JP3933856B2 - Phase difference amplification system - Google Patents

Description

【００２６】
反射ビームの電界分布は各々次式のようになる。
【００２７】
【数２】

【００２８】
これらの各式において、ｎ次のベッセル関数はＬＣＳＬＭ１に書きこまれたキャリア干渉縞のｎ次の回折ビームに相当する。ＬＣＳＬＭ１から出射する各回折ビームはシステムの光軸に対し異なる角度で進行する。入射ビームの角度を光軸について対称になるように調整することにより、一方の入射ビームについてｎ＝−β／α〔式（６）において〕、他方の入射ビームについてｎ＝β／α〔式（７）において〕とすることができる。２本の回折ビーム〔式（６）から得られる−ｎ次ビーム及び式（７）から得られるｎ次ビーム〕は光軸に沿って進み、次式のように表すことができる。
Ｅ₁ （ｘ，ｙ）＝Ｊ_n （２Ａ² Ｋ）ｅｘｐ[ −ｊｎ〔φ（ｘ，ｙ）＋π／２〕]
…（８）
Ｅ₂ （ｘ，ｙ）＝Ｊ_n （２Ａ² Ｋ）ｅｘｐ[ ｊｎ〔φ（ｘ，ｙ）＋π／２〕]
…（９）
これらの２本のビームは干渉計で合成されて、次式で示される強度分布を持つ干渉縞パターンを生成する。
【００２９】
Ｉ_Out ＝｜Ｅ₁ ＋Ｅ₂ ｜²
＝２〔Ｊ_n （２Ａ² Ｋ）〕² [ １＋ｃｏｓ〔２ｎψ（ｘ，ｙ）〕 ]
…（１０）
これは、当初、供試物４から発生した位相分布の２ｎ倍の位相分布に相当する干渉縞パターンである。この干渉縞パターンは、試験干渉計を波長がλではなくλ／２ｎである光を用いて動作させた場合に生成される干渉縞パターンと同じであると考えることができる。
【００３０】
得られる結果は、乾板に書かれたホログラムを用いた従来の位相差増幅システムで得られる結果と同じである。写真乾板を用いた場合も干渉縞の輪郭が経路差間隔λ／２ｎで現れる。ただし、本発明では位相ＬＣＳＬＭの使用によりシステムのリアルタイム動作が可能となる。さらに、位相変調度は変調器の書き込み側における入射光強度の変更により調整できる（また、変調器駆動電圧及び周波数の調整によっても位相変調度は調整できる）ので、回折次数間の強度のバランスは、ある特定の次数の回折（ｎ）及び位相増幅率でのシステムの動作を最適にするように調整が可能である。
【００３１】
以下、本発明の実施例について説明する。
【００３２】
図２は本発明の実施例を示す液晶空間光変調器を用いた実時間位相差増幅システムを示す図である。
【００３３】
この図において、２１はＨｅＮｅレーザー（レーザー１）、２２は空間フィルタ、２３はビームスプリッタ（ＢＳ₁ ）、２４は供試物（位相物体）、２５は反射ミラー（Ｍ₁ ）、２６は反射ミラー（Ｍ₂ ）、２７は干渉縞を投射するイメージングレンズ（Ｌ₂ ）、２８は平行配向液晶空間光変調器（ＬＣＳＬＭ）、３１はレーザー（レーザー２）、３２は空間フィルタ、３３はビームスプリッタ（ＢＳ₂ ）、３４は反射ミラー（Ｍ₃ ）、３５は反射ミラー（Ｍ₄ ）、３６は反射ミラー（Ｍ₅ ）、３７はビームスプリッタ（ＢＳ₃ ）、３８はフーリエ変換レンズ（Ｌ₄ ）、３９は反射ミラー（Ｍ₆ ）、４０はピンホール、４１はレンズ（Ｌ₅ ）、４２はＣＣＤカメラ、４３はコンピュータである。
【００３４】
図２に示されるシステムを用いて、光アドレス型位相ＬＣＳＬＭに書き込まれたホログラムを用いた位相差増幅を実証した。
【００３５】
本システムは、ＬＣＳＬＭに書き込まれる元の干渉縞パターンを生成するための試験干渉計と、位相増幅した最終的な干渉縞を生成する読み出し干渉計との２つの干渉計を有する。
【００３６】
試験干渉計はＨｅＮｅレーザー（レーザー１）２１により駆動されるマイケルソン干渉計で、ビームスプリッター（ＢＳ₁ ）２３、ミラー（Ｍ₁ ）２５及びミラー（Ｍ₂ ）２６、ＬＣＳＬＭの書き込み側Ａに干渉縞を投射するイメージングレンズ（Ｌ₂ ）２７を有する。透明なガラス板で構成された供試物２４を用いて既知の位相分布を干渉計に入力した。このガラス板は高精度の平面を持ち、既知の厚さのＳｉＯ₂ のストライプパターンが蒸着されている。ストライプは幅３ｍｍ、隙間３ｍｍである。高さはＴａｌｙｓｔｅｐで計測され、５０ｎｍであった（結果を図３に示す）。供試物２４は、ＬＣＳＬＭ２８の書き込み側Ａと〔レンズ（Ｌ₂ ）２７を介し〕共役の関係にある（ｃｏｎｊｕｇａｔｅ）平面上で、一方のマイケルソンミラーの前に設置される。キャリア干渉縞を生成するようにマイケルソンミラーの内の一方を傾けた〔式（３）及び（４）に必要な位相勾配αに相当する〕。
【００３７】
使用したＬＣＳＬＭ２８は平行配向液晶空間光変調器（ＰＡＬ−ＳＬＭ、Ｘ５６４１シリーズ、浜松フォトニクス株式会社製）で、アモルファスシリコンの感光性層を有する。この感光性層は、多層誘電体ミラーの前に配置された平行配向液晶層のＡＣ電界振幅を制御するために使用される。ＡＣ電界は、αシリコン、ミラー、及び液晶から成る層の外側に配置された透明電極から生じる。ＬＣＳＬＭデバイスへの書き込み光はアモルファス層に入り、その局部伝導率を減少させる。これにより、液晶層中の電界の増大が引き起こされ、液晶分子の配向及び屈折率が変化する。読み出し光はＬＣ層に入り、誘電体ミラーで反射しＬＣ層を再び通過してＬＣＳＬＭデバイスから出て行く。変調層を２回通過することにより、ＬＣディレクタ（ＬＣ ｄｉｒｅｃｔｏｒ）に平行な出射光ビームの偏光成分（ｐｏｌａｒｉｚａｔｉｏｎ ｃｏｍｐｏｎｅｎｔ）に空間位相変調が現れる。ＬＣＳＬＭデバイスは１８×１８ｍｍの正方形の開口全体に亘って３０ｌｐ／ｍｍの空間解像度を持ち、最大位相変調度は波長６３３ｎｍで１．５λである。
【００３８】
ＬＣＳＬＭ２８に対する読み出し干渉計は、ＨｅＮｅレーザー（レーザー２）、ビームスプリッタ（ＢＳ₂ ）３３、ミラー（Ｍ₃ ）３４、（Ｍ₄ ）３５、（Ｍ₅ ）３６から構成された、サニャック干渉計によく似たシステムで発生させた。反射ミラー（Ｍ₄ ）３５を垂直軸の回りで傾けることにより、システムの２本の出射ビームは光軸の両側に等角度でＬＣＳＬＭ２８の読み出し側に向かって進行し、位相増幅率の選択に必要な位相勾配−βとβが与えられる〔式（６）、（７）〕。
【００３９】
サニャックシステムからの出射ビームは、ＬＣＳＬＭ２８での反射（回折）後、３番目のビームスプリッタ３７（ＢＳ₃ ）で分けられ、フーリエ変換レンズ３８（Ｌ₄ ）を通過する。２本のサニャックビームによる異なる回折次数に対応する一連のスポット（ｓｐｏｔ）がレンズ３８（Ｌ₄ ）の焦点面に現れる。光軸に最も近い場所に現れる２つの次数のみを選択するためにピンホールを使用した。レンズ４１（Ｌ₅ ）は、レンズ３８（Ｌ₄ ）とともに無限焦点イメージングシステムを形成し、ＬＣＳＭＬ３８の出力面をＣＣＤカメラ４２上に映した。ＣＣＤカメラ４２には位相増幅された出力干渉縞が現れる。ＣＣＤカメラ４２の前に２つの偏光子を設けて強度を適当なレベルに調整し、これによって過負荷を避けた。
【００４０】
図４はマイケルソン干渉計から直接得られたＬＣＳＬＭの書き込み側Ａの干渉縞を示す。ストライプによる光路差はかなり小さく、約４８ｎｍ、即ちλ／１３であり（ＳｉＯ₂の反射率を１．４８と仮定し、マイケルソン干渉計内のサンプルを２回通過するものとする）、ストライプのエッジにおける干渉縞偏差もかなり小さい。
【００４１】
図５はＣＣＤカメラで得られた干渉縞を示す。複数の異なる位相増幅率を生成するために、異なる次数の回折がピンホールを通過するように、反射ミラー３５（Ｍ₄ ）の角度を調整した。ＬＣＳＭＬ２８への入射強度とその駆動電圧は、強度−位相変換特性の線形部分で動作するように調整した。図５（ａ）はピンホールを通過した両方の読み出しビームの０次について得られる結果である。予期したように、位相増幅率は零で、干渉縞偏差は無かった。図５（ｂ）、図５（ｃ）、図５（ｄ）、図５（ｅ）は、各々±１次、±２次、±３次、±４次成分がＣＣＤカメラ４８で干渉するように反射ミラー３５（Ｍ₄ ）を調整した場合に得られた結果である。±４次成分については、ＬＣＳＬＭ２８からの回折光の強度は非常に低く、識別可能な干渉縞は得られなかった。
【００４２】
位相変調度、非線形性、つまり高次回折ビームの強度を増加するために、ＬＣＳＬＭ２８の書き込み側Ａにおける強度を増加し、図６に示す結果を得た。図６（ａ）、図６（ｂ）、図６（ｃ）、図６（ｄ）はＣＣＤカメラ４２での±３、±４、±５、±６次の回折の干渉の結果を示す。この図から明らかなように、このシステムで可能な最大増幅率は±５次の干渉で得られ１０倍である。
【００４３】
上記した図５及び図６に示された結果は、明らかに本発明のシステムの位相増幅能力を実証するものであるが、マイケルソン干渉計に位相物体（供試物）を置かずに得られた干渉縞を詳細に調査し、いくつかの興味深い影響を発見した。
【００４４】
このように、本発明によれば、供試物を含むマイケルソン干渉計により生成され、光アドレス型位相空間光変調器に書かれたホログラムを用いたほぼリアルタイムで動作する位相差増幅を実証した。光アドレス型位相空間光変調器からの読み出しは、サニャック干渉計に似た装置から発生させる２本のビームを用いて行った。また、フーリエフィルタリングテクニックを用いてホログラムから所望の干渉次数を選択し、位相増幅率を決定する。本発明を用いて、１０倍までの位相増幅を実証することができた。
【００４５】
本発明は、従来の位相ステッピングやヘテロダイン干渉計を用いて計測を行う時間が容認されない製造／検査の場面で精度及び解像度を大幅に向上させることができる有意義なものである。
【００４６】
なお、本発明は上記実施例に限定されるものではなく、本発明の趣旨に基づいて種々の変形が可能であり、これらを本発明の範囲から排除するものではない。
【００４７】
【発明の効果】
以上、詳細に説明したように、本発明によれば、ＬＣＳＬＭ（液晶空間光変調素子）を用いてほぼ実時間で位相差増幅法を実現することができる。
【図面の簡単な説明】
【図１】 本発明の実施例を示す光アドレス型位相ＬＣＳＬＭを用いる位相差増幅の原理を示す概要図である。
【図２】 本発明の実施例を示す液晶空間光変調器を用いた実時間位相差増幅システムを示す図である。
【図３】 Ｔａｌｙｓｔｅｐによる供試物のステップ高の測定結果を示す図である。ステップ高は約５０ｎｍであり、マイケルソン干渉計において４８ｎｍの光路差を生じる（変調器を２回通過、ＳｉＯ₂の反射率は１．４８）。
【図４】 位相対象物（供試物）を設置したマイケルソン干渉計によって生成される干渉縞を示す図である。
【図５】 ＣＣＤカメラで得られた異なる位相増幅率の干渉縞を示す図である。
【図６】 ＬＣＳＬＭへの入射強度を増加させ、非線形性を大きくした場合にＣＣＤカメラで得られた干渉縞を示す図である。図６（ａ）は増幅率＝６（±３次）、図６（ｂ）は増幅率＝８（±４次）、図６（ｃ）は増幅率＝１０（±５次）、図６（ｄ）ｈ増幅率＝１２（±６次）を示している。
【符号の説明】
１ 光アドレス型位相液晶空間光変調器（ＬＣＳＬＭ）
２ 波長λの光
３ ビームスプリッタ
４，２４ 供試物（位相物体）
５，６ 反射ミラー
７ イメージングレンズ
Ａ 書き込み側
Ｂ 読み出し側
１１ 読み出しビームｅｘｐ（ｊβｘ）
１２ 読み出しビームｅｘｐ（−ｊβｘ）
１３ ｎ次回折ビーム
１４ −ｎ次回折ビーム
２１ ＨｅＮｅレーザー（レーザー１）
２２ 空間フィルタ
２３ ビームスプリッタ（ＢＳ₁ ）
２５ 反射ミラー（Ｍ₁ ）
２６ 反射ミラー（Ｍ₂ ）
２７ 干渉縞を投射するイメージングレンズ（Ｌ₂ ）
２８ 平行配向液晶空間光変調器（ＬＣＳＬＭ）
３１ レーザー（レーザー２）
３２ 空間フィルタ
３３ ビームスプリッタ（ＢＳ₂ ）
３４ 反射ミラー（Ｍ₃ ）
３５ 反射ミラー（Ｍ₄ ）
３６ 反射ミラー（Ｍ₅ ）
３７ ビームスプリッタ（ＢＳ₃ ）
３８ フーリエ変換レンズ（Ｌ₄ ）
３９ 反射ミラー（Ｍ₆ ）
４０ ピンホール
４１ レンズ（Ｌ₅ ）
４２ ＣＣＤカメラ
４３ コンピュータ[0001]
BACKGROUND OF THE INVENTION
The present invention, real-time using a liquid crystal spatial light modulator (real time) relates to wide systems increase retardation.
[0002]
[Prior art]
Conventionally, examples of prior literature in such a field include the following.
[0003]
(1) K.K. Creath “Phase Measurement interferometry techniques” in Progress in Optics 026 249, Elsevier Science Publishers, Amsterdam (1988).
(2) O. Bringdahl and A.M. W. Lohmann “Interferograms are image holograms” J. Am. Opt. Soc. Am. 58 141-142 (1968).
(3) K. Matsumoto and M.M. Takashima “Phase difference amplification by nonlinear holograms”, J. Mol. Opt. Soc. Am. 60 30-33 (1970).
(4) O.I. Brynddahl “Longitudinally reversed sharing interferometry” J. Am. Opt. Soc. Am. 59 142-145 (1969).
(5) K.K. Matsuda, C.I. H. Freund, and P.M. Hariharan “Phase-difference amplification using longitudinally reversed shared interferometry: an experimental study” Appl. Opt. 20 2763-2765 (1981).
Conventionally, optical interferometry is used to measure the deformation and shape of an object. However, in this method, in any case, the contour line for each wavelength is obtained in the case of a transmission object, and the contour line for each half wavelength is obtained in the case of a reflection object.
[0004]
On the other hand, the phase difference amplification method is a technology that draws a contour line of every one-hundredth or several tens of wavelengths, and plays an important role in the measurement of the nanometer region and the sub-nanometer region. Can do.
[0005]
However, the phase difference amplification method has so far been performed using holography technology that utilizes nonlinearity caused by black-and-white amplitude modulation of the photographic plate, but it takes too much time to develop and fix the photographic plate, which is a practical viewpoint. It was far from the technology.
[0006]
There is a fringe stepping method as a competing technology. This method is a technique in which, after obtaining the respective interference fringes by shifting the phase at least three times of π / 2, π, and 3π / 2, the interference fringes are computer processed to create a highly accurate interference pattern.
[0007]
[Problems to be solved by the invention]
As described above, the conventional methods are not technically satisfactory.
[0008]
The present invention is, in view of the above situation, with a LCSLM (liquid crystal spatial light modulator), and an object thereof is to provide a phase difference amplification system that can realize a phase difference amplification method in near real-time .
[0009]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides
[ 1 ] In a phase difference amplification system, an interference fringe pattern from an interferometer including a test sample is written in an optical address type phase liquid crystal spatial light modulator, and the written interference fringe pattern is written. Two diffracted beams are generated by irradiating light beams incident substantially symmetrically at approximately equal angles on both sides of the optical axis on the readout side of the liquid crystal spatial light modulator, and these two diffracted beams are synthesized by an interferometer. Thus, an interference fringe pattern having a predetermined intensity distribution is obtained.
[0010]
[ 2 ] The phase difference amplification system according to [ 1 ], wherein the liquid crystal spatial light modulator is a parallel alignment liquid crystal spatial light modulator.
[0011]
[ 3 ] The phase difference amplification system according to [ 1 ] or [ 2 ], wherein the phase modulation degree can be adjusted by changing the incident light intensity on the writing side of the liquid crystal spatial light modulator.
[0012]
[ 4 ] The phase difference amplification system according to [ 1 ] or [ 2 ], wherein the phase modulation degree can be adjusted by changing the modulator driving voltage and frequency.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail.
[0014]
In the present invention, a phase difference amplification system operating almost in real time, in which a hologram is directly written on a liquid crystal spatial light modulator (LCSLM), will be described.
[0015]
The use of phase modulators arranged in parallel reveals a major non-linearity that produces higher order diffracted beams. This is because the intensity distribution of the original interference fringes is converted into a phase distribution in which light incident on the hologram is diffracted. When the phase modulation changes in a cosine manner, the intensity of each of the different diffraction orders is determined by a Bessel function that appears in normal communication phase modulation theory. The intensity can be controlled by changing the input writing intensity to the modulator and the driving voltage. This is the first time that near real-time operation has been demonstrated in a phase difference amplification system.
[0016]
FIG. 1 is a schematic diagram showing the principle of phase difference amplification using an optical address type phase LCSLM according to an embodiment of the present invention.
[0017]
In this figure, 1 is an optical address type phase liquid crystal spatial light modulator (LCSLM), A is a writing side, 3 is a beam splitter, 4 is a specimen (phase object), 5 and 6 are reflection mirrors, and 7 is an imaging lens. , B is a readout side, 11 is a readout beam exp (jβx), 12 is a readout beam exp (−jβx), 13 is an n-order diffraction beam, and 14 is a −n-order diffraction beam.
[0018]
In FIG. 1, the writing side A irradiates the beam splitter 3 with the light 2 having the wavelength λ, passes through the signal light from the specimen (phase object) 4, passes through the beam splitter 3, and is reflected from the reflection mirror 5. The reference light reflected by the beam splitter 3 is written to the LCSLM 1 via the imaging lens 7.
[0019]
On the other hand, on the reading side B, the reading beam exp (jβx) 11 and the reading beam exp (−jβx) 12 can be read as the n-order diffracted beam 13 and the −n-order diffracted beam 14 by irradiating the LCSLM 1 with the reading beam exp (−jβx) 12.
[0020]
As described above, the interference fringe pattern I (x, y) from the interferometer (test interferometer) including the specimen 4 is projected onto the writing side A of the LCSLM 1 by operating with the light 2 having the wavelength λ. Assuming that the interference fringe contrast is 1, the intensity distribution of the interference fringe pattern is expressed by the following equation.
[0021]
I (x, y) = 2A ² [1 + cos ψ (x, y)] (1)
Here, the electric field strength of the two interference beams from the test interferometer is A, and the phase difference is ψ (x, y). Assume that ψ (x, y) consists of two components.
[0022]
ψ (x, y) = φ (x, y) + αx (2)
Here, φ (x, y) is a phase distribution generated from the specimen 4 (that is, a phase distribution to be measured), and α indicates an inclination introduced between wavefronts that interfere with each other in the test interferometer, Carrier interference fringes are generated.
[0023]
The phase LCSLM converts the intensity distribution on the writing side A into a phase distribution. This phase distribution is reflected in the readout beams 11 and 12 reflected on the readout side B of the device. Since these readout beams 11 and 12 are incident on the LCSLM 1 at equal angles on both sides of the optical axis, the phase distribution of these beams on the readout side B is ± βx. Assuming that the reflected amplitude is 1, the electric field distribution of the reflected beams 13 and 14 is expressed by the following two equations.
[0024]
E ₁ (x, y) = exp (jβx) exp {j2A ² K
[1 + cos [φ (x, y) + αx]]} (3)
E ₂ (x, y) = exp (−jβx) exp {j2A ² K
[1 + cos [φ (x, y) + αx]]} (4)
In accordance with the analysis of ordinary phase modulation performed in communication theory, the second exponential term of each equation is expanded as follows using a Bessel function.
[0025]
[Expression 1]

[0026]
The electric field distribution of the reflected beam is as follows:
[0027]
[Expression 2]

[0028]
In each of these equations, the nth order Bessel function corresponds to the nth order diffracted beam of the carrier interference fringe written in the LCSLM1. Each diffracted beam emerging from the LCSLM 1 travels at a different angle with respect to the optical axis of the system. By adjusting the angle of the incident beam so as to be symmetric with respect to the optical axis, n = −β / α (in the equation (6)) for one incident beam and n = β / α for the other incident beam (equation ( 7)]. The two diffracted beams [the n-order beam obtained from the equation (6) and the n-order beam obtained from the equation (7)] travel along the optical axis and can be expressed as the following equation.
E ₁ (x, y) = J _n (2A ² K) exp [−jn [φ (x, y) + π / 2]]
(8)
E ₂ (x, y) = J _n (2A ² K) exp [jn [φ (x, y) + π / 2]]
... (9)
These two beams are combined by an interferometer to generate an interference fringe pattern having an intensity distribution represented by the following equation.
[0029]
I _Out = | E ₁ + E ₂ | ²
= 2 [J _n (2A ² K)] ² [1 + cos [2nψ (x, y)]]
(10)
This is an interference fringe pattern corresponding to a phase distribution of 2n times the phase distribution generated from the specimen 4 at the beginning. This interference fringe pattern can be considered to be the same as the interference fringe pattern generated when the test interferometer is operated using light having a wavelength of λ / 2n instead of λ.
[0030]
The results obtained are the same as those obtained with a conventional phase difference amplification system using a hologram written on a dry plate. Even when a photographic plate is used, the outline of interference fringes appears at a path difference interval λ / 2n. However, in the present invention, the real-time operation of the system is possible by using the phase LCSLM. Furthermore, the phase modulation degree can be adjusted by changing the incident light intensity on the writing side of the modulator (and the phase modulation degree can also be adjusted by adjusting the modulator driving voltage and frequency), so the intensity balance between diffraction orders is Adjustments can be made to optimize system operation at certain orders of diffraction (n) and phase gain.
[0031]
Examples of the present invention will be described below.
[0032]
FIG. 2 is a diagram showing a real-time phase difference amplification system using a liquid crystal spatial light modulator according to an embodiment of the present invention.
[0033]
In this figure, 21 is a HeNe laser (laser 1), 22 is a spatial filter, 23 is a beam splitter (BS ₁ ), 24 is a specimen (phase object), 25 is a reflecting mirror (M ₁ ), and 26 is a reflecting mirror. (M ₂ ), 27 is an imaging lens (L ₂ ) for projecting interference fringes, 28 is a parallel alignment liquid crystal spatial light modulator (LCSLM), 31 is a laser (laser 2), 32 is a spatial filter, 33 is a beam splitter ( BS ₂ ), 34 is a reflecting mirror (M ₃ ), 35 is a reflecting mirror (M ₄ ), 36 is a reflecting mirror (M ₅ ), 37 is a beam splitter (BS ₃ ), 38 is a Fourier transform lens (L ₄ ), 39 is a reflection mirror (M ₆ ), 40 is a pinhole, 41 is a lens (L ₅ ), 42 is a CCD camera, and 43 is a computer.
[0034]
The system shown in FIG. 2 was used to demonstrate phase difference amplification using a hologram written in an optically addressed phase LCSLM.
[0035]
The system has two interferometers, a test interferometer for generating the original fringe pattern written to the LCSLM and a readout interferometer for producing the final phase-amplified fringe.
[0036]
The test interferometer is a Michelson interferometer driven by a HeNe laser (laser 1) 21. It interferes with the beam splitter (BS ₁ ) 23, mirror (M ₁ ) 25 and mirror (M ₂ ) 26, and the writing side A of the LCSLM. An imaging lens (L ₂ ) 27 for projecting stripes is included. A known phase distribution was input to the interferometer using the specimen 24 composed of a transparent glass plate. This glass plate has a high-precision plane, and a SiO ₂ stripe pattern having a known thickness is deposited thereon. The stripe has a width of 3 mm and a gap of 3 mm. The height was measured by Talystep and was 50 nm (results are shown in FIG. 3). The specimen 24 is placed in front of one of the Michelson mirrors on a conjugated plane with the writing side A of the LCSLM 28 (via a lens (L ₂ ) 27). One of the Michelson mirrors was tilted so as to generate carrier interference fringes [corresponding to the phase gradient α required for equations (3) and (4)].
[0037]
The LCSLM 28 used is a parallel alignment liquid crystal spatial light modulator (PAL-SLM, X5641 series, manufactured by Hamamatsu Photonics Co., Ltd.) and has a photosensitive layer of amorphous silicon. This photosensitive layer is used to control the AC electric field amplitude of the parallel alignment liquid crystal layer disposed in front of the multilayer dielectric mirror. The AC electric field arises from a transparent electrode placed outside the layer consisting of alpha silicon, mirror and liquid crystal. Write light to the LCSLM device enters the amorphous layer and reduces its local conductivity. This causes an increase in the electric field in the liquid crystal layer and changes the alignment and refractive index of the liquid crystal molecules. The readout light enters the LC layer, is reflected by a dielectric mirror, passes through the LC layer again, and exits the LCSLM device. By passing through the modulation layer twice, spatial phase modulation appears in the polarization component of the outgoing light beam parallel to the LC director. The LCSLM device has a spatial resolution of 30 lp / mm over the entire 18 × 18 mm square aperture, and the maximum phase modulation is 1.5λ at a wavelength of 633 nm.
[0038]
The readout interferometer for the LCSLM 28 is often a Sagnac interferometer composed of a HeNe laser (Laser 2), a beam splitter (BS ₂ ) 33, a mirror (M ₃ ) 34, (M ₄ ) 35, and (M ₅ ) 36. It was generated on a similar system. By tilting the reflective mirror (M ₄ ) 35 around the vertical axis, the two outgoing beams of the system travel at equal angles on both sides of the optical axis toward the readout side of the LCSLM 28, which is necessary for the selection of the phase gain. Phase gradients -β and β are given [Equations (6) and (7)].
[0039]
The outgoing beam from the Sagnac system is reflected (diffracted) by the LCSLM 28, is divided by the _third beam splitter 37 (BS ₃ ), and passes through the Fourier transform lens 38 (L ₄ ). A series of spots corresponding to the different diffraction orders by the two Sagnac beams appear in the focal plane of the lens 38 (L ₄ ). A pinhole was used to select only the two orders that appear in the location closest to the optical axis. The lens 41 (L ₅ ) and the lens 38 (L ₄ ) formed an infinite focus imaging system, and the output surface of the LCSML 38 was projected on the CCD camera 42. In the CCD camera 42, phase-amplified output interference fringes appear. Two polarizers were provided in front of the CCD camera 42 to adjust the intensity to an appropriate level, thereby avoiding overload.
[0040]
FIG. 4 shows the fringes on the write side A of the LCSLM obtained directly from the Michelson interferometer. The optical path difference due to the stripe is quite small, about 48 nm, ie λ / 13 (assuming the reflectivity of SiO ₂ is 1.48 and the sample in the Michelson interferometer is passed twice) The fringe deviation at the edge is also quite small.
[0041]
FIG. 5 shows interference fringes obtained with a CCD camera. In order to generate a plurality of different phase amplification factors, the angle of the reflection mirror 35 (M ₄ ) was adjusted so that different orders of diffraction pass through the pinhole. The incident intensity to the LCSML 28 and its driving voltage were adjusted so as to operate in the linear part of the intensity-phase conversion characteristics. FIG. 5A shows the results obtained for the 0th order of both readout beams that have passed through the pinhole. As expected, the phase gain was zero and there was no interference fringe deviation. 5 (b), FIG. 5 (c), FIG. 5 (d), and FIG. 5 (e) show that ± 1st order, ± 2nd order, ± 3rd order, and ± 4th order components interfere with each other at the CCD camera 48. This is the result obtained when adjusting the reflection mirror 35 (M ₄ ). For the ± 4th order component, the intensity of the diffracted light from the LCSLM 28 was very low, and no distinguishable interference fringes were obtained.
[0042]
In order to increase the phase modulation degree, nonlinearity, that is, the intensity of the higher-order diffraction beam, the intensity on the writing side A of the LCSLM 28 was increased, and the result shown in FIG. 6 was obtained. 6 (a), 6 (b), 6 (c), and 6 (d) show the results of ± 3, ± 4, ± 5, and ± 6th order diffraction interference in the CCD camera 42. FIG. As is apparent from this figure, the maximum amplification factor possible with this system is 10 times obtained with ± 5th order interference.
[0043]
The results shown in FIGS. 5 and 6 above clearly demonstrate the phase amplification capability of the system of the present invention, but are obtained without placing a phase object (sample) on the Michelson interferometer. The interference fringes were investigated in detail and some interesting effects were discovered.
[0044]
As described above, according to the present invention, phase difference amplification using a hologram generated by a Michelson interferometer including a specimen and written in an optical address type phase spatial light modulator has been demonstrated. . Reading from the optical addressed phase spatial light modulator was performed using two beams generated from a device similar to a Sagnac interferometer. In addition, a desired interference order is selected from the hologram using a Fourier filtering technique, and the phase amplification factor is determined. Using the present invention, up to 10 times phase amplification could be demonstrated.
[0045]
The present invention is significant in that it can significantly improve accuracy and resolution in manufacturing / inspection situations where the time to perform measurements using conventional phase stepping or heterodyne interferometers is unacceptable.
[0046]
In addition, this invention is not limited to the said Example, A various deformation | transformation is possible based on the meaning of this invention, and these are not excluded from the scope of the present invention.
[0047]
【The invention's effect】
As described above in detail, according to the present invention, the phase difference amplification method can be realized in almost real time by using an LCSLM (Liquid Crystal Spatial Light Modulator).
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing the principle of phase difference amplification using an optical address type phase LCSLM showing an embodiment of the present invention.
FIG. 2 is a diagram showing a real-time phase difference amplification system using a liquid crystal spatial light modulator according to an embodiment of the present invention.
FIG. 3 is a diagram showing a measurement result of a step height of a test sample by Tallystep. The step height is about 50 nm, resulting in an optical path difference of 48 nm in a Michelson interferometer (2 passes through the modulator, SiO ₂ reflectivity is 1.48).
FIG. 4 is a diagram showing interference fringes generated by a Michelson interferometer provided with a phase object (sample).
FIG. 5 is a diagram showing interference fringes with different phase gains obtained by a CCD camera.
FIG. 6 is a diagram showing interference fringes obtained by a CCD camera when the incident intensity to the LCSLM is increased and the nonlinearity is increased. 6A shows amplification factor = 6 (± 3rd order), FIG. 6B shows amplification factor = 8 (± 4th order), FIG. 6C shows amplification factor = 10 (± 5th order), FIG. (D) h amplification factor = 12 (± 6th order).
[Explanation of symbols]
1 Optical address type phase liquid crystal spatial light modulator (LCSLM)
2 Light of wavelength λ 3 Beam splitter 4,24 Specimen (phase object)
5, 6 Reflecting mirror 7 Imaging lens A Writing side B Reading side 11 Reading beam exp (jβx)
12 Read beam exp (−jβx)
13 n-order diffracted beam 14 -n-order diffracted beam 21 HeNe laser (Laser 1)
22 Spatial filter 23 Beam splitter (BS ₁ )
25 Reflective mirror (M ₁ )
26 Reflection mirror (M ₂ )
27 Imaging lens for projecting interference fringes (L ₂ )
28 Parallel alignment liquid crystal spatial light modulator (LCSLM)
31 Laser (Laser 2)
32 Spatial filter 33 Beam splitter (BS ₂ )
34 Reflection mirror (M ₃ )
35 Reflective mirror (M ₄ )
36 Reflective mirror (M ₅ )
37 Beam splitter (BS ₃ )
38 Fourier transform lens (L ₄ )
39 reflection mirror (M ₆₎
40 Pinhole 41 lens (L ₅₎
42 CCD camera 43 Computer

Claims (4)

  Operates with light of a predetermined wavelength, writes an interference fringe pattern from an interferometer including a specimen to an optical address type phase liquid crystal spatial light modulator, and writes the written interference fringe pattern to the reading side of the liquid crystal spatial light modulator Two diffracted beams are generated by irradiating light beams incident substantially symmetrically at approximately equal angles on both sides of the optical axis, and the two diffracted beams are combined by an interferometer to generate interference having a predetermined intensity distribution. A phase difference amplification system characterized by obtaining a fringe pattern. 2. The phase difference amplification system according to claim 1 , wherein the liquid crystal spatial light modulator is a parallel alignment liquid crystal spatial light modulator. 3. The phase difference amplification system according to claim 1 , wherein the phase modulation degree is adjustable by changing the incident light intensity on the writing side of the liquid crystal spatial light modulator. 3. The phase difference amplification system according to claim 1, wherein the phase modulation degree can be adjusted by changing the modulator driving voltage and the frequency.

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