JP2004198839A - Objective lens for optical disk, objective optical system and optical head device using the above - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize an objective lens which is used for recording and reproducing a plurality of kinds of optical disks having different disk thicknesses and has small aberration variation caused by the temperature change. <P>SOLUTION: An objective lens 1 for optical disks is a single lens made of resin. The incident plane of the lens 1 is divided into four regions by the circles whose approximate centers are located on an optical axis. A circular shaped first region 3 including the optical axis is made to have a structure on which diffraction lenses are integrally formed on the rotatively symmetrical non-spherical surface of the refractive lenses. Ring shaped second, third and fourth regions 4, 5 and 6 which are located outside the first region 3 are respectively constructed by rotationally symmetrical non-spherical surfaces. Differences in level 7 and 8 are provided at the boundary section between the second region 4 and the third region 5 and the boundary section between the third region 5 and the fourth region 6 to generate phase differences with respect to the respective transmissive luminous fluxes. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００３８】
但し、上記（数７）中、ｈ（ｍｍ）は光軸からの高さ、Ｓ（ｍｍ）は光軸からの高さがｈの箇所における非球面上の点の非球面頂点からの距離（サグ）、Ｒｍ（ｍｍ）は第ｍ番目の各領域の非球面頂点の曲率半径（以下『頂点曲率半径』という）、Ｋｍは第ｍ番目の各領域の円錐定数、Ａｍは第ｍ番目の各領域の光軸上でのオフセット量、Ｄｍ、Ｅｍ、Ｆｍ、Ｇｍは第ｍ番目の各領域の４次、６次、８次及び１０次の非球面係数をそれぞれ表している。尚、本明細書中においては、Ｄｍ、Ｅｍ、Ｆｍ、Ｇｍだけでなく、Ｒｍ、Ｋｍをも含めて『非球面係数』と呼ぶことにする。
【００３９】
第２の領域４と第３の領域５との境界部、及び第３の領域５と第４の領域６との境界部には、それぞれ透過光束に対して位相差を生じさせる段差７、８が設けられている。
【００４０】
また、対物レンズ１の出射面９は、下記（数８）で定義される回転対称非球面によって構成されている。
【００４１】
【数８】

【００４２】
但し、上記（数８）中、ｈ（ｍｍ）は光軸からの高さ、Ｓ（ｍｍ）は光軸からの高さがｈの箇所における非球面上の点の非球面頂点からの距離（サグ）、Ｒ（ｍｍ）は頂点曲率半径、Ｋは円錐定数、Ｄ、Ｅ、Ｆ、Ｇは４次、６次、８次及び１０次の非球面係数をそれぞれ表している。
【００４３】
光ディスク２がＤＶＤである場合、入射光束１０は、対物レンズ１の入射面側の第１〜第４の領域３〜６（対物レンズ１の全域）を用いてＤＶＤの情報記録面上に集光される（図１の上半分参照）。
【００４４】
一方、光ディスク２がＣＤである場合には、対物レンズ１の入射面側の第１の領域３を透過する光束１１のみがＣＤの情報記録面上に集光される（図１の下半分参照）。この場合、第１の領域３以外の領域を透過する光束（図示せず）は、当該領域部分のレンズの収差が大きいために結像点の周辺に大きく拡がった光量分布を持つこととなり、実質上結像には寄与しなくなる。
【００４５】
すなわち、本実施の形態の対物レンズ１は、入射面側の内周の第１の領域３がＤＶＤとＣＤの両方に対して良好な集光性能を有する回折一体型の非球面レンズとなっている。また、本実施の形態の対物レンズ１は、入射面側の第２〜第４の領域４〜６がＤＶＤに対して良好に収差が補正された非球面を、透過光束に対して位相差を生じさせる段差７、８を設けてつなぎ合わせた構成となっている。
【００４６】
光ディスク用の対物レンズに用いられる樹脂材料の、温度変化に対する屈折率の変化量は、一般に、ほぼ−１０×１０^-5／℃程度である。また、光ディスク装置用の半導体レーザ光源も、温度変化に伴って発振波長が変化し、その変化量は０．２ｎｍ／℃程度である。さらに、樹脂材料は、当該樹脂材料に入射する光の波長が長くなると屈折率が小さくなる波長特性を示す。従って、光ヘッド装置を動作させている環境の温度が上昇すると、光源の波長が長波長側にシフトし、樹脂材料の屈折率は、光源の波長が長くなることと、温度変化による屈折率の低下の双方の影響によって低下する。仮に対物レンズが一般的な回転対称非球面によって構成された樹脂レンズであったとすると、これらの影響によって３次球面収差は負側にシフトする。特に、ＤＶＤ用等に用いられるような開口数（ＮＡ）が０．６程度の対物レンズの場合には、１℃当たり１ｍλ程度の収差変化となるため、３５℃の温度変化に対しては３５ｍλもの球面収差が発生してしまう。
【００４７】
上記したように、本実施の形態の対物レンズ１における入射面側の内周の第１の領域３は、屈折レンズの非球面上に回折レンズが一体化された構造となっている。このとき、回折レンズ及び屈折レンズの双方が正の屈折力を有する構成の場合には、回折レンズの温度変化による球面収差が屈折レンズのそれを緩和する方向に発生する。つまり、屈折レンズによって球面収差が負側にシフトする場合には、回折レンズは球面収差を正側にシフトさせ、逆に、屈折レンズによって球面収差が正側にシフトする場合には、回折レンズは球面収差を負側にシフトさせる。このように、本実施の形態の対物レンズ１における入射面側の内周の第１の領域３は、温度変化による収差の発生量が通常の非球面樹脂レンズの場合に比べて小さくなっている。このことは、集光しようとする光ディスクがＤＶＤ、ＣＤのいずれの場合にも言えることである。
【００４８】
一方、本実施の形態の対物レンズ１における入射面側の第２〜第４の領域４〜６によって構成される外周部は、ＤＶＤ用に最適設計された非球面を、透過光束に対して位相差を生じさせる段差７、８を設けてつなぎ合わせた構成となっている。これらの領域を透過する光は、ＣＤに対する情報の記録あるいは再生には用いられない。すなわち、第２〜第４の領域４〜６を上記（数７）によって定義される回転対称非球面とし、第２の領域４と第３の領域５との境界部、及び第３の領域５と第４の領域６との境界部に、それぞれ透過光束に対して位相差を生じさせる段差７、８を設けることにより、ＣＤ用の波長の光がこの領域に入射した場合には、大きな収差が発生し、ＣＤの情報記録面上で当該光は実質上結像に寄与しない程度に拡がったスポットとなる。
【００４９】
第２の領域４と第３の領域５との境界部、及び第３の領域５と第４の領域６との境界部に設けた段差７、８は、当該段差７、８によって生じる透過光束の位相差が、ＤＶＤの設計波長の透過光束に対して、その波長の略整数倍となる量であるのが望ましい。このような構成にすれば、ＤＶＤの設計波長において、段差７、８が収差に影響を与えることを防止することができる。しかし、光ヘッド装置を動作させている環境の温度が変化すると、光源である半導体レーザの発振波長の変化や対物レンズ１の材料である樹脂材料の屈折率の変化により、この段差７、８によって生じる透過光束の位相差がＤＶＤの設計波長の略整数倍からずれた値となる。そして、段差７、８によって生じる透過光束の位相差がＤＶＤの設計波長の略整数倍からずれると、球面収差が発生し、当該球面収差により、温度変化による球面収差の変化が緩和される。
【００５０】
また、段差７、８によって区切られた各領域（第２の領域４、第３の領域５、第４の領域６）のうち、透過光束の位相が最も進む領域と最も遅れる領域との間の位相差が、ＤＶＤの設計波長の６倍以上であるのが望ましく、これにより十分な温度補償効果が得られる。
【００５１】
ここで、ＤＶＤの設計波長λ１としては、下記（数９）で規定される範囲にあるものが用いられる。
［数９］
６５０ｎｍ＜λ１＜６８０ｎｍ
尚、本実施の形態においては、段差の数を２としているが（段差７、８）、段差の数は３以上であってもよい。
【００５２】
また、本実施の形態においては、対物レンズ１の入射面が光軸を略中心とする円により４つの領域に分割されているが、対物レンズ１の出射面を、光軸を略中心とする円により４つの領域に分割してもよい。また、分割の数は３以上であればよい。
【００５３】
以下、本実施の形態における光ディスク用対物レンズの具体的な設計について、好適な数値実施例を挙げてさらに詳細に説明する。
【００５４】
（数値実施例１）
本実施例において、設計温度は３５℃であり、ＤＶＤ用の光源の設計温度における発振波長は６６０ｎｍ、ＣＤ用の光源の設計温度における発振波長は７９０ｎｍである。尚、光源として用いる半導体レーザの発振波長は、温度が１℃上昇するごとに０．２ｎｍだけ長くなる方向に変化するものとする。
【００５５】
対物レンズ１の入射面は、第ｍ番目の各領域の頂点曲率半径をＲｍ（ｍｍ）、第ｍ番目の各領域の円錐定数をＫｍ、第ｍ番目の各領域の４次、６次、８次及び１０次の非球面係数をそれぞれＤｍ、Ｅｍ、Ｆｍ、Ｇｍ、第ｍ番目の各領域の光軸上でのオフセット量をＡｍとして、光軸からの高さがｈ（ｍｍ）の箇所における非球面上の点の非球面頂点からの距離（サグ）Ｓ（ｍｍ）が上記（数７）によって定義される回転対称非球面を用いて設計されている。また、対物レンズ１の出射面は、頂点曲率半径をＲ（ｍｍ）、円錐定数をＫ、４次、６次、８次及び１０次の非球面係数をそれぞれＤ、Ｅ、Ｆ、Ｇとして、光軸からの高さがｈ（ｍｍ）の箇所における非球面上の点の非球面頂点からの距離（サグ）Ｓ（ｍｍ）が上記（数８）によって定義される回転対称非球面を用いて設計されている。それぞれの面の係数は、下記（表１）、（表２）に示す通りである。尚、下記（表１）の設計データにおいて、第１の領域３の回折レンズ部は、高屈折率法を用いて表現されている。そのため、第１の領域３の実際の加工形状は、樹脂面で表される非球面形状に高屈折率面で表されるデータから算出されるレリーフ形状を重ね合わせた形状となる。
［表１］入射面側の面形状データ
第１の領域３ 領域半径 1.05mm
第１の領域の非球面係数（高屈折率面）
Ｒ１： 1.3667152
Ｋ１： -0.40340044
Ａ１： 0.0
Ｄ１： -0.014833817
Ｅ１： -0.014183695
Ｆ１： 0.0050402632
Ｇ１： -0.0020063491
第１の領域の非球面係数（樹脂面）
Ｒ１： 1.3667129
Ｋ１： -0.40340312
Ａ１： 0.0
Ｄ１： -0.014834096
Ｅ１： -0.014184697
Ｆ１： 0.0050406379
Ｇ１： -0.0020064273
第２の領域４ 領域半径 1.2mm
Ｒ２： 1.3952
Ｋ２： -0.46563672
Ａ２： -0.00495
Ｄ２： -0.0075278349
Ｅ２： 0.0002166981
Ｆ２： -0.0012772982
Ｇ２： -0.00024732574
第３の領域５ 領域半径 1.3mm
Ｒ３： 1.3963
Ｋ３： -0.46563672
Ａ３： -0.01003
Ｄ３： -0.0075278349
Ｅ３： 0.0002166981
Ｆ３： -0.0012772982
Ｇ３： -0.00024732574
第４の領域６
Ｒ４： 1.3974
Ｋ４： -0.46563672
Ａ４： -0.0151
Ｄ４： -0.0075278349
Ｅ４： 0.0002166981
Ｆ４： -0.0012772982
Ｇ４： -0.00024732574
［表２］射出面側の面形状データ
Ｒ： -6.402302
Ｋ： -17.72508
Ｄ： 0.0094256721
Ｅ： -0.0010662053
Ｆ： -0.0015622112
Ｇ： 0.00056196144
対物レンズ１の厚みは１．２４５ｍｍであり、設計温度におけるＤＶＤ用の光源での焦点距離は２．２５ｍｍである。また、対物レンズ１のＮＡは、ＤＶＤ用の光学系で０．６、ＣＤ用の光学系で０．４７となっている。また、対物レンズ１の入射面側の有効径は、ＤＶＤ用の光学系で２．７６ｍｍ、ＣＤ用の光学系で２．０９５ｍｍである。また、設計温度及びそれから±３５℃温度が変化したときの光源の発振波長と、その状態での対物レンズ１の樹脂材料及び光ディスクの透明基材の屈折率は、下記（表３）に示す通りである。
【００５６】
【表３】

【００５７】
このとき、第２の領域４と第３の領域５との境界部に設けられた段差７は、当該段差７によって生じる透過光束の位相差が、波長６６０ｎｍの光束に対して、その波長の略４倍となる量である。また、第３の領域５と第４の領域６との境界部に設けられた段差８も、当該段差８によって生じる透過光束の位相差が、波長６６０ｎｍの光束に対して、その波長の略４倍となる量である。従って、本実施例の体物レンズ１において、第２の領域４（透過光束の位相の最も進む領域）と第４の領域６（透過光束の位相の最も遅れる領域）との間の位相差は、波長６６０ｎｍの光束に対して、その波長の略８倍となる。
【００５８】
図２に、設計温度（３５℃）における本実施例の対物レンズ１のＤＶＤに対するＯＰＤ（光路長収差：Optical path difference）特性の計算結果を示す。図２において、縦軸がＯＰＤであり、波長を単位として表されている。また、横軸は口径である。図２に示すように、設計温度（３５℃）における本実施例の対物レンズ１のＤＶＤに対するＯＰＤは、透過光束に対して位相差を生じさせる段差７、８の影響により、段差が４λの階段状となっている。ここで、光源の波長のちょうど整数倍の位相差を無視すれば、図３（ａ）に示すようなＯＰＤ特性が得られる。また、同様に、温度が０℃及び７０℃の場合においても、図３（ｂ）、（ｃ）に示すようなＯＰＤ特性が得られる。尚、図３（ｂ）、（ｃ）において、破線は、対物レンズ１の外周部が位相差を生じさせる段差の存在しない単独の非球面によって構成されている場合の、ＯＰＤ特性の計算結果を示している。図３（ｂ）、（ｃ）に示すように、外周部に透過光束に対して位相差を生じさせる段差が存在しない場合には、光ヘッド装置を動作させている環境の温度が変化すると、ＯＰＤが大きくずれるために大きな球面収差が発生するが、外周部に透過光束に対して位相差を生じさせる段差７、８を設けることにより、この球面収差が実使用上問題のない程度にまで軽減されることが分かる。
【００５９】
図４に、本実施例の対物レンズ１のＣＤに対するＯＰＤ特性の計算結果を示す。図４において、縦軸はＯＰＤを波長を単位として表しており、図４中のＷは波長（wavelength）を示している。図４（ａ）が設計温度（３５℃）の場合、図４（ｂ）が０℃の場合、図４（ｃ）が７０℃の場合をそれぞれ示している。光ディスク２がＣＤである場合には、対物レンズ１の内周の第１の領域３を透過する光束のみがその情報記録面上に集光されるので、透過光束に対して位相差を生じさせる段差７、８の効果は関係ないが、図４に示すように、温度変化に対する収差の発生量はそれほど大きくないことが分かる。
【００６０】
下記（表４）に、これらのＯＰＤの波面収差を計算した結果を示す。
【００６１】
【表４】

【００６２】
以上より、本実施例の対物レンズによれば、設計温度（３５℃）に対して±３５℃の範囲で３次球面収差の変動を改善できることが分かる。
【００６３】
［第２の実施の形態］
図５は本発明の第２の実施の形態における光ディスク用の対物レンズを示す概略断面図である。
【００６４】
図５において、１２は樹脂製の単レンズからなる対物レンズ、１３は光ディスクをそれぞれ示している。尚、図５においても、便宜上、上半分に光ディスク１３がＤＶＤである場合の光路を、下半分に光ディスク１３がＣＤである場合の光路をそれぞれ示している。また、図５においても、光ディスク１３として、上半分にＤＶＤを、下半分にＣＤをそれぞれ示している。
【００６５】
図５に示すように、本実施の形態の対物レンズ１２は、その入射面が、光軸を略中心とする円により２つの領域に分割されている。そして、光軸を含む円状の領域１４は、上記（数７）で定義される屈折レンズの回転対称非球面上に回折レンズが一体化された構造を有し、これによりＤＶＤとＣＤの両方に対して良好に収差が補正されている。また、円状の領域１４の外側に位置する円環状の領域１５は、上記（数７）で定義される回転対称非球面によって構成され、ＤＶＤ用に最適設計されている。
【００６６】
また、対物レンズ１２の出射面は、光軸を略中心とする同心円により３つの領域１６、１７、１８に分割されている。これら３つの領域１６、１７、１８は、ＤＶＤ用に最適設計された、上記（数７）で定義される回転対称非球面によって構成されている。また、領域１６と領域１７との境界部、及び領域１７と領域１８との境界部には、それぞれ光軸に平行な段差が設けられている。これらの段差は、対物レンズ１２の入射面側の外周部（円環状の領域１５部分）を透過した光が通る光路中に設けられている。尚、これらの段差は、ＤＶＤ用の波長の透過光束に対して、当該波長の略整数倍の位相差を生じさせる段差であるのが望ましい。また、これらの段差によって区切られた各領域１６〜１８のうち、透過光束の位相が最も進む領域と最も遅れる領域との間の位相差が、ＤＶＤ用の波長の透過光束に対して、当該波長の６倍以上であるのが望ましい。また、段差の数は２以上であるのが望ましい。
【００６７】
以上のような構成とした場合においても、上記第１の実施の形態の場合と同様に、対物レンズ１２の入射面側の内周部（円状の領域１４部分）は、回折レンズの作用により、ＤＶＤとＣＤの両方に対して良好な集光性能を有し、かつ、温度変化に対する球面収差の変化が小さくなる。また、対物レンズ１２の入射面側の外周部（円環状の領域１５部分）を透過するＤＶＤ用の光束は、対物レンズ１２の出射面側に設けられた段差の効果により、温度変化による球面収差の変化が緩和される。その結果、本実施の形態の対物レンズ１２は、レンズ全体として温度変化に対する球面収差の変化の小さい対物レンズとなる。
【００６８】
尚、対物レンズ１２の入射面と出射面の構成を逆にしても、本実施の形態と同様の効果を得ることができる。
【００６９】
［第３の実施の形態］
図６は本発明の第３の実施の形態における光ディスク用の対物光学系を示す概略断面図である。
【００７０】
図６において、２０は収差補正素子、２１は樹脂製の単レンズからなる対物レンズ、２２は光ディスクをそれぞれ示している。尚、図６においても、便宜上、上半分に光ディスク２２がＤＶＤである場合の光路を、下半分に光ディスク２２がＣＤである場合の光路をそれぞれ示している。また、図６においても、光ディスク２２として、上半分にＤＶＤを、下半分にＣＤをそれぞれ示している。
【００７１】
図６に示すように、本実施の形態の対物レンズ２１は、その入射面が、光軸を略中心とする円により２つの領域に分割されている。そして、光軸を含む円状の第１の領域２３は、上記（数７）で定義される屈折レンズの回転対称非球面上に回折レンズが一体化された構造を有し、これによりＤＶＤとＣＤの両方に対して良好な集光性能を有するようにされている。また、円状の第１の領域２３の外側に位置する円環状の第２の領域２４は、上記（数７）で定義される回転対称非球面によって構成され、ＤＶＤ用に最適設計されている。
【００７２】
また、対物レンズ２１の出射面２５は、上記（数８）で定義される回転対称非球面によって構成されている。
【００７３】
収差補正素子２０は、対物レンズ２１の入射面側に配置された樹脂製の略平行平板により構成されている。収差補正素子２０の出射面（対物レンズ２１との対向面）には、対物レンズ２１の外周部に相当する部分に光軸を略中心とする同心円により階段状の段差２６が設けられている。この場合、段差２６は、ＤＶＤの設計波長の透過光束に対して、その波長の略整数倍の位相差を生じさせる段差であるのが望ましい。また、段差２６によって区切られた各領域のうち、透過光束の位相が最も進む領域と最も遅れる領域との間の位相差は、ＤＶＤの設計波長の透過光束に対して、その波長の６倍以上であるのが望ましい。さらに、段差２６の数は２以上であるのが望ましい。
【００７４】
光源である半導体レーザからの入射光束２７は、本実施の形態の対物レンズ２１により、光ディスク２２の情報記録面上に集光される。また、光ディスク２２の情報記録面からの反射光束は、対物レンズ２２によって平行光束に変換された後、図示しない光ヘッドの検出光学系に導かれる。
【００７５】
ＣＤに対して記録あるいは再生を行う場合には、対物レンズ２１の内周部（円状の第１の領域２３部分）を透過する光束のみがＣＤの情報記録面上に集光される（図６の下半分参照）。すなわち、対物レンズ２１の全域に光が入射しても、対物レンズ２１の外周部（円環状の第２の領域２４部分）は、ＤＶＤ用に収差補正された（ＤＶＤ用に最適設計された）非球面であるため、ＣＤ用の光学系において用いた場合には大きな球面収差が発生する。従って、対物レンズ２１の外周部（円環状の第２の領域２４部分）を透過する光束（図示せず）は、結像点の周辺に大きく拡がった光量分布を持つこととなり、実質上結像には寄与しなくなる。また、対物レンズ２１の内周部（円状の第１の領域２３部分）は、屈折レンズの非球面上に回折レンズが一体化された構成となっているため、上記第１の実施の形態の場合と同様に、温度変化に対する球面収差の変化が小さくなる。
【００７６】
一方、ＤＶＤに対して記録あるいは再生を行う場合には、対物レンズ２１の有効径全体を透過する光束がＤＶＤの情報記録面上に集光される（図６の上半分参照）。すなわち、対物レンズ２１の内周部（円状の第１の領域２３部分）は、回折レンズの作用により、ＤＶＤに対しても良好な集光性能を有しており、また、外周部（円環状の第２の領域２４部分）は、ＤＶＤ用に収差補正された（ＤＶＤ用に最適設計された）非球面であるため、ＤＶＤに対しては対物レンズ２１の全域で収差が良好に補正される。また、対物レンズ２１の内周部（円状の第１の領域２３部分）が、屈折レンズの非球面上に回折レンズが一体化された構成となっているので、上記第１の実施の形態の場合と同様に、温度変化による球面収差の発生が緩和される。また、対物レンズ２１の外周部（円環状の第２の領域２４部分）においては、光ヘッド装置を動作させている環境の温度が変化すると、光源である半導体レーザの発振波長の変化や対物レンズ２１の材料である樹脂材料の屈折率の変化により、球面収差が発生するが、前記変化により収差補正素子２０に設けた段差２６によって生じる透過光束の位相差がＤＶＤの設計波長の略整数倍からずれ、対物レンズ２１の外周部で発生する球面収差を緩和するような球面収差が発生する。
【００７７】
以上のように、本実施の形態の対物光学系によれば、例えば、ＤＶＤ用、ＣＤ用の両方の光学系において、温度変化に対する収差変動を抑え、良好な結像性能を発揮することができる。
【００７８】
以下、本実施の形態における光ディスク用対物光学系の具体的な設計について、好適な数値実施例を挙げてさらに詳細に説明する。
【００７９】
（数値実施例２）
本実施例において、設計温度は３５℃であり、ＤＶＤ用の光源の設計温度における発振波長は６６０ｎｍ、ＣＤ用の光源の設計温度における発振波長は７９０ｎｍである。尚、光源として用いる半導体レーザの発振波長は、温度が１℃上昇するごとに０．２ｎｍだけ長くなる方向に変化するものとする。尚、ここで説明する実施例は、設計温度においては上記数値実施例１で示した光学系と等価で、段差２６は、ＤＶＤの設計波長の透過光束に対して、その波長の５倍の位相差を生じさせる段差となっている。
【００８０】
対物レンズ２１の入射面側は、第ｍ番目の各領域の頂点曲率半径をＲｍ（ｍｍ）、第ｍ番目の各領域の円錐定数をＫｍ、第ｍ番目の各領域の４次、６次、８次及び１０次の非球面係数をそれぞＤｍ、Ｅｍ、Ｆｍ、Ｇｍ、第ｍ番目の各領域の光軸上でのオフセット量をＡｍとして、光軸からの高さがｈ（ｍｍ）の箇所における非球面上の点の非球面頂点からの距離（サグ）Ｓ（ｍｍ）が上記（数７）によって定義される回転対称非球面を用いて設計されている。また、対物レンズ２１の射出面は、頂点曲率半径をＲ（ｍｍ）、円錐定数をＫ、４次、６次、８次及び１０次の非球面係数をそれぞれＤ、Ｅ、Ｆ、Ｇとして、光軸からの高さがｈ（ｍｍ）の箇所における非球面上の点の非球面頂点からの距離（サグ）Ｓ（ｍｍ）が上記（数８）によって定義される回転対称非球面となっている。それぞれの面の係数は、下記（表５）、（表６）に示す通りである。尚、下記（表５）の設計データにおいて、第１の領域２３の回折レンズ部は、高屈折率法を用いて表現されている。そのため、第１の領域２３の実際の加工形状は、樹脂面で表される非球面形状に高屈折率面で表されるデータから算出されるレリーフ形状を重ね合わせた形状となる。
［表５］入射面側の面形状データ
第１の領域２３ 領域半径 1.05mm
第１の領域の非球面係数（高屈折率面）
Ｒ１： 1.3667152
Ｋ１： -0.40340044
Ａ１： 0.0
Ｄ１： -0.014833817
Ｅ１： -0.014183695
Ｆ１： 0.0050402632
Ｇ１： -0.0020063491
第１の領域の非球面係数（樹脂面）
Ｒ１： 1.3667129
Ｋ１： -0.40340312
Ａ１： 0.0
Ｄ１： -0.014834096
Ｅ１： -0.014184697
Ｆ１： 0.0050406379
Ｇ１： -0.0020064273
第２の領域２４
Ｒ２： 1.39525
Ｋ２： -0.46563672
Ａ２： -0.00492
Ｄ２： -0.0075278349
Ｅ２： 0.0002166981
Ｆ２： -0.0012772982
Ｇ２： -0.00024732574
［表６］射出面側の面形状データ
Ｒ： -6.402302
Ｋ： -17.72508
Ｄ： 0.0094256721
Ｅ： -0.0010662053
Ｆ： -0.0015622112
Ｇ： 0.00056196144
対物レンズ２１の厚みは１．２４５ｍｍであり、設計温度におけるＤＶＤ用の光源での焦点距離は２．２５ｍｍである。また、対物レンズ２１のＮＡは、ＤＶＤ用の光学系で０．６、ＣＤ用の光学系で０．４７となっている。また、対物レンズ２１の入射面側の有効径は、ＤＶＤ用の光学系で２．７６ｍｍ、ＣＤ用の光学系で２．０９５ｍｍである。
【００８１】
図７に、収差補正素子２０の具体的設計例を示す。図７に示す収差補正素子２０において、２箇所の段差はいずれも６．１２５μｍであり、これは、当該段差によって生じる透過光束の位相差が、波長６６０ｎｍの光束に対して、その波長の略５倍となる量である。従って、本実施例の収差補正素子２０において、段差によって区切られた各領域間の最大の位相差は、波長６６０ｎｍの光束に対して、その波長の略１０倍となる。また、収差補正素子２０は、対物レンズ２１と同じ樹脂材料で構成されており、対物レンズ２１及び光ディスク２２の透明基材の屈折率は、数値実施例１の上記（表３）に示した値と同じである。
【００８２】
下記（表７）に、本実施例の対物光学系の波面収差を計算した結果を示す。
【００８３】
【表７】

【００８４】
以上より、本実施例の対物光学系によれば、設計温度に対して±３５℃の範囲で３次球面収差の変動を改善できることが分かる。
【００８５】
［第４の実施の形態］
図８は本発明の第４の実施の形態における光ヘッド装置を示す概略構成図である。
【００８６】
図８中、３０はＤＶＤ用の波長の光を射出する半導体レーザであり、３１はＣＤ用の波長の光を射出する半導体レーザである。また、３２はコリメートレンズ、３３、３４はビームスプリッタ、３５は対物レンズ、３６は光ディスク、３７は検出光学系、３８は受光素子をそれぞれ示している。ここで、対物レンズ３５としては、上記第１又は第２の実施の形態で説明した本発明の対物レンズが用いられている。
【００８７】
図８に示すように、半導体レーザ３０からの出射光束は、ビームスプリッタ３３に入射した後、そのまま当該ビームスプリッタ３３を透過する。また、半導体レーザ３１からの出射光束は、ビームスプリッタ３３に入射した後、光路を折り返されて、当該ビームスプリッタ３３から射出される。
【００８８】
ビームスプリッタ３３からの出射光束は、コリメートレンズ３２によってそれぞれ略平行光束となり、ビームスプリッタ３４へと入射する。ビームスプリッタ３４からの出射光束は、本発明の対物レンズ３５によって光ディスク３６の情報記録面上に集光される。光ディスク３６からの反射光束は、本発明の対物レンズ３５によって略平行光束となり、当該略平行光束は、ビームスプリッタ３４で反射された後、検出光学系３７によって受光素子３８に集光される。
【００８９】
本実施の形態の光ヘッド装置において温度変化があった場合、光源である半導体レーザ３０あるいは３１の発振波長は、温度が１℃上昇するごとに約０．２ｎｍだけ長波長側にシフトする。このとき、対物レンズ３５の内周部は、回折レンズの作用により、ＤＶＤとＣＤの両方に対して良好な集光性能を有し、かつ、温度変化に対する球面収差の変化が小さくなる。また、対物レンズ３５の外周部を透過するＤＶＤ用の光束は、対物レンズ３５に設けられた段差の効果により、温度変化による球面収差の変化が緩和される。
【００９０】
以上のように、本実施の形態の光ヘッド装置においては、対物レンズ３５として上記第１又は第２の実施の形態で説明した本発明の対物レンズが用いられていることにより、例えば、ＤＶＤ用、ＣＤ用の両方の光学系において温度変化に対する収差変動を抑えることができるので、１つの対物レンズを用いてディスク厚みの異なる光ディスクの記録再生を良好に行なうことのできる光ヘッド装置を実現することができる。
【００９１】
［第５の実施の形態］
図９は本発明の第５の実施の形態における光ヘッド装置を示す概略構成図である。
【００９２】
図９中、４０はＤＶＤ用の波長の光を射出する半導体レーザであり、４１はＣＤ用の波長の光を射出する半導体レーザである。また、４２はコリメートレンズ、４３、４４はビームスプリッタ、４５は収差補正素子、４６は対物レンズ、４７は鏡筒、４８は光ディスク、４９は検出光学系、５０は受光素子をそれぞれ示している。ここで、収差補正素子４５と対物レンズ４６とからなる対物光学系としては、上記第３の実施の形態で説明した本発明の対物光学系が用いられており、収差補正素子４５と対物レンズ４６とは鏡筒４７に固定されて一体駆動できるように構成されている。
【００９３】
図９に示すように、半導体レーザ４０からの出射光束は、ビームスプリッタ４３に入射した後、そのまま当該ビームスプリッタ４３を透過する。また、半導体レーザ４１からの出射光束は、ビームスプリッタ４３に入射した後、光路を折り返されて、当該ビームスプリッタ４３から射出される。
【００９４】
ビームスプリッタ４３からの出射光束は、コリメートレンズ４２によってそれぞれ略平行光束となり、ビームスプリッタ４４へと入射する。ビームスプリッタ４４からの出射光束は、収差補正素子４５と対物レンズ４６とからなる本発明の対物光学系によって光ディスク４８の情報記録面上に集光される。光ディスク４８からの反射光束は、対物レンズ４６によって略平行光束となり、当該略平行光束は、収差補正素子４５を透過し、ビームスプリッタ４４で反射された後、検出光学系４９によって受光素子５０に集光される。
【００９５】
本実施の形態の光ヘッド装置において温度変化があった場合、光源である半導体レーザ４０あるいは４１の発振波長は、温度が１℃上昇するごとに約０．２ｎｍだけ長波長側にシフトする。このとき、対物レンズ４６の内周部は、回折レンズの作用により、ＤＶＤとＣＤの両方に対して良好な集光性能を有し、かつ、温度変化に対する球面収差の変化が小さくなる。また、対物レンズ４６の外周部を透過するＤＶＤ用の光束は、収差補正素子４５に設けられた段差の効果により、温度変化による球面収差の変化が緩和される。
【００９６】
以上のように、本実施の形態の光ヘッド装置においては、収差補正素子４５と対物レンズ４６とからなる対物光学系として上記第３の実施の形態で説明した本発明の対物光学系が用いられていることにより、例えば、ＤＶＤ用、ＣＤ用の両方の光学系において温度変化に対する収差変動を抑えることができるので、１つの対物光学系を用いてディスク厚みの異なる光ディスクの記録再生を良好に行なうことのできる光ヘッド装置を実現することができる。
【００９７】
尚、上記各実施の形態においては、対物レンズあるいは収差補正素子における各領域の境界部分が真円である場合を例に挙げて説明したが、加工上の制約や意図的に非点収差を発生させる等の設計上の理由により楕円にしても構わない。この場合には、楕円の長軸と短軸の平均値を真円の径ととらえればよい。
【００９８】
【発明の効果】
以上説明したように、本発明の光ディスク用の対物レンズあるいは対物光学系によれば、例えば、ＤＶＤ用、ＣＤ用の両方の光学系において用いることができ、かつ、温度変化に対する収差変動の小さい対物レンズあるいは対物光学系を実現することができる。
【００９９】
また、本発明の光ヘッド装置によれば、例えば、ＤＶＤ用、ＣＤ用の両方の光学系において用いることができ、かつ、温度変化に対する収差変動の小さい前記本発明の光ディスク用対物レンズあるいは光ディスク用対物光学系が搭載されていることにより、１つの対物光学系を用いてディスク厚みの異なる光ディスクの記録再生を良好に行なうことのできる光ヘッド装置を実現することができる。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態における光ディスク用の対物レンズを示す概略断面図
【図２】本発明の第１の実施の形態の数値実施例１における対物レンズのＤＶＤに対するＯＰＤ特性図
【図３】本発明の第１の実施の形態の数値実施例１における対物レンズのＤＶＤに対する、光源の波長の整数倍の位相差を無視したＯＰＤ特性図（（ａ）は設計温度３５℃の場合、（ｂ）は温度０℃の場合、（ｃ）は温度７０℃の場合）
【図４】本発明の第１の実施の形態の数値実施例１における対物レンズのＣＤに対する、光源の波長の整数倍の位相差を無視したＯＰＤ特性図（（ａ）は設計温度３５℃の場合、（ｂ）は温度０℃の場合、（ｃ）は温度７０℃の場合）
【図５】本発明の第２の実施の形態における光ディスク用の対物レンズを示す概略断面図
【図６】本発明の第３の実施の形態における光ディスク用の対物光学系を示す概略断面図
【図７】本発明の第３の実施の形態の数値実施例２における収差補正素子の具体的設計例を示す断面図
【図８】本発明の第４の実施の形態における光ヘッド装置を示す概略構成図
【図９】本発明の第５の実施の形態における光ヘッド装置を示す概略構成図
【符号の説明】
１、１２、２１、３５、４６ 対物レンズ
２、１３、２２、３６、４８ 光ディスク
３ 対物レンズの第１の領域
４ 対物レンズの第２の領域
５ 対物レンズの第３の領域
６ 対物レンズの第４の領域
７、８、２６ 段差
９ 対物レンズの出射面
１０ ＤＶＤの入射光束
１１ ＣＤの入射光束
１４ 対物レンズの光軸を含む円状の領域
１５ 対物レンズの円環状の領域
１６、１７、１８ 対物レンズの出射面側に設けられた領域
２０ 収差補正素子
２３ 対物レンズの光軸を含む円状の第１の領域
２４ 対物レンズの円環状の第２の領域
２５ 対物レンズの出射面
２７ 入射光束
３０、４０ ＤＶＤ用半導体レーザ
３１、４１ ＣＤ用半導体レーザ
３２、４２ コリメートレンズ
３３、３４、４３、４４ ビームスプリッタ
３７、４９ 検出光学系
３８、５０ 受光素子
３５ 収差補正素子
４７ 鏡筒[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an objective lens and an objective optical system for an optical disc, and an optical head device using the objective lens and the objective optical system. More specifically, the present invention relates to, for example, an objective lens and an objective optical system for compatibility between a DVD (digital versatile disk) and a CD (compact disk), and an optical head device using the objective lens and the objective optical system.
[0002]
[Prior art]
2. Description of the Related Art In an optical disk apparatus, an optical head optical system including an objective lens having a diffraction-limited light-collecting performance is used for recording or reproduction of an information recording medium such as a DVD (hereinafter referred to as an "optical disk"). By the way, the thickness of the transparent substrate (the thickness from the surface of the transparent substrate to the information recording layer) is 1.2 mm in the case of CD, whereas it is 0.6 mm in the case of DVD, and both thicknesses (hereinafter referred to as “disc thickness”). Is also different. Further, the NA (numerical aperture) of the objective lens and the wavelength of the light source are also different between the CD device and the DVD device. The NA of the CD device is 0.45 and the wavelength of the light source is around 780 nm. 0.6, and the wavelength of the light source is around 660 nm.
[0003]
As described above, the CD device and the DVD device have different standards for the disk thickness of the optical disk used, the NA of the objective lens, and the wavelength of the light source. Therefore, when trying to reproduce a CD using an objective lens optimally designed for a DVD, spherical aberration occurs due to a difference in the disk thickness, and information cannot be read. In order to perform recording / reproduction on optical discs of both standards using one optical disc apparatus, for example, the NA optimally designed for an optical disc having a transparent base material having a thickness of 1.2 mm is set to 0. An optical head device (optical pickup) equipped with both an objective lens of 45 and an objective lens with an NA of 0.6 optimally designed for an optical disk having a transparent substrate with a thickness of 0.6 mm may be used. However, in this case, since a lens switching mechanism is required, the structure of the optical head device is complicated, and it is difficult to reduce the size and cost. Therefore, there is a demand for a method of recording and reproducing information on optical discs of both standards using one objective lens.
[0004]
As a method of reproducing an optical disk having a different disk thickness by using one objective lens, a zone in which two or three regions are divided by a concentric circle having the optical axis substantially at the center, and each region has a different aspherical shape. A method using a split type objective lens has been proposed (for example, refer to Patent Literature 1, Patent Literature 2, and Patent Literature 3). In this case, each of the divided areas of the objective lens is designed such that the aberration is properly corrected with respect to the thickness of the transparent base material of the DVD or CD or the wavelength of the light source. In some cases, it is optimally designed for an intermediate thickness between the thickness of the transparent substrate of DVD and the thickness of the transparent substrate of CD, or an intermediate wavelength between the wavelength of the light source for DVD and the wavelength of the light source for CD. In some cases, the amount of aberration generated when one objective lens is used in an optical system for DVD and an optical system for CD is adjusted using the lens surface.
[0005]
Another method of reproducing an optical disc having a different disc thickness by using one objective lens is to use a phase plate to measure spherical aberration generated when an objective lens optimally designed for DVD is used in an optical system for CD. There is also proposed a method of performing correction by using (for example, see Patent Document 4). In this method, a phase plate having a step which generates a phase difference of an integral multiple of the wavelength of a light beam emitted from a light source for a DVD is used. The aberration can be corrected for the optical system for CD without changing.
[0006]
Still another method for reproducing an optical disk having a different disk thickness using one objective lens is to use a diffractive lens that has a larger change in performance with respect to the wavelength to be used than a refractive lens. There has been proposed a method of using an objective lens in which a refraction lens and a diffraction lens are integrated with each other (for example, see Patent Document 5). If this method is adopted, an optimum light-converging spot is formed on the information recording surface of the DVD when a light source for DVD is used, and an optimal light-converging spot is formed on the information recording surface of CD when a light source for CD is used. A focused spot can be formed.
[0007]
[Patent Document 1] Japanese Patent Application Laid-Open No. 9-184975
[0008]
[Patent Document 2] JP-A-10-55564
[0009]
[Patent Document 3] Japanese Patent Application Laid-Open No. 9-219035
[0010]
[Patent Document 4] JP-A-10-334504
[0011]
[Patent Document 5] JP-A-2000-81566
[0012]
[Problems to be solved by the invention]
The above-mentioned objective lens is generally formed of a resin material. Here, when the temperature of the environment in which the optical head device operates increases, the oscillation wavelength of the semiconductor laser as the light source shifts to the longer wavelength side. Further, the refractive index of the resin material also decreases as the temperature increases. For this reason, when the objective lens is mounted on an apparatus having a large temperature change, the aberration of the lens changes, and there is a problem that good light-collecting performance cannot be obtained.
[0013]
The present invention has been made in order to solve the above-mentioned problems in the prior art, and can be used for recording and reproducing a plurality of types of optical discs having different disc thicknesses, and has an objective for an optical disc which has a small aberration fluctuation with respect to a temperature change. It is an object to provide a lens, an objective optical system, and an optical head device using the same.
[0014]
[Means for Solving the Problems]
In order to achieve the above object, a first configuration of an objective lens for an optical disc according to the present invention includes a first optical disc having a first thickness and a light flux emitted from a first light source that emits light of a first wavelength. The light flux emitted from the second light source that emits light of the second wavelength longer than the first wavelength and is condensed on the information recording surface of the second optical disc is converted into a second light beam thicker than the first optical disc. An objective lens for an optical disk for condensing light on an information recording surface of an optical disk, comprising a single lens made of resin, wherein an entrance surface or an exit surface of the single lens has three or more circles having a center substantially at an optical axis. The circular region including the optical axis is divided into regions, and has a structure in which a diffractive lens is integrated on an aspheric surface. The regions are separated by a step that causes a phase difference with respect to the transmitted light beam. Cut is characterized in that is.
[0015]
According to the first configuration of the objective lens for an optical disk, for example, an objective lens that can be used in both optical systems for DVD and CD and that has small aberration fluctuation with respect to temperature change can be realized.
[0016]
In the first configuration of the objective lens for an optical disk of the present invention, the step is such that a phase difference of a transmitted light beam generated by the step is substantially an integer of the wavelength of the transmitted light beam of the first wavelength. Preferably, the amount is doubled. According to this preferred example, for example, when the optical head device is used for compatibility with a DVD and a CD, if the temperature of the environment in which the optical head device is operated changes, the oscillation wavelength of the semiconductor laser as the light source changes or the objective lens changes. Due to the change in the refractive index of the resin material, the phase difference of the transmitted light flux caused by the step becomes a value deviated from substantially an integral multiple of the design wavelength of the DVD. Then, when the phase difference of the transmitted light beam caused by the step deviates from substantially an integral multiple of the design wavelength of the DVD, spherical aberration occurs, and the change in spherical aberration due to temperature change is reduced by the spherical aberration.
[0017]
In the first configuration of the optical disc objective lens of the present invention, in each of the areas divided by the step, the phase difference between the area in which the phase of the transmitted light flux advances and the area in which the phase of the transmitted light flux advances is the same. Preferably, the transmitted light flux of the first wavelength is at least six times that wavelength. According to this preferred example, a sufficient temperature compensation effect can be obtained in a general optical resin material.
[0018]
In the first configuration of the objective lens for an optical disk of the present invention, when the first wavelength is λ1, it is preferable that λ1 is in a range defined by the following (Equation 4).
[Equation 4]
650 nm <λ1 <680 nm
In the first configuration of the optical disc objective lens of the present invention, it is preferable that the number of the steps is two or more. According to this preferred example, occurrence of higher-order aberrations can be reduced.
[0019]
Further, the second configuration of the objective lens for an optical disc according to the present invention is that the light flux emitted from the first light source that emits the light of the first wavelength is formed on the information recording surface of the first optical disc having the first thickness. A light beam emitted from a second light source that emits light of a second wavelength longer than the first wavelength is focused on an information recording surface of a second optical disk that is thicker than the first optical disk. An optical disc objective lens for converging light thereon, which is a single lens made of resin, and a first surface, which is either an entrance surface or an exit surface, of the single lens is defined by a circle having a center substantially at an optical axis. The circular area divided into two areas and including the optical axis of the first surface has a structure in which a diffractive lens is integrated on an aspheric surface, and the annular area of the first surface is A second surface facing the first surface is a concentric circle having a center substantially at the optical axis. It is further divided into a plurality of regions, each region of the second surface is an aspheric surface, and adjacent regions of the second surface are separated by a step parallel to the optical axis. And
[0020]
According to the second configuration of the objective lens for an optical disk, for example, an objective lens that can be used in both optical systems for DVD and CD and that has small aberration fluctuation with respect to temperature change can be realized.
[0021]
In the second configuration of the objective lens for an optical disk of the present invention, the step is a step that generates a phase difference of substantially an integral multiple of the wavelength of the transmitted light beam of the first wavelength. Is preferred.
[0022]
Further, in the second configuration of the optical disc objective lens of the present invention, in each of the areas divided by the step, the phase difference between the area where the phase of the transmitted light flux advances the most and the area where the phase of the transmitted light flux lags most is the above-mentioned. Preferably, the transmitted light flux of the first wavelength is at least six times that wavelength.
[0023]
In the second configuration of the objective lens for an optical disk of the present invention, when the first wavelength is λ1, it is preferable that λ1 is in a range defined by the following (Equation 5).
[Equation 5]
650 nm <λ1 <680 nm
Further, in the second configuration of the objective lens for an optical disk of the present invention, it is preferable that the number of the steps is two or more.
[0024]
The objective optical system for an optical disc according to the present invention is configured such that a light beam emitted from a first light source that emits light of a first wavelength is collected on an information recording surface of a first optical disc having a first thickness. A light beam emitted from a second light source that emits light and emits light of a second wavelength longer than the first wavelength is placed on an information recording surface of a second optical disc that is thicker than the first optical disc. An optical disc objective optical system for condensing, comprising a resin single lens having a condensing power and an aberration correction element, wherein the first surface, which is either the entrance surface or the exit surface of the single lens, A circular area including the optical axis of the first surface, the circular area including the optical axis of the first surface having a structure in which a diffractive lens is integrated on an aspheric surface; An annular area of the first surface is an aspheric surface, and a second surface facing the first surface A rotationally symmetric aspherical surface, wherein the aberration correction element is formed of a substantially parallel flat plate, and has a step-like step structure on at least one of an entrance surface and an exit surface. It is provided in a portion corresponding to the portion.
[0025]
According to the configuration of the objective optical system for an optical disk, for example, it is possible to use the optical system for both a DVD and a CD, and to realize an objective optical system with small aberration fluctuation with respect to a temperature change.
[0026]
In the configuration of the objective optical system for an optical disk of the present invention, it is preferable that the step is a step that generates a phase difference of substantially an integral multiple of the wavelength of the transmitted light beam of the first wavelength. .
[0027]
In the configuration of the objective optical system for an optical disc according to the present invention, in each of the areas divided by the step, the phase difference between the area where the phase of the transmitted light flux advances the most and the area where the phase lags the most is the first phase difference. It is preferable that the wavelength is 6 times or more of the transmitted light flux of the wavelength.
[0028]
In the configuration of the objective optical system for an optical disc according to the present invention, when the first wavelength is λ1, it is preferable that λ1 is in a range defined by the following (Equation 6).
[Equation 6]
650 nm <λ1 <680 nm
Further, in the configuration of the objective optical system for an optical disk of the present invention, it is preferable that the number of the steps is two or more.
[0029]
The first configuration of the optical head device according to the present invention includes a first light source that emits light of a first wavelength and a second light source that emits light of a second wavelength longer than the first wavelength. A light source, a light collecting means for collecting light beams emitted from the first and second light sources on an information recording surface of an optical disk, and a light receiving means for receiving a transmitted light beam or a reflected light beam from the optical disk. An optical head device, wherein the objective lens for an optical disk of the present invention is used as the light-collecting means.
[0030]
According to the first configuration of the optical head device, for example, the optical disk objective lens of the present invention, which can be used in both optical systems for DVD and CD and has small aberration fluctuation with respect to temperature change, is mounted. By doing so, it is possible to realize an optical head device that can perform good recording and reproduction of optical disks having different disk thicknesses using one objective lens.
[0031]
A second configuration of the optical head device according to the present invention includes a first light source that emits light of a first wavelength and a second light source that emits light of a second wavelength longer than the first wavelength. A light source, an aberration correction element, a light condensing means for condensing light beams emitted from the first and second light sources on an information recording surface of an optical disk, and a light receiving device for receiving a transmitted light beam or a reflected light beam from the optical disk. Means, wherein the objective optical system for an optical disc according to the present invention is used as the aberration correction element and the light condensing means.
[0032]
According to the second configuration of the optical head device, for example, the objective optical system for an optical disk of the present invention, which can be used in both optical systems for DVD and CD and has small aberration variation with respect to temperature change, can be used. By being mounted, it is possible to realize an optical head device that can perform good recording and reproduction of optical disks having different disk thicknesses using one objective optical system.
[0033]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described more specifically with reference to embodiments.
[0034]
[First Embodiment]
FIG. 1 is a schematic sectional view showing an objective lens for an optical disk according to the first embodiment of the present invention.
[0035]
In FIG. 1, reference numeral 1 denotes an objective lens made of a resin single lens, and 2 denotes an optical disk. Examples of the resin in this case include ZONEX (manufactured by Zeon Corporation), Apel (manufactured by Mitsui Petrochemical Co., Ltd.), ARTON (manufactured by JSR Corporation), and the like. In FIG. 1, for convenience, the optical path in the case where the optical disk 2 is a DVD (thickness of the transparent base material: 0.6 mm) is shown in the upper half, and the optical disk 2 is the CD (thickness of the transparent base material: 1.times.) In the lower half. 2 mm). In FIG. 1, the upper half of the optical disk 2 shows a DVD and the lower half shows a CD.
[0036]
As shown in FIG. 1, in the objective lens 1 of the present embodiment, the incident surface is divided into four regions by a circle having the optical axis substantially at the center. The first circular region 3 including the optical axis has a structure in which a diffractive lens is integrated on a rotationally symmetric aspheric surface of a refractive lens defined by the following (Equation 7). The annular second, third, and fourth regions 4, 5, and 6 located outside the first region 3 are each constituted by a rotationally symmetric aspheric surface defined by the following (Formula 7). I have.
[0037]
(Equation 7)

[0038]
However, in the above (Equation 7), h (mm) is the height from the optical axis, and S (mm) is the distance from the aspherical vertex of the point on the aspherical surface at the position where the height from the optical axis is h. Sag), Rm (mm) is the radius of curvature of the aspherical vertex of each m-th region (hereinafter referred to as “vertex radius of curvature”), Km is the conic constant of each m-th region, and Am is each m-th region. The offset amounts Dm, Em, Fm, and Gm on the optical axis of the region represent the fourth-order, sixth-order, eighth-order, and tenth-order aspherical coefficients of the m-th region, respectively. In this specification, not only Dm, Em, Fm, and Gm but also Rm and Km are referred to as "aspheric coefficients".
[0039]
Steps 7 and 8 that generate a phase difference with respect to a transmitted light beam are respectively provided at a boundary between the second region 4 and the third region 5 and a boundary between the third region 5 and the fourth region 6. Is provided.
[0040]
The exit surface 9 of the objective lens 1 is constituted by a rotationally symmetric aspheric surface defined by the following (Equation 8).
[0041]
(Equation 8)

[0042]
Here, in the above (Equation 8), h (mm) is the height from the optical axis, and S (mm) is the distance from the aspherical vertex of a point on the aspherical surface at a height h from the optical axis (h). Sag) and R (mm) are the radii of the vertices of curvature, K is the conic constant, and D, E, F, and G are the fourth-, sixth-, eighth-, and tenth-order aspherical coefficients.
[0043]
When the optical disk 2 is a DVD, the incident light beam 10 is condensed on the information recording surface of the DVD using the first to fourth regions 3 to 6 (entire regions of the objective lens 1) on the incident surface side of the objective lens 1. (See the upper half of FIG. 1).
[0044]
On the other hand, when the optical disc 2 is a CD, only the light flux 11 passing through the first area 3 on the incident surface side of the objective lens 1 is focused on the information recording surface of the CD (see the lower half of FIG. 1). ). In this case, the luminous flux (not shown) transmitted through the area other than the first area 3 has a light amount distribution greatly spread around the image forming point due to the large aberration of the lens in the area, and the light flux is substantially reduced. It does not contribute to upper imaging.
[0045]
That is, in the objective lens 1 of the present embodiment, the first area 3 on the inner periphery on the incident surface side is a diffraction-integrated aspheric lens having good light-collecting performance for both DVD and CD. I have. Further, in the objective lens 1 of the present embodiment, the second to fourth regions 4 to 6 on the incident surface side have an aspheric surface whose aberration has been well corrected for a DVD and a phase difference for a transmitted light beam. Steps 7 and 8 to be generated are provided and connected.
[0046]
The amount of change in the refractive index of a resin material used for an objective lens for an optical disk with respect to a temperature change is generally about −10 × 10 ^-Five / ° C. The oscillation wavelength of a semiconductor laser light source for an optical disk device also changes with a change in temperature, and the amount of change is about 0.2 nm / ° C. Further, the resin material exhibits a wavelength characteristic in which the refractive index decreases as the wavelength of light incident on the resin material increases. Therefore, when the temperature of the environment in which the optical head device is operated rises, the wavelength of the light source shifts to the longer wavelength side, and the refractive index of the resin material increases as the wavelength of the light source increases and the refractive index of the resin material changes due to temperature change. Decrease due to both effects of decline. If the objective lens is a resin lens constituted by a general rotationally symmetric aspherical surface, the tertiary spherical aberration shifts to the negative side due to these effects. In particular, in the case of an objective lens having a numerical aperture (NA) of about 0.6 as used for a DVD or the like, an aberration change of about 1 mλ per 1 ° C., and therefore, 35 mλ for a temperature change of 35 ° C. Spherical aberration occurs.
[0047]
As described above, the first region 3 on the inner surface on the incident surface side of the objective lens 1 of the present embodiment has a structure in which a diffractive lens is integrated on an aspheric surface of a refractive lens. At this time, in the case where both the diffractive lens and the refractive lens have a positive refractive power, the spherical aberration due to the temperature change of the diffractive lens is generated in a direction to reduce that of the refractive lens. That is, when the spherical aberration is shifted to the negative side by the refractive lens, the diffractive lens shifts the spherical aberration to the positive side, and conversely, when the spherical aberration is shifted to the positive side by the refractive lens, the diffractive lens is shifted. Shifts spherical aberration to the negative side. As described above, in the first region 3 on the inner surface on the incident surface side in the objective lens 1 of the present embodiment, the amount of generation of aberration due to a temperature change is smaller than that of a normal aspherical resin lens. . This is true whether the optical disk to be focused is a DVD or a CD.
[0048]
On the other hand, the outer peripheral portion constituted by the second to fourth regions 4 to 6 on the incident surface side of the objective lens 1 of the present embodiment is arranged such that the aspherical surface optimally designed for DVD is positioned with respect to the transmitted light beam. Steps 7 and 8 for generating a phase difference are provided and connected. Light transmitted through these areas is not used for recording or reproducing information on a CD. That is, the second to fourth regions 4 to 6 are rotationally symmetric aspheric surfaces defined by the above (Equation 7), and the boundary between the second region 4 and the third region 5 and the third region 5 By providing steps 7 and 8 for generating a phase difference with respect to a transmitted light beam at the boundary between the first and fourth regions 6, respectively, when light of a wavelength for CD is incident on this region, a large aberration occurs. Is generated, and the light becomes a spot spread on the information recording surface of the CD to such an extent that the light does not substantially contribute to image formation.
[0049]
Steps 7 and 8 provided at the boundary between the second region 4 and the third region 5 and at the boundary between the third region 5 and the fourth region 6 are transmitted light beams generated by the steps 7 and 8. Is desirably an amount that is substantially an integral multiple of the transmitted light flux of the DVD design wavelength. With such a configuration, it is possible to prevent the steps 7, 8 from affecting the aberration at the DVD design wavelength. However, when the temperature of the environment in which the optical head device is operated changes, the steps 7, 8 change due to the change in the oscillation wavelength of the semiconductor laser, which is the light source, and the change in the refractive index of the resin material, which is the material of the objective lens 1. The resulting phase difference of the transmitted light flux deviates from substantially an integral multiple of the design wavelength of the DVD. When the phase difference of the transmitted light flux generated by the steps 7 and 8 deviates from substantially an integral multiple of the design wavelength of the DVD, spherical aberration occurs, and the spherical aberration reduces the change in spherical aberration due to temperature change.
[0050]
In each of the areas (the second area 4, the third area 5, and the fourth area 6) demarcated by the steps 7, 8, the distance between the area in which the phase of the transmitted light flux advances most and the area in which the phase of the transmitted light flux lags most is determined. It is desirable that the phase difference is at least six times the design wavelength of the DVD, whereby a sufficient temperature compensation effect can be obtained.
[0051]
Here, as the design wavelength λ1 of the DVD, a wavelength within the range defined by the following (Equation 9) is used.
[Equation 9]
650 nm <λ1 <680 nm
In the present embodiment, the number of steps is set to 2 (steps 7 and 8), but the number of steps may be 3 or more.
[0052]
Further, in the present embodiment, the entrance surface of the objective lens 1 is divided into four regions by a circle having the optical axis substantially at the center, but the exit surface of the objective lens 1 is set at the optical axis at the center. It may be divided into four regions by a circle. The number of divisions may be three or more.
[0053]
Hereinafter, a specific design of the objective lens for an optical disk according to the present embodiment will be described in more detail by way of preferred numerical examples.
[0054]
(Numerical Example 1)
In this embodiment, the design temperature is 35 ° C., the oscillation wavelength at the design temperature of the light source for DVD is 660 nm, and the oscillation wavelength at the design temperature of the light source for CD is 790 nm. It is assumed that the oscillation wavelength of the semiconductor laser used as the light source changes in a direction that becomes longer by 0.2 nm each time the temperature rises by 1 ° C.
[0055]
The entrance surface of the objective lens 1 has a vertex radius of curvature of each m-th region Rm (mm), a conical constant of each m-th region Km, a fourth-order, a sixth-order, and an eighth-order of each m-th region. Assuming that the aspherical coefficients of the tenth and tenth orders are Dm, Em, Fm, and Gm, and the offset amount on the optical axis of each of the m-th region is Am, the height from the optical axis is h (mm). The distance (sag) S (mm) of a point on the aspherical surface from the aspherical vertex is designed using a rotationally symmetric aspherical surface defined by the above (Equation 7). The exit surface of the objective lens 1 has an apex curvature radius of R (mm), a conic constant of K, a fourth-order, sixth-order, eighth-order, and tenth-order aspherical coefficients of D, E, F, and G, respectively. Using a rotationally symmetric aspherical surface whose distance (sag) S (mm) from the aspherical vertex of a point on the aspherical surface at a height of h (mm) from the optical axis is defined by the above (Equation 8) Designed. The coefficients of each surface are as shown in (Table 1) and (Table 2) below. In the following design data (Table 1), the diffraction lens portion in the first region 3 is expressed by using a high refractive index method. Therefore, the actual processed shape of the first region 3 is a shape obtained by superimposing the relief shape calculated from the data expressed by the high refractive index surface on the aspherical shape expressed by the resin surface.
[Table 1] Surface shape data on incident surface side
First area 3 Area radius 1.05mm
Aspheric coefficient of the first region (high refractive index surface)
R1: 1.3667152
K1: -0.40340044
A1: 0.0
D1: -0.014833817
E1: -0.014183695
F1: 0.0050402632
G1: -0.0020063491
Aspheric coefficient of first area (resin surface)
R1: 1.3667129
K1: -0.40340312
A1: 0.0
D1: -0.014834096
E1: -0.014184697
F1: 0.0050406379
G1: -0.0020064273
Second area 4 Area radius 1.2mm
R2: 1.3952
K2: -0.46563672
A2: -0.00495
D2: -0.0075278349
E2: 0.0002166981
F2: -0.0012772982
G2: -0.00024732574
Third area 5 Area radius 1.3mm
R3: 1.3963
K3: -0.46563672
A3: -0.01003
D3: -0.0075278349
E3: 0.0002166981
F3: -0.0012772982
G3: -0.00024732574
Fourth area 6
R4: 1.3974
K4: -0.46563672
A4: -0.0151
D4: -0.0075278349
E4: 0.0002166981
F4: -0.0012772982
G4: -0.00024732574
[Table 2] Surface shape data on the emission surface side
R: -6.402302
K: -17.72508
D: 0.0094256721
E: -0.0010662053
F: -0.0015622112
G: 0.00056196144
The thickness of the objective lens 1 is 1.245 mm, and the focal length of the light source for DVD at the design temperature is 2.25 mm. The NA of the objective lens 1 is 0.6 in the optical system for DVD and 0.47 in the optical system for CD. The effective diameter on the incident surface side of the objective lens 1 is 2.76 mm for a DVD optical system and 2.095 mm for a CD optical system. In addition, the design temperature and the oscillation wavelength of the light source when the temperature changes by ± 35 ° C., and the refractive index of the resin material of the objective lens 1 and the transparent base material of the optical disk in that state are as shown in Table 3 below. It is.
[0056]
[Table 3]

[0057]
At this time, the step 7 provided at the boundary between the second region 4 and the third region 5 causes the phase difference of the transmitted light beam generated by the step 7 to be substantially equal to that of the light beam having a wavelength of 660 nm. The amount is quadrupled. In addition, the step 8 provided at the boundary between the third region 5 and the fourth region 6 also causes the phase difference of the transmitted light beam generated by the step 8 to be approximately 4 times smaller than the light beam having a wavelength of 660 nm. The amount is doubled. Therefore, in the body lens 1 of the present embodiment, the phase difference between the second region 4 (the region where the phase of the transmitted light beam advances the most) and the fourth region 6 (the region where the phase of the transmitted light beam is the latest) is , For a light beam having a wavelength of 660 nm, is approximately eight times that wavelength.
[0058]
FIG. 2 shows a calculation result of an OPD (Optical path difference) characteristic for the DVD of the objective lens 1 of this embodiment at the design temperature (35 ° C.). In FIG. 2, the vertical axis is the OPD, which is expressed in units of wavelength. The horizontal axis is the diameter. As shown in FIG. 2, at the design temperature (35 ° C.), the OPD for the DVD of the objective lens 1 according to the present embodiment has a step having a step of 4λ due to the steps 7 and 8 that generate a phase difference with respect to the transmitted light beam. It has a shape. Here, if the phase difference that is exactly an integral multiple of the wavelength of the light source is ignored, an OPD characteristic as shown in FIG. 3A can be obtained. Similarly, when the temperature is 0 ° C. and 70 ° C., the OPD characteristics as shown in FIGS. 3B and 3C are obtained. 3 (b) and 3 (c), the broken line indicates the calculation result of the OPD characteristic in the case where the outer peripheral portion of the objective lens 1 is constituted by a single aspherical surface having no step which causes a phase difference. Is shown. As shown in FIGS. 3B and 3C, when there is no step on the outer peripheral portion that causes a phase difference with respect to the transmitted light flux, when the temperature of the environment in which the optical head device is operated changes, Although a large spherical aberration occurs due to a large shift of the OPD, the spherical aberration is reduced to such a level that there is no problem in practical use by providing steps 7 and 8 for generating a phase difference with respect to the transmitted light beam on the outer peripheral portion. It is understood that it is done.
[0059]
FIG. 4 shows a calculation result of the OPD characteristic of the objective lens 1 of the present example with respect to the CD. In FIG. 4, the vertical axis represents OPD in units of wavelength, and W in FIG. 4 represents a wavelength (wavelength). 4A shows a case where the design temperature (35 ° C.), FIG. 4B shows a case where the temperature is 0 ° C., and FIG. 4C shows a case where the temperature is 70 ° C. When the optical disc 2 is a CD, only a light beam transmitted through the first area 3 on the inner periphery of the objective lens 1 is condensed on the information recording surface, so that a phase difference is generated with respect to the transmitted light beam. Although the effects of the steps 7 and 8 are irrelevant, as shown in FIG. 4, it can be seen that the amount of occurrence of aberration with respect to the temperature change is not so large.
[0060]
The following Table 4 shows the results of calculating the wavefront aberration of these OPDs.
[0061]
[Table 4]

[0062]
From the above, it can be seen that according to the objective lens of the present example, the fluctuation of the third-order spherical aberration can be improved within a range of ± 35 ° C. with respect to the design temperature (35 ° C.).
[0063]
[Second embodiment]
FIG. 5 is a schematic sectional view showing an objective lens for an optical disc according to the second embodiment of the present invention.
[0064]
In FIG. 5, reference numeral 12 denotes an objective lens composed of a single lens made of resin, and reference numeral 13 denotes an optical disk. In FIG. 5, for convenience, the optical path when the optical disk 13 is a DVD is shown in the upper half, and the optical path when the optical disk 13 is a CD is shown in the lower half. Also in FIG. 5, as the optical disk 13, a DVD is shown in the upper half and a CD is shown in the lower half.
[0065]
As shown in FIG. 5, the objective lens 12 of the present embodiment has its incident surface divided into two regions by a circle having the optical axis substantially at the center. The circular region 14 including the optical axis has a structure in which a diffractive lens is integrated on a rotationally symmetric aspheric surface of a refractive lens defined by the above (Equation 7). Are corrected satisfactorily. The annular area 15 located outside the circular area 14 is constituted by a rotationally symmetric aspherical surface defined by the above (Equation 7), and is optimally designed for DVD.
[0066]
The exit surface of the objective lens 12 is divided into three regions 16, 17, and 18 by concentric circles having the optical axis substantially at the center. These three regions 16, 17, and 18 are constituted by rotationally symmetric aspheric surfaces defined by the above (Equation 7), which are optimally designed for DVD. Steps parallel to the optical axis are provided at the boundary between the region 16 and the region 17 and at the boundary between the region 17 and the region 18. These steps are provided in the optical path through which light transmitted through the outer peripheral portion (the annular region 15) on the incident surface side of the objective lens 12 passes. Note that these steps are desirably steps that cause a phase difference of substantially an integral multiple of the wavelength for a transmitted light beam of a DVD wavelength. Further, of the regions 16 to 18 divided by these steps, the phase difference between the region in which the phase of the transmitted light beam advances and the region in which the phase of the transmitted light beam is the most delayed is the wavelength of the transmitted light beam for the DVD. It is desirably six times or more. Further, it is desirable that the number of steps is two or more.
[0067]
Even in the case of the above-described configuration, similarly to the case of the first embodiment, the inner peripheral portion (circular region 14) on the incident surface side of the objective lens 12 is formed by the action of the diffraction lens. , It has good light-collecting performance for both DVD and CD, and the change in spherical aberration with temperature change is small. In addition, the DVD light flux transmitted through the outer peripheral portion (the annular region 15) on the incident surface side of the objective lens 12 has spherical aberration due to temperature change due to the effect of a step provided on the exit surface side of the objective lens 12. Changes are mitigated. As a result, the objective lens 12 according to the present embodiment is an objective lens having a small change in spherical aberration with respect to a temperature change as a whole lens.
[0068]
Note that the same effects as in the present embodiment can be obtained even if the configuration of the entrance surface and the exit surface of the objective lens 12 is reversed.
[0069]
[Third Embodiment]
FIG. 6 is a schematic sectional view showing an objective optical system for an optical disc according to the third embodiment of the present invention.
[0070]
In FIG. 6, reference numeral 20 denotes an aberration correction element, 21 denotes an objective lens made of a single lens made of resin, and 22 denotes an optical disk. In FIG. 6, for convenience, the optical path when the optical disk 22 is a DVD is shown in the upper half, and the optical path when the optical disk 22 is a CD is shown in the lower half. 6, the upper half of the optical disk 22 shows a DVD, and the lower half shows a CD.
[0071]
As shown in FIG. 6, the objective lens 21 of the present embodiment has an entrance surface divided into two regions by a circle having the optical axis substantially at the center. The first circular region 23 including the optical axis has a structure in which a diffractive lens is integrated on a rotationally symmetric aspherical surface of a refractive lens defined by the above (Equation 7), whereby the DVD and It has good light-collecting performance for both CDs. Further, the annular second region 24 located outside the circular first region 23 is constituted by a rotationally symmetric aspheric surface defined by the above (Formula 7), and is optimally designed for DVD. .
[0072]
The exit surface 25 of the objective lens 21 is constituted by a rotationally symmetric aspheric surface defined by the above (Equation 8).
[0073]
The aberration correction element 20 is configured by a resin-made substantially parallel flat plate disposed on the incident surface side of the objective lens 21. On the emission surface of the aberration correction element 20 (the surface facing the objective lens 21), a step 26 having a concentric circle with the optical axis substantially at the center is provided at a portion corresponding to the outer peripheral portion of the objective lens 21. In this case, it is desirable that the step 26 is a step that generates a phase difference of substantially an integral multiple of the wavelength of a transmitted light beam having a design wavelength of the DVD. The phase difference between the region where the phase of the transmitted light beam advances and the region where the phase of the transmitted light beam is the slowest among the regions divided by the step 26 is at least 6 times the wavelength of the transmitted light beam at the DVD design wavelength. It is desirable that Further, it is desirable that the number of steps 26 be two or more.
[0074]
An incident light beam 27 from a semiconductor laser as a light source is focused on the information recording surface of the optical disk 22 by the objective lens 21 of the present embodiment. The reflected light beam from the information recording surface of the optical disk 22 is converted into a parallel light beam by the objective lens 22 and then guided to a detection optical system of an optical head (not shown).
[0075]
When performing recording or reproduction on a CD, only a light beam transmitted through the inner peripheral portion (the circular first region 23) of the objective lens 21 is focused on the information recording surface of the CD (FIG. 6 lower half). That is, even when light is incident on the entire area of the objective lens 21, the outer peripheral portion (the annular second area 24) of the objective lens 21 is aberration-corrected for DVD (optimally designed for DVD). Since it is an aspheric surface, a large spherical aberration occurs when used in an optical system for CD. Therefore, a light beam (not shown) transmitted through the outer peripheral portion (the annular second region 24) of the objective lens 21 has a light amount distribution greatly spread around the image forming point, and substantially forms an image. Will no longer contribute. The inner peripheral portion (the circular first region 23) of the objective lens 21 has a configuration in which a diffractive lens is integrated on an aspheric surface of a refractive lens, and thus the first embodiment described above. As in the case of the above, the change of the spherical aberration with respect to the temperature change becomes small.
[0076]
On the other hand, when performing recording or reproduction on a DVD, a light beam transmitted through the entire effective diameter of the objective lens 21 is focused on the information recording surface of the DVD (see the upper half of FIG. 6). That is, the inner peripheral portion (the circular first region 23) of the objective lens 21 has good light-collecting performance even for DVD due to the action of the diffractive lens, and the outer peripheral portion (the circular portion). The annular second region 24) is an aspheric surface that is aberration-corrected for DVD (optimally designed for DVD), so that aberration is well corrected for DVD over the entire area of the objective lens 21. You. Further, the inner peripheral portion (the circular first region 23) of the objective lens 21 has a configuration in which the diffractive lens is integrated on the aspherical surface of the refractive lens, and thus the first embodiment is described. As in the case of 1, the occurrence of spherical aberration due to a temperature change is reduced. Further, in the outer peripheral portion (the annular second region 24) of the objective lens 21, when the temperature of the environment in which the optical head device is operated changes, the oscillation wavelength of the semiconductor laser as a light source changes and the objective lens changes. A change in the refractive index of the resin material 21 causes spherical aberration. The change in the phase difference of the transmitted luminous flux caused by the step 26 provided in the aberration correction element 20 is caused by a substantially integer multiple of the design wavelength of the DVD. In this case, a spherical aberration is generated which reduces the spherical aberration generated at the outer peripheral portion of the objective lens 21.
[0077]
As described above, according to the objective optical system of the present embodiment, for example, in both the optical system for DVD and the optical system for CD, it is possible to suppress the fluctuation of the aberration with respect to the temperature change and exhibit good imaging performance. .
[0078]
Hereinafter, a specific design of the objective optical system for an optical disk according to the present embodiment will be described in more detail with reference to preferred numerical examples.
[0079]
(Numerical example 2)
In this embodiment, the design temperature is 35 ° C., the oscillation wavelength at the design temperature of the light source for DVD is 660 nm, and the oscillation wavelength at the design temperature of the light source for CD is 790 nm. It is assumed that the oscillation wavelength of the semiconductor laser used as the light source changes in a direction that becomes longer by 0.2 nm each time the temperature rises by 1 ° C. Note that the embodiment described here is equivalent to the optical system shown in Numerical Embodiment 1 at the design temperature, and the step 26 is about five times as large as the transmitted light flux at the design wavelength of the DVD. This is a step that causes a phase difference.
[0080]
On the incident surface side of the objective lens 21, the vertex radius of curvature of each m-th region is Rm (mm), the conic constant of each m-th region is Km, the fourth, sixth, and Assuming that the 8th-order and 10th-order aspherical coefficients are Dm, Em, Fm, Gm, and the offset amount on the optical axis of each of the m-th region is Am, the height from the optical axis is h (mm). The distance (sag) S (mm) of the point on the aspherical surface at the point from the aspherical vertex is designed using the rotationally symmetric aspherical surface defined by the above (Equation 7). The exit surface of the objective lens 21 has an apex curvature radius of R (mm), a conic constant of K, and fourth, sixth, eighth, and tenth order aspherical coefficients of D, E, F, and G, respectively. The distance (sag) S (mm) from the aspherical vertex of a point on the aspherical surface at a height of h (mm) from the optical axis is a rotationally symmetric aspherical surface defined by the above (Equation 8). I have. The coefficients of each surface are as shown in (Table 5) and (Table 6) below. In the following design data (Table 5), the diffraction lens portion in the first region 23 is expressed by using a high refractive index method. Therefore, the actual processed shape of the first region 23 is a shape in which the relief shape calculated from the data expressed by the high refractive index surface is superimposed on the aspherical shape expressed by the resin surface.
[Table 5] Surface shape data on the incident side
First area 23 Area radius 1.05mm
Aspheric coefficient of the first region (high refractive index surface)
R1: 1.3667152
K1: -0.40340044
A1: 0.0
D1: -0.014833817
E1: -0.014183695
F1: 0.0050402632
G1: -0.0020063491
Aspheric coefficient of first area (resin surface)
R1: 1.3667129
K1: -0.40340312
A1: 0.0
D1: -0.014834096
E1: -0.014184697
F1: 0.0050406379
G1: -0.0020064273
Second area 24
R2: 1.39525
K2: -0.46563672
A2: -0.00492
D2: -0.0075278349
E2: 0.0002166981
F2: -0.0012772982
G2: -0.00024732574
[Table 6] Surface shape data on the emission surface side
R: -6.402302
K: -17.72508
D: 0.0094256721
E: -0.0010662053
F: -0.0015622112
G: 0.00056196144
The thickness of the objective lens 21 is 1.245 mm, and the focal length of the light source for DVD at the design temperature is 2.25 mm. The NA of the objective lens 21 is 0.6 in the optical system for DVD and 0.47 in the optical system for CD. The effective diameter of the objective lens 21 on the incident surface side is 2.76 mm for the optical system for DVD and 2.095 mm for the optical system for CD.
[0081]
FIG. 7 shows a specific design example of the aberration correction element 20. In the aberration correction element 20 shown in FIG. 7, each of the two steps has a step of 6.125 μm. This is because the phase difference of the transmitted light beam caused by the step is approximately 5 times the wavelength of the light beam having a wavelength of 660 nm. The amount is doubled. Therefore, in the aberration correction element 20 of the present embodiment, the maximum phase difference between the regions divided by the step is approximately 10 times the wavelength of a light beam having a wavelength of 660 nm. Further, the aberration correction element 20 is made of the same resin material as the objective lens 21, and the refractive indices of the objective lens 21 and the transparent base material of the optical disk 22 are the values shown in the above (Table 3) of Numerical Example 1. Is the same as
[0082]
The following (Table 7) shows the calculation results of the wavefront aberration of the objective optical system of this example.
[0083]
[Table 7]

[0084]
From the above, it can be seen that according to the objective optical system of the present embodiment, the fluctuation of the third-order spherical aberration can be improved in a range of ± 35 ° C. with respect to the design temperature.
[0085]
[Fourth Embodiment]
FIG. 8 is a schematic configuration diagram showing an optical head device according to the fourth embodiment of the present invention.
[0086]
In FIG. 8, reference numeral 30 denotes a semiconductor laser that emits light of a DVD wavelength, and reference numeral 31 denotes a semiconductor laser that emits light of a CD wavelength. Reference numeral 32 denotes a collimator lens, reference numerals 33 and 34 denote beam splitters, reference numeral 35 denotes an objective lens, reference numeral 36 denotes an optical disk, reference numeral 37 denotes a detection optical system, and reference numeral 38 denotes a light receiving element. Here, as the objective lens 35, the objective lens of the present invention described in the first or second embodiment is used.
[0087]
As shown in FIG. 8, a light beam emitted from the semiconductor laser 30 enters the beam splitter 33 and then passes through the beam splitter 33 as it is. Further, the light beam emitted from the semiconductor laser 31 enters the beam splitter 33, then returns its optical path, and is emitted from the beam splitter 33.
[0088]
The light beams emitted from the beam splitter 33 are converted into substantially parallel light beams by the collimator lens 32 and enter the beam splitter 34. The light beam emitted from the beam splitter 34 is focused on the information recording surface of the optical disk 36 by the objective lens 35 of the present invention. The light beam reflected from the optical disk 36 is converted into a substantially parallel light beam by the objective lens 35 of the present invention. The substantially parallel light beam is reflected by the beam splitter 34 and then condensed on the light receiving element 38 by the detection optical system 37.
[0089]
When the temperature changes in the optical head device of the present embodiment, the oscillation wavelength of the semiconductor laser 30 or 31 as the light source shifts to the longer wavelength side by about 0.2 nm each time the temperature rises by 1 ° C. At this time, the inner peripheral portion of the objective lens 35 has good light-collecting performance for both DVD and CD due to the action of the diffraction lens, and the change in spherical aberration with respect to temperature change is small. Further, in the DVD light beam transmitted through the outer peripheral portion of the objective lens 35, a change in spherical aberration due to a temperature change is reduced by the effect of a step provided in the objective lens 35.
[0090]
As described above, in the optical head device according to the present embodiment, since the objective lens of the present invention described in the first or second embodiment is used as the objective lens 35, for example, for the DVD Therefore, it is possible to realize an optical head device capable of favorably performing recording and reproduction of optical disks having different disk thicknesses using a single objective lens because aberration fluctuations due to temperature changes can be suppressed in both optical systems for CD. Can be.
[0091]
[Fifth Embodiment]
FIG. 9 is a schematic configuration diagram showing an optical head device according to the fifth embodiment of the present invention.
[0092]
In FIG. 9, reference numeral 40 denotes a semiconductor laser that emits light having a wavelength for DVD, and 41 denotes a semiconductor laser that emits light having a wavelength for CD. Reference numeral 42 denotes a collimating lens, 43 and 44 denote beam splitters, 45 denotes an aberration correction element, 46 denotes an objective lens, 47 denotes a lens barrel, 48 denotes an optical disk, 49 denotes a detection optical system, and 50 denotes a light receiving element. Here, as the objective optical system including the aberration correction element 45 and the objective lens 46, the objective optical system of the present invention described in the third embodiment is used, and the aberration correction element 45 and the objective lens 46 are used. Is fixed to the lens barrel 47 so as to be able to be driven integrally.
[0093]
As shown in FIG. 9, the light beam emitted from the semiconductor laser 40 enters the beam splitter 43 and then passes through the beam splitter 43 as it is. The light beam emitted from the semiconductor laser 41 is incident on the beam splitter 43, then the optical path is turned back, and emitted from the beam splitter 43.
[0094]
The light beams emitted from the beam splitter 43 are converted into substantially parallel light beams by the collimating lens 42, and are incident on the beam splitter 44. The light beam emitted from the beam splitter 44 is condensed on the information recording surface of the optical disk 48 by the objective optical system of the present invention including the aberration correction element 45 and the objective lens 46. The light beam reflected from the optical disk 48 is converted into a substantially parallel light beam by the objective lens 46. The substantially parallel light beam passes through the aberration correction element 45, is reflected by the beam splitter 44, and is collected by the detection optical system 49 on the light receiving element 50. Is lighted.
[0095]
When there is a temperature change in the optical head device of the present embodiment, the oscillation wavelength of the semiconductor laser 40 or 41, which is the light source, shifts to the longer wavelength side by about 0.2 nm each time the temperature rises by 1 ° C. At this time, the inner peripheral portion of the objective lens 46 has good light-collecting performance for both DVD and CD due to the action of the diffractive lens, and changes in spherical aberration with respect to temperature change are small. Further, in the DVD light beam transmitted through the outer peripheral portion of the objective lens 46, a change in spherical aberration due to a temperature change is reduced by the effect of the step provided in the aberration correction element 45.
[0096]
As described above, in the optical head device of the present embodiment, the objective optical system of the present invention described in the third embodiment is used as the objective optical system including the aberration correction element 45 and the objective lens 46. Thus, for example, aberration fluctuations due to temperature changes can be suppressed in both the optical system for DVD and the optical system for CD, so that recording and reproduction of optical disks having different disk thicknesses can be favorably performed using one objective optical system. An optical head device that can be realized.
[0097]
In each of the above embodiments, the case where the boundary between the respective regions in the objective lens or the aberration correction element is a perfect circle has been described as an example. However, processing constraints or intentional astigmatism may occur. An ellipse may be used for design reasons such as making the shape elliptical. In this case, the average value of the major axis and the minor axis of the ellipse may be taken as the diameter of the perfect circle.
[0098]
【The invention's effect】
As described above, according to the objective lens or objective optical system for an optical disk of the present invention, it can be used in, for example, both optical systems for DVDs and CDs, and has a small aberration variation with temperature change. A lens or an objective optical system can be realized.
[0099]
Further, according to the optical head device of the present invention, for example, it can be used in both optical systems for DVD and CD, and has a small variation in aberration with respect to temperature change. Since the objective optical system is mounted, it is possible to realize an optical head device that can perform good recording and reproduction of optical disks having different disk thicknesses using one objective optical system.
[Brief description of the drawings]
FIG. 1 is a schematic sectional view showing an objective lens for an optical disc according to a first embodiment of the present invention.
FIG. 2 is an OPD characteristic diagram of an objective lens for a DVD in Numerical Data Example 1 according to the first embodiment of the present invention;
3A and 3B are OPD characteristics diagrams for a DVD of an objective lens in Numerical Example 1 according to the first embodiment of the present invention in which a phase difference of an integral multiple of the wavelength of a light source is ignored ((a) shows a design temperature of 35 ° C.). In the case, (b) is at a temperature of 0 ° C., (c) is at a temperature of 70 ° C.)
FIG. 4 is an OPD characteristic diagram ignoring a phase difference of an integral multiple of the wavelength of a light source with respect to a CD of an objective lens in Numerical Example 1 of the first embodiment of the present invention ((a) shows a design temperature of 35 ° C.). In this case, (b) is at a temperature of 0 ° C., (c) is at a temperature of 70 ° C.)
FIG. 5 is a schematic sectional view showing an objective lens for an optical disk according to a second embodiment of the present invention.
FIG. 6 is a schematic sectional view showing an objective optical system for an optical disc according to a third embodiment of the present invention.
FIG. 7 is a cross-sectional view showing a specific design example of an aberration correction element in Numerical Example 2 of the third embodiment of the present invention.
FIG. 8 is a schematic configuration diagram showing an optical head device according to a fourth embodiment of the present invention.
FIG. 9 is a schematic configuration diagram showing an optical head device according to a fifth embodiment of the present invention.
[Explanation of symbols]
1, 12, 21, 35, 46 Objective lens
2, 13, 22, 36, 48 Optical discs
3 First area of objective lens
4 Second area of objective lens
5. Third area of objective lens
6. Fourth area of objective lens
7, 8, 26 steps
9 Emission surface of objective lens
10 Incident light flux of DVD
11 CD incident light flux
14 circular area including the optical axis of the objective lens
15 Toroidal area of objective lens
16, 17, 18 Areas provided on the exit surface side of the objective lens
20 Aberration correction element
23 first circular region including optical axis of objective lens
24 annular second area of objective lens
25 Exit surface of objective lens
27 Incident light flux
30, 40 DVD semiconductor laser
31, 41 Semiconductor laser for CD
32, 42 Collimating lens
33, 34, 43, 44 Beam splitter
37, 49 Detection optical system
38, 50 light receiving element
35 Aberration correction element
47 lens barrel

Claims (17)

A light beam emitted from a first light source that emits light of a first wavelength is condensed on an information recording surface of a first optical disk having a first thickness, and a second wavelength longer than the first wavelength. An optical disc objective lens for condensing an emitted light beam from a second light source that emits light on an information recording surface of a second optical disc that is thicker than the first optical disc;
It consists of a single lens made of resin,
The entrance surface or the exit surface of the single lens is divided into three or more regions by a circle having the optical axis substantially at the center,
The circular region including the optical axis has a structure in which the diffraction lens is integrated on the aspheric surface,
Each of the annular regions is aspherical,
The optical disk objective lens according to claim 1, wherein the adjacent annular regions are separated by a step that causes a phase difference with respect to a transmitted light beam. 2. The optical disc objective lens according to claim 1, wherein the step is an amount by which a phase difference of a transmitted light beam caused by the step is substantially an integral multiple of a wavelength of the transmitted light beam of the first wavelength. The phase difference between the region where the phase of the transmitted light beam advances most and the region where the phase of the transmitted light beam advances the most among the regions separated by the step is 6 times or more the wavelength of the transmitted light beam of the first wavelength. The objective lens for an optical disk according to claim 1. 2. The objective lens for an optical disk according to claim 1, wherein, when the first wavelength is λ1, λ1 is in a range defined by the following (Equation 1).
[Equation 1]
650 nm <λ1 <680 nm The optical disc objective lens according to claim 1, wherein the number of the steps is two or more. A light beam emitted from a first light source that emits light of a first wavelength is condensed on an information recording surface of a first optical disk having a first thickness, and a second wavelength longer than the first wavelength. An optical disc objective lens for condensing an emitted light beam from a second light source that emits light on an information recording surface of a second optical disc that is thicker than the first optical disc;
It consists of a single lens made of resin,
The first surface, which is either the entrance surface or the exit surface of the single lens, is divided into two regions by a circle having the optical axis substantially at the center,
The circular area including the optical axis of the first surface has a structure in which a diffractive lens is integrated on an aspheric surface.
The annular area of the first surface is aspherical,
A second surface facing the first surface is divided into a plurality of regions by concentric circles having the optical axis substantially at the center,
Each region of the second surface is aspherical,
An object lens for an optical disk, wherein adjacent regions of the second surface are separated by a step parallel to an optical axis. 7. The optical disc objective lens according to claim 6, wherein the step is a step that causes a phase difference of substantially an integral multiple of the wavelength of the transmitted light beam of the first wavelength. The phase difference between the region where the phase of the transmitted light beam advances most and the region where the phase of the transmitted light beam advances the most among the regions separated by the step is 6 times or more the wavelength of the transmitted light beam of the first wavelength. The objective lens for an optical disk according to claim 6. The objective lens for an optical disk according to claim 6, wherein, when the first wavelength is λ1, λ1 is in a range defined by the following (Equation 2).
[Equation 2]
650 nm <λ1 <680 nm The objective lens for an optical disk according to claim 6, wherein the number of the steps is two or more. A light beam emitted from a first light source that emits light of a first wavelength is condensed on an information recording surface of a first optical disk having a first thickness, and a second wavelength longer than the first wavelength. An optical disc objective optical system for condensing a light flux emitted from a second light source that emits light on an information recording surface of a second optical disc thicker than the first optical disc,
Equipped with a resin single lens having condensing power and an aberration correction element,
The first surface, which is either the entrance surface or the exit surface of the single lens, is divided into two regions by a circle having the optical axis substantially at the center,
The circular area including the optical axis of the first surface has a structure in which a diffractive lens is integrated on an aspheric surface.
The annular area of the first surface is aspherical,
A second surface facing the first surface is a rotationally symmetric aspheric surface;
The aberration correction element is formed of a substantially parallel flat plate, and has a step-like step structure on at least one of an entrance surface and an exit surface,
The objective optical system for an optical disk, wherein the step is provided at a portion corresponding to an outer peripheral portion of the single lens. The objective optical system for an optical disk according to claim 11, wherein the step is a step that generates a phase difference of substantially an integral multiple of the wavelength of the transmitted light beam of the first wavelength. The phase difference between the region where the phase of the transmitted light beam advances most and the region where the phase of the transmitted light beam advances the most among the regions separated by the step is 6 times or more the wavelength of the transmitted light beam of the first wavelength. An objective optical system for an optical disk according to claim 11. The objective optical system for an optical disk according to claim 11, wherein, when the first wavelength is λ1, λ1 is in a range defined by the following (Equation 3).
[Equation 3]
650 nm <λ1 <680 nm The objective optical system for an optical disk according to claim 11, wherein the number of the steps is two or more. A first light source for emitting light of a first wavelength, a second light source for emitting light of a second wavelength longer than the first wavelength, and light beams emitted from the first and second light sources An optical head device comprising: a light-collecting unit that collects light on an information recording surface of an optical disk; and a light-receiving unit that receives a transmitted light beam or a reflected light beam from the optical disk,
11. An optical head device using the objective lens for an optical disk according to claim 1 as the light collecting means. A first light source that emits light of a first wavelength, a second light source that emits light of a second wavelength longer than the first wavelength, an aberration correction element, and the first and second light sources. An optical head device comprising: a light condensing unit that converges a light beam emitted from a light source on an information recording surface of an optical disc;
An optical head device using the objective optical system for an optical disk according to any one of claims 11 to 15 as the aberration correction element and the light condensing unit.

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