JP2690943C

JP2690943C -

Info

Publication number: JP2690943C
Authority: JP
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Publication date: 1999-07-05

Description

【発明の詳細な説明】〔産業上の利用分野〕本発明は光デイスク装置に係り、特に焦点検出に好適な装置に関する。〔従来の技術〕従来、光デイスク装置における焦点検出は第２図に示す方法で行なわれていた
。すなわち、半導体レーザ１から出射するレーザ光はカツプリングレンズ２で平
行光となりオブジエクテイブレンズ４で光デイスク６上の微小光スポツトに絞り
こまれる。光デイスク６上に絞りこまれた光スポツトの反射光はビーム分割器３
で光検出器９に導かれる。オブジエクテイブレンズ４と光デイスク位置の焦点誤差は第２図(ｂ)に示す光
学系で検知される。光デイスクからの反射光は凸レンズ７,円筒レンズ８により
絞りこむと、焦点付近では第２図10,11,12で示すような光強度分布が得られる。
この焦点付近に４分割の光検知素子を配置すると、光デイスク６とオブジエクテ
イブレンズ４間の距離に従い、第２図ｃに示すような光強度分布10,11,12が得ら
れる。したがつて４分割光検知素子に入射する光強度をＩ＝（Ｉ（9a）＋Ｉ（9b
））−（Ｉ（9c）＋Ｉ（9d））に従つて演算されると第２図ｄに示すように焦点
誤差信号が得られ、オブジエクテイブレンズ位置を合焦状態に常に保持するよう
にレンズ駆動コイル５で可能となる。〔発明が解決しようとする課題〕上記従来技術は光学系を小型化また調整のし易さの点について配慮がされてお
らず問題があつた。すなわち、第２図ｃに示すように光強度分布と４分割光検知素子を精度良く位
置合わせをする必要があるが、第２図の光学系を小型化するためにレンズ７およ
び８の焦点距離を短くすると光分布が微小なものとなり、４分割検知素子との位
置調整が困難となるという問題があつた。本発明の目的は小型でかつ調整のし易い光デイスク光学系を提供することにあ
る。〔課題を解決するための手段〕上記目的は、焦点誤差検出のために光デイスクから反射してもどつてくる光の
微細なパターンの形状変化を検出することは行なわず、反射してもどつてくる光の強度を検
出することで可能とする。このため本発明では焦点深さ方向の異なる位置に少なくとも３点以上の光スポ
ットを形成させる回折格子と、上記光スポットの反射光を検出する検出手段と、
上記検出手段の出力信号の変調度の大きさを得る変調度検出手段と、上記３点以
上の光スポットのうちの１つである第１の光スポットの焦点誤差を検出するため
に上記３点以上の光スポットのうち上記第１の光スポットを挟む第２及び第３の
光スポットの変調度の大きさの差をとる差動手段とを具備することにより達成さ
れる。〔実施例〕以下、本発明の一実施例を第１図により説明する。半導体レーザ１からの光はカツプリングレンズ２で平行光となり焦点検出用回
折格子16により３本の光に分岐される。３つの分岐光はオブジエクテイブレンズ
４で絞りこまれると焦点深さ方向の異なる３点の位置に絞りこむようになつてい
る（17a〜c）。これらのもどり光はビーム分割器β，レンズ７を介して３分割光
検知素子13a,13b,13cに絞りこまれる。３つの分岐光による光スポツト17a〜cの
うちの２つのサイドスポツト17a,17cは焦点検出に用いられる。第１図（ｂ），（ｃ）はサイドスポツトによる光検知信号を示している。第１
図（ｂ）はサイドスポツト17aの方が17cよりも光デイスク６の位置に近く、光デ
イスク６に記録された信号を各光検知素子13a〜cで検知した場合、光デイスク位
置近くにあるサイドスポツト17aにより検知素子13aが検知する信号の変調度の方
が17cのものより大きくなる。この場合はサイドスポツトの中間位置にある主スポツトの位置も光デイスク位
置よりずれたものになる。したがつて第１図（ｃ）に示すようにサイドスポツト
による検知信号の変調度が等しくなるようにすると主スポツト17bの位置は合焦
状態となる。この制御を自動的に行なうために、サイドスポツトを検知する光検
知素子13a,13cの出力はそれぞれ包絡線検波回路14a,14bで検波され、その信号を
差動増幅器15で差動増幅し、レンズ駆動コイル5を駆動する。本発明で用いる焦点検出用回折格子について以下で説明する。第３図（ａ）は
焦点検出用回折格子設計のために用いる図である。オブジエクテイブレンズ４の
焦点距離をｆとし、サイドスポツトの結像位置をレンズからａの距離，光軸から
ｃはなれた位置とする。簡単のために回折格子はオブジエクテイブレンズの面に
するように考えても一般性を失なわない。Ｘ点に点光源があるとした時、回折格
子作成のための位相分布Ａ（c,δ）を求める。回折格子に与えるべき位相Ｐは、Ａ（c,δ）とＡ（−c,−δ）から次式となる
。２値化したパターンで回折格子を作成する場合は、ｍは整数。で与えられる。すなわち、となり、第３図（ｂ）に示すような曲線群となる。〔発明の効果〕以上の如き本発明によれば、回折格子によるサイドスポツト形成と、サイドス
ポツトからの反射光強度比較によるオブジエクテイブレンズの合焦制御がなされ
るため、従来技術の如き、集光レンズとシリンドリカルレンズの組合わせのよう
な光学系が不要となり、光学系の位置合わせ精度や、小型化への難点が解消され
る。Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disc device, and more particularly to an apparatus suitable for focus detection. [Prior Art] Conventionally, focus detection in an optical disk device has been performed by a method shown in FIG. That is, the laser light emitted from the semiconductor laser 1 becomes parallel light by the coupling lens 2 and is narrowed down to a minute light spot on the optical disk 6 by the objective lens 4. The reflected light of the light spot focused on the optical disk 6 is reflected by the beam splitter 3.
To the photodetector 9. The focus error between the objective lens 4 and the optical disk position is detected by the optical system shown in FIG. 2 (b). When the reflected light from the optical disk is converged by the convex lens 7 and the cylindrical lens 8, a light intensity distribution as shown in FIGS.
When a four-divided light sensing element is arranged near the focal point, light intensity distributions 10, 11, and 12 as shown in FIG. 2c are obtained according to the distance between the optical disk 6 and the objective lens 4. Accordingly, the light intensity incident on the four-divided photodetector is expressed as I = (I (9a) + I (9b
))-(I (9c) + I (9d)), a focus error signal is obtained as shown in FIG. 2d, and the lens is set so that the objective lens position is always kept in focus. This is made possible by the drive coil 5. [Problems to be Solved by the Invention] The prior art described above has a problem in that the size of the optical system and the ease of adjustment are not considered. That is, as shown in FIG. 2c, it is necessary to accurately align the light intensity distribution and the four-divided photodetector. However, in order to reduce the size of the optical system shown in FIG. When the distance is shortened, the light distribution becomes minute, and it is difficult to adjust the position with respect to the four-divided detection element. SUMMARY OF THE INVENTION An object of the present invention is to provide an optical disk optical system which is small and easy to adjust. [Means for Solving the Problems] The above-described object is to detect and return a fine pattern shape change of light coming back from an optical disk for focus error detection. This is made possible by detecting the intensity of light. Therefore, in the present invention, a diffraction grating for forming at least three or more light spots at different positions in the focal depth direction, a detecting means for detecting reflected light of the light spot,
Modulation degree detection means for obtaining the magnitude of the modulation degree of the output signal of the detection means;
To detect a focus error of a first light spot which is one of the above light spots
Of the three or more light spots sandwiching the first light spot.
And differential means for obtaining a difference in the degree of modulation of the light spot . Embodiment An embodiment of the present invention will be described below with reference to FIG. The light from the semiconductor laser 1 becomes parallel light by the coupling lens 2 and is split into three lights by the focus detection diffraction grating 16. When the three branched lights are narrowed down by the objective lens 4, they are narrowed down to three different positions in the focal depth direction (17a to 17c). These return lights are condensed to the three-division light detection elements 13a, 13b, and 13c via the beam splitter β and the lens 7. Two side spots 17a and 17c of the three light spots 17a to 17c are used for focus detection. FIGS. 1 (b) and 1 (c) show a light detection signal by a side spot. First
FIG. 7B shows that the side spot 17a is closer to the position of the optical disc 6 than the side spot 17c, and when the signals recorded on the optical disc 6 are detected by the respective photodetectors 13a to 13c, the side spot near the optical disc position is detected. The modulation degree of the signal detected by the detecting element 13a by the spot 17a is larger than that of the signal of 17c. In this case, the position of the main spot at the intermediate position between the side spots is also shifted from the optical disc position. Accordingly, as shown in FIG. 1 (c), when the modulations of the detection signals by the side spots are made equal, the position of the main spot 17b is in focus. In order to perform this control automatically, the outputs of the light detecting elements 13a and 13c that detect the side spots are detected by envelope detection circuits 14a and 14b, respectively, and the signals are differentially amplified by a differential amplifier 15, and The drive coil 5 is driven. The focus detection diffraction grating used in the present invention will be described below. FIG. 3A is a diagram used for designing a diffraction grating for focus detection. The focal length of the objective lens 4 is f, and the imaging position of the side spot is a distance a from the lens and a position c away from the optical axis. For simplicity, the generality is not lost even if the diffraction grating is considered to be the surface of the objective lens. Assuming that there is a point light source at point X, a phase distribution A (c, δ) for forming a diffraction grating is obtained. The phase P to be given to the diffraction grating is given by the following equation from A (c, δ) and A (−c, −δ). When creating a diffraction grating with a binarized pattern, m is an integer. Given by That is, And a curve group as shown in FIG. 3 (b). [Effects of the Invention] According to the present invention as described above, the side spot formation by the diffraction grating and the focusing control of the objective lens by comparing the reflected light intensity from the side spot are performed. An optical system such as a combination of a lens and a cylindrical lens is not required, and difficulties in positioning accuracy of the optical system and miniaturization are eliminated.

【図面の簡単な説明】第１図は本発明の光デイスク装置の光学系の模式図、および検出光強度の波形
図、第２図は従来の装置の光学系の模式図と合焦状態検出の説明図、第３図は本
発明の焦点検出用回折格子設計のための説明図である。BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of an optical system of an optical disk device according to the present invention, and a waveform diagram of detected light intensity, and FIG. 2 is a schematic diagram of an optical system of a conventional device and focus state detection. FIG. 3 is an explanatory diagram for designing a diffraction grating for focus detection according to the present invention.

Claims

Claims 1. A diffraction grating for forming at least three light spots at different positions in a focal depth direction, a detecting means for detecting reflected light of the light spot, and an output of the detecting means.
Modulation degree detecting means for obtaining the magnitude of the modulation degree of the force signal;
In order to detect a focus error of the first light spot which is one of the above three or more points,
Modulation of the second and third light spots sandwiching the first light spot among the light spots
An optical disc device comprising: a differential means for obtaining a difference between degrees .