JP3573367B2 - Micro displacement measuring device - Google Patents

Abstract

PURPOSE: To improve detection sensitivity of a focus error signal by setting the grating shape of double diffraction gratings constituting an interference fringes generation means in a prescribed relation. CONSTITUTION: The recording surface of an optical disk 13 is irradiated with a beam emitted from a semiconductor laser (LD) 9 through a collimate lens 10, a beam splitter 11 and an objective lens 12, and the reflected light is made incident on a photodetector(PD) 14 through the objective lens 12, the beam splitter 11. The double diffraction grating 15 consisting of a first diffraction grating 15a generating a first diffracted beam k1 and a second diffraction grating 15b generating plural second diffracted beams k2 is arranged between the beam splitter 11 and the PD 14, and a movement amount in the axial direction of the optical disk 13 is measured from the phase change of the interference fringes of the second diffracted beams k2 . At this time, the grating shape is set so that the phase is shifted by 1/4 wavelength from the second diffracted beams k2 becoming the interference fringes. For instance, the relative phases of the first, second diffraction gratings 15a, (b) are made to have 1/8 pitch of the grating.

Description

となる。ここで、二重回折格子１５による位相差をπ／２＋δ₀ としたとき、光路長の位相差δは、

として表わされる。そして、２つの点光源Ｐ₁ ，Ｐ₂ による干渉縞の強度Ｉは、
Ｉ∝ｃｏｓ²（δ／２） …（５）
として表わされる。さらに、（４）式を（５）式に代入すると、干渉縞強度分布Ｉ（ｘ）は、
Ｉ（ｘ）∝ｃｏｓ²（πｌ₁ ｘ／λｙ₀＋π／４＋δ₀／２） ＝〔−ｓｉｎ（２πｌ₁ ｘ／λｙ₀＋δ₀）＋１〕／２ …（６）
として表わされる。
【００１７】
次に、光ディスク１３にデフォーカスが生じた場合におけるｙ_０ の値を求める。今、対物レンズ１２の焦点距離をｆ_０ とし、対物レンズ１２からの反射光の検出光路側の焦点位置をｙ_１ とすると、
１／（ｆ_０ ＋ｄ）＋１／ｙ_１ ＝１／ｆ_０ …（７）
の関係が成立する。これにより、ｙ_１ は、
ｙ_１ ≒ｆ_０ ^２／ｄ …（８）
として表わされる。そして、ｙ_０ ＝ｙ_１ −ｙ_２ とし、（８）式を（６）式に代入すると、

を得る。このＩ（ｘ，ｄ）が干渉縞分布を示す基本式である。そして、（９）式中、ｙ_２ が小さいとし、ｙ_２ ＝０を（９）式に代入すると、
Ｉ（ｘ，ｄ）∝−ｓｉｎ〔２πｌ_１ ｘｄ／λｆ_０ ^２＋δ_０〕＋１ …（１０）
となる。これにより、干渉縞分布Ｉ（ｘ，ｄ）は、ｘ，ｄに対して対称であり、二重回折格子１５に位相差をもたせた効果を得ることができ、波長変動の影響によりその干渉縞分布も対称的に変化する。なお、デフォーカスがないときは波長に拘らず干渉縞分布は平坦になり、ジャストオンフォーカス時には影響がない。
【００１８】
図３は、その干渉縞分布Ｉ（ｘ，ｄ）をｘ方向に積分することにより得られるデフォーカス量に対するフォーカスエラー信号ＦｅのＳ字曲線Ｓ（ｄ）を示す。Ｓ字曲線Ｓ（ｄ）の波形Ａ（実線）は実験値を示し、波形Ｂ（破線）は理論的な計算値を示す。この場合、ＰＤ１４の面をｘ座標にして、−ａから０、そして、０からａの２つの領域に分けられるため、その場の干渉縞分布Ｉ（ｘ，ｄ）を平坦な光量分布を仮定してそれぞれを積分してその差をとると、Ｓ（ｄ）は、
Ｓ（ｄ）∝λｆ₀ ²・ ｃｏｓδ₀〔−１＋ｃｏｓ（２πｌ₁ ａｄ／λｆ₀ ² ）〕／πｌ₁ ｄ …（１１）
として表わすことができる。この（１１）式中には、ｃｏｓδ₀の項があり、δ₀ ＝０のときには、ｃｏｓδ₀＝１となり、δ₀ ＝πのときにはｃｏｓδ₀＝−１となり、これによりＳ（ｄ）の絶対値は最大となる。
【００１９】
従って、δ_０ ＝０のとき、すなわち、干渉する２つの第二回折光Ｋ_２ の位相がλ／４だけずれるように、第一回折格子１５ａと第二回折格子１５ｂとの格子形状を形成することによって、フォーカスエラー信号ＦｅのＳ字曲線を最大にすることができ、これにより、デフォーカス量を容易に求めることができ、信号の検出感度を向上させることができる。
【００２０】
次に、本発明の第二の実施例を図４〜図６に基づいて説明する（請求項２，３記載の発明に対応する）。なお、前述した第一の実施例と同一部分についての説明は省略し、その同一部分については同一符号を用いる。
【００２１】
本実施例では、第一回折格子１５ａと第二回折格子１５ｂとの相対的な位相は、その回折格子の１／４ピッチを（ｎ_１ −ｎ_２ ）で割った値、又は、その値にさらに１ピッチを（ｎ_１ −ｎ_２ ）で割った値を１個ないしは複数個加算又は減算した値に設定されている。この場合、特に、第一回折格子１５ａと第二回折格子１５ｂとの相対的な位相を、その回折格子の１／８ピッチ、又は、５／８ピッチの値に設定するようにしてもよい。
【００２２】
以下、回折格子１５ａ，１５ｂの相対的な位相関係を、上記条件に設定することにより、フォーカスエラー信号ＦｅのＳ字曲線を最大にすることができる理由について述べる。図４は、単一の回折格子１６に光を入射させ、その回折格子１６が移動するときに回折光の波面も移動することを示す例である。今、回折格子１６が１ピッチ移動し、その移動方向に回折する光をｎ_１ 次光とし、反対方向に回折する光をｎ_２ 次光とする。この場合、回折格子１６の移動方向に対して同一方向に進む光は波面１７ａも同一方向に進み、次数のｎ_１ に対して１ピッチ当たり波長のｎ_１ 倍進む。一方、回折格子１６の移動方向に対して反対方向に進む光は波面１７ｂが後退し、次数のｎ_２ に対して１ピッチ当たり波長のｎ_２ 倍後退する（ｎ_２ が負の数とすると、−ｎ_２ 倍進む）。また、図５に示すように、２枚の回折格子、例えば第一、第二回折格子１５ａ，１５ｂからなる二重回折格子１５の場合、一方の第一回折格子１５ａを１ピッチ移動させ、他方の第二回折格子１５ｂを固定したとすると、ｎ_１ 次光の波面１７ａとｎ_２ 次光の波面１７ｂとの進行が逆になるため、（ｎ_１ −ｎ_２ ）倍×２πの位相変化となり、この変化分だけ波面１７ａ，１７ｂが重なることになる。例として、ｎ_１ ＝１、ｎ_２ ＝−１とすると、（ｎ_１ −（−ｎ_２ ））×２π＝（１−（−１））×２π＝２×２πなる位相変化を生じる。
【００２３】
また、図６に示すように、第一、第二回折格子１５ａ，１５ｂの相対的な位相がピッタリ合っていると波面１７ａ，１７ｂも揃う。このような状態から、一方の第一回折格子１５ａが移動して、波面１７ａを波面１７ｂに対してλ／４だけ異ならせるためには、回折格子の４分の１ピッチを（ｎ_１ −ｎ_２ ）で割った距離だけ移動させることによってその目的を達成できる。例として、ｎ_１ ＝１、ｎ_２ ＝−１の場合は、４分の１ピッチを（１−（−１））＝２で割った１／８ピッチだけ回折格子を相対的に移動させることによって、λ／４だけ波面の位相を異ならせることができる。なお、回折格子を移動させる方向は左右どちらでもよい。
【００２４】
さらに、第一、第二回折格子１５ａ，１５ｂのうちの一方を１ピッチ移動させると、（ｎ_１ −ｎ_２ ）倍×２πの位相変化となることから、２つの波面１７ａ，１７ｂの位相差は回折格子の１ピッチの移動の間に（ｎ_１ −ｎ_２ ）回の同じ位相となり、これにより１ピッチを（ｎ_１ −ｎ_２ ）で割った位相分だけ回折格子を移動させることで同一の位相となる。例として、ｎ_１ ＝１、ｎ_２ ＝−１の場合は、１ピッチを（１−（−１））＝２で割った値、すなわち、１／２ピッチの移動で２つの波面１７ａ，１７ｂの位相関係が同一となる。従って、波面１７ａを波面１７ｂに対してλ／４だけ異ならせるためには、１ピッチを（ｎ_１ −ｎ_２ ）で割った値に、回折格子の４分の１ピッチを（ｎ_１ −ｎ_２ ）で割った距離（ｎ_１ −ｎ_２ ）で割った値を加算又は減算させるようにすればよい。この例では、１／２ピッチと１／８ピッチとを加算して５／８ピッチだけ移動させることによって、λ／４だけ波面の位相を異ならせることができる。
【００２５】
この他の高次数を例にとると、ｎ_１ ＝２、ｎ_２ ＝−３の場合は、１／４ピッチを（２−（−３））＝５で割って、１／２０ピッチの分だけ回折格子の位相変化をもたせることによって、λ／４だけ波面の位相を異ならせることができる。さらにこのことは、１ピッチを（２−（−３））＝５で割った１／５ピッチの値と、１／２０ピッチとを加算した１／４ピッチだけ移動させることによって、λ／４だけ波面の位相を異ならせることができる。
【００２６】
上述したように、第一回折格子１５ａと第二回折格子１５ｂとの相対的な位相を、１／４ピッチを（ｎ_１ −ｎ_２ ）で割った値か、又は、その１／４ピッチを（ｎ_１ −ｎ_２ ）で割った値に、１ピッチを（ｎ_１ −ｎ_２ ）で割った値を１個ないしは複数個加算又は減算した値に設定したので、干渉縞となる第二回折光の位相を１／４波長だけ正確にしかも容易にずらすことができ、これにより、フォーカスエラー信号ＦｅのＳ字曲線を最大にして、デフォーカス量を正確に求めることができる。特に、±１次光を用い、１／８ピッチ、５／８ピッチの値に設定することによって、第一，第二回折格子１５ａ，１５ｂや受光素子１４の形状が単純化されて作成が容易となり、生産コストを低減することができる。
【００２７】
次に、本発明の第三の実施例を図７〜図９に基づいて説明する（請求項４記載の発明に対応する）。なお、前記各実施例と同一部分についての説明は省略し、その同一部分については同一符号を用いる。
【００２８】
本実施例では、二重回折格子１５における第一回折格子１５ａのピッチはΛ_１ とされ、第二回折格子１５ｂのピッチはΛ_２ とされている。そして、第一回折格子１５ａにより回折される第一回折光Ｋ_１ はｎ_１ 次光とｎ_２ 次光とされ、また、第二回折格子１５ｂにより得られる第二回折光Ｋ_２ はｍ_１ 次光とｍ_２ 次光とされている。この場合、各回折格子のピッチと回折光の次数との間は、
Λ_２ ｎ_１ ＋Λ_１ ｍ_１ ＝Λ_２ ｎ_２ ＋Λ_１ ｍ_２ …（１２）
の関係式に設定されている。
【００２９】
以下、回折格子１５ａ，１５ｂの相対的な位相関係を、上記（１２）式の条件に設定することにより、フォーカスエラー信号ＦｅのＳ字曲線を最大にすることができる理由について述べる。図７は、ピッチΛ_１ の第一回折格子１５ａとピッチΛ_２ の第二回折格子１５ｂとが形成された二重回折格子１５での回折条件を示す。今、光ディスク１３からの反射光が入射角θ_０ で二重回折格子１５に入射し、第一回折格子１５ａ側で出射角θ_１ のｎ_１ 次光，出射角θ_３ のｎ_２ 次光が発生し、第二回折格子１５ｂ側で出射角θ_２ のｍ_１ 次光，出射角θ_４ のｍ_２ 次光が発生するものとする。この場合、第一回折格子１５ａ側では、
ｓｉｎθ_０ ＋ｓｉｎθ_１ ＝ｎ_１ λ／Λ_１ …（１３）
ｓｉｎθ_０ ＋ｓｉｎθ_３ ＝ｎ_２ λ／Λ_１ …（１４）
が成り立つ。一方、第二回折格子１５ｂ側では、
ｓｉｎθ_１ ＋ｓｉｎθ_２ ＝−ｍ_１ λ／Λ_２ …（１５）
ｓｉｎθ_３ ＋ｓｉｎθ_４ ＝−ｍ_２ λ／Λ_２ …（１６）
が成り立つ。これにより、二重回折格子１５から出射したｍ_１ 次光、ｍ_２ 次光の２つの回折光を平行にするために、θ_２ ＝θ_４ とすると、
Λ_２ ｎ_１ ＋Λ_１ ｍ_１ ＝Λ_２ ｎ_２ ＋Λ_１ ｍ_２ …（１７）
の関係が得られる。
【００３０】
例として、図８に示すように、第一回折格子１５ａのΛ_１ ＝１μｍ、第二回折格子１５ｂのΛ_２ ＝２μｍ、ｎ_１ ＝２、ｎ_２ ＝−１として、（１７）式に代入すると、
２・２＋１・ｍ_１ ＝２・（−１）＋１・ｍ_２
すなわち、
ｍ_１ −ｍ_２ ＝−６ …（１８）
の関係式を得る。この（１８）式から、例えば、図９に示すように、ｍ_１ ＝−２、ｍ_２ ＝４とすると、第一回折格子１５ａにより２次光と−１次光が発生し、第二回折格子１５ｂにより−２次光と４次光が発生する。これにより、回折されたｍ_１ ，ｍ_２ 次光の位相をλ／４だけずらすことが可能となる。
【００３１】
上述したように、（１２）式の関係を満たすように設定することによって、干渉縞となる第二回折光Ｋ_２ の位相をλ／４だけずらすことができ、これにより、フォーカスエラー信号ＦｅのＳ字曲線を最大にして、デフォーカス量を正確に求めることができる。なお、ここでは、回折格子に入射する光が垂直であることを前提としているが、垂直に入射しない場合にも、２つの回折光の位相がλ／４だけずれるように調整すればよい。
【００３２】
【発明の効果】
請求項１記載の発明は、互いに干渉し合う第二回折光の位相が１／４波長だけずれるように、干渉縞発生手段の各回折格子を形成したので、フォーカスエラー信号のＳ字曲線を最大にしてデフォーカス量を容易に求めることができ、これにより、従来のナイフエッジ法や非点収差法に比べて、信号の検出感度を格段に向上させて微小変位の測定を正確に行うことができる。また、このような干渉縞を用いた測定方式においては、検出光路長を従来よりも短くとることができるため、光ピックアップ部の小型化を図ることができる。さらに、このような干渉縞による検出においてはビーム形状を比較的大きくとれるため、光学素子の位置調整を極めてラフに行うことができ、耐環境性を向上させ、常に安定した信号検出を行うことができる。
【００３３】
請求項２記載の発明は、第一回折格子と第二回折格子との相対的な位相を、４分の１ピッチを（ｎ_１ −ｎ_２ ）で割った値か、又は、その４分の１ピッチを（ｎ_１ −ｎ_２ ）で割った値に、１ピッチを（ｎ_１ −ｎ_２ ）で割った値を１個ないしは複数個加算又は減算した値に設定したので、干渉縞となる第二回折光の位相を１／４波長だけ正確にずらすことができ、これにより、フォーカスエラー信号のＳ字曲線を最大にして、デフォーカス量を正確に求めることができる。
【００３４】
請求項３記載の発明は、第一回折格子と第二回折格子との相対的な位相を、８分の１ピッチ、又は、８分の５ピッチの値に設定したので、干渉縞となる第二回折光の位相を１／４波長だけ正確にしかも容易にずらすことができ、これにより、フォーカスエラー信号のＳ字曲線を最大にして、デフォーカス量を一段と正確に求めることができる。また、第一回折格子と第二回折格子とを等ピッチすなわち同一の格子ピッチとし、±１次光の第二回折光を用いて干渉縞を発生させるようにしたので、回折格子や受光素子の作成が容易となり、生産コストを一段と削減することができる。
【００３５】
請求項４記載の発明は、ピッチがΛ_１ の第一回折格子により得られる第一回折光をｎ_１ 次光とｎ_２ 次光とし、ピッチがΛ_２ の第二回折格子により得られる第二回折光をｍ_１ 次光とｍ_２ 次光としたとき、各回折格子のピッチと回折光の次数とを、
Λ_２ ｎ_１ ＋Λ_１ ｍ_１ ＝Λ_２ ｎ_２ ＋Λ_１ ｍ_２
の関係式を満たすように設定したので、干渉縞となる第二回折光の位相を１／４波長だけずらし、フォーカスエラー信号のＳ字曲線を最大にしてデフォーカス量を一段と正確に求めることができる。
【図面の簡単な説明】
【図１】本発明の第一の実施例として２つの回折光により１／４波長の位相差が生じた場合の理論的解析の等価光学系を示す模式図である。
【図２】微小変位測定装置の全体構成を示す光路図である。
【図３】デフォーカス量に対するフォーカスエラー信号のＳ字曲線を示す波形図である。
【図４】本発明の第二の実施例として回折格子の移動により回折光の波面も移動することを示す側面図である。
【図５】回折格子を２枚重ねて構成し、回折格子を１ピッチだけ移動した場合における回折光に位相差が生じている様子を示す側面図である。
【図６】回折格子の相対的な位相が一致している場合に回折光の波面も一致する場合の様子を示す側面図である。
【図７】本発明の第三の実施例である回折格子のピッチが異なっている場合の回折光の発生の様子を示す側面図である。
【図８】ピッチが異なっている回折格子を用いた場合に回折光に位相差が生じている様子を示す側面図である。
【図９】高次の回折光が発生した場合の様子を示す側面図である。
【図１０】従来の装置におけるフォーカスエラー信号の検出方式を示す光路図である。
【符号の説明】
９ 光源
１２ 対物レンズ
１３ 測定物
１４ 受光素子
１５ 干渉縞発生手段
１５ａ 第一回折格子
１５ｂ 第二回折格子
Ｋ_１ 第一回折光
Ｋ_２ 第二回折光[0001]
[Industrial applications]
[0002]
[Prior art]
Conventionally, as a focus servo method using a focus error signal used in an optical head for a magneto-optical disk, an astigmatism method, a critical angle method, a knife edge method, and the like are known. Among them, the astigmatism method is used not only for magneto-optical disks but also for all optical disks including compact disks and laser disks. As known techniques relating to the astigmatism method, Japanese Patent Publication No. 53-39123 discloses an "automatic focus adjustment device" and Japanese Patent Publication No. 57-12188 discloses a "device for focusing a reading light beam on a moving data carrier". Japanese Patent Publication No. 60-48949 discloses a "device for reading information with a light beam", and Japanese Patent Publication No. 61-61178 discloses an "automatic focus adjustment method".
[0003]
FIG. 10 shows the operation principle of the astigmatism method in the optical pickup device. Light emitted from a semiconductor laser (not shown) is collimated by a collimating lens (not shown), passes through a beam splitter 1, is condensed by an objective lens 2, and is irradiated on the surface of an optical disc 3, Information recording and the like are performed. The reflected light from the optical disk 3 passes through the objective lens 2 and is reflected by the beam splitter 1 this time, and sequentially passes through the condenser lens 4 and the cylindrical lens 5 to form a beam 6 in which astigmatism has occurred. The beam 6 is guided to a light receiving element 7 having four divided light receiving surfaces a, b, c, and d. Then, the output of the light receiving element 7 is sent to the amplifier 8 to detect the focus error signal Fe.
[0004]
In this case, when the optical disk 3 is focused, the shape of the beam 6 of the reflected light from the optical disk 3 is circular on the four-divided light receiving surfaces a, b, c, and d of the light receiving element 7. At this time, the value of the differential output {(a + c)-(b + d)} is zero, and the value of the focus error signal Fe is zero, and is not detected. When the optical disk 3 moves farther or closer to the objective lens 2, the shape of the beam 6 changes from a circle to an ellipse, and the differential output does not become zero, so that the value of the focus error signal is positive (far). Or, it becomes negative (close), and the position of the objective lens is adjusted.
[0005]
[Problems to be solved by the invention]
In recent years, in this type of optical disc device, a faster access time has been demanded, and in order to achieve such a purpose, it is indispensable to reduce the size and weight of the optical pickup unit. However, in the focus servo method using the astigmatism method as described above, in order to detect a change in the shape of the beam, it is not sufficient to increase the distance (detection length) to the light receiving element 7 to some extent (several cm). No detection sensitivity can be obtained. Therefore, there is a limit in miniaturization of the conventional optical disk device. In addition, since the spot diameter on the light receiving element 7 is considerably small from several microns to several tens of microns, adjustment is difficult, and an offset is generated depending on the environment, so that the detected signal becomes unstable.
[0006]
[Means for Solving the Problems]
According to the first aspect of the present invention, the first diffraction grating that generates the first diffracted light of the n _1st order light and the n _2nd order light, and the second diffracted light diffracted when the first diffracted light enters. second consists of a diffraction grating, an interference fringe generating means for each diffraction grating is shaped to deviate by a phase 1/4 wavelength of the second diffracted light interfere with each other, the interference fringes generated light a light receiving element composed of at least two divided regions for receiving light from generating means is provided, said receiving a change in the phase of the interference fringe by the interference between the second diffraction light generated by the interference fringe generating means The amount of movement of the measurement object in the optical axis direction is measured by detecting the element.
[0007]
According to a second aspect of the present invention, in the first aspect of the present invention, the relative phase between the first diffraction grating and the second diffraction grating is set such that a quarter pitch of the diffraction grating is (n ₁ −n ₂ ). divided by, or set a value obtained by dividing a further one pitch to the value (n 1 _{-n 2)} to one or a plurality adding or subtracting a value.
[0008]
In the invention according to claim 3, in the invention according to claim 1, when the first diffraction grating and the second diffraction grating have an equal pitch, and interference fringes are generated using the second diffraction light of ± 1st order light, The relative phase between the first diffraction grating and the second diffraction grating was set to a value of 1/8 pitch or 5/8 pitch of the diffraction grating.
[0009]
In the invention of claim 4, wherein, in the invention according to the first aspect, the first diffracted light pitch obtained by the first diffraction grating of lambda ₁ and n ₁ order light and n ₂ order light, the pitch is lambda ₂ second When the second diffracted light obtained by the two diffraction gratings is m ₁ order light and m ₂ order light, the pitch of each diffraction grating and the order of the diffracted light are
_{Λ 2 n 1 + Λ 1 m} 1 = Λ 2 n 2 + Λ 1 m 2
Was set to satisfy the relational expression.
[0010]
[Action]
According to the first aspect of the present invention, the respective diffraction gratings of the interference fringe generating means are formed in such a shape that the phase of the second diffracted light is shifted by ４ wavelength, so that it can be obtained from the interference fringe distribution of the second diffracted light. The S-shaped curve of the focus error signal to be obtained can be maximized, and the defocus amount can be easily obtained from the maximized S-shaped curve.
[0011]
In the invention according to claim 2, a value obtained by dividing a quarter pitch by (n ₁ −n ₂ ) or a value obtained by dividing the quarter pitch by (n ₁ −n ₂ ) is given by: By setting the value obtained by dividing one pitch by (n ₁ −n ₂ ) to a value obtained by adding or subtracting one or more, the phases of the second diffracted lights that interfere with each other are accurately shifted by １ ／ wavelength. be able to.
[0012]
According to the third aspect of the present invention, since the relative phase between the first diffraction grating and the second diffraction grating is set to a value of 1/8 pitch or 5/8 pitch, they interfere with each other. The phase of the matched second diffracted light can be shifted precisely and easily by ４ wavelength. Further, the first diffraction grating and the second diffraction grating have the same pitch, that is, the same grating pitch, and the interference fringes are generated using the second diffraction light of the ± 1st order light, so that the diffraction grating can be easily formed. Therefore, the shape of the light receiving element that receives interference fringes can be simplified.
[0013]
According to the fourth aspect of the present invention, the pitch between the first diffraction grating and the second diffraction grating is made different, and by satisfying a certain conditional expression between the pitch of each diffraction grating and the order of the diffracted light, interference can be achieved. The phase of the second diffracted light that becomes a stripe can be shifted by 波長 wavelength.
[0014]
【Example】
A first embodiment of the present invention will be described with reference to FIGS. 1 to 3 (corresponding to the first embodiment). First, the overall configuration of the minute displacement measuring device will be described with reference to FIG. Light emitted from a semiconductor laser 9 (hereinafter, referred to as an LD) as a light source is collimated by a collimator lens 10, reflected by a beam splitter 11, and becomes a beam condensed by an objective lens 12 to be measured. Is irradiated on the surface of the optical disk 13. Then, the reflected light from the optical disk 13 passes through the objective lens 12 again, and then passes through the beam splitter 11 and travels to the light receiving element 14 (hereinafter, referred to as PD).
[0015]
On the optical path between the beam splitter 11 and the PD 14, a double diffraction grating 15 as interference fringe generating means is arranged. This double diffraction grating 15, a first diffraction grating 15a for generating a first diffracted beam K ₁ and n ₁ order light and n ₂ order light, a plurality of diffraction by the first diffraction light K ₁ is incident consists a second diffraction grating 15b for generating a second diffracted light K ₂ which is. Then, the interference fringes of the second diffracted beam K ₂ is generated by the reflected light from the optical disc 13 passes through such a double diffraction grating 15, the change in phase of the interference fringe is detected in PD 14, the optical disk The measurement of the amount of movement of the optical disc 13 in the optical axis direction is performed.
[0016]
In this embodiment, the first diffraction grating 15a constituting the double diffraction grating 15 and the second diffraction grating 15b is, the second diffracted light K ₂ the phase of the interference fringe shifts 1/4 wavelength (lambda) It is formed in such a lattice shape. Hereinafter, the reason why the phases of these two diffracted lights K ₁ and K ₂ are shifted by λ / 4 will be described. FIG. 1 shows coordinate axes x, y, and z with the center on the surface of the PD 14 as the coordinate O. Now, let L ₁ and L ₂ be the distances from the _two point light sources P ₁ and P ₂ with respect to the surface of the PD 14, and let the distance in the x direction between these point light sources be l ₁ and the center of the point light sources P ₁ and P ₂ . Let (y, z) coordinates be (y ₀ , z ₀ ). At this time, L _{1 ^2,} L ^{2 2} values,
^{_{L 1 2 = (l 1/}} 2 + x) 2 + y 0 2 + z 2 ... (1)
^{_{L 2 2 = (- l 1}} /2 + x) 2 + y 0 2 + z 2 ... (2)
It is represented as Accordingly, the optical path length difference dL when the phase difference due to the double diffraction grating 15 is not given is:

It becomes. Here, when the phase difference due to the double diffraction grating 15 is π / 2 + δ ₀ , the phase difference δ of the optical path length is

It is represented as Then, the intensity I of the interference fringes due to the two point light sources P ₁ and P ₂ is
I∝cos ² (δ / 2) (5)
It is represented as Further, by substituting equation (4) into equation (5), the interference fringe intensity distribution I (x) becomes
^{I (x) αcos 2 (πl} 1 x / λy 0 + π / 4 + δ 0/2) = _{[- sin (2πl 1 x / λy} 0 + δ 0) +1 ] / 2 ... (6)
It is represented as
[0017]
Next, find the value of y ₀ in the case where defocusing occurs in the optical disk 13. Now, assuming that the focal length of the objective lens 12 is f ₀ and the focal position on the detection optical path side of the reflected light from the objective lens 12 is y ₁ ,
1 / (f ₀ + d) + 1 / y ₁ = 1 / f ₀ (7)
Is established. Thus, y ₁ becomes
^{_{y 1 ≒ f 0 2 / d}} ... (8)
It is represented as Then, when y ₀ = y ₁ −y ₂ and equation (8) is substituted into equation (6),

Get. This I (x, d) is a basic expression showing the interference fringe distribution. Then, in equation (9), if y ₂ is small and y ₂ = 0 is substituted into equation (9),
I (x, d) ∝-sin [2πl ₁ xd / λf ₀ ² + δ ₀ ] +1 (10)
It becomes. As a result, the interference fringe distribution I (x, d) is symmetric with respect to x and d, and an effect can be obtained in which the double diffraction grating 15 has a phase difference. The fringe distribution also changes symmetrically. Note that when there is no defocus, the interference fringe distribution becomes flat regardless of the wavelength, and there is no effect during just-on focus.
[0018]
FIG. 3 shows an S-shaped curve S (d) of the focus error signal Fe with respect to a defocus amount obtained by integrating the interference fringe distribution I (x, d) in the x direction. The waveform A (solid line) of the S-shaped curve S (d) indicates an experimental value, and the waveform B (dashed line) indicates a theoretical calculated value. In this case, since the surface of the PD 14 is divided into two regions from -a to 0 and from 0 to a when the surface of the PD 14 is set as the x coordinate, the interference fringe distribution I (x, d) at that position is assumed to be a flat light amount distribution. And integrating each of them and taking the difference, S (d) becomes
S (d) ∝λf ₀ ² · cos δ ₀ [−1 + cos (2πl ₁ ad / λf ₀ ² ) ] / πl ₁ d (11)
Can be represented as During this (11), some sections of cos [delta] _0, when [delta] ₀ = 0 is, cos [delta] ₀ = 1 becomes, cos [delta] ₀ = -1 becomes when the [delta] ₀ = [pi, thereby absolute S (d) The value is maximum.
[0019]
Therefore, when [delta] _{0 =} 0, i.e., as interfering two second diffracted light K ₂ phases shifted by lambda / 4, to form a lattice shape of the first diffraction grating 15a and the second diffraction grating 15b Thereby, the S-shaped curve of the focus error signal Fe can be maximized, whereby the defocus amount can be easily obtained, and the signal detection sensitivity can be improved.
[0020]
Next, a second embodiment of the present invention will be described with reference to FIGS. 4 to 6 (corresponding to claims 2 and 3). The description of the same parts as those in the first embodiment is omitted, and the same reference numerals are used for the same parts.
[0021]
In this embodiment, the relative phase between the first diffraction grating 15a and the second diffraction grating 15b is a value obtained by dividing a quarter pitch of the diffraction grating by (n ₁ −n ₂ ), or a value obtained by dividing the value by (n ₁ −n ₂ ). It is further set a value obtained by dividing a pitch (n 1 _{-n 2)} to one or a plurality adding or subtracting a value. In this case, in particular, the relative phase between the first diffraction grating 15a and the second diffraction grating 15b may be set to a value of 1/8 pitch or 5/8 pitch of the diffraction grating.
[0022]
Hereinafter, the reason why the S-shaped curve of the focus error signal Fe can be maximized by setting the relative phase relationship between the diffraction gratings 15a and 15b under the above condition will be described. FIG. 4 shows an example in which light is incident on a single diffraction grating 16 and the wavefront of the diffracted light also moves when the diffraction grating 16 moves. Now, a diffraction grating 16 1 and a pitch movement, the light diffracted in the moving direction and n _1-order light, the light diffracted in the opposite direction and n ₂ order light. In this case, light traveling in the same direction to the moving direction of the diffraction grating 16 is wavefront 17a also proceeds in the same direction, the process proceeds ₁ times n of one pitch per wavelength with respect to n ₁ in order. On the other hand, when the light traveling in the opposite direction to the moving direction of the diffraction grating 16 is wavefront 17b is retracted, relative to n ₂ orders retracted _twice n wavelengths per pitch (n ₂ is a negative number, -N ₂ times). Also, as shown in FIG. 5, in the case of a double diffraction grating 15 including two diffraction gratings, for example, first and second diffraction gratings 15a and 15b, one of the first diffraction gratings 15a is moved by one pitch, When fixing the other of the second diffraction grating 15b, since the progression of the wave front 17b of _{n 1-order} light wavefront 17a and _{n 2} order light is _reversed, the phase change in the _(n 1 -n ₂₎ times × 2 [pi The wavefronts 17a and 17b overlap by this change. For example, if n ₁ = 1 and n ₂ = −1, a phase change of (n ₁ − (− n ₂ )) × 2π = (1 − (− 1)) × 2π = 2 × 2π occurs.
[0023]
Also, as shown in FIG. 6, when the relative phases of the first and second diffraction gratings 15a and 15b are perfectly matched, the wavefronts 17a and 17b are also aligned. From such a state, in order to move one of the first diffraction gratings 15a to make the wavefront 17a different from the wavefront 17b by λ / 4, the quarter pitch of the diffraction grating is set to (n ₁ −n). The object can be achieved by moving by a distance divided by ₂ ). As an example, when n ₁ = 1 and n ₂ = -1, the diffraction grating is relatively moved by １ ／ pitch obtained by dividing a quarter pitch by (1 − (− 1)) = 2. Thus, the phase of the wavefront can be made different by λ / 4. The direction in which the diffraction grating is moved may be either left or right.
[0024]
Further, when one of the first and second diffraction gratings 15a and 15b is moved by one pitch, a phase change of (n ₁ −n ₂ ) × 2π occurs, so that the phase difference between the two wavefronts 17a and 17b is obtained. Is the same phase (n ₁ −n ₂ ) times during one pitch movement of the diffraction grating, whereby the same pitch is obtained by moving the diffraction grating by the phase obtained by dividing one pitch by (n ₁ −n ₂ ). Phase. As an example, when n ₁ = 1 and n ₂ = -1, a value obtained by dividing one pitch by (1 − (− 1)) = 2, that is, two wavefronts 17a and 17b by a movement of ピ ッ チ pitch Have the same phase relationship. Therefore, in order to vary the wavefront 17a only lambda / 4 with respect to the wavefront 17b is 1 divided by the pitch _(n 1 -n _2), a ¼ pitch of the diffraction grating _(n 1 -n ₂ ) The value divided by the distance (n ₁ −n ₂ ) divided by ₂ ) may be added or subtracted. In this example, the phase of the wavefront can be changed by λ / 4 by adding １ ／ pitch and ８ pitch and moving by ／ pitch.
[0025]
Taking another higher order as an example, in the case of n ₁ = 2 and n ₂ = −3, the quarter pitch is divided by (2 − (− 3)) = 5 to obtain the 1/20 pitch. Only by changing the phase of the diffraction grating, the phase of the wavefront can be changed by λ / 4. Further, this is achieved by moving the pitch by ４ pitch obtained by adding 1/5 pitch value obtained by dividing one pitch by (2 − (− 3)) = 5 and 1/20 pitch. Only the phase of the wavefront can be different.
[0026]
As described above, the relative phase between the first diffraction grating 15a and the second diffraction grating 15b is determined by dividing a quarter pitch by (n ₁ −n ₂ ) or by setting the quarter pitch to divided by the (n 1 _{-n 2),} since a value obtained by dividing the one pitch in (n 1 _{-n 2)} to one or a plurality adding or subtracting a value, the second diffraction an interference fringe The phase of the light can be shifted precisely and easily by 波長 wavelength, whereby the S-curve of the focus error signal Fe is maximized, and the defocus amount can be determined accurately. In particular, by setting the values to 1/8 pitch and 5/8 pitch using ± 1st order light, the shapes of the first and second diffraction gratings 15a and 15b and the light receiving element 14 are simplified, and the fabrication is easy. And the production cost can be reduced.
[0027]
Next, a third embodiment of the present invention will be described with reference to FIGS. 7 to 9 (corresponding to the invention described in claim 4). The description of the same parts as those in the above embodiments is omitted, and the same reference numerals are used for the same parts.
[0028]
In this embodiment, the pitch of the first diffraction grating 15a of the double diffraction grating 15 is set to lambda _1, the pitch of the second diffraction grating 15b is a lambda _2. The first diffracted beam K ₁ diffracted by the first diffraction grating 15a is the n _1-order light and n ₂ order light and second diffracted light K ₂ obtained by the second diffraction grating 15b is m ₁ primary there is a light and m ₂ order light. In this case, between the pitch of each diffraction grating and the order of the diffracted light,
_{Λ 2 n 1 + Λ 1 m} 1 = Λ 2 n 2 + Λ 1 m 2 ... (12)
Is set in the relational expression.
[0029]
Hereinafter, the reason why the S-curve of the focus error signal Fe can be maximized by setting the relative phase relationship between the diffraction gratings 15a and 15b under the condition of the above equation (12) will be described. Figure 7 shows the diffraction condition of the double diffraction grating 15 and the second diffraction grating 15b of the first diffraction grating 15a and the pitch lambda ₂ pitches lambda ₁ is formed. Now, the reflected light incident on the incident angle theta ₀ double diffraction grating 15 from the optical disk 13, the emission angle theta ₁ of n _1-order light in the first diffraction grating 15a side, n ₂ order light emission angle theta ₃ There occurs, it is assumed that the second diffraction grating 15b side exit angle theta ₂ of the m _1-order light, m ₂ order light emission angle theta ₄ occurs. In this case, on the first diffraction grating 15a side,
sin θ ₀ + sin θ ₁ = n ₁ λ / Λ ₁ (13)
sin θ ₀ + sin θ ₃ = n ₂ λ / Λ ₁ (14)
Holds. On the other hand, on the second diffraction grating 15b side,
sin θ ₁ + sin θ ₂ = −m ₁ λ / Λ ₂ (15)
sin θ ₃ + sin θ ₄ = −m ₂ λ / Λ ₂ (16)
Holds. Thus, in order to make two diffracted lights of m ₁ -order light and m ₂ -order light emitted from the double diffraction grating 15 parallel, θ ₂ = θ ₄ ,
_{Λ 2 n 1 + Λ 1 m} 1 = Λ 2 n 2 + Λ 1 m 2 ... (17)
Is obtained.
[0030]
As an example, as shown in FIG. 8, Λ ₁ = 1 μm of the first diffraction grating 15a, Λ ₂ = 2 μm, n ₁ = 2, and n ₂ = −1 of the second diffraction grating 15b are substituted into the equation (17). Then
_{2 · 2 + 1 · m 1} = 2 · (-1) +1 · m 2
That is,
m ₁ −m ₂ = −6 (18)
Is obtained. From this equation (18), for example, as shown in FIG. 9, when m ₁ = −2 and m ₂ = 4, the first diffraction grating 15a generates the second-order light and the −1st-order light, and the second diffraction light. The grating 15b generates -2nd order light and 4th order light. This makes it possible to shift the diffracted m _1, m ₂ order light phase by lambda / 4.
[0031]
As described above, by setting so as to satisfy the equation (12) in relation to the second diffracted light K ₂ the phase of the interference fringes can be shifted by lambda / 4, thereby, the focus error signal Fe The defocus amount can be accurately obtained by maximizing the S-shaped curve. Here, it is assumed that the light incident on the diffraction grating is vertical. However, even when the light does not enter vertically, adjustment may be made so that the phases of the two diffracted lights are shifted by λ / 4.
[0032]
【The invention's effect】
According to the first aspect of the present invention, since each diffraction grating of the interference fringe generating means is formed so that the phases of the second diffracted lights that interfere with each other are shifted by ４ wavelength, the S-shaped curve of the focus error signal is maximized. The amount of defocus can be easily obtained by using this method, which makes it possible to significantly improve the signal detection sensitivity and accurately measure minute displacement compared to the conventional knife edge method and astigmatism method. it can. Further, in the measurement method using such interference fringes, the length of the detection optical path can be made shorter than in the past, so that the size of the optical pickup unit can be reduced. Furthermore, since the beam shape can be made relatively large in the detection using such interference fringes, the position of the optical element can be adjusted extremely rough, the environment resistance can be improved, and stable signal detection can always be performed. it can.
[0033]
According to a second aspect of the present invention, the relative phase between the first diffraction grating and the second diffraction grating is a value obtained by dividing a quarter pitch by (n ₁ −n ₂ ), or a quarter of that value. the value obtained by dividing a pitch (n 1 _{-n 2),} since a value obtained by dividing the one pitch in (n 1 _{-n 2)} to one or a plurality adding or subtracting a value, an interference fringe The phase of the second diffracted light can be accurately shifted by １ ／ wavelength, so that the S-curve of the focus error signal can be maximized and the defocus amount can be accurately obtained.
[0034]
According to the third aspect of the present invention, since the relative phase between the first diffraction grating and the second diffraction grating is set to a value of 1/8 pitch or 5/8 pitch, the second phase becomes an interference fringe. The phase of the two diffracted lights can be accurately and easily shifted by １ ／ wavelength, so that the S-shaped curve of the focus error signal can be maximized, and the defocus amount can be obtained more accurately. Further, since the first diffraction grating and the second diffraction grating have the same pitch, that is, the same grating pitch, and interference fringes are generated using the second diffraction light of ± 1st order light, the diffraction grating and the light receiving element The production is easy, and the production cost can be further reduced.
[0035]
Fourth aspect of the present invention, the first diffracted light pitch obtained by the first diffraction grating of lambda ₁ and n ₁ order light and n ₂ order light, the second pitch is obtained by the second diffraction grating of lambda ₂ When the diffracted lights are m _1st order light and m _2nd order light, the pitch of each diffraction grating and the order of the diffracted light are
_{Λ 2 n 1 + Λ 1 m} 1 = Λ 2 n 2 + Λ 1 m 2
, The phase of the second diffracted light, which becomes an interference fringe, is shifted by を wavelength, and the S-curve of the focus error signal is maximized to obtain the defocus amount more accurately. it can.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an equivalent optical system of a theoretical analysis when a phase difference of 差 wavelength is generated by two diffracted lights as a first embodiment of the present invention.
FIG. 2 is an optical path diagram showing the entire configuration of the minute displacement measuring device.
FIG. 3 is a waveform diagram showing an S-shaped curve of a focus error signal with respect to a defocus amount.
FIG. 4 is a side view showing that a wavefront of diffracted light also moves by moving the diffraction grating as a second embodiment of the present invention.
FIG. 5 is a side view illustrating a state in which a phase difference is generated in diffracted light when two diffraction gratings are stacked and the diffraction grating is moved by one pitch.
FIG. 6 is a side view showing a state where the wavefronts of the diffracted light also match when the relative phases of the diffraction gratings match.
FIG. 7 is a side view showing a state of generation of diffracted light when the pitch of the diffraction grating according to the third embodiment of the present invention is different.
FIG. 8 is a side view showing a state in which a phase difference occurs in diffracted light when diffraction gratings having different pitches are used.
FIG. 9 is a side view showing a state where high-order diffracted light is generated.
FIG. 10 is an optical path diagram showing a focus error signal detection method in a conventional device.
[Explanation of symbols]
Reference Signs List 9 light source 12 objective lens 13 measured object 14 light receiving element 15 interference fringe generating means 15a first diffraction grating 15b second diffraction grating K ₁ first diffraction light K ₂ second diffraction light

Claims (4)

The light from the light source is condensed by the objective lens, irradiates the object to be measured, and the light reflected by the object and passed through the objective lens again is incident, whereby the first diffraction of the n _1st light and the n _2nd light is performed. A first diffraction grating that generates light, and a second diffraction grating that generates a plurality of diffracted second diffracted light by the incidence of the first diffracted light, the second diffracted light interferes with each other An interference fringe generating means in which each diffraction grating is formed in such a shape that the phase is shifted by ４ wavelength; and a light receiving element comprising at least two divided areas for receiving light from the interference fringe generating means , to measure the amount of movement in the optical axis direction of the measured object by sensing changes in the phase of the interference fringe in the light receiving element by the interference between the second diffraction light generated by the interference fringe generating means Small changes characterized by Measuring device. The relative phase between the first diffraction grating and the second diffraction grating is obtained by dividing a quarter pitch of the diffraction grating by (n ₁ −n ₂ ), or further adding one pitch to the value (n _{1 2.} The minute displacement measuring device according to claim 1, wherein a value obtained by adding or subtracting one or a plurality of values divided by −n ₂ ) is set. The first and second diffraction gratings that generate interference fringes of ± 1st-order light have the same pitch, and the relative phase between the first and second diffraction gratings is ピ ッ チ pitch of the diffraction grating. The minute displacement measuring device according to claim 1, wherein the value is set to a value of 5/8 pitch. A first diffracted light pitch obtained by the first diffraction grating of lambda ₁ and n ₁ order light and n ₂ order light, a second diffracted light pitch obtained by the second diffraction grating of lambda ₂ and m _1-order light m _{Assuming the} secondary light, the pitch of each diffraction grating and the order of the diffracted light are:
_{Λ 2 n 1 + Λ 1 m} 1 = Λ 2 n 2 + Λ 1 m 2
2. The small displacement measuring device according to claim 1, wherein the relationship is set so as to satisfy the following relationship.

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