JP3628615B2 - Heterodyne beat image synchronous measurement device - Google Patents

Description

のようになる。さらに、３つのタイミングで照射した画像、つまり、３つの時間領域で時間積分した画像を、Ｉ₀ 、Ｉ₁ 、Ｉ₂ とする。すなわち、
Ｉ_HB（ｔ）に対して以下の３つの時間領域で積分したものをＩ₀ ，Ｉ₁ ，Ｉ₂ とする。
【００２０】
Ｉ： ｔ₀ ＋ＮＴ／４−τ／２≦ｔ≦ｔ₀ ＋ＮＴ／４＋τ／２ （Ｎ＝０）
II： ｔ₀ ＋ＮＴ／４−τ／２≦ｔ≦ｔ₀ ＋ＮＴ／４＋τ／２ （Ｎ＝１）
III ：ｔ₀ ＋ＮＴ／４−τ／２≦ｔ≦ｔ₀ ＋ＮＴ／４＋τ／２ （Ｎ＝２）
…（３）
変調信号に対する全体の時間遅延ｔ₀ で、それぞれの時間遅延は、０、周期／４、周期／２であり、積分時間はτである。その結果、画像信号Ｉ₀ ，Ｉ₁ ，Ｉ₂ はそれぞれ、
【００２１】
【数２】

【００２２】
【数３】

【００２３】
【数４】

になる。ここで、
【００２４】
【数５】

【００２５】
【数６】

のようにＳ₁ 、Ｓ₂ を求める。
【００２６】
ここで、ｔ₀ については、ｔ₀ ＝Ｔ／４が１つのよい条件である。
【００２７】
そこで、ｔ ₀ ＝Ｔ／４が望ましい理由について、以下説明する。
【００２８】
まず、第１に、δによらずＩ_S が求められる条件としてＳ₂ のｓｉｎδの項（第２番目の項）が０になる必要がある。つまり、ｃｏｓ（２ｎ＋１）ωｔ₀ ＝０である。
【００２９】
このとき、
（２ｎ＋１）ωｔ₀ ＝（２ｎ＋１）２πｔ₀ ／Ｔ＝（ｎ′＋１／２）π
（全てのｎに対してｎ′が対応すればよい）

ｔ₀ がｎにならず、ある一定値になるには、ｎ＝ｎ′であり、
∴ｔ₀ ＝Ｔ／４
となればよい。
【００３０】
このように、変調信号に対する全体の時間遅延ｔ₀ は、周期／４が好ましいことが解析的に得られる。
【００３１】
更に、ｔ ₀ ＝Ｔ／４であれば、第２に、上記Ｓ ₂ 式において、δによらずＩ _S が求められる条件としてＳ ₂ のｃｏｓδの項（第１番目の項）も０にすることができる。
【００３２】
以下、その点にＩついて説明する。
【００３３】
また、τについても周期／４にすると値が安定することが、図３に示すように、数値計算より明らかとなる。
【００３４】
なお、図３（Ａ）において
τ＝ａＴ、ｓｉｎ２ｎπ（τ／Ｔ）＝ｓｉｎ２ｎπａ
ｎ＝１，２，３，４，…
であり、ｎが偶数のときはｓｉｎ２ｎπａ＝０、奇数のときはｓｉｎ２ｎπａ≠０である。
【００３５】
このように、周期／４にすると、ｓｉｎ２ｎπａ＝０を得ることができ、上記Ｓ ₂ 式において、δによらずＩ _S が求められる条件としてＳ ₂ のｃｏｓδの項（第１番目の項）も０にすることができる。
【００３６】
また、図３（Ｂ）において、
τ＝ａＴ、ｓｉｎ（２ｎ＋１）π（τ／Ｔ）
ｎ＝１，２，３，４，…，に対して、
ｓｉｎ（２ｎ＋１）π（τ／Ｔ）＝ｓｉｎ（２ｎ＋１）πａ≠０
以上より、ａ＝０．２５又は０．７５〔τ＝（Ｔ／４）又は（３Ｔ／４）〕のとき値は０とならずに安定に求まる。
【００３７】
これより、Ｓ₁ ，Ｓ₂ は、
【００３８】
【数７】

【００３９】
【数８】

になる。この時、
Ｉ_A ² ＝（Ｓ₁ ／Ａ）² ＋（Ｓ₂ ／Ｂ）² ＝４Ｉ_r Ｉ_S …（７）
より、求めるＩ_S は、
∴Ｉ_S ＝（１／４Ｉ_r ）｛（Ｓ₁ ／Ａ）² ＋（Ｓ₂ ／Ｂ）² ｝ …（８）
のように求められる。Ｉ_S は、高次高調波を含んだ解析にも関らずδを含んでいないので、位相揺らぎに対して安定であることがわかる。
【００４０】
次に、本発明の測定システムについて説明する。
【００４１】
図１は本発明のヘテロダインビート画像同期測定システムの構成図である。
【００４２】
この図に示されるように、断続的なＯＮ、ＯＦＦが出来る光源１、そのドライバー２、サンプル４からの干渉画像を撮像デバイス（ＣＣＤカメラ）７面上に結像する結像干渉光学系３、参照光の位相変調器５、その位相変調器５のドライバー６、撮像デバイス７のコントローラ８、画像処理とシステム全体制御を行うコンピュータ９で構成される。
【００４３】
図２は本発明にかかるヘテロダインビート画像同期測定タイミングチャートである。つまり、図２（ａ）は全体のシーケンスを示しており、Ｎ周期の画像データは蓄積・表示・クリアを繰り返す。図２（ｂ）はそのシーケンスのエレメントの拡大図、図２（ｃ）はシーケンス図であり、Ｅｘｐは照射、Ｓは画像データセーブ、Ｃａｌは演算を示している。図２（ｄ）は照射信号のタイミングチャート、図２（ｅ）は変調信号の波形図、図２（ｆ）はヘテロダインビート信号の波形図である。
【００４４】
このヘテロダインビート信号の計測には、浜松ホトニクスＣ−４８８０−８０を用い、ビニング（２×２）３２８（Ｈ）×２４７（Ｖ）、フレームレート５３Ｈｚ（１９ｍｓ）、Ｔ／４＝４．７５ｍｓ、変調周波数５３Ｈｚ、１フレーム／ｓｅｃとすると５３回の積算である。
【００４５】
変調信号〔図２（ｅ）〕に同期して、照射信号〔図２（ｄ）〕により光源は断続的にスイッチングされる。最初の照射信号〔図２（ｄ）〕の最初のパルスの中心は、変調信号〔図２（ｅ）〕より周期／４だけ遅れており、パルス幅はほぼ周期／４である。Ｅｘｐ１により画像Ｉ₀ が得られセーブ（Ｓ１）、Ｅｘｐ２により画像Ｉ₁ が得られセーブ（Ｓ２）、Ｅｘｐ３により画像Ｉ₂ が得られ、式（８）に従って演算処理され、セーブ（Ｓ３）される。この一連の処理を１周期（Ｐ１）とする。以後、Ｓ３によって得られた画像をＮ回積算して最終画像とする。つまり、Ｎ回積算・表示・クリア・Ｎ回積算・表示・クリアを繰り返す。最終的にＮ回積算を行った画像を断層画像として出力する。
【００４６】
上記したように、
位相揺らぎ抑制方法としては、
〔方法１〕
もし、Ａ（φ_o ，τ＝Ｔ／４）＝Ｂ（φ_o ，τ＝Ｔ／４）を満たすφ_o が求まれば、

∴Ｉ_S ＝Ｉ_C ² ／８Ａ² Ｉ_r
として、Ｉ_S が求まる。
〔方法２〕
Ｉ_A ² ＝（Ｓ₁ ／Ａ）² ＋（Ｓ₂ ／Ｂ）² ＝４Ｉ_r Ｉ_S …（７）
∴Ｉ_S ＝（１／４Ｉ_r ）｛（Ｓ₁ ／Ａ）² ＋（Ｓ₂ ／Ｂ）² ｝ …（８）
として、Ｉ_S が求められる。これが１画素でのサンプルからの後方散乱光強度であり、この処理を全ての画素に対して行うことにより画像データが得られる。
【００４７】
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。
【００４８】
【発明の効果】
以上、詳細に説明したように、本発明によれば、高次高調波を考慮した上で、画像全体に有効な位相揺らぎを抑制する方法を講じたために、大変有効である。
【００４９】
したがって、実時間表示の生体用鉛直断面画像測定装置が実現され、基礎医学から臨床医学の分野にわたって、いままでわからなかった様々なことが解明可能である。よって、医学分野、さらに、半導体や他の産業分野への波及効果は多大である。
【図面の簡単な説明】
【図１】本発明のヘテロダインビート画像同期測定システムの構成図である。
【図２】本発明にかかるヘテロダインビート画像同期測定タイミングチャートである。
【図３】変調信号に対する全体の時間遅延の数値結果を示す図である。
【図４】従来のヘテロダインビート画像測定システムの模式図である。
【図５】従来のへテロダインビート画像検出原理の説明図である。
【符号の説明】
１ 光源
２ ドライバー
３ 結像干渉光学系
４ サンプル
５ 参照光の位相変調器
６ 位相変調器のドライバー
７ 撮像デバイス（ＣＣＤカメラ）
８ 撮像デバイスのコントローラ
９ 画像処理とシステム全体制御を行うコンピュータ[0001]
BACKGROUND OF THE INVENTION
The present invention, even in the presence of random phase fluctuations in the heterodyne beat signal, it relates to a heterodyne beat image sync measurement TeiSo location of the heterodyne beat image can be measured stably.
[0002]
[Prior art]
Conventionally, there have been the following technologies in such fields.
[0003]
FIG. 4 is a schematic diagram of such a conventional heterodyne beat image measurement system, and FIG. 5 is an explanatory diagram of its detection principle.
[0004]
FIG. 4 shows an experimental optical system based on a reflective Mach-Zehnder interferometer. A super luminescent diode (SLD) having a wavelength λ = 813 nm and a wavelength width Δλ = 16 nm is used as a low coherence light source. Using. Incident light passes through the lens L1, and is divided into signal light and reference light by a beam splitter (BS1). The signal light and the reference light are frequency-shifted to 79.9 MHz and 79.96 MHz by two acousto-optic elements (AOM), respectively, to obtain a beat frequency (Δf = 40 kHz). The signal light back-scattered from the subject is combined with the reference light again at BS2, and is imaged on the CCD camera through the lens pair L2, L3 that expands to a magnification of 2 times.
[0005]
In order to apply the CCD array, which is a charge storage sensor, to optical heterodyne detection, a frequency synchronization method was newly introduced in the optical heterodyne detection method. In the interferometer of FIG. 4, both AOMs are driven by a square wave having a frequency equal to the beat frequency Δf, so that both the signal light and the reference light are made into a pulse train having the frequency Δf. As shown in FIG. 5, the time waveform of the optical interference signal is sampled by Δf on the detection surface of the CCD. Due to the low response characteristics (˜30 Hz) of the CCD element, each element functions as a low-frequency pass filter, and accumulates and outputs the charge amount corresponding to the heterodyne signal having a high frequency within the observation time.
[0006]
[Problems to be solved by the invention]
When an imaging interference optical system is used for two-dimensional tomographic image measurement, the conventional AOM is not suitable because the beam system is thin, and phase modulation by a half mirror with a piezo or non-linear type by an electro-optic effect Phase modulation using crystals.
[0007]
In the latter case, the beam width is several millimeters in consideration of the beam width, a high voltage is required, and a high voltage power supply having a sufficiently high frequency characteristic is not easy.
[0008]
Accordingly, the heterodyne beat signal includes high-order harmonics. Further, the phase fluctuation between the reference wave and the signal wave easily occurs due to fluctuations in the environment such as temperature. These are important issues for practical application.
[0009]
As described above, the above-described conventional frequency synchronization method cannot cope with a case where a higher-order harmonic is included in the heterodyne beat signal.
[0010]
In addition, there is a problem that the heterodyne beat signal cannot be dealt with when phase fluctuation is randomly included.
[0011]
In view of the above situation, the present invention performs a study in consideration of higher-order harmonics included in a heterodyne beat signal, and even if there is a random phase fluctuation in the heterodyne beat signal, the heterodyne beat image is stably obtained. There is an object to provide a heterodyne beat image synchronization measurement TeiSo location that can be measured.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides
[ 1 ] In a heterodyne beat image synchronous measurement apparatus, a light source capable of controlling light intensity, a driver of the light source , a beam splitter for dividing light from the light source into signal light and reference light, and a phase of the reference light A phase modulator to modulate, a driver of the phase modulator, the signal light is irradiated onto the sample, the signal light from the sample and the modulated reference light are combined by the beam splitter, and an interference image is obtained. An imaging device that forms an image on a surface, a controller for the imaging device, and a computer that performs image processing and overall system control, and is synchronized with a modulation signal [FIG. 2 (e)] from the driver of the phase modulator Then, the light source is intermittently switched by the irradiation signal from the driver of the light source (FIG. 2 (d)), and the first pulse of the first irradiation signal (FIG. 2 (d)) is changed. The modulated signal [Fig. 2 (e)] is delayed by the period / 4 from the pulse width is approximately periodic / 4, the image I ₀ is obtained by the first irradiation, the image I ₁ is obtained by the second irradiation is, the image I ₂ is obtained by the third irradiation, by the computer, by performing arithmetic processing according to the following equation, as well as consider the high-order harmonics of the heterodyne beat signal on a pixel of the imaging device The signal light intensity I _S is obtained without depending on the phase fluctuation component δ, and a heterodyne beat image is stably measured even when random phase fluctuation exists.
[0013]
I _S = (1 / 4I _r ) {(S ₁ / A) ² + (S ₂ / B) ² }
Here, I _r is the reference light intensity, S ₁ is I ₀ -I ₁ , S ₂ is I ₀ + I ₂ -2I ₁ , and A and B are constants.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail.
[0015]
First, the principle of the heterodyne beat image synchronization measurement method [ILM (Image Lock-in Measurement)] of the present invention will be described.
[0016]
In the interference imaging optical system, one pixel is the same as a normal heterodyne beat signal, I _HB (t) = I _r + I _S + 2√ (I _r · I _S ) × cos [φ (t) + δ] (1 )
It is represented by
[0017]
Here, Ir: reference light intensity, Is: signal light intensity, φ (t): phase modulation component, and δ: phase fluctuation component.
[0018]
The final purpose here is to obtain the signal intensity I _S without depending on the phase fluctuation component δ, that is, to represent I _S without using δ. Assuming standard sinusoidal phase modulation and using the Bessel function:
[Expression 1]

become that way. Furthermore, an image irradiated at three timings, that is, an image that is time-integrated in three time regions is defined as I ₀ , I ₁ , and I ₂ . That is,
Those integrated in three time domain with respect to I _HB (t) and _{I 0, I 1, I 2} .
[0020]
I: t ₀ + NT / 4−τ / 2 ≦ t ≦ t ₀ + NT / 4 + τ / 2 (N = 0)
II: t ₀ + NT / 4−τ / 2 ≦ t ≦ t ₀ + NT / 4 + τ / 2 (N = 1)
III: t ₀ + NT / 4−τ / 2 ≦ t ≦ t ₀ + NT / 4 + τ / 2 (N = 2)
... (3)
With an overall time delay t ₀ for the modulation signal, the respective time delays are 0, period / 4, period / 2, and the integration time is τ. As a result, the image signals I ₀ , I ₁ , I ₂ are respectively
[0021]
[Expression 2]

[0022]
[Equation 3]

[0023]
[Expression 4]

become. here,
[0024]
[Equation 5]

[0025]
[Formula 6]

S ₁ and S ₂ are obtained as follows.
[0026]
Here, for t ₀ , t ₀ = T / 4 is one good condition.
[0027]
The reason why t ₀ = T / 4 is desirable will be described below.
[0028]
First, as a condition for obtaining I _S regardless of δ, the sin δ term (second term) of S ₂ needs to be zero. That is, cos (2n + 1) ωt ₀ = 0.
[0029]
At this time,
(2n + 1) ωt ₀ = (2n + 1) 2πt ₀ / T = (n ′ + 1/2) π
(N ′ should correspond to all n)

For t ₀ not to be n but to be a certain value, n = n ′,
∴t ₀ = T / 4
If it becomes.
[0030]
Thus, it can be analytically obtained that the overall time delay t ₀ for the modulation signal is preferably a period / 4.
[0031]
Moreover, if t ₀ = T / 4, the second, in the S ₂ expression, the term cosδ of S ₂ as a condition of I _S is required regardless of the [delta] (1st term) is also zero be able to.
[0032]
Hereinafter, this point will be described.
[0033]
Further, as shown in FIG. 3, it becomes clear from numerical calculation that the value of τ is stable when the period is / 4.
[0034]
In FIG. 3A, τ = aT, sin2nπ (τ / T) = sin2nπa
n = 1, 2, 3, 4,.
When n is an even number, sin2nπa = 0, and when n is an odd number, sin2nπa ≠ 0.
[0035]
Thus, when the period / 4, it is possible to obtain a sin2nπa = 0, in the S ₂ equation, terms (1st term) of cosδ of S ₂ as a condition of I _S is required regardless of the δ even Can be zero.
[0036]
In FIG. 3B,
τ = aT, sin (2n + 1) π (τ / T)
For n = 1, 2, 3, 4,.
sin (2n + 1) π (τ / T) = sin (2n + 1) πa ≠ 0
From the above, when a = 0.25 or 0.75 [τ = (T / 4) or (3T / 4)], the value does not become 0 but can be obtained stably.
[0037]
From this, S ₁ and S ₂ are
[0038]
[Expression 7]

[0039]
[Equation 8]

become. This time,
I _A ² = (S ₁ / A) ² + (S ₂ / B) ² = 4I _r I _S (7)
Therefore, the required I _S is
∴I _S = (1 / 4I _r ) {(S ₁ / A) ² + (S ₂ / B) ² } (8)
It is required as follows. Since I _S does not include δ in spite of analysis including high-order harmonics, it is understood that I _S is stable against phase fluctuation.
[0040]
Next, the measurement system of the present invention will be described.
[0041]
FIG. 1 is a configuration diagram of a heterodyne beat image synchronous measurement system of the present invention.
[0042]
As shown in this figure, an imaging interference optical system 3 that forms an interference image from a light source 1 that can be intermittently turned on and off, its driver 2 and a sample 4 on the surface of an imaging device (CCD camera) 7, A reference light phase modulator 5, a driver 6 for the phase modulator 5, a controller 8 for the imaging device 7, and a computer 9 for performing image processing and overall system control.
[0043]
FIG. 2 is a heterodyne beat image synchronization measurement timing chart according to the present invention. That is, FIG. 2A shows the entire sequence, and N-cycle image data is repeatedly accumulated, displayed, and cleared. 2B is an enlarged view of the elements of the sequence, FIG. 2C is a sequence diagram, Exp is irradiation, S is image data saving, and Cal is calculation. 2D is a timing chart of the irradiation signal, FIG. 2E is a waveform diagram of the modulation signal, and FIG. 2F is a waveform diagram of the heterodyne beat signal.
[0044]
This heterodyne beat signal is measured using Hamamatsu Photonics C-4880-80, binning (2 × 2) 328 (H) × 247 (V), frame rate 53 Hz (19 ms), T / 4 = 4.75 ms, If the modulation frequency is 53 Hz and 1 frame / sec, the integration is 53 times.
[0045]
In synchronization with the modulation signal [FIG. 2 (e)], the light source is intermittently switched by the irradiation signal [FIG. 2 (d)]. The center of the first pulse of the first irradiation signal [FIG. 2D] is delayed by a period / 4 from the modulation signal [FIG. 2E], and the pulse width is substantially a period / 4. The image I ₀ is obtained and saved by Exp1 (S1), the image I ₁ is obtained and saved by Exp2 (S2), the image I ₂ is obtained by Exp3, and is processed according to the equation (8) and saved (S3). . This series of processing is defined as one cycle (P1). Thereafter, the images obtained in S3 are integrated N times to obtain the final image. That is, N times integration / display / clear / N times integration / display / clear are repeated. The image that has been finally integrated N times is output as a tomographic image.
[0046]
As mentioned above,
As a method for suppressing phase fluctuation,
[Method 1]
If φ _o satisfying A (φ _o , τ = T / 4) = B (φ _o , τ = T / 4) is obtained,

∴I _S = I _C ² / 8A ² I _r
As, I _S is obtained.
[Method 2]
I _A ² = (S ₁ / A) ² + (S ₂ / B) ² = 4I _r I _S (7)
∴I _S = (1 / 4I _r ) {(S ₁ / A) ² + (S ₂ / B) ² } (8)
As, I _S is required. This is the intensity of backscattered light from the sample at one pixel, and image data can be obtained by performing this process on all pixels.
[0047]
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.
[0048]
【The invention's effect】
As described above in detail, according to the present invention, in consideration of the high-order harmonics, in order to have taken a method of inhibiting effective phase fluctuation to the entire image, it is very effective.
[0049]
Therefore, a real-time display vertical cross-sectional image measuring apparatus for living bodies is realized, and various things that have not been understood in the field of basic medicine to clinical medicine can be clarified. Therefore, the ripple effect on the medical field and further on the semiconductor and other industrial fields is great.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a heterodyne beat image synchronous measurement system of the present invention.
FIG. 2 is a heterodyne beat image synchronization measurement timing chart according to the present invention.
FIG. 3 is a diagram illustrating a numerical result of an overall time delay with respect to a modulation signal.
FIG. 4 is a schematic diagram of a conventional heterodyne beat image measurement system.
FIG. 5 is an explanatory diagram of a conventional heterodyne beat image detection principle.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Light source 2 Driver 3 Imaging interference optical system 4 Sample 5 Reference light phase modulator 6 Phase modulator driver 7 Imaging device (CCD camera)
8 Image pickup device controller 9 Computer for image processing and overall system control

Claims (1)

(A) a light source whose light intensity can be controlled;
(B) a driver for the light source;
(C) a beam splitter that divides light from the light source into signal light and reference light;
And (d) a phase modulator for modulating the phase of the reference light,
(E) a driver for the phase modulator;
( F ) An imaging device that irradiates the sample with the signal light, combines the signal light from the sample and the modulated reference light with the beam splitter, and forms an interference image on the surface ;
(G) a controller imaging device,
( H ) a computer for image processing and overall system control;
(I) The light source is intermittently switched by the irradiation signal from the driver of the light source in synchronization with the modulation signal from the driver of the phase modulator, and the center of the first pulse of the first irradiation signal is from the modulation signal Delayed by period / 4, the pulse width is approximately period / 4, the image I ₀ is obtained by the first irradiation, the image I ₁ is obtained by the second irradiation, and the image I ₂ is obtained by the third irradiation. , by the computer, by performing arithmetic processing according to the following equation, with a high-order harmonic considered to take into contained in the heterodyne beat signal on a pixel of the imaging device, a signal light intensity I _S of the phase fluctuation component δ A heterodyne beat image synchronous measurement apparatus characterized in that a heterodyne beat image is obtained without being dependent and stably measures a heterodyne beat image even in the presence of random phase fluctuations.
I _S = (1 / 4I _r ) {(S ₁ / A) ² + (S ₂ / B) ² }
Here, I _r is the reference light intensity, S ₁ is I ₀ -I ₁ , S ₂ is I ₀ + I ₂ -2I ₁ , and A and B are constants.

Images