JP3688608B2 - Optical coherence tomographic image measuring device with spectroscopic function - Google Patents

Description

ただし、ｋ＝２πｆ／ｃは波数、ｆは光周波数、ｃは光の速度、２Δｌ＝２（ｌ₁ −ｌ₂ ）は光路長差、Ｓ（ｆ）は広帯域光源のパワースペクトル関数である。
【００３５】
単一光検出器７を用いて光干渉信号を検出する場合、上記式（１）における周波数積分は光源のスペクトル全域にわたって行われると考えることができる。ここで、説明の便宜上Ｓ（ｆ）を中心周波数ｆ₀ 、半値全幅Δｆのガウス型とすると、干渉信号はガウス関数によって振幅変調を受け、その振幅はΔｌ＝０の時最大値となるが、ΔｆΔｌに比例して指数的に減衰する〔例えば、陳建培、丹野直弘、“Ｏ ｐｌｕｓ Ｅ”、Ｖｏｌ．２２，Ｎｏ．４，４６１（２０００）参照〕。すなわち、可干渉距離はΔｆによって制限されている。
【００３６】
また、被測定体５からの反射光強度（＝Ｒ・Ｅ₂ ² ）を測定するためには、上記式（１）で与えられる干渉信号の振幅ｉ_l を二乗すればよく、そこで、ｉ_l ² 半値全幅に対応する深さを干渉測定の空間分解能Δｚと定義すると、Δｚが光源のコヒーレント長の半分に等しいことが察知できる。ただし、ｌ_C は、
【００３７】
【数２】

で与えられる。この原理から、連続光を用いた光反射実験でも、光源のスペクトルが広ければ、優れた深さ（距離）分解能が得られることが分かる。小型で便利な半導体光源であるスーパールミネセントダイオード（ＳＬＤ）を例とすると、市販品（中心波長０．８μｍ、半値幅３０ｎｍ）では、ｌ_C ＝１４μｍとなり、約７μｍの深さ分解能が期待できる。
【００３８】
次に、図１に示した光干渉断層計測装置において被測定体５が例えば生体組織のような散乱媒質である場合について説明する。被測定体内深さｚにある反射点からの反射光と参照鏡からの参照光との干渉信号は次のように与えられる。
【００３９】
【数３】

ただし、ψ（ｆ，ｚ）は位相差、Ｅ₂ （ｆ，ｚ）は反射光波の振幅であって、
【００４０】
【数４】

ここで、Ｐ₀ （ｆ）は被測定体の表面から反射されて干渉に寄与する信号光（周波数ｆ）の強度、μ_t は吸収係数μ_a と散乱係数μ_s を含めた減衰係数で、
【００４１】
【数５】

となる。式（２）で説明した原理から、光断層画像化計測は例えば生体組織のような散乱媒質を、
【００４２】
【数６】

の厚みで光学的に切り出して画像化すると考えることができる。ただし、ｎは媒質の光屈折率である。従って、式（３）より、光断層画像を構成する反射光強度を次のように近似できる。
【００４３】
【数７】

式（７）から明らかなように、従来の単一光検出器を用いた光コヒーレンス断層画像計測装置では、周波数積分が光源のスペクトル全域にわたって行われるため、分解能が式（６）で与えられる値に達するが、分光情報が抽出されないことになる。
【００４４】
本発明は、図１に示すように、二分割された干渉光の一方を単一検出器で検出して高分解能の断層画像計測を行うと同時に、もう一方の干渉光ビームを例えば回折格子のような波長分散素子で分光させてセンサアレイで検出することを特徴とする。センサアレイの１素子の受光面で観測される干渉光のスペクトル幅をδλとし、またｊ番目の素子に入射する干渉光の周波数をｆ_j とすると、該素子で検出される反射光の強度は次のようになる。
【００４５】
【数８】

ここで、分解能δｚはアレイ素子で観測されるスペクトル幅δλで制限されて低下し、
【００４６】
【数９】

となる。
【００４７】
本発明では、分光分解能を上げることは光コヒーレンス断層画像の空間分解能の低下につながることを十分に認識した上で、図１に示すように分光画像計測を別途に行う。また、上記式（８）から明らかなように、本発明による分光画像測定は時間領域で行われるものであり、フーリエ変換などの演算処理を用いた従来の周波数領域での分光画像計測方法と根本的に異なる。
【００４８】
以下、本発明による分光情報の抽出方法について説明する。
【００４９】
上記式（８）によれば、センサアレイのｊ番目の素子で検出される、ｍ−１番層〔深さ（ｍ−１）δｚ〕およびｍ番層（深さｍδｚ）からの反射光強度の比を取ることで、ｍ番層における光減衰係数を次のように算出することが出来る。
【００５０】
【数１０】

そこで、μ_t の測定をｍ＝１，２，３．．．Ｎの順に行えば、μ_t （ｆ_j ）の深さ方向分布が計測されることになる。同様の測定をセンサアレイの各素子からの出力について行えば、めいめいの光周波数における減衰係数の分布を計測することができる。このように求められた減衰係数の分光情報を画像化するために、例えば図２に示す３次元表示法が有効である。
【００５１】
さらに、本発明によれば、２つもしくは２つ以上の光周波数における減衰度の差分値を画像化することも容易である。以下、その方法について説明する。
【００５２】
被測定体のｍ番層（深さｍδｚ）に局在する、周波数ｆ_j とｆ_k 間の差分減衰度α_jk,mを次のように定義する。
【００５３】
【数１１】

一方、式（８）から、それれぞれの光周波数で検出される反射光の強度比は次のようである。
【００５４】
【数１２】

ただし、初期値〔Ｉ_n （Ｐ_oj／Ｐ_ok）〕は予め測定できるもので、Ｐ_oj＝Ｐ_okの場合ゼロとなる。式（１２）で与えられる強度比から、α_jk,mを次のように算出することができる。
【００５５】
【数１３】

上記のように求められた差分減衰度を画像化するために、例えば図３に示す３次元表示法が有効である。
【００５６】
単一光検出器を用いた従来の光コヒーレンス断層画像計測法によって得られる光断層像（図７参照）は、構造に依存した反射光強度の変化を画像情報としているが、図３は吸収および散乱特性の波長依存性による差分減衰度の変化を画像情報としている点が大きな特徴である。
【００５７】
さらに、図１に示す、本発明による光干渉断層画像計測装置は、単一光検出器を用いた断層画像計測機能と、波長分散素子及びセンサアレイを用いた分光画像計測機能を両備することから、一測定で被測定体の内部に関する構造的及び分光的情報を同時に得ることが可能となり、分光的構造を可視化する画像が構成できる特徴を有する。
〔実施例１〕
図４は、本発明による図１の光干渉断層画像計測装置に低コヒーレント光源として連続出力のＳＬＤを用いた実施例を示す。
【００５８】
この図において、２１はＳＬＤ（光源）、２２は第１のレンズ、２３は光ファイバからなる光ファイバカプラ２４の第１のアーム、２４は光ファイバカプラ、２５は光ファイバからなる光ファイバカプラ２４の第２のアーム、２６，２７は第２及び第３のレンズ、２８はガルバノスキャナ、２９は被測定体（被検体）、３０は光ファイバからなる光ファイバカプラの第３のアーム、３１は第４のレンズ、３２はＺスキャンの鏡（反射鏡）、３３は光ファイバからなる光ファイバカプラの第４のアーム、３４は第５のレンズ、３５はビームスプリッタ、３６は単一光検出器、３７は回折格子、３８はセンサアレイである。
【００５９】
ここで、市販されている近赤外域ＳＬＤ２１を用いる場合、コヒーレント長はｌｃ≒３０μｍ、発光ダイオード（ＬＥＤ）の場合はｌｃ≒１０μｍ程度である。
【００６０】
さらに、図４の実施例は、本発明による図１の信号光及び参照光の伝送手段として光ファイバを用いることにより、被測定体２９の配置を光画像計測装置から分離することを特徴とする。
【００６１】
ＳＬＤ（光源）２１からの出力光は第１のレンズ２２によって光ファイバカプラ２４の第１のアーム２３に結合された後、信号光と参照光に二分される。信号光は第２のアーム２５によって被測定体２９側へ伝搬される。第２のアーム２５から出射する信号光は第２のレンズ２６と第３のレンズ２７によって被測定体２９へフォーカスされて入射する。横方向スキャナを行うために、例えばガルバノスキャナ２８を使用することが望ましい。被測定体２９からの反射光は第３のレンズ２７と第２のレンズ２６によって再び光ファイバカプラ２４の第２のアーム２５に結合されて光ファイバカプラ２４へ返送される。
【００６２】
他方、参照光は光ファイバカプラ２４の第３のアーム３０によって反射鏡３２へ伝送される。反射鏡３２からの反射光は光ファイバカプラ２４の第３のアーム３０によって光ファイバカプラ２４へ返送されて被測定体２９からの反射光と合波される。
【００６３】
低コヒーレント光源２１を用いた図４の実施例では、信号光と参照光との光路長差が光源の極めて短いコヒーレント長以内にあるときのみ、光干渉が生成される。そのため、例えば反射鏡３２の位置をスキャンして参照光の光路長を信号光の光路長に等しくなるように差調節することが望ましい。
【００６４】
また、前述したように、本発明はセンサアレイ３８、例えば空間的に受光素子が配置される１次元ないし２次元光検出器アレイを用いて回折格子３７によって分光された干渉光を並列検出することを特徴としている。そのための並列信号処理系として複数の整流検波器を並列に配置されるエンベロープ検出器アレイが有用であると考えられる。
〔実施例２〕
図５は本発明による図１に示した光断層画像計測装置にセンサアレイとしてイメージセンサ、例えばＣＣＤ（電荷結合素子）カメラを用いた実施例を示す。
【００６５】
図５において、４１は広いスペクトル幅をもつ低コヒーレント光源（広帯域光源）であり、例えばスーパールミネセントダイオード（ＳＬＤ）、４２は第１のビームスプリッタ、４３は鏡、４４はＰＺＴ、４５は被測定体、４６は第２のビームスプリッタ、４７は単一光検出器、４８は回折格子、４９はレンズ、５０は第３のビームスプリッタ、５１は第１のシャッター、５２は第１のＣＣＤカメラ、５３は第２のシャッター、５４は第２のＣＣＤカメラである。
【００６６】
ここで、市販品のＣＣＤカメラは数十万ないし数百万の受光素子を有するので、これら極めて多数の受光素子を有効に利用すれば、高分解能の分光画像計測が可能となる。
【００６７】
図５に示すように、回折格子４８によって分光された干渉光はレンズ４９でコリメートされた後、第３のビームスプリッタ５０によって二分割される。二分割された両光ビームはそれぞれ第１のＣＣＤカメラ５２と第２のＣＣＤカメラ５４に入射する。市販品のカメラは一般的に応答性が低く（３０Ｈｚ程度）、数１００Ｈｚもしくはそれ以上のビート周波数をもつ干渉信号の検出に適用するために、本実施例はＣＣＤカメラの前に高速シャッターを配置することを特徴とする。
【００６８】
図５では、第１の光シャッター５１と第２の光シャッター５３としたが、例えば液晶素子または電気光学素子を用いた高速シャッターが望ましい。この両シャッター５１，５３は干渉光のビート周波数と同じ周波数で干渉光を周期的に遮断して、互いに位相差が９０度である２列の干渉光パルスを生成する。この２つの光パルス列はそれぞれ第１のＣＣＤカメラ５２と第２のＣＣＤカメラ５４に入射し、各ＣＣＤカメラ５２，５４の受光時間内で光電荷が蓄積される。
【００６９】
なお、本発明は上記実施例に限定されるものではなく、本発明の趣旨に基づいて種々の変形が可能であり、これらを本発明の範囲から排除するものではない。
【００７０】
【発明の効果】
以上、詳細に説明したように、本発明によれば、フーリエ変換などの信号処理を行う必要がなく、実時間で反射光の深さおよび分光情報を同時に取得することができる。
【図面の簡単な説明】
【図１】 本発明の分光機能を備えた光コヒーレンス断層画像計測装置の構成図である。
【図２】 本発明の光コヒーレンス断層画像計測装置における３次元表示例（その１）を示す図である。
【図３】 本発明の光コヒーレンス断層画像計測装置における３次元表示例（その２）を示す図である。
【図４】 本発明による光断層画像計測装置に低コヒーレント光源として連続出力のＳＬＤを用いた実施例を示す図である。
【図５】 本発明による光断層画像計測装置にセンサアレイとしてイメージセンサを用いた実施例を示す図である。
【図６】 従来のマイケルソン干渉計を用いた光コヒーレンス断層画像計測装置の基本構成図である。
【図７】 従来の断層画像計測装置における３次元表示例を示す図である。
【図８】 従来のマイケルソン干渉計を用いた光コヒーレンス断層画像計測装置の他の構成を示す図である。
【図９】 従来のマイケルソン干渉計を用いた光コヒーレンス断層画像計測装置の更なる他の構成を示す図である。
【符号の説明】
１，４１ 低コヒーレント光源（広帯域光源）
１Ａ，１Ｂ，４９ レンズ
２ 第１のビームスプリッタ（ＢＳ１）
３，４３ 鏡
４，４４ ＰＺＴ
５，２９，４５ 被測定体（被検体）
６ 第２のビームスプリッタ（ＢＳ２）
７，３６，４７ 単一光検出器
８，３７，４８ 回折格子
９，３８ センサアレイ
１０ エンベロープ検出器
１１ 断層画像処理系
１２ 分光画像処理系
２１ ＳＬＤ
２２ 第１のレンズ
２３ 光ファイバからなる光ファイバカプラの第１のアーム
２４ 光ファイバカプラ
２５ 光ファイバからなる光ファイバカプラの第２のアーム
２６ 第２のレンズ
２７ 第３のレンズ
２８ ガルバノスキャナ
３０ 光ファイバからなる光ファイバカプラの第３のアーム
３１ 第４のレンズ
３２ Ｚスキャンの鏡（反射鏡）
３３ 光ファイバからなる光ファイバカプラの第４のアーム
３４ 第５のレンズ
３５ ビームスプリッタ
４２ 第１のビームスプリッタ
４６ 第２のビームスプリッタ
５０ 第３のビームスプリッタ
５１ 第１のシャッター
５２ 第１のＣＣＤカメラ
５３ 第２のシャッター
５４ 第２のＣＣＤカメラ[0001]
BACKGROUND OF THE INVENTION
The present invention uses an image of a subject, in particular, a light scattering medium irradiated with a light beam and propagated (reflected or transmitted) through the subject to form an image of the surface or internal form and functional information of the subject. The present invention relates to an optical coherence tomographic image measuring apparatus having a spectroscopic function.
[0002]
[Prior art]
In recent years, with the aging of society, promotion of health management corresponding to the needs of consumers and prevention of illness have been demanded. Against this background, in the field of medical diagnostic imaging, assuming that the test is repeated for asymptomatic and unconscious persons, it is essential to consider that the test method is simple and less painful and burdening the patient. .
[0003]
From this point of view, great expectations are placed on biological sensing of light waves. Optical sensing is non-invasive and safe (de-isotope, de-radiation), high accuracy and high resolution, and has the potential for identification and quantitative analysis of substances by spectroscopic measurement. In particular, the realization of functional imaging by spectroscopy is expected to greatly contribute to the diagnosis of the actual condition of diseases.
[0004]
However, for example, a sample (subject) having a non-uniform constituent material such as a human body or a living tissue significantly scatters light inside thereof, and thus the internal form thereof is generally invisible. The biggest difficulty in optical measurement of such a scattering medium lies in how to extract signal light that follows a traceable optical path from transmitted light or reflected light emitted in all directions from the subject. .
[0005]
One method that makes this possible is optical coherence tomography with excellent distance resolution (see, for example, Naohiro Tanno, 'Optics', Vol. 28, No. 3, 116 (1999)).
[0006]
Optical coherence tomography is focused on the low coherence of the light source with a wide spectral width in the time domain (also expressed as a short coherence distance in the spatial domain), and the reflected light wave from the inside of the living body in the interferometer is on the order of μm. It is a method of detecting with the distance resolution of.
[0007]
FIG. 6 shows a basic configuration of an optical coherence tomographic image measurement apparatus using a Michelson interferometer. In this figure, 61 is a low-coherent light source (broadband light source), 62 is a beam splitter (translucent mirror), 63 is a mirror, 64 is an object to be measured (subject), 65 is a photodetector, and 66 is an envelope detector. is there.
[0008]
Therefore, the light beam from the low coherent light source (broadband light source) 61 is divided into two by the beam splitter 62. One light beam is given a Doppler frequency shift by, for example, position scanning (Z-scan) of the mirror 63 to obtain a reference light wave, and the other is irradiated to the measured object 64 to obtain backscattered light from the deep layer of the object. When the measured object 64 is a scattering medium, the reflected light wave is considered to be a diffuse wavefront having a messy phase including multiple scattering.
[0009]
In the optical coherence tomographic image measurement, due to the low coherence of the light source 61, the optical path length difference between the signal light and the reference light is within the coherent length of the light source 61 on the order of μm, and a component having a phase correlation with the reference light wave, that is, coherent. Only the signal light component selectively interferes with the reference light wave. Therefore, the light reflection distribution image can be measured by changing the reference optical path length by scanning the position of the mirror 63. FIG. 7 shows such a situation.
[0010]
However, as can be seen in FIG. 7, the current optical coherence tomographic image measurement is to sequentially detect the reflected light wave from each part by scanning the optical path length difference and the light beam. At this time, optical interference between the reflected light wave and the reference light wave occurs in the entire spectrum region of the light source, and the amplitude of the interference signal becomes the intensity of the image signal. Therefore, in the result of FIG. 7, it is obvious that the intensity of the reflected light wave maintaining the optical coherence is detected, but the amount of change in the spectrum of the signal light propagated through the sample is not extracted.
[0011]
For example, there is a method shown in FIG. 8 in order to simultaneously analyze not only the internal structure of the sample but also the light absorption and scattering characteristics depending on the wavelength by analyzing the spectral distribution of the signal light (spectral analysis). Here, the time waveform of the optical interference signal is recorded and the intensity of the reflected light is measured from the waveform envelope, and the frequency information of the reflected light, that is, the spectral information is extracted by performing Fourier transform on the waveform. [For example, U.S. Pat. Morganer, W.M. Drexler, F.M. X. Kartner, X. et al. D. Li, C.I. Pitris, E .; P. Ippen, J. et al. G. Fujimoto, “Optics Letters”, Vol. 25, 111 (2000)].
[0012]
This method is understood to be a measurement method in the time domain.
[0013]
On the other hand, as a measurement method in the frequency domain, as shown in FIG. 9, as shown in FIG. 9, the interference light is dispersed using, for example, a wavelength dispersion element such as a diffraction grating 69, and the dispersed interference light is, for example, a CCD (charge-coupled device). ) An example of detection by an image sensor such as the camera 71 has been reported [for example, W, Watanabe, Y. et al. Matsuda, K .; Itoh, “Optical Review”, Vol. 6, 455 (1999)]. In FIG. 9, reference numerals 67, 68, and 70 denote lenses.
[0014]
In principle, by performing Fourier transform on the interference light intensity detected for each wavelength, the depth and spectral information of the reflected light can be acquired simultaneously without performing a Z-scan due to the displacement of the mirror 63. At this time, the resolution in the depth direction is inversely proportional to the window function width of the Fourier transform.
[0015]
[Problems to be solved by the invention]
However, the conventional optical coherence tomographic imaging apparatus having the spectroscopic analysis function uses an arithmetic processing method such as Fourier transform as shown in FIG. 8 or FIG. 9, for example. There is a limit to speedup.
[0016]
On the other hand, since the resolution Δz in the depth direction in the optical coherence tomographic imaging method is inversely proportional to the window function width Δk of Fourier transform, increasing the spectral resolution by narrowing Δk leads to a decrease in Δz.
[0017]
In view of the above situation, an object of the present invention is to provide an optical coherence tomographic image measurement apparatus having a spectral function capable of simultaneously obtaining the depth of reflected light and spectral information in real time.
[0018]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides
[1] In an optical coherence tomographic image measurement apparatus having a spectroscopic function, a light source that emits a light beam, and a light beam emitted from the light source and a signal light that passes through a subject placement position where the subject is placed The reference light passing through a different optical path from the optical path passing through the subject arrangement position is divided into two, and the signal light after passing through the subject placement position and the reference light passing through the different optical path are mutually connected. An interference optical system that generates interference light by superimposing, a frequency shifter that relatively shifts the frequency of the signal light and the frequency of the reference light, and the interference optical system includes the interference light In order to receive the light, the interference light is divided into two, and one of the two divided interference lights is received, and the optical interference between the signal light and the reference light is performed in the entire spectral range of the light source. Generate high A single photodetector for performing tomographic image measurement resolution is, said bisected interference light wavelength dispersion element for spectrally other of, for heterodyne detection of the split interference light independently An optical sensor and the optical sensor are spatially arranged, each of which has a plurality of light receiving elements that independently detect heterodyne, and further detects amplitudes of a plurality of heterodyne signals obtained by the optical sensor. Each interest on the propagation path of the signal light of the amplitude detector and the surface or inner layer of the subject arranged at the subject placement position by integrating the plurality of heterodyne signal amplitudes obtained by the amplitude detector A signal processing unit that generates a signal corresponding to a point is provided.
[0019]
[2] In the optical coherence tomographic image measurement apparatus having the spectral function described in [1] above, the wavelength dispersion element is a diffraction grating.
[0020]
[3] In the optical coherence tomographic image measurement apparatus having the spectroscopic function described in [1], a Michelson interferometer is used as the interference optical system.
[0021]
[4] In the optical coherence tomographic image measurement apparatus having the spectral function described in [1] above, the light source is a superluminescent diode.
[0022]
[5] The optical coherence tomographic image measuring apparatus having the spectral function described in [1] above, wherein the light source is a light emitting diode.
[0023]
[6] In the optical coherence tomographic image measurement apparatus having the spectral function described in [1] above, the light source is a femto laser light source.
[0024]
[7] The optical coherence tomographic image measurement apparatus having the spectral function described in [1] above, wherein the subject is a light scattering medium.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0026]
FIG. 1 is a configuration diagram of an optical coherence tomographic image measuring apparatus having a spectral function according to the present invention.
[0027]
In this figure, 1 is a low coherent light source (broadband light source) having a wide spectral width, for example, a superluminescent diode (SLD), 1A and 1B are lenses for expanding the beam diameter of light, and 2 is a first beam. Splitter (BS1), 3 is a mirror (total reflection mirror), 4 is PZT for Z-direction scanning, 5 is an object to be measured (subject), 6 is a second beam splitter (BS2), and 7 is a single unit A photodetector, 8 is a diffraction grating, 9 is a sensor array, 10 is an envelope detector, 11 is a tomographic image processing system, and 12 is a spectral image processing system.
[0028]
A light beam emitted from a low-coherent light source 1 having a wide spectral width, for example, a super luminescent diode (SLD), has a beam diameter expanded by a lens 1A and a lens 1B, and further a signal beam by a first beam splitter (BS1) 2. And the reference light. The signal light separated from the reference light is incident on the measurement object 5 made of, for example, a scattering medium. A part of the reflected light wave from the measurement object 5 is reflected by the first beam splitter (BS1) 2 and transmitted to the second beam splitter (BS2) 6.
[0029]
On the other hand, the reference light is transmitted to the mirror (total reflection mirror) 3, and the reflected light wave undergoes a frequency shift of the Doppler frequency due to position scanning of the mirror 3, for example. Part of the reference light that has undergone the frequency shift passes through the first beam splitter (BS1) 2 and is superimposed on the signal light to generate interference light.
[0030]
FIG. 1 is constructed based on the principle of a Michelson interferometer. In the present invention, an optical coherence tomographic image measurement capable of simultaneously obtaining the depth of reflected light and spectral information by providing the interferometer with the following mechanism. Providing equipment. That is, here, the interference light obtained by the light wave superimposing means by the first beam splitter (BS1) 2 is divided into two by the second beam splitter (BS2) 6. One of the two divided interference light beams is detected by a single photodetector 7, and the other is incident on a wavelength dispersion element such as a diffraction grating 8. The interference light dispersed by the diffraction grating 8 is detected by a one-dimensional or two-dimensional photosensor array 9.
[0031]
According to the present invention, optical interference between a reflected light wave and a reference light wave is generated in the whole spectrum region of the light source by the above method to perform tomographic image measurement with high depth resolution, while detecting the dispersed interference light with a sensor array. Thus, it is characterized by measuring a tomographic image at each wavelength. Thereby, it is not necessary to perform signal processing such as Fourier transform, and spectroscopic optical tomographic image measurement in real time is possible.
[0032]
The measurement principle according to the present invention will be described below.
[0033]
In the low coherent interferometer shown in FIG. 1, consider a case where the measured object 5 is a hard target such as a mirror. The surface reflectance of the measured object 5 is R (Z), and the signal light reflected from the surface of the measured object 5 is e ₂ (t) = E ₂ √Rexp [−j (2πft−2kl ₂ −ψ )], The interference signal generated by the interference with the reference light e ₁ (t) = E ₁ exp [−j (2πft−2kl ₁ −ψ)] from the mirror 3 is given as follows.
[0034]
[Expression 1]

Where k = 2πf / c is the wave number, f is the optical frequency, c is the speed of light, 2Δl = 2 (l ₁ −l ₂ ) is the optical path length difference, and S (f) is the power spectrum function of the broadband light source.
[0035]
When an optical interference signal is detected using the single photodetector 7, it can be considered that the frequency integration in the above equation (1) is performed over the entire spectrum of the light source. Here, for convenience of explanation, if S (f) is a Gaussian type having a center frequency f ₀ and a full width at half maximum Δf, the interference signal is subjected to amplitude modulation by a Gaussian function, and the amplitude becomes a maximum value when Δl = 0. Decreases exponentially in proportion to ΔfΔl [for example, Chen Jianmei, Naohiro Tanno, “O plus E”, Vol. 22, no. 4, 461 (2000)]. That is, the coherence distance is limited by Δf.
[0036]
Further, in order to measure the reflected light intensity (= R · E ₂ ² ) from the measured object 5, it is only necessary to square the amplitude i _{l of the} interference signal given by the above equation (1), where i _l ^If the depth corresponding to the full width at half maximum is defined as the spatial resolution Δz of interferometry, it can be seen that Δz is equal to half the coherent length of the light source. Where l _C is
[0037]
[Expression 2]

Given in. From this principle, it can be seen that even in a light reflection experiment using continuous light, an excellent depth (distance) resolution can be obtained if the spectrum of the light source is wide. Taking a superluminescent diode (SLD), which is a small and convenient semiconductor light source, as an example, a commercially available product (center wavelength 0.8 μm, half width 30 nm) has l _C = 14 μm, and a depth resolution of about 7 μm can be expected. .
[0038]
Next, the case where the measured object 5 is a scattering medium such as a living tissue in the optical coherence tomography measuring apparatus shown in FIG. 1 will be described. The interference signal between the reflected light from the reflection point at the measurement object depth z and the reference light from the reference mirror is given as follows.
[0039]
[Equation 3]

Where ψ (f, z) is the phase difference, E ₂ (f, z) is the amplitude of the reflected light wave,
[0040]
[Expression 4]

Here, P ₀ (f) is the intensity of the signal light (frequency f) reflected from the surface of the object to be measured and contributes to interference, μ _t is the attenuation coefficient including the absorption coefficient μ _a and the scattering coefficient μ _s ,
[0041]
[Equation 5]

It becomes. From the principle described in Equation (2), optical tomographic imaging measurement is performed using a scattering medium such as biological tissue,
[0042]
[Formula 6]

It can be considered that an image is optically cut out with a thickness of 10 mm. Here, n is the optical refractive index of the medium. Therefore, from the formula (3), the reflected light intensity constituting the optical tomographic image can be approximated as follows.
[0043]
[Expression 7]

As apparent from the equation (7), in the optical coherence tomographic image measurement apparatus using the conventional single photodetector, the frequency integration is performed over the entire spectrum of the light source, and therefore the resolution is a value given by the equation (6). However, spectral information is not extracted.
[0044]
In the present invention, as shown in FIG. 1, one of the two divided interference lights is detected by a single detector and high-resolution tomographic image measurement is performed. At the same time, the other interference light beam is, for example, a diffraction grating. It is characterized in that it is dispersed by such a wavelength dispersion element and detected by a sensor array. If the spectral width of the interference light observed on the light receiving surface of one element of the sensor array is δλ, and the frequency of the interference light incident on the j-th element is f _j , the intensity of the reflected light detected by the element is It becomes as follows.
[0045]
[Equation 8]

Here, the resolution δz is limited and decreased by the spectral width δλ observed by the array element,
[0046]
[Equation 9]

It becomes.
[0047]
In the present invention, it is fully recognized that increasing the spectral resolution leads to a reduction in the spatial resolution of the optical coherence tomographic image, and then spectral image measurement is separately performed as shown in FIG. Further, as is clear from the above equation (8), the spectral image measurement according to the present invention is performed in the time domain, and the conventional spectral image measurement method in the frequency domain using arithmetic processing such as Fourier transform and the fundamental method. Is different.
[0048]
The spectral information extraction method according to the present invention will be described below.
[0049]
According to the above equation (8), the intensity of reflected light from the m-1th layer [depth (m-1) δz] and the mth layer (depth mδz) detected by the jth element of the sensor array. by taking the ratio of, it is possible to calculate the light attenuation coefficient as follows in the m-th layer.
[0050]
[Expression 10]

Therefore, the measurement of μ _t is performed with m = 1, 2, 3,. . . If it is performed in the order of N, the distribution in the depth direction of μ _t (f _j ) is measured. If the same measurement is performed on the output from each element of the sensor array, the distribution of the attenuation coefficient at each of the optical frequencies can be measured. For example, the three-dimensional display method shown in FIG. 2 is effective for imaging the spectral information of the attenuation coefficient thus obtained.
[0051]
Furthermore, according to the present invention, it is also easy to image the difference value of attenuation at two or more optical frequencies. The method will be described below.
[0052]
The differential attenuation α _{jk, m} between the frequencies f _j and f _k that is localized in the m-th layer (depth mδz) of the object to be measured is defined as follows.
[0053]
[Expression 11]

On the other hand, from the equation (8), the intensity ratio of reflected light detected at each optical frequency is as follows.
[0054]
[Expression 12]

However, the initial value [I _n (P _oj / P _ok )] can be measured in advance, and becomes zero when P _oj = P _ok . From the intensity ratio given by equation (12), α _{jk, m} can be calculated as follows.
[0055]
[Formula 13]

For example, the three-dimensional display method shown in FIG. 3 is effective for imaging the difference attenuation degree obtained as described above.
[0056]
An optical tomographic image (see FIG. 7) obtained by a conventional optical coherence tomographic image measurement method using a single photodetector uses the change in reflected light intensity depending on the structure as image information. A major feature is that image information is a change in differential attenuation due to the wavelength dependence of the scattering characteristics.
[0057]
Furthermore, the optical coherence tomographic image measurement apparatus according to the present invention shown in FIG. 1 has both a tomographic image measurement function using a single photodetector and a spectral image measurement function using a wavelength dispersion element and a sensor array. It is possible to obtain structural and spectral information on the inside of the measured object at the same time in one measurement, so that an image for visualizing the spectral structure can be constructed.
[Example 1]
FIG. 4 shows an embodiment in which a continuous output SLD is used as a low coherent light source in the optical coherence tomographic image measurement apparatus of FIG. 1 according to the present invention.
[0058]
In this figure, 21 is an SLD (light source), 22 is a first lens, 23 is a first arm of an optical fiber coupler 24 made of an optical fiber, 24 is an optical fiber coupler, and 25 is an optical fiber coupler 24 made of an optical fiber. The second arm 26, 27 are second and third lenses, 28 is a galvano scanner, 29 is an object to be measured (subject), 30 is a third arm of an optical fiber coupler made of an optical fiber, 31 is The fourth lens, 32 is a Z-scan mirror (reflecting mirror), 33 is a fourth arm of an optical fiber coupler made of an optical fiber, 34 is a fifth lens, 35 is a beam splitter, and 36 is a single photodetector. , 37 is a diffraction grating, and 38 is a sensor array.
[0059]
Here, when a commercially available near-infrared region SLD21 is used, the coherent length is lc≈30 μm, and in the case of a light emitting diode (LED), lc≈10 μm.
[0060]
Further, the embodiment of FIG. 4 is characterized in that the arrangement of the measured object 29 is separated from the optical image measuring device by using the optical fiber as the signal light and reference light transmission means of FIG. 1 according to the present invention. .
[0061]
The output light from the SLD (light source) 21 is coupled to the first arm 23 of the optical fiber coupler 24 by the first lens 22 and then divided into signal light and reference light. The signal light is propagated to the measured object 29 side by the second arm 25. The signal light emitted from the second arm 25 is focused and incident on the measured object 29 by the second lens 26 and the third lens 27. For example, a galvano scanner 28 is preferably used to perform a lateral scanner. The reflected light from the measured object 29 is again coupled to the second arm 25 of the optical fiber coupler 24 by the third lens 27 and the second lens 26 and returned to the optical fiber coupler 24.
[0062]
On the other hand, the reference light is transmitted to the reflecting mirror 32 by the third arm 30 of the optical fiber coupler 24. The reflected light from the reflecting mirror 32 is returned to the optical fiber coupler 24 by the third arm 30 of the optical fiber coupler 24 and combined with the reflected light from the measured object 29.
[0063]
In the embodiment of FIG. 4 using the low-coherent light source 21, optical interference is generated only when the optical path length difference between the signal light and the reference light is within an extremely short coherent length of the light source. Therefore, for example, it is desirable to scan the position of the reflecting mirror 32 and adjust the difference so that the optical path length of the reference light becomes equal to the optical path length of the signal light.
[0064]
Further, as described above, the present invention detects in parallel the interference light dispersed by the diffraction grating 37 using a sensor array 38, for example, a one-dimensional or two-dimensional photodetector array in which light receiving elements are spatially arranged. It is characterized by. For this purpose, an envelope detector array in which a plurality of rectifier detectors are arranged in parallel is considered useful as a parallel signal processing system.
[Example 2]
FIG. 5 shows an embodiment in which an image sensor such as a CCD (Charge Coupled Device) camera is used as a sensor array in the optical tomographic image measuring apparatus shown in FIG. 1 according to the present invention.
[0065]
In FIG. 5, 41 is a low coherent light source (broadband light source) having a wide spectral width, for example, a superluminescent diode (SLD), 42 is a first beam splitter, 43 is a mirror, 44 is PZT, and 45 is measured. Body, 46 is a second beam splitter, 47 is a single photodetector, 48 is a diffraction grating, 49 is a lens, 50 is a third beam splitter, 51 is a first shutter, 52 is a first CCD camera, 53 is a second shutter, and 54 is a second CCD camera.
[0066]
Here, since a commercially available CCD camera has hundreds of thousands to millions of light receiving elements, high-resolution spectral image measurement can be performed by effectively using such a large number of light receiving elements.
[0067]
As shown in FIG. 5, the interference light dispersed by the diffraction grating 48 is collimated by a lens 49 and then divided into two by a third beam splitter 50. The two split light beams are incident on the first CCD camera 52 and the second CCD camera 54, respectively. Commercially available cameras generally have low responsiveness (about 30 Hz), and in this embodiment, a high-speed shutter is disposed in front of the CCD camera in order to apply to detection of interference signals having beat frequencies of several hundred Hz or higher. It is characterized by doing.
[0068]
Although the first optical shutter 51 and the second optical shutter 53 are shown in FIG. 5, a high-speed shutter using, for example, a liquid crystal element or an electro-optical element is desirable. The shutters 51 and 53 periodically block the interference light at the same frequency as the beat frequency of the interference light, and generate two rows of interference light pulses having a phase difference of 90 degrees. These two optical pulse trains are incident on the first CCD camera 52 and the second CCD camera 54, respectively, and photocharges are accumulated within the light receiving time of the CCD cameras 52 and 54.
[0069]
In addition, this invention is not limited to the said Example, A various deformation | transformation is possible based on the meaning of this invention, and these are not excluded from the scope of the present invention.
[0070]
【The invention's effect】
As described above in detail, according to the present invention, it is not necessary to perform signal processing such as Fourier transform, and the depth of reflected light and spectral information can be simultaneously acquired in real time.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an optical coherence tomographic image measurement apparatus having a spectral function according to the present invention.
FIG. 2 is a diagram showing a three-dimensional display example (part 1) in the optical coherence tomographic image measurement apparatus of the present invention.
FIG. 3 is a diagram showing a three-dimensional display example (No. 2) in the optical coherence tomographic image measurement apparatus of the present invention.
FIG. 4 is a diagram showing an embodiment in which a continuous output SLD is used as a low coherent light source in the optical tomographic image measurement apparatus according to the present invention.
FIG. 5 is a diagram showing an embodiment in which an image sensor is used as a sensor array in the optical tomographic image measurement apparatus according to the present invention.
FIG. 6 is a basic configuration diagram of an optical coherence tomographic image measurement apparatus using a conventional Michelson interferometer.
FIG. 7 is a diagram illustrating a three-dimensional display example in a conventional tomographic image measurement apparatus.
FIG. 8 is a diagram showing another configuration of an optical coherence tomographic image measurement apparatus using a conventional Michelson interferometer.
FIG. 9 is a diagram showing still another configuration of an optical coherence tomographic image measurement apparatus using a conventional Michelson interferometer.
[Explanation of symbols]
1,41 Low coherent light source (broadband light source)
1A, 1B, 49 Lens 2 First beam splitter (BS1)
3,43 mirror 4,44 PZT
5, 29, 45 Object to be measured (subject)
6 Second beam splitter (BS2)
7, 36, 47 Single light detector 8, 37, 48 Diffraction grating 9, 38 Sensor array 10 Envelope detector 11 Tomographic image processing system 12 Spectral image processing system 21 SLD
DESCRIPTION OF SYMBOLS 22 1st lens 23 1st arm of the optical fiber coupler which consists of optical fibers 24 Optical fiber coupler 25 2nd arm of the optical fiber coupler which consists of optical fibers 26 2nd lens 27 3rd lens 28 Galvano scanner 30 Light Third arm of optical fiber coupler made of fiber 31 Fourth lens 32 Z-scan mirror (reflecting mirror)
33 4th arm of optical fiber coupler comprising optical fiber 34 5th lens 35 beam splitter 42 1st beam splitter 46 2nd beam splitter 50 3rd beam splitter 51 1st shutter 52 1st CCD camera 53 Second shutter 54 Second CCD camera

Claims (7)

(A ) a light source that emits a light beam;
(B) The light beam emitted from the light source is converted into signal light that passes through the subject placement position where the subject is placed and reference light that passes through an optical path different from the optical path that passes through the subject placement position. And an interference optical system that generates interference light by superimposing the signal light after passing through the subject arrangement position and the reference light passing through the different optical paths,
(C) the interference optical system includes a frequency shifter that relatively shifts the frequency of the signal light and the frequency of the reference light;
(D) The interference optical system divides the interference light into two parts to receive the interference light, and further receives one of the two divided interference lights, and the signal light and the reference A single photodetector for measuring tomographic images with high depth resolution by generating optical interference with light in the entire spectral range of the light source;
( E ) a wavelength dispersion element for splitting the other of the two divided interference lights;
( F ) an optical sensor for heterodyne detection of the separated interference light independently;
( G ) The optical sensor has a plurality of light receiving elements that are spatially arranged and independently detect heterodyne, and further detects an amplitude of a plurality of heterodyne signals obtained by the optical sensor. A detection unit;
(H) A plurality of heterodyne signal amplitudes obtained by the amplitude detection unit are integrated to each point of interest on the propagation path of the signal light on the surface or inner layer of the subject placed at the subject placement position. An optical coherence tomographic image measurement apparatus having a spectroscopic function, comprising a signal processing unit that generates a corresponding signal.   2. The optical coherence tomographic image measuring apparatus having a spectroscopic function according to claim 1, wherein the wavelength dispersion element is a diffraction grating.   2. The optical coherence tomographic image measuring apparatus having a spectroscopic function according to claim 1, wherein a Michelson interferometer is used as the interference optical system.   2. The optical coherence tomographic image measuring apparatus having a spectroscopic function according to claim 1, wherein the light source is a superluminescent diode.   2. The optical coherence tomographic image measuring apparatus having a spectroscopic function according to claim 1, wherein the light source is a light emitting diode.   2. The optical coherence tomographic image measurement apparatus having a spectroscopic function according to claim 1, wherein the light source is a femto laser light source.   The optical coherence tomographic image measurement apparatus having a spectroscopic function according to claim 1, wherein the subject is a light scattering medium.

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