JP2004028970A

JP2004028970A - Polarization-sensitive optical spectrum interference coherence tomography apparatus and method for measuring polarization information inside sample using the apparatus

Info

Publication number: JP2004028970A
Application number: JP2002217274A
Authority: JP
Inventors: Yoshiaki Yasuno; 安野　嘉晃; Toyohiko Yatagai; 谷田貝　豊彦; Masahide Ito; 伊藤　雅英
Original assignee: Institute of Tsukuba Liaision Co Ltd
Current assignee: Institute of Tsukuba Liaision Co Ltd
Priority date: 2002-06-21
Filing date: 2002-06-21
Publication date: 2004-01-29
Anticipated expiration: 2022-06-21
Also published as: JP4045140B2

Abstract

【課題】複屈折分布を含む魚の骨や人間の皮膚といった生物試料の断面構造の計測について、従来のＯＣＴ計測では得ることができなかった複屈折分布を含む微細な構造を確認可能とする。
【解決手段】参照光と物体光の偏光状態を、それぞれ水平直線偏光、垂直直線偏光、４５°直線偏光、右回り円偏光の４通りのいずれかに選択的に調整し、それぞれの偏光状態をビームスプリッター１０で組み合わせて４通り×４通りの１６通りの組み合わせた状態で、回折格子２０及びレンズから成る分光器を通し、その干渉縞をＣＣＤカメラ２２で撮影して１６枚のコヒーレンストモグラフィー像を取得し、これらの１６枚のコヒーレンストモグラフィー像から、試料の偏光特性を表示することができるミュラー行列の各成分を求めることにより、試料内部の偏光情報の測定を行う。
【選択図】　　　図１An object of the present invention is to measure a cross-sectional structure of a biological sample such as a fish bone or a human skin including a birefringence distribution, and to confirm a fine structure including a birefringence distribution which cannot be obtained by conventional OCT measurement.
A polarization state of a reference light and an object light are selectively adjusted to one of four types of horizontal linear polarization, vertical linear polarization, 45 ° linear polarization, and clockwise circular polarization, respectively, and each polarization state is adjusted. In a state of being combined by the beam splitter 10 in 16 ways of 4 ways × 4 ways, the light passes through a spectroscope including a diffraction grating 20 and a lens, and the interference fringes are photographed by a CCD camera 22 to form 16 coherence tomographic images. The polarization information inside the sample is measured by obtaining each component of the Muller matrix that can acquire and acquire the polarization characteristics of the sample from these 16 coherence tomography images.
[Selection diagram] Fig. 1

Description

【０００１】
【発明の属する技術分野】
本発明は、光コヒーレンストモグラフィー（ＯＣＴ：Ｏｐｔｉｃａｌ　ｃｏｈｅｒｅｎｃｅ　ｔｏｍｏｇｒａｐｈｙ）を用いて物体のもつ偏光情報を捉え、より微細な構造を計測するための偏光感受型光コヒーレンストモグラフィー装置及び該装置による試料内部の偏光情報の測定方法に関する。
【０００２】
【従来の技術】
従来、物体の内部情報、つまり屈折率分布の微分構造を非破壊、高分解能で捉えるために、ＯＣＴを用いることが行われている。
【０００３】
しかし、従来のＯＣＴでは、物体の屈折率分布の微分構造は、非破壊、高分解能で捉えることはできるものの、物体そのものが本来持っている偏光依存性を捉えることはできない。特にＯＣＴを生体計測へ応用することを考える場合、繊維状の構造に起因する複屈折による偏光依存性を持つ生物試料の測定においては、解像度の低下とともに、構造を捉えられないなどの問題が生じてしまう。
【０００４】
【発明が解決しようとする課題】
一般に、散乱光成分はほとんど偏光特性を持っていないため、その偏光情報を捉えるのは困難である。本発明者らは、ＯＣＴのような低コヒーレンス干渉計で、ある特定部分からの散乱光成分とある偏光状態の参照光とを干渉させると、その干渉成分には偏光特性が強く反映され、その結果、奥行き方向の断面のある特定部分の偏光情報を捉えることが可能となるという点を想到するに至った。
【０００５】
本発明は、試料（物体）の持つ偏光情報を捉えて微細な構造を計測する物体の持つ偏光情報を捉えて微細な構造を計測することを目的とし、機械的な走査なしに深さ方向の断面構造を計測できるスペクトル干渉型のトモグラフィー装置に偏光感受性を導入した。
【０００６】
【課題を解決するための手段】
本発明は上記課題を解決するために、入射光の入射光路上に設けられ、該入射光の偏光状態を、互いに異なる４通りの偏光状態のいずれかに選択的に調整可能な入射光学系と、上記入射光路上に設けられ、上記４通りの偏光状態のいずれかに選択的に調整された入射光を参照光と計測対象とする試料に対する入射光とに分けるビームスプリッターと、上記参照光の偏光状態を、上記４通りの偏光状態のいずれかに選択的に調整して上記ビームスプリッターに戻して透過させるための参照光光学系と、それぞれの４通りの偏光状態に調整された上記参照光と上記試料から反射されてきた物体光とを互いに組み合わせて４通り×４通りの１６通りの組み合わせた状態で、参照光と物体とを干渉させる回折格子及びレンズから成る分光器と、上記干渉で生じたスペクトル干渉縞を撮影して１６枚のコヒーレンストモグラフィー像を取得するＣＣＤカメラとを備え、上記物体光のうち参照光と同じ偏光成分に基づく信号を取り出すことを特徴とする偏光感受型光コヒーレンストモグラフィー装置を提供する。
【０００７】
本発明は上記課題を解決するために、入射光の入射光路上に設けられ、該入射光の偏光状態を、水平直線偏光、垂直直線偏光、４５°直線偏光及び右周り円偏光の４通りのいずれかに選択的に調整する第１の１／２波長板及び第１の１／４波長板と、上記入射光路上に設けられ、上記４通りの偏光状態のいずれかに選択的に調整された入射光を参照光と計測対象とする試料に対する入射光とに分けるビームスプリッターと、上記参照光の偏光状態を水平直線偏光、垂直直線偏光、４５°直線偏光及び右周り円偏光の４通りのいずれかに選択的に調整して上記ビームスプリッターに戻して透過させるための第２の１／４波長板、第３の１／４波長板及びミラーと、上記４通りの偏光状態のいずれかにそれぞれ調整され上記試料で反射され、さらに上記ビームスプリッターで反射された物体光と上記４通りの偏光状態のいずれかにそれぞれ調整された参照光が重ね合わせられた光を水平直線偏光に制御する第４の１／４波長板及び第２の１／２波長板と、それぞれの４通りの偏光状態に調整された上記参照光と上記試料から反射されてきた物体光とを互いに組み合わせて４通り×４通りの１６通りの組み合わせた状態で、参照光と物体とを干渉させる回折格子及びレンズから成る分光器と、上記干渉で生じたスペクトル干渉縞を撮影して１６枚のコヒーレンストモグラフィー像を取得するＣＣＤカメラとを備え、上記物体光のうち参照光と同じ偏光成分に基づく信号を取り出すことを特徴とする偏光感受型光コヒーレンストモグラフィー装置を提供する。
【０００８】
本発明は上記課題を解決するために、参照光と物体光の偏光状態を、互いに異なる４通り偏光状態のいずれかに選択的に調整し、それぞれの偏光状態を組み合わせて４通り×４通りの１６通りの組み合わせた状態で、参照光と物体光の干渉縞を撮影して１６枚のコヒーレンストモグラフィー像を取得し、これらの１６枚のコヒーレンストモグラフィー像から、上記試料の偏光特性を表示することができるミュラー行列の各成分を求めることにより、試料内部の偏光情報の測定を行うことを特徴とする偏光感受型光コヒーレンストモグラフィーによる試料内部の偏光情報の測定方法を提供する。
【０００９】
本発明は上記課題を解決するために、入射光の偏光状態を、水平直線偏光、垂直直線偏光、４５°直線偏光及び右周り円偏光の４通りのいずれかに選択的に調整し、上記４通りの偏光状態のいずれかに選択的に調整された入射光を、ビームスプリッターによって参照光と計測対象とする試料に対する入射光とに分け、上記参照光の偏光状態を水平直線偏光、垂直直線偏光、４５°直線偏光及び右周り円偏光の４通りのいずれかに選択的に調整して上記ビームスプリッターに戻して透過させ、上記４通りの偏光状態のいずれかに選択的に調整された上記試料からの物体光と、上記４通りの偏光状態にいずれかに選択的に調整された参照光とを、回折格子及びレンズから成る分光器で干渉させ、上記干渉で生じたスペクトル干渉縞をＣＣＤカメラで撮影して物体光のうち参照光と同じ偏光成分に基づく信号を取り出すことを特徴とする偏光感受型光コヒーレンストモグラフィーによる試料内部の偏光情報を計測する方法であって、上記それぞれの偏光状態を組み合わせて４通り×４通りの１６通りの組み合わせた状態で、参照光と物体光の干渉縞を撮影して１６枚のコヒーレンストモグラフィー像を取得し、これらの１６枚のコヒーレンストモグラフィー像から、上記試料の偏光特性を表示することができる　ミュラー行列の各成分を求めることにより、上記偏光情報の測定を行うことを特徴とする偏光感受型光コヒーレンストモグラフィーによる試料内部の偏光情報の測定方法を提供する。
【００１０】
本発明は上記課題を解決するために、入射光の偏光状態を、第１の１／２波長板及び第１の１／４波長板によって、水平直線偏光、垂直直線偏光、４５°直線偏光及び右周り円偏光の４通りのいずれかに選択的に調整し、上記４通りの偏光状態のいずれかに選択的に調整された入射光を、ビームスプリッターによって、参照光と計測対象とする試料に対する入射光とに分け、上記参照光の偏光状態を、第２の１／４波長板、第３の１／４波長板及びミラーで、水平直線偏光、垂直直線偏光、４５°直線偏光及び右周り円偏光の４通りのいずれかに選択的に調整して上記ビームスプリッターに戻して透過させ、上記試料で反射され、さらに上記ビームスプリッターで反射された物体光と上記参照光が重ね合わせられた光を、第４の１／４波長板及び第２の１／２波長板で参照光が水平直線偏光になるように制御し、これらの物体光と参照光を回折格子及びレンズから成る分光器で干渉させ、上記分光器で形成されたスペクトル干渉縞をＣＣＤカメラで撮影し、物体光のうち参照光と同じ偏光成分に基づく信号を取り出し、該ＣＣＤカメラからの上記信号を、画像処理装置に入力して、フーリエ変換された参照光と物体光との相関信号を得ることを特徴とする偏光感受型光コヒーレンストモグラフィーによる試料内部の偏光情報の測定方法であって、上記それぞれの偏光状態を組み合わせて４通り×４通りの１６通りの組み合わせた状態で、参照光と物体光の干渉縞を撮影して１６枚のコヒーレンストモグラフィー像を取得し、これらの１６枚のコヒーレンストモグラフィー像から、上記試料の偏光特性を表示することができる　ミュラー行列の各成分を求めることにより、上記偏光情報の測定を行うことを特徴とする偏光感受型光コヒーレンストモグラフィーによる試料内部の偏光情報の測定方法を提供する。
【００１１】
【発明の実施の形態】
本発明に係る偏光感受型光コヒーレンストモグラフィー装置の実施の形態を実施例に基づいて図面を参照して説明する。
【００１２】
本発明に係る偏光感受型光コヒーレンストモグラフィー装置の基本的な原理は、参照光と物体光の偏光状態をそれぞれ水平直線偏光（Ｈ）、垂直直線偏光（Ｖ）、４５°直線偏光（Ｐ）、右回り円偏光（Ｒ）の４通りに制御し、それぞれの偏光状態を組み合わせた状態（４通り×４通りの１６通りの組み合わせ）で通常のＯＣＴ計測を行う。
【００１３】
これにより披検物体（試料）の偏光特性が反映された１６枚のＯＣＴ像が取得できる。これらの１６枚のＯＣＴ像から、披検物体の偏光特性を表示することができる　ミュラー行列の各成分を求めることにより、偏光情報の測定を行うものである。
【００１４】
（実施例１）
以下、図１に示す偏光感受型スペクトル干渉トモグラフィー装置を利用した本発明に係る実施例１を説明する。
【００１５】
図１は、本発明に係る偏光感受型スペクトル干渉トモグラフィー装置１の光学系の全体構成を説明する図である。図１において、この光学系は、その概要としては、マイケルソン干渉計と分光器の２つの光学系から構成されている。
【００１６】
本発明に係る偏光感受型光コヒーレンストモグラフィー装置１は、光源２にパルスレーザ、或いはパルスレーザーと同様に広帯域スペクトルを有するスーパールミネッセントダイオード（ＳＬＤ：Ｓｕｐｅｒ　Ｌｕｍｉｎｅｓｓｅｎｔ　Ｄｉｏｄｅ）を使用する。このように、本発明では、光パルスのもつ高いピーク強度を利用していないため、光源に必ずしもパルスレーザを用いる必要は無く、ＳＬＤを使用することが可能である。
【００１７】
但し、パルスレーザーは、ＳＬＤに比べ安定した空間モード（横モード）を持ち、中心波長が可視域に近く、系の作製や調整が容易であることに加え、出力強度が高く最適化なしでも低反射・高散乱物体の計測が可能であるから、実施例では、パルスレーザーを用いる。
【００１８】
まず、光源から出たパルスレーザー光の光路上に、光のパワーを減少させる光ウェッジ３を設け、さらにミラー４、５、６を介して、マイケルソン干渉計が設けられている。マイケルソン干渉計は、入射光路上に設けられた偏光子７、１／２波長板８（第１の１／２波長板）、１／４波長板９（第１の１／４波長板）及びビームスプリッター１０と、参照光光学系を構成する１／４波長板１１（第２の１／４波長板）、１／４波長板１２（第３の１／４波長板）及びミラー１３と、入射光を集光するレンズ１４とから構成される。
【００１９】
ビームスプリッター１０は、１／４波長板９から出た入射光を、参照光と試料台１６に支持された試料に向かう入射光に分ける働きをする。試料台１６に支持された試料に向かう入射光は、レンズ１４により試料の一点に集光して反射され、物体光としてビームスプリッター１０に向かい、そこで反射され参照光光学系からの参照光と重ね合わせられ、マイケルソン干渉計から出射する。
【００２０】
さらに、マイケルソン干渉計から出射された光の光路上にミラー１７を介して１／４波長板１８（第４の１／４波長板）、１／２波長板１９（第２の１／２波長板）を設け、さらに１／２波長板１９から出射された光の光路上に回折格子２０を設け、回折格子２０で反射され回折された光の光路上にレンズ２１とＣＣＤカメラ２２を設ける。この回折格子２０及びレンズ２１で分光器を構成する。
【００２１】
（実施例１の作用）
光源２から出た光（光パルスを利用する。）は、光ウェッジ３によりパワーを減少された後、偏光子７により水平直線偏光（Ｈ）となる。そして、入射光の偏光状態を、１／２波長板８と１／４波長板９により、水平直線偏光（以下「Ｈ」という）、垂直直線偏光（以下「Ｖ」という）、４５°直線偏光（以下「Ｐ」という）及び右周り円偏光（以下「Ｒ」という）の４通りのいずれかに選択的に調整し、参照光と試料に入射する光とに分ける。
【００２２】
ビームスプリッター１０で分けられた参照光は、参照光光学系の２枚の１／４波長板１１、１２により、偏光状態がＨ、Ｖ、Ｐ、Ｒとなるように調整されビームスプリッター１０に入射される。一方、試料に入射する光はレンズ１４により試料１５上の１点に集光され、反射され物体光としてビームスプリッター１０に向かう。ビームスプリッター１０は、上記入射してくる参照光を透過させ物体光を４５度反射させて、両者を重ね合わせる。
【００２３】
このようにして重ね合わせられてビームスプリッター１０から出てくる偏光状態がＨ、Ｖ、Ｐ、Ｒとなるように調整された参照光と、試料から反射してきた物体光は、ミラー１７で反射されてから１／４波長板１８及び１／２波長板１９を通して偏光状態をＨにされ、回折格子２０及びレンズ２１からなる分光器に入射する。なお、回折格子の高い回折効率を使うために回折格子に入射する偏光状態をＨに制御している。
【００２４】
このように特定偏光の参照光（Ｈ、Ｖ、Ｐ、Ｒのいずれかの偏光状態の参照光）と物体光を干渉させることにより、物体光の特定偏光成分だけがＣＣＤカメラ上にスペクトル干渉縞を作り、その結果、物体光のうち参照光と同じ偏光状態をもつ成分だけを信号として取り出すことができる。そして、このスペクトル干渉縞をコンピュータに取り込み、画像のｙ軸のある１点から横１行を抜き取って離散フーリエ変換（ＤＦＴ：Ｄｉｓｃｒｅｔｅ　ｆｏｕｒｉｅｒ　ｔｒａｎｓｆｏｒｍ）により空間的なフーリエ変換を計算する。
【００２５】
これにより、参照光と物体光との一次元相関信号が得られる。さらにこれらの信号強度を組み合わせてミュラー行列（Ｍｕｅｌｌｅｒ　ｍａｔｒｉｘ）を求めることにより、試料内部の偏光情報を捉えることができる。
【００２６】
さらに実施例１を詳細に説明する。図１に示す偏光感受型スペクトル干渉トモグラフィー装置の光学系において、光の進行方向にｚ軸、ｚ軸と垂直かつ紙面上になるようにｘ軸、紙面垂直方向にｙ軸を設定している。光源として中心波長（λｃ）７７５ｎｍ、パルスの繰り返し周波数１ＫＨｚ、パルス幅１５０ｆｓ（ＦＷＨＭ）のＴｉ：　Ｓａｐｐｈｉｒｅ再生増幅パルス（Ｃｌｅｒｋ社製　ＣＰＡ２００１）を用いている。
【００２７】
まず、光源から出た光パルスは、光ウェッジ（Ｗｅｄｇｅ　光くさび）　によりパワーを減少させた後、偏光子（Ｐｏｌ．）７により水平直線偏光（Ｈ）となる。ここで、系に入射する光パルスの電場を次の数式１のように定義する。
【００２８】
【数１】

【００２９】
次に、入射光の偏光状態を入射光学系を構成する１／２波長板８と１／４波長板９によって　Ｈ、Ｖ、Ｐ、Ｒ　の４通りのいずれかに選択的に調整し、ビームスプリッター１０で試料に入射する光と参照光とに分離する。そして、参照光は、参照光光学系を構成する２枚の１／４波長板１１、１２及びミラー１３により、参照アーム透過後の偏光状態がＨ、Ｖ、Ｐ、Ｒのいずれかに選択的に調整されるるようにしてビームスプリッター１０に戻され、これを透過する。一方、試料１５に入射する光は、レンズ１５により試料１５上の一点に集光されて反射される。この時の参照光の電場Ｅ_Ａ，ｒ（ｘ、ｔ）と試料による反射された後の物体光の電場Ｅ_Ａ，ｓ（ｘ、ｔ）はそれぞれ次の数式２で表される。
【００３０】
【数２】

【００３１】
ここで、Δｌ（本明細書記載の「Δｌ」の「ｌ」はＬ（エル）の小文字を表しているものとする。）は参照アームと物体アームの光路差、ｃは光速、添字のＡは偏光状態、ｒとｓはそれぞれ参照光及び物体光を表している。
【００３２】
次に、試料１５から反射された物体光と、上記偏光状態が選択的に調整された参照光をビームスプリッターにより再度重ね合わせる。そして、マイケルソン干渉計から出力された光（ビームスプリッター１０から出力された光）をミラー１７で反射し、これを偏光状態を１／４波長板１８及び１／２波長板１９を通してＨにし、回折格子２０とレンズ２１からなる分光器に入射させる。
【００３３】
ここで、回折格子１０に入射する偏光状態をＨにするのは、Ｈの場合に回折格子の回折効率が一番高くなるためである。そして分光器による分光、すなわち時間的なフーリエ変換を行うと、参照光と物体光のスペクトルがＣＣＤカメラ２２上に空間的に展開される。ＣＣＤカメラ２２上でのそれぞれの電場は次の数式３で表される。
【００３４】
【数３】

【００３５】
また、β＝λｃ／（ｃｄｃｏｓθｄ）は、中心波長λｃ及び回折格子１０の格子間隔ｄ、光速ｃ、回折折角度θｄにより決まる定数、α＝ｃｏｓθｉ／ｃｏｓθｄは、回折格子への入射角度θｉ及び回折角度θｄにより決まる定数、ｆはシリンドリカルレンズｘ−ＣＬの焦点距離であり、＊はコンボリューションを表している。また、次の数式４で示すものの幅は、数式５で示すものの幅に較べて十分小さいため、デルタ関数として無視することにより、上記数式３のように近似して表現することができる。
【００３６】
【数４】

【００３７】
【数５】

【００３８】
次に、ＣＣＤカメラ２２上（フーリエ変換面）のｘ軸方向に空間的に展開された参照光と物体光のスペクトルはＣＣＤカメラ２２上で重なり合い、スペクトル干渉縞を形成する。ここでは特定の偏光状態の参照光と物体光とが重なり合っているため、物体光の特定の偏光成分だけが干渉していることになる。このスペクトル干渉縞をＣＣＤカメラ２２で撮影して、スペクトル強度分布に変換する。ＣＣＤカメラで得られる強度分布は数式６に示すように、数式３におけるそれぞれの式の電場の和の強度で表される。
【００３９】
【数６】

【００４０】
数式６において、第１項と第２項はそれぞれ参照光と物体光のスペクトル強度を表し、第３項と第４項は物体光のスペクトルに窓関数として参照光のスペクトルが掛けられたものが、参照アームと物体アームの光路差Δｌに比例する周波数をもった正弦関数で変調されていることを表している。
【００４１】
最後に、ＣＣＤカメラ２２により撮影し、画像ボード（サイバーテック社製ＣＴ３０００Ａ）を介してコンピュータに取り込んだ干渉縞の強度分布（数式６参照）の空間的なフーリエ変換を離散フーリエ変換（ＤＦＴ）により計算すると、次の数式７で示すように、参照光と物体光の強度相関信号が得られる。
【００４２】
但し、コンピュータで計算する際の標本点数をＮ、標本間隔をＴとし、χ（＝０，１，・・・，　Ｎ−１）　番目の空間周波数をν＝χ／（ＮＴ）で表している。本装置では、数式６で示すスペクトル強度分布をＣＣＤカメラ２２により計測しているため、ＮはＣＣＤカメラ２２のｘ軸方向の画素数、ＴはＣＣＤカメラ２２のｘ軸方向の画素の大きさにより決定される。
【００４３】
【数７】

【００４４】
ここで、第１項と第２項はそれぞれ参照光と物体光の自己相関信号でありν＝０を中心として表れる。第３項と第４項はそれぞれ参照光と物体光の相互相関信号であり、自己相関信号からν＝±Δｌ／（λｃβｆｃ）離れた位置に表れる。この相互相関信号をみると、参照アームと物体アームの光路差Δｌ、すなわち深さ情報が含まれている。これにより、マイケルソン干渉計のアームの片方に置かれた試料の深さ情報を得ることができる。
【００４５】
ところで、通常、偏光状態を表示するには、ＪｏｎｅｓベクトルやＳｔｏｋｅｓベクトルが用いられる。Ｊｏｎｅｓベクトルは光がいくつかの光学素子を通る各段において、その偏光状態を表すことができる。だが、完全な偏光しか記述することができず、部分偏光あるいは自然光のような非偏光な光の状態を扱うことができない。一方、ＳｔｏｋｅｓベクトルはＪｏｎｅｓベクトルに比べ、各段における偏光状態の見通しがつきにくくなるものの、自然光のような部分偏光を含む幅広い偏光特性を表すことが可能である。
【００４６】
一般に、光ビームのＳｔｏｋｅｓベクトルＳは検出器に入射する６つの偏光状態Ｈ（水平直線偏光）、Ｖ（垂直直線偏光）、Ｐ（４５°直線偏光）、Ｍ（−４５°直線偏光）、Ｒ（右回り円偏光）、Ｌ（左回り円偏光。以下「Ｌ」という）での光強度により定義することができる。このとき、それぞれの偏光状態での光強度の関係は、Ｉ_Ｈ＋Ｉ_Ｖ＝Ｉ_Ｐ＋Ｉ_Ｍ＝Ｉ_Ｒ＋Ｉ_Ｌとなるため、実際には４つの独立した状態で光強度を測定することによりＳｔｏｋｅｓベクトルを決定することができる。今回用いる偏光状態　Ｈ、Ｖ、Ｐ、Ｒを用いるとＳｔｏｋｅｓベクトルは次の数式８のように定義される。
【００４７】
【数８】

【００４８】
例として　Ｈ、Ｖ、Ｐ、Ｍ、Ｒ、Ｌ　及び自然光すなわち無偏光のＳｔｏｋｅｓベクトルを次の数式９に示す。
【００４９】
【数９】

【００５０】
さらに、光学素子や測定物体への入力ＳｔｏｋｅｓベクトルをＳｉｎ、出力ＳｔｏｋｅｓベクトルをＳｏｕｔとすると、それらの光学素子や測定物体の偏光特性は、ＳｉｎとＳｏｕｔの線形変換行列をＭとして次の数式１０のように表すことができる。
【００５１】
【数１０】

【００５２】
上記数式１０に用いられる線形変換行列Ｍがミュラー行列（Ｍｕｅｌｌｅｒ　Ｍａｔｒｉｘ）と呼ばれているものである。また、Ｍ_０、Ｍ_１、Ｍ_２、Ｍ_３はそれぞれ４行１列のミュラーベクトル要素を表している。この４×４のミュラー行列は１６個の独立した要素から構成されているため、１６個の独立した光強度の測定により決定することができる。
【００５３】
そこで、上記数式９のＨ、Ｖ、Ｐ、Ｒの４つを入力Ｓｔｏｋｅｓベクトルとする場合を考える。この時、それぞれの出力Ｓｔｏｋｅｓベクトルは数式１０から、次の数式１１と表すことができる。
【００５４】
【数１１】

【００５５】
したがって、これらの出力Ｓｔｏｋｅｓベクトルからミュラー行列を計算すると、数式１２のようになる。
【００５６】
【数１２】

【００５７】
ここで、添字は参照光と物体光の偏光状態の組み合わせを表しており、例えばＩ_Ｈｖならば参照光の偏光状態がＨで物体光の偏光状態がＶの場合の干渉信号強度である。図１に示す装置では、それぞれの強度はスペクトル干渉縞の強度を離散フーリエ変換（ＤＦＴ）によりフーリエ変換した後の信号強度すなわち相関信号強度となる。本発明に係る装置ではまた、試料１５の一点について一つのミュラー行列を決定する。つまり、最終的には計測物体におけるミュラー行列の分布を計測するのである。
【００５８】
その結果は、Ｍ_００〜Ｍ_３３までの合計１６枚のＯＣＴ画像を得ることになる。それぞれのＯＣＴ画像は、例えばＭ_００であれば通常のＯＣＴ装置で得ることができる偏光情報を含まない分布を表し、Ｍ_２３であれば４５°直線偏光の光を円偏光に変換するような分布を表している。つまり、それぞれの成分を観察することにより、試料（物体）の偏光特性を捉えることができるのである。以下に代表的な偏光素子のミュラー行列を数式１３〜１６に示す。
【００５９】
ここに、ｘ軸方向の振幅透過率ｐ_ｘ、ｙ軸方向の振幅透過率がｐ_ｙである部分偏光子のミュラー行列は数式１３で示し、ｘ軸方向に透過軸をもつ完全偏光子のミュラー行列は数式１４で示し、ｚ軸方向の位相がδだけ進む移相子のミュラー行列は数式１５で示し、偏光方位をθ回転させる旋光子のミュラー行列は数式１６で示す。
【００６０】
【数１３】

【００６１】
【数１４】

【００６２】
【数１５】

【００６３】
【数１６】

【００６４】
（実験例１）
本発明者等は、図１に示す装置により奥行き方向の情報がどの程度の分解能で得られるかを確認するために、マイケルソン干渉計の試料台１６に平面鏡をおいて光路差を測定した。偏光状態は試料、ここでは平面鏡に入射した光と参照光、両方とも水平直線偏光（ＨＨ）の状態で行った。測定した結果を図２に示す。
【００６５】
図２（ａ）において左側の像はＣＣＤカメラ２２で撮影されたスペクトル干渉縞の像である。右側の分布は、スペクトル干渉縞の強度分布の横一列を抜き取り、離散フーリエ変換（ＤＦＴ：　Ｄｉｓｃｒｅａｔ　ｆｏｕｒｉｅｒ　ｔｒａｎｓｆｏｒｍ）によってスペクトル強度のフーリエ変換を計算して得られた参照光と物体光の相関強度分布である。また、図２（ｂ）は、（ａ）の場合よりも６００μｍ物体光側の光路長が長い場合の結果である。
【００６６】
図２（ａ）と図２（ｂ）それぞれの相関強度分布は、干渉縞画像のｙ＝１０７〜４０６の部分の相関強度分布の平均値をとったものであり、参照光に起因するノイズを除去するためにＤＦＴを行う前に干渉縞画像を参照光のみの画像で割っている。これら２つの相関強度分布において、ｚ＝０付近の分布は参照光と物体光それぞれの自己相関分布（０次光分布）を、その右側及び左側の分布が参照光と物体光の相互相関分布（１次光成分及び−１次光成分）をそれぞれ表している。
【００６７】
これらの結果から、相関分布における１次または−１次のピーク位置のずれが、干渉計の光路差に比例していることが分かる。ピーク位置のずれと光路差の比例関係から強度相関分布の横軸（ｚ軸に対応）の係数を計算すると、１６μｍ／ｐｉｘｅｌとなる。さらに、相互相関分布の半値全幅（ＦＷＨＭ：Ｆｕｌｌ　ｗｉｄｔｈ　ａｎｄ　ｈａｌｆｍａｘｉｍｕｍ）から装置の深さ方向（ｚ軸）の分解能を求めた結果、本装置では３２μｍの分解能をもつことが分かった。
【００６８】
この装置１は、奥行き方向の走査を行うことなく１回の測定で深さ情報を得ることができるため、ｘ軸方向の一次元走査のみで測定物体の断面構造を計測することができる。そのため、ｘ軸方向の分解能は、走査間隔により決まると言える。
【００６９】
（実験例２）
魚の骨の計測：
生物試料として、図３（ａ）に示す魚（鮭）の中骨の断面構造の計測を行った。試料はスライドガラス上に両面テープにより張り付けて固定し、図３（ａ）中のＡ−Ｂの部分を５μｍ間隔で５０点走査した。（ｂ）の写真は、図３（ａ）の断面部分を、微分干渉顕微鏡（ＤＩＣ：Ｄｉｆｆｅｒｅｎｔｉａｌ　ｉｎｔｅｒｆｅｒｅｎｃｅ　ｃｏｎｔｒａｓｔ　ｍｉｃｒｏｓｃｏｐｅ）により計測し
たものである。
【００７０】
Ｈ、Ｖ、Ｐ、Ｒ　の４つの偏光状態の組み合わせにより得られる１６枚の生のＯＣＴ像を図４に、そのＯＣＴ計測データから数式１２によりミュラー行列を計算しその要素ごとに表示したミュラー行列像を図５に示す。ＨＨやＨＶなどは試料に入射する光と参照光の偏光状態を、Ｍの添字はミュラー行列の各成分を表しており、それぞれの画像は各画像の最大値で規格化している。また、これらの画像の大きさは、横２５０μｍ×縦８００μｍである。
【００７１】
図４のＯＣＴ像において、ＨＰやＰＰ、ＲＰなどに微細な構造が観測されていることがわかる。しかし、ＯＣＴ像では−４５°直線偏光や左回り円偏光に変換する性質をもつ部分の分布を確認することはできない。一方、図５のミュラー行列の各成分を表示した像を見ると、Ｍ_００は通常のＯＣＴ像と同様の偏光特性情報を含まない像であるため、全体の大まかな構造を確認することができるが、複屈折性を示す細かな構造は分からない。しかし、ＯＣＴ像でははっきりしていなかった骨のコラーゲン小繊維が要因と思われる複屈折性をもつ細かな構造がＭ_００以外の像にはっきりと表れているのが確認できる。
【００７２】
例えば、Ｍ_２１では、水平直線偏光を４５°直線偏光に変換する性質をもつ部分（図５では明示されないが赤色を呈する部分。以下、色については図５では明示されない。）と−４５°直線偏光に変換する性質（青色）をもつ部分の構造が表れており、Ｍ_２３では右回り円偏光を４５°直線偏光に変換する性質（赤色）をもつ部分と−４５°直線偏光に変換する性質（青色）をもつ部分の構造が、さらにＭ_２２では４５°直線偏光を保存する性質（赤色）をもつ部分と４５°直線偏光を−４５°直線偏光に変換する性質（青色）をもつ部分の分布、Ｍ_３３では右回り円偏光成分を保存する性質（赤色）をもつ部分と右回り円偏光を左回り円偏光に変換する性質（青色）をもつ部分の微細な構造が表れている。
【００７３】
（実験例３）
人間の皮膚の計測：
本発明の装置の生体への応用の可能性を調べるために、人間の皮膚切片の断面構造の計測を行った。計測した皮膚切片（大きさ２．０ｃｍ×２．５ｃｍ）は、図６（ａ）のようにスライドガラス上にテープで固定し、５μｍ間隔で５０点走査を行った。図６は、微分干渉顕微鏡により測定された人間の皮膚の断面像である。
【００７４】
偏光状態を制御して得られたＯＣＴ像を図７に示し、そのＯＣＴ像からミュラー行列の各成分の像を求めて表示した結果を図８に示す。画像の大きさは横　２５０μｍ×縦１０００μｍであり、各画像の最大値で規格化している。
【００７５】
偏光情報を含まないＭ_００の像では、皮膚切片の表層付近の大まかな構造は分かるものの、微細構造がはっきりとは確認できない。その他の像について見てみると、角質層の構造によるものと思われる複雑な微細構造を確認することができる。
【００７６】
例えば、Ｍ_１１では垂直直線偏光を保存する成分（図８では明示されないが赤色を呈する部分。以下、色については図８では明示されない）の構造が、Ｍ_３０では水平直線偏光を右回り円偏光に変換する性質をもつ部分（赤色）の微細な構造がはっきりと表れている。また、Ｍ_３２では、４５°直線偏光を左回り円偏光に変換する成分（青色）をもつ部分の分布、Ｍ_３３では右回り円偏光を左回り円偏光に変換する成分（青色）をもつ部分の分布といった微細な構造がはっきりと表れていることが分かる。
【００７７】
実際の皮膚の断面構造は、図６（ａ）を見るとわかるように、繊維状の構造が複雑に重なりあっている。今回の計測結果においては、皮膚の表層部分つまり角質層部分の複雑な繊維状の構造が要因とみられる偏光状態の変化を捉えることができていると考えられる。
【００７８】
以上の結果から、本装置により生体がもつ複屈折性を含んだ微細な構造を計測することが十分可能である。
【００７９】
以上、本発明に係る偏光感受型光コヒーレンストモグラフィー装置の実施形態を実施例に基づいて説明したが、本発明は特にこのような実施例に限定されることなく、特許請求の範囲記載の技術的事項の範囲内でいろいろな実施例があることはいうまでもない。
【００８０】
【発明の効果】
本発明は以上の通り、深さ方向の機械走査を必要としないスペクトル干渉型のＯＣＴに偏光感受性を導入することで、偏光感受型スペクトル干渉ＯＣＴ装置を作製したものであり、次のような顕著な効果を奏する。
【００８１】
（１）１／４波長板の偏光特性を計測した結果、水平直線偏光を垂直直線偏光に変換、垂直直線偏光を水平直線偏光に変換、４５°直線偏光はそのまま保持、右回り円偏光を左回り円偏光に変換するミュラー行列の各成分を計測できる。
【００８２】
（２）複屈折分布を含む魚の骨や人間の皮膚といった生物試料の断面構造の計測について、従来のＯＣＴ計測では得ることができなかった複屈折分布を含む微細な構造を確認できる。そして、本装置では、深さ方向の空間分解能は３２μｍであり、空気中では２ｍｍのダイナミックレンジを確認し、きわめて高い分解能で計測が可能である。
【図面の簡単な説明】
【図１】本発明に係る偏光感受型光コヒーレンストモグラフィー装置の光学系を説明する図である。
【図２】実験例１の測定結果を示す図である。
【図３】実験例２で行った魚の断面構造の計測の生物試料として使用する魚（鮭）を示す図である。
【図４】実験例２のそれぞれの偏光状態の組み合わせにより得られる１６枚の生のＯＣＴ像を示す。
【図５】図４のＯＣＴ計測データからミュラー行列を計算しその要素ごとに表示したミュラー行列像を示す。
【図６】実験例３で行った試料である人間の皮膚切片をスライドガラス上にテープで固定した状態を示す図である。
【図７】実験例３で偏光状態を制御して得られたＯＣＴ像を示す。
【図８】図７のＯＣＴ像からミュラー行列の各成分の像を求めて表示した結果を示す。
【符号の説明】
１　　偏光感受型光コヒーレンストモグラフィー装置
２　　光源
３　　光ウェッジ
４、５、６、１３、１７　　ミラー
７　　偏光子
８、１９　　１／２波長板
９、１１、１２、１８　　１／４波長板
１０　　ビームスプリッター
１４　　入射光を集光するレンズ
１５　　試料
１６　　試料台
２０　　回折格子
２１　　レンズ
２２　　ＣＣＤカメラ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a polarization-sensitive optical coherence tomography apparatus for capturing polarization information of an object using optical coherence tomography (OCT) and measuring a finer structure, and polarization information inside a sample by the apparatus. The measurement method.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, OCT is used to capture internal information of an object, that is, a differential structure of a refractive index distribution in a non-destructive and high-resolution manner.
[0003]
However, in the conventional OCT, although the differential structure of the refractive index distribution of the object can be captured with high resolution and non-destructive, it cannot capture the polarization dependency inherent in the object itself. Especially when considering the application of OCT to biological measurement, in the measurement of a biological sample having polarization dependence due to birefringence caused by a fibrous structure, there are problems such as a decrease in resolution and the inability to capture the structure. Would.
[0004]
[Problems to be solved by the invention]
Generally, the scattered light component has almost no polarization characteristics, so that it is difficult to capture the polarization information. The present inventors, when a low-coherence interferometer such as OCT causes a scattered light component from a specific portion to interfere with a reference light having a certain polarization state, the interference component strongly reflects the polarization characteristics, As a result, they have come to think that it is possible to capture polarization information of a specific portion of a cross section in the depth direction.
[0005]
An object of the present invention is to measure a fine structure by capturing polarization information of a sample (object) and measure a minute structure by capturing polarization information of an object. Polarization sensitivity was introduced to a spectral interference type tomography device that can measure the cross-sectional structure.
[0006]
[Means for Solving the Problems]
In order to solve the above problems, the present invention provides an incident optical system that is provided on an incident optical path of incident light and that can selectively adjust the polarization state of the incident light to any one of four different polarization states. A beam splitter provided on the incident optical path and dividing incident light selectively adjusted to one of the four polarization states into reference light and incident light with respect to a sample to be measured; A reference light optical system for selectively adjusting the polarization state to one of the four polarization states and returning the beam to the beam splitter for transmission; and the reference light adjusted to the four polarization states. And a spectroscope comprising a diffraction grating and a lens for interfering the reference light with the object in a state where the object light reflected from the sample and the object light reflected from the sample are combined with each other in 16 combinations of 4 ways × 4 ways; And a CCD camera for acquiring 16 coherence tomographic images by photographing the spectral interference fringes generated in step (a), and extracting a signal based on the same polarization component as the reference light among the object light. A coherence tomography device is provided.
[0007]
In order to solve the above problem, the present invention is provided on an incident optical path of incident light, and changes the polarization state of the incident light into four types of horizontal linearly polarized light, vertical linearly polarized light, 45 ° linearly polarized light, and clockwise circularly polarized light. A first half-wave plate and a first quarter-wave plate that are selectively adjusted to any one of them, and are provided on the incident optical path and selectively adjusted to any one of the four polarization states. And a beam splitter that divides the incident light into a reference light and an incident light with respect to the sample to be measured. The polarization state of the reference light is divided into four types of horizontal linear polarization, vertical linear polarization, 45 ° linear polarization, and right-handed circular polarization. A second quarter-wave plate, a third quarter-wave plate, and a mirror for selectively adjusting any one of them and returning the beam to the beam splitter and transmitting the light to the beam splitter; Each is adjusted and reflected by the sample, and A fourth quarter-wave plate and a second quarter-wave plate for controlling the light in which the object light reflected by the beam splitter and the reference light adjusted to any one of the four polarization states are superimposed to be horizontally linearly polarized; １／ wavelength plate, the reference light adjusted to each of the four polarization states and the object light reflected from the sample are combined with each other in a state of 16 combinations of 4 × 4 combinations. A spectroscope comprising a diffraction grating and a lens for causing the reference light to interfere with the object, and a CCD camera for photographing spectral interference fringes generated by the interference to obtain 16 coherence tomographic images, Provided is a polarization-sensitive optical coherence tomography apparatus, wherein a signal based on the same polarization component as the reference light is extracted.
[0008]
In order to solve the above-mentioned problem, the present invention selectively adjusts the polarization state of the reference light and the object light to one of four different polarization states, and combines the respective polarization states into four × 4 different polarization states. In the 16 combinations, the interference fringes of the reference light and the object light are photographed to obtain 16 coherence tomography images, and from these 16 coherence tomography images, the polarization characteristics of the sample can be displayed. A method for measuring polarization information inside a sample by polarization-sensitive optical coherence tomography, wherein polarization information inside the sample is measured by obtaining each component of a possible Mueller matrix.
[0009]
In order to solve the above-mentioned problems, the present invention selectively adjusts the polarization state of incident light to one of four types of horizontal linear polarization, vertical linear polarization, 45 ° linear polarization, and right-handed circular polarization. The incident light selectively adjusted to one of the different polarization states is divided into a reference light and an incident light with respect to a sample to be measured by a beam splitter, and the polarization state of the reference light is determined by horizontal linear polarization and vertical linear polarization. , 45 ° linearly polarized light and clockwise circularly polarized light, selectively adjusted to any one of the four types, returned to the beam splitter and transmitted, and selectively adjusted to any of the four types of polarization states. And the reference light selectively adjusted to any one of the four polarization states described above are caused to interfere by a spectroscope including a diffraction grating and a lens, and the spectral interference fringes generated by the interference are detected by a CCD camera. so A method for measuring polarization information inside a sample by polarization-sensitive optical coherence tomography, characterized by taking out a signal based on the same polarization component as the reference light among the object light, and combining the respective polarization states. In the state of 16 combinations of 4 × 4 combinations, the interference fringes of the reference light and the object light are photographed to obtain 16 coherence tomography images, and from these 16 coherence tomography images, Provided is a method for measuring polarization information inside a sample by polarization-sensitive optical coherence tomography, wherein the polarization information is measured by obtaining each component of a Muller matrix capable of displaying polarization characteristics.
[0010]
In order to solve the above problem, the present invention changes the polarization state of incident light by using a first half-wave plate and a first quarter-wave plate, to obtain horizontal linearly polarized light, vertical linearly polarized light, 45 ° linearly polarized light, The incident light selectively adjusted to any one of the four types of right-handed circularly polarized light and selectively adjusted to any one of the above-mentioned four polarization states is supplied to the reference light and the sample to be measured by the beam splitter. The polarization state of the above-mentioned reference light is divided into a horizontal linearly polarized light, a vertical linearly polarized light, a 45 ° linearly polarized light and a clockwise direction by a second quarter-wave plate, a third quarter-wave plate and a mirror. Light that is selectively adjusted to one of four types of circularly polarized light, returned to the beam splitter and transmitted, reflected by the sample, and further superimposed on the object light and the reference light reflected by the beam splitter. To the fourth quarter-wave plate And the second half-wave plate controls the reference light so that it becomes horizontal linearly polarized light. The object light and the reference light interfere with each other in a spectroscope including a diffraction grating and a lens. The spectral interference fringes are photographed by a CCD camera, a signal based on the same polarization component as the reference light is taken out of the object light, and the signal from the CCD camera is input to an image processing device, and the Fourier-transformed reference light and A method of measuring polarization information inside a sample by polarization-sensitive optical coherence tomography, characterized by obtaining a correlation signal with object light, wherein 16 combinations of 4 × 4 combinations of the above polarization states are provided. In this state, the interference fringes of the reference light and the object light are photographed to obtain 16 coherence tomography images, and from these 16 coherence tomography images, The present invention provides a method of measuring polarization information inside a sample by polarization-sensitive optical coherence tomography, wherein the polarization information is measured by determining each component of a Muller matrix capable of displaying the polarization characteristics of the sample. .
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of a polarization-sensitive optical coherence tomography apparatus according to the present invention will be described based on examples with reference to the drawings.
[0012]
The basic principle of the polarization-sensitive optical coherence tomography apparatus according to the present invention is that the polarization states of the reference light and the object light are respectively set to horizontal linear polarization (H), vertical linear polarization (V), 45 ° linear polarization (P), Normal OCT measurement is performed in a state where each of the four polarization states (R) is controlled in the clockwise direction and the respective polarization states are combined (16 combinations of 4 × 4).
[0013]
Thus, 16 OCT images reflecting the polarization characteristics of the test object (sample) can be obtained. The polarization information is measured by obtaining, from these 16 OCT images, each component of a Muller matrix capable of displaying the polarization characteristics of the object to be examined.
[0014]
(Example 1)
Hereinafter, Embodiment 1 according to the present invention using the polarization-sensitive spectral interference tomography apparatus shown in FIG. 1 will be described.
[0015]
FIG. 1 is a diagram illustrating the overall configuration of an optical system of a polarization-sensitive spectral interference tomography apparatus 1 according to the present invention. In FIG. 1, this optical system is generally composed of two optical systems, a Michelson interferometer and a spectroscope.
[0016]
The polarization-sensitive optical coherence tomography apparatus 1 according to the present invention uses a pulse laser as a light source 2 or a super luminescent diode (SLD: Super Luminescent Diode) having a broadband spectrum like the pulse laser. As described above, since the present invention does not use the high peak intensity of the light pulse, it is not necessary to use a pulse laser as a light source, and an SLD can be used.
[0017]
However, the pulse laser has a spatial mode (transverse mode) that is more stable than the SLD, the center wavelength is close to the visible region, the system can be easily manufactured and adjusted, and the output intensity is high and low even without optimization. In the embodiment, a pulsed laser is used because measurement of a reflected / highly scattered object is possible.
[0018]
First, an optical wedge 3 for reducing the power of light is provided on the optical path of the pulse laser light emitted from the light source, and a Michelson interferometer is provided via

mirrors

4, 5, and 6. The Michelson interferometer includes a polarizer 7, a half-wave plate 8 (first half-wave plate), and a quarter-wave plate 9 (first quarter-wave plate) provided on an incident optical path. And a beam splitter 10, a quarter-wave plate 11 (second quarter-wave plate), a quarter-wave plate 12 (third quarter-wave plate), and a mirror 13, which constitute a reference light optical system. , And a lens 14 for condensing incident light.
[0019]
The beam splitter 10 functions to split incident light emitted from the quarter-wave plate 9 into reference light and incident light directed to a sample supported on the sample stage 16. The incident light directed toward the sample supported by the sample stage 16 is condensed and reflected at one point of the sample by the lens 14, travels toward the beam splitter 10 as object light, is reflected there, and overlaps with the reference light from the reference light optical system. And exit from the Michelson interferometer.
[0020]
Further, a quarter-wave plate 18 (fourth quarter-wave plate) and a half-wave plate 19 (second half-wave plate 19) are placed on the optical path of the light emitted from the Michelson interferometer via a mirror 17. Wavelength plate), a diffraction grating 20 is provided on the optical path of the light emitted from the half-wave plate 19, and a lens 21 and a CCD camera 22 are provided on the optical path of the light reflected and diffracted by the diffraction grating 20. . The diffraction grating 20 and the lens 21 constitute a spectroscope.
[0021]
(Operation of First Embodiment)
The light (using a light pulse) emitted from the light source 2 is reduced in power by the light wedge 3, and then becomes a horizontal linearly polarized light (H) by the polarizer 7. The polarization state of the incident light is changed by the half-wave plate 8 and the quarter-wave plate 9 into horizontal linearly polarized light (hereinafter referred to as “H”), vertical linearly polarized light (hereinafter referred to as “V”), and 45 ° linearly polarized light. (Hereinafter referred to as “P”) and right-handed circularly polarized light (hereinafter referred to as “R”) are selectively adjusted to one of four types, and are divided into reference light and light incident on the sample.
[0022]
The reference light split by the beam splitter 10 is adjusted by two １／

wavelength plates

11 and 12 of the reference light optical system so that the polarization state becomes H, V, P, and R, and is incident on the beam splitter 10. Is done. On the other hand, the light incident on the sample is condensed by the lens 14 at one point on the sample 15 and is reflected and directed to the beam splitter 10 as object light. The beam splitter 10 transmits the incident reference light and reflects the object light by 45 degrees, and superimposes the two.
[0023]
The reference light, which is superposed in this way and adjusted so that the polarization state coming out of the beam splitter 10 becomes H, V, P, and R, and the object light reflected from the sample, are reflected by the mirror 17. After that, the polarization state is changed to H through the 波長 wavelength plate 18 and the 波長 wavelength plate 19, and the light enters the spectroscope including the diffraction grating 20 and the lens 21. The polarization state incident on the diffraction grating is controlled to H in order to use the high diffraction efficiency of the diffraction grating.
[0024]
By causing the reference light having a specific polarization (reference light having any one of the polarization states of H, V, P, and R) and the object light to interfere with each other, only the specific polarization component of the object light is displayed on the CCD camera by spectral interference fringes. As a result, of the object light, only a component having the same polarization state as the reference light can be extracted as a signal. Then, the spectral interference fringes are taken into a computer, one horizontal line is extracted from a certain point on the y-axis of the image, and a spatial Fourier transform is calculated by a Discrete Fourier Transform (DFT: Discrete Fourier Transform).
[0025]
Thus, a one-dimensional correlation signal between the reference light and the object light is obtained. Further, by obtaining a Mueller matrix by combining these signal intensities, polarization information inside the sample can be captured.
[0026]
Example 1 will be further described in detail. In the optical system of the polarization-sensitive spectral interference tomography apparatus shown in FIG. 1, the z-axis is set in the traveling direction of light, the x-axis is perpendicular to the z-axis, and the y-axis is set in the direction perpendicular to the paper. As the light source, a Ti: Sapphire reproduction amplification pulse (CPA 2001 manufactured by Clerk) having a center wavelength (λc) of 775 nm, a pulse repetition frequency of 1 KHz, and a pulse width of 150 fs (FWHM) is used.
[0027]
First, the light pulse emitted from the light source is reduced in power by a light wedge, and then becomes a horizontal linearly polarized light (H) by a polarizer (Pol.) 7. Here, the electric field of the light pulse incident on the system is defined as in the following Expression 1.
[0028]
(Equation 1)

[0029]
Next, the polarization state of the incident light is selectively adjusted to one of four types of H, V, P, and R by a half-wave plate 8 and a quarter-wave plate 9 constituting the incident optical system, and the beam is adjusted. The splitter 10 separates the light into the sample and the reference light. Then, the reference light is selectively polarized into any one of H, V, P, and R after passing through the reference arm by the two quarter-

wave plates

11 and 12 and the mirror 13 constituting the reference light optical system. The beam is returned to the beam splitter 10 in such a manner as to be adjusted, and is transmitted therethrough. On the other hand, light incident on the sample 15 is condensed by the lens 15 at one point on the sample 15 and reflected. The electric field E of the reference light at this time _{A, r} (X, t) and the electric field E of the object light after being reflected by the sample _{A, s} (X, t) is represented by the following Equation 2, respectively.
[0030]
(Equation 2)

[0031]
Here, Δl (“l” of “Δl” described in this specification represents the lowercase letter of L), the optical path difference between the reference arm and the object arm, c is the speed of light, and the subscript A Represents a polarization state, and r and s represent reference light and object light, respectively.
[0032]
Next, the object light reflected from the sample 15 and the reference light whose polarization state has been selectively adjusted are superimposed again by the beam splitter. Then, the light output from the Michelson interferometer (the light output from the beam splitter 10) is reflected by the mirror 17, and the polarization state is changed to H through the １／ wavelength plate 18 and the １／ wavelength plate 19, and The light is incident on a spectroscope including a diffraction grating 20 and a lens 21.
[0033]
Here, the reason why the state of polarization incident on the diffraction grating 10 is H is that the diffraction efficiency of the diffraction grating becomes highest in the case of H. Then, when spectroscopy is performed, that is, when a temporal Fourier transform is performed, the spectra of the reference light and the object light are spatially developed on the CCD camera 22. Each electric field on the CCD camera 22 is represented by the following equation (3).
[0034]
[Equation 3]

[0035]
Β = λc / (cdcos θd) is a constant determined by the center wavelength λc, the grating interval d of the diffraction grating 10, the light speed c, and the diffraction angle θd, and α = cos θi / cos θd is the incidence angle θi to the diffraction grating and the diffraction A constant determined by the angle θd, f is a focal length of the cylindrical lens x-CL, and * represents convolution. Further, the width of the following expression 4 is sufficiently smaller than the width of the expression 5, so that it can be approximated and expressed as the above expression 3 by ignoring it as a delta function.
[0036]
(Equation 4)

[0037]
(Equation 5)

[0038]
Next, the spectra of the reference light and the object light spatially developed on the CCD camera 22 (Fourier transform plane) in the x-axis direction overlap on the CCD camera 22 to form spectral interference fringes. Here, since the reference light and the object light in a specific polarization state overlap with each other, only a specific polarization component of the object light interferes. This spectral interference fringe is photographed by the CCD camera 22 and converted into a spectral intensity distribution. As shown in Equation 6, the intensity distribution obtained by the CCD camera is represented by the sum of the electric fields of each equation in Equation 3.
[0039]
(Equation 6)

[0040]
In Equation 6, the first and second terms represent the spectral intensities of the reference light and the object light, respectively, and the third and fourth terms are obtained by multiplying the spectrum of the object light by the spectrum of the reference light as a window function. , Which is modulated by a sine function having a frequency proportional to the optical path difference Δl between the reference arm and the object arm.
[0041]
Finally, the spatial Fourier transform of the interference fringe intensity distribution (see Equation 6) captured by the CCD camera 22 via an image board (CT3000A manufactured by Cybertec) is calculated by the discrete Fourier transform (DFT). When the calculation is performed, an intensity correlation signal between the reference light and the object light is obtained as shown in the following Expression 7.
[0042]
Here, the number of sample points when calculating with a computer is N, the sample interval is T, and the χ (= 0, 1,..., N−1) th spatial frequency is represented by ν = χ / (NT). . In this apparatus, since the spectral intensity distribution represented by Expression 6 is measured by the CCD camera 22, N is the number of pixels of the CCD camera 22 in the x-axis direction, and T is the size of the pixels of the CCD camera 22 in the x-axis direction. It is determined.
[0043]
(Equation 7)

[0044]
Here, the first and second terms are autocorrelation signals of the reference light and the object light, respectively, and appear around ν = 0. The third and fourth terms are cross-correlation signals of the reference light and the object light, respectively, and appear at positions ν = ± Δl / (λcβfc) apart from the autocorrelation signal. Looking at this cross-correlation signal, it includes the optical path difference Δl between the reference arm and the object arm, that is, depth information. Thereby, depth information of the sample placed on one side of the arm of the Michelson interferometer can be obtained.
[0045]
By the way, usually, a Jones vector or a Stokes vector are used to display the polarization state. The Jones vector can represent its polarization state at each stage where light passes through several optical elements. However, it can describe only completely polarized light, and cannot handle the state of unpolarized light such as partially polarized light or natural light. On the other hand, the Stokes vector is less visible in the polarization state at each stage than the Jones vector, but can exhibit a wide range of polarization characteristics including partial polarization such as natural light.
[0046]
In general, the Stokes vector S of a light beam has six polarization states H (horizontal linear polarization), V (vertical linear polarization), P (45 degree linear polarization), M (-45 degree linear polarization), R (Right-handed circularly polarized light) and L (left-handed circularly polarized light; hereinafter referred to as “L”). At this time, the relationship between the light intensity in each polarization state is I _H + I _V = I _P + I _M = I _R + I _L Thus, the Stokes vector can be determined by actually measuring the light intensity in four independent states. Using the polarization states H, V, P, and R used this time, the Stokes vector is defined as in the following Expression 8.
[0047]
(Equation 8)

[0048]
As an example, H, V, P, M, R, L and natural light, that is, an unpolarized Stokes vector are shown in the following Expression 9.
[0049]
(Equation 9)

[0050]
Further, assuming that the input Stokes vector to the optical element or the measurement object is Sin and the output Stokes vector is Sout, the polarization characteristic of the optical element or the measurement object is represented by the following equation (10), where M is a linear transformation matrix of Sin and Sout. It can be expressed as follows.
[0051]
(Equation 10)

[0052]
The linear transformation matrix M used in the above equation (10) is called a Mueller matrix. Also, M ₀ , M ₁ , M ₂ , M ₃ Represents Mueller vector elements of 4 rows and 1 column, respectively. Since this 4 × 4 Mueller matrix is composed of 16 independent elements, it can be determined by measuring 16 independent light intensities.
[0053]
Therefore, a case is considered in which four of H, V, P, and R in Expression 9 are set as input Stokes vectors. At this time, each output Stokes vector can be expressed by the following equation 11 from the equation 10.
[0054]
[Equation 11]

[0055]
Therefore, when the Mueller matrix is calculated from these output Stokes vectors, Equation 12 is obtained.
[0056]
(Equation 12)

[0057]
Here, the suffix represents a combination of the polarization states of the reference light and the object light. _Hv Then, it is the interference signal intensity when the polarization state of the reference light is H and the polarization state of the object light is V. In the apparatus shown in FIG. 1, the respective intensities are signal intensities after Fourier transform of the intensities of the spectral interference fringes by a discrete Fourier transform (DFT), that is, correlation signal intensities. The apparatus according to the present invention also determines one Mueller matrix for one point of the sample 15. That is, the distribution of the Mueller matrix in the measurement object is finally measured.
[0058]
The result is M ₀₀ ~ M ₃₃ Thus, a total of 16 OCT images are obtained. Each OCT image is, for example, M ₀₀ Represents a distribution that does not include polarization information that can be obtained with a normal OCT device, ₂₃ Represents a distribution that converts 45 ° linearly polarized light into circularly polarized light. That is, by observing each component, the polarization characteristics of the sample (object) can be grasped. Equations 13 to 16 show Mueller matrices of typical polarizing elements.
[0059]
Here, the amplitude transmittance p in the x-axis direction _x , The amplitude transmittance in the y-axis direction is p _y The Mueller matrix of the partial polarizer is expressed by Equation 13, the Mueller matrix of the perfect polarizer having the transmission axis in the x-axis direction is expressed by Equation 14, and the Mueller matrix of the phase shifter whose phase in the z-axis direction advances by δ is The Mueller matrix of the optical rotator that rotates the polarization direction by θ is represented by Expression (16).
[0060]
(Equation 13)

[0061]
[Equation 14]

[0062]
[Equation 15]

[0063]
(Equation 16)

[0064]
(Experimental example 1)
The present inventors measured an optical path difference by placing a plane mirror on a sample stage 16 of a Michelson interferometer in order to confirm how much resolution in the depth direction can be obtained by the apparatus shown in FIG. The polarization was performed in the state of the sample, in this case, the light incident on the plane mirror and the reference light, both of which were horizontal linearly polarized light (HH). FIG. 2 shows the measurement results.
[0065]
In FIG. 2A, the image on the left is an image of the spectral interference fringes captured by the CCD camera 22. The distribution on the right side is a correlation intensity distribution of the reference light and the object light obtained by extracting a horizontal row of the intensity distribution of the spectral interference fringes and calculating the Fourier transform of the spectral intensity by a discrete Fourier transform (DFT). is there. FIG. 2B shows the result when the optical path length on the 600 μm object light side is longer than that in FIG.
[0066]
Each of the correlation intensity distributions in FIGS. 2A and 2B is obtained by averaging the correlation intensity distribution in the portion of y = 107 to 406 in the interference fringe image. The interference fringe image is divided by the image of only the reference light before performing the DFT for removal. In these two correlation intensity distributions, the distribution near z = 0 is the autocorrelation distribution (zero-order light distribution) of each of the reference light and the object light, and the distribution on the right and left sides is the cross-correlation distribution of the reference light and the object light ( (First-order light component and minus first-order light component).
[0067]
From these results, it can be seen that the shift of the primary or −1st order peak position in the correlation distribution is proportional to the optical path difference of the interferometer. When the coefficient on the horizontal axis (corresponding to the z-axis) of the intensity correlation distribution is calculated from the proportional relationship between the shift of the peak position and the optical path difference, it is 16 μm / pixel. Further, the resolution in the depth direction (z-axis) of the apparatus was obtained from the full width and half maximum (FWHM) of the cross-correlation distribution. As a result, it was found that the apparatus had a resolution of 32 μm.
[0068]
Since the apparatus 1 can obtain depth information by one measurement without performing scanning in the depth direction, the cross-sectional structure of the measurement object can be measured only by one-dimensional scanning in the x-axis direction. Therefore, it can be said that the resolution in the x-axis direction is determined by the scanning interval.
[0069]
(Experimental example 2)
Fish bone measurement:
As a biological sample, the cross-sectional structure of the middle bone of a fish (salmon) shown in FIG. 3A was measured. The sample was adhered and fixed on a slide glass with a double-sided tape, and 50 points were scanned at AB intervals in FIG. 3A at 5 μm intervals. In the photograph of (b), the cross section of FIG. 3 (a) was measured by a differential interference contrast microscope (DIC).
It is something.
[0070]
FIG. 4 shows 16 raw OCT images obtained by a combination of the four polarization states of H, V, P, and R. A Mueller matrix is calculated from the OCT measurement data by Expression 12, and is displayed for each element. The image is shown in FIG. HH and HV indicate the polarization states of the light incident on the sample and the reference light, the subscript M indicates each component of the Mueller matrix, and each image is normalized by the maximum value of each image. The size of these images is 250 μm in width × 800 μm in height.
[0071]
It can be seen from the OCT image of FIG. 4 that fine structures are observed in HP, PP, RP, and the like. However, in the OCT image, it is not possible to confirm the distribution of a portion having a property of converting to -45 ° linearly polarized light or counterclockwise circularly polarized light. On the other hand, when an image showing each component of the Mueller matrix in FIG. ₀₀ Is an image that does not include polarization characteristic information similar to a normal OCT image, so that the overall structure can be confirmed roughly, but the fine structure showing birefringence is not known. However, the fine structure having birefringence, which is considered to be due to bone collagen fibrils, which was not clear in the OCT image ₀₀ It can be clearly seen in other images.
[0072]
For example, M ₂₁ Then, a portion having a property of converting horizontal linearly polarized light into 45 ° linearly polarized light (a portion not shown in FIG. 5 but presenting a red color. Hereinafter, the color is not explicitly shown in FIG. 5) and a −45 ° linearly polarized light are converted. The structure of the part having the property (blue) appears. ₂₃ The structure of the portion having the property of converting clockwise circularly polarized light into 45 ° linearly polarized light (red) and the portion having the property of converting it into −45 ° linearly polarized light (blue) are further M. ₂₂ In M, the distribution of a portion having a property of preserving 45 ° linear polarized light (red) and a portion having a property of converting 45 ° linear polarized light to −45 ° linear polarized light (blue), M ₃₃ In FIG. 2, a fine structure of a portion having a property of retaining right-handed circularly polarized light (red) and a portion having a property of converting right-handed circularly polarized light to left-handed circularly polarized light (blue) appears.
[0073]
(Experimental example 3)
Measurement of human skin:
In order to investigate the possibility of applying the device of the present invention to a living body, the cross-sectional structure of a human skin slice was measured. The measured skin section (2.0 cm × 2.5 cm) was fixed on a slide glass with a tape as shown in FIG. 6A, and 50 points were scanned at 5 μm intervals. FIG. 6 is a cross-sectional image of human skin measured by a differential interference microscope.
[0074]
FIG. 7 shows an OCT image obtained by controlling the polarization state, and FIG. 8 shows a result of obtaining and displaying images of each component of the Mueller matrix from the OCT image. The size of the image is 250 μm in width × 1000 μm in height, and is standardized by the maximum value of each image.
[0075]
M without polarization information ₀₀ In the image of, although the rough structure near the surface layer of the skin section can be seen, the fine structure cannot be clearly confirmed. Looking at the other images, one can see a complex microstructure, probably due to the stratum corneum structure.
[0076]
For example, M ₁₁ In FIG. 8, the structure of a component that preserves vertical linearly polarized light (a part that exhibits red although not explicitly shown in FIG. 8; the color is not explicitly shown in FIG. 8) is represented by M ₃₀ In the figure, the fine structure of the portion (red) having the property of converting horizontal linearly polarized light into clockwise circularly polarized light is clearly shown. Also, M ₃₂ Then, the distribution of a portion having a component (blue) that converts 45 ° linearly polarized light into left-handed circularly polarized light, M ₃₃ It can be seen that a fine structure such as a distribution of a portion having a component (blue) that converts right-handed circularly polarized light into left-handed circularly polarized light is clearly shown.
[0077]
As can be seen from FIG. 6 (a), the cross-sectional structure of the actual skin is such that fibrous structures overlap in a complicated manner. It is considered that the results of this measurement have been able to capture the change in the polarization state, which is considered to be caused by the complex fibrous structure of the surface layer of the skin, that is, the stratum corneum.
[0078]
From the above results, it is sufficiently possible to measure a fine structure including birefringence of a living body by using this apparatus.
[0079]
As described above, the embodiments of the polarization-sensitive optical coherence tomography apparatus according to the present invention have been described based on the examples. However, the present invention is not particularly limited to such examples, and the technical features described in the claims are not limited. It goes without saying that there are various embodiments within the scope of the matter.
[0080]
【The invention's effect】
As described above, the present invention is to produce a polarization-sensitive spectral interference OCT apparatus by introducing polarization sensitivity into a spectral interference OCT that does not require mechanical scanning in the depth direction. Effect.
[0081]
(1) As a result of measuring the polarization characteristics of the quarter-wave plate, horizontal linear polarized light is converted to vertical linear polarized light, vertical linear polarized light is converted to horizontal linear polarized light, 45 ° linear polarized light is kept as it is, clockwise circular polarized light is left. Each component of the Mueller matrix that converts to circularly polarized light can be measured.
[0082]
(2) Regarding measurement of a cross-sectional structure of a biological sample such as a fish bone or human skin including a birefringent distribution, a fine structure including a birefringent distribution that cannot be obtained by conventional OCT measurement can be confirmed. In the present apparatus, the spatial resolution in the depth direction is 32 μm, and a dynamic range of 2 mm is confirmed in the air, and measurement can be performed with an extremely high resolution.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating an optical system of a polarization-sensitive optical coherence tomography apparatus according to the present invention.
FIG. 2 is a diagram showing measurement results of Experimental Example 1.
FIG. 3 is a diagram showing a fish (salmon) used as a biological sample for measuring the cross-sectional structure of a fish performed in Experimental Example 2.
FIG. 4 shows 16 raw OCT images obtained by combining respective polarization states in Experimental Example 2.
5 shows a Mueller matrix image displayed by calculating a Mueller matrix from the OCT measurement data of FIG. 4 and displaying each element.
FIG. 6 is a view showing a state in which a human skin slice, which is a sample performed in Experimental Example 3, is fixed on a slide glass with a tape.
FIG. 7 shows an OCT image obtained by controlling the polarization state in Experimental Example 3.
8 shows a result of obtaining and displaying an image of each component of the Mueller matrix from the OCT image of FIG. 7;
[Explanation of symbols]
1. Polarization-sensitive optical coherence tomography apparatus
2 Light source
3 Light wedge
4, 5, 6, 13, 17 mirrors
7 Polarizer
8, 19 1/2 wave plate
9, 11, 12, 18 quarter wave plate
10 Beam splitter
14 Lens that collects incident light
15 samples
16 Sample table
20 diffraction grating
21 lenses
22 CCD camera

Claims

An incident optical system that is provided on an incident optical path of the incident light and that can selectively adjust a polarization state of the incident light to any one of four different polarization states;
A beam splitter that is provided on the incident light path and divides incident light selectively adjusted to one of the four polarization states into reference light and incident light with respect to a sample to be measured;
A reference light optical system for selectively adjusting the polarization state of the reference light to one of the four polarization states and transmitting the reference light back to the beam splitter;
The reference light and the object are interfered with each other in a state where the reference light adjusted to each of the four polarization states and the object light reflected from the sample are combined with each other in 16 combinations of 4 × 4 combinations. A spectroscope comprising a diffraction grating and a lens,
A CCD camera that captures spectral interference fringes generated by the interference and obtains 16 coherence tomographic images,
A polarization-sensitive optical coherence tomography apparatus, wherein a signal based on the same polarization component as the reference light is extracted from the object light.

A first type, which is provided on an incident optical path of incident light and selectively adjusts the polarization state of the incident light to one of four types of horizontal linear polarization, vertical linear polarization, 45 ° linear polarization, and clockwise circular polarization. A half-wave plate and a first quarter-wave plate;
A beam splitter that is provided on the incident light path and divides incident light selectively adjusted to one of the four polarization states into reference light and incident light with respect to a sample to be measured;
A second mode for selectively adjusting the polarization state of the reference light to one of four types of horizontal linearly polarized light, vertical linearly polarized light, 45 ° linearly polarized light, and right-handed circularly polarized light and returning the beam to the beam splitter for transmission. A quarter-wave plate, a third quarter-wave plate and a mirror;
The object light adjusted to one of the four polarization states and reflected by the sample, further reflected by the beam splitter, and the reference light adjusted to one of the four polarization states are superimposed. A fourth quarter-wave plate and a second half-wave plate for controlling the reflected light to horizontal linear polarization,
The reference light and the object are interfered with each other in a state where the reference light adjusted to each of the four polarization states and the object light reflected from the sample are combined with each other in 16 combinations of 4 × 4 combinations. A spectroscope comprising a diffraction grating and a lens,
A CCD camera that captures spectral interference fringes generated by the interference and obtains 16 coherence tomographic images,
A polarization-sensitive optical coherence tomography apparatus, wherein a signal based on the same polarization component as the reference light is extracted from the object light.

The polarization states of the reference light and the object light are selectively adjusted to one of four different polarization states, and the respective polarization states are combined, and the reference light and the reference light are combined in four combinations × 4 combinations of 16 combinations. The interference fringes of the object light are photographed to obtain 16 coherence tomography images,
Polarization-sensitive optical coherence characterized by measuring polarization information inside a sample by obtaining each component of a Muller matrix capable of displaying the polarization characteristics of the sample from the 16 coherence tomography images. A method for measuring polarization information inside a sample by tomography.

The polarization state of the incident light is selectively adjusted to one of four types of horizontal linear polarization, vertical linear polarization, 45 ° linear polarization, and clockwise circular polarization,
The incident light selectively adjusted to any one of the above four polarization states is divided into reference light and incident light with respect to a sample to be measured by a beam splitter,
The polarization state of the reference light is horizontally linearly polarized light, vertically linearly polarized light, 45 ° linearly polarized light, and selectively adjusted to one of four types of right-handed circularly polarized light, returned to the beam splitter and transmitted,
The object light from the sample selectively adjusted to one of the four polarization states and the reference light selectively adjusted to one of the four polarization states are transmitted from the diffraction grating and the lens. Interference with a spectroscope consisting of
Spectral interference fringes generated by the interference are photographed by a CCD camera, and a signal based on the same polarization component as the reference light is extracted from the object light, and polarization information inside the sample is measured by polarization-sensitive optical coherence tomography. The method,
In each of the 16 combinations of the above-mentioned polarization states and 4 × 4 combinations, the interference fringes of the reference light and the object light are photographed to obtain 16 coherence tomography images,
Polarization-sensitive optical coherence tomography characterized in that the polarization information is measured by obtaining each component of a Muller matrix capable of displaying the polarization characteristics of the sample from these 16 coherence tomography images. A method for measuring polarization information inside a sample.

The polarization state of the incident light is changed into one of four types of horizontal linear polarization, vertical linear polarization, 45 ° linear polarization, and clockwise circular polarization by the first half-wave plate and the first quarter-wave plate. Selectively adjust,
The incident light selectively adjusted to one of the above four polarization states is divided into reference light and incident light with respect to the sample to be measured by the beam splitter,
The polarization state of the reference light is changed by a second quarter-wave plate, a third quarter-wave plate and a mirror into four types of horizontal linearly polarized light, vertically linearly polarized light, 45 ° linearly polarized light and clockwise circularly polarized light. Selectively adjust to either, return to the beam splitter and transmit,
The light in which the object light reflected by the sample and further reflected by the beam splitter and the reference light are superimposed is horizontally reflected by the fourth quarter-wave plate and the second half-wave plate. Control to be linearly polarized, these object light and reference light interfere with a spectroscope consisting of a diffraction grating and a lens,
The spectral interference fringes formed by the spectroscope are photographed by a CCD camera, and a signal based on the same polarization component as the reference light is extracted from the object light,
The signal from the CCD camera is input to an image processing device, and a correlation signal between the Fourier-transformed reference light and the object light is obtained to obtain polarization information inside the sample by polarization-sensitive optical coherence tomography. A measuring method,
In each of the 16 combinations of the above-mentioned polarization states and 4 × 4 combinations, the interference fringes of the reference light and the object light are photographed to obtain 16 coherence tomography images,
Polarization-sensitive optical coherence tomography characterized in that the polarization information is measured by obtaining each component of a Muller matrix capable of displaying the polarization characteristics of the sample from these 16 coherence tomography images. A method for measuring polarization information inside a sample.