).
[0178]
The correction of spherical aberration at the time of wavelength fluctuation or temperature change in the present embodiment is the same as that in the first embodiment, and thus description thereof is omitted. As is apparent from Table 6 showing the correction result of the spherical aberration, the spherical aberration at the time of wavelength fluctuation or temperature change is good. Further, by using a plastic material for the objective lens 3, the weight of the optical system is reduced and the burden on the movable mechanism is reduced.
[Table 6]

[0179]
(Example 4)
Table 7 shows data related to the optical system including the negative lens 5, the positive lens 4, and the objective lens 3 in Example 4.
[Table 7]

[0180]
8 and 9 are optical system configuration diagrams of the negative lens 5, the positive lens 4, and the objective lens 3 according to the fourth embodiment. 10 and 11 are spherical aberration diagrams concerning the objective lens 3 when information is recorded or reproduced on different optical information recording media. In Example 4, using the same optical system, a combination of a first light source 11 with a wavelength of 405 nm and an optical information recording medium with a transparent substrate thickness of 0.1 mm, or a second light source 11 with a wavelength of 655 nm, and a transparent substrate thickness This is an example of an optical pickup device that records or reproduces information in combination with a 0.6 mm optical information recording medium. In Example 4, axial chromatic aberration is corrected by selecting materials of νdN = 30.0 and νdP = 56.5 as materials of the negative lens 5 and the positive lens 4, respectively. Further, in this embodiment, fN = −3.82 (mm), fP = 6.85 (mm), f1 = 1.765 (mm), and fD1 = 500000.02 (mm). Note that the focal length of the objective lens at the oscillation wavelength λ2 = 655 nm is f2 = 1.804.
[0181]
In the fourth embodiment, the variation in spherical aberration caused by the difference in the thickness of the transparent substrate in different optical information recording media is composed of one negative lens 5 and one positive lens 4 in order from the light source side. Correction is performed by changing the interval of the divergence angle changing means (corresponding to the means for correcting the fluctuation of the spherical aberration of the present invention or the means for correcting the fluctuation of the spherical aberration and the longitudinal chromatic aberration). Further, by providing a diffractive surface on the light source side surface of the objective lens 3, the spherical aberration is corrected more favorably. Furthermore, the spherical aberration deterioration of the objective lens when the light source wavelength is changed or when the temperature and humidity are changed is well corrected by changing the interval of the divergence changing means. That is, as apparent from Table 8, by changing the distance between the negative lens 5 and the positive lens 4 to an appropriate distance, the spherical aberration of the objective lens 3 is deteriorated when the substrate thickness is changed, the wavelength is changed, and the temperature and humidity are changed. Is corrected well. Further, by using a plastic material for the objective lens 3, the negative lens 5, and the positive lens 4, the weight of the optical system is reduced and the burden on the movable mechanism is reduced.
[Table 8]

[0182]
(Example 5)
Table 9 shows data related to the optical system including the negative lens 5, the positive lens 4, and the objective lens 3 in Example 5.
[Table 9]

[0183]
12 and 13 are optical system configuration diagrams of the negative lens 5, the positive lens 4, and the objective lens 3 according to the fifth embodiment. FIGS. 14 and 15 are spherical aberration diagrams concerning the objective lens 3 when information is recorded or reproduced on different optical information recording media, respectively. In Example 5, using the same optical system, a combination of a first light source 11 with a wavelength of 405 nm and an optical information recording medium with a transparent substrate thickness of 0.1 mm, or a second light source 11 with a wavelength of 655 nm, and a transparent substrate thickness This is an example of an optical pickup device that records or reproduces information in combination with a 0.6 mm optical information recording medium. In Example 5, axial chromatic aberration is corrected by selecting materials of νdN = 30.0 and νdP = 56.5 as materials of the negative lens 5 and the positive lens 4, respectively. In this embodiment, fN = −6.59 (mm), fP = 9.85 (mm), f1 = 3.011 (mm), and fD1 = 849964.33 (mm). Note that the focal length of the objective lens at the oscillation wavelength λ2 = 655 nm is f2 = 3.076.
[0184]
As is clear from Table 10, by changing the distance between the negative lens 5 and the positive lens 4 to an appropriate distance, the objective at the time of changing the thickness of the transparent substrate, changing the wavelength, and changing the temperature and humidity is clear. The spherical aberration deterioration of the lens can be corrected well. Further, by using a plastic material for the objective lens 3, the negative lens 5, and the positive lens 4, the weight of the optical system is reduced and the burden on the movable mechanism is reduced.
[Table 10]

[0185]
Note that the light beam incident on the negative lens 5 as a means for correcting the fluctuation of the spherical aberration is not only parallel light but also divergent light or convergent light as in the above-described embodiment, and the optical system of the present invention is used. Can be applied as well. Although not shown in the present embodiment, a coupling lens for changing the divergence of the light beam from the light source can be provided between the light source and the spherical aberration correcting means. By adding a diffractive surface to such a coupling lens so as to shorten the back focus on the long wavelength side, axial chromatic aberration generated in the objective lens can be corrected.
[0186]
The coupling lens used in the optical system according to the present invention is not limited to the above-described form, and if it is as in Japanese Patent Application No. 2000-060843 by the same applicant, the axial chromatic aberration generated in the objective lens 3 is better. Can be corrected.
[0187]
Further, the astigmatic difference of the light beam from the light source is reduced between the coupling lens and the means for correcting the fluctuation of the spherical aberration (the negative lens 5 and the positive lens 4), and a substantially circular light beam is applied to the spherical aberration correcting means. When a beam shaping element that can be made incident is provided, the divergence of the light beam from the coupling lens changes due to the focal point movement of the coupling lens due to temperature and humidity changes, and astigmatism is generated by the beam shaping element. End up. In order to suppress this, astigmatism due to the beam shaping element can be suppressed by using a coupling lens as disclosed in Japanese Patent Application No. 2000-053858 by the same applicant.
[0188]
In Examples 4 and 5, aberration diagrams for optical information recording media having a light source wavelength of 655 nm and a transparent substrate thickness of 0.6 mm are shown up to NA 0.65. However, at this time, the objective lens 3 is incident with a light beam that passes through all the apertures determined by the light source wavelength of 405 nm and NA of 0.85. A light beam of NA 0.65 or more that does not contribute to image formation is used as a flare component by utilizing the effect of the diffractive surface provided on the objective lens 3, so that the spot diameter is not excessively reduced on the information recording surface, and the optical pickup device It is possible to prevent detection of unnecessary signals in the light receiving element.
[0189]
(Example 6)
Table 11 shows data related to the optical system including the negative lens 5, the positive lens 4, and the objective lens 3 in Example 6.
[Table 11]

[0190]
FIG. 16 is an optical system configuration diagram of the negative lens 5, the positive lens 4, and the objective lens 3 according to the sixth embodiment. FIG. 17 is a spherical aberration diagram concerning the objective lens 3. In the sixth embodiment, information is recorded or reproduced by a combination of the first light source 11 having a wavelength of 405 nm and the image-side numerical aperture NA 0.85 of the objective lens 3. In the sixth embodiment, a diffractive surface is added to the surface of the positive lens 4 on the side of the optical information recording medium, and the diffractive lens has a short back focus on the long wavelength side. It is corrected. In this embodiment, fN = −5.03 (mm), fP = 6.81 (mm), and f = 1.765 (mm).
[0191]
The correction of the fluctuation of the light source wavelength or the variation of the spherical aberration when the temperature changes in this embodiment is the same as that in the first embodiment, and thus the description thereof is omitted. As can be seen from Table 12, the spherical aberration at the time of wavelength change or temperature change is good. Further, by using a plastic material for the objective lens 3, the negative lens 5, and the positive lens 4, the weight of the optical system is reduced and the burden on the movable mechanism is reduced.
[Table 12]

[0192]
(Example 7)
Table 13 shows data related to the optical system including the negative lens 5, the positive lens 4, and the objective lens 3 in Example 7.
[Table 13]

[0193]
FIG. 18 is an optical system configuration diagram of the negative lens 5, the positive lens 4, and the objective lens 3 according to the seventh embodiment. FIG. 19 is a spherical aberration diagram concerning the objective lens 3. In Example 7, information is recorded or reproduced by a combination of the first light source 11 having a wavelength of 405 nm and the image-side numerical aperture NA 0.85 of the objective lens 3. In Example 7, axial chromatic aberration of the objective lens 3 is corrected by adding a diffractive surface to both surfaces of the positive lens 4 and forming a diffractive lens that shortens the back focus on the long wavelength side. In this embodiment, fN = −4.89 (mm), fP = 5.83 (mm), and f = 1.765 (mm).
[0194]
The correction of the fluctuation of the light source wavelength or the variation of the spherical aberration when the temperature changes in this embodiment is the same as that in the first embodiment, and thus the description thereof is omitted. As is apparent from Table 14, the spherical aberration at the time of wavelength variation or temperature change is good. Further, by using a plastic material for the objective lens 3, the negative lens 5, and the positive lens 4, the weight of the optical system is reduced and the burden on the movable mechanism is reduced.
[Table 14]

[0195]
(Example 8)
Table 15 shows data related to the optical system including the negative lens 5, the positive lens 4, and the objective lens 3 in Example 8.
[Table 15]

[0196]
FIG. 20 is an optical system configuration diagram of the negative lens 5, the positive lens 4, and the objective lens 3 according to the eighth embodiment. FIG. 21 is a spherical aberration diagram concerning the objective lens 3. In Example 8, information is recorded or reproduced by a combination of the first light source 11 having a wavelength of 405 nm and the image-side numerical aperture NA 0.85 of the objective lens 3. In the eighth embodiment, axial chromatic aberration of the objective lens 3 is corrected by adding diffractive surfaces to both surfaces of the negative lens 5 and the positive lens 4 so as to shorten the back focus on the long wavelength side. ing. In this embodiment, fN = −5.54 (mm), fP = 7.42 (mm), and f = 1.765 (mm).
[0197]
Since correction of spherical aberration at the time of light source length fluctuation or temperature change in this embodiment is the same as that in Embodiment 1, description thereof is omitted. As is apparent from Table 16, the spherical aberration at the time of wavelength fluctuation or temperature change is good. Further, by using a plastic material for the objective lens 3, the negative lens 5, and the positive lens 4, the weight of the optical system is reduced and the burden on the movable mechanism is reduced.
[Table 16]

[0198]
Example 9
Table 17 shows data related to the optical system including the negative lens 5, the positive lens 4, and the objective lens 3 in Example 9.
[Table 17]

[0199]
FIG. 22 is an optical system configuration diagram of the negative lens 5, the positive lens 4, and the objective lens 3 according to the ninth embodiment. FIG. 23 is a spherical aberration diagram concerning the objective lens 3. In the ninth embodiment, information is recorded or reproduced by a combination of the first light source 11 having a wavelength of 405 nm and the image-side numerical aperture NA 0.85 of the objective lens 3. In Example 9, a diffractive surface is added to the surface of the positive lens 4 on the side of the optical information recording medium, and a diffractive lens that shortens the back focus on the long wavelength side is used. It is corrected. Further, by selecting materials of N = 30.0 and P = 56.5 as materials of the negative lens 5 and the positive lens 4 of the spherical aberration correcting means, the axial chromatic aberration of the objective lens can be corrected more satisfactorily. ing. In this embodiment, fN = −4.15 (mm), fP = 5.91 (mm), and f = 1.765 (mm).
[0200]
The correction of the fluctuation of the light source wavelength or the variation of the spherical aberration when the temperature changes in this embodiment is the same as that in the first embodiment, and thus the description thereof is omitted. As is apparent from Table 18, the spherical aberration at the time of wavelength change or temperature change is good. Further, by using a plastic material for the objective lens 3, the negative lens 5, and the positive lens 4, the weight of the optical system is reduced and the burden on the movable mechanism is reduced.
[Table 18]

[0201]
(Example 10)
Table 19 shows data related to the optical system including the negative lens 5, the positive lens 4, and the objective lens 3 in Example 10.
[Table 19]

[0202]
FIG. 24 is an optical system configuration diagram of the negative lens 5, the positive lens 4, and the objective lens 3 according to the tenth embodiment. FIG. 25 is a spherical aberration diagram concerning the objective lens 3. In Example 10, information is recorded or reproduced by a combination of the first light source 11 having a wavelength of 405 nm and the image-side numerical aperture NA0.85 of the objective lens 3. In Example 10, axial chromatic aberration of the objective lens 3 is corrected by adding a diffractive surface to both surfaces of the positive lens 4 and forming a diffractive lens that shortens the back focus on the long wavelength side. At this time, as shown in FIG. 25, the axial chromatic aberration of the combined system including the objective lens 3 and the negative lens 5 and the positive lens 4 serving as spherical aberration correcting means is overcorrected. The spherical aberration curve of the oscillation wavelength (405 nm) of one light source 11 and the spherical aberration curve on the long / short wavelength side are crossed. As a result, the mode hop phenomenon of the light source and the deterioration of the wavefront aberration at the time of high frequency superposition are very small. For example, even when the oscillation wavelength of the light source slightly fluctuates, the movement of the optimum writing position can be kept small. Furthermore, the negative lens 5 that is a movable element as a spherical aberration correcting means is a double-sided aspheric lens, so that the decentering of the negative lens 5 and the deterioration of wavefront aberration at the time of tracking error are suppressed to a minimum. Further, the axial chromatic aberration of the objective lens 3 is corrected by selecting materials of νdN = 24.3 and νdP = 56.5 as materials of the negative lens 5 and the positive lens 4, respectively, and added to the positive lens 4. This reduces the burden on the diffractive structure. In this embodiment, fN = −7.78 (mm), fP = 9.95 (mm), and f = 1.765 (mm).
[0203]
In the present embodiment, the diaphragm for restricting the light beam is disposed on the optical information recording medium side from the apex of the surface on the light source side of the objective lens 3, so that when the divergent light beam is incident, the most light source side of the objective lens 3. The height of light passing through the surface can be kept small. This is preferable for reducing the diameter of the objective lens 3 or correcting aberrations.
[0204]
The correction of the fluctuation of the wavelength of the light source or the change of the spherical aberration at the time of the temperature change in this embodiment is the same as that in the first embodiment, and the description thereof will be omitted. As can be seen from Table 20, the spherical aberration at the time of wavelength change or temperature change is good. Further, by using a plastic material for the objective lens 3, the negative lens 5, and the positive lens 4, the weight of the optical system is reduced and the burden on the movable mechanism is reduced. In addition, since a plastic material having a high transmittance with respect to light having a short wavelength is used, an optical system that can be mass-produced at low cost and has high light utilization efficiency has been achieved. Note that the movable mechanism is a shifting device for the negative lens 5 and a focusing mechanism for the objective lens 3 in the embodiments in this specification.
[Table 20]

[0205]
In this embodiment, although not shown in FIG. 25, as shown in the embodiment of FIG. 1, in the actual optical pickup device, a collimator or the like is provided between the light source and the spherical aberration correcting means. A coupling lens is provided. In that case, axial chromatic aberration generated in the coupling lens can also be corrected by the configuration of the present embodiment, and a condensing optical system with good chromatic aberration can be obtained.
[0206]
Furthermore, the recording capacity of the optical information recording medium is substantially reduced by providing two phase change films of the first information recording layer and the second information recording layer on one side of the optical information recording medium and recording information on each of them. A so-called double-layer recording type optical information recording medium is known which is doubled. In this embodiment, information is recorded or reproduced on such a double-layer recording type optical information recording medium. The spherical aberration caused by the difference in thickness up to the information recording surface of each information recording layer can be corrected. For example, when the first information recording layer and the second information recording layer are sequentially formed from the surface side of the optical information recording medium, as shown in FIG. 26, the distance between the negative lens 5 and the positive lens 4 as spherical aberration correcting means is set. By reducing the size, information can be recorded or reproduced on the information recording surface of the second information recording layer.
[0207]
(Example 11)
Table 21 shows data related to the optical system including the negative lens 5, the positive lens 4, and the objective lens 3 in Example 11.
[Table 21]

[0208]
FIG. 27 is an optical system configuration diagram of the negative lens 5, the positive lens 4, and the objective lens 3 according to the eleventh embodiment. FIG. 28 is a spherical aberration diagram concerning the objective lens 3. In Example 11, information is recorded or reproduced by a combination of the first light source 11 having a wavelength of 405 nm and the image-side numerical aperture NA0.85 of the objective lens 3. In Example 11, a diffractive surface is added to the surface of the objective lens 3 on the light source side so that the back focus is shortened on the long wavelength side, thereby correcting the axial chromatic aberration of the objective lens 3. Yes. Furthermore, the negative lens 5 that is a movable element as a spherical aberration correcting means is a double-sided aspheric lens, so that the decentering of the negative lens 5 and the deterioration of wavefront aberration at the time of tracking error are suppressed to a minimum. In this example, fN = −8.32 (mm), fP = 12.30 (mm), f = 1.765 (mm), and fD = 28.417 (mm).
[0209]
The correction of the fluctuation of the wavelength of the light source or the change of the spherical aberration at the time of the temperature change in this embodiment is the same as that in the first embodiment, and the description thereof will be omitted. As apparent from Table 22, the spherical aberration at the time of wavelength change or temperature change is good. Further, by using a plastic material for the objective lens 3, the negative lens 5, and the positive lens 4, the weight of the optical system is reduced and the burden on the movable mechanism is reduced. In addition, since a plastic material having a high transmittance with respect to light having a short wavelength is used, an optical system that can be mass-produced at low cost and has high light utilization efficiency has been achieved.
[Table 22]

[0210]
(Example 12)
Table 23 shows data related to the optical system including the negative lens 5, the positive lens 4, and the objective lens 3 in Example 12.
[Table 23]

[0211]
29 and 30 are optical system configuration diagrams of the negative lens 5, the positive lens 4, and the objective lens 3 according to the twelfth embodiment. FIGS. 31 and 32 are spherical aberration diagrams concerning the objective lens 3 when information is recorded or reproduced on different optical information recording media, respectively. In Example 12, using the same optical system, a combination of a first light source 11 with a wavelength of 405 nm and an optical information recording medium with a transparent substrate thickness of 0.1 mm, or a second light source 11 with a wavelength of 655 nm, and a transparent substrate thickness This is an example of an optical pickup device that records or reproduces information in combination with a 0.6 mm optical information recording medium. In the twelfth embodiment, by providing a diffractive structure on the surface of the objective lens 3 on the light source side, spherical aberration and chromatic spherical aberration caused by the difference in the thickness of the transparent substrate are corrected. Specifically, by moving the negative lens 5 as the spherical aberration correction means in the optical axis direction, the divergence angle of the light beam incident on the objective lens 3 is changed corresponding to the transparent substrate thickness of the optical information recording medium. Do. In this embodiment, fN = −6.39 (mm), fP = 10.51 (mm), f1 = 1.765 (mm), and fD1 = 45.46 (mm). Note that the focal length of the objective lens at the oscillation wavelength λ2 = 655 nm is f2 = 1.79.
[0212]
The correction of the fluctuation of the wavelength of the light source or the change of the spherical aberration at the time of the temperature change in this embodiment is the same as that in the first embodiment, and the description thereof will be omitted. As is apparent from Table 24, the spherical aberration at the time of wavelength change or temperature change is good. Further, by using a plastic material for the objective lens 3, the negative lens 5, and the positive lens 4, the weight of the optical system is reduced and the burden on the movable mechanism is reduced. In addition, since a plastic material having a high transmittance with respect to light having a short wavelength is used, an optical system that can be mass-produced at low cost and has high light utilization efficiency has been achieved.
[Table 24]

[0213]
As in Examples 4 and 5, a light flux of NA 0.65 or more with respect to an optical information recording medium having a light source wavelength of 655 nm and a transparent substrate thickness of 0.6 mm flare using the effect of the diffractive surface provided on the objective lens 3. By using the component, the spot diameter is not excessively reduced on the information recording surface, and detection of an unnecessary signal at the light receiving element of the optical pickup device can be prevented.
[0214]
(Example 13)
Table 25 shows data related to the optical system including the collimator corresponding to the coupling lens 21 or the coupling lenses 15 and 21, the negative lens 5, the positive lens 4, and the objective lens 3 in Example 13.
[Table 25]

[0215]
FIG. 33 is an optical system configuration diagram of the collimator, the negative lens 5, the positive lens 4, and the objective lens 3 according to the thirteenth embodiment. FIG. 34 is a spherical aberration diagram concerning the objective lens 3. In Example 13, information is recorded or reproduced by a combination of the first light source 11 having a wavelength of 405 nm and the image-side numerical aperture NA 0.85 of the objective lens 3. In the thirteenth embodiment, the negative lens 5 in the spherical aberration correcting means is shifted along the optical axis direction, thereby changing the divergence angle of the light beam incident on the objective lens 3, and the condensing optical system (collimator and objective lens). The variation of spherical aberration occurring on each optical surface in 3) is corrected. In this embodiment, fN = -10.71 (mm), fP = 13.18 (mm), and f = 1.765 (mm).
[0216]
Further, by adding diffractive surfaces to both surfaces of the positive lens 4, the spherical aberration correcting means itself generates axial chromatic aberration having a polarity opposite to the axial chromatic aberration generated on the optical surface of the condensing optical system, thereby condensing light. The axial chromatic aberration generated on the optical surface of the optical system was corrected to improve the axial chromatic aberration of the wavefront when focused on the information recording surface. In the condensing optical system of this example, when the axial chromatic aberration amounts generated by the collimator and the objective lens 3 as the optical elements are respectively ΔfB1 and ΔfB2, and the ratios thereof are roughly calculated, the focal length of the collimator is Since 12 mm, the magnification of the spherical aberration correction means is 1.23 times, and the distance between the objective lenses is 1.765 mm, ΔfB1 / ΔfB2 = 1/30. That is, if the absolute value of the axial chromatic aberration of the reverse polarity generated by the spherical aberration correcting means is substantially the same as the absolute value of the axial chromatic aberration generated by the objective lens, the axis of the wavefront when focused on the information recording surface The upper chromatic aberration can be improved. At this time, as shown in FIG. 34, the axial chromatic aberration of the combined system that combines the condensing optical system and the negative lens 5 and the positive lens 4 as spherical aberration correcting means is overcorrected. The spherical aberration curve of the oscillation wavelength (405 nm) of the first light source 11 and the spherical aberration curve on the long / short wavelength side are crossed. As a result, the mode hop phenomenon of the light source and the deterioration of the wavefront aberration at the time of high frequency superposition are very small. For example, even when the oscillation wavelength of the light source slightly fluctuates, the movement of the optimum writing position can be kept small. Further, the negative lens 5 which is a movable element in the spherical aberration correcting means is a double-sided aspheric lens, so that the decentering of the negative lens 5 and the deterioration of the wavefront aberration at the time of tracking error are suppressed to a minimum.
[0217]
As can be seen from Table 26, the variation of spherical aberration that occurs on each optical surface of the condensing optical system can be corrected due to various factors, such as when the wavelength varies or when the temperature varies, resulting in good spherical aberration. Further, by using plastic materials for all of the collimator and objective lens 3 constituting the condensing optical system, and the negative lens 5 and the positive lens 4 constituting the spherical aberration correcting means, the weight of the optical system can be reduced and the movable mechanism can be reduced. We are trying to reduce the burden. In addition, since a plastic material having a high transmittance with respect to light having a short wavelength is used, an optical system that can be mass-produced at low cost and has high light utilization efficiency has been achieved.
[Table 26]

[0218]
In this embodiment, the negative lens 5 in the spherical aberration correcting means can be shifted. However, the positive lens 4 may be movable, and both the lenses can be shifted. The variation of the spherical aberration can be corrected. In this embodiment, the axial chromatic aberration of the condensing optical system and the spherical aberration correcting means is corrected by the diffractive structure provided on the positive lens 4 in the spherical aberration correcting means. Alternatively, an optical element having a surface provided with a diffractive structure may be separately added.
[0219]
(Example 14)
Table 27 shows data related to the optical system including the coupling lens 15, the negative lens 5, the positive lens 4, and the objective lens 3 in Example 14.
[Table 27]

[0220]
FIG. 35 is an optical system configuration diagram of the coupling lens 15, the negative lens 5, the positive lens 4, and the objective lens 3 according to the fourteenth embodiment. The coupling lens 15 of the present embodiment has a function of converting a strong divergent light beam from the first light source 11 into a weak divergent light beam. FIG. 36 is a spherical aberration diagram concerning the objective lens 3. In Example 14, information is recorded or reproduced by a combination of the first light source 11 having a wavelength of 405 nm and the image-side numerical aperture NA0.85 of the objective lens 3. In the fourteenth embodiment, the negative lens 5 in the spherical aberration correcting means is shifted along the optical axis direction, thereby changing the divergence angle of the light beam incident on the objective lens 3, and the condensing optical system (coupling lens 15). And variations in spherical aberration occurring on each optical surface of the objective lens 3) are corrected. In this embodiment, fN = -14.67 (mm), fP = 11.66 (mm), and f = 1.765 (mm).
[0221]
Further, by adding diffractive surfaces to both surfaces of the positive lens 4, the spherical aberration correcting means itself generates axial chromatic aberration having a polarity opposite to the axial chromatic aberration generated on the optical surface of the condensing optical system, thereby condensing light. The axial chromatic aberration generated on the optical surface of the optical system was corrected to improve the axial chromatic aberration of the wavefront when focused on the information recording surface. At this time, as shown in FIG. 36, the axial chromatic aberration of the combined system combining the condensing optical system and the negative lens 5 and the positive lens 4 as spherical aberration correcting means is overcorrected. The spherical aberration curve of the oscillation wavelength (405 nm) of the first light source 11 and the spherical aberration curve on the long / short wavelength side are crossed. As a result, the mode hop phenomenon of the light source and the deterioration of the wavefront aberration at the time of high frequency superposition are very small. For example, even when the oscillation wavelength of the light source slightly fluctuates, the movement of the optimum writing position can be kept small.
[0222]
As is apparent from Table 28, the spherical aberration variation generated on each optical surface of the condensing optical system can be corrected due to various factors such as the wavelength variation or the temperature variation, and the spherical aberration is satisfactory. Further, the plastic lens is used for all of the coupling lens 15 and the objective lens 3 constituting the condensing optical system, and the negative lens 5 and the positive lens 4 constituting the spherical aberration correcting means, thereby reducing the weight and moving the optical system. The burden on the mechanism is reduced. In addition, since a plastic material having a high transmittance with respect to light having a short wavelength is used, an optical system that can be mass-produced at low cost and has high light utilization efficiency has been achieved. Further, in this embodiment, since the incident light to the spherical aberration correction means is a weak divergent light beam, the power of the coupling lens 15 and the power of the negative lens 5 in the spherical aberration correction means can be reduced, and each lens can be reduced. Deterioration of wavefront aberration due to decentration of the lens was suppressed to a small level.
[Table 28]

[0223]
In this embodiment, the negative lens 5 in the spherical aberration correcting means can be shifted. However, the positive lens 4 may be movable, and both the lenses can be shifted. The variation of the spherical aberration can be corrected. In this embodiment, the axial chromatic aberration of the condensing optical system and the spherical aberration correcting means is corrected by the diffractive structure provided on the positive lens 4 in the spherical aberration correcting means. Alternatively, an optical element having a surface provided with a diffractive structure may be separately added.
[0224]
Each example illustrated above uses a beam expander as spherical aberration correction means, and the beam expander shows an example constituted by a single lens negative lens that can be shifted and a single lens positive lens. However, it is needless to say that the present invention is not limited to this, and may be configured by two or more lens groups including a plurality of lenses, and various modifications are possible without departing from the present invention.
[0225]
FIG. 37 is a diagram showing optical systems according to different embodiments. An element SE that corrects the variation of spherical aberration is inserted between the coupling lens CL and the objective lens OL. Such an optical system can be used in place of the negative lens 5, the positive lens 4, and the objective lens 3 of FIG.
[0226]
The element SE sandwiches the X-direction liquid crystal element SE1, the half-wave plate SE2, and the Y-direction liquid crystal element SE3 from the coupling glass CL side between the four glass plates SE4. By electrically driving both liquid crystal elements SE1 and SE2, it is possible to correct variations in spherical aberration. Further, by providing an annular diffractive structure (not shown) on the surface of the coupling lens CL on the objective lens side, the chromatic aberration having the opposite phase to the axial chromatic aberration generated in the objective lens OL, that is, over the short wave town side. On the long wavelength side, under-axial chromatic aberration can be generated. As a result, since the axial chromatic aberration is canceled, the wavefront when passing through the element SE that corrects the variation of the spherical aberration and the objective lens OL and focusing on the optical information recording medium (not shown) is The bearing chromatic aberration is reduced.
[0227]
FIG. 38 is a diagram showing an optical system according to a modification of the present embodiment. In FIG. 38, the objective lens OL and the element SE for correcting the variation of the spherical aberration are the same as those in the embodiment shown in FIG. In FIG. 38, the coupling lens CL has a configuration in which the negative lens CL1 and the positive lens CL2 are bonded together, and the Abbe number νdN of the negative lens CL1 and the Abbe number νdP of the positive lens CL2 are νdN <νdP. The relationship is established.
[0228]
By adjusting the Abbe numbers of the negative lens CL1 and the positive lens CL2 in this way, the chromatic aberration of the opposite phase to the axial chromatic aberration generated in the objective lens OL, that is, the axis on the short wave side is over and the axis on the long wavelength side is under. Chromatic aberration can be generated. As a result, since the axial chromatic aberration is canceled, the wavefront when passing through the element SE that corrects the variation of the spherical aberration and the objective lens OL and focusing on the optical information recording medium (not shown) is The bearing chromatic aberration is reduced.
[0229]
FIG. 39 is a cross-sectional view (a) schematically showing an objective lens 3 ′ that can be used in the optical pickup device of the present embodiment, and a front view (b) seen from the light source side. [The chain line indicates the optical axis. ]
[0230]
This objective lens 3 ′ can correct spherical aberration variation due to the difference in thickness of the transparent substrate of different optical information recording media. In FIG. 36, the light source side refractive surface S1 and the optical disk side refractive surface S2 are both convex lenses having an aspherical shape and having positive refractive power. Further, the refractive surface S1 on the light source side of the objective lens is composed of four divided surfaces b1 to b4 concentrically with the optical axis. The boundary of the dividing surface is provided with a step to form each dividing surface. Accordingly, the spherical aberration and the wavefront aberration of the objective lens have a step at a position corresponding to the boundary portion.
[0231]
In a normal objective lens, the occurrence of spherical aberration due to the difference in the transparent substrate thickness of different optical information recording media is inevitable. However, although the objective lens 3 ′ used in the present embodiment cannot perform complete spherical aberration correction, it is designed to relax such aberration as will be described below.
[0232]
First, when information is reproduced and / or recorded on the first optical information recording medium, the refractive surface S1 and the refractive surface S2 so that the spherical aberration component of the wavefront aberration is within 0.05λ1 rms at the best image plane position. To design. The refracting surface S1 thus designed is applied to the first divided surface b1 and the fourth divided surface b4. A new refracting surface S1 ′ is used without using the refracting surface S2 as a variable so that the spherical aberration component of the wavefront aberration is within 0.05λ2 rms at the best image plane position at the transparent substrate thickness t3 (t1 ≦ t3 ≦ t2). To design.
[0233]
The refracting surface S1 ′ is the second divided surface b2 and the third divided surface b3. Since the refractive surface S1 ′ is optimized by the transparent substrate thickness t3, when the first optical disk 10 is used, The best image plane position is apparently formed at a position different from the best image plane position formed by the fourth divided plane b4. However, the wavefront aberration changes the inclination of the wavefront aberration in the dividing plane. For example, in the first optical information recording medium (for example, a next-generation optical disk having a higher density and a larger capacity than the DVD), the wavefront aberration drops to the right. On the contrary, in the second optical information recording medium (for example, DVD), it is slightly increased. Providing a part of two or more such dividing surfaces on the refracting surface S1 facilitates coexistence of wavefront aberrations in different optical information recording media.
[0234]
By appropriately designing the boundary positions of these divided surfaces and the axial thicknesses of the divided surfaces, the wave front at the minimum beam spot circle of confusion and the front pin position on the DVD for next-generation optical disks with higher density and larger capacity than DVD. Aberration correction is possible. That is, in the next-generation optical disk having a higher density and larger capacity than DVD, the light beam in the first to fourth light beams LB1 to LB4 is condensed at the position of the minimum circle of confusion by the objective lens. It has an integer multiple of λ1, ie, miλ1 (mi is an integer, i = 1, 2,..., k).
[0235]
Further, since the required numerical aperture NA2 is smaller than NA1 in a DVD, it is not necessary to effectively use all of the first to fourth light beams LB1 to LB4. In the optical pickup device of the present embodiment, the first to third light beams LB1 to LB1 are used. The light beam in LB3 has a wavefront aberration of approximately an integral multiple niλ1 (ni is an integer, i = 1, 2,..., K) of the wavelength λ2 at the front pin position. The fourth light beam LB4 is unnecessary light in the case of DVD, and is irradiated as a flare on a recording surface of the optical disc at a position spaced from the main spot light. Since this flare is sufficiently small with respect to the main spot light, a means for changing the numerical aperture of the diaphragm 8 is required only by making the diaphragm 8 equivalent to the required numerical aperture of a next-generation optical disk having a higher density and larger capacity than DVD. DVD playback is possible. Of course, a diaphragm 8 having a function of shielding the fourth light beam LB4 when using a DVD may be used.
[0236]
Therefore, although the optical pickup device of the present embodiment is provided with four divided surfaces b1 to b4, unlike the objective lens of the prior art, each disk does not have a plurality of focal positions, so that it is possible to reduce the spot light amount loss. And, when using each optical disk, the wavefront aberration of the light beam within the required numerical aperture is made an integer multiple of the wavelength, and the light beams that have passed through the required numerical aperture interfere with each other and strengthen each other. A sufficient amount of reflected light can be obtained from the optical disc, and a stable operation can be performed as a compatible optical pickup device.
[0237]
In the present embodiment, the objective lens is provided with four dividing surfaces, but basically has three portions for dividing the incident light beam into three light beams so as to form three dividing surfaces. An objective lens having a surface can also be used for the objective lens of the present invention. For example, at least a first portion, a second portion, and a third portion that divide a light beam emitted from a light source by refraction into a plurality of light beams in order from the optical axis side toward the outer periphery thereof on at least one surface The first part and the third part are from a light source so that information can be recorded on or reproduced from the information recording surface of the first optical information recording medium having a transparent substrate thickness t1. Of the first optical information recording surface of the second optical information recording medium having a transparent substrate thickness t2 (t1 <t2). This is a well-known objective lens configured to be able to collect a light beam from a light source on an information recording surface so that information can be recorded or reproduced.
[0238]
According to the present embodiment described above, the optical pickup device and the optical system that can effectively correct the variation of the spherical aberration caused by the mode hop of the semiconductor laser, the spherical aberration of the objective lens caused by the temperature / humidity change, etc. An optical pickup device and an optical system capable of effectively correcting fluctuations, an optical pickup device including a short wavelength laser and a high NA objective lens and capable of recording or reproducing information on different optical information recording media can be provided. . Of course, the present invention is not limited to the above embodiment and various examples.
[0239]
【The invention's effect】
According to the present invention, fluctuations in spherical aberration can be effectively corrected in an optical pickup device.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an optical pickup device according to an embodiment.
2 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 1. FIG.
3 is a spherical aberration diagram concerning the objective lens 3 according to the optical system of Example 1. FIG.
4 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 2. FIG.
5 is a spherical aberration diagram relating to the objective lens 3 according to the optical system of Example 2. FIG.
6 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 3. FIG.
7 is a spherical aberration diagram concerning the objective lens 3 according to the optical system of Example 3. FIG.
8 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 4. FIG.
9 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 4. FIG.
10 is a spherical aberration diagram concerning the objective lens 3 according to the optical system of Example 4. FIG.
11 is a spherical aberration diagram concerning the objective lens 3 according to the optical system of Example 4. FIG.
12 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 5. FIG.
13 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 5. FIG.
14 is a spherical aberration diagram concerning the objective lens 3 according to the optical system of Example 5. FIG.
15 is a spherical aberration diagram concerning the objective lens 3 according to the optical system of Example 5. FIG.
16 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 6. FIG.
17 is a spherical aberration diagram concerning the objective lens 3 according to the optical system of Example 6. FIG.
18 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 7. FIG.
19 is a spherical aberration diagram concerning the objective lens 3 according to the optical system of Example 7. FIG.
20 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 8. FIG.
21 is a spherical aberration diagram relating to the objective lens 3 according to the optical system of Example 8. FIG.
22 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 9. FIG.
23 is a spherical aberration diagram concerning the objective lens 3 according to the optical system of Example 9. FIG.
24 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 10. FIG.
25 is a spherical aberration diagram concerning the objective lens 3 according to the optical system of Example 10. FIG.
26 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 10. FIG.
27 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 11. FIG.
28 is a spherical aberration diagram concerning the objective lens 3 according to the optical system of Example 11. FIG.
29 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 12. FIG.
30 is a configuration diagram of an optical system of a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 12. FIG.
FIG. 31 is a spherical aberration diagram concerning the objective lens 3 according to the optical system of Example 12;
32 is a spherical aberration diagram concerning the objective lens 3 according to the optical system of Example 12. FIG.
33 is a configuration diagram of an optical system of a collimator, a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 13. FIG.
34 is a spherical aberration diagram concerning the objective lens 3 according to the optical system of Example 13. FIG.
35 is a configuration diagram of an optical system of a collimator, a negative lens 5, a positive lens 4, and an objective lens 3 according to Example 14. FIG.
36 is a spherical aberration diagram concerning the objective lens 3 according to the optical system of Example 14; FIG.
FIG. 37 is a diagram showing optical systems according to different embodiments.
FIG. 38 is a diagram showing an optical system according to a modification of the present embodiment.
FIG. 39 is a cross-sectional view (a) schematically showing an objective lens 3 ′ usable in the optical pickup device of the present embodiment, and a front view (b) seen from the light source side.
[Explanation of symbols]
3 Objective Lens 4 Positive Lens 5 Negative Lens 6 Objective Lens Actuator 7
Negative lens actuator 8 Aperture 9 Cylindrical lens 11 First light source 12 Second light source 15 Coupling lens 16 Concave lens 17 Hologram 21
Coupling lenses 41, 42 Photo detector 62 Beam splitter 71, 72 1/4 wavelength plate

Claims (22)

A condensing optical system including a light source, an objective lens for condensing a light beam emitted from the light source on an information recording surface via a transparent substrate of the optical information recording medium, and reflected light from the optical information recording medium An optical pickup device having a photodetector for receiving light,
The light source has an oscillation wavelength at least at a wavelength of 500 nm or less;
The objective lens is a single objective lens having an aspheric surface on at least one surface,
An image-side numerical aperture NA of the objective lens is 0.75 or more,
The optical pickup device is capable of recording and / or reproducing information with respect to an optical information recording medium in which a plurality of transparent substrates and information recording layers are laminated in order from the surface side of the optical information recording medium.
A divergence changing means is provided between the light source and the objective lens to change the divergence of the light beam incident on the objective lens according to the information recording layer when condensing each information recording layer. ,
The objective lens satisfy | fills the following formula | equation, The optical pick-up apparatus characterized by the above-mentioned .
1.1 ≦ d1 / f ≦ 3.0
However,
d1: On-axis lens thickness (mm)
f: Focal length (mm) at the oscillation wavelength of the light source (however, if the light source has a plurality of light sources having different oscillation wavelengths, the focal length at the oscillation wavelength with the shortest wavelength, and a diffraction surface on the objective lens) If equipped, the total focal length combining the refractive power and diffraction power) The divergence changing means is a means for correcting the variation of the spherical aberration,
2. The optical pickup device according to claim 1, wherein the means for correcting the variation of the spherical aberration can correct the spherical aberration up to 0.2λrms to 0.07λrms or less.   3. The optical pickup device according to claim 2, wherein the means for correcting the variation of the spherical aberration can correct the spherical aberration up to 0.5λrms to 0.07λrms or less. 4. The optical pickup according to claim 1, wherein the divergence changing unit corrects a variation in spherical aberration generated in the objective lens due to a minute variation in the oscillation wavelength of the light source. apparatus. The objective lens is a plastic lens;
The divergence changing means is provided between the light source and the objective lens, to claim 1, characterized in that for correcting the variation of the spherical aberration generated in the light converging optical system due to the temperature change 5. The optical pickup device according to any one of 4 above . The divergence changing means is provided between the light source and the objective lens, to claim 1, characterized in that for correcting the variation of the spherical aberration generated in due the condensing optical system to the humidity change 6. The optical pickup device according to any one of 5 above . The divergence changing means includes at least one negative lens and at least one positive lens, to claim 1, at least one of which is characterized in that has a transition moveable element in the optical axis direction The optical pickup device according to claim 6 . The optical pickup device according to claim 7 , wherein the divergence changing unit satisfies the following expression .
νdP> νdN
However,
νdP: average of Abbe number of d-line of all positive lenses including the positive lens
νdN: average of Abbe number of d-line of all negative lenses including the negative lens The optical pickup device according to claim 8 , wherein the νdP and the νdN satisfy the following expression .
νdP> 55
νdN <35 10. The optical pickup device according to claim 7, wherein the following expression is established .
Δd · | fP / fN | /Δνd≦0.05
However,
Δd: Amount of movement (mm) of the movable element when information is recorded or reproduced on one information recording surface of any one optical information recording medium capable of recording or reproducing information
fP: focal length (mm) of the positive lens (however, when the positive lens is provided with a diffractive surface, the total focal length combining the refractive power and the diffractive power)
fN: focal length (mm) of the negative lens (however, when the negative lens is provided with a diffractive surface, the total focal length combining refractive power and diffractive power)
Δνd: difference between the maximum Abbe number of the positive lens and the minimum Abbe number of the negative lens The optical pickup device can record and / or reproduce information with respect to at least two types of optical information recording media.
The divergence changing means changes the divergence of a light beam incident on the objective lens according to the thickness of each transparent substrate with respect to at least two types of optical information recording media having different transparent substrate thicknesses. The optical pickup device according to claim 1 . When the transparent substrate thicknesses of the two types of optical information recording media are a and b (a <b), respectively, when information is recorded on or reproduced from the information recording surface of the optical information recording medium of the transparent substrate thickness a. the in claim 11, characterized in that to increase the distance of the positive lens and the negative lens than when recording or reproducing information on the information recording surface of the optical information recording medium of the transparent substrate thickness b The optical pickup device described. The divergence changing means includes at least one positive lens and at least one negative lens, at least one of which is a movable element that is movable in the optical axis direction, and satisfies the following expression: The optical pickup device according to claim 11 or 12 .
| FP / fN | ≧ 1.3
However,
fP: focal length of the positive lens (however, when the positive lens is provided with a diffractive surface, the total focal length combining the refractive power and the diffractive power)
fN: focal length of the negative lens (however, when the negative lens is provided with a diffractive surface, the total focal length combining the refractive power and the diffractive power) 14. The optical pickup device according to claim 7, wherein at least one of the positive lens and the negative lens is made of a plastic material . The optical pickup apparatus according to claim 7, wherein the divergence changing unit includes the one positive lens and the one negative lens . The divergence changing means has the one negative lens to the light source side, to any one of claims 7 to 15, characterized in Rukoto to have a said one positive lens on the objective lens side The optical pickup device described. The optical pickup device according to claim 7 , wherein the negative lens is a movable element that can be displaced in an optical axis direction . 18. The optical pickup device according to claim 7 , wherein an on-axis chromatic aberration of a combined system including the objective lens, the negative lens, and the positive lens is overcorrected . The divergence changing means includes at least one positive lens and at least one negative lens, at least one of which is a movable element that can be shifted in the optical axis direction ,
When the paraxial power at the oscillation wavelength of the light source is P1, the paraxial power at a wavelength 10 nm shorter than the oscillation wavelength is P2, and the paraxial power at a wavelength 10 nm longer than the oscillation wavelength is P3, the following equation is satisfied. The optical pickup device according to claim 1 , wherein the optical pickup device is an optical pickup device.
P2 <P1 <P3 Wherein among the spherical aberration of the objective lens, when the third-order spherical aberration components SA1,5 and seventh order and ninth order of the sum of the spherical aberration component SA2, claim 1, characterized in that the following expression is satisfied 20. An optical pickup device according to any one of items 19 to 19 .
| SA1 / SA2 |> 1.0
However,
SA1: Third-order spherical aberration component when the aberration function is expanded into a Zernike polynomial
SA2: Square root of the sum of squares of the fifth-order spherical aberration component, the seventh-order spherical aberration component, and the ninth-order spherical aberration component when the aberration function is expanded into a Zernike polynomial Any said diaphragm to determine the numerical aperture of the objective lens, according to claim 1 to 19 wherein the optical information recording medium from a surface apex on the most light source side surface of the objective lens is characterized in that located on the side to be arranged the optical pickup device according to any. The divergence changing means, the optical pickup device according to any one of claims 1 to 21, characterized in that a beam expander.

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