JP2005017525A - Objective lens for optical disk - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an objective lens for an optical disk having a numerical aperture of ≥0.75, superior in on-axis aberration, off-axis aberration and eccentric aberration between surfaces and constituted of a bi-aspherical single lens. <P>SOLUTION: The objective lens 11 for an optical disk is the bi-aspherical single lens having a numerical aperture of ≥0.75, and the center thickness (t) and the focal distance of the objective lens 11 satisfy a expression of t>(1+E)f, and an angle formed by a normal line N to the 1st surface 1 and an optical axis at the highest incident point of light L0 is controlled to be equal to or smaller than one of 55°, 56° and 57°, and E is a number ≥0. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００８１】
図３には、開口数と入射角の関係を図示して示す。αは５３．２５１６度である。図中の符号◆は実際の設計値を示し、実線は回帰式による値を示す。
【００８２】
また、図には、レンズの厚さが１．５ｍｍで、硝材の屈折率が１．７５のレンズに関する他の例のデータを示す。図中の符号▲は、他の例に対する実際の設計値を示し、破線は回帰式による値を示す。
【００８３】
いずれの場合も、回帰式は、実際の設計値を良く反映していることが見て取れる。なお、他の多数のレンズ設計データでも同様な結果が得られており、前記回帰式（６）は、一般的な式として十分な精度を有している。
【００８４】
ここで、開口数が０．８５より低い場合に対する角度の条件を求める。まず、開口数が下がると、当然レンズ最外周での面の傾き（第１面への入射角）は緩くなる。さらに、このために前記条件（１）〜（３）に対する制限も緩くなるので、例えば製造公差も厳しくなくなる。
【００８５】
しかしながら、開口数が０．８５より低い場合も、開口数が０．８５の場合と同様に、第１面への入射角が増加するとともに収差特性が劣化するという一般的な特性を有する。
【００８６】
したがって、開口数が０．８５より小さいレンズを開口数が０．８５と同様に、好ましくは５７度、より好ましくは５６度、さらにより好ましくは５５度以下の条件で設計すれば良好なレンズが出来る。さらに、開口数が低いことによる上記した有利さを加味して、回帰式が示す角度だけ設計の目標値を小さくすれば公差と性能を向上させることが出来る。
【００８７】
このことにより、開口数が０．８５より低い場合は、次の条件式で決まる範囲に、第１面への入射角θを設定することで、さらによい結果が得られる。
【００８８】
θ＜α−（０．８５−ＮＡ）／０．１５×７．１（度） ・・・（７）
ここに、角度αは、好ましくは５７度、より好ましくは５６度、さらにより好ましくは５５度である。
【００８９】
ところで、上記したレンズは、偏芯公差は確保されたもの、条件（４）と（５）を未考慮であるため、色収差特性が確保できる十分条件を満たしていない。次に、色収差特性に関して詳しい説明をする。
【００９０】
ここで、レンズの中心厚さ（軸上厚さともいう）と焦点距離が、次の式を満たす。
【００９１】
ｔ＞（１＋Ｅ）ｆ
ここで、ｆは焦点距離、ｔはレンズの中心厚さであり、Ｅは、０以上の数であり、好ましくは０、より好ましくは０．１、さらにより好ましくは０．２である。
【００９２】
上記関係を持つ場合、条件（４）と条件（５）の満足度が高まる。
【００９３】
まず条件（４）の波長誤差がある場合の各波長での最良像面の収差増加が小さいことに関しては、レンズの中心厚さが厚いほうがレンズ第１面（入射面）の半径を比較的大きくできるからである。より詳細には、第１面の曲率半径が大きくなると、レンズの外側の端部を通る光線の、レンズへの入射角θ（レンズ面の法線と光線のなす角度）が小さくなり、これにより非線形現象としての屈折の効果が小さくなり、その結果として波長が変化した場合の球面収差の増加が少なくなるからである。
【００９４】
図４は、レンズの中心厚さと、波長誤差（５ｎｍ）による残留収差の関係である。残留収差は球面収差である。この図は、ＮＡが０．８５で、焦点距離が２．５ｍｍのレンズを多数設計して描いた。硝材は、オハラ製のＬＡＭ７０である。またレンズ設計では、偏芯公差を比較的大きめに取る設計を採用している。
【００９５】
図４によれば、レンズの厚さが、焦点距離より薄くなると、０．０４λ以上と、大きな収差が発生することがわかる。また、厚さが焦点距離の１．２倍の３ｍｍ以下で収差の増加が大きいこともわかる。
【００９６】
次に、条件（５）の波長拡がりがある場合の収差増加に関しては、波長拡がりがある場合は、その拡がりに中心波長の最良像面を観測面とした場合、他の波長では、前述の球面収差に加えて、焦点誤差が発生する。実際には、球面収差に比べて焦点誤差の影響の方が大きいが、特に、波長が０．４５μｍ以下の場合は、ガラスの屈折率の分散が大きくなるため、焦点誤差の影響が非常に大きくなる。
【００９７】
この焦点誤差は、波長が変化した場合の、レンズのバックフォーカス距離の変化に起因する。レンズのバックフォーカス距離ｆｂは、近軸近似による光線追跡式で求めることが出来る。それが、Ｒ１、ｔ、ｎと次の関係式である。
【００９８】
ｆｂ＝ｆ（１−ｔ（ｎー１）／ｎ／Ｒ１）
ガラスの分散に応じて、ｎを変化させた場合の、ｆｂの値の差が、焦点誤差となる。
【００９９】
図５は、焦点距離が２ｍｍで屈折率が１．７５のレンズにおいて、ガラスの屈折率が１．７４８６に変化した場合の、ｆｂの変化量を示している。ｆｂの変化量は、軸上色収差である。また、この屈折率の変化は、アッベ数が４５程度のガラスを、４００ｎｍ付近の波長で用いた場合の、約５ｎｍの波長変化時の屈折率変化に相当する。レンズ形状は平凸レンズであり、Ｒ１は１．５ｍｍである。現実のレンズは、平凸レンズではなく両面が球面とされている。より正確には非球面であるが、ｆ、ｆｂ等の近軸諸量は、頂点の半径で決まるので球面レンズとして問題はない。しかし、ｆｂの変化は、焦点距離を保ってＲ１とＲ２を変化させる、レンズのベンディングにあまり影響されず平凸レンズの場合と非常に近い結果になるので、図５により判断することで問題ない。図によれば、軸上色収差は、レンズの厚さに比例して小さくなる。したがって、レンズの厚さは出来るだけ厚いことが望まれる。
【０１００】
上記をまとめると、レンズの第１面での最大光線の入射角とレンズの厚さがこれらの条件を満たすと、前記条件（１）〜（３）、すなわち、軸上収差特性、軸外収差特性、偏芯公差による収差増加、さらに前記条件（４）、（５）、すなわち波長誤差による面収差と色収差が小さいレンズの条件を同時に満足することが出来る。
【０１０１】
さらに補足すると、この非球面レンズは、光軸に対して回転対称なレンズ（共軸光学系）であっても、方向により僅かに非球面形状を変化させたトーリックレンズ（ｔｏｒｉｃ ｌｅｎｓ）のような形状であっても良い。後者の場合も、最大高さの光線が通る各点で、前記した範囲に入っている必要があるのは言うまでもない。
【０１０２】
以下、本発明に係る光ディスク用対物レンズの実施例を示す。
【０１０３】
実施例では、次のような多項式を用いて非球面を表す。
【０１０４】
Ｚ＝Ｃｈ^２／（１＋（１−（１＋Ｋ）Ｃ^２ｈ^２）^０．５）＋Ａ_４ｈ^４＋Ａ_６ｈ^６＋Ａ_８ｈ^８＋Ａ_１０ｈ^１０＋Ａ_１２ｈ^１２＋Ａ_１４ｈ^１４
ここで、Ｚは面の頂点からの距離、ｈは光軸からの高さ、Ｋはコーニック定数、Ｃは曲率（＝１／Ｒ）、Ａ_４〜Ａ_１４は４次から１４次の非球面係数である。たとえば、Ａ_４は、ｈの４乗の係数に相当する。
【０１０５】
＜実施例１＞
図６は、実施例１の対物レンズの断面図である。
【０１０６】
対物レンズ１１に入射した光束Ｌは、第１面１と第２面２で屈折し、光ディスク２１の第３面３と透過層を透過して信号記録面に集光される。
【０１０７】
レンズ仕様は、表２の通りである。
【０１０８】
【表２】

【０１０９】
レンズの設計値は、表３の通りである。なお、半径及び厚さの単位はｍｍである。以下でも同様である。
【０１１０】
【表３】

【０１１１】
第１面の非球面係数は、表４の通りである。
【０１１２】
【表４】

【０１１３】
第２面の非球面係数は、表５の通りである。
【０１１４】
【表５】

【０１１５】
このレンズの第１面での最大高さの光線の入射角は５３．２５度である。このレンズは、条件（１）と（２）を略満足するアプラナートであり、条件（３）に僅かに誤差が残る。
【０１１６】
波面収差については、軸上の波面収差は０．００２λと小さく、実用上は無収差と言える値である。軸外０．５度の入射光線に対する波面収差は、０．０２３λと、良好な特性を有している。さらに、製造公差で重要な面間の偏芯に関しては、偏芯が３μｍの時、波面収差０．０３６μｍと非常に良好な値を有している。
【０１１７】
図７は縦収差図であり、図８は正弦条件不満足量を示す図であり、図９は非点収差図である。
【０１１８】
レンズの厚さは焦点距離の１．３７５倍である。このレンズの硝材は屈折率を固定して設計してあるが、波長が５ｎｍ変化した場合に相当する屈折率変化として、屈折率が１．８４８６となった場合の、最良像面における収差は０．０１λと小さな値に押さえられている。また、軸上色収差の量は、２．１７μｍであり低く押さえられている。
【０１１９】
＜実施例２＞
図１０は、実施例２の対物レンズの断面図である。
【０１２０】
対物レンズ１１に入射した光束Ｌは、第１面１と第２面２で屈折し、光ディスク２１の第３面３と透過層を透過して信号記録面に集光される。
【０１２１】
レンズ仕様は、表６の通りである。
【０１２２】
【表６】

【０１２３】
レンズの設計値は、表７の通りである。
【０１２４】
【表７】

【０１２５】
第１面の非球面係数は、表８の通りである。
【０１２６】
【表８】

【０１２７】
第２面の非球面係数は、表９の通りである。
【０１２８】
【表９】

【０１２９】
このレンズの第１面での最高高さの光線の入射角は、５１．４１度である。開口数０．８に対する条件（７）による角度は５２．６３度であるので、この条件を満足している。
【０１３０】
このレンズは、条件（１）と（２）を略満足したアプラナートであり、条件（３）に僅かに誤差が残るが、軸上での波面収差は、０．００１λと非常に小さく、実用上は無収差と言える。
【０１３１】
入射角が０．５度の軸外収差は、波面収差が０．０１３λと良好な特性を有している。また、製造公差で重要となる面間の偏芯に関しては、偏芯が３μｍの時、波面収差が０．０２３λと非常に良好な特性を有している。
【０１３２】
図１１は縦収差図であり、図１２は正弦条件不満足量を示す図であり、図１３は非点収差図である。
【０１３３】
レンズの厚さは焦点距離の１．４２９倍である。このレンズの硝材は屈折率を固定して設計してあるが、波長が５ｎｍ変化した場合に相当する屈折率変化として、屈折率が１．７４８６となった場合の、最良像面における収差は０．０１λと小さな値に押さえられている。また、軸上色収差の量は、２．１０μｍであり低く押さえられている。
【０１３４】
＜実施例３＞
図１４は、実施例３の対物レンズの断面図である。
【０１３５】
対物レンズ１１に入射した光束Ｌは、第１面１と第２面２で屈折し、光ディスク２１の第３面３と透過層を透過して信号記録面に集光される。
【０１３６】
レンズ仕様は、表１０の通りである。
【０１３７】
【表１０】

【０１３８】
レンズの設計値は、表１１の通りである。
【０１３９】
【表１１】

【０１４０】
第１面の非球面係数は、表１２の通りである。
【０１４１】
【表１２】

【０１４２】
第２面の非球面係数は、表１３の通りである。
【０１４３】
【表１３】

【０１４４】
各硝材の屈折率は表１４の通りである。
【０１４５】
【表１４】

【０１４６】
このレンズの第１面での最高高さの光線の入射角は、５５．０度である。
【０１４７】
このレンズの特性は、ほぼ条件（１）を満足していて、（２）は多少の不満足を残し、その分実施例１のレンズよりも、偏芯時の収差増加を抑えたレンズとなっている、そして、条件（３）には、僅かに誤差の残るアプラナートに非常に近いレンズである。
【０１４８】
軸上での波面収差は、０．００６λと非常に小さく、実用は無収差と言える値である。軸外０．５度の入射光線に対する波面収差は、０．０６９λと良好な特性を示している。さらに、製造公差で重要な面間の偏芯に関しては、偏芯が５μｍの時に波面収差０．０３４λと、非常に良好な値を有している。
【０１４９】
図１５は縦収差図であり、図１６は正弦条件不満足量を示す図であり、図１７は非点収差図である。
【０１５０】
レンズの厚さは焦点距離の１．４１１倍である。波長が５ｎｍ変化して４１０ｎｍになった場合の、最良像面における収差は０．０２９λと小さな値に押さえられている。また、軸上色収差の量は、２．２１μｍであり低く押さえられている。
【０１５１】
なお、本実施の形態では、光ディスク用対物レンズについて具体的数値を用い説明したが、本発明はこれらの数値に限定されない。本発明は、本発明を逸脱しない範囲で種種の光ディスク用対物レンズに対して適用できる。
【０１５２】
あえて数値の具体例を挙げると、本実施の形態においては、光ディスクには、例えば範囲０．０１〜０．３ｍｍの厚さの透過層を有するものを用いることができる。また、対物レンズには、例えばＮＢＦ１のような光学ガラスを用い、例えば１．５〜２．０の範囲の屈折率を有するものを用いることができる。
【０１５３】
【発明の効果】
上述のように、本発明によると、開口数が０．７５以上で、軸上収差、軸外収差と面間の偏芯収差にすぐれた両面非球面の単レンズによる光ディスクの対物レンズを提供することができる。
【図面の簡単な説明】
【図１】レンズの幾何学的な関係を示す図である。
【図２】最大高さの光線の第１面への入射角度と収差特性の関係を示す図である。
【図３】実際の設計値と回帰式による値を比較して示す図である。
【図４】レンズの中心厚さと、波長誤差（５ｎｍ）による残留収差の関係を示す図である。
【図５】焦点距離が２ｍｍで屈折率が１．７５のレンズにおいて、ガラスの屈折率が１．７４８６に変化した場合の、ｆｂの変化量を示している。
【図６】実施例１の対物レンズの断面図である。
【図７】実施例１の対物レンズの縦収差図である。
【図８】実施例１の対物レンズの正弦条件不満足量を示す図である。
【図９】実施例１の対物レンズの非点収差図である。
【図１０】実施例２の対物レンズの断面図である。
【図１１】実施例２の対物レンズの縦収差図である。
【図１２】実施例２の対物レンズの正弦条件不満足量を示す図である。
【図１３】実施例２の対物レンズの非点収差図である。
【図１４】実施例３の対物レンズの断面図である。
【図１５】実施例３の対物レンズの縦収差図である。
【図１６】実施例３の対物レンズの正弦条件不満足量を示す図である。
【図１７】実施例３の対物レンズの非点収差図である。
【符号の説明】
１ 第１面
２ 第２面
１１ 対物レンズ
２１ 光ディスク[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an objective lens for an optical disc having a high numerical aperture (NA) that realizes a large-capacity optical disc.
[0002]
[Prior art]
Conventionally, a CD disk is read or written with a laser beam having a wavelength of about 780 nm using an objective lens having a numerical aperture (NA) of 0.45 to 0.5. In addition, a DVD disk is read or written with a laser beam having a wavelength of about 650 nm using an objective lens having a numerical aperture of about 0.6.
[0003]
By the way, in order to increase the capacity of the optical disk, development of a next-generation optical disk pickup system using a light source having a shorter wavelength and a lens having a higher numerical aperture has been advanced.
[0004]
As a laser having a shorter wavelength, a so-called blue laser having a wavelength of about 400 nm is considered.
[0005]
For example, the following system has been reported as the objective lens having a high numerical aperture.
[0006]
(A) Jpn. J. et al. Appl. Phys. Vol. 39 (2000) pp. 978-979 M.I. Itonaga et al. “Optical Disk System Using High-Numerical Aperture Single Objective Lens and Blue LD”.
(B) Jpn. J. et al. Appl. Phys. Vol. 39 (2000) pp. 937-942. Ichimura et al.
“Optical Disk Recording Using a GaN Blue-Violet Laser Diode”.
Here, (A) reports a system using a single lens with a numerical aperture of 0.7, and (B) reports a system using a two-group lens with a numerical aperture of 0.85.
[0007]
Another feature of these is that the thickness of the transmission layer of the disk is reduced from 1.2 mm for CD or 0.6 mm for DVD in order to counter the decrease in system margin due to high NA. The thickness of the transmission layer is 0.12 mm according to (A) and 0.1 mm according to (B). Although it depends on how the margin of the system is distributed, it is desired that the transmission layer is thinner than about 0.3 mm.
[0008]
Although the system using the two-group lens of (B) has a larger numerical aperture than that of (A), it requires an assembly process and requires two lenses, so it is inferior in mass production and high in cost. Become.
[0009]
Therefore, an optical disk objective lens using a single lens having a numerical aperture of 0.7 or more is desired for the next generation system.
[0010]
Japanese Patent Laid-Open No. 4-163510 discloses an objective lens using a single lens having a numerical aperture of about 0.6 to 0.8.
[0011]
[Problems to be solved by the invention]
It is conventionally known that a lens having a high numerical aperture can be designed. For example, “Research on Aspherical Aplanato Lenses with a Large Aperture Ratio” (Shotaro Yoshida, Tohoku University Research Institute for Scientific Research, March 1958) describes in detail how to design a double-sided aspherical lens with a high numerical aperture. It is written.
[0012]
However, a lens having a high numerical aperture cannot be manufactured simply by saying that the design is possible. In order to actually manufacture such a lens, it is necessary to have a design that ensures manufacturing tolerances. Furthermore, in order to reduce the influence when the wavelength of the light source fluctuates or when there is a width in the wavelength, the lens needs to be less affected by chromatic aberration.
[0013]
Here, in the case of a double-sided aspheric lens, the most severe and important manufacturing tolerance is the eccentricity between the surfaces (eccentricity between the surfaces). Therefore, it is necessary to satisfy the design tolerance of the objective lens represented by the on-axis aberration, which is an aberration in the case of normal incidence to the objective lens, and the off-axis aberration, which is an aberration in the case of oblique incidence, and manufacturing tolerances at the same time. .
[0014]
However, it is difficult to achieve both lens design performance and manufacturing tolerance, especially when the numerical aperture is higher than 0.75.
[0015]
In fact, in such a double-sided aspheric lens, the off-axis aberration is deteriorated as the numerical aperture is increased even when designed without considering the manufacturing tolerance described above, and becomes worse when the manufacturing tolerance is taken into consideration. That is, in order to ensure a large eccentricity tolerance, it is necessary to sacrifice on-axis aberrations and off-axis aberrations.
[0016]
On-axis aberrations are only slightly degraded even when considering eccentricity tolerances, but off-axis aberrations can be manufactured for lenses with high numerical apertures that exceed 0.6. If manufacturing tolerances of the order of microns are secured, it will be considerably sacrificed.
[0017]
With respect to chromatic aberration, priority is given to the fact that the lens itself can be manufactured. Therefore, it is necessary to obtain a lens shape having as good a chromatic aberration characteristic as possible while satisfying manufacturing tolerances.
[0018]
For the reasons described above, a search for a shape of a double-sided aspheric lens with good performance has been made conventionally, and various documents have been reported. JP-A-5-241069 and JP-A-4-163510 are examples thereof.
[0019]
Japanese Patent Application Laid-Open No. 4-163510 describes a shape range of a lens having good performance. However, this document does not mention securing an eccentricity tolerance. The lens of Example 2 having a numerical aperture of more than 0.75 (specification with a wavelength of 532 nm and a numerical aperture of 0.8) has a problem that a large aberration occurs even with a slight decentration. There is no description about chromatic aberration.
[0020]
Furthermore, the range shown by these prior documents is quite wide, and there is a problem that it is not always possible to actually design a good lens within these ranges.
[0021]
The present invention has been proposed in view of the above-described problems, and has a numerical aperture of 0.75 or more, excellent on-axis aberration, off-axis aberration and decentering aberration between surfaces, and excellent chromatic aberration characteristics. An object of the present invention is to provide an objective lens for an optical disk using a single lens having both aspheric surfaces.
[0022]
[Means for Solving the Invention]
In order to solve the above-described problem, the objective lens for an optical disk according to the present invention is a single lens having both surfaces aspherical and a numerical aperture NA of 0.75 or more, and a light beam having a maximum height is incident thereon. The angle formed between the normal line of the first surface and the optical axis is less than or equal to a predetermined angle. The predetermined angle is preferably 57 degrees, more preferably 56 degrees, and even more preferably 55 degrees.
[0023]
In addition, the objective lens for an optical disk according to the present invention is a single lens having both surfaces aspheric and a numerical aperture NA of 0.75 or more, and the normal of the first surface at the point where the light beam having the maximum height is incident. And the angle θ formed by the optical axis satisfies the following formula.
[0024]
θ <α− (0.85−NA) /0.15×7.1 (degrees)
Here, α is preferably 57 degrees, more preferably 56 degrees, and even more preferably 55 degrees.
[0025]
Preferably, the axial thickness t of the lens and the focal length f satisfy the following formula.
[0026]
t> (1 + E) f
Here, E is a number of 0 or more, preferably 0, more preferably 0.1, and even more preferably 0.2.
[0027]
Further preferably, the objective lens for an optical disc according to the present invention has an imaging magnification of zero. That is, it is preferable that the objective lens collects parallel light when it is manufactured at least without error and the wavelength of the light source matches the reference wavelength.
[0028]
Preferably, the objective lens for an optical disc according to the present invention is designed in conformity with a light source having a wavelength of 450 nm or less.
[0029]
The present invention has good characteristics for DVD discs, transmission layers thinner than CD discs, especially for optical discs having a thickness of 0.4 mm or less.
[0030]
In the present invention, the focal length f is preferably 10 mm or less, and more preferably 3.5 mm or less.
[0031]
That is, the size (diameter) φ of the light beam is given by the following equation and depends on the numerical aperture (NA) and the focal length f.
[0032]
φ = 2 × NA × f
When the focal length is 10 mm and NA is 0.75, φ = 15 mm. This diameter can be said to be large compared to the fact that many optical pickup devices use a light flux of about φ <5 mm. Therefore, it is desirable that the focal length is 10 mm or less. Furthermore, if φ = 5 mm, NA = 0.75 and f = 3.33 mm, so that the focal length is more preferably 3.5 mm or less.
[0033]
Further, the focal length is preferably 0 or more, and more preferably 0.2 mm or more.
[0034]
In other words, the working distance depends on the thickness of the optical disk, and becomes large for a thin disk. A system using a thin disk and using a very small working distance with a very short lens with a short focal length is conceivable. For example, if the disk has a front reading structure, the lens can be designed even if the focal length is 0.1 mm. Therefore, the lower limit of the focal length is sufficient if f> 0. However, in reality, no means for producing a lens that is too small has been established at this time. Considering this point, it can be said that f> 0.2 mm is the current lower limit.
[0035]
Preferably, the upper limit of the lens thickness t is determined so that the working distance dw defined by the following formula is positive.
[0036]
dw = fb−d / n ′
Here, d is the thickness of the optical disc, and n ′ is the refractive index of the optical disc. fb is defined by the following equation. R1 is defined by the above formula.
[0037]
fb = f (1-t (n-1) / n / R1)
That is, as the lens becomes thicker, the working distance becomes shorter. However, in order to establish a lens, the working distance needs to be finite. Therefore, the upper limit of the lens thickness is in a range where the working distance is a finite value. This range is determined by the focal length, thickness, and disc thickness of the lens.
[0038]
The range of the lens thickness can be set to, for example, 1.5 mm or more and 3.5 mm or less.
[0039]
The present invention can be applied to an optical pickup device including the above-described objective lens for an optical disk. Preferably, the optical pickup device records or reproduces an information signal by collecting and irradiating a light beam along a track of the optical disk using the objective lens for the optical disk. Preferably, the imaging magnification of the optical pickup device is zero.
[0040]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of an objective lens for an optical disc according to the present invention will be described in detail with reference to the drawings.
[0041]
First, prior to the description of the conditional expressions satisfied by the objective lens for an optical disk according to the present embodiment, basic balance of on-axis aberration characteristics, off-axis aberration characteristics, and eccentricity tolerance regarding the design of the lens of the present embodiment will be described To do. Here, the eccentricity tolerance is defined as an increase in wavefront aberration when there is an eccentricity.
[0042]
In the present embodiment, it is required to balance the following three conditions in order to ensure on-axis aberration, off-axis aberration, and eccentricity tolerance.
[0043]
(1) The spherical aberration of the lens is corrected to ensure axial aberration.
[0044]
(2) The lens satisfies the sine condition to ensure off-axis aberrations.
[0045]
(3) In order to ensure eccentricity tolerance, the second surface alone satisfies the sine condition.
[0046]
In addition, in order to prevent deterioration of chromatic aberration characteristics, it is required to satisfy the following conditions.
[0047]
(4) The increase in aberration of the best image plane at each wavelength when there is a wavelength error is small.
[0048]
This is called spherical aberration due to wavelength error.
[0049]
(5) In order to suppress an increase in aberration when the light source has a wavelength spread, it is required that the change in the focal position due to the wavelength change be small. Wavelength broadening occurs when the laser is multimoded by applying high frequency superposition to improve the noise characteristics of the semiconductor laser. Here, the fact that the change in the focal length is small due to the wavelength change means that the axial chromatic aberration is required to be small.
[0050]
In the following, a basic form of a lens that secures on-axis aberration and off-axis aberration will be described in detail, and then a lens system pair with good chromatic aberration characteristics will be described.
[0051]
The double-sided aspherical lens can simultaneously satisfy the two conditions (1) and (2) for ensuring on-axis aberration and off-axis aberration. A lens that satisfies the conditions (1) and (2) at the same time is called an aplanat.
[0052]
However, generally, if the conditions (1) and (2) are satisfied, the condition (3) for securing the eccentricity tolerance cannot be satisfied.
[0053]
On the other hand, when the conditions (2) and (3) are satisfied, the entire lens satisfies the sine condition, and the second surface also satisfies the sine condition. The condition is satisfied.
[0054]
In the present embodiment, the conditions (1) and (2) for securing the on-axis aberration and the off-axis aberration and the condition (3) for securing the eccentricity tolerance are balanced and almost satisfied. By satisfying the satisfaction of the condition (3), it is possible to secure an eccentricity tolerance that enables the production of a lens while ensuring on-axis aberrations and off-axis aberrations.
[0055]
According to the above-mentioned “Research on Aspherical Aperato Lenses with a Large Aperture Ratio” (Shotaro Yoshida, Report of the Institute of Scientific Research, Tohoku University, March 1958) It has been clarified that when the radius is changed by bending, a lens satisfying the conditions (1) and (2) at the same time can be obtained in the range of a combination of considerably wide vertex radii.
[0056]
Furthermore, according to Yasuhiro Tanaka “Aplanatic Single Lens Design and Application to Disc Optical System”, Optics 27, 12 (1998) p720, lenses that are resistant to decentering between surfaces must satisfy condition (3). It is shown.
[0057]
Here, among the designs of the aspherical lenses that satisfy the conditions (1) and (2), if there is a lens that satisfies the condition (3), it can be said that the lens is strong against eccentricity tolerance. However, as described above, these cannot be completely satisfied at the same time. This is because the design freedom of the lens is only two aspheric surfaces with respect to the three conditions, and there are two design freedoms.
[0058]
Furthermore, according to the analysis of the inventor of the present application, it has been found that the larger the numerical aperture, the greater the deviation from completeness for the conditions (1) to (3).
[0059]
In fact, if the conventional numerical aperture lens for DVD discs is 0.6 or the numerical aperture for CD discs is 0.45, the numerical aperture is so low that the apex radius can be set in a fairly wide range. Even if it is changed, the increase in aberration is small, and a balance between the on-axis aberration and the off-axis aberration can be easily obtained. That is, regardless of the radius, the eccentricity tolerance can be increased if the off-axis aberration or the on-axis aberration is slightly sacrificed.
[0060]
On the other hand, when the numerical aperture is increased and the wavelength is shortened, the aberration increases in inverse proportion to the wavelength, so there is no design margin. Therefore, it has been necessary to determine the shape (paraxial shape) more strictly for such a lens.
[0061]
It is also possible to slightly change the aspherical shape to increase the eccentricity tolerance. In this case, deterioration of on-axis aberration or off-axis aberration is unavoidable, but a practical lens having a sufficiently large manufacturing tolerance is obtained. This is an important point.
[0062]
In other words, it can be said that the design is performed with a balance to ensure the eccentricity tolerance by appropriately degrading the on-axis aberration and the off-axis aberration. In other words, it can be said that the degree of satisfaction of the above three conditions (1) to (3) is an appropriate work.
[0063]
When searching for an aspherical shape in this way, an angle formed by the normal of the surface at the maximum ray height of the first surface and the optical axis (hereinafter simply referred to as an incident angle) is a predetermined condition. If the expression is not satisfied, an eccentricity tolerance, off-axis aberration, or on-axis aberration increases, and a design that balances the aberrations cannot be achieved. This will be described in detail below.
[0064]
The inventor of the present application designs a large number of lenses that are substantially completely satisfying the conditions (1) and (2) and satisfy the condition (3) as much as possible. The lens thickness and the refractive index of the glass material of the lens were variously examined. As a result, it was found that the incident angle of the light beam having the maximum height to the first surface of the lens dominates the relationship between the on-axis aberration, the off-axis aberration, and the aberration at the time of decentering. The design wavelength of the lens is preferably 450 nm or less, and specifically 405 nm.
[0065]
FIG. 1 is a diagram illustrating a geometric relationship in a lens.
[0066]
The light ray L0 having the maximum height incident on the first surface 1 of the objective lens 11 parallel to the optical axis and the normal N of the first surface at the point where this light ray is incident form an incident angle θ.
[0067]
FIG. 2 is a diagram illustrating a relationship between an incident angle of the light beam having the maximum height to the first surface and aberration characteristics. A in FIG. 2 is an off-axis aberration with respect to an oblique incident light beam of 0.5 degrees, and increases as the incident angle of the first surface increases. In the figure, symbol ♦ represents the refractive index of the glass material of 1.55, symbol ◇ 1.65, symbol Δ 1.75, symbol ◯ 1.8, and symbol □ 1.85.
[0068]
B in the figure is the aberration when the decentering between the surfaces is 3 μm. In the figure, the symbol ▪ represents the refractive index of the glass material of 1.55, the symbol x is 1.65, the symbol Δ is 1.75, the symbol □ is 1.8, and the symbol ♦ is 1.85.
[0069]
According to the figure, the increase in aberrations varies somewhat depending on lens design specifications such as lens focal length, lens thickness, and refractive index of lens glass material, and individual design differences such as the approximation method of the aspheric coefficient. However, it can be said that it is substantially linear with respect to the incident angle of the first surface. Note that the axial aberration is corrected well in any case, and the aberration is 0.006λ or less.
[0070]
This relationship is a general relationship. That is, even if the refractive index of the glass and the thickness of the lens are different, or even if the radius at the apex of the first surface is different, the same aberration characteristics can be obtained if the angles are equal.
[0071]
Here, in order to obtain a lens having good decentration tolerance and off-axis aberration, the aberration at the time of decentering 3 μm is 0.04λ or less, and the off-axis aberration with respect to 0.5 ° oblique incidence is 0.03λ or less. Based on the lens shape, the above conditions (1) to (3) need to be appropriately designed.
[0072]
Here, as described above, the conditions (1) to (3) are balanced so that off-axis aberrations or on-axis aberrations are sacrificed to some extent instead of ensuring eccentricity tolerance, for example. That is.
[0073]
As described above, the lens according to the present embodiment is an aperat that substantially satisfies the conditions (1) and (2), and the on-axis aberration and off-axis aberration are almost ideally corrected. Since the aberration correction at the time is slightly insufficient, such a proposition is performed.
[0074]
According to such a standard, in a lens having a numerical aperture NA of 0.85, the incident angle of the light beam having the maximum height to the first surface is preferably 57 degrees, more preferably 56 degrees, and even more preferably 55 degrees. It is necessary to be smaller. In addition, the change of the shape by the appropriation of said conditions (1)-(3) is slight.
[0075]
When the numerical aperture is lower than 0.85, the increase in aberration with respect to the error is small. Similarly, it is preferably 57 degrees, more preferably 56 degrees, and even more preferably 55 degrees or less. That is, it is possible to provide a good lens for the conditions (1) to (3).
[0076]
By the way, when the lens of this embodiment is manufactured by molding with a mold, the incident angle is directly related to the difficulty of processing the mold. Therefore, it is desirable that the incident angle be as small as possible.
[0077]
Further, in such a molded lens, there is molding shrinkage caused by a molding process at a high temperature between the mold and the molded product, and the shape of the molded product is slightly different from the mold. Therefore, when the numerical aperture is lower than 0.85, it is more convenient in manufacturing to reduce the incident angle according to the numerical aperture.
[0078]
Therefore, when comparing the design of a large number of lenses, when the numerical aperture decreases, the incident angle θ of the first surface has the following relationship with respect to the angle α when the numerical aperture is 0.85. I found it. Α is a value obtained by actual design.
[0079]
θ = α− (0.85-NA) /0.15×7.1 (degrees) (6)
Table 1 shows, as an example, the relationship between the numerical aperture and the incident angle for a lens having the specifications of Example 1 described later. The equation (6) is a regression equation calculated using Table 1.
[0080]
[Table 1]

[0081]
FIG. 3 illustrates the relationship between the numerical aperture and the incident angle. α is 53.2516 degrees. The symbol ◆ in the figure indicates the actual design value, and the solid line indicates the value obtained from the regression equation.
[0082]
The figure also shows data of another example relating to a lens having a lens thickness of 1.5 mm and a glass material having a refractive index of 1.75. A symbol ▲ in the figure indicates an actual design value for another example, and a broken line indicates a value based on a regression equation.
[0083]
In any case, it can be seen that the regression equation well reflects the actual design value. Similar results are obtained with many other lens design data, and the regression equation (6) has sufficient accuracy as a general equation.
[0084]
Here, the angle condition for the case where the numerical aperture is lower than 0.85 is obtained. First, when the numerical aperture decreases, the inclination of the surface (incident angle to the first surface) at the outermost periphery of the lens naturally becomes gentle. Further, because of this, the restrictions on the conditions (1) to (3) are relaxed, so that, for example, manufacturing tolerances are not severe.
[0085]
However, when the numerical aperture is lower than 0.85, as in the case where the numerical aperture is 0.85, there is a general characteristic that the incident angle to the first surface increases and the aberration characteristic deteriorates.
[0086]
Therefore, if a lens having a numerical aperture smaller than 0.85 is designed under the condition that the numerical aperture is 0.85, preferably 57 degrees, more preferably 56 degrees, and even more preferably 55 degrees or less, a good lens can be obtained. I can do it. Furthermore, in consideration of the above-mentioned advantages due to the low numerical aperture, tolerance and performance can be improved by reducing the design target value by the angle indicated by the regression equation.
[0087]
Accordingly, when the numerical aperture is lower than 0.85, a better result can be obtained by setting the incident angle θ to the first surface within a range determined by the following conditional expression.
[0088]
θ <α− (0.85-NA) /0.15×7.1 (degrees) (7)
Here, the angle α is preferably 57 degrees, more preferably 56 degrees, and even more preferably 55 degrees.
[0089]
By the way, the lens described above does not satisfy a sufficient condition for ensuring chromatic aberration characteristics because the eccentricity tolerance is ensured and the conditions (4) and (5) are not considered. Next, the chromatic aberration characteristics will be described in detail.
[0090]
Here, the center thickness of the lens (also referred to as the axial thickness) and the focal length satisfy the following expression.
[0091]
t> (1 + E) f
Here, f is the focal length, t is the center thickness of the lens, and E is a number of 0 or more, preferably 0, more preferably 0.1, and even more preferably 0.2.
[0092]
When the above relationship is satisfied, the satisfaction level of the condition (4) and the condition (5) is increased.
[0093]
First, regarding the small increase in aberration of the best image plane at each wavelength when there is a wavelength error in the condition (4), the radius of the first lens surface (incident surface) is relatively large when the center thickness of the lens is thick. Because it can. More specifically, as the radius of curvature of the first surface increases, the incident angle θ (the angle formed between the normal of the lens surface and the light beam) of the light beam passing through the outer edge of the lens decreases. This is because the effect of refraction as a non-linear phenomenon is reduced, and as a result, an increase in spherical aberration is reduced when the wavelength is changed.
[0094]
FIG. 4 shows the relationship between the center thickness of the lens and the residual aberration due to the wavelength error (5 nm). Residual aberration is spherical aberration. In this figure, a large number of lenses having an NA of 0.85 and a focal length of 2.5 mm are designed and drawn. The glass material is OHARA LAM70. The lens design employs a design with a relatively large eccentricity tolerance.
[0095]
According to FIG. 4, it can be seen that when the lens thickness is thinner than the focal length, a large aberration of 0.04λ or more occurs. It can also be seen that the increase in aberration is large when the thickness is 3 mm or less, which is 1.2 times the focal length.
[0096]
Next, regarding the increase in aberration when there is a wavelength broadening of the condition (5), if there is a wavelength broadening, the best image plane of the central wavelength is used as the observation plane for the broadening, and the spherical surface described above at other wavelengths. In addition to aberrations, focus errors occur. Actually, the influence of the focus error is larger than that of the spherical aberration. In particular, when the wavelength is 0.45 μm or less, the dispersion of the refractive index of the glass becomes large, so the influence of the focus error is very large. Become.
[0097]
This focus error is caused by a change in the back focus distance of the lens when the wavelength is changed. The back focus distance fb of the lens can be obtained by a ray tracing formula based on paraxial approximation. That is R1, t, n and the following relational expression.
[0098]
fb = f (1−t (n−1) / n / R1)
The difference in the value of fb when n is changed according to the dispersion of the glass becomes the focus error.
[0099]
FIG. 5 shows the amount of change in fb when the refractive index of glass is changed to 1.7486 in a lens having a focal length of 2 mm and a refractive index of 1.75. The amount of change in fb is axial chromatic aberration. This change in refractive index corresponds to a change in refractive index when a wavelength of about 5 nm is used when glass having an Abbe number of about 45 is used at a wavelength near 400 nm. The lens shape is a plano-convex lens, and R1 is 1.5 mm. The actual lens is not a plano-convex lens, but both surfaces are spherical. More precisely, although it is an aspherical surface, the paraxial quantities such as f and fb are determined by the radius of the apex, so there is no problem as a spherical lens. However, the change in fb changes R1 and R2 while maintaining the focal length, and the result is very similar to that of the plano-convex lens without much influence from the lens bending. According to the figure, the axial chromatic aberration decreases in proportion to the lens thickness. Therefore, it is desirable that the thickness of the lens is as thick as possible.
[0100]
In summary, when the incident angle of the maximum light ray on the first surface of the lens and the thickness of the lens satisfy these conditions, the above conditions (1) to (3), that is, the on-axis aberration characteristic, the off-axis aberration, Aberration increase due to characteristics and decentration tolerance, and the above conditions (4) and (5), that is, the conditions of a lens with small surface aberration and chromatic aberration due to wavelength error can be satisfied simultaneously.
[0101]
Furthermore, even if this aspherical lens is a rotationally symmetric lens (coaxial optical system) with respect to the optical axis, it is like a toric lens whose aspherical shape is slightly changed depending on the direction. It may be a shape. Needless to say, the latter case also needs to be within the above-described range at each point where the light beam having the maximum height passes.
[0102]
Examples of the objective lens for optical disc according to the present invention will be described below.
[0103]
In the embodiment, an aspherical surface is represented using the following polynomial.
[0104]
Z = Ch²/ (1+ (1- (1 + K) C²h²)^0.5) + A₄h⁴+ A₆h⁶+ A₈h⁸+ A₁₀h¹⁰+ A₁₂h¹²+ A₁₄h¹⁴
Here, Z is the distance from the vertex of the surface, h is the height from the optical axis, K is the conic constant, C is the curvature (= 1 / R), A₄~ A₁₄Is a fourth to fourteenth aspheric coefficient. For example, A₄Corresponds to the fourth power of h.
[0105]
<Example 1>
6 is a cross-sectional view of the objective lens of Example 1. FIG.
[0106]
The light beam L incident on the objective lens 11 is refracted by the first surface 1 and the second surface 2, passes through the third surface 3 and the transmission layer of the optical disk 21, and is condensed on the signal recording surface.
[0107]
The lens specifications are as shown in Table 2.
[0108]
[Table 2]

[0109]
Table 3 shows the lens design values. The unit of radius and thickness is mm. The same applies to the following.
[0110]
[Table 3]

[0111]
Table 4 shows the aspheric coefficients of the first surface.
[0112]
[Table 4]

[0113]
Table 5 shows the aspherical coefficients of the second surface.
[0114]
[Table 5]

[0115]
The incident angle of the light beam having the maximum height on the first surface of the lens is 53.25 degrees. This lens is an aplanat that substantially satisfies the conditions (1) and (2), and a slight error remains in the condition (3).
[0116]
As for wavefront aberration, the on-axis wavefront aberration is as small as 0.002λ, which is practically no aberration. The wavefront aberration for an incident light beam with an off-axis angle of 0.5 degrees is 0.023λ, which is a good characteristic. Further, regarding the eccentricity between the planes, which is important in terms of manufacturing tolerance, when the eccentricity is 3 μm, the wavefront aberration has a very good value of 0.036 μm.
[0117]
FIG. 7 is a longitudinal aberration diagram, FIG. 8 is a diagram showing an unsatisfactory sine condition, and FIG. 9 is an astigmatism diagram.
[0118]
The lens thickness is 1.375 times the focal length. The glass material of this lens is designed with a fixed refractive index. However, when the refractive index is 1.8486 as a refractive index change corresponding to a wavelength change of 5 nm, the aberration in the best image plane is 0. It is suppressed to a small value of .01λ. The amount of axial chromatic aberration is 2.17 μm, which is kept low.
[0119]
<Example 2>
FIG. 10 is a cross-sectional view of the objective lens according to the second embodiment.
[0120]
The light beam L incident on the objective lens 11 is refracted by the first surface 1 and the second surface 2, passes through the third surface 3 and the transmission layer of the optical disk 21, and is condensed on the signal recording surface.
[0121]
The lens specifications are as shown in Table 6.
[0122]
[Table 6]

[0123]
Table 7 shows the lens design values.
[0124]
[Table 7]

[0125]
Table 8 shows the aspherical coefficients of the first surface.
[0126]
[Table 8]

[0127]
The aspherical coefficients of the second surface are as shown in Table 9.
[0128]
[Table 9]

[0129]
The incident angle of the highest light beam on the first surface of this lens is 51.41 degrees. Since the angle according to the condition (7) for the numerical aperture of 0.8 is 52.63 degrees, this condition is satisfied.
[0130]
This lens is an aplanat that substantially satisfies the conditions (1) and (2), and a slight error remains in the condition (3). It can be said that there is no aberration.
[0131]
Off-axis aberrations with an incident angle of 0.5 degrees have a good wavefront aberration of 0.013λ. Further, regarding the decentering between the planes, which is important for manufacturing tolerance, when the decentering is 3 μm, the wavefront aberration is 0.023λ, which has a very good characteristic.
[0132]
11 is a longitudinal aberration diagram, FIG. 12 is a diagram showing an unsatisfactory sine condition, and FIG. 13 is an astigmatism diagram.
[0133]
The lens thickness is 1.429 times the focal length. The glass material of this lens is designed with a fixed refractive index, but the aberration at the best image plane is 0 when the refractive index is 1.7486 as a refractive index change corresponding to a change in wavelength of 5 nm. It is suppressed to a small value of .01λ. The amount of axial chromatic aberration is 2.10 μm, which is kept low.
[0134]
<Example 3>
FIG. 14 is a sectional view of the objective lens of Example 3.
[0135]
The light beam L incident on the objective lens 11 is refracted by the first surface 1 and the second surface 2, passes through the third surface 3 and the transmission layer of the optical disk 21, and is condensed on the signal recording surface.
[0136]
The lens specifications are as shown in Table 10.
[0137]
[Table 10]

[0138]
Table 11 shows the lens design values.
[0139]
[Table 11]

[0140]
Table 12 shows the aspheric coefficients of the first surface.
[0141]
[Table 12]

[0142]
The aspheric coefficient of the second surface is as shown in Table 13.
[0143]
[Table 13]

[0144]
The refractive index of each glass material is as shown in Table 14.
[0145]
[Table 14]

[0146]
The incident angle of the highest light beam on the first surface of this lens is 55.0 degrees.
[0147]
The characteristics of this lens almost satisfied the condition (1), and (2) left some dissatisfaction, and the lens in which the increase in aberrations at the time of decentering was suppressed compared with the lens of Example 1 correspondingly. In condition (3), the lens is very close to an aplanat with a slight error.
[0148]
The wavefront aberration on the axis is as very small as 0.006λ, and it can be said that there is no aberration in practical use. The wavefront aberration for an incident light beam with an off-axis angle of 0.5 degrees is 0.069λ, which is a good characteristic. Further, the decentering between the planes, which is important in terms of manufacturing tolerance, has a very good value of wavefront aberration of 0.034λ when the decentering is 5 μm.
[0149]
15 is a longitudinal aberration diagram, FIG. 16 is a diagram showing an unsatisfactory sine condition, and FIG. 17 is an astigmatism diagram.
[0150]
The lens thickness is 1.411 times the focal length. When the wavelength is changed to 5 nm by 410 nm, the aberration on the best image plane is suppressed to a small value of 0.029λ. The amount of axial chromatic aberration is 2.21 μm, which is kept low.
[0151]
In the present embodiment, the optical disk objective lens has been described using specific numerical values, but the present invention is not limited to these numerical values. The present invention can be applied to various types of objective lenses for optical discs without departing from the present invention.
[0152]
As a specific example of numerical values, in the present embodiment, an optical disk having a transmission layer with a thickness in the range of 0.01 to 0.3 mm can be used. For the objective lens, for example, an optical glass such as NBF1, for example, having a refractive index in the range of 1.5 to 2.0 can be used.
[0153]
【The invention's effect】
As described above, according to the present invention, there is provided an optical disk objective lens having a double-sided aspherical single lens having a numerical aperture of 0.75 or more and excellent in on-axis aberration, off-axis aberration and decentering aberration between surfaces. be able to.
[Brief description of the drawings]
FIG. 1 is a diagram showing a geometric relationship of lenses.
FIG. 2 is a diagram illustrating a relationship between an incident angle of a light beam having a maximum height to a first surface and aberration characteristics.
FIG. 3 is a diagram showing a comparison between an actual design value and a value based on a regression equation.
FIG. 4 is a diagram showing a relationship between a lens center thickness and a residual aberration due to a wavelength error (5 nm).
FIG. 5 shows the amount of change in fb when the refractive index of glass is changed to 1.7486 in a lens having a focal length of 2 mm and a refractive index of 1.75.
6 is a sectional view of the objective lens according to Example 1. FIG.
7 is a longitudinal aberration diagram of the objective lens according to Example 1. FIG.
FIG. 8 is a diagram illustrating an unsatisfactory amount of the sine condition of the objective lens according to Example 1;
9 is an astigmatism diagram of the objective lens according to Example 1. FIG.
10 is a cross-sectional view of an objective lens according to Example 2. FIG.
FIG. 11 is a longitudinal aberration diagram of the objective lens according to Example 2;
12 is a graph showing an unsatisfactory amount of the sine condition of the objective lens according to Example 2. FIG.
FIG. 13 is an astigmatism diagram of the objective lens according to Example 2;
14 is a cross-sectional view of an objective lens according to Example 3. FIG.
15 is a longitudinal aberration diagram of the objective lens according to Example 3. FIG.
FIG. 16 is a diagram illustrating a sine condition unsatisfactory amount of the objective lens according to Example 3;
FIG. 17 is an astigmatism diagram of the objective lens according to Example 3;
[Explanation of symbols]
1 First side
2 Second side
11 Objective lens
21 Optical disc

Claims (5)

A single lens having both surfaces aspherical and having a numerical aperture NA of 0.75 or more, and the angle formed by the normal of the first surface and the optical axis at the point where the light beam having the maximum height is incident is 57 degrees or less. An optical disk objective lens. The angle θ between the normal of the first surface and the optical axis at the point of incidence of the maximum height of the light beam is a single lens with both surfaces being aspherical and having a numerical aperture NA of 0.75 or more. An objective lens for an optical disc, characterized in that:
θ <α− (0.85−NA) /0.15×7.1 (degrees)
Here, α is 57 degrees. The objective lens for an optical disk according to claim 1 or 2, wherein the center thickness t and the focal length f satisfy the following expression.
t> (1 + E) f
Here, E is a number of 0 or more. The objective lens for an optical disk according to any one of claims 1 to 3, wherein an imaging magnification is 0. 5. The objective lens for an optical disk according to claim 1, wherein the wavelength of light from a light source incident on the objective lens for an optical disk is 450 nm or less.

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