JP4590776B2 - Optical pickup device and objective lens for optical pickup device - Google Patents

Description

【表２】

【００３５】
表１、表２を考察するに、ＤＶＤ使用時における第２光束の透過率を小さくすることで、サイドローブ比が小さくなることがわかった。表２の場合は、サイドローブは小さくなるが、表１と比較してスポット径が大きくなることが傾向にある。表２の例（Ｄ）のケースの場合、ビーム径が１．７％増加しており、開口数が見かけ上０．５９となるが、ＤＶＤの開口数の公差として、０．６±０．０１が一般的なのでこの範囲ならば間題ないと考えられる。また、通常半導体レーザからの発散角が大きい程、光強度が小さくなるので、アポダイゼーション効果があらわれビーム径が大きくなり、サイドローブが小さくなるが、そのような場合でもサイドローブをさらに減少させる効果がある。以下、第２光束の透過率を低減させる手法について説明する。
【００３６】
（１）対物レンズ３の内側光学領域、中間光学領域、外側光学領域とも回折部を設ける（図３（ｂ）参照）。
内側光学領域、外側光学領域の回折部は、ＤＶＤ使用時の波長λ１に対しては、一次回折光が最大となるようにブレーズ化（回折部の形状・ピッチを最適設計すること）されており、内側光学領域と外側光学領域のブレーズ化に用いた波長（以下、ブレーズ化波長という）をそれぞれλＢ１、λＢ３としたときに、内側光学領域はＤＶＤとＣＤの共用領域であるので、λ１≦λＢ１≦λ２とする。一方、外側光学領域は主にＤＶＤに使用する領域なので、λ１≦λＢ３≦（λ１×λ２）^l/2とする。中間光学領域は、主にＣＤに使用する領域なので、波長λ２に対しては一次回折光が最大となるようにブレーズ化されており、そのブレーズ化波長をそれぞれλＢ２としたときに、λＢ２≧λ２とする。
具体的には、λ１＝６５０ｎｍ、λ２＝７８０ｎｍとしたときに、各ブレーズ化波長を以下のように設定する。
λＢ１＝７１０ｎｍ
λＢ２＝９２０ｎｍ
λＢ３＝６５０ｎｍ
このとき、透過率は表３で示すものとなる。尚、光学系の吸収、散乱、反射等の影響を無視して考える（以下、同様）。
【表３】

【００３７】
ここで、回折次数ｍにおいて、ブレーズ化波長をλＢとすると、波長λＢ’における回折効率ηは次式で表せる。
η＝ｓｉｎｃ²（α−ｍ） （３）
但し、ｓｉｎｃ（ｘ）＝（ｓｉｎ（πｘ）／πｘ）、α＝λＢ／λＢ’
【００３８】
（２）対物レンズ３の内側光学領域、中間光学領域、外側光学領域とも回折部を設ける（図３（ｂ）参照）。
内側光学領域、外側光学領域の回折部は、ＤＶＤ使用時の波長λ１に対しては、一次回折光が最大となるようにブレーズ化されており、内側光学領域と外側光学領域のブレーズ化波長をそれぞれλＢ１、λＢ３としたときに、内側光学領域はＤＶＤとＣＤの共用領域であるので、λ１≦λＢ１≦λ２とする。一方、外側光学領域は、主にＤＶＤに使用する領域なので、λ１≦λＢ３≦（λ１×λ２）^l/2とする。中間光学領域は、主にＣＤに使用する領域なので、波長λ２に対しては２次回折光が最大となるようにブレーズ化されており、そのブレーズ化波長をそれぞれλＢ２としたときに、０．８λ１＜λＢ２＜１．２λ２とする。
具体的には、λ１＝６５０ｎｍ、λ２＝７８０ｎｍとしたときに、各ブレーズ化波長を以下のように設定する。
λＢ１＝７１０ｎｍ
λＢ２＝７８０ｎｍ
λＢ３＝６５０ｎｍ
このとき、透過率は表４で示すものとなる。
【表４】

【００３９】
（３）対物レンズ３の内側光学領域、中間光学領域、外側光学領域とも回折部を設ける（図３（ｂ）参照）。
内側光学領域の回折部は、ＤＶＤ使用時の波長λ１に対しては、一次回折光が最大となるようにブレーズ化されており、内側光学領域と外側光学領域のブレーズ化波長をそれぞれλＢ１としたときに、内側光学領域はＤＶＤとＣＤの共用領域であるので、λ１≦λＢ１≦λ２とする。中間光学領域は、主にＣＤに使用する領域なので、波長λ２に対しては２次回折光が最大となるようにブレーズ化されており、そのブレーズ化波長をそれぞれλＢ２としたときに、０．８λ１＜λＢ２＜１．２λ２とする。更に、外側光学領域は、主にＤＶＤに使用する領域なので、ＤＶＤ使用時の波長λ１に対しては、２次回折光が最大となるようにブレーズ化化されており、そのブレーズ化化波長をそれぞれλＢ３としたときに、０．８λ１＜λＢ３＜１．２λ１とする。
具体的には、λ１＝６５０ｎｍ、λ２＝７８０ｎｍとしたときに、各ブレーズ化波長を以下のように設定する。
λＢ１＝７１０ｎｍ
λＢ２＝７８０ｎｍ
λＢ３＝６５０ｎｍ
このとき、透過率は表５で示すものとなる。尚、表５においてＣＤ使用時における外側光学領域の透過率が低くなっているが、この領域を通過する光束は、ＣＤ使用時にはフレア光となるので、問題はない。
【表５】

【００４０】
（４）対物レンズ３において、第１光束のみが通過する内側光学領域に屈折面を形成し、第２光束のみが通過する中間光学領域に回折部を形成し、第３光束のみが通過する外側光学領域に屈折面を形成する（図３（ｄ）参照）。
中間光学領域は、主にＣＤに使用する領域なので、その回折部は、ＣＤ使用時の波長λ２に対しては２次回折光が最大となるようにブレーズ化されており、そのブレーズ化波長をそれぞれλＢ２としたときに、０．８λ２＜λＢ２＜１．２λ２とする。
具体的には、λ１＝６５０ｎｍ、λ２＝７８０ｎｍとしたときに、λＢ２＝７８０ｎｍとする。
このとき、透過率は表６で示すものとなる。
【表６】

【００４１】
（５）対物レンズ３において、第１光束のみが通過する内側光学領域に屈折面を形成し、第２光束のみが通過する中間光学領域に回折部を形成し、第３光束のみが通過する外側光学領域に回折部を形成する（図３（ｅ）参照）。
中間光学領域は、主にＣＤに使用する領域なので、その回折部は、ＣＤ使用時の波長λ２に対しては２次回折光が最大となるようにブレーズ化されており、そのブレーズ化波長をそれぞれλＢ２としたときに、０．８λ２＜λＢ２＜１．２λ２とする。外側光学領域は、主にＤＶＤに使用する領域なので、その回折部は、ＣＤ使用時の波長λ１に対しては２次回折光が最大となるようにブレーズ化化されており、そのブレーズ化波長をそれぞれλＢ３としたときに、０．８１＜λＢ３＜１．２λ１とする。
具体的には、λ１＝６５０ｎｍ、λ２＝７８０ｎｍとしたときに、λＢ２＝７８０ｎｍ、λＢ３＝６５０ｎｍとする。
このとき、透過率は表７で示すものとなる。
【表７】

【００４２】
（６）対物レンズ３の内側光学領域、中間光学領域、外側光学領域とも回折部を設ける（図３（ｂ）参照）。
各領域の回折部とも、ＤＶＤ使用時の波長λ１に対しては一次回折光が最大となるようにブレーズ化されており、金型作成のしやすさから一定のブレーズ化波長とすると好ましい。ここでは、各領域のブレーズ化波長をそれぞれλＢ１、λＢ２、λＢ３としたときに、λ１≦λＢ１＝λＢ２＝λＢ３≦λ２とする。
具体的には、λ１＝６５０ｎｍ、λ２＝７８０ｎｍとしたときに、各ブレーズ化波長を以下のように設定する。
λＢ１＝７１０ｎｍ
λＢ２＝７１０ｎｍ
λＢ３＝７１０ｎｍ
このとき回折部での透過率は、表８で表されるとおり、各領域で等しいものとなる。
【表８】

【００４３】
そこで、ここでは図３（ｂ）に示すように、中間光学領域を通る第２光束を、さらに透過率が波長に依存するダイクロコートＤＣを施した光学面Ｓ２を通過させる。このダイクロコートＤＣの特性として、波長λ１、λ２の光束の透過率をＴ（λ１）、Ｔ（λ２）としたときに、Ｔ（λ１）＜Ｔ（λ２）とし、Ｔ（λ２）を１００％に近づける構成とすることが好ましい。プラスチック対物レンズの光学面にダイクロコートを施す場合、Ｔ（λ１）＝０とするのは難しいが、比較的少ない層数でＴ（λ１）＝５０％前後とすることは可能である。ここでは、Ｔ（λ１）＝５０％、Ｔ（λ２）＝１００％のダイクロコートＤＣを用いたとして、各領域の透過率は、表９に示すようなものとなる。
【表９】

【００４４】
以上のシュミレーションにおいては、対物レンズに入射する光束を一定の光量としているが、実際の半導体レーザーの出射光は、周辺光量が少ないことから、第１光束より第２光束の方が光量が少なく、第２光束より第３光束の方が光量が少ないことが多いが、同様な考え方により第２光束の光量を低下させることは可能である。又、本実施の形態では、光量低下を５０％を目標としているが、ブレーズ化波長や、最も効率の高い次数を変更することで、幅広い制御が可能となる。加えて、本実施の形態では、対物レンズに回折部を設けることで第２光束の光量低下を達成しているが、対物レンズとは別個の平行平板に、回折部やダイクロコート、或いは吸収膜を設けたり、液晶光学素子を用いることでも、同様に第２光束の光量低下を達成できる
【００４５】
次に、透過率分布の測定法について説明する。図４に測定装置の一例の概要を示す。この例で、被検レンズとしては、平行光入射に最適化された対物レンズの場合である。半導体レーザ１０１からの光束を、コリメ一タ１０２で平行光束とし、絞り１０３を介して被検レンズ１０４に入射させる。被検レンズ１０４を通過した光束は、光ディスクの透明基板に相当するカバーガラス１０５を介して、集光され、集光光束は、再び顕微鏡対物レンズ１０６によって、後述する被検レンズ１０４の位置調整を経て平行光となり、ビームスプリッタ１０７を通過し、ビームエキスパンダ１０８で光束径が拡大され、第１ＣＣＤ撮像装置１０９に入射する。第１ＣＣＤ撮像装置１０９からの出力は、第１モニター１１０に表示されるとともに、画像処理装置１１１へと転送される。第１モニター１１０には、例えば対物レンズを被検レンズ１０４とした場合、ほば同心円上の３領域にわかれ、中間の領域が暗くなっているのが観察される。また、画像処理装置１１１により、円の中心の光量を１００％とした光量分布が算出される。
【００４６】
被検レンズ１０４の位置調整は、ビームスプリッタ１０７で反射された光束を利用して行われる。ビームスプリッタ１０７で反射された光束は、集光レンズ１１２で第２ＣＣＤ撮像装置１１３の撮像面上に入射される。第２ＣＣＤ撮像装置１１３からの出力は、第２モニター１１４に表示される。被検レンズ１０４と顕微鏡対物レンズ１０６を光路からはずした状態で、第２モニター１１４を見ながら、第２モニター１１４のほぼ中心に集光スポットが観察されるように集光レンズ１１２を調整する。次に、被検レンズ１０４と顕微鏡対物レンズ１０６をセットし、前記集光スポットが観察された位置に、集光スポットが来るようにすれば、被検レンズ１０４の位置を調整することができる。
【００４７】
【発明の効果】
本発明によれば、異なる光情報記録媒体に対して情報の記録又は再生を行う場合に、いずれの光情報記録媒体に対しても情報の再生を的確に行える光ピックアップ装置及びそ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】情報記録面上のスポットプロファイルの例を示す図である。
【図２】本実施の形態の光ピックアップ装置の概略構成図である。
【図３】対物レンズの断面図である。
【図４】透過率測定装置の概略構成図である。
【符号の説明】
３ 対物レンズ
８ 絞り
９ シリンドリカルレンズ
１１ 第１光源
１２ 第２光源
１５ カップリングレンズ
１６ 凹レンズ
１７ ホログラム
２１、２２ カップリングレンズ
４１，４２ 光検出器
６１，６２ ビームスプリッタ
７１，７２ １／４波長板[0001]
[Industrial application fields]
The present invention relates to an optical pickup device for recording and / or reproducing information on different optical information recording media, and an objective lens for the optical pickup device.
[0002]
[Prior art]
In recent years, the thickness of the transparent substrate on the recording surface and the laser beam for recording / reproduction, such as a CD-R that can be recorded / reproduced and a DVD with a high recording density, which is about the same size as a CD that is a conventional optical information recording medium With the development of a plurality of optical information recording media having different wavelengths, it is required to record or reproduce information on these optical information recording media using the same optical pickup device. For this reason, various types of optical pickup devices have been proposed that converge a laser beam with a necessary numerical aperture on a recording surface using the same objective lens while having a plurality of laser light sources according to the wavelength used.
[0003]
In such an optical pickup device, an objective lens as described in JP-A-11-96585 is widely used. In this objective lens, one optical surface of the objective lens is divided into a first divided surface near the optical axis and a third divided surface located with the second divided surface between the first divided surface. When the DVD is recorded or reproduced, the light beam that has passed through the first divided surface and the third divided surface is mainly used, and when the CD is recorded or reproduced, the first divided surface is mainly used. And the light beam that has passed through the second split surface is used.
[0004]
Here, in such an optical pickup device, if the second dividing surface is made small, the wavefront aberration during recording or reproduction with respect to a CD type optical disk (CD-R, CD-RW, CD-Video, CD-ROM, etc.) becomes large. On the other hand, if the second dividing surface is increased, the light utilization efficiency of the DVD deteriorates, and it becomes difficult to record on a DVD-type recordable optical disc (DVD-RAM, DVD-R, DVD-RW, DVD + RW, etc.). In addition, it is necessary to increase the power of the laser light source even during reproduction.
[0005]
Various optical systems for optical pickup devices using diffraction elements have also been proposed. Japanese Patent Application Laid-Open No. 9-54973 discloses an optical system using a hologram optical element that uses transmitted light (0th order diffracted light) for a light beam with a wavelength of 635 nm, and uses a −1st order diffracted light for a light beam with a wavelength of 785 nm, and An optical system using a hologram optical element that uses + 1st order diffracted light for a light beam and transmitted light (0th order diffracted light) for a light beam having a wavelength of 785 nm is disclosed.
[0006]
Further, Japanese Patent Laid-Open No. 10-283668 discloses an optical system including a hologram type ring lens that transmits 100% of a light beam having a wavelength of 650 nm and first-order diffracts a light beam having a wavelength of 780 nm. Japanese Patent Application No. 2000-013071 filed by the present applicant proposes an optical pickup device using a diffraction element that uses the same diffraction order for two light sources having different wavelengths.
[0007]
The optical pickup device described in the above-mentioned application specification uses the + 1st order diffracted light from the diffraction pattern of the objective lens for the light flux from the first light source to record and / or record information on the DVD. Alternatively, information is recorded and / or reproduced with respect to the CD by performing reproduction and using the + 1st order diffracted light from the diffraction pattern of the objective lens for the light beam from the second light source.
[0008]
In a more specific embodiment, the wavelength λ1 of the first light source is about 650 nm, and the wavelength λ2 of the second light source is about 780 nm. The diffraction pattern provided on at least one surface of the objective lens is rotationally symmetric with respect to the optical axis, and is farthest from the optical axis of the diffraction pattern of the objective lens in the light flux from the first light source. The + 1st order diffracted light from the circumference is converted into a light beam with a numerical aperture of NAH1 on the optical information recording medium side, and from the circumference on the most optical axis side of the diffraction pattern of the objective lens in the light beam from the first light source. + 1st order diffracted light is converted into a light flux with a numerical aperture on the optical information recording medium side of NAL1,
NAH1 <0.6 (1)
0 ≤ NAL1 ≤ 0.45 (2)
The above conditions are satisfied.
[0009]
At this time, light fluxes having a numerical aperture NAH1 to a numerical aperture 0.6 pass only through the refractive surface of the objective lens or through the second diffractive surface. Any optical system using the above-described diffractive element can minimize the spherical aberration and maximize the transmittance over the entire luminous flux during DVD recording or reproduction while maintaining the desired optical characteristics of the CD. This is an attempt to improve the performance of DVD.
[0010]
[Problems to be solved by the invention]
However, the lenses of Japanese Patent Application Laid-Open No. 11-96585 and Japanese Patent Application No. 2000-013071 were prototyped and the light utilization efficiency and reproduction performance of DVD and CD were compared. As a result, the latter was related to DVD and CD signal levels and CD reproduction performance. Although it is excellent, it has been found that the reproduction performance of the DVD generally deteriorates.
[0011]
The present invention has been made in view of such problems, and when information is recorded or reproduced on different optical information recording media, the information is accurately reproduced on any optical information recording medium. An object of the present invention is to provide an optical pickup device that can be used and an objective lens thereof.
[0012]
[Means for Solving the Problems]
The optical pickup device according to claim 1, wherein the thickness of the transparent substrate is t. ₁ The wavelength λ is adapted to record or reproduce information by irradiating the first optical information recording medium with a light beam. ₁ The thickness of the first light source and the transparent substrate is t ₂ (T ₁ <T ₂ ) Is a wavelength λ for recording or reproducing information by irradiating the second optical information recording medium with a light beam. ₂ (Λ ₁ <Λ ₂ ) And the condensing optics for condensing the light beams emitted from the first and second light sources onto the information recording surface via the transparent substrates of the first and second optical information recording media. The optical surface of the optical element constituting the condensing optical system is made to have a different distance from the optical axis in a direction intersecting the optical axis. 3 One more than The optical region is formed, Further, in the inner optical region near the optical axis of the optical element constituting the condensing optical system, the outer optical region far from the optical axis, and the intermediate optical region between the inner optical region and the outer optical region, A diffraction part having a diffraction ring zone is formed, When recording or reproducing information on the first optical information recording medium using a light beam from the first light source, Said A first light beam passing through the inner optical region; Said In the image plane where the wavefront aberration of the wavefront formed by the third light flux passing through the outer optical region is minimized, By passing through the diffraction part of the intermediate optical region , Said Transmittance of the second light flux through the intermediate optical region The Lower than the transmittance of the first light flux and the transmittance of the third light flux Comb and at least the first light flux passes through the diffractive portion of the inner optical region, whereby aberration correction is performed on both the first optical information recording medium and the second optical information recording medium. be able to It is characterized by that. Here, the image plane in which the wavefront aberration of the wavefront formed by the first light flux and the third light flux is minimized is, for example, recording on the first optical information recording medium when the focusing optical system is in focus. Say the face. The present invention will be described below. The three or more optical regions formed with different distances from the optical axis in the direction intersecting the optical axis may all be formed on one optical surface in the same optical element. May be formed on different optical surfaces, or may be formed over a plurality of optical surfaces of different optical elements. That is, it is sufficient if three or more optical regions are formed in the entire condensing optical system.
[0013]
First, a DVD will be described as an example of the first optical information recording medium, and a CD will be described as an example of the second optical information recording medium. It has been found that the improvement of the reproduction performance of the DVD can be achieved by reducing the spot diameter of the imaging spot on the information surface of the DVD and reducing the side lobe.
[0014]
FIG. 1A is a diagram showing an example of a spot profile on the information recording surface when recording or reproducing information on a CD, and FIG. 1B is a spot on the information recording surface when recording or reproducing information on a DVD. It is a figure which shows the example of a profile. In each figure, the vertical axis indicates the amount of light, and the horizontal axis indicates the position from the center.
[0015]
As is clear from FIG. 1A, the CD spot profile has a large main lobe Mc because of the need to increase the spot diameter, and side lobes to form flare light outside the numerical aperture of the CD. Sc is also clear.
[0016]
On the other hand, as shown in FIG. 1B, in the spot profile of DVD, the main lobe Mv is narrowed because the spot diameter needs to be reduced. However, if a method of increasing the numerical aperture is employed to reduce the spot diameter, coma aberration generated when the recording surface of the DVD is tilted increases, thereby increasing the side lobe Sv. It is known that when the side lobe Sv is increased, jitter is increased in the optical pickup device and the reproduction performance of the DVD is deteriorated.
[0017]
On the other hand, as a result of earnest research, the present inventors have performed recording or reproduction of information on the first optical information recording medium (here, DVD) using the light beam from the first light source. In the image plane in which the wavefront aberration of the wavefront formed by the first light beam passing through the inner optical region near the optical axis and the third light beam passing through the outer optical region far from the optical axis is minimized, The side lobe Sv is suppressed by making the transmittance of the second light beam passing through the intermediate optical region between the inner optical region and the outer optical region lower than the transmittance of the first light beam and the transmittance of the third light beam. I discovered what I could do. The technique for reducing the transmittance of the second light flux will be specifically described later by taking an example as an example.
[0018]
When the optical pickup apparatus according to claim 2 records or reproduces information on the second optical information recording medium (here, a CD) by using a light beam from the second light source, Since the transmittance of the third light flux is lower than the transmittance of the first light flux and the transmittance of the second light flux in the image plane where the wavefront aberration of the wavefront formed by the light flux and the second light flux is minimized, The amount of the third light flux that should be flare light can be reduced, thereby suppressing the possibility of so-called false detection in which the third light flux is detected by the photodetector during information storage or reproduction. Here, the image surface where the wavefront aberration of the wavefront formed by the first light beam and the second light beam is minimized is, for example, recording on the second optical information recording medium when the focusing optical system is in focus. Say the face.
[0019]
The optical pickup device according to claim 3, wherein the condensing optical system is changed to reduce the divergence of the light flux from the first light source or the second light source, and at least in the optical axis direction. And an objective lens that can be transferred to The optical element is the objective lens Compared to the case where the objective lens is used at a finite magnification, the objective lens can be designed more easily. However, one optical region may be formed on one optical surface of the objective lens, and two or more optical regions may be formed on the other optical surface, or two optical regions may be formed on one optical surface of the objective lens. A region may be formed, and one or more optical regions may be formed on another optical surface.
[0023]
The objective lens for an optical pickup device according to claim 4, wherein the thickness of the transparent substrate is t. ₁ The wavelength λ is adapted to record or reproduce information by irradiating the first optical information recording medium with a light beam. ₁ The thickness of the first light source and the transparent substrate is t ₂ (T ₁ <T ₂ ) Is a wavelength λ for recording or reproducing information by irradiating the second optical information recording medium with a light beam. ₂ (Λ ₁ <Λ ₂ And an objective lens for condensing the light beams emitted from the first and second light sources onto the information recording surface via the transparent substrates of the first and second optical information recording media. And an optical pickup device having a condensing optical system, wherein the distance from the optical axis is varied in the direction intersecting the optical axis on the optical surface of the objective lens. 3 One more than The optical region is formed, Further, a diffractive portion having a diffraction ring zone in an inner optical region near the optical axis of the objective lens, an outer optical region far from the optical axis, and an intermediate optical region between the inner optical region and the outer optical region. Is formed When recording or reproducing information with respect to the first optical information recording medium using a light beam from the first light source, Said A first light beam passing through the inner optical region; Said In the image plane where the wavefront aberration of the wavefront formed by the third light flux passing through the outer optical region is minimized, By passing through the diffraction part of the intermediate optical region , Said Transmittance of the second light flux through the intermediate optical region The Lower than the transmittance of the first light flux and the transmittance of the third light flux Comb and at least the first light flux passes through the diffractive portion of the inner optical region, whereby aberration correction is performed on both the first optical information recording medium and the second optical information recording medium. be able to It is characterized by that. The operational effects of the present invention are the same as the operational effects of the invention described in claim 1.
[0024]
Claim 5 The objective lens for an optical pickup device according to the item 1, wherein the first light beam and the first light beam are recorded when information is recorded on or reproduced from the second optical information recording medium using the light beam from the second light source. The transmittance of the third light flux is lower than the transmittance of the first light flux and the transmittance of the second light flux on the image plane where the wavefront aberration of the wavefront formed by the two light fluxes is minimized. The operational effects of the present invention are the same as the operational effects of the invention described in claim 2.
[0029]
The “diffractive part” used in the present specification refers to a part provided with a relief on the surface of an optical element so as to condense or diverge a light beam by diffraction. As the shape of the relief, for example, as shown in FIG. 3B to be described later, the surface of the objective lens is formed as a substantially concentric annular zone centering on the optical axis, and the cross section is a plane including the optical axis. Each ring zone is known to have a sawtooth-like shape, but includes such a shape, and such a shape is particularly referred to as a “diffraction ring zone”.
[0030]
In this specification, the objective lens is, in a narrow sense, a light collecting action that is arranged to face the optical information recording medium at the position closest to the optical information recording medium when the optical information recording medium is loaded in the optical pickup device. In a broad sense, it refers to a lens group that can be operated at least in the optical axis direction by an actuator together with the lens. Here, the lens group refers to at least one lens (for example, two lenses). Therefore, in this specification, the numerical aperture NA on the optical information recording medium side (image side) of the objective lens means the numerical aperture NA of the light beam passing through the lens surface located closest to the optical information recording medium of the objective lens. It is what you point to. Further, in this specification, the required numerical aperture NA is the numerical aperture specified by the standard of each optical information recording medium, or the information of the information depending on the wavelength of the light source used for each optical information recording medium. The numerical aperture of an objective lens having a diffraction limited performance capable of obtaining a spot diameter necessary for recording or reproduction is shown.
[0031]
In the present specification, the second optical information recording medium refers to various optical discs such as CD-R, CD-RW, CD-Video, CD-ROM, etc., and the first optical information recording medium. The term “DVD-ROM”, “DVD-RAM”, “DVD-R”, “DVD-RW”, and “DVD-Video” refers to various types of DVD optical disks. Further, in this specification, the thickness t of the transparent substrate includes t = 0.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail with reference to the drawings. FIG. 2 is a schematic configuration diagram of the optical pickup device according to the present embodiment. In FIG. 2, the optical pickup device includes a first light source 11 for performing recording and / or reproduction with respect to a first optical information recording medium (for example, DVD), and a second optical information recording medium (for example, CD). A cup for recording and / or reproducing with respect to the first light source 11 and a second light source 12 having a wavelength different from that of the first light source 11 and converting a divergence angle of a divergent light beam emitted from each light source into a desired divergence angle. Ring lenses 21 and 22, a beam splitter 62 which is a light combining means for combining the light beams so as to travel in almost one direction, and an object for condensing the light beams from the beam splitter 62 on the information recording surface 5 of the optical information recording medium. A lens 3 and photodetectors 41 and 42 for receiving reflected light from the optical information recording medium are provided. In the figure, 8 is a stop, 9 is a cylindrical lens, 71 and 72 are quarter-wave plates, 15 is a coupling lens for reducing the divergence of a divergent light beam from the light source 11, 16 is a concave lens, and 17 is a reflected light beam. , 61 is a beam splitter for emitting reflected light from the information recording surface and directing it toward the detector 41. The condensing optical system in the present embodiment is composed of the coupling lenses 15 and 21, the beam splitters 61 and 62, the quarter wavelength plate 71 and the objective lens 3 when using a DVD, and the coupling when using a CD. The lens 22 includes a beam splitter 62, a quarter-wave plate 72, and the objective lens 3. The objective lens 3 can be shifted in the optical axis direction by an actuator (not shown).
[0033]
FIG. 3 is a diagram schematically illustrating an example of a diffraction portion provided in the objective lens. Here, when the wavefront aberration at the best image point of the light beam emitted from the condensing optical system is 0.001 λrms or less, that is, when it can be considered that there is no geometrical aberration, the maximum numerical aperture NA1 of the condensing optical system is 0. .6, the luminous flux is divided into a first luminous flux in the vicinity of the optical axis (0 ≦ NA <NAL), a second luminous flux outside the first luminous flux (NAL ≦ NA ≦ NAH), and an outer side of the second luminous flux. To the third light flux (NAH <NA ≦ 0.6). For example, in the objective lens shown in FIG. 3B, the first light beam passes through the inner optical region, the second light beam passes through the intermediate optical region, and the third light beam passes through the outer optical region.
[0034]
Tables 1 and 2 show the calculation results of the beam diameter and the side lobe ratio when uniform transmittance is given to each light beam. Table 1 shows the case of NAL = 0.34 and NAH = 0.47, and Table 2 shows the case of NAL = 0.39 and NAH = 0.47. The beam diameter is defined as the diameter at which the intensity is 13.5% of the peak intensity, and the side lobe ratio is the ratio of the peak intensity of the primary side lobe to the peak intensity of the main lobe. The transmittance in the table is based on the simulation calculation result. Actually, the transmittance tends to slightly decrease due to the transmittance of the material itself, the shape of the die cutting tool, the draft angle, and the like. .
[Table 1]

[Table 2]

[0035]
Considering Tables 1 and 2, it was found that the sidelobe ratio is reduced by reducing the transmittance of the second light flux when using the DVD. In the case of Table 2, the side lobe is small, but the spot diameter tends to be large as compared with Table 1. In the case of the example (D) in Table 2, the beam diameter is increased by 1.7% and the numerical aperture is apparently 0.59. However, the tolerance of the numerical aperture of DVD is 0.6 ± 0. Since 01 is general, there is no problem in this range. In general, the larger the divergence angle from a semiconductor laser is, the smaller the light intensity becomes. Therefore, the apodization effect appears, the beam diameter increases, and the side lobe becomes smaller. is there. Hereinafter, a method for reducing the transmittance of the second light beam will be described.
[0036]
(1) A diffraction part is provided in each of the inner optical region, intermediate optical region, and outer optical region of the objective lens 3 (see FIG. 3B).
The diffractive parts in the inner and outer optical areas are blazed (optimally designed for the shape and pitch of the diffractive parts) to maximize the first-order diffracted light for the wavelength λ1 when using DVD. When the wavelengths used for blazing the inner optical region and the outer optical region (hereinafter referred to as blazed wavelengths) are λB1 and λB3, respectively, the inner optical region is a shared region for DVD and CD, so λ1 ≦ λB1 ≦ λ2. On the other hand, since the outer optical area is an area mainly used for DVD, λ1 ≦ λB3 ≦ (λ1 × λ2) ^{l / 2} And Since the intermediate optical region is a region mainly used for CD, it is blazed so as to maximize the first-order diffracted light with respect to the wavelength λ2, and when the blazed wavelength is λB2, λB2 ≧ λ2 And
Specifically, when λ1 = 650 nm and λ2 = 780 nm, each blazed wavelength is set as follows.
λB1 = 710nm
λB2 = 920 nm
λB3 = 650 nm
At this time, the transmittance is as shown in Table 3. Note that the effects of absorption, scattering, reflection, etc. of the optical system are ignored (hereinafter the same).
[Table 3]

[0037]
Here, when the blazed wavelength is λB at the diffraction order m, the diffraction efficiency η at the wavelength λB ′ can be expressed by the following equation.
η = sinc ² (Α-m) (3)
Where sinc (x) = (sin (πx) / πx), α = λB / λB ′
[0038]
(2) A diffraction part is provided in each of the inner optical region, intermediate optical region, and outer optical region of the objective lens 3 (see FIG. 3B).
The diffractive portions of the inner optical region and the outer optical region are blazed so as to maximize the first-order diffracted light with respect to the wavelength λ1 when using the DVD, and the blazed wavelengths of the inner optical region and the outer optical region are changed. When λB1 and λB3, respectively, the inner optical area is a shared area for DVD and CD, so that λ1 ≦ λB1 ≦ λ2. On the other hand, since the outer optical area is an area mainly used for DVD, λ1 ≦ λB3 ≦ (λ1 × λ2) ^{l / 2} And Since the intermediate optical region is a region mainly used for CD, it is blazed so as to maximize the second-order diffracted light with respect to the wavelength λ2, and when the blazed wavelength is λB2, 0.8λ1 <ΛB2 <1.2λ2.
Specifically, when λ1 = 650 nm and λ2 = 780 nm, each blazed wavelength is set as follows.
λB1 = 710nm
λB2 = 780 nm
λB3 = 650 nm
At this time, the transmittance is as shown in Table 4.
[Table 4]

[0039]
(3) A diffraction part is provided in each of the inner optical region, intermediate optical region, and outer optical region of the objective lens 3 (see FIG. 3B).
The diffractive portion of the inner optical region is blazed so as to maximize the first-order diffracted light with respect to the wavelength λ1 when using the DVD, and the blazed wavelengths of the inner optical region and the outer optical region are set to λB1, respectively. Sometimes, the inner optical area is a shared area for DVD and CD, so λ1 ≦ λB1 ≦ λ2. Since the intermediate optical region is a region mainly used for CD, it is blazed so as to maximize the second-order diffracted light with respect to the wavelength λ2, and when the blazed wavelength is λB2, 0.8λ1 <ΛB2 <1.2λ2. Further, since the outer optical area is an area mainly used for DVD, it is blazed so that the second-order diffracted light is maximized with respect to the wavelength λ1 when using the DVD, and the blazed wavelength is set respectively. When λB3, 0.8λ1 <λB3 <1.2λ1.
Specifically, when λ1 = 650 nm and λ2 = 780 nm, each blazed wavelength is set as follows.
λB1 = 710nm
λB2 = 780 nm
λB3 = 650 nm
At this time, the transmittance is as shown in Table 5. In Table 5, the transmittance of the outer optical region when using the CD is low. However, since the light beam passing through this region becomes flare light when using the CD, there is no problem.
[Table 5]

[0040]
(4) In the objective lens 3, a refractive surface is formed in the inner optical region through which only the first light beam passes, and a diffractive portion is formed in the intermediate optical region through which only the second light beam passes, and the outer side through which only the third light beam passes. A refractive surface is formed in the optical region (see FIG. 3D).
Since the intermediate optical region is mainly used for a CD, the diffractive portion is blazed so that the second-order diffracted light is maximized with respect to the wavelength λ2 when the CD is used. When λB2, 0.8λ2 <λB2 <1.2λ2.
Specifically, when λ1 = 650 nm and λ2 = 780 nm, λB2 = 780 nm.
At this time, the transmittance is as shown in Table 6.
[Table 6]

[0041]
(5) In the objective lens 3, a refractive surface is formed in the inner optical region through which only the first light beam passes, and a diffractive portion is formed in the intermediate optical region through which only the second light beam passes, and the outer side through which only the third light beam passes. A diffraction part is formed in the optical region (see FIG. 3E).
Since the intermediate optical region is mainly used for a CD, the diffractive portion is blazed so that the second-order diffracted light is maximized with respect to the wavelength λ2 when the CD is used. When λB2, 0.8λ2 <λB2 <1.2λ2. Since the outer optical region is a region mainly used for DVD, the diffractive portion is blazed so that the second-order diffracted light becomes maximum with respect to the wavelength λ1 when the CD is used. When λB3 is set, 0.81 <λB3 <1.2λ1.
Specifically, when λ1 = 650 nm and λ2 = 780 nm, λB2 = 780 nm and λB3 = 650 nm.
At this time, the transmittance is as shown in Table 7.
[Table 7]

[0042]
(6) Diffraction portions are provided in the inner optical region, intermediate optical region, and outer optical region of the objective lens 3 (see FIG. 3B).
The diffractive portions in each region are blazed so as to maximize the first-order diffracted light with respect to the wavelength λ1 when the DVD is used, and it is preferable to have a constant blazed wavelength for ease of mold production. Here, λ1 ≦ λB1 = λB2 = λB3 ≦ λ2, where blazed wavelengths in each region are λB1, λB2, and λB3, respectively.
Specifically, when λ1 = 650 nm and λ2 = 780 nm, each blazed wavelength is set as follows.
λB1 = 710nm
λB2 = 710nm
λB3 = 710 nm
At this time, as shown in Table 8, the transmittance in the diffraction part is equal in each region.
[Table 8]

[0043]
Therefore, here, as shown in FIG. 3B, the second light flux passing through the intermediate optical region is further passed through the optical surface S2 on which dichroic coating DC whose transmittance depends on the wavelength is applied. As a characteristic of the dichroic coat DC, T (λ1) <T (λ2) where T (λ1) <T (λ2), and T (λ2) is 100%, where T (λ1), T (λ2) It is preferable to make it the structure which approaches. When dichroic coating is applied to the optical surface of the plastic objective lens, it is difficult to set T (λ1) = 0, but it is possible to set T (λ1) = about 50% with a relatively small number of layers. Here, assuming that a dichroic coat DC of T (λ1) = 50% and T (λ2) = 100% is used, the transmittance of each region is as shown in Table 9.
[Table 9]

[0044]
In the above simulation, the light beam incident on the objective lens has a constant light amount. However, since the actual light emitted from the semiconductor laser has a small amount of peripheral light, the second light beam has less light than the first light beam. Although the amount of light of the third light beam is often smaller than that of the second light beam, it is possible to reduce the light amount of the second light beam by the same concept. In the present embodiment, the reduction in the amount of light is targeted to be 50%, but a wide range of control is possible by changing the blazed wavelength and the most efficient order. In addition, in the present embodiment, the light quantity of the second light flux is reduced by providing the objective lens with a diffractive part, but the diffractive part, dichroic coat, or absorption film is formed on a parallel plate separate from the objective lens. Or using a liquid crystal optical element can similarly achieve a reduction in the amount of light of the second light beam.
[0045]
Next, a method for measuring the transmittance distribution will be described. FIG. 4 shows an outline of an example of the measuring apparatus. In this example, the test lens is an objective lens optimized for parallel light incidence. The light beam from the semiconductor laser 101 is converted into a parallel light beam by the collimator 102 and is incident on the lens 104 to be examined through the stop 103. The light beam that has passed through the test lens 104 is collected through a cover glass 105 corresponding to a transparent substrate of the optical disk, and the focused light beam is again adjusted by the microscope objective lens 106 to adjust the position of the test lens 104 to be described later. After that, it becomes parallel light, passes through the beam splitter 107, the beam diameter is enlarged by the beam expander 108, and enters the first CCD image pickup device 109. The output from the first CCD imaging device 109 is displayed on the first monitor 110 and transferred to the image processing device 111. In the first monitor 110, for example, when the objective lens is the lens 104 to be examined, it is observed that the middle region is darkened, and is roughly divided into three regions on concentric circles. Further, the image processing device 111 calculates a light amount distribution with the light amount at the center of the circle as 100%.
[0046]
The position of the test lens 104 is adjusted using the light beam reflected by the beam splitter 107. The light beam reflected by the beam splitter 107 is incident on the imaging surface of the second CCD imaging device 113 by the condenser lens 112. The output from the second CCD image pickup device 113 is displayed on the second monitor 114. While the test lens 104 and the microscope objective lens 106 are removed from the optical path, the condensing lens 112 is adjusted so that the condensing spot is observed at the approximate center of the second monitor 114 while looking at the second monitor 114. Next, the position of the test lens 104 can be adjusted by setting the test lens 104 and the microscope objective lens 106 so that the focus spot comes to the position where the focus spot is observed.
[0047]
【The invention's effect】
According to the present invention, there is provided an optical pickup device and its objective lens capable of accurately reproducing information on any optical information recording medium when information is recorded or reproduced on different optical information recording media. can do.
[Brief description of the drawings]
FIG. 1 is a diagram showing an example of a spot profile on an information recording surface.
FIG. 2 is a schematic configuration diagram of an optical pickup device according to the present embodiment.
FIG. 3 is a cross-sectional view of an objective lens.
FIG. 4 is a schematic configuration diagram of a transmittance measuring device.
[Explanation of symbols]
3 Objective lens
8 Aperture
9 Cylindrical lens
11 First light source
12 Second light source
15 coupling lens
16 Concave lens
17 Hologram
21, 22 Coupling lens
41, 42 photodetector
61, 62 Beam splitter
71,72 1/4 wave plate

Claims (5)

A first light source having a wavelength λ ₁ for recording or reproducing information by irradiating a light beam onto a first optical information recording medium having a thickness t ₁ of the transparent substrate; Wavelength λ ₂ (λ ₁ <λ) where information is recorded or reproduced by irradiating a second optical information recording medium having a thickness of t ₂ (t ₁ <t ₂ ) with a light beam. ₂ ) and a light condensing light beam emitted from the first and second light sources on the information recording surface through the transparent substrates of the first and second optical information recording media. In an optical pickup device having an optical system,
Three or more optical regions are formed on the optical surface of the optical element constituting the condensing optical system at different distances from the optical axis in a direction intersecting the optical axis, and the condensing optical system A diffractive portion having a diffraction ring zone in an inner optical region on the side close to the optical axis of the optical element constituting the optical element, an outer optical region on the side far from the optical axis, and an intermediate optical region between the inner optical region and the outer optical region Is formed,
When recording or reproducing information for the first optical information recording medium with a light beam from the first light source, the first light flux passing through the inner optical zone, passes through the outer optical zone in the image plane wavefront aberration of the wavefront and the third light flux forms is minimized, the intermediate by passing through the diffraction portion of the optical region, the first light flux transmittance of the second light flux passing through the intermediate optical zone the transmission and the third lower comb than the transmittance of the light beam, and at least the first light flux, the by passing through the diffraction portion of the inner optical zone, wherein the first optical information recording medium and the second An optical pickup device capable of correcting aberrations for both optical information recording media .   When recording or reproducing information to or from the second optical information recording medium using the light beam from the second light source, the wavefront aberration of the wavefront formed by the first light beam and the second light beam is minimized. 3. The optical pickup device according to claim 2, wherein the transmittance of the third light flux is lower than the transmittance of the first light flux and the transmittance of the second light flux in the image plane. The condensing optical system includes a coupling optical system that changes so as to reduce the divergence of a light beam from the first light source or the second light source, and an objective lens that can be shifted at least in the optical axis direction. The optical pickup device according to claim 1, wherein the optical element is the objective lens . A first light source having a wavelength λ1 for recording or reproducing information by irradiating a light beam onto a first optical information recording medium having a thickness t1 of the transparent substrate, and a thickness of the transparent substrate A second light source having a wavelength λ2 (λ1 <λ2) for recording or reproducing information by irradiating the second optical information recording medium with t2 (t1 <t2) with a light beam; A condensing optical system comprising an objective lens for condensing the light beams emitted from the first and second light sources onto the information recording surface via the transparent substrates of the first and second optical information recording media; In an objective lens for an optical pickup device having
Three or more optical regions are formed on the optical surface of the objective lens at different distances from the optical axis in the direction intersecting the optical axis, and the inner optical side closer to the optical axis of the objective lens is formed. A diffractive portion having a diffraction ring zone is formed in the region, the outer optical region far from the optical axis, and the intermediate optical region between the inner optical region and the outer optical region ;
When recording or reproducing information for the first optical information recording medium with a light beam from the first light source, the first light flux passing through the inner optical zone, passes through the outer optical zone in the image plane wavefront aberration of the wavefront and the third light flux forms is minimized, the intermediate by passing through the diffraction portion of the optical region, the first light flux transmittance of the second light flux passing through the intermediate optical zone the transmission and the third lower comb than the transmittance of the light beam, and at least the first light flux, the by passing through the diffraction portion of the inner optical zone, wherein the first optical information recording medium and the second An objective lens for an optical pickup device , wherein aberration correction can be performed on both optical information recording media .   When recording or reproducing information to or from the second optical information recording medium using the light beam from the second light source, the wavefront aberration of the wavefront formed by the first light beam and the second light beam is minimized. 5. The objective lens for an optical pickup device according to claim 4, wherein a transmittance of the third light beam is lower than a transmittance of the first light beam and a transmittance of the second light beam in an image plane to be .

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