KR101050369B1 - Coherent Vanstock Raman Scattering Endoscope Optical Bandgap Optical Fiber - Google Patents

Abstract

The present invention proposes a design of a new photonic bandgap fibers structure for implementing a coherent anti-stokes Raman Scattering (CARS) endoscope system. The proposed fiber is a solid core double cladding optical bandgap optical fiber, which has a function of guiding light only at a specific wavelength (pump and Stokes light source wavelength) by the optical bandgap effect. In addition, it has a large core area to suppress the nonlinearity of the pump and Stokes light to be guided, and at the same time shows a low dispersion characteristic in the operating region due to the bandgap waveguide effect. Finally, the proposed fiber includes a double cladding fiber structure with high numerical aperture (Numerical Aperture> 0.5).

Spontaneous Raman scattering, coherent antistock Raman scattering, nonlinear optical endoscope, wide bandgap optical fiber

Description

Photonic Bandgap Fibers for High Efficiency Coherent Anti-stokes Raman Scattering Endoscope

The present invention relates to an optical waveguide (optical fiber) design, which is a key component of the CARS endoscope, and more specifically, a pump for generating a CARS signal, a Stokes beam, and without distortion, using the optical bandgap effect, and a generated CARS signal. Design and computational simulation of a new optical bandgap optical fiber capable of collecting highly efficient

Traditional optical microscopes have had difficulty acquiring morphological and chemical distribution images of biostructures (cells or tissues) and various structures within cells that have almost similar linear characteristics of light between the object and the background material. This is because optical contrast such as linear absorption and reflection of light is not large in the bio sample. In order to overcome this problem, studies have been actively conducted to detect the characteristics and behavior of cells by staining fluorescent markers on samples and acquiring fluorescence distribution of irradiated light. However, it was difficult to obtain a distribution image of the intact sample because the fluorescent material changed the characteristics of the bio sample itself or the fluorescence property decreased over time.

Recently, a technique for obtaining a cell image by detecting a unique property of the material itself without a marker such as two-photon emission fluorescence, secondary, tertiary harmonic generation, and Raman scattering spectroscopy has been attracting attention. In particular, Raman scattering spectroscopy is a molecular vibration frequency. Irrespective of this, any single wavelength light source can be used, so there is an advantage in that the selection of the laser light source is easy and the operation is simple. However, the intensity of the Raman scattering signal is extremely weak, and it takes a long time to acquire an image, and there is a limit in observing the dynamic characteristics of cells in living biological samples. Coherent Anti-stokes Raman scattering (CARS) microscopy, designed to overcome this limitation, utilizes the Raman nonlinear effect of light to allow three incident laser beams to interact within a sample, resulting in a single CARS signal beam. It uses the principle of mixed light wave to generate

1 shows the principle of CARS spectroscopy using Raman transition. When two laser beams (pump beam and Stokes beam) having a frequency difference by the Raman shift of a specific molecule are incident on a biosample to be measured, a beat waveform corresponding to the difference frequency is generated. This waveform induces coherent forced harmonic oscillation, when a third laser light (search light) is incident on molecules oscillating in phase, which is then pumped after interaction with the molecular vibration. Induces anti-Stokes Raman signals in the shorter wavelength range. The generated signals become coherent light sources with mutually matched phases and specific orientations. Precise high-speed mapping of such nonlinear optical signals in sample space results in CARS microscopic images.

CARS microscope, which operates on the basis of this principle, has a very high measurement sensitivity compared to the existing Raman scattering microscope, which enables fast image acquisition. Since the intensity of the reproduced nonlinear signal is determined by the third order nonlinear effect of the incident light, the CARS signal can be obtained in proportion to the cube of the applied electric field strength. Therefore, in order to maximize the nonlinear effect of light, a high peak power pulse laser is generally used. CARS microscopy is a non-invasive measurement method that avoids thermal damage caused by laser in principle because it does not leave any laser energy on the sample after laser interaction. It is a three-dimensional image of the inside of the sample through the focusing and scanning of light. Can be obtained with high spatial resolution. Based on these advantages, CARS microscopy has attracted great attention in the field of nonlinear bioimaging.

However, in order to observe tissue images of living organisms in real time or to actively use them in pharmacy, such as clinical diagnosis, using CARS-based bio-imaging technology, it is desirable to implement them in an endoscopic form rather than a microscope form. Endoscopic optical imaging has been successfully implemented in classical optical microscopy and has recently been shown to include optical coherence tomography, two photon emission fluorescence, and second harmonic generation imaging. Endoscopic image acquisition is also active in these fields.

Figure 2 shows a schematic diagram for implementing the CARS system with an endoscope. As shown in the figure, an optical fiber waveguide capable of efficiently collecting the generated CARS signal after focusing a strong intensity pump and Stokes laser pulses on a sample is essential. However, the general single mode optical fiber operating in the 600 ~ 1000 nm band, which is the operating region of the CARS microscope, has the nonlinearity of the high power pulsed laser (pump and stokes light source) traveling through the waveguide due to the limited size of the core mode (6 mm or less in diameter of the core mode). It may cause distortion. In addition, the intrinsic dispersion characteristic of silica optical fiber causes the chirping to spread the pulses, which reduces the peak power and at the same time spatially separate the positions of each wavelength within a single pulse. Due to this linear and nonlinear nature of the fiber, changes in the pump Stokes light source can significantly reduce the efficiency of CARS signal generation. In addition, the CARS endoscope needs to collect backscattered CARS signals having a small signal strength and a large scattering angle due to the characteristics of the structure. Therefore, a new type of optical waveguide has been required to realize a highly sensitive and efficient CARS endoscope.

Photonic crystal fiber, which was first experimentally implemented in the late 1990s, can easily change the waveguide characteristics such as light dispersion and mode size through the fiber by appropriately designing the cross-sectional structure of the silica / air hole inside the fiber. Has received great attention. In particular, it can operate in a single mode over all wavelengths, or can be guided to a low refractive index core by the optical bandgap effect, and bipolar nonlinearity can be realized to realize high power fiber lasers, optical bandgap optical fibers, and soliton progression. , Pulse compression and broadband light source reproduction have been studied with great interest. In order to implement CARS endoscopes, studies using single-mode optical fibers or photonic crystal fibers have been proposed, but until now, photonic crystal fibers that have overcome the limitations of existing optical fibers have not been proposed yet. It is only experimentally investigated the laser beam transmission characteristics. Therefore, it is urgent to design a new photonic crystal fiber for high efficiency CARS endoscope.

The present invention first proposes the use of a new optical bandgap optical fiber as a photonic crystal optical fiber for constructing an efficient CARS endoscope device.

By using the optical bandgap optical fiber proposed in the present invention, the pump and Stokes light are guided without loss by the optical bandgap effect.

In addition, it maintains the fiber core mode which is 3 ~ 5 times larger than single mode fiber and at the same time design waveguide with little group velocity dispersion characteristic to reduce the linear and nonlinear distortion of pulses that can occur when using conventional single mode fiber. We want to reduce it as much as possible.

In addition, the center wavelength of the light source in which the CARS signal is generated is deliberately excluded outside the optical bandgap region so that the CARS signal incident to the cladding is not reflected by the bandgap effect. Finally, to provide a CARS endoscopy imaging apparatus capable of collecting backscattered CARS signals with high efficiency by efficiently collecting CARS signals scattered in all directions using a high numerical aperture double cladding structure.

In the present invention, when constructing a high performance coherent anti-stock Raman scattering endoscope dual cladding optical bandgap optical fiber 100, the core 110 and the core (110) for transmitting the Stokes light and the pump light to the measurement sample located at the center It characterized in that it has an inner clad 120 including a circular circular medium 121 having a refractive index higher than the refractive index of 110. This allows pump light and Stokes light to pass through the core 110 without loss due to the optical bandgap effect. In addition, the outer clad 130 structure composed of an annular edge-shaped air layer on the outside of the inner clad 120 so that the reproduced and reflected CARS signal can be transmitted to the photodetector through total reflection, and the outer clad 130 outside the cladding. It is characterized in that it comprises a support for supporting the outer silica 140 wrapped. Here, the core 110 is formed by removing one or three circular media 121 located at the center of the inner clad 120 structure.

)은 7~10 ㎛이고, 원형 매질(121)의 직경(d)과 거리(

)와의 비(

)는 0.2~0.45의 값을 가지는 것을 특징으로 한다. In this case, the core 110 is composed of silica having a refractive index of 1.45, the circular medium 121 included in the inner clad 120 is made of Ge-doped silica and the like to have a high refractive index in the range of about 1.47 to 1.7. Have In addition, the circular medium 121 forms a triangular lattice shape having a rotational symmetry of an angle of 60 ° at a predetermined interval with respect to the center of the core 110, wherein the interval between the circular mediums 121

) Is 7 ~ 10 ㎛, the diameter (d) and the distance (

Ratio with)

) Has a value of 0.2 ~ 0.45.

This structure forms a discontinuous optical bandgap in the 600-1100 nm light wavelength region so that the Stokes and the pump light are guided to the optical fiber core 110 by the bandgap effect, and the reproduced and reflected CARS signal exists outside the bandgap region to In the clad 120 region is characterized in that the transfer to the total reflection principle.

As described, the present invention proposes a new optical fiber structure necessary for implementing an CARS microscope with an endoscope. Using the double cladding solid core optical bandgap optical fiber proposed in the present invention, the high power laser pulse (pump, stokes light) used in the CARS endoscope is guided by the optical bandgap effect. In this case, the inherent characteristics of the bandgap waveguide minimize the nonlinear and scattering distortion caused by the pulse propagation so that the pump and Stokes light traveling through the optical fiber can reach the sample with characteristics similar to the initial laser pulse to generate a stable CARS signal. Do it. In addition, the generated backscattered CARS signal can be placed outside the optical bandgap structure to reduce reflection upon incident cladding, and at the same time, a double cladding structure having a high numerical aperture can be used to efficiently collect signals, thereby greatly improving the efficiency of the CARS endoscope.

The present invention proposes a design of a new photonic bandgap fibers structure for implementing a coherent anti-stokes Raman Scattering (CARS) endoscope system. The proposed fiber is a solid core double cladding optical bandgap optical fiber, which has a function of guiding light only at a specific wavelength (pump and Stokes light source wavelength) by the optical bandgap effect. In addition, it has a large core area to suppress the nonlinearity of the pump and Stokes light to be guided, and at the same time shows a low dispersion characteristic in the operating region due to the bandgap waveguide effect. Finally, the proposed fiber includes a double cladding fiber structure with high numerical aperture (Numerical Aperture> 0.5).

The solid core optical bandgap optical fiber designed as described above has an advantage of selectively guiding only light sources having a desired wavelength (pumps and Stokes light sources) due to discontinuous bandgap characteristics. In particular, the optical bandgap effect, unlike the conventional waveguide, has the great advantage of suppressing the group velocity dispersion of light propagating from the pump and the Stokes light source as much as possible. In addition, the proposed optical fiber maintains spatial single mode in the operating area even though it has core mode size more than several times compared with general single mode fiber. These characteristics prevent the degradation of the CARS signal generation efficiency due to waveguide propagation by suppressing the linear / nonlinear distortion of the high power pump and Stokes laser pulses used for CARS imaging. The center wavelength of the CARS signal generated in the proposed optical fiber is out of the bandgap by the optimized design, which makes it possible to collect the CARS signal efficiently by the double cladding structure with high numerical aperture. In addition, the proposed solid core optical bandgap optical fiber does not have air holes in the optical fiber cross section compared with the conventional optical bandgap optical fiber, so it is physically stable when combining the optical fiber end with other optical elements to realize the endoscope system. Prevents waveguide degradation. By using the optical bandgap optical fiber having these characteristics, it is possible to implement a high sensitivity and high quality CARS endoscope system.

Hereinafter, the operation principle of the optical bandgap optical fiber and the CARS endoscope including the same proposed by the present invention with reference to the accompanying drawings.

Figure 2 shows the configuration of the CARS endoscope. The CARS endoscope includes a light source 1 for generating Stokes light having an arbitrary frequency band and pump light for exciting the sample; An optical fiber 100 which simultaneously transmits signals generated from the light source 1 without signal distortion and collects backscattered CARS signals (epi-CARS signals) generated from a sample; A scanning device (2) for focusing and spatially scanning the two signals transmitted from the optical fiber (100) to a biosample; It consists of a bandpass filter (3) for filtering only the backscattered CARS signal detected through the optical fiber and a photodetector (4) composed of a photomultiplier tube (PMT) for amplifying and detecting the backscattered CARS signal.

The light source 1 includes a Stokes beam, a Pump beam, and a Probe beam. In order to generate an efficient CARS signal of a sample, a laser pulse having a peak power of several to several tens of kW is generally used. I use it.

FIG. 3 (a) depicts the characteristics of the optical fiber waveguide portion and the direction of pulse propagation of the CARS endoscope system. As described above, in order to efficiently implement a CARS endoscope system using a pulse laser having a high peak power, the nonlinearity and dispersion characteristics of an optical fiber used as a waveguide must be considered simultaneously.

값을 가지고 있다. 그러나 시스템의 비선형 효율을 가늠하는 비선형 성능지수(figure of merit)는 Bulk 매질과 비교하여 광섬유에서 약 10⁵ 이상 향상된 값을 보이게 되며 이는 원치 않은 비선형 효과를 축적하게 된다. 광섬유에서 비선형 특성은 구체적으로 다음과 같은 비선형 계수(

)로 정의할 수 있다. The silica medium constituting the optical fiber has a relatively small Kerr-nonlinear coefficient

Has a value. However, the figure of merit, which measures the system's nonlinear efficiency, shows an improvement of about 10 ⁵ over the optical fiber compared to bulk media, which accumulates unwanted nonlinear effects. Non-linear properties in optical fibers are specifically defined as

Can be defined as

Where n ₂ is the nonlinear constant of the medium and A _eff represents the cross-sectional size (or effective area) of the light traveling through the waveguide at a given wavelength ( l ). From the nonlinear coefficients defined above, it can be seen that the nonlinear characteristics of the light traveling through the waveguide can be controlled by the effective area A _eff of the light. Therefore, waveguides with small A _eff show large nonlinear characteristics under the same conditions, while waveguides with large cross-sectional area of light show relatively small nonlinearities. In order to apply an optical fiber to a CARS endoscope, it is advantageous to have a large A _eff as large as possible because the waveguide has the smallest nonlinear effect and the maximum nonlinear effect must occur in the optical fiber carrying the pump and stokes signal.

Dispersion in the optical fiber refers to a phenomenon in which a laser pulse including several wavelengths spreads through the optical fiber as the effective refractive index ( n _eff ) of the core mode changes with the wavelength, and may be classified into medium dispersion and waveguide dispersion. Dispersion characteristics of the optical fiber are expressed by group refractive index dispersion value D (ps / nm / km) , and a general silica optical fiber has a zero dispersion value (zero GVD ) around 1310 nm. Therefore, the CARS endoscope has a large normal variance in the region around 800 nm to 1100 nm in which the CARS endoscope operates ( D = -100 ~ -200 ps / nm / km ), which can cause severe distortion in the microwave pulse progression. Conventionally, researches to realize the zero dispersion value in the operating region (800 nm ~ 1100 nm) by adjusting the waveguide dispersion value have been attempted in the nonlinear optics field, but for this purpose, the core size of the photonic crystal fiber should be reduced to match the wavelength of light. This greatly reduces the previously mentioned A _eff value, which is inconsistent with the object of the present invention as dispersion control can cause severe nonlinear distortion.

In order to overcome the above nonlinearity and dispersion characteristics in the optical fiber used in the CARS endoscope, the present invention proposes the use of an optical bandgap optical fiber. In the optical bandgap optical fiber, the light is guided by the bandgap effect, not total reflection, that is, the light can be transmitted without loss at high refractive index due to the difference in refractive index. Nonlinearity and dispersion problems can be solved simultaneously.

Figure 3 (b) depicts the transmission characteristics of the optical bandgap optical fiber for CARS endoscope implementation. As shown in the figure, when the optical fiber with discontinuous optical bandgap transmission characteristics is implemented in the operating region, the pump and stokes light are inside the optical bandgap and guided by the bandgap effect, and the CARS signal is located outside the bandgap. Design to be guided through total reflection with cladding. The mode size of the light propagated by the band gap effect is determined by the core and the cladding (bandgap) structure surrounding the core. In the structure proposed in the future, the core mode size is 3 to 5 times larger than that of a general single mode fiber. It has an area, which does not cause a noticeable nonlinear phenomenon for short length (<2 m) optical fibers used in endoscopes.

One of the important characteristics of waveguides by the bandgap effect is that the light shows zero dispersion at the center of each bandgap and changes to a large dispersion value as it moves to the bandgap boundary regardless of the dispersion characteristics of the waveguide medium. Therefore, if each bandgap is properly adjusted, the advancing pump, Stokes light will have zero dispersion, or the soliton of light (the nonlinear interaction of the interface and the wave around the medium will not dissipate easily and the scattering will be nearly Can be induced). This has the advantage that it is possible to implement soliton propagation of light which prevents linear distortion of the pulse due to dispersion and further overcomes nonlinear phenomenon.

On the other hand, the generated CARS signal is designed to exist outside the bandgap as shown in the figure so that the CARS signal, which is scattered backwards at various angles, can be transmitted without being reflected by the bandgap structure of the cladding. The light incident on the cladding is guided through a double cladding structure having a large numerical aperture so that the CARS signal can be efficiently collected.

4 illustrates a cross-sectional structure of the optical fiber proposed in the present invention. The optical fiber is composed of ① core core, ② inner optical band gap cladding, and ③ outer air cladding part. The outer air cladding has an annular structure, which is supported by the supporting outer silica 140, which has a large refractive index difference with the inner structure. Considering the average refractive index of the inner silica part, the numerical aperture of light to be guided is a large value of 0.5 or more, which effectively collects and guides the backscattered CARS signal (epi CARS signal) scattered from the sample. .

) 간격으로 이루어진 모양을 가진다. 클래딩 구조에서 안쪽 하나의 구멍을 제거하면 이 부분이 광결정 광섬유의 코어(110) 부분이 된다. 내부 클래드(120)의 밴드갭 특성은 직경 d와 굴절률 차이(n₂-n₁)에 의하여 결정되고, 코어모드의 특성은 원형 매질(121) 사이의 거리(

)와 직경(d)에 의하여 결정된다. The optical fiber core 110 is composed of a silica medium (n ₁ to 1.45), which is an optical bandgap cladding structure including a circular medium 121 of high refractive index (n ₂ : 1.47 to 1.7), that is, to the inner clad 120. Is surrounded by. The inner cladding 120 structure described in FIG. 4 is a triangular lattice, and a circular structure having a high refractive index n ₂ diameter d in a silica is rotationally symmetrical at a 60 ° angle as shown in the figure, and has a high refractive index circular medium. Distance between 121

) It has a shape composed of intervals. If one inner hole is removed from the cladding structure, this becomes the core 110 portion of the photonic crystal fiber. The bandgap characteristics of the inner clad 120 are determined by the diameter d and the refractive index difference n ₂ -n ₁ , and the core mode characteristic is the distance between the circular media 121.

) And diameter (d).

는 7~10 mm,

은 0.2~0.45, n₂=1.47~1.7 사이의 값을 가지도록 설계한다. 이는 빛이 동작영역(700 nm~1500 nm)에서 불연속적인 밴드갭을 생성할 수 있도록 해준다. 생성된 밴드갭은 도 4의 하단 그림과 같이 펌프광 및 스톡스 광에 대하여는 코어(110)로, CARS 광은 내부 클래드(120)로 진행 할 수 있게 해준다. 이때 펌프광 및 스톡스광을 진행하는 코어(110)는 비교적 큰 유효 면적 A _eff (100 ~ 200 mm²)을 가짐으로써 작은 비선형 특성을 유지시켜 준다. In the present invention

Is 7-10 mm,

Is designed to have a value between 0.2 ~ 0.45, n ₂ = 1.47 ~ 1.7. This allows light to produce discrete bandgaps in the operating region (700 nm to 1500 nm). The generated bandgap allows the core 110 to travel to the pump light and Stokes light and the CARS light to the inner clad 120 as shown in the bottom of FIG. 4. At this time, the core 110, which propagates the pump light and the stokes light, has a relatively large effective area A _eff (100 to 200 mm ² ) to maintain a small nonlinear characteristic.

Therefore, the structure as shown in FIG. 4 proceeds the pump and Stokes light with an optical band gap effect without nonlinear distortion and dispersion problems, and the CARS light becomes an optimized waveguide structure that can be efficiently collected by the inner clad 120.

=7 mm, n₂=1.64 이다. 최적화 설계된 광섬유에서 펌프(~817 nm)와 스톡스 파장(~1064 nm)은 밴드갭 안 즉, 코어(110)에 존재하며, CARS 신호의 파장 (~660nm)은 밴드갭의 바깥쪽에 존재하여 밴드갭 효과로 진행되지 않고 내부 클래드(120)로 진행하는 것을 예상할 수 있다. 도면은 각각의 밴드갭을 진행하는 최저차 모드의 유효굴절률을 같이 보여 주고 있는데 이를 통하여 밴드갭 내부를 진행하는 코어모드의 빛의 분산 특성을 예측 할 수 있다. 전산모사를 통하여 계산된 분산 값은 펌프, 스톡스 광에 대하여 ±30 ps/km/nm 사이의 값을 보이고 있으며 이는 좀 더 정교한 설계를 통하여 줄일 수 있을 것으로 예상한다.FIG. 5 shows the bandgap characteristics and dispersion characteristics of the light that proceeds through the structure of FIG. 4 through computer simulation. The values used in computer simulation are d = 3 mm,

= 7 mm, n ₂ = 1.64. In an optimized optical fiber, the pump (~ 817 nm) and Stokes wavelength (~ 1064 nm) are present in the bandgap, i.e., core 110, and the wavelength of the CARS signal (~ 660nm) is outside the bandgap, resulting in a bandgap. It can be expected to proceed to the inner clad 120 without going into effect. The figure shows the effective refractive index of the lowest-difference mode that progresses through each bandgap. Through this, it is possible to predict the dispersion of light in the core mode that progresses inside the bandgap. The dispersion values calculated by computer simulation show values between ± 30 ps / km / nm for pump and Stokes light, which can be reduced by more sophisticated design.

1 is a schematic diagram showing the principle of CARS optical signal generation at the virtual energy level (dotted line) and the vibration energy level of the sample molecule (solid line)

2 is a schematic diagram of a CARS endoscope using an optical fiber

Figure 3 is a (a) the pump, Stokes light and CARS signal that proceeds through the optical bandgap optical fiber (b) the conceptual diagram of the transmission characteristics according to the wavelength of the optical bandgap optical fiber.

Figure 4 is an optimization design of optical bandgap optical fiber structure based on computer simulation and distribution characteristics of the pump, Stokes, CARS light that proceeds to the optical bandgap optical fiber.

5 is a computer simulation result of the transmission and dispersion characteristics of the optical bandgap optical fiber according to the present invention.

* Description of the symbols for the main parts of the drawings *

100: optical fiber 110: core

120: inner clad 121: circular medium

130: outer clad 140: support outer silica

Claims (5)

delete delete Coherent anti-Stokes Raman Scattering (CARS) signal, which transmits the beam overlapped with Stokes and pump light to the measurement sample, and transmits the Coherent Anti-Stokes Raman Scattering (CARS) signal to the photo detector. In the optical bandgap optical fiber 100, A core 110 positioned at the center and transmitting the stokes light and the pump light to the measurement sample; It is formed on the outside of the core 110, the circular medium 121 having a periodic structure in the inner portion is formed long in the longitudinal direction of the optical fiber so as to have a refractive index higher than the refractive index of the core 110 to the optical band gap effect An inner clad 120 that allows stokes and pump lights to be transmitted through the core 110 without loss and totally reflects the re-reflected CARS signal to a photodetector; An outer cladding 130 having an annular rim shape and an air layer outside the inner clad 120, It comprises a support outer silica 140 for wrapping and supporting the outside of the outer clad 130, The core 110 is made of silica, The circular medium 121 included in the inner clad 120 is made of Ge-doped silica having a higher refractive index than the core 110. The circular medium 121 of the inner clad 120 has a triangular lattice in which the triangular lattice is rotated symmetrically at an angle of 60 ° at a predetermined interval about the core 110 and the distance between the circular mediums 121. (

) Is 7 ~ 10 ㎛, the diameter (d) and the distance (of the circular medium 121)

Ratio with)

) Is an optical bandgap optical fiber for the coherent antistock Raman scattering endoscope, characterized in that 0.2 ~ 0.45. The method of claim 3, wherein The refractive index of the circular medium 121 included in the inner clad 120 is n ₂ = 1.47 to 1.7, and the discontinuous optical band gap effect in the region where the wavelength of the light passing through the inner clad 120 is 600 to 1100 nm. An optical bandgap optical fiber for a coherent antistock Raman scattering endoscope characterized in that for transmitting the re-reflected CARS signal. delete

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