JP2004251924A - Beam expander and optical head using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a beam expander capable of correcting large chromatic aberration caused in a high NA objective in a short wavelength region, and correcting spherical aberration caused at the focus of the objective, and to provide an optical head using the same. <P>SOLUTION: In the beam expander where a 1st lens group 10 having negative power and a 2nd lens group 20 having positive power are arranged, the lens groups 10 and 20 are doublets obtained by sticking a convex lens and a concave lens, and at least either the lens group 10 or 20 can move in an optical axis direction. Assuming that the Abbe number of the convex lens of the lens group 10 is ν11, the Abbe number of the concave lens thereof is ν12, the Abbe number of the convex lens of the lens group 20 is ν21, the Abbe number of the concave lens thereof is ν22, the focal distance of the lens group 10 is f1 and the focal distance of the lens group 20 is f2, they satisfy relation; 10≤¾ν11-ν12¾, 10≤¾ν21-ν22¾ and 1.0<-f2/f1≤2.0. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００４６】
表１中、曲率半径（ｍｍ）を示したｒｉ（ｉ＝１〜８）面は、図１に示した各面であり、Ｗは使用波長（設計波長）、Ｗ２はシフト波長（光源のパワー変化による波長シフト時の波長）、Ｄは出射側の平行光束の直径である。屈折率については下記の通りである。
【００４７】
ｎ１１：第１レンズ１の使用波長（Ｗ）に対する屈折率
ｎ１２：第２レンズ２の使用波長（Ｗ）に対する屈折率
ｎ２１：第３レンズ３の使用波長（Ｗ）に対する屈折率
ｎ２２：第４レンズ４の使用波長（Ｗ）に対する屈折率
ｎ´１１：第１レンズ１のシフト波長（Ｗ２）に対する屈折率
ｎ´１２：第２レンズ２のシフト波長（Ｗ２）に対する屈折率
ｎ´２１：第３レンズ３のシフト波長（Ｗ２）に対する屈折率
ｎ´２２：第４レンズ４のシフト波長（Ｗ２）に対する屈折率
表２に本実施例の諸数値、及び測定結果を以下の表２に示す。
【００４８】
【表２】

【００４９】
表２中、ｄ１〜ｄ４は各レンズのレンズ厚（ｍｍ）であり、その位置は図１に示している。Ｌ１（ｍｍ）は図１に示したレンズ間隔（第２レンズ２と第３レンズ３との間の空気間隔）であり、Ｌ２（ｍｍ）はＬ１と同じ位置の空気間隔であり、レンズ移動後の空気間隔である。
【００５０】
また、θ１、θ２、ＣＡ１、ＣＡ２は下記の通りである。
【００５１】
θ１：出射角度（度）
θ２：レンズ移動時の出射角度（度）
ＣＡ１：色収差補正量（μｍ／ｎｍ）
ＣＡ２：レンズ移動時の色収差補正量（μｍ／ｎｍ）
また、本実施例では、第４面であるｒ４面（レンズ２の対物レンズ側）、及び第８面であるｒ８面（レンズ４の対物レンズ側）を非球面としており、各係数、定数の値を以下の表３に示す。
【００５２】
【表３】

【００５３】
表３中、Ａ４、Ａ６、Ａ８、Ａ１０は、それぞれ４次、６次、８次、１０次の非球面係数であり、ＣＣは非球面の円錐定数である。
【００５４】
また、式（４）、（５）、及び（６）の各式に対応する値は、νｏｂ＝４５．６、ｎｏｂ＝１．７５５１２、ＮＡ＝０．８５とした。
【００５５】
図２は、本実施例の第１レンズ群１０の収差図であり、図２（ａ）は球面収差を、図２（ｂ）は正弦条件違反量を示している。図３は、本実施例の第２レンズ群２０の収差図を示しており、図３（ａ）は球面収差を、図３（ｂ）は正弦条件違反量を示している。
【００５６】
図２（ａ）から分るように、第１群レンズの球面収差はほぼ良好に補正されており、図２（ｂ）から分るように、正弦条件も良好に補正されている。また、図３（ａ）から分るように、第２レンズ群の球面収差はほぼ良好に補正されており、図３（ｂ）から分るように、正弦条件も良好に補正されている。
【００５７】
（実施の形態２）
図４は、実施の形態２に係るビームエクスパンダーの構成図を示している。光源（図示せず）側から順に、負パワーの第１レンズ群１０ａと、正パワーの第２レンズ群２０ａとが配置されている。第１レンズ群１０ａは、凹レンズである第１レンズ１ａの１枚構成である。第２レンズ群２０ａは、凸レンズである第２レンズ２ａと、凹レンズである第３レンズ３ａとの２枚構成であり、これらを張り合わせた張り合わせレンズである。
【００５８】
平行光である入射光線８は第１レンズ１ａに入射し、さらに第２レンズ群２０ａを通って平行光９となる。第２レンズ群２０ａは、光軸方向（矢印ａ方向）に移動できる。第１レンズ１ａに近づくように移動すると、平行光束９は発散光束となって対物レンズに入射し球面収差が発生する。逆に対物レンズ側に移動すると、平行光束９は収束光となって対物レンズに入射し、第１レンズ１ａに近づけた場合とは逆の球面収差を発生する。このことにより、光ディスクの基材厚みが変化したことによって生じる球面収差を補正することができる。
【００５９】
また、本実施の形態では、第１レンズ群１０ａと第２レンズ群２０ａとがともにレンズ群単体で球面収差が補正されている。このため、光学系全体として球面収差が補正されているだけでなく、各レンズ群単独で収差測定できるので、性能の確認が容易となる。
【００６０】
さらに、本実施の形態では、第１レンズ群１０ａ及び第２レンズ群２０ａのうち、一方のレンズ群は正弦条件が補正不足であり、他方のレンズ群は正弦条件が補正されている。例えば、第１レンズ１ａの正弦条件が大きく補正不足になっている場合、入射側の平行光８を光軸に平行に等間隔に分割した時、出射側ではその間隔が等間隔ではなく、光軸付近では間隔が粗になり、光軸から離れるに従って間隔が密になる。
【００６１】
ここで、半導体レーザを光源とした場合、その強度分布はほぼガウス分布となっているため、光軸から離れるに従って強度が低くなる。したがって光量をなるべく多く取り込もうとすると、光軸近傍と最周縁とでは光の強度が大きく異なることになり、スポットを十分に絞れない現象が生じる。
【００６２】
本実施の形態では、前記のように光軸付近の光は密度が粗になるため、その強度が下がり、逆に周縁の光は密度が上がるので強度を上げることができる。すなわち半導体レーザからの取り込み効率を上げたまま、光強度を均一な方向へ近づけることができ、結果として良く絞られたスポットを形成することができる。
【００６３】
さらに、球面収差を補正するためにレンズを光軸方向に移動する際に、正弦条件を補正した方のレンズ群（レンズ群２０ａ）を移動させることで、移動時に生じるレンズの位置誤差、例えば傾きなどによって生じる収差の発生を低く抑えることができる。
【００６４】
正弦条件が補正不足であるレンズ群の最大光線高さにおける正弦条件違反量（ｍｍ）をＳＣとすると、下記の式（１１）の関係を満足することが好ましい。
【００６５】
式（１１） −０．１≦ＳＣ≦−０．０２
式（１１）の関係を満足することにより、半導体レーザのビーム強度の均一化が図れるとともに、レンズを位置決め精度が厳しくなり過ぎず組立てが容易になる。すなわち、ＳＣが−０．０２より大きいと（ＳＣの絶対値が０．０２より小さいと）、光強度を均一に近づける作用が小さくなり過ぎてしまい、逆に−０．１より小さいと（ＳＣの絶対値が０．１より大きいと）レンズの傾きに対して敏感になり過ぎるため、光学系としての位置決めが困難となる。
【００６６】
また、第１レンズ群１０ａを、図４に示したように、単レンズであるレンズ１ａで構成し、これを両面非球面とすることにより、球面収差をほぼ完全に補正したまま、前記式（１１）の関係を満足するような正弦条件補正が可能となる。
【００６７】
また、第２レンズ群２０ａは、凸レンズ（レンズ２ａ）と凹レンズ（レンズ３ａ）との張り合わせレンズであり、凸レンズのアッベ数を５０以上、凹レンズのアッベ数を３５以下とすることにより、色収差を過剰補正としている。
【００６８】
このことにより、最小のレンズ枚数で色収差補正し、かつ第２レンズ群２０ａ単独では色収差補正過剰でありながら、対物レンズなどを含む光学系全体での色収差補正が可能となる。すなわち、できるだけシンプルな構成が好ましい対物レンズの色収差を適正に補正することができる。
【００６９】
アッベ数の範囲が前記範囲より外れると、色収差が補正不足となるか、張り合わせ面の曲率半径が小さくなり過ぎて、製造が困難になる。
【００７０】
また、張り合わせ面（ｒ４面、ｒ５面）が球面であり、張り合わせ面以外の他の面のうち、少なくとも１面が非球面であることが好ましい。張り合わせ面を球面とすることで張り合わせ時の芯出しが容易となり、張り合わせ面以外を非球面とすることで第２レンズ群２０ａ単独での球面収差補正が容易となる。
【００７１】
次に、本実施の形態に係るビームエクスパンダーの実施例の数値を以下の表４に示す。
【００７２】
【表４】

【００７３】
表４中、曲率半径（ｍｍ）を示したｒｉ（ｉ＝１〜６）面は、図４に示した各面であり、Ｗは使用波長（設計波長）、Ｄは出射側の平行光束の直径である。また、空気間隔部の屈折率は１．０である。
【００７４】
以下の表５に、レンズ厚およびレンズ間隔を示す。
【００７５】
【表５】

【００７６】
表５中、ｄ１、ｄ３、ｄ４は各レンズのレンズ厚（ｍｍ）であり、その位置は図４に示している。ｄ２は、図４に示したレンズ間隔（第１２レンズ１ａと第２レンズ２ａとの間の空気間隔）である。
【００７７】
本実施例では、非球面を採用しており、非球面形状は、以下の（数１）で与えられる。
【００７８】
【数１】

【００７９】
ただし、各符号の意味は以下の通りであり、ｊ＝１，２である。
【００８０】
Ｘ：光軸からの高さがｈの非球面上の点の非球面頂点の接平面からの距離
ｈ：光軸からの高さ
Ｃｊ：対物レンズの第ｊ面の非球面頂点の曲率（Ｃｊ＝１／Ｒｊ）
Ｋｊ：対物レンズの第ｊ面の円錐定数
Ａｊ，ｎ：対物レンズの第ｊ面のｎ次の非球面係数
本実施例における非球面の係数を以下の表６に示す。
【００８１】
【表６】

【００８２】
図５に、本実施例の第１レンズ１ａの収差図を示している。図５（ａ）は球面収差を、図５（ｂ）は正弦条件違反量を示している。図６に、第２レンズ群２０ａの収差図を示している。図６（ａ）は球面収差を、図６（ｂ）は正弦条件違反量を示す。
【００８３】
第１レンズ１ａは、図５（ａ）に示したように、球面収差は良好に補正されているが、図５（ｂ）に示したように、正弦条件は大きく補正不足になっており、最大光線高さでの正弦条件違反量は約−０．０４ｍｍである。
【００８４】
第２レンズ群２０ａは、図６（ａ）に示したように、球面収差は良好に補正されており、図６（ｂ）に示したように、正弦条件もほぼ良好に補正されている。
【００８５】
第２レンズ群２０ａの後方にＮＡ０．８５、焦点距離２ｍｍの光ディスク用対物レンズを置いた場合、ビームエクスパンダー間の間隔を±１．７ｍｍ移動させると±２５０ｍλの球面収差を補正することができた。これはＮＡ０．８５の場合、光ディスクの基材厚みにして約±０．２５ｍｍの厚み変化を補正できることになる。
【００８６】
ここで、レンズを光軸方向に移動させるときに、完全に光軸を保つことは困難である。本実施例では、出射側平行ビーム９の波面収差は１．２ｍλと非常に小さく抑えられている。しかしながら、第１レンズ１ａが０．１度傾いたときの波面収差は、２８．６ｍλとなる。一方、第２レンズ群２０ａが同じく０．１度傾いたときの波面収差は、３．０ｍλとなり、ほとんど変化しない。
【００８７】
すなわち第２レンズ群２０ａの正弦条件が補正されているために、レンズの位置誤差に対して許容度が非常に大きくなっているのが分かる。
【００８８】
（実施の形態３）
図７は、実施の形態３に係る光ヘッドの構成図を示している。本図の構成は、実施の形態１に係るビームエクスパンダーを備えており、第１レンズ群２５と第２レンズ群２６とで、ビームエクスパンダーを構成している。光源である半導体レーザ２２から出射した光束は、ビームスプリッター２３を透過し、コリメートレンズ２４により略平行光となる。
【００８９】
ビームエクスパンダーの第１レンズ第２５と第２レンズ群２６とによって、そのビーム径が拡げられ対物レンズ２７により、光ディスク２８の情報媒体面２８ａ上に集光される。集光スポットは情報媒体面２８ａに形成された反射率の異なるピットで反射する。情報媒体面２８ａで反射されたレーザ光は、対物レンズ２７、第２レンズ群２６、第１レンズ第２５、及びコリメートレンズ２４を透過しする。その後、ビームスプリッター２３で反射し、検出レンズ２９で屈折して受光素子３０上に集光される。受光素子３０の電気信号により、情報媒体面２８ａで変調された光量変化を検出し、データを読み取る。
【００９０】
ここで、ビームエクスパンダーの第１レンズ群２５は光軸方向（矢印ａ方向）に可動になるように設置されており、受光素子３０の検出値と第１レンズ群２５との移動とを連動させて、光ディスク２８の基板厚みや他の光ヘッド光学系の球面収差を補正する。また半導体レーザ２２の波長が変動した場合でもビームエクスパンダー色収差補正効果により、対物レンズ２７のフォーカス位置の移動量が許容範囲内になるように補正されている。また、第２レンズ群２６のレンズは、正弦条件が十分に補正されているため移動時に発生するレンズの傾きに対しても収差がほとんど発生しない。
【００９１】
なお、図７は実施の形態１に係るビームエクスパンダーを備えた例であるが、実施の形態２に係るビームエクスパンダーを備えた構成としてもよい。この場合は、半導体レーザ２２の取り込み効率を上げるためにコリメートレンズ２４の焦点距離を短くしても対物レンズ２７の周縁における光強度の低下が防止できる。
【００９２】
また、図７のコリメートレンズ２４は、例えば非球面単レンズとしてもよいが、複数枚の組合せレンズにし、かつビームエクスパンダーの第１レンズ群の機能を同時に持たせるようにしてもよい。
【００９３】
また、実施の形態１、３において、光軸方向に移動させるレンズ群が第１レンズ群である例で説明したが、移動させるレンズ群は、正弦条件を満たしかつレンズの大きさが小さいレンズ群であることが好ましい。このようなレンズ群は、重量が軽く、可動機構の負担が小さくなるからである。
【００９４】
また、前記各実施の形態における張り合わせレンズは、張り合わせずにレンズ間に空気間隔があるようにしてもよく、この場合でも同様の効果を持つ。
【００９５】
【発明の効果】
以上のように、本発明の第１のビームエクスパンダーによれば、２群４枚のビームエクスパンダーにより、短波長域において、高ＮＡの対物レンズで発生する大きな色収差を補正しつつ、張り合わせレンズ間距離を可変することにより対物レンズ焦点で発生する球面収差補正を行うことができる。
【００９６】
また、本発明の第２のビームエクスパンダーによれば、光学系における球面収差をビームエクスパンダー間の間隔を可変することで補正し、かつ半導体レーザのビーム強度を均一な方向に近づけることができる。
【図面の簡単な説明】
【図１】本発明の実施の形態１に係るビームエキスパンダーの構成図
【図２】本発明の実施の形態１に係る第１レンズ群の収差図
【図３】本発明の実施の形態１に係る第２レンズ群の収差図
【図４】本発明の実施の形態２に係るビームエキスパンダーの構成図
【図５】本発明の実施の形態２に係る第１レンズの収差図
【図６】本発明の実施の形態２に係る第２レンズ群の収差図
【図７】本発明の実施の形態３に係る光ヘッドの構成図
【符号の説明】
１，１ａ 第１レンズ
２，２ａ 第２レンズ
３，３ａ 第３レンズ
４ 第４レンズ
５，８入射光束
６ 発散光束
７、９ 出射光束
１０，１０ａ 第１レンズ群
２０，２０ａ 第２レンズ群[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a beam expander and an optical head device used for an optical head such as a DVD and an optical recording device for a computer.
[0002]
[Prior art]
With the increase in capacity of optical storage media, shorter wavelength light sources for optical heads and higher NA of objective lenses have been developed. However, as the NA becomes higher, the change of the spherical aberration due to the change of the base material thickness of the optical disk becomes more remarkable. For example, when the wavelength is 650 nm used for DVD and the NA of the objective lens is 0.6, a spherical aberration of about 10 mλ is generated for a change of the substrate thickness of 10 μm. On the other hand, when the wavelength is 400 nm used for the next-generation optical disc and the NA of the objective lens is 0.85, the change in the base material thickness is about 100 mλ for 10 μm and about 10 mλ for DVD. Double spherical aberration occurs.
[0003]
Therefore, as the wavelength of the light source becomes shorter, a means for correcting spherical aberration is required as an optical head. Patent Document 1 below discloses that an afocal beam expander combining a concave lens and a convex lens is inserted immediately before an objective lens having two lenses, and one lens is moved in the optical axis direction to correct spherical aberration. Proposed.
[0004]
[Patent Document 1]
JP 2000-131603 A
[0005]
[Problems to be solved by the invention]
However, in order to further simplify the configuration of the optical system, it is conceivable to configure the objective lens with a single lens, but in order to configure the objective lens with a single lens, it is necessary to use a glass material having a high refractive index. I have to use glass material with large dispersion.
[0006]
Further, in a short wavelength region of about 400 nm, the dispersion of the lens material is very large as compared with the vicinity of 650 nm, and the refractive index changes significantly due to a slight difference in wavelength. For this reason, if an attempt is made to further shorten the wavelength used by using a single-lens objective lens, chromatic aberration will become extremely large.
[0007]
In a recordable optical disk, the power at the time of recording and the power at the time of reproduction of the semiconductor laser, which is a light source, are greatly different, and the wavelength changes at the time of switching between recording and reproduction. At this time, if there is chromatic aberration in the optical system, the focal position fluctuates at a speed much faster than the band in which the focus control is effective, so that problems such as incorrect writing and reading of information occur.
[0008]
In order to prevent this chromatic aberration, a cemented lens of a concave lens and a convex lens made of materials having different dispersions is generally known. However, in order to correct large chromatic aberration, it is necessary to make the joining surface as small as possible in radius of curvature, resulting in large aberration.
[0009]
Further, in order to correct the spherical aberration of the entire optical system as described above, at least one of the lenses of the beam expander must be moved in the optical axis direction. In this case, if the lens is tilted with respect to the optical axis during the movement, there is a problem that a large aberration occurs.
[0010]
Further, if a large spherical aberration remains in the concave lens unit or the convex lens unit, it becomes a great obstacle to confirm the performance of each lens unit independently.
[0011]
Also, when a semiconductor laser is used as the light source of the optical head, when trying to capture a large amount of light in order to increase the light use efficiency, the light intensity at the periphery of the objective lens decreases and the spot cannot be narrowed, and conversely the light at the periphery There was a problem in that if the strength was to be increased, the loading efficiency would decrease.
[0012]
The present invention solves the conventional problems as described above, and corrects spherical aberration generated at the objective lens focal point while correcting large chromatic aberration generated at a high NA objective lens in a short wavelength range. It is an object of the present invention to provide a beam expander that can perform the operation and an optical head using the same.
[0013]
[Means for Solving the Problems]
In order to achieve the above object, a first beam expander of the present invention has a wavelength range of 390 nm to 420 nm, and a first lens group of negative power and a second lens of positive power in order from the light source side. And a group are arranged,
The first lens group and the second lens group are cemented lenses in which two lenses, a convex lens and a concave lens, are cemented, respectively.
At least one of the first lens group and the second lens group is movable in the optical axis direction,
The Abbe number of the concave lens of the first lens group is ν11, the Abbe number of the convex lens is ν12, the Abbe number of the convex lens of the second lens group is ν21, the Abbe number of the concave lens is ν22, the focal length of the first lens group is f1, and the second is If the focal length of the lens group is f2,
10 ≦ | ν11−ν12 |
10 ≦ | ν21−ν22 |
1.0 <−f2 / f1 ≦ 2.0
Is satisfied.
[0014]
Next, in the second beam expander of the present invention, a first lens group having a negative power and a second lens group having a positive power are arranged in this order from the light source side, and are arranged in parallel from the first lens group side. A beam expander into which light is incident and which is converted into parallel light again through the second lens group,
At least one of the first lens group and the second lens group is movable in the optical axis direction,
Among the first lens group and the second lens group, one of the lens groups is insufficiently corrected for the sine condition, and the other lens group is corrected for the sine condition.
[0015]
An optical head device of the present invention is an optical head device including any of the beam expanders of the present invention,
A light source, achromatizing means for the light source, means for expanding a light beam for the light source, light condensing means for condensing a light beam emitted from the light source on an information recording surface of the optical disk, and an information recording surface of the optical disk. A light beam separating unit that separates a modulated light beam, and a light receiving unit that receives a light beam that is modulated on an information recording surface of the optical disc,
The means for achromatizing and the means for expanding the light beam are the beam expanders.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
The first beam expander of the present invention uses a two-group, four-beam expander to correct the large chromatic aberration generated by the high-NA objective lens in the short wavelength range and change the distance between the bonding lenses. In addition, it is possible to correct spherical aberration generated at the focus of the objective lens.
[0017]
The second beam expander of the present invention can correct spherical aberration in the optical system by changing the interval between the beam expanders, and can make the beam intensity of the semiconductor laser nearly uniform. Further, when the lens is moved in the optical axis direction to correct the spherical aberration, the lens whose sine condition is corrected is moved, so that the position error of the lens caused at the time of movement, for example, the occurrence of the aberration caused by the inclination or the like is reduced. It can be kept low.
[0018]
In the first beam expander of the present invention, the objective lens used on the emission light side of the beam expander is formed of a single lens, the Abbe number of the objective lens is νob, and the Assuming that the refractive index is nob and the numerical aperture of the objective lens is NA,
40 ≦ νob ≦ 60
1.5 ≦ nob ≦ 2.0
0.8 ≦ NA
Is preferably satisfied.
[0019]
Further, at least one surface of the lenses constituting the first lens group and the second lens group is aspherical, and spherical aberration is corrected by each of the first lens group alone and the second lens group alone. Is preferred. According to this configuration, not only the spherical aberration is corrected in the entire optical system, but also the aberration can be measured by each lens group alone, so that the performance can be easily confirmed.
[0020]
Further, at least one surface of the lenses constituting the first lens group and the second lens group is aspherical, and spherical aberration is corrected by each of the first lens group alone and the second lens group alone. And the sine condition is preferably corrected. According to this configuration, the evaluation becomes easy, and the inclination tolerance becomes strong.
When the chromatic aberration correction amount (μm / nm) of the first lens group is CA10 and the chromatic aberration correction amount (μm / nm) of the second lens group 20 is CA20,
CA10 ≦ −10
CA20 ≦ −10
Is preferably satisfied.
[0021]
Further, assuming that a moving distance (mm) of the first group lens or the second group lens in the optical axis direction (mm) is ΔL and an emission angle (degree) of a light beam emitted from the second group lens is θ1,
−2.0 ≦ ΔL ≦ 2.0
-1 ≦ θ1 ≦ 1
Is preferably satisfied.
[0022]
In the second beam expander of the present invention, it is preferable that spherical aberration is corrected in each of the first lens group alone and the second lens group alone. According to this configuration, not only the spherical aberration is corrected in the entire optical system, but also the aberration can be measured by each lens group alone, so that the performance can be easily confirmed.
[0023]
Further, when the sine condition violation amount (mm) at the maximum ray height of the lens group whose sine condition is undercorrected is SC,
−0.1 ≦ SC ≦ −0.02
Is preferably satisfied. According to this configuration, the beam intensity of the semiconductor laser can be made uniform, and the positioning accuracy of the lens does not become too strict, thereby facilitating assembly.
[0024]
Further, it is preferable that the first lens group is a single lens having both surfaces aspherical. According to this configuration, a predetermined sine condition can be corrected while spherical aberration is almost completely corrected.
[0025]
Further, the second lens group is a cemented lens of two lenses of a positive power lens and a negative power lens,
Abbe number of the lens having the positive power is 50 or more, Abbe number of the lens having the negative power is 35 or less,
Preferably, the bonding surface is a spherical surface, and at least one of the surfaces other than the bonding surface is an aspheric surface. According to this configuration, the chromatic aberration can be corrected with the minimum number of lenses, and the chromatic aberration can be corrected in the entire optical system including the objective lens while the second lens group alone is overcorrecting the chromatic aberration. In addition, centering during bonding is facilitated by making the bonding surface spherical, and spherical aberration correction is facilitated by the second lens unit alone by making the surface other than the bonding surface aspheric.
[0026]
Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
[0027]
(Embodiment 1)
FIG. 1 shows a configuration diagram of the beam expander according to Embodiment 1 of the present invention. A first lens group 10 of negative power and a second lens group 20 of positive power are arranged in this order from the light source (not shown) side. The first lens group 10 is composed of a first lens 1 that is a concave lens and a second lens 2 that is a convex lens, and is a laminated lens in which these are laminated.
[0028]
The second lens group 20 is composed of a third lens 3 that is a convex lens and a fourth lens 4 that is a concave lens, and is a laminated lens in which these are laminated. More specifically, the r2 surface of the first lens 1 and the r3 surface of the second lens 2 are bonded with an adhesive, and the r6 surface of the third lens 3 and the r7 surface of the fourth lens 4 are bonded with an adhesive. It is glued. In the present embodiment, chromatic aberration is corrected by using the first lens group 10 and the second lens group 20 each as a cemented lens as described above.
[0029]
The parallel light beam 5 enters from the r1 surface, which is the light source side surface of the first lens 1, passes through the r2 surface, the r3 surface, and the r4 surface of the second lens 2, and emits a divergent light beam 6. The emitted light enters from the r5 surface of the third lens 3 and passes through the r6 surface and the r7 surface of the fourth lens 4, and then is parallel from the r8 surface of the fourth lens 4 on the objective lens (not shown) side. The light beam 7 is emitted.
[0030]
The first lens group 10 can move in the optical axis direction (the direction of arrow a). When moving so as to approach the second lens group 20 side, the parallel light flux 7 becomes a divergent light flux and is incident on the objective lens to generate spherical aberration. Conversely, when the parallel light beam 7 moves to the opposite side to the objective lens, the parallel light flux 7 becomes convergent light and enters the objective lens, and generates a spherical aberration opposite to that when the parallel light beam 7 approaches the second lens group 20 side. This makes it possible to correct the spherical aberration caused by the change in the thickness of the base material of the optical disc.
[0031]
Here, as the Abbe number difference is larger, a laminated lens with larger color correction can be formed, and the power of the lens can be reduced, so that the lens can be easily manufactured. For this reason, the beam expander according to the present embodiment satisfies the following equations (1) and (2). If the Abbe number difference is smaller than 10, the amount of color correction is insufficient, and the power of the lens must be increased, resulting in a lens that is difficult to manufacture.
[0032]
Further, the relationship of the following expression (3) is satisfied. When the magnification of the beam expander exceeds 2, the chromatic aberration increases. However, it is preferable for the magnification of the beam expander to be larger than 1 because expanding the beam is advantageous for an optical system.
[0033]
Equation (1) 10 ≦ | ν11−ν12 |
Equation (2) 10 ≦ | ν21−ν22 |
Formula (3) 1.0 <−f2 / f1 ≦ 2.0
However, the meaning of each code is as follows, and the same applies to Tables 1 and 2 below.
[0034]
ν11: Abbe number of the first lens 1 (concave lens)
ν12: Abbe number of the second lens 2 (convex lens)
ν21: Abbe number of the third lens 3 (convex lens)
ν22: Abbe number of the fourth lens 4 (concave lens)
f1: focal length of 10 lenses in the first lens group
f2: focal length of the second lens group 20 lens
In the present embodiment, the chromatic aberration correction that occurs at the focal point of the objective lens at the time of wavelength shift and the spherical surface that occurs at the focal point of the objective lens by changing the distance between the bonding lenses are provided by providing the above-described configuration. Both aberration correction can be performed.
[0035]
In this case, the distance that the lens is moved in the optical axis direction for correcting spherical aberration can be reduced while effectively maintaining chromatic aberration during wavelength shift. That is, the configuration is such that a larger spherical aberration correction amount can be obtained with a smaller lens movement amount. The smaller the movable distance of the movable part on the optical system is, the smaller the optical head is, and the smaller the aberration that occurs when moving the optical head, the more advantageous the production is.
[0036]
Here, the spherical aberration indicates a spherical aberration generated from each component of the optical system and a spherical aberration with respect to a change in the base material thickness of the optical disc.
[0037]
In the present embodiment, large chromatic aberration generated by an objective lens with a high NA is corrected in a short wavelength range of 390 nm to 420 nm using a beam expander of two groups and four beams. For this reason, it is preferable that the objective lens used on the emission light side is a single lens that satisfies the following equations (4) to (6).
[0038]
Formula (4): 40 ≦ νob ≦ 60
Formula (5) 1.5 ≦ nob ≦ 2.0
Formula (6) 0.8 ≦ NA
However, the meaning of each code is as follows.
[0039]
νob: Abbe number of objective lens
nob: refractive index of the objective lens in the used wavelength range
NA: Objective lens numerical aperture
In the present embodiment, the first lens group 10 and the second lens group 20 are configured such that the chromatic aberration is corrected by using a cemented lens of a convex lens and a concave lens having different dispersions. It is preferable that the following expressions (7) and (8) are satisfied. By satisfying the expressions (7) and (8), it becomes easy to manufacture, and sufficient color correction can be performed at the focal point of the objective lens.
[0040]
Formula (7) CA10 ≦ −10
Formula (8) CA20 ≦ −10
Here, CA10 (μm / nm) is the chromatic aberration correction amount of the first lens group 10, and CA20 (μm / nm) is the chromatic aberration correction amount of the second lens group 20.
[0041]
Further, in the present embodiment, it is advantageous for miniaturization if the relationship of the following expressions (9) and (10) is satisfied. Also, by reducing the dynamic range, manufacturing errors can be suppressed.
[0042]
Formula (9) -2.0 ≦ ΔL ≦ 2.0
Formula (10) -1 ≦ θ1 ≦ 1
Here, ΔL is a moving distance (mm) of the first lens group 10 in the optical axis direction, and θ1 (degree) is an emission angle (an angle formed with the optical axis) of the light beam emitted from the second lens group 20.
[0043]
In addition, if at least one of the lens surfaces r1 to r8 is provided with an aspheric surface so that the spherical aberration is corrected by the cemented lens alone, not only the spherical aberration is corrected as a whole optical system, but also Since the aberration can be measured by each lens group alone, it is easy to confirm the performance. Further, if the first lens group 10 and the second lens group 20 alone satisfy the sine condition, the inclination tolerance becomes strong.
[0044]
Next, Table 1 below shows numerical values of examples of the beam expander according to the present embodiment.
[0045]
[Table 1]

[0046]
In Table 1, the ri (i = 1 to 8) planes showing the radius of curvature (mm) are the respective planes shown in FIG. 1, W is the operating wavelength (design wavelength), and W2 is the shift wavelength (power of the light source). D) is the diameter of the parallel light beam on the emission side. The refractive index is as follows.
[0047]
n11: refractive index of the first lens 1 with respect to the used wavelength (W)
n12: Refractive index for the working wavelength (W) of the second lens 2
n21: Refractive index for the working wavelength (W) of the third lens 3
n22: refractive index of the fourth lens 4 with respect to the used wavelength (W)
n′11: refractive index of the first lens 1 with respect to the shift wavelength (W2)
n′12: Refractive index for the shift wavelength (W2) of the second lens 2
n'21: Refractive index of third lens 3 with respect to shift wavelength (W2)
n'22: refractive index of the fourth lens 4 with respect to the shift wavelength (W2)
Table 2 shows various numerical values and measurement results of this example in Table 2 below.
[0048]
[Table 2]

[0049]
In Table 2, d1 to d4 are the lens thickness (mm) of each lens, and their positions are shown in FIG. L1 (mm) is the lens interval (the air interval between the second lens 2 and the third lens 3) shown in FIG. 1, L2 (mm) is the air interval at the same position as L1, and Is the air spacing.
[0050]
Further, θ1, θ2, CA1, CA2 are as follows.
[0051]
θ1: Outgoing angle (degree)
θ2: Outgoing angle (degree) during lens movement
CA1: chromatic aberration correction amount (μm / nm)
CA2: chromatic aberration correction amount during lens movement (μm / nm)
In the present embodiment, the r4 surface (the objective lens side of the lens 2) as the fourth surface and the r8 surface (the objective lens side of the lens 4) as the eighth surface are aspherical surfaces. The values are shown in Table 3 below.
[0052]
[Table 3]

[0053]
In Table 3, A4, A6, A8, and A10 are fourth-order, sixth-order, eighth-order, and tenth-order aspherical coefficients, respectively, and CC is an aspherical conical constant.
[0054]
The values corresponding to the expressions (4), (5), and (6) were νob = 45.6, nob = 1.75512, and NA = 0.85.
[0055]
2A and 2B are aberration diagrams of the first lens unit 10 according to the present embodiment. FIG. 2A illustrates spherical aberration, and FIG. 2B illustrates a sine condition violation amount. 3A and 3B show aberration diagrams of the second lens group 20 of the present embodiment. FIG. 3A shows the spherical aberration, and FIG. 3B shows the sine condition violation amount.
[0056]
As can be seen from FIG. 2A, the spherical aberration of the first lens unit has been corrected almost satisfactorily, and as can be seen from FIG. 2B, the sine condition has also been corrected well. Further, as can be seen from FIG. 3A, the spherical aberration of the second lens group has been corrected almost satisfactorily, and as can be seen from FIG. 3B, the sine condition has also been well corrected.
[0057]
(Embodiment 2)
FIG. 4 shows a configuration diagram of the beam expander according to the second embodiment. A first lens group 10a having a negative power and a second lens group 20a having a positive power are arranged in this order from the light source (not shown). The first lens group 10a has a single lens configuration of a first lens 1a that is a concave lens. The second lens group 20a has a two-lens configuration including a second lens 2a that is a convex lens and a third lens 3a that is a concave lens, and is a laminated lens in which these are laminated.
[0058]
The incident light beam 8 which is a parallel light is incident on the first lens 1a and further passes through the second lens group 20a to become the parallel light 9. The second lens group 20a can move in the optical axis direction (the direction of arrow a). When moving so as to approach the first lens 1a, the parallel light flux 9 becomes a divergent light flux and is incident on the objective lens to cause spherical aberration. Conversely, when moving to the objective lens side, the parallel light flux 9 becomes convergent light and enters the objective lens, and generates spherical aberration opposite to that when the first lens 1a is approached. This makes it possible to correct the spherical aberration caused by the change in the thickness of the base material of the optical disc.
[0059]
Further, in the present embodiment, the spherical aberration of both the first lens group 10a and the second lens group 20a is corrected by the single lens group. Therefore, not only the spherical aberration is corrected as a whole in the optical system, but also the aberration can be measured by each lens group alone, so that the performance can be easily confirmed.
[0060]
Further, in the present embodiment, of the first lens group 10a and the second lens group 20a, the sine condition of one lens group is insufficiently corrected, and the sine condition of the other lens group is corrected. For example, when the sine condition of the first lens 1a is largely under-corrected, when the parallel light 8 on the incident side is divided into equal intervals in parallel with the optical axis, the interval on the exit side is not equal, and The interval becomes coarse near the axis, and becomes closer as the distance from the optical axis increases.
[0061]
Here, when a semiconductor laser is used as a light source, its intensity distribution is substantially Gaussian distribution, and thus the intensity decreases as the distance from the optical axis increases. Therefore, when trying to capture as much light as possible, the light intensity greatly differs between the vicinity of the optical axis and the outermost periphery, and a phenomenon occurs in which the spot cannot be sufficiently focused.
[0062]
In the present embodiment, as described above, light near the optical axis has a low density, so that its intensity decreases, and conversely, light at the peripheral edge increases, so that its intensity can be increased. That is, the light intensity can be made closer to a uniform direction while the efficiency of taking in light from the semiconductor laser is increased, and as a result, a well-focused spot can be formed.
[0063]
Further, when the lens group is moved in the optical axis direction to correct the spherical aberration, the lens group (the lens group 20a) for which the sine condition is corrected is moved, so that the positional error of the lens caused at the time of movement, for example, the inclination Occurrence of aberrations caused by such factors can be suppressed low.
[0064]
Assuming that the sine condition violation amount (mm) at the maximum ray height of the lens group whose sine condition is undercorrected is SC, it is preferable to satisfy the relationship of the following expression (11).
[0065]
Formula (11): −0.1 ≦ SC ≦ −0.02
By satisfying the relationship of the expression (11), the beam intensity of the semiconductor laser can be made uniform, and the positioning accuracy of the lens is not too severe, so that the assembly becomes easy. That is, when SC is larger than -0.02 (when the absolute value of SC is smaller than 0.02), the effect of making the light intensity close to uniform becomes too small, and when SC is smaller than -0.1 (SC). (When the absolute value is larger than 0.1), the lens is too sensitive to the tilt of the lens, and positioning as an optical system becomes difficult.
[0066]
Further, as shown in FIG. 4, the first lens group 10a is composed of a single lens 1a, which is made aspherical on both surfaces, so that the spherical aberration can be corrected almost completely while the spherical aberration is almost completely corrected. Sine condition correction that satisfies the relationship of 11) can be performed.
[0067]
The second lens group 20a is a cemented lens composed of a convex lens (lens 2a) and a concave lens (lens 3a). By setting the Abbe number of the convex lens to 50 or more and the Abbe number of the concave lens to 35 or less, the chromatic aberration becomes excessive. It has been corrected.
[0068]
As a result, chromatic aberration can be corrected with the minimum number of lenses, and chromatic aberration can be corrected in the entire optical system including the objective lens while the second lens group 20a alone is overcorrecting chromatic aberration. That is, the chromatic aberration of the objective lens, which is preferably as simple as possible, can be appropriately corrected.
[0069]
If the Abbe number is out of the above range, the chromatic aberration will be insufficiently corrected, or the radius of curvature of the bonding surface will be too small, making the production difficult.
[0070]
In addition, it is preferable that the bonding surfaces (the r4 surface and the r5 surface) are spherical, and at least one of the surfaces other than the bonding surface is an aspheric surface. The centering at the time of bonding is facilitated by making the bonding surface spherical, and the spherical aberration correction by the second lens group 20a alone becomes easy by making the surface other than the bonding surface aspherical.
[0071]
Next, Table 4 below shows numerical values of examples of the beam expander according to the present embodiment.
[0072]
[Table 4]

[0073]
In Table 4, ri (i = 1 to 6) planes showing the radius of curvature (mm) are the respective planes shown in FIG. 4, W is the working wavelength (design wavelength), and D is the parallel light flux on the emission side. Is the diameter. The refractive index of the air gap is 1.0.
[0074]
Table 5 below shows the lens thickness and the lens interval.
[0075]
[Table 5]

[0076]
In Table 5, d1, d3, and d4 are the lens thickness (mm) of each lens, and the positions are shown in FIG. d2 is the lens interval (the air interval between the twelfth lens 1a and the second lens 2a) shown in FIG.
[0077]
In this embodiment, an aspherical surface is employed, and the aspherical shape is given by the following (Equation 1).
[0078]
(Equation 1)

[0079]
However, the meaning of each code is as follows, and j = 1, 2.
[0080]
X: distance from the tangent plane of the aspherical vertex of a point on the aspherical surface having a height h from the optical axis
h: Height from optical axis
Cj: curvature of the aspherical vertex of the j-th surface of the objective lens (Cj = 1 / Rj)
Kj: conical constant of the j-th surface of the objective lens
Aj, n: n-th aspherical coefficient of the j-th surface of the objective lens
Table 6 below shows the coefficients of the aspheric surface in this embodiment.
[0081]
[Table 6]

[0082]
FIG. 5 shows an aberration diagram of the first lens 1a of the present embodiment. FIG. 5A shows the spherical aberration, and FIG. 5B shows the sine condition violation amount. FIG. 6 shows an aberration diagram of the second lens group 20a. FIG. 6A shows the spherical aberration, and FIG. 6B shows the sine condition violation amount.
[0083]
In the first lens 1a, as shown in FIG. 5A, the spherical aberration is satisfactorily corrected, but as shown in FIG. 5B, the sine condition is largely undercorrected. The amount of violation of the sine condition at the maximum ray height is about -0.04 mm.
[0084]
In the second lens group 20a, the spherical aberration is satisfactorily corrected as shown in FIG. 6A, and the sine condition is almost satisfactorily corrected as shown in FIG. 6B.
[0085]
When an objective lens for an optical disk having a NA of 0.85 and a focal length of 2 mm is placed behind the second lens group 20a, moving the interval between the beam expanders by ± 1.7 mm can correct the spherical aberration of ± 250 mλ. Was. This means that in the case of NA of 0.85, it is possible to correct a thickness change of about ± 0.25 mm in the thickness of the base material of the optical disk.
[0086]
Here, it is difficult to completely maintain the optical axis when moving the lens in the optical axis direction. In this embodiment, the wavefront aberration of the exit side parallel beam 9 is suppressed to a very small value of 1.2 mλ. However, the wavefront aberration when the first lens 1a is inclined by 0.1 degrees is 28.6 mλ. On the other hand, the wavefront aberration when the second lens group 20a is also tilted by 0.1 degree is 3.0 mλ, and hardly changes.
[0087]
That is, since the sine condition of the second lens group 20a has been corrected, it can be seen that the tolerance for the positional error of the lens is very large.
[0088]
(Embodiment 3)
FIG. 7 shows a configuration diagram of the optical head according to the third embodiment. The configuration in the figure includes the beam expander according to Embodiment 1, and the first lens group 25 and the second lens group 26 constitute a beam expander. A light beam emitted from a semiconductor laser 22 as a light source passes through a beam splitter 23 and is converted into substantially parallel light by a collimator lens 24.
[0089]
The beam diameter is expanded by the first lens 25 and the second lens group 26 of the beam expander, and the beam is focused on the information medium surface 28a of the optical disk 28 by the objective lens 27. The condensed spot is reflected by pits formed on the information medium surface 28a and having different reflectivities. The laser light reflected by the information medium surface 28a passes through the objective lens 27, the second lens group 26, the first lens 25, and the collimator lens 24. Thereafter, the light is reflected by the beam splitter 23, refracted by the detection lens 29, and condensed on the light receiving element 30. The change in the amount of light modulated on the information medium surface 28a is detected based on the electric signal of the light receiving element 30, and the data is read.
[0090]
Here, the first lens group 25 of the beam expander is installed so as to be movable in the optical axis direction (the direction of arrow a), and the detection value of the light receiving element 30 and the movement of the first lens group 25 are linked. Thus, the substrate thickness of the optical disk 28 and the spherical aberration of other optical head optical systems are corrected. Even when the wavelength of the semiconductor laser 22 fluctuates, the amount of movement of the focus position of the objective lens 27 is corrected to be within an allowable range by the beam expander chromatic aberration correction effect. Further, since the sine condition of the lenses of the second lens group 26 is sufficiently corrected, almost no aberration occurs even with respect to the tilt of the lens that occurs during movement.
[0091]
Although FIG. 7 illustrates an example including the beam expander according to the first embodiment, a configuration including the beam expander according to the second embodiment may be employed. In this case, even if the focal length of the collimator lens 24 is shortened in order to increase the efficiency of taking in the semiconductor laser 22, it is possible to prevent the light intensity at the periphery of the objective lens 27 from decreasing.
[0092]
Further, the collimator lens 24 in FIG. 7 may be, for example, an aspherical single lens, or may be a combination of a plurality of lenses and have the function of the first lens group of the beam expander at the same time.
[0093]
Further, in the first and third embodiments, an example has been described in which the lens group to be moved in the optical axis direction is the first lens group, but the lens group to be moved satisfies the sine condition and has a small lens size. It is preferable that This is because such a lens group is light in weight and reduces the load on the movable mechanism.
[0094]
Further, the laminated lenses in each of the above embodiments may have an air gap between the lenses without being laminated, and in this case, the same effect is obtained.
[0095]
【The invention's effect】
As described above, according to the first beam expander of the present invention, the two-group four-beam expander corrects the large chromatic aberration generated by the high-NA objective lens in the short wavelength range, and combines the laminated lens. The spherical aberration generated at the focal point of the objective lens can be corrected by changing the distance.
[0096]
Further, according to the second beam expander of the present invention, the spherical aberration in the optical system can be corrected by changing the interval between the beam expanders, and the beam intensity of the semiconductor laser can be made closer to a uniform direction. .
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a beam expander according to Embodiment 1 of the present invention.
FIG. 2 is an aberration diagram of a first lens group according to Embodiment 1 of the present invention.
FIG. 3 is an aberration diagram of a second lens unit according to the first embodiment of the present invention.
FIG. 4 is a configuration diagram of a beam expander according to Embodiment 2 of the present invention.
FIG. 5 is an aberration diagram of a first lens according to Embodiment 2 of the present invention.
FIG. 6 is an aberration diagram of a second lens unit according to Embodiment 2 of the present invention.
FIG. 7 is a configuration diagram of an optical head according to Embodiment 3 of the present invention.
[Explanation of symbols]
1,1a First lens
2,2a Second lens
3,3a Third lens
4 Fourth lens
5.8 incident light flux
6 divergent luminous flux
7, 9 Outgoing light flux
10, 10a First lens group
20, 20a Second lens group

Claims (12)

The wavelength range of use is 390 nm or more and 420 nm or less, and a first lens group of negative power and a second lens group of positive power are arranged in order from the light source side,
The first lens group and the second lens group are cemented lenses in which two lenses, a convex lens and a concave lens, are cemented, respectively.
At least one of the first lens group and the second lens group is movable in the optical axis direction,
The Abbe number of the concave lens of the first lens group is ν11, the Abbe number of the convex lens is ν12, the Abbe number of the convex lens of the second lens group is ν21, the Abbe number of the concave lens is ν22, the focal length of the first lens group is f1, and the second is If the focal length of the lens group is f2,
10 ≦ | ν11−ν12 |
10 ≦ | ν21−ν22 |
1.0 <−f2 / f1 ≦ 2.0
A beam expander characterized by satisfying the above relationship. The objective lens used on the outgoing light side of the beam expander is a single lens, the Abbe number of the objective lens is νob, the refractive index of the objective lens in a used wavelength region is nob, and the numerical aperture of the objective lens is NA. Then
40 ≦ νob ≦ 60
1.5 ≦ nob ≦ 2.0
0.8 ≦ NA
The beam expander according to claim 1, which satisfies the following relationship: At least one surface of the lenses constituting the first lens group and the second lens group is aspherical, and the spherical aberration is corrected by the first lens group alone and the second lens group alone. The beam expander according to claim 1. At least one surface of the lenses constituting the first lens group and the second lens group is aspheric, and spherical aberration is corrected by the first lens group alone and the second lens group alone. 3. The beam expander according to claim 1, wherein a sine condition is corrected. If the chromatic aberration correction amount (μm / nm) of the first lens group is CA10 and the chromatic aberration correction amount (μm / nm) of the second lens group 20 is CA20,
CA10 ≦ −10
CA20 ≦ −10
The beam expander according to any one of claims 1 to 4, which satisfies the following relationship. Assuming that a moving distance (mm) of the first group lens or the second group lens in the optical axis direction (mm) is ΔL, and an emission angle (degree) of a light beam emitted from the second group lens is θ1,
−2.0 ≦ ΔL ≦ 2.0
-1 ≦ θ1 ≦ 1
The beam expander according to any one of claims 1 to 5, which satisfies the following relationship. A first lens group having a negative power and a second lens group having a positive power are arranged in this order from the light source side, and parallel light enters from the first lens group side, passes through the second lens group, and becomes parallel again. A beam expander that is converted to light,
At least one of the first lens group and the second lens group is movable in the optical axis direction,
A beam expander characterized in that one of the first lens group and the second lens group has a sine condition that is undercorrected for a sine condition, and the other lens group has a sine condition corrected. The beam expander according to claim 7, wherein spherical aberration is corrected in each of the first lens group alone and the second lens group alone. When the sine condition violation amount (mm) at the maximum ray height of the lens group whose sine condition is undercorrected is SC,
−0.1 ≦ SC ≦ −0.02
The beam expander according to claim 7, wherein the following relationship is satisfied. The beam expander according to claim 7, wherein the first lens group is a single lens having two aspheric surfaces. The second lens group is a cemented lens of two lenses, a positive power lens and a negative power lens,
Abbe number of the lens having the positive power is 50 or more, Abbe number of the lens having the negative power is 35 or less,
The beam expander according to claim 7, wherein the bonding surface is a spherical surface, and at least one of the surfaces other than the bonding surface is an aspheric surface. An optical head device including the beam expander according to any one of claims 1 to 11,
A light source, achromatizing means for the light source, means for expanding a light beam for the light source, light condensing means for condensing a light beam emitted from the light source on an information recording surface of the optical disk, and an information recording surface of the optical disk. A light beam separating unit that separates a modulated light beam, and a light receiving unit that receives a light beam that is modulated on an information recording surface of the optical disc,
The optical head device, wherein the achromatic means and the means for expanding the light beam are the beam expanders.

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