JP2004020343A - Birefringence measuring apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To inexpensively provide a birefringence measuring apparatus that can measure the amount of birefringence and the orientation of a birefringence principal axis by a relatively simple configuration even in a specimen such as a polymer material indicating a birefringence of λ/4 or more or the property of a strong scatterer. <P>SOLUTION: In the birefringence measuring apparatus, a light signal (a wavelength λ) from a light source 1 is detected by a photodetector 4 via a linear polarizer 21 and a quarter wavelength plate 22 that form a polarizer 2 on the light-source side, the sample OB, and a quarter wavelength plate 31 and a linear polarizer 32 that form a polarizer 3 on the detector side. Meanwhile, the linear polarizer 21 and quarter wavelength plate 22 on the light-source side maintain specific rotation ratio (1:2) and at the same time rotate around an optical axis by a motor driver 5, and the light signal detected by the photodetector 4 is fetched by a PC 7 via an A/D converter 6. The PC 7 calculates the amount of birefringence in the sample OB by the orientation of the birefringence principal axis and accuracy in a measurement range obtained by a tangent function, namely 0 to λ/2. <P>COPYRIGHT: (C)2004,JPO

Description

の算出式から求める演算手段を備えることが可能である。
【００１５】
本発明の別の側面では、前記光源側の偏光素子は、前記光源からの光信号を受ける直線偏光子と、この直線偏光子からの光信号を受けて前記被測定試料側に出射する２つの四分の一波長板とを有し、前記制御手段は、前記２つの四分の一波長板を互いに所定の回転比を保ちつつ光軸回りに回転させる手段を備えることが可能である。
【００１６】
この側面では、前記処理手段は、前記２つの四分の一波長板の回転比を１対２としたときの当該２つの四分の一波長板の進相軸の方位をそれぞれθ及び２θとし、前記被測定試料の複屈折量及びその複屈折主軸の方位をそれぞれΔ及びφとし、前記光源で生成される光信号の光強度をａとし、前記光検出器により検出される光信号の光強度の内の前記四分の一波長板の１回転θに対しｎ（ｎ＝２、４、６、８）倍周期で変化する正弦波成分及び余弦波成分の各振幅をそれぞれＳ_（ｎ）及びＣ_（ｎ）としたとき、前記被測定試料の複屈折量Δ及びその複屈折主軸の方位φを、
【数５】

の算出式から求める演算手段を備えることが可能である。
【００１７】
本発明のさらに別の側面では、前記光源側の偏光素子は、前記光源からの光信号を受ける直線偏光子と、この直線偏光子からの光信号を受けて前記被測定試料側に出射する２つの位相差可変の位相子とを有し、前記制御手段は、前記２つの位相子の位相差を互いに所定の割合で変化させる手段を備えることが可能である。
【００１８】
この側面では、前記処理手段は、前記２つの位相子の位相差を変化させる割合を１対２としたときの当該２つの位相子の位相差をそれぞれδ及び２δとし、前記被測定試料の複屈折量及びその複屈折主軸の方位をそれぞれΔ及びφとし、前記光源で生成される光信号の光強度をａとし、前記光検出器により検出される光信号の光強度の内の前記位相子の位相変化δに対しｎ（ｎ＝１、２、３）倍周期で変化する正弦波成分及び余弦波成分の各振幅をそれぞれＳ_（ｎ）及びＣ_（ｎ）としたとき、前記被測定試料の複屈折量Δ及びその複屈折主軸の方位φを、
【数６】

の算出式から求める演算手段を備えることができる。
【００１９】
【発明の実施の形態】
以下、本発明に係る複屈折測定装置の実施の形態を添付図面を参照して説明する。
【００２０】
（第１実施形態）
図１は、第１実施形態に係る複屈折測定装置を示す。
【００２１】
図１に示す複屈折測定装置は、光信号を放出する光源１と、この光源１からの光信号を所定の偏光状態に変化させて被測定試料ＯＢに入射させる光源側の偏光素子２と、被測定試料ＯＢから射出される光信号を受ける検出器側（受光側）の偏光素子３と、この検出器側の偏光素子３からの光信号をその光強度に応じたアナログ量の電気信号に変換して検出する光検出器４と、この光検出器４による信号検出が行なわれる間、光源側の偏光素子２を駆動させて被測定試料ＯＢに入射すべき光信号の偏光状態を制御するモータドライバ（本発明の制御手段の要部を成す）５と、光検出器４の信号出力側にＡ（Ａｎａｌｏｇ）／Ｄ（Ｄｉｇｉｔａｌ）コンバータ（Ａ／Ｄ変換器）６を介して接続されるＰＣ（Ｐｅｒｓｏｎａｌ　Ｃｏｍｐｕｔｅｒ）（本発明の処理手段の要部を成す）７とを備える。ＰＣ７の出力側は図示しない信号線を介してモータドライバ５に接続される。
【００２２】
光源１は、本例では所定波長（例えば、６３２ｎｍ）のレーザ光を生成するレーザ装置で構成されるが、これに限定されず、例えばＬＥＤ（Ｌｉｇｈｔ　Ｅｍｉｔｔｉｎｇ　Ｄｅｖｉｃｅ）等やその他のランプ等から出射した光束をレンズやミラーなど適当な光学系により平行光束とするタイプのものでも構わない。
【００２３】
光源側の偏光素子２は、本例では光源１から出射される光信号の光路上に配置される直線偏光子（以下、「光源側の直線偏光子２１」）及びその射出光の光路上に配置される四分の一波長板（以下、「光源側の四分の一波長板２２）で構成され、いずれもモータドライバ５の駆動動作により図示しない回転機構を介して光軸回りに回転可能となっている。このときの光源側の直線偏光子２１及びその四分の一波長板２２の回転比は、例えば本例では「１：２」を保つ（光源側の直線偏光子２１の偏光透過軸が基準方位（初期方位）に対し方位θの位置にあるときに光源側の四分の一波長板２２の複屈折主軸（進相軸）が基準方位に対し方位２θの位置にある）ように設定されている。
【００２４】
受光側の偏光素子３は、本例では被測定試料ＯＢから射出される光信号の光路上に配置される四分の一波長板（以下、「検出器側の四分の一波長板３１」）及びその射出光の光路上に配置される直線偏光子（以下、「検出器側の直線偏光子３２」）で構成される。本例では、検出器側の四分の一波長板３１はその主軸（進相軸）方位が光源側の直線偏光子２１の基準方位に対し方位４５度の位置に、また検出器側の直線偏光子３２はその偏光透過軸方位が基準方位に対し方位０度の位置にそれぞれ固定配置される。
【００２５】
ＰＣ７は、本例では本発明の複屈折測定原理に基づく偏光解析アルゴリズムを実行する制御・演算処理装置として機能するもので、図示しないＣＰＵがその偏光解析アルゴリズム用のプログラムの命令を逐次実行することにより、複屈折測定時にはモータドライバ７に対しその駆動動作を制御するための制御指令を与えると共に、光検出器２から出力される光信号の光強度に相当するアナログ量の電気信号をＡ／Ｄコンバータ６を介してデジタル信号として取り込み、その波形解析（ＤＦＴ解析等）による偏光解析により被測定試料ＯＢの複屈折Δ及びその主軸方位φを演算するようになっている。
【００２６】
ここで、本実施形態の全体動作を説明する。
【００２７】
まず、光源１から出射した光は、光源側の直線偏光子２１及びその四分の一波長板２２、被測定試料ＯＢ、検出器側の四分の一波長板３１及びその直線偏光子３２を通過し、光検出器４にて検出される。この間、モータドライバ５の駆動により光源側の直線偏光子２１及びその四分の一波長板２２が互いに１：２の回転比を保ちながら回転し、これにより光検出器４にて検出された光信号が、逐次、Ａ／Ｄコンバータ６を介してＰＣ７に取り込まれる。
【００２８】
このときの光検出器４で得られる光信号（光強度信号）は、ミューラー行列を用いたストークス・パラメータ（Ｓｔｏｋｅｓ　Ｐａｒａｍｅｔｅｒ）による行列計算式では、次の（１）式で表現される。なお、ストークス・パラメータは、光の強度と偏りを表す４つの量、即ち光の強度Ｓ_０、水平垂直直線偏光成分Ｓ_１、±４５度直線偏光成分Ｓ_２、及び左右円偏光成分Ｓ_３を１組として光の全ての偏光状態を記述するもので、またミューラー行列は、各種の偏光素子を、入射偏光のストークス・パラメータを出射偏光のそれに変換する素子として考えた場合の４×４の行列に相当するものである。
【００２９】
【数７】

上記（１）式において、Ｓ’は最終的に得られるストークス・パラメータ（４つの成分、即ちＳ’_０〜Ｓ’_３から構成）、Ｓは光源１のストークス・パラメータ（４つの成分、即ちＳ_０〜Ｓ_３から構成）、ＬＰ_θ、ＱＷ_２θ、Ｘ_Δ、φ、ＱＷ_４５、及びＬＰ_０は、それぞれ光源側の直線偏光子（方位θ度）２１、光源側の四分の一波長板（方位２θ度）２２、被測定試料（複屈折位相差Δ及び主軸方位φ）ＯＢ、検出器側の四分の一波長板（方位４５度）３１、及び検出器側の直線偏光子（方位０度）３２の各ミューラー行列を示す。
【００３０】
上記（１）式に従って逐次計算を行うと、ストークス・パラメータＳ’の光強度を表すＳ’_０項は、次の（２）式〜（７）式で求めることができる。
【００３１】
【数８】

上記（３）式〜（７）式において、ａは、光源１から出射される光信号の光強度、ＤＣは光検出器４で検出される光信号の光強度の内の直流成分、Ｓ_（ｎ）及びＣ_（ｎ）（ｎ＝２、６）は、光検出器４で検出される光信号の光強度の内の光源側の直線偏光子２１の１回転に対しｎ倍周期で変化する正弦波成分及び余弦波成分の各振幅をそれぞれ示す。
【００３２】
上記（３）式〜（７）式を連立して解くと、被測定試料ＯＢの複屈折Δとその主軸方位φは、次の（８）式及び（９）式で求めることができる。
【００３３】
【数９】

【００３４】
上記（８）式によれば、複屈折Δは、従来例のように正弦（ｓｉｎ）関数からではなく、正接関数（ｔａｎ）から得ることができるため、その測定ダイナミックレンジを従来例の「０〜λ／４」（λ：測定光の波長）から「０〜λ／２」に拡大させることができる。しかも、複屈折Δと同時にその主軸方位φを測定できるため、この点を勘案すれば、実質的に「０〜λ」の測定（フルレンジの測定）が可能となる。
【００３５】
また、上記（８）式によれば、ＤＣ成分を使用しないで複屈折Δを求めることができるため、光検出素子に迷光等が入射した場合や熱雑音等による光強度信号中のノイズ成分の影響を未然に防ぐことができ、明室内での光学系の設置も可能になるといった利点もある。
【００３６】
従って、ＰＣ７の処理により、光検出器４で得られる光信号を入力して、上記各式に基づく偏光解析アルゴリズム用のプログラムを実行することにより、正接関数で得られる測定範囲（０〜λ／２）の精度で、被測定試料ＯＢの複屈折Δをその主軸方位φと共に同時に演算することができる。
【００３７】
また、本実施形態では、検出器側の四分の一波長板及びその直線偏光子は回転動作を伴わない構成に構築できるため、フィルム状の偏光素子（偏光子及び波長板）を採用して光検出器の入射側に直接貼り付ける等、光検出器に密着させることができ、これにより、散乱を生じる被測定試料でも、光検出器を被測定試料に近接させることで、より安定した計測ができるといった利点も得られる。
【００３８】
上記複屈折測定原理の有効性を検証するため、被測定試料ＯＢとして複屈折量を可変可能なバビネソレイユ補償子（ＢＳＣ）を使用し、そのＢＳＣの位相差を変化させて複屈折位相差量（リターデーション：Ｒｅｔａｒｄａｔｉｏｎ）及びその複屈折主軸の方位を実測した。その測定結果を図２に示す。図２中の横軸は、ＢＳＣの位相差を変化させる調整ネジの送り量を、また縦軸は、そのＢＳＣ調整ネジ送り量に対応して実測された複屈折位相差量及び主軸方位をそれぞれ示す。
【００３９】
図２に示す測定結果では、ＢＳＣの調整ネジの送り量に対して、複屈折量が線形に変化し、ＢＳＣの構造に起因する複屈折特性（調整ネジの送り量に対し複屈折量が線形に変化）とよく調和すると共に、複屈折位相差量が０〜３１６ｎｍ（測定光の波長６３２ｎｍに対し２分の一波長）の範囲で線形に増加及び減少を繰り返し（０ｎｍ及び３１６ｎｍの位置で折り返し）、その増減に応じて、主軸方位が９０度（＋４０度、−５０度）変化していることが確認された。従って、この測定結果からも、従来例の測定方法と比べ複屈折の測定ダイナミックレンジが広くとれていることが確認された。
【００４０】
以上、本実施形態による複屈折測定の特徴をまとめると、次の通りである。
１）複屈折量（複屈折位相差）を正接関数を用いた算出式により求めることができるため、従来の回転偏光子法等の偏光解析法と比べ、複屈折測定のダイナミックレンジが広がる。
２）複屈折主軸方位を同時に求めることができる。
３）受光側の偏光子（被測定試料から検出器側に配置する素子）を回転させる必要がないため、検出部の小型化が可能である。
４）２次元測定に拡張しやすい。
【００４１】
なお、本実施形態における計測方法は、光源側の偏光素子（直線偏光子及び四分の一波長板）を回転させながら逐次光強度を測定してＤＦＴ解析する方法のほか、偏光素子を１：２の回転比を保ちつつ高速に回転させて直線偏光子の１回転に対する２倍周期及び６倍周期の各成分をロックインアンプ等で同期検出する方法等を用いることが可能である。
【００４２】
また、本実施形態では、回転すべき光源側の直線偏光子及び四分の一波長板の回転比を１：２としたが、本発明はこれに限らず、この回転比を任意に設定することが可能である。これは、回転比を変化させると、その回転に伴う透過光強度の変化の割合が変化するが、ＤＦＴ解析で得られる情報（上記（３）〜（７）式）の内容は基本的に変わらないためである。
【００４３】
さらに、本実施形態では、光源側の直線偏光子の初期方位と、検出器側の直線偏光子の固定方位との関係を平行ニコル（いずれも方位０度）となるように設定しているが、本発明はこれに限らず、両者をいずれも方位９０度の位置にセットしても構わないし、互いに直交する位置（クロスニコルの関係となる位置）に配置しても構わない。これと同様に、検出器側の四分の一波長板の初期方位は、検出器側の直線偏光子に対し方位４５度の位置に設定されているが、これは相対的な方位であればよく、例えば方位１３５度の位置でも構わない。これらの設定方位の違いは、上記（３）〜（７）式では、符号の違いとなって表れる。
【００４４】
また、本実施形態では、光源にレーザを使用しているが、本発明はこれに限らず、例えばＬＥＤやその他のランプ等から出射した光束をレンズやミラーなど適当な光学系を用いて平行光束にしたものでも構わない。また、光源側の直線偏光子と光源の間に拡散板を置き、検出器側にレンズを配置して光検出を行うような構成であっても構わない。
【００４５】
（第２実施形態）
図３は、第２実施形態に係る複屈折測定装置を示す。図３に示す複屈折測定装置は、上記第１実施形態と比べると、光源側の偏光素子の構成及び検出器側の偏光素子の方位を一部変更したもので、その他の構成は実質的に同様である。
【００４６】
すなわち、図３に示すように、光源側の偏光素子２ａは、方位０度に固定される直線偏光子２３と、モータドライバ５からの駆動により図示しない回転機構を介して所定の回転比（例えば１対２）で光軸回りに回転可能な２つの四分の一波長板２４、２５とから構成され、検出器側の偏光素子３ａを成す四分の一波長板３３及び直線偏光子３４は、それぞれ方位４５度及び方位９０度の位置に固定される。
【００４７】
この構成によれば、光源１から出射した光は、光源側の偏光素子２ａを成す直線偏光子２３及び２つの四分の一波長板２４、２５、被測定試料ＯＢ、検出器側の偏光素子３ａを成す四分の一波長板３３及び直線偏光子３４を通過し、光検出器４にて検出される。この間、モータドライバ５からの駆動により図示しない回転機構を介して光源側の２つの四分の一波長板２４、２５が互いに１：２の回転比を保ちつつ光軸回りに回転し、これにより光検出器４にて検出される光信号が、逐次、Ａ／Ｄコンバータ６を介してＰＣ７に取り込まれる。
【００４８】
このときの光検出器４で得られる光信号（光強度信号）は、前述したミューラー行列を用いたストークス・パラメータによる行列計算式では、次の（１２）式で表現される。
【００４９】
【数１０】

上記（１２）式において、Ｓ’は最終的に得られるストークス・パラメータ（４つの成分、即ちＳ’_０〜Ｓ’_３から構成）、Ｓは光源１のストークス・パラメータ（４つの成分、即ちＳ_０〜Ｓ_３から構成）、ＬＰ_０、ＱＷ_θ、ＱＷ_２θ、Ｘ_Δ、φ、ＱＷ_４５、及びＬＰ_９０は、それぞれ光源側の直線偏光子（方位０度）２３、光源側の一方の四分の一波長板（方位θ）２４、その他方の四分の一波長板（方位２θ）２５、被測定試料（複屈折位相差Δ及び主軸方位φ）ＯＢ、検出器側の四分の一波長板（方位４５度）３３、及び検出器側の直線偏光子（方位９０度）３４の各ミューラー行列を示す。
【００５０】
上記（１２）式に従って逐次計算を行うと、ストークス・パラメータＳ’の光強度を表すＳ’_０項は、次の（１３）式〜（２２）式で求めることができる。
【００５１】
【数１１】

上記（１４）式〜（２２）式において、ａは、光源１から出射される光信号の光強度、ＤＣは光検出器４で検出される光信号の光強度の内の直流成分、Ｓ_（ｎ）及びＣ_（ｎ）（ｎ＝２、４、６、８）は、光検出器４で検出される光信号の光強度の内の光源側側の四分の一波長板２４の１回転に対しｎ倍周期で変化する正弦波成分及び余弦波成分の各振幅をそれぞれ示す。
【００５２】
上記（１４）式〜（２２）式を連立して解くと、被測定試料ＯＢの複屈折Δとその主軸方位φは、次の（２３）式及び（２４）式で求めることができる。
【００５３】
【数１２】

【００５４】
上記（２３）式によれば、上記第１実施形態と同様に、複屈折Δを正弦関数ではなく、正接関数から得ることができ、これにより複屈折の測定ダイナミックレンジを拡大することができる。また、上記（２３）式以外の方法でも同様に連立方程式を解く方法が複数存在することは、上記（１４）〜（２２）式を見れば明らかである。
【００５５】
従って、本実施形態でも、ＰＣ７の処理により、光検出器４で得られる光信号を入力して、上記各式に基づく偏光解析アルゴリズム用のプログラムを実行することにより、正接関数で得られる測定範囲（λ／２）の精度で、被測定試料ＯＢの複屈折Δとその主軸方位φを演算することができる。
【００５６】
また、上記（２３）式は、前述した（８）式と同様に、ＤＣ成分を使わないで複屈折が求められることを示し、これにより光検出素子に迷光等が入射した場合や熱雑音などのノイズ成分の影響を未然に防ぐことができ、明室内での光学系の設置が可能となるといった利点もある。
【００５７】
さらに、本実施形態の計測方法は、２つの四分の一波長板を回転させながら逐次光強度を測定し、ＤＦＴ解析する方法のほか、２つの四分の一波長板を１：２の回転比を保ちつつ高速に回転させて波長板の１回転に対する２倍周期、４倍周期、６倍周期、８倍周期の各成分をロックインアンプ等で同期検出する方法を用いることが可能である。
【００５８】
（第３実施形態）
図４は、第３実施形態に係る複屈折測定装置を示す。図４に示す複屈折測定装置は、前記第１及び第２の実施形態と比べると、光源側の偏光素子として位相差板（直線偏光子、四分の一波長板）をＰＣに接続されたモータドライバにより光軸回りに回転させる構成の代わりに、位相差可変の素子（可変位相子、可変波長板）を配置し、その位相差をＰＣに接続された位相ドライバにより変化させる構成を採用したもので、その他の構成は実質的に同様である。
【００５９】
すなわち、図４において、光源側の偏光素子２ｂは、方位０度に固定される直線偏光子２６と、位相ドライバ５ａからの駆動により所定の割合（例えば１：２）で位相差を可変可能な２つの可変位相子２７、２８とから構成される。
【００６０】
２つの可変位相子２７、２８は、例えば液晶を用いたものや、バビネソレイユ補償子等で構成され、その進相軸（複屈折主軸）がそれぞれ方位４５度及び０度の位置に固定される。また、検出器側の偏光素子３ｂは、上記第２実施形態と同様に、方位４５度の位置に固定される四分の一波長板３５と、方位９０度の位置に固定される直線偏光子３６とから構成される。
【００６１】
この構成によれば、光源１から出射した光は、光源側の偏光素子２ｂを成す直線偏光子２６及び２つの可変位相子２７、２８、被測定試料ＯＢ、検出器側の偏光素子３ｂを成す四分の一波長板３５及び直線偏光子３６を通過し、光検出器４にて検出される。この間、位相ドライバ５ａの駆動制御により２つの可変位相子２７、２８の位相差が１：２の変化量の割合で変化し、これにより光検出器４にて検出される光信号が、逐次、Ａ／Ｄコンバータ６を介してＰＣ７に取り込まれる。
【００６２】
このときの光検出器４で得られる光信号（光強度信号）は、前述したミューラー行列を用いたストークス・パラメータによる行列計算式では、次の（２５）式で表現される。
【００６３】
【数１３】

上記（２５）式において、Ｓ’は最終的に得られるストークス・パラメータ（４つの成分、即ちＳ’_０〜Ｓ’_３から構成）、Ｓは光源１のストークス・パラメータ（４つの成分、即ちＳ_０〜Ｓ_３から構成）、ＬＰ_０、Ｒ_δ、４５、Ｒ_２δ、０、Ｘ_Δ、φ、ＱＷ_４５、及びＬＰ_９０は、それぞれ光源側の直線偏光子（方位０度）２６、光源側の一方の可変位相子（位相差δ）２７、その他方の可変位相子（位相差２δ）２８、被測定試料（複屈折位相差Δ及び主軸方位φ）ＯＢ、検出器側の四分の一波長板（方位４５度）３５、及び検出器側の直線偏光子（方位９０度）３６の各ミューラー行列を示す。
【００６４】
上記（２５）式に従って逐次計算を行うと、ストークス・パラメータＳ’の光強度を表すＳ’_０項は、次の（２６）式〜（３２）式で求めることができる。
【００６５】
【数１４】

上記（２７）〜（３２）式において、ａは、光源１から出射される光信号の光強度、ＤＣは光検出器４で検出される光信号の光強度の内の直流成分、Ｓ_（ｎ）及びＣ_（ｎ）（ｎ＝１、２、３）は、光検出器４で検出される光信号の光強度の内の光源側の可変位相子２７の位相変化δに対しｎ倍周期で変化する正弦波成分及び余弦波成分の各振幅をそれぞれ示す。
【００６６】
上記（２７）〜（３２）式を連立して解くと、被測定試料ＯＢの複屈折Δとその主軸方位φは、次の（３３）式及び（３４）式で求めることができる。
【００６７】
【数１５】

【００６８】
上記（３３）式によれば、上記第１及び第２の実施形態と同様に、ＰＣ７の処理により、複屈折Δを正弦関数ではなく、正接関数から得ることができ、これにより複屈折の測定ダイナミックレンジを拡大することができる。また、上記（３３）式以外の方法でも同様に連立方程式を解く方法が複数存在することは、上記（２７）〜（３２）式を見れば明らかである。
【００６９】
従って、本実施形態でも、ＰＣ７の処理により、光検出器４で得られる光信号を入力して、上記各式に基づく偏光解析アルゴリズム用のプログラムを実行することにより、正接関数で得られる測定範囲（λ／２）の精度で、被測定試料ＯＢの複屈折Δとその主軸方位φを演算することができる。
【００７０】
また、上記（３３）式は、前述した（８）式及び（２３）式と同様に、ＤＣ成分を使わないで複屈折が求められることを示し、これにより光検出素子に迷光等が入射した場合や熱雑音などのノイズ成分の影響を未然に防ぐことができ、明室内での光学系の設置が可能となるといった利点もある。
【００７１】
また、本実施形態の計測方法としては、２つの可変位相子の位相差を変化させながら逐次光強度を測定してＤＦＴ解析する方法を例示できる。また、２つの可変位相子における位相差の変化の割合は、上述の１：２に限らず、２：１でも、１：３でも同様の解析が可能である。
【００７２】
（第４実施形態）
図５は、第４実施形態に係る複屈折測定装置を示す。図５に示す複屈折測定装置は、上記第１実施形態と比べると、計測原理は同じであるが、光検出器４の代わりにＣＣＤカメラ等の２次元画像取得器４ａを配置し、これに伴い、光源側の偏光素子２を成す直線偏光子２１及び四分の一波長板２２側にその光路を拡大させるためのビームエキスパンダ２９を挿入配置し、さらに２次元画像取得器４ａにて取得される画像をＰＣ７に転送するのためにフレームメモリ８を配置した点が相違する。それ以外の構成（検出器側の偏光素子３を成す四分の一波長板３１及び直線偏光子３２、モータドライバ７等）は、上記第１実施形態と本質的に同様である。
【００７３】
この構成によれば、光源１からの光信号は、光源側の直線偏光子２１及びその四分の一波長板２２を通過し、ビームエキスパンダ２９にてその光路が拡大され、その拡大ビームがそのまま被測定試料ＯＢ、検出器側の四分の一波長板３１、及び直線偏光子３２を通過し、２次元画像取得器４ａにて検出される。この間、モータドライバ５の駆動により光源側の直線偏光子２１及びその四分の一波長板２２が互いに１：２の回転比を保ちながら回転し、これにより２次元画像取得器４ａにて検出された光信号の光強度を反映した２次元画像データが、逐次、Ａ／Ｄコンバータ６を介してＰＣ７に取り込まれる。
【００７４】
従って、本実施形態でも、ＰＣ７の処理により、２次元画像取得器４ａで得られる２次元画像データを入力して、上記各式に基づく偏光解析アルゴリズム用のプログラムを実行することにより、正接関数で得られる測定範囲（λ／２）の精度で、被測定試料ＯＢの複屈折Δとその主軸方位φを２演算することができる。これにより、第１実施形態と同様の測定ダイナミックレンジを持つ２次元の複屈折測定を実施可能となる。
【００７５】
なお、図５に示す光学配置例では、光源側の四分の一波長板２２と被測定試料ＯＢとの間の光路上にビームエキスパンダ２９を配置しているが、これに限らず、例えば、図６に示すように、光源側の直線偏光子２１と光源１の間にビームエキスパンダ２９を配置しても原理的には上記と同様に考えることができる。
【００７６】
また、本実施形態では、第１実施形態における光学配置を適用しているが、第２及び第３実施形態における光学配置でも同様に適用可能である。
【００７７】
【発明の効果】
以上説明したように、本発明によれば、複屈折の大きさがλ／４（測定光の四分の一波長）以上を示したり、或いは強散乱体の性質を示したりする高分子材料等の被測定試料であっても、複屈折量及びその複屈折主軸の方位を比較的簡素な構成で測定できる複屈折測定装置を安価に提供できる。
【図面の簡単な説明】
【図１】本発明の第１実施形態に係る複屈折測定装置の全体構成及びその偏光素子配置を示す概略図。
【図２】バビネソレイユ補償子（ＢＳＣ）を用いた測定結果を示すグラフ。
【図３】本発明の第２実施形態に係る複屈折測定装置の全体構成及びその偏光素子配置を示す概略図。
【図４】本発明の第３実施形態に係る複屈折測定装置の全体構成及びその偏光素子配置を示す概略図。
【図５】本発明の第４実施形態に係る複屈折測定装置の全体構成及びその偏光素子配置を示す概略図。
【図６】図５に示す構成の変形例を示す概略図。
【符号の説明】
１　光源
２、２ａ、２ｂ、２ｃ　光源側の偏光素子
３、３ａ、３ｂ、３ｃ　検出器側の偏光素子
４　光検出器
４ａ　２次元画像取得器４ａ
５　モータドライバ
５ａ　位相ドライバ
６　Ａ／Ｄコンバータ
７　ＰＣ
８　フレームメモリ
２１　光源側の直線偏光子（回転）
２２、２４、２５　光源側の四分の一波長板（回転）
２３、２６　光源側の直線偏光子（固定）
２７、２８　可変位相子
２９　ビームエキスパンダ
３１、３３、３５　検出器側の四分の一波長板
３２、３４、３６　検出器側の直線偏光子[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a birefringence measurement device, and more particularly to a device configuration including a polarization element arrangement of a polarization measurement optical system and a driving method thereof, and a device for a polarization analysis algorithm.
[0002]
[Prior art]
In recent years, with the development of optoelectronics, in the field of engineering, demands for products such as injection molded products, optical disks, thin film products, optical elements using crystals, polymer films, and liquid crystals have been rapidly increasing. Therefore, in these engineering fields, birefringence measurement, which measures the birefringence of these products, is becoming more and more expected as one of devices and methods for quantitatively evaluating the quality of each product. I have.
[0003]
On the other hand, many polymer materials and crystals cause scattering when light is incident due to the influence of optical scatterers present inside. Generally, it is said that in a sample that causes such scattering, a phenomenon such as multiple reflection of light occurs due to the scattering, and this causes a decrease in the accuracy of birefringence measurement. Therefore, in the case of such a sample to be measured, analysis algorithms such as a measurement method using pulsed light and an analysis method using an autocorrelation function are used as a method of birefringence measurement to avoid a decrease in accuracy due to scattering. Has been proposed.
[0004]
On the other hand, if the thickness is small, as represented by a polymer film, or if it is used in a special environment such as high temperature, strong electric field, or strong magnetic field, use a measuring device with a simple device configuration as much as possible. There are also requests to do so. Therefore, as a method of measuring birefringence in the case of such a sample to be measured, it is expected to use a generally known polarization analysis method or the like without depending on a complicated device configuration or a special device.
[0005]
[Problems to be solved by the invention]
However, the conventional birefringence measurement described above has the following problems.
[0006]
First, in a commonly known polarization analysis method, for example, in a method such as a rotation analyzer method or a rotation phaser method, the principal axis of birefringence of a sample to be measured is set at a predetermined angle with respect to the polarization axis of the measurement optical system. In the method of simultaneously measuring the principal axes of birefringence such as the rotating polarizer method, the dynamic range (measurement accuracy) of the birefringence measurement is obtained as "0 to λ / 4" (where λ is (Wavelength of light).
[0007]
Also, when trying to measure birefringence for a sample with large scattering, it is important to increase the measurement accuracy by making the distance between the sample and the photodetector as close as possible. However, the method of dynamically moving or modulating the polarizing element between the sample to be measured and the light detecting element by some method has a limitation that the shortest distance is determined by the physical size of the mechanism. Was. Moreover, when a two-dimensional distribution measurement is performed using a CCD camera or the like, the elements (optical systems) immediately before the camera become large, so that driving these elements tends to impair the measurement stability. There is also a problem that the mechanical size increases and the manufacturing cost increases.
[0008]
Furthermore, in recent years, birefringence evaluation has been performed in polymer research fields, biological research fields, and the like in order to quantitatively evaluate orientation control and molecular alignment states. The magnitude of refraction may be λ / 4 (quarter wavelength of the measuring light) or more, and in many cases, it is a strong scatterer. Therefore, even in the evaluation of birefringence in these research fields, the measurement accuracy determined by the optical arrangement of the polarizing element between the sample to be measured and the light detecting element and the driving method thereof is limited by the dynamic range in the above-described birefringence measurement. It is expected that the above restrictions will apply.
[0009]
The present invention has been made in consideration of the above-described problems of the related art, and has a birefringence of λ / 4 (quarter wavelength of the measurement light) or more, or a property of a strong scatterer. An object of the present invention is to provide an inexpensive birefringence measuring apparatus capable of measuring the amount of birefringence and the direction of the main axis of birefringence with a relatively simple configuration even for a sample to be measured such as a polymer material to be shown.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, a birefringence measuring apparatus according to the present invention includes a light source that generates an optical signal, and a light source side that causes the optical signal from the light source to enter a sample to be measured while changing the optical signal to a predetermined polarization state. A polarizing element, a light-receiving-side polarizing element that receives an optical signal from the sample to be measured, a light detector that detects an optical signal from the light-receiving-side polarizing element, and the light signal is detected by the light detector. Control means for controlling the polarization state of the optical signal incident on the sample to be measured by driving at least a part of the polarizing element on the light source side, and the light of the optical signal detected by the photodetector. Processing means for calculating the amount of birefringence of the sample to be measured together with the azimuth of the principal axis of the birefringence with an accuracy of a measurement range determined by a tangent function based on the intensity.
[0011]
In the present invention, the accuracy of the measurement range obtained by the tangent function of the amount of birefringence refers to “0 to λ / 2 (λ: wavelength of the light signal of the light source)”. Since the amount of birefringence can be calculated together with the azimuth of the main axis of birefringence, the accuracy of the substantial measurement range is improved to “0 to λ”.
[0012]
As a preferred example of the present invention, the polarizing element on the light receiving side is a quarter-wave plate for receiving an optical signal from the sample to be measured, and receiving the optical signal from the quarter-wave plate to receive the light signal. Preferably, a linear polarizer that emits light to the detector side is provided, and the fast axis of the quarter-wave plate and the polarization transmission axis of the linear polarizer are preferably fixed at predetermined azimuth positions.
[0013]
Further, as one aspect of the present invention, the polarizing element on the light source side receives a light signal from the light source, and outputs the light signal from the linear polarizer to the sample to be measured. A control means for rotating the linear polarizer and the quarter-wave plate about the optical axis while maintaining a predetermined rotation ratio with respect to each other. is there.
[0014]
In this aspect, the processing means includes a polarization transmission axis of the linear polarizer and a fast axis of the quarter-wave plate when the rotation ratio of the linear polarizer and the quarter-wave plate is 1: 2. Are defined as θ and 2θ, respectively, the birefringence amount of the sample to be measured and the azimuth of the birefringent principal axis thereof are respectively Δ and φ, the light intensity of an optical signal generated by the light source is a, and the light detection is performed. The amplitudes of a sine wave component and a cosine wave component that change in a cycle of n (n = 2, 6) times with respect to one rotation θ of the linear polarizer in the light intensity of the optical signal detected by the optical signal are represented by S, respectively. _(N) And C _(N) When, the amount of birefringence Δ of the sample to be measured and the azimuth φ of the principal axis of the birefringence,
(Equation 4)

It is possible to provide a calculation means for obtaining from the calculation formula.
[0015]
In another aspect of the present invention, the polarizing element on the light source side includes a linear polarizer that receives an optical signal from the light source, and two linear light polarizers that receive an optical signal from the linear polarizer and emit the optical signal to the sample to be measured. The control means may include means for rotating the two quarter-wave plates about an optical axis while maintaining a predetermined rotation ratio with respect to each other.
[0016]
In this aspect, the processing means sets the directions of the fast axes of the two quarter-wave plates to θ and 2θ when the rotation ratio of the two quarter-wave plates is 1: 2, respectively. , The birefringence amount of the sample to be measured and the azimuth of the birefringent principal axis thereof are Δ and φ, respectively, the light intensity of the optical signal generated by the light source is a, and the light of the optical signal detected by the photodetector is The amplitudes of a sine wave component and a cosine wave component that change in a cycle of n (n = 2, 4, 6, 8) times one rotation θ of the quarter-wave plate among the intensities are represented by S _(N) And C _(N) When, the amount of birefringence Δ of the sample to be measured and the azimuth φ of the principal axis of the birefringence,
(Equation 5)

It is possible to provide a calculation means for obtaining from the calculation formula.
[0017]
In still another aspect of the present invention, the polarizing element on the light source side includes a linear polarizer that receives an optical signal from the light source, and an optical signal that receives the optical signal from the linear polarizer and emits the optical signal to the sample to be measured. And a control unit that changes the phase difference between the two phase shifters at a predetermined ratio from each other.
[0018]
In this aspect, the processing means sets the phase difference between the two phase shifters to δ and 2δ when the ratio of changing the phase difference between the two phase shifters is 1: 2, The amount of refraction and the azimuth of the birefringent main axis are respectively Δ and φ, the light intensity of the optical signal generated by the light source is a, and the phase shifter of the light intensity of the optical signal detected by the photodetector is The amplitudes of a sine wave component and a cosine wave component that change in a cycle of n (n = 1, 2, 3) times the phase change δ of _(N) And C _(N) When, the amount of birefringence Δ of the sample to be measured and the azimuth φ of the principal axis of the birefringence,
(Equation 6)

Calculation means for obtaining from the calculation formula of
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of a birefringence measuring device according to the present invention will be described with reference to the accompanying drawings.
[0020]
(1st Embodiment)
FIG. 1 shows a birefringence measuring device according to the first embodiment.
[0021]
The birefringence measuring device shown in FIG. 1 includes a light source 1 that emits an optical signal, a light source-side polarizing element 2 that changes the optical signal from the light source 1 to a predetermined polarization state and causes the optical signal to enter the sample OB to be measured. A polarizing element 3 on the detector side (light receiving side) that receives an optical signal emitted from the sample OB to be measured, and an optical signal from the polarizing element 3 on the detector side is converted into an analog electrical signal corresponding to the light intensity. The photodetector 4 for conversion and detection, and while the signal detection by the photodetector 4 is performed, the polarization state of the optical signal to be incident on the sample OB is controlled by driving the polarization element 2 on the light source side. A motor driver (which forms a main part of the control means of the present invention) 5 is connected to a signal output side of the photodetector 4 via an A (Analog) / D (Digital) converter (A / D converter) 6. PC (Personal Computer) ( 7 which constitutes a main part of the processing means of the present invention. The output side of the PC 7 is connected to the motor driver 5 via a signal line (not shown).
[0022]
In this example, the light source 1 is configured by a laser device that generates a laser beam having a predetermined wavelength (for example, 632 nm), but is not limited to this. For example, the light source 1 emits light from an LED (Light Emitting Device) or another lamp. A type in which a light beam is converted into a parallel light beam by an appropriate optical system such as a lens or a mirror may be used.
[0023]
In this example, the light source-side polarizing element 2 includes a linear polarizer (hereinafter, “light source-side linear polarizer 21”) arranged on the optical path of an optical signal emitted from the light source 1 and an optical path of the emitted light. A quarter-wave plate (hereinafter referred to as a “quarter-wave plate 22 on the light source side”) is disposed, and can be rotated around the optical axis by a driving operation of the motor driver 5 via a rotation mechanism (not shown). At this time, the rotation ratio of the linear polarizer 21 on the light source side and the quarter-wave plate 22 thereof is maintained at, for example, “1: 2” in this example (the polarization of the linear polarizer 21 on the light source side). When the transmission axis is at the position of azimuth θ with respect to the reference azimuth (initial azimuth), the birefringent main axis (fast axis) of quarter-wave plate 22 on the light source side is at azimuth 2θ with respect to the reference azimuth. It is set as follows.
[0024]
In this example, the light-receiving-side polarizing element 3 is a quarter-wave plate (hereinafter, referred to as a “detector-side quarter-wave plate 31”) arranged on the optical path of an optical signal emitted from the sample OB to be measured. ) And a linear polarizer (hereinafter, “linear polarizer 32 on the detector side”) disposed on the optical path of the emitted light. In the present example, the quarter-wave plate 31 on the detector side has its main axis (fast axis) azimuth at 45 degrees with respect to the reference azimuth of the linear polarizer 21 on the light source side. The polarizer 32 is fixedly disposed at a position where its polarization transmission axis azimuth is 0 degree with respect to the reference azimuth.
[0025]
In this example, the PC 7 functions as a control / arithmetic processing unit that executes a polarization analysis algorithm based on the birefringence measurement principle of the present invention, and a CPU (not shown) sequentially executes instructions of a program for the polarization analysis algorithm. Thus, at the time of birefringence measurement, a control command for controlling the driving operation is given to the motor driver 7 and an analog electric signal corresponding to the light intensity of the light signal output from the photodetector 2 is A / D-converted. The birefringence Δ of the sample OB to be measured and its principal axis direction φ are calculated by a polarization analysis based on a waveform analysis (DFT analysis or the like) via the converter 6 as a digital signal.
[0026]
Here, the overall operation of the present embodiment will be described.
[0027]
First, the light emitted from the light source 1 passes through the linear polarizer 21 on the light source side and its quarter-wave plate 22, the sample to be measured OB, the quarter-wave plate 31 on the detector side, and its linear polarizer 32. It passes and is detected by the photodetector 4. During this time, the linear polarizer 21 and the quarter-wave plate 22 on the light source side rotate while maintaining a rotation ratio of 1: 2 with respect to each other by the driving of the motor driver 5, whereby the light detected by the photodetector 4 is rotated. The signal is sequentially taken into the PC 7 via the A / D converter 6.
[0028]
The optical signal (light intensity signal) obtained by the photodetector 4 at this time is expressed by the following equation (1) in a matrix calculation formula based on a Stokes parameter using a Mueller matrix. It should be noted that the Stokes parameters are four quantities representing light intensity and bias, ie, light intensity S ₀ , Horizontal and vertical linearly polarized light components S ₁ , ± 45 degree linearly polarized light component S ₂ , And left and right circularly polarized light components S ₃ Is described as a set, and all the polarization states of light are described. The Mueller matrix is a 4 × 4 matrix when various polarization elements are considered as elements for converting the Stokes parameter of incident polarization into that of output polarization. It is equivalent to a matrix.
[0029]
(Equation 7)

In the above equation (1), S ′ is a Stokes parameter (four components, that is, S ′) finally obtained. ₀ ~ S ' ₃ ), S is the Stokes parameter of light source 1 (four components, ie, S ₀ ~ S ₃ ), LP _θ , QW _2θ , X _{Δ, φ} , QW ₄₅ , And LP ₀ Are a linear polarizer (azimuth θ degree) 21 on the light source side, a quarter-wave plate (azimuth 2θ degree) 22 on the light source side, a sample to be measured (birefringence phase difference Δ and principal axis direction φ) OB, a detector Each of the Mueller matrices of a quarter-wave plate (azimuth 45 degrees) 31 on the side and a linear polarizer (azimuth 0 degrees) 32 on the detector side is shown.
[0030]
By performing successive calculations according to the above equation (1), S ′ representing the light intensity of the Stokes parameter S ′ is obtained. ₀ The term can be obtained by the following equations (2) to (7).
[0031]
(Equation 8)

In the above equations (3) to (7), a is the light intensity of the optical signal emitted from the light source 1, DC is the DC component of the optical intensity of the optical signal detected by the photodetector 4, S _(N) And C _(N) (N = 2, 6) are a sine wave component and a cosine wave component that change at a period n times the rotation of the linear polarizer 21 on the light source side in the light intensity of the optical signal detected by the photodetector 4. Are shown, respectively.
[0032]
By simultaneously solving the above equations (3) to (7), the birefringence Δ of the sample OB to be measured and the principal axis direction φ can be obtained by the following equations (8) and (9).
[0033]
(Equation 9)

[0034]
According to the above equation (8), the birefringence Δ can be obtained not from the sine function (sin) as in the conventional example but from the tangent function (tan). .About..lambda. / 4 "(.lambda .: wavelength of the measurement light) to" 0.lambda./2 ". Moreover, since the principal axis direction φ can be measured simultaneously with the birefringence Δ, taking this point into account, measurement of “0 to λ” (measurement in the full range) can be substantially performed.
[0035]
Further, according to the above equation (8), the birefringence Δ can be obtained without using the DC component. Therefore, the noise component of the light intensity signal due to stray light or the like incident on the photodetector or thermal noise due to thermal noise or the like can be obtained. There is also an advantage that the influence can be prevented beforehand, and the optical system can be installed in a bright room.
[0036]
Therefore, by inputting an optical signal obtained by the photodetector 4 by the processing of the PC 7 and executing a program for an ellipsometric algorithm based on the above equations, the measurement range (0 to λ / With the accuracy of 2), the birefringence Δ of the sample OB to be measured can be calculated simultaneously with the principal axis direction φ.
[0037]
Further, in the present embodiment, since the quarter-wave plate on the detector side and its linear polarizer can be constructed so as not to rotate, a film-shaped polarizing element (polarizer and wave plate) is employed. It can be stuck to the photodetector, for example, by directly attaching it to the incident side of the photodetector, so that even if the sample causes scattering, the measurement can be performed more stably by bringing the photodetector closer to the sample. The advantage that can be obtained is also obtained.
[0038]
In order to verify the effectiveness of the above-described birefringence measurement principle, a Babinet Soleil compensator (BSC) that can vary the amount of birefringence is used as the sample OB to be measured, and the phase difference of the BSC is changed to change the birefringence phase difference. (Retardation) and the orientation of its birefringent principal axis were measured. FIG. 2 shows the measurement results. The horizontal axis in FIG. 2 indicates the feed amount of the adjustment screw for changing the BSC phase difference, and the vertical axis indicates the birefringence phase difference amount and the principal axis direction actually measured corresponding to the BSC adjustment screw feed amount. Show.
[0039]
In the measurement results shown in FIG. 2, the amount of birefringence linearly changes with respect to the feed amount of the adjustment screw of the BSC, and the birefringence characteristic caused by the structure of the BSC (the amount of birefringence is linear with respect to the feed amount of the adjustment screw). ), And increase and decrease linearly in the range of the birefringence phase difference in the range of 0 to 316 nm (half the wavelength of the measurement light of 632 nm) (turning back at the positions of 0 nm and 316 nm). ), It was confirmed that the principal axis direction changed by 90 degrees (+40 degrees, -50 degrees) according to the increase or decrease. Therefore, from the measurement results, it was confirmed that the measurement dynamic range of the birefringence was wider than that of the conventional measurement method.
[0040]
The characteristics of the birefringence measurement according to the present embodiment are summarized as follows.
1) Since the amount of birefringence (birefringence phase difference) can be obtained by a calculation formula using a tangent function, the dynamic range of the birefringence measurement is expanded as compared with a conventional polarization analysis method such as a rotating polarizer method.
2) The birefringent principal axis direction can be determined simultaneously.
3) Since it is not necessary to rotate the light-receiving-side polarizer (the element arranged from the sample to be measured to the detector side), the size of the detection unit can be reduced.
4) Easy expansion to two-dimensional measurement.
[0041]
The measurement method according to the present embodiment includes a method of sequentially measuring the light intensity while rotating a polarizing element (a linear polarizer and a quarter-wave plate) on the light source side and performing DFT analysis. It is possible to use a method of rotating the linear polarizer at a high speed while maintaining the rotation ratio of 2 and synchronously detecting each component of the double period and the six times period with respect to one rotation of the linear polarizer using a lock-in amplifier or the like.
[0042]
In the present embodiment, the rotation ratio of the linear polarizer and the quarter-wave plate on the light source side to be rotated is set to 1: 2, but the present invention is not limited to this, and the rotation ratio is set arbitrarily. It is possible. This is because, when the rotation ratio is changed, the rate of change of the transmitted light intensity due to the rotation is changed, but the contents of the information (Equations (3) to (7)) obtained by the DFT analysis are basically changed. Because there is no.
[0043]
Furthermore, in the present embodiment, the relationship between the initial orientation of the linear polarizer on the light source side and the fixed orientation of the linear polarizer on the detector side is set to be parallel Nicols (both directions are 0 degrees). However, the present invention is not limited to this, and both of them may be set at a position of 90 degrees in azimuth, or may be arranged at positions orthogonal to each other (positions having a crossed Nicols relationship). Similarly, the initial direction of the quarter-wave plate on the detector side is set at a position of 45 degrees with respect to the linear polarizer on the detector side, but this is a relative direction. For example, a position at an azimuth of 135 degrees may be used. These differences in the set azimuths are represented by differences in the signs in the above equations (3) to (7).
[0044]
Further, in the present embodiment, a laser is used as a light source. However, the present invention is not limited to this. For example, a light beam emitted from an LED or another lamp is converted into a parallel light beam using an appropriate optical system such as a lens or a mirror. You can use anything you like. Further, a configuration may be adopted in which a diffusion plate is placed between the linear polarizer on the light source side and the light source, and a lens is disposed on the detector side to perform light detection.
[0045]
(2nd Embodiment)
FIG. 3 shows a birefringence measuring device according to the second embodiment. The birefringence measuring device shown in FIG. 3 is different from the first embodiment in that the configuration of the polarizing element on the light source side and the orientation of the polarizing element on the detector side are partially changed, and the other configurations are substantially the same. The same is true.
[0046]
That is, as shown in FIG. 3, the polarization element 2a on the light source side has a predetermined rotation ratio (for example, a rotation ratio not shown) through a rotation mechanism (not shown) driven by the motor driver 5 and the linear polarizer 23 fixed at an azimuth of 0 degree. The two-quarter wave plates 24 and 25 rotatable about the optical axis (1: 2), and the quarter wave plate 33 and the linear polarizer 34 that constitute the detector-side polarizing element 3a are: , Are fixed at positions of azimuth 45 degrees and azimuth 90 degrees, respectively.
[0047]
According to this configuration, the light emitted from the light source 1 is divided into the linear polarizer 23 and the two quarter-wave plates 24 and 25, which constitute the polarizing element 2a on the light source side, the sample OB to be measured, and the polarizing element on the detector side. The light passes through a quarter-wave plate 33 and a linear polarizer 34 forming 3a, and is detected by the photodetector 4. During this time, the two quarter-wave plates 24 and 25 on the light source side rotate around the optical axis while maintaining a rotation ratio of 1: 2 with each other via a rotation mechanism (not shown) by the drive from the motor driver 5. Optical signals detected by the photodetector 4 are sequentially taken into the PC 7 via the A / D converter 6.
[0048]
The optical signal (light intensity signal) obtained by the photodetector 4 at this time is expressed by the following equation (12) in the matrix calculation equation based on the Stokes parameter using the Mueller matrix described above.
[0049]
(Equation 10)

In the above equation (12), S ′ is a Stokes parameter (four components, ie, S ′) finally obtained. ₀ ~ S ' ₃ ), S is the Stokes parameter of light source 1 (four components, ie, S ₀ ~ S ₃ ), LP ₀ , QW _θ , QW _2θ , X _{Δ, φ} , QW ₄₅ , And LP ₉₀ Are a linear polarizer (azimuth 0 degree) 23 on the light source side, a quarter-wave plate (azimuth θ) 24 on one side of the light source side, a quarter-wave plate (azimuth 2θ) 25 on the other side, and a measured object. Each of the Mueller matrices of the sample (birefringence phase difference Δ and principal axis direction φ) OB, a quarter-wave plate (direction 45 degrees) 33 on the detector side, and a linear polarizer (direction 90 degrees) 34 on the detector side is Show.
[0050]
By performing the sequential calculation according to the above equation (12), S ′ representing the light intensity of the Stokes parameter S ′ is obtained. ₀ The term can be obtained by the following equations (13) to (22).
[0051]
[Equation 11]

In the above equations (14) to (22), a is the light intensity of the optical signal emitted from the light source 1, DC is the DC component of the optical intensity of the optical signal detected by the photodetector 4, S _(N) And C _(N) (N = 2, 4, 6, 8) is the light intensity of the optical signal detected by the photodetector 4 and changes at a period of n times with respect to one rotation of the quarter-wave plate 24 on the light source side. The respective amplitudes of the sine wave component and the cosine wave component are shown.
[0052]
When the above equations (14) to (22) are simultaneously solved, the birefringence Δ of the sample OB to be measured and the principal axis direction φ can be obtained by the following equations (23) and (24).
[0053]
(Equation 12)

[0054]
According to the above equation (23), similarly to the first embodiment, the birefringence Δ can be obtained not from a sine function but from a tangent function, whereby the measurement dynamic range of birefringence can be expanded. It is apparent from the above equations (14) to (22) that there are a plurality of methods for solving the simultaneous equations in the same manner other than the equation (23).
[0055]
Therefore, also in the present embodiment, by inputting the optical signal obtained by the photodetector 4 by the processing of the PC 7 and executing the program for the polarization analysis algorithm based on each of the above equations, the measurement range obtained by the tangent function is obtained. With the accuracy of (λ / 2), the birefringence Δ of the sample OB to be measured and its principal axis direction φ can be calculated.
[0056]
Equation (23) indicates that birefringence can be obtained without using a DC component, similar to equation (8), whereby stray light or the like enters the photodetector, thermal noise, etc. There is also an advantage that the influence of the noise component can be prevented beforehand, and the optical system can be installed in a bright room.
[0057]
Further, the measurement method of the present embodiment measures the light intensity sequentially while rotating the two quarter-wave plates and performs DFT analysis. In addition, the two quarter-wave plates are rotated by 1: 2. It is possible to use a method of rotating at a high speed while maintaining the ratio and synchronously detecting each component of a double cycle, a quadruple cycle, a six-fold cycle, and an eight-fold cycle with respect to one rotation of the wave plate by a lock-in amplifier or the like. .
[0058]
(Third embodiment)
FIG. 4 shows a birefringence measuring device according to the third embodiment. The birefringence measuring device shown in FIG. 4 has a retardation plate (linear polarizer, quarter-wave plate) connected to a PC as a light source-side polarizing element as compared with the first and second embodiments. Instead of using a motor driver to rotate around the optical axis, a variable phase difference element (variable phase shifter, variable wavelength plate) is arranged, and the phase difference is changed by a phase driver connected to a PC. The other configuration is substantially the same.
[0059]
That is, in FIG. 4, the polarization element 2b on the light source side can change the phase difference at a predetermined ratio (for example, 1: 2) by driving from the linear polarizer 26 fixed at the azimuth of 0 degree and the phase driver 5a. It is composed of two variable phase shifters 27 and 28.
[0060]
The two variable phase shifters 27 and 28 are composed of, for example, one using liquid crystal, a Babinet Soleil compensator, and the like, and their fast axes (birefringent main axes) are fixed at positions of azimuth 45 degrees and 0 degrees, respectively. . The detector-side polarizing element 3b includes a quarter-wave plate 35 fixed at a position of 45 degrees azimuth and a linear polarizer fixed at a position of 90 degrees azimuth as in the second embodiment. 36.
[0061]
According to this configuration, the light emitted from the light source 1 forms the linear polarizer 26 and the two variable phase shifters 27 and 28 that form the polarizing element 2b on the light source side, the sample OB to be measured, and the polarizing element 3b on the detector side. The light passes through the quarter-wave plate 35 and the linear polarizer 36 and is detected by the photodetector 4. During this time, the phase difference between the two variable phase shifters 27 and 28 changes at a rate of a change amount of 1: 2 by the drive control of the phase driver 5a, whereby the optical signal detected by the photodetector 4 is sequentially changed. The data is taken into the PC 7 via the A / D converter 6.
[0062]
The optical signal (light intensity signal) obtained by the photodetector 4 at this time is expressed by the following equation (25) in the matrix calculation formula based on the Stokes parameter using the Mueller matrix described above.
[0063]
(Equation 13)

In the above equation (25), S ′ is a Stokes parameter finally obtained (four components, that is, S ′). ₀ ~ S ' ₃ ), S is the Stokes parameter of light source 1 (four components, ie, S ₀ ~ S ₃ ), LP ₀ , R _{δ, 45} , R _{2δ, 0} , X _{Δ, φ} , QW ₄₅ , And LP ₉₀ Are the linear polarizer (azimuth 0 degree) 26 on the light source side, one variable phase shifter (phase difference δ) 27 on the light source side, the other variable phase shifter (phase difference 2δ) 28, and the sample to be measured (birefringence). The Mueller matrix of the phase difference Δ and the principal axis direction φ) OB, the quarter-wave plate (direction 45 degrees) 35 on the detector side, and the linear polarizer (direction 90 degrees) 36 on the detector side are shown.
[0064]
By performing successive calculations according to the above equation (25), S ′ representing the light intensity of the Stokes parameter S ′ is obtained. ₀ The term can be obtained by the following equations (26) to (32).
[0065]
[Equation 14]

In the above equations (27) to (32), a is the light intensity of the optical signal emitted from the light source 1, DC is the DC component of the optical intensity of the optical signal detected by the photodetector 4, S _(N) And C _(N) (N = 1, 2, 3) is a sine wave component that changes at a period n times the phase change δ of the variable phase shifter 27 on the light source side in the light intensity of the optical signal detected by the photodetector 4 and Each amplitude of the cosine wave component is shown.
[0066]
When the above equations (27) to (32) are simultaneously solved, the birefringence Δ of the sample OB to be measured and the principal axis direction φ can be obtained by the following equations (33) and (34).
[0067]
[Equation 15]

[0068]
According to the above equation (33), the birefringence Δ can be obtained not from the sine function but from the tangent function by the processing of the PC 7 as in the first and second embodiments. The dynamic range can be expanded. It is apparent from the above equations (27) to (32) that there are a plurality of methods for solving the simultaneous equations in a similar manner other than the equation (33).
[0069]
Therefore, also in the present embodiment, by inputting the optical signal obtained by the photodetector 4 by the processing of the PC 7 and executing the program for the polarization analysis algorithm based on each of the above equations, the measurement range obtained by the tangent function is obtained. With the accuracy of (λ / 2), the birefringence Δ of the sample OB to be measured and its principal axis direction φ can be calculated.
[0070]
In addition, the above equation (33) indicates that birefringence can be obtained without using a DC component, similarly to the above-described equations (8) and (23), whereby stray light or the like enters the photodetector. There is also an advantage that the influence of noise components such as noise and thermal noise can be prevented beforehand, and the optical system can be installed in a bright room.
[0071]
Further, as a measurement method of the present embodiment, a method of sequentially measuring the light intensity while changing the phase difference between two variable phase shifters and performing DFT analysis can be exemplified. Further, the same analysis is possible when the ratio of the change in the phase difference between the two variable phase shifters is not limited to 1: 2 as described above, but is 2: 1 or 1: 3.
[0072]
(Fourth embodiment)
FIG. 5 shows a birefringence measuring device according to a fourth embodiment. The measurement principle of the birefringence measuring apparatus shown in FIG. 5 is the same as that of the first embodiment, but a two-dimensional image acquisition device 4a such as a CCD camera is arranged in place of the photodetector 4, and Accordingly, a linear polarizer 21 constituting the polarizing element 2 on the light source side and a beam expander 29 for enlarging the optical path are inserted and arranged on the quarter wavelength plate 22 side, and further acquired by the two-dimensional image acquisition device 4a. The difference is that a frame memory 8 is arranged to transfer the image to be transmitted to the PC 7. Other configurations (the quarter-wave plate 31 and the linear polarizer 32, the motor driver 7, etc., constituting the detector-side polarizing element 3) are essentially the same as those in the first embodiment.
[0073]
According to this configuration, the optical signal from the light source 1 passes through the linear polarizer 21 on the light source side and the quarter-wave plate 22 thereof, the optical path is expanded by the beam expander 29, and the expanded beam is The light passes through the sample OB as it is, the quarter-wave plate 31 on the detector side, and the linear polarizer 32, and is detected by the two-dimensional image acquisition device 4a. During this time, the linear polarizer 21 and the quarter-wave plate 22 on the light source side rotate while maintaining a rotation ratio of 1: 2 with respect to each other by the driving of the motor driver 5, whereby the two-dimensional image acquisition unit 4 a detects the rotation. The two-dimensional image data reflecting the light intensity of the light signal is sequentially taken into the PC 7 via the A / D converter 6.
[0074]
Therefore, also in the present embodiment, by inputting the two-dimensional image data obtained by the two-dimensional image acquisition unit 4a by the processing of the PC 7, and executing the program for the polarization analysis algorithm based on the above equations, the tangent function is obtained. The birefringence Δ of the sample OB to be measured and its principal axis azimuth φ can be calculated twice with the accuracy of the obtained measurement range (λ / 2). This makes it possible to perform two-dimensional birefringence measurement having the same measurement dynamic range as in the first embodiment.
[0075]
In the example of the optical arrangement shown in FIG. 5, the beam expander 29 is arranged on the optical path between the quarter-wave plate 22 on the light source side and the sample OB to be measured, but is not limited to this. As shown in FIG. 6, even if a beam expander 29 is arranged between the light source side linear polarizer 21 and the light source 1, the same principle can be considered.
[0076]
Further, in the present embodiment, the optical arrangement in the first embodiment is applied, but the optical arrangement in the second and third embodiments can be similarly applied.
[0077]
【The invention's effect】
As described above, according to the present invention, a polymer material having a birefringence of λ / 4 (quarter wavelength of the measurement light) or more, or exhibiting the properties of a strong scatterer, etc. With respect to the sample to be measured, a birefringence measuring apparatus capable of measuring the amount of birefringence and the direction of the principal axis of the birefringence with a relatively simple configuration can be provided at low cost.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an entire configuration of a birefringence measuring device according to a first embodiment of the present invention and an arrangement of polarizing elements thereof.
FIG. 2 is a graph showing measurement results obtained using a Babinet Soleil compensator (BSC).
FIG. 3 is a schematic diagram showing an entire configuration of a birefringence measuring device according to a second embodiment of the present invention and an arrangement of polarizing elements thereof.
FIG. 4 is a schematic diagram showing an entire configuration of a birefringence measuring device according to a third embodiment of the present invention and an arrangement of polarizing elements thereof.
FIG. 5 is a schematic diagram showing an entire configuration of a birefringence measuring device according to a fourth embodiment of the present invention and an arrangement of polarizing elements thereof.
FIG. 6 is a schematic diagram showing a modification of the configuration shown in FIG. 5;
[Explanation of symbols]
1 light source
2, 2a, 2b, 2c Light source side polarizing element
3, 3a, 3b, 3c Polarizing element on the detector side
4 Photodetector
4a Two-dimensional image acquisition device 4a
5 Motor driver
5a Phase driver
6 A / D converter
7 PC
8 frame memory
21 Light source side linear polarizer (rotation)
22, 24, 25 Quarter-wave plate on the light source side (rotation)
23, 26 Linear polarizer on the light source side (fixed)
27, 28 Variable phase shifter
29 Beam Expander
31, 33, 35 Quarter-wave plate on the detector side
32, 34, 36 Linear polarizer on the detector side

Claims (8)

A light source for generating an optical signal;
A polarizing element on the light source side that is incident on the sample to be measured while changing the optical signal from the light source to a predetermined polarization state,
A light-receiving-side polarizing element that receives an optical signal from the sample to be measured,
A photodetector for detecting an optical signal from the light-receiving side polarizing element,
While the optical signal is detected by the photodetector, control means for controlling the polarization state of the optical signal incident on the sample to be measured by driving at least a part of the polarizing element on the light source side,
Processing means for calculating the amount of birefringence of the sample to be measured with the accuracy of a measurement range determined by a tangent function together with the azimuth of the principal axis of the birefringence, based on the light intensity of the optical signal detected by the photodetector. A birefringence measuring device characterized by the above-mentioned. The light-receiving side polarizing element is a quarter-wave plate that receives an optical signal from the sample to be measured, and a linearly polarized light that receives an optical signal from the quarter-wave plate and emits the light to the photodetector. 2. The birefringence measurement apparatus according to claim 1, wherein a fast axis of the quarter-wave plate and a polarization transmission axis of the linear polarizer are fixed at predetermined azimuth positions, respectively. . The polarizing element on the light source side has a linear polarizer that receives an optical signal from the light source, and a quarter-wave plate that receives an optical signal from the linear polarizer and emits the optical signal to the sample to be measured. 3. The birefringence measurement according to claim 2, wherein the control unit includes a driving unit that rotates the linear polarizer and the quarter-wave plate around the optical axis while maintaining a predetermined rotation ratio with respect to each other. apparatus. The processing means sets the respective directions of the polarization transmission axis of the linear polarizer and the fast axis of the quarter-wave plate when the rotation ratio of the linear polarizer and the quarter-wave plate is 1: 2. Θ and 2θ, respectively, the amount of birefringence of the sample to be measured and the azimuth of the main axis of the birefringence are Δ and φ, respectively, the light intensity of the optical signal generated by the light source is a, and the light intensity is detected by the photodetector. The amplitudes of a sine wave component and a cosine wave component that change in a cycle of n (n = 2, 6) times one rotation θ of the linear polarizer in the light intensity of the optical signal are represented by S _(n) and C, respectively. _{When (n)} , the birefringence amount Δ of the sample to be measured and the azimuth φ of the main axis of the birefringence are

4. A birefringence measuring apparatus according to claim 3, further comprising a calculating means for calculating from a calculation formula. The polarizing element on the light source side includes a linear polarizer that receives an optical signal from the light source, and two quarter-wave plates that receive the optical signal from the linear polarizer and emit the optical signal to the sample to be measured. 3. A birefringence measuring apparatus according to claim 2, wherein said control means includes means for rotating said two quarter-wave plates about an optical axis while maintaining a predetermined rotation ratio with respect to each other. . Birefringence measurement device. The processing means sets the azimuths of the fast axes of the two quarter-wave plates to θ and 2θ when the rotation ratio of the two quarter-wave plates is 1: 2, The amount of birefringence of the sample and the azimuth of the birefringent principal axis are respectively Δ and φ, the light intensity of the optical signal generated by the light source is a, and the light intensity of the optical signal detected by the photodetector is The amplitudes of a sine wave component and a cosine wave component that change in a cycle of n (n = 2, 4, 6, 8) times one rotation θ of the quarter-wave plate are S _(n) and C _{(n )} , The birefringence amount Δ of the sample to be measured and the azimuth φ of the principal axis of the birefringence are

4. A birefringence measuring apparatus according to claim 3, further comprising a calculating means for calculating from a calculation formula. The polarizing element on the light source side includes a linear polarizer that receives an optical signal from the light source, and two phase difference variable phasers that receive an optical signal from the linear polarizer and emit the optical signal to the sample to be measured. 3. The birefringence measuring apparatus according to claim 2, wherein the control means includes means for changing a phase difference between the two phase shifters at a predetermined ratio. The processing means sets the phase difference between the two phase shifters to δ and 2δ when the ratio of changing the phase difference between the two phase shifters is 1: 2, and the birefringence amount of the sample to be measured and its The directions of the birefringent principal axes are respectively Δ and φ, the light intensity of the optical signal generated by the light source is a, and the phase change δ of the phaser in the light intensity of the optical signal detected by the photodetector. When the amplitudes of a sine wave component and a cosine wave component that change in a cycle of n (n = 1, 2, 3) times are S _(n) and C _{(n )} , respectively, the birefringence of the sample to be measured is Δ and the orientation φ of the birefringent principal axis,

4. A birefringence measuring apparatus according to claim 3, further comprising a calculating means for calculating from a calculation formula.

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