JP2004296048A - Objective lens for optical recording medium and optical pickup device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To converge use light beams onto each of different kind of optical recording media in the state an aberration is greatly satisfactory, and to mitigate an allowable value of a lens manufacturing error, by forming a light incidence/emission plane to a free curved surface and a luminous flux reflecting plane to a diffracted optical surface and also making larger than 1 a ratio of paraxial curvature radius in a meridian plane of the light emitting plane with respect to the light incident plane, in the prism type objective lens. <P>SOLUTION: This objective lens consisting of an almost triangular prism type lens 2 is provided with a 1st plane shaped to a free curved surface having the positive refraction, a 2nd plane formed to the reflective surface consisting of the diffracted optical surface (DOE surface) and a 3rd plane shaped to the free curved surface having the positive refraction, in this order from the light source side. Also, it is constituted so as to satisfy the conditional expression of R<SB>3</SB>/R<SB>1</SB>>1, when the paraxial curvature radius in the meridian plane on the 1st plane 21 is expressed as R<SB>1</SB>and the paraxial curvature radius in the meridian plane on the 3rd plane 23 is expressed as R<SB>3</SB>. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００２１】
ここで、Ｘ、Ｙ、Ｚは各々各面におけるＸ座標値、Ｙ座標値、Ｚ座標値、ａ（ｉ，ｊ）は面形状多項式係数、Ｒは子午面内における近軸曲率半径、ｉおよびｊは０および正の整数である。ただし、Ｘの次数ｉは子午面に対する対称性から偶数とされている。
【００２２】
また、第２面は、下記（４）式で表される回折光学面からなる反射面とされており、この回折光学面において反射回折された各光束は、使用光の波長が互いに異なり、互いに厚みの異なる複数の光記録媒体の各々に対応する照射光束とされる。
【００２３】
【数２】

【００２４】
ここで、Ｘ、Ｙは各々各面におけるＸ座標値、Ｙ座標値、ｂ（ｉ，ｊ）は位相差関数の係数、ｉおよびｊは０および正の整数である。ただし、Ｘの次数ｉは子午面に対する対称性から偶数とされている。なお、単位はラディアンである。
【００２５】
なお、この回折光学面におけるレンズ基板形状は平面であっても、所定形状の曲面であってもよい。また、回折光学面を構成する回折格子は、矩形状、階段状、三角波状等の種々の形状を選択しうる。
【００２６】
このように、本実施形態の光ピックアップレンズ２によれば、第２面を所定形状の回折光学面からなる反射面とし、さらに、第１面と第３面については自由曲面としているため、互いに異なる使用波長および互いに異なる厚みとされた複数の光記録媒体の記録面の各々に、収差を良好に補正した状態で光束を収束させることができる。
また、このように複雑な形状からなる光ピックアップレンズ２をガラスまたはプラスチックによる一体成形により製造すれば、その製造に要するコストおよび労力を軽減することができ、好ましい。
【００２７】
なお、本実施形態においては、例えばＤＶＤとＣＤ−Ｒの２つの光記録媒体に対応するように構成される。すなわち、ＤＶＤは、使用光波長６５０ｎｍ、ディスク基板厚０．６ｍｍ、開口数０．６のものが一般的であり、ＣＤ−Ｒは、使用光波長７８０ｎｍ、ディスク基板厚１．２ｍｍ、開口数０．４５で規格統一されているから、これらの条件に対応して収差を良好に補正し得るようになっている。しかし、本発明の光記録媒体用対物レンズとしては、本実施形態のものに限られるものではなく、その他の異種の光記録媒体の組合わせに対応する構成とすることが可能である。
【００２８】
また、本実施形態の光ピックアップレンズ２においては、上記第１面の子午面内における近軸曲率半径をＲ_１、上記第３面の子午面内における近軸曲率半径をＲ_３としたとき、下記条件式（５）を満足するように構成されている。
Ｒ_３／Ｒ_１＞１ ……（５）
【００２９】
上記条件式（５）を満足することにより、球面収差および正弦条件を同時に制御することが容易になり、比較的、光学設計および製造が容易となるとともにコストも安価なものとすることができる。すなわち、製造誤差の感度に与える影響を緩和することができ、製造コストの低下および製造時間の短縮化を図ることができる。また、歩留まりも向上させることができる。
【００３０】
次に、この光ピックアップレンズ２を搭載した光ピックアップ装置について説明する。図４に示すように、ＬＤ電源１１Ａからの電力供給により半導体レーザ１１Ｂ、１１Ｃから出力されたレーザ光５２がハーフミラー５３により反射され、コリメータレンズ５４により平行光とされて光ピックアップレンズ２に入射し、この光ピックアップレンズ２により光路を折り曲げられるとともに収束光とされて光ディスク１の記録領域６Ａ上に各収差が良好に補正された状態で収束される。なお、光ディスク１は、記録領域６Ａを保護するためのＰＣ板等からなる透明な保護板を有している。
【００３１】
なお、半導体レーザ１１Ｂは、例えばＣＤ−Ｒ（追記型コンパクトディスク）用の、波長７８０ｎｍの近赤外域のレーザ光を出力する光源であり、半導体レーザ１１Ｃは、例えばＤＶＤ（ディジタル・バーサタイル・ディスク）用の、波長６５０ｎｍの可視域のレーザ光を出力する光源であり、ハーフミラー１１Ｄを介していずれかの半導体レーザ１１Ｂ、１１Ｃから出力されたレーザ光５２がハーフミラー５３に照射されるようになっている。また、ＬＤ電源１１Ａと半導体レーザ１１Ｂ、１１Ｃとの間には切替スイッチ１１Ｅが配されており、この切替スイッチ１１Ｅの操作によりいずれかの半導体レーザ１１Ｂ、１１Ｃに電力が供給されるようになっている。
【００３２】
上記記録領域６Ａには信号情報を担持したピットがトラック状に配列されるようになっており、この記録領域６Ａからの上記レーザ光５２の再生反射光は信号情報を担持した状態で光ピックアップレンズ２およびコリメータレンズ５４を介してハーフミラー５３に入射し、このハーフミラー５３を透過して４分割のフォトダイオード１７に入射する。このフォトダイオード１７では分割された４つのダイオード位置の各受光量を演算してデータ信号、およびフォーカスとトラッキングの各エラー信号を得る。
【００３３】
なお、ハーフミラー５３は光ディスク１からの戻り光の光路に対して４５°傾いた状態で挿入されているのでシリンドリカルレンズと同等の作用をなし、このハーフミラー５３を透過した光ビームは非点収差を有することとなり、４分割のフォトダイオード１７上におけるこの戻り光のビームスポットの形状に応じてフォーカスのエラー量が決定されることとなる。なお、上記コリメータレンズ５４は状況に応じて省略することも可能であり、さらにフォトダイオード１７とハーフミラー５３との間にグレーティングを挿入して３ビームによりトラッキングエラーを検出することも可能である。
【００３４】
この光ピックアップ装置では、例えばＣＤ−ＲとＤＶＤのいずれの光ディスク１についても信号の記録再生が可能となるように構成されている。
【００３５】
以下、実施例１〜２を用いて、本発明の光記録媒体用対物レンズについて、さらに具体的に説明する。
＜実施例１＞
この光ピックアップレンズ２は、ＤＶＤ１ａおよびＣＤ−Ｒ１ｂという規格の異なる光ディスクの記録および／または再生に好適な光記録媒体用対物レンズである。図２はこの実施例１に係る光ピックアップレンズ２を示す図である。この光ピックアップレンズ２の第１面２１および第３面２３には、各々上述のように自由曲面式で表される自由曲面が形成されている。
【００３６】
また、この光ピックアップレンズ２の第２面２２は、ＤＶＤ１ａ用の波長６５０ｎｍのレーザ光を反射し、ＤＶＤ１ａの記録領域上に収束させるとともに、ＣＤ−Ｒ１ｂ用の波長７８０ｎｍのレーザ光を反射し、ＣＤ−Ｒ１ｂの記録領域上に収束させるように機能する。この第２面２２は上述のように位相差関数式で表される回折光学反射面とされている。
【００３７】
図２に示すように、光ピックアップ装置として用いられ、ＤＶＤ１ａ（ディスク基板厚０．６ｍｍ；使用光に対する屈折率１．５８）が所定位置（ターンテーブル上）に配されてその記録・再生が行われる場合には、波長６５０ｎｍのレーザ光（実線で表された光束Ｉａ）が光ピックアップレンズ２に入射される。この後このレーザ光は、第２面２２により回折反射され、開口数が０．６となり、ＤＶＤ１ａの記録領域上に収束せしめられる。なお、波長６５０ｎｍのレーザ光に対する光ピックアップレンズ２の構成材料の屈折率は１．５８５３７である。
【００３８】
他方、図２に示すように、光ピックアップ装置として用いられ、ＣＤ−Ｒ１ｂ（ディスク基板厚１．２ｍｍ；使用光に対する屈折率１．５７）が所定位置（ターンテーブル上）に配されてその記録・再生が行われる場合には、波長７８０ｎｍのレーザ光（点線で表された光束Ｉｂ）が光ピックアップレンズ２に入射される。この後このレーザ光は、第２面２２により回折反射され、開口数が０．４５相当となり、光ピックアップレンズ２によりＣＤ−Ｒ１ｂの記録領域上に収束せしめられる。なお、波長７８０ｎｍのレーザ光に対する光ピックアップレンズ２の構成材料の屈折率は１．５８１４７である。
【００３９】
なお、ＤＶＤ用の焦点距離は３．５ｍｍに設定され、ＣＤ−Ｒ用の焦点距離も３．５ｍｍに設定されている。
光ピックアップレンズ２に入射されるレーザ光は、いずれの波長の場合も略平行光とされた状態で入射されるが、各面の屈折作用および反射作用により、厚さの異なるいずれの光ディスクの記録領域にも収差が補正された状態で良好に収束される。
【００４０】
下記表１に、この実施例１に係る光ピックアップレンズ２のレンズデータ（子午面内における近軸曲率半径Ｒ、λ＝６５０ｎｍ、７８０ｎｍに対する面間隔Ｄおよびλ＝６５０ｎｍ、７８０ｎｍに対する屈折率Ｎ）をＤＶＤ１ａおよびＣＤ−Ｒ１ｂの各場合について示す。
さらに、光記録媒体としてＤＶＤ１ａおよびＣＤ−Ｒ１ｂをセットした各場合における使用光について、この実施例１に係る光ピックアップレンズ２の焦点距離およびＲＭＳ−ＯＰＤの各値を示す。
また、表２および４に各々第１面２１および第３面２３の面形状多項式係数を示す。
さらに、表３に第２面２２の位相差関数の係数を示す。
【００４１】
なお、下記表１から明らかなように、第１面２１の、子午面内における近軸曲率半径Ｒ_１は２．９４１２とされ、第３面２３の、子午面内における近軸曲率半径Ｒ_３は３．０７２５とされているので、Ｒ_３／Ｒ_１は約１．０４５となり、上記条件式（５）を満足する。
【００４２】
【表１】

【００４３】
【表２】

【００４４】
【表３】

【００４５】
【表４】

【００４６】
＜実施例２＞
実施例２の光ピックアップレンズ２は、図３に示すように、基本的構成は上記実施例１と略同様に構成されているので、実施例１と同様な部分については説明を省略する。
【００４７】
次に、表５に、この実施例２に係る光ピックアップレンズ２のレンズデータ（子午面内における近軸曲率半径Ｒ、λ＝６５０ｎｍ、７８０ｎｍに対する面間隔Ｄおよびλ＝６５０ｎｍ、７８０ｎｍに対する屈折率Ｎ）をＤＶＤ１ａおよびＣＤ−Ｒ１ｂの各場合について示す。
さらに、光記録媒体としてＤＶＤ１ａおよびＣＤ−Ｒ１ｂをセットした各場合における使用光について、この実施例２に係る光ピックアップレンズ２の焦点距離およびＲＭＳ−ＯＰＤの各値を示す。
また、表６および８に各々第１面２１および第３面２３の面形状多項式係数を示す。
さらに、表７に第２面２２の位相差関数の係数を示す。
【００４８】
なお、下記表５から明らかなように、第１面２１の、子午面内における近軸曲率半径Ｒ_１は２．６３１６とされ、第３面２３の、子午面内における近軸曲率半径Ｒ_３は４．６０５０とされているので、Ｒ_３／Ｒ_１は約１．７５０となり、上記条件式（５）を満足する。
【００４９】
【表５】

【００５０】
【表６】

【００５１】
【表７】

【００５２】
【表８】

【００５３】
【発明の効果】
以上に説明したように、本発明の光記録媒体用対物レンズおよび光ピックアップ装置によれば、光源側より順に、正の屈折力を有する自由曲面からなる第１面と、所定の回折光学面からなる反射面とされた第２面と、正の屈折力を有する自由曲面からなる第３面とを備えている。また、第１面の子午面内における近軸曲率半径Ｒ_１に対する、第３面の子午面内における近軸曲率半径Ｒ_３の比であるＲ_３／Ｒ_１を１より大きくしている。これにより異種の光記録媒体の記録面の各々に、収差が極めて良好とされた状態で使用光を収束させることができ、また、球面収差および正弦条件を同時にかつ容易に制御可能とすることで光学設計およびレンズ製造を容易なものとすることができる。また、これにより製造誤差の感度に与える影響を緩和することが可能となる。
【００５４】
したがって、異種の光記録媒体の記録面の各々に、収差が極めて良好にされた状態で使用光を収束させることができるとともに、レンズ製造時における、製造コストの低下および製造時間の短縮化を図ることが可能である。
【図面の簡単な説明】
【図１】本発明の実施形態に係る光記録媒体用対物レンズ（光ピックアップレンズ）を説明するためのレンズ構成図
【図２】本発明の実施例１に係る光ピックアップレンズを示すレンズ構成図
【図３】本発明の実施例２に係る光ピックアップレンズを示すレンズ構成図
【図４】本発明の光ピックアップ装置を示す概略図
【符号の説明】
１ 光ディスク
１ａ ＤＶＤ
１ｂ ＣＤ−Ｒ
２ 光ピックアップレンズ
６Ａ 記録領域
１１Ａ ＬＤ光源
１１Ｂ、１１Ｃ 半導体レーザ
１１Ｄ、５３ ハーフミラー
１１Ｅ 切替スイッチ
１７ フォトダイオード
２１ 第１面
２２ 第２面
２３ 第３面
５２ レーザ光
５４ コリメータレンズ
Ｉａ、Ｉｂ 光束[0001]
TECHNICAL FIELD OF THE INVENTION
According to the present invention, when information is recorded or reproduced, with respect to two or more optical recording media having different wavelengths and different substrate thicknesses of the used light, each used light is recorded on the recording surface of the corresponding optical recording medium. The present invention relates to an objective lens for an optical recording medium that can efficiently converge, and an optical pickup device using the objective lens.
[0002]
[Prior art]
In recent years, various optical recording media have been developed, and an optical pickup device capable of recording / reproducing by sharing a plurality of types of optical recording media is known. For example, a DVD (digital versatile disc; including -ROM, -R, -RW, etc.) and a CD (compact disc, including -ROM, -R, -RW, etc.) using one optical pickup device. 2. Description of the Related Art There is known a device for recording and reproducing data. In such two optical recording media, for example, visible light of about 650 nm is used for DVD in order to improve the recording density, whereas near-red light of about 780 nm is used for CD. It is supposed to use outside light. Therefore, an optical pickup device that can be shared by both uses a so-called two-wavelength beam system that uses light of two different wavelengths as irradiation light.
[0003]
In addition, the DVD has a disc thickness of 0.6 mm (numerical aperture is 0.6), and the CD has a disc thickness of 1.2 mm (numerical aperture is 0.45). The disk substrate thicknesses are different from each other.
If the used optical wavelengths, the disk thicknesses, and the numerical apertures of the two optical recording media are different from each other as in the case of the DVD and the CD, the respective wavelengths for performing reproduction or recording in such an optical pickup device. It is necessary to have different convergence functions for the light of the same.
[0004]
On the other hand, in recent years, it has been generalized that the above-described optical disk is loaded into a notebook personal computer or the like and recorded / reproduced. Also, there is a strong demand for compact optical disk recording / reproducing devices for home and business use. In such a case, it is desirable to arrange most of the optical system for recording / reproducing the optical disk and the turntable for holding the disk so as to be parallel to the bottom surface of the housing in order to reduce the size of the entire apparatus. In order to converge the light beam emitted from the optical system on the recording surface of the optical disk, it is necessary to bend the light beam at a right angle.
[0005]
A reflecting mirror for bending a light beam at a right angle can be provided independently, for example, as shown in Patent Document 1 below. However, it is preferable that a part of an objective lens of an optical pickup also has a reflecting mirror function. If this is the case, it is possible to further meet the demand for compactness and the like.
[0006]
As an objective lens for a two-wavelength optical pickup having such a reflection mirror function, Japanese Patent Application Laid-Open No. H11-163873 discloses a light having a substantially right-angled triangular cross section and an aspherical lens on one of two sides sandwiching the right angle. It is known that the incident surface and the other are a light emitting surface provided with an aspherical lens, and the oblique side is a flat light reflecting surface.
[0007]
In the prior art described in Patent Document 2, the first surface and the third surface, which are refraction surfaces, are made aspherical, and the second surface, which is a plane reflection surface, is provided with a HOE (hologram optical element). It is configured to correspond to different types of optical recording media, that is, different types of disk substrate thicknesses and different types of used light wavelengths.
[0008]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2001-256661 [Patent Document 2]
Japanese Patent No. 3077737 [0009]
[Problems to be solved by the invention]
However, Patent Literature 2 does not provide a more detailed description, and states that aberrations are favorably corrected on each of the recording surfaces of the different types of optical recording media described above using only such abstract information. It is difficult to converge light at
[0010]
In recent years, there has been a rapid demand for higher density of optical recording media and higher accuracy of recording / reproducing. In order to improve aberration correction, the shape of an optical pickup objective lens is complicated. Inevitably, the sensitivity to manufacturing errors has become extremely high. Such strict tolerances have a significant effect on manufacturing costs and manufacturing time, and the development of technologies that can simplify optical design and lens manufacturing that can reduce manufacturing tolerances while maintaining optical performance. Was desired.
[0011]
The present invention has been made in view of such circumstances, and has been described in the state where the aberrations are extremely excellent on each of the recording surfaces of a plurality of optical recording media having different use light wavelengths and different thicknesses. It is an object of the present invention to provide an objective lens for an optical recording medium and an optical pickup device using the objective lens, which can reduce the production cost and the production time while converging the optical pickup.
[0012]
[Means for Solving the Problems]
The objective lens for an optical recording medium according to the present invention, for a plurality of optical recording media respectively corresponding to mutually different substrate thicknesses and mutually different used light wavelengths, applies each used light on the recording surface of the optical recording medium related to the used light. In a prism type optical recording medium objective lens having a substantially triangular cross section to converge on
A first surface provided to include the substantially triangular first side, which is a free-form surface having a positive refractive power as a light incident surface, and a diffractive optical reflection surface for reflecting incident light. A second surface provided so as to include the substantially triangular second side and a free-form surface having a positive refractive power as a light emitting surface, provided so as to include the substantially triangular third side. With the third surface,
When the paraxial curvature radius in the meridional plane of the first surface and R _1, a paraxial curvature radius in the meridional plane of the third surface and R _3, and characterized by satisfying the following conditional expression (2) Is what you do.
R ₃ / R ₁ > 1 (2)
[0013]
Here, the “substantially triangular shape” means that each side may have a curved shape, and has an appearance similar to a triangular shape as a whole.
[0014]
It is preferable that the objective lens for an optical recording medium is integrally formed of glass or plastic.
[0015]
Further, an optical pickup device according to the present invention includes any one of the above-described optical recording medium objective lenses.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an objective lens for an optical recording medium according to an embodiment of the present invention (hereinafter, referred to as an optical pickup lens in the present embodiment) will be described with reference to the drawings.
[0017]
FIG. 1 is a lens configuration diagram of an optical pickup lens 2 according to the embodiment of the present invention. As shown in FIG. 1, the optical pickup lens 2 according to the present embodiment includes a substantially triangular prism type lens in a cross section including the incident optical axis and the exit optical axis of the optical pickup lens 2, and is sequentially arranged from the light source side. A first surface of a free-form surface having a positive refractive power, a second surface serving as a reflection surface composed of a diffractive optical surface (DOE surface), and a third surface of a free-form surface having a positive refractive power. According to the cross-sectional shape shown in FIG. 1, the second surface is configured to include the longest side of a substantially triangular shape.
[0018]
In addition, among the respective coordinate axes that define the shape of each surface (first to third surfaces), the Y axis and the Z axis are described in FIG. 1, and the remaining X axis is the Y axis and the Z axis of each surface. At the intersection position, they are arranged in a direction perpendicular to the plane of the drawing.
[0019]
Here, the first surface and the third surface are each a free-form surface represented by the following equation (3) so that each aberration can be favorably corrected at the convergence position of the used light.
[0020]
(Equation 1)

[0021]
Here, X, Y, and Z are X coordinate values, Y coordinate values, and Z coordinate values on each surface, a (i, j) is a surface shape polynomial coefficient, R is a paraxial radius of curvature in the meridional plane, i and j is 0 and a positive integer. However, the degree i of X is an even number because of the symmetry with respect to the meridian plane.
[0022]
Further, the second surface is a reflecting surface composed of a diffractive optical surface represented by the following equation (4). The light beams reflected and diffracted on this diffractive optical surface have different wavelengths of light used and are different from each other. Irradiation light fluxes corresponding to a plurality of optical recording media having different thicknesses are used.
[0023]
(Equation 2)

[0024]
Here, X and Y are X and Y coordinate values on each surface, b (i, j) is a coefficient of the phase difference function, and i and j are 0 and a positive integer. However, the degree i of X is an even number because of the symmetry with respect to the meridian plane. The unit is radian.
[0025]
The shape of the lens substrate on the diffractive optical surface may be a flat surface or a curved surface having a predetermined shape. Further, the diffraction grating constituting the diffractive optical surface can be selected from various shapes such as a rectangular shape, a step shape, and a triangular wave shape.
[0026]
As described above, according to the optical pickup lens 2 of the present embodiment, the second surface is a reflection surface composed of a diffractive optical surface having a predetermined shape, and the first surface and the third surface are free-form surfaces. The luminous flux can be converged on each of the recording surfaces of the plurality of optical recording media having different use wavelengths and different thicknesses in a state in which the aberration is satisfactorily corrected.
It is preferable to manufacture the optical pickup lens 2 having such a complicated shape by integral molding of glass or plastic, because the cost and labor required for the manufacturing can be reduced.
[0027]
The present embodiment is configured to correspond to, for example, two optical recording media, DVD and CD-R. That is, a DVD generally has an operating light wavelength of 650 nm, a disk substrate thickness of 0.6 mm and a numerical aperture of 0.6, and a CD-R has an operating light wavelength of 780 nm, a disk substrate thickness of 1.2 mm and a numerical aperture of 0. .45 standardized, aberrations can be satisfactorily corrected in accordance with these conditions. However, the objective lens for an optical recording medium of the present invention is not limited to the one of the present embodiment, and may have a configuration corresponding to a combination of other types of optical recording media.
[0028]
Further, in the optical pickup lens 2 of the present embodiment, when the paraxial radius of curvature in the meridional plane of the first surface is R ₁ , and the paraxial radius of curvature in the meridional plane of the third surface is R ₃ , The zoom lens is configured to satisfy the following conditional expression (5).
R ₃ / R ₁ > 1 (5)
[0029]
By satisfying the conditional expression (5), it is easy to simultaneously control the spherical aberration and the sine condition, so that the optical design and manufacturing are relatively easy and the cost is relatively low. That is, the influence of the manufacturing error on the sensitivity can be reduced, and the manufacturing cost and the manufacturing time can be reduced. Also, the yield can be improved.
[0030]
Next, an optical pickup device equipped with the optical pickup lens 2 will be described. As shown in FIG. 4, the laser light 52 output from the semiconductor lasers 11B and 11C by the power supply from the LD power supply 11A is reflected by the half mirror 53, converted into parallel light by the collimator lens 54, and incident on the optical pickup lens 2. Then, the optical path is bent by the optical pickup lens 2 and converged, so that the light is converged on the recording area 6A of the optical disc 1 in a state where the aberrations are properly corrected. The optical disc 1 has a transparent protective plate such as a PC plate for protecting the recording area 6A.
[0031]
The semiconductor laser 11B is a light source for outputting near-infrared laser light having a wavelength of 780 nm for a CD-R (write-once compact disc), for example. The semiconductor laser 11C is a DVD (digital versatile disc), for example. For outputting a laser beam in the visible region having a wavelength of 650 nm, and the laser beam 52 output from one of the semiconductor lasers 11B and 11C via the half mirror 11D is applied to the half mirror 53. ing. Further, a changeover switch 11E is provided between the LD power supply 11A and the semiconductor lasers 11B and 11C, and power is supplied to one of the semiconductor lasers 11B and 11C by operating the changeover switch 11E. I have.
[0032]
In the recording area 6A, pits carrying signal information are arranged in a track shape, and the reproduction reflected light of the laser beam 52 from the recording area 6A is used as an optical pickup lens while carrying the signal information. The light enters the half mirror 53 via the second and collimator lenses 54, passes through the half mirror 53, and enters the four-division photodiode 17. The photodiode 17 calculates the amount of light received at each of the four divided diode positions to obtain a data signal and error signals for focus and tracking.
[0033]
Since the half mirror 53 is inserted at an angle of 45 ° with respect to the optical path of the return light from the optical disk 1, it performs the same function as a cylindrical lens, and the light beam transmitted through the half mirror 53 has astigmatism. And the amount of focus error is determined according to the shape of the beam spot of the return light on the four-division photodiode 17. The collimator lens 54 can be omitted depending on the situation, and a tracking error can be detected by three beams by inserting a grating between the photodiode 17 and the half mirror 53.
[0034]
This optical pickup device is configured so that signals can be recorded and reproduced with respect to, for example, both optical disks 1 of CD-R and DVD.
[0035]
Hereinafter, the objective lens for an optical recording medium of the present invention will be described more specifically with reference to Examples 1 and 2.
<Example 1>
This optical pickup lens 2 is an objective lens for an optical recording medium suitable for recording and / or reproduction of optical discs having different standards such as DVD1a and CD-R1b. FIG. 2 is a diagram illustrating the optical pickup lens 2 according to the first embodiment. On the first surface 21 and the third surface 23 of the optical pickup lens 2, a free-form surface represented by a free-form surface formula is formed as described above.
[0036]
The second surface 22 of the optical pickup lens 2 reflects the laser beam of 650 nm wavelength for DVD1a and converges it on the recording area of DVD1a, and reflects the laser beam of 780 nm wavelength for CD-R1b. It functions to converge on the recording area of the CD-R 1b. The second surface 22 is a diffractive optical reflecting surface represented by a phase difference function as described above.
[0037]
As shown in FIG. 2, used as an optical pickup device, a DVD 1a (disk substrate thickness: 0.6 mm; refractive index for use light: 1.58) is arranged at a predetermined position (on a turntable), and recording / reproduction is performed. In this case, a laser beam (wavelength Ia represented by a solid line) having a wavelength of 650 nm is incident on the optical pickup lens 2. Thereafter, the laser light is diffracted and reflected by the second surface 22 to have a numerical aperture of 0.6, and is converged on the recording area of the DVD 1a. The refractive index of the constituent material of the optical pickup lens 2 with respect to the laser beam having a wavelength of 650 nm is 1.58537.
[0038]
On the other hand, as shown in FIG. 2, used as an optical pickup device, CD-R1b (disc substrate thickness: 1.2 mm; refractive index for use light: 1.57) is arranged at a predetermined position (on a turntable) and recorded. When the reproduction is performed, a laser beam having a wavelength of 780 nm (a light flux Ib indicated by a dotted line) is incident on the optical pickup lens 2. Thereafter, the laser light is diffracted and reflected by the second surface 22 to have a numerical aperture equivalent to 0.45, and is converged by the optical pickup lens 2 on the recording area of the CD-R 1b. The refractive index of the constituent material of the optical pickup lens 2 with respect to the laser beam having a wavelength of 780 nm is 1.58147.
[0039]
The focal length for DVD is set to 3.5 mm, and the focal length for CD-R is also set to 3.5 mm.
The laser light incident on the optical pickup lens 2 is incident on the optical pickup lens 2 in a state where the laser light is substantially collimated at any wavelength. The region is well converged with the aberration corrected.
[0040]
Table 1 below shows the lens data (the paraxial radius of curvature R in the meridional plane, the surface spacing D for λ = 650 nm and 780 nm, and the refractive index N for λ = 650 nm and 780 nm) in the optical pickup lens 2 according to Example 1. The case of the DVD 1a and the case of the CD-R 1b will be described.
Further, the values of the focal length and RMS-OPD of the optical pickup lens 2 according to the first embodiment are shown for the light used in each case where the DVD 1a and the CD-R 1b are set as the optical recording medium.
Tables 2 and 4 show surface shape polynomial coefficients of the first surface 21 and the third surface 23, respectively.
Table 3 shows coefficients of the phase difference function of the second surface 22.
[0041]
As is apparent from Table 1, the first surface 21, is a paraxial radius of curvature R ₁ in the meridional plane is a 2.9412, the third surface 23, the paraxial curvature at the meridional plane a radius R ₃ Is 3.0725, so R ₃ / R ₁ is about 1.045, which satisfies the conditional expression (5).
[0042]
[Table 1]

[0043]
[Table 2]

[0044]
[Table 3]

[0045]
[Table 4]

[0046]
<Example 2>
As shown in FIG. 3, the optical pickup lens 2 according to the second embodiment has a basic configuration substantially the same as that of the first embodiment, and a description of the same parts as those of the first embodiment will be omitted.
[0047]
Next, Table 5 shows the lens data (the paraxial radius of curvature R in the meridional plane, the surface interval D for λ = 650 nm and 780 nm, and the refractive index N for λ = 650 nm and 780 nm in the meridional plane). ) Are shown for each case of DVD1a and CD-R1b.
Further, the values of the focal length and RMS-OPD of the optical pickup lens 2 according to the second embodiment are shown for the light used in each case where the DVD 1a and the CD-R 1b are set as the optical recording medium.
Tables 6 and 8 show surface shape polynomial coefficients of the first surface 21 and the third surface 23, respectively.
Table 7 shows the coefficients of the phase difference function of the second surface 22.
[0048]
As is apparent from Table 5, the first surface 21, is a paraxial radius of curvature R ₁ in the meridional plane is a 2.6316, the third surface 23, the paraxial curvature at the meridional plane a radius R ₃ Is set to 4.6050, so R ₃ / R ₁ is about 1.750, which satisfies the conditional expression (5).
[0049]
[Table 5]

[0050]
[Table 6]

[0051]
[Table 7]

[0052]
[Table 8]

[0053]
【The invention's effect】
As described above, according to the objective lens and the optical pickup device for an optical recording medium of the present invention, in order from the light source side, the first surface consisting of a free-form surface having a positive refractive power and the predetermined diffractive optical surface And a third surface formed of a free-form surface having a positive refractive power. Further, R ₃ / R ₁ , which is the ratio of the paraxial radius of curvature R ₃ in the meridional plane of the third surface to the paraxial radius of curvature R ₁ in the meridional plane of the first surface, is set to be larger than 1. This makes it possible to converge the used light on each of the recording surfaces of the different types of optical recording media in a state where the aberration is extremely good, and to simultaneously and easily control the spherical aberration and the sine condition. Optical design and lens manufacturing can be facilitated. This also makes it possible to reduce the influence of the manufacturing error on the sensitivity.
[0054]
Therefore, it is possible to converge the light to be used on each of the recording surfaces of the different types of optical recording media in a state in which the aberration is extremely excellent, and to reduce the manufacturing cost and the manufacturing time when manufacturing the lens. It is possible.
[Brief description of the drawings]
FIG. 1 is a lens configuration diagram illustrating an objective lens (optical pickup lens) for an optical recording medium according to an embodiment of the present invention. FIG. 2 is a lens configuration diagram illustrating an optical pickup lens according to Example 1 of the present invention. FIG. 3 is a lens configuration diagram showing an optical pickup lens according to Embodiment 2 of the present invention. FIG. 4 is a schematic diagram showing an optical pickup device of the present invention.
1 Optical disk 1a DVD
1b CD-R
2 Optical pickup lens 6A Recording area 11A LD light source 11B, 11C Semiconductor laser 11D, 53 Half mirror 11E Changeover switch 17 Photodiode 21 First surface 22 Second surface 23 Third surface 52 Laser beam 54 Collimator lens Ia, Ib Light flux

Claims (3)

A prism having a substantially triangular cross section for converging each used light on a recording surface of the optical recording medium related to the used light for a plurality of optical recording media respectively corresponding to different substrate thicknesses and different used light wavelengths. Type optical recording medium objective lens,
A first surface provided to include the substantially triangular first side, which is a free-form surface having a positive refractive power as a light incident surface, and a diffractive optical reflection surface for reflecting incident light. A second surface provided so as to include the substantially triangular second side and a free-form surface having a positive refractive power as a light emitting surface, provided so as to include the substantially triangular third side. With the third surface,
When the paraxial curvature radius in the meridional plane of the first surface and R _1, a paraxial curvature radius in the meridional plane of the third surface and R _3, and characterized by satisfying the following conditional expression (1) Objective lens for optical recording media.
R ₃ / R ₁ > 1 (1) 2. The objective lens for an optical recording medium according to claim 1, wherein the objective lens is integrally formed of glass or plastic. An optical pickup device comprising the objective lens for an optical recording medium according to claim 1.

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