KR100982556B1 - High Performance Coherent Banstock Raman Scattering Endoscope - Google Patents

Abstract

The present invention is directed to the design of a novel photonic crystal fibers structure for implementing a coherent anti-stokes Raman Scattering (CARS) endoscope system. The optical fiber of the present invention is a double cladding polarization maintaining photonic crystal optical fiber having a large area, specifically, a photonic crystal fiber core mode having a large area (more than 15 μm in diameter of the core mode), and having a large area core, but also an operating region (600 to 1100). nm), which includes the ability to operate in spatial single core mode, a polarization sustaining waveguide structure that maintains the polarization state of light propagating through the single mode photonic crystal fiber, and finally a double cladding fiber structure of high numerical aperture (Numerical Aperture> 0.5) Doing.

The new photonic crystal fiber according to the present invention maintains a spatial single mode in the operating region even though it has a core mode of several times to ten times larger than a general single mode optical fiber. This property can suppress the nonlinear distortion of the high power pump and Stokes laser pulses used for CARS imaging as much as possible to prevent the degradation of the CARS signal generation efficiency due to the waveguide progression. In addition, through the optical fiber design that can maintain the polarization state of the pump Stokes light traveling through the waveguide, it is possible to implement an efficient nonlinear coupling of the pump and Stokes beam on the biosample. At the same time, a weak CARS signal generated by introducing a double cladding structure with a high numerical aperture can be collected with high efficiency. Using a photonic crystal fiber and to offer when compared to the case using a normal single mode optical fiber can be implemented a high-performance CAR endoscope system having a greatly improved sensitivity.

Spontaneous Raman scattering, coherent antistock Raman scattering, nonlinear optical endoscope, photonic crystal fiber

Description

Polarization Maintaining Photonic Crystal Fiber with Large Scale for High Efficienty Coherent Anti-stokes Raman Scattering Endoscope

The present invention relates to the design of a photonic crystal fiber, which is a key component of the CARS endoscope, and more specifically, a pump for making a CARS signal, a new photonic crystal capable of transmitting the Stokes beam without distortion, and collecting the generated CARS signal with high efficiency. The present invention relates to the design and computational simulation of optical fibers.

Traditionally, general optical microscopes have difficulty acquiring morphological and chemical distribution images of transparent biosamples (cells or tissues) and various structures within cells, which have almost similar linear characteristics of the light of the subject and the background material. This is because optical contrast does not occur. In order to overcome this problem, studies have been actively conducted to detect the characteristics and behavior of cells by dyeing fluorescent markers on the sample and acquiring the fluorescence distribution of the irradiated light. In addition, over time, the fluorescence property was degraded, making it difficult to obtain a distribution image of an intact sample.

Recently, a technique of acquiring a cell image by detecting a unique characteristic of the material itself without a fluorescent marker has been attracting attention. Typically, Raman scattering spectroscopy is used to measure molecular images of microstructures. Raman microscope has the advantage that it is easy to select the laser light source and the operation is easy because it can use any single wavelength light source irrespective of the molecular vibration frequency. 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 is a method designed to overcome these limitations. Using the Raman nonlinear effect of light, three incident laser beams interact in a sample It uses the principle of the mixed light wave to generate the CARS signal light. 1 shows the principle of CARS spectroscopy using Raman transition. Two incident lasers (pump beam and Stokes beam) with a frequency difference by the Raman shift of the sample specific molecule cause a beat, forcing a coherent forced harmonic oscillation. . In this case, when the third laser light (scanning light) is incident on the molecules having the oscillated phase coincidence, anti-Stokes Raman scattering, which shortens the laser wavelength after interaction, has the same phase and has a specific propagation direction. Coherent signal light having? Is outputted. Precise high-speed mapping of such nonlinear optical signals in sample space results in CARS microscopic images.

The advantage of CARS microscopy is that, compared to Raman scattering microscopy, fast image acquisition speed can be obtained based on very high measurement sensitivity. In addition, since the intensity of the CARS signal is determined by the third-order nonlinear interaction of the incident light with the biosample, the intensity of the CARS signal can be further amplified by increasing the intensity of the light. Therefore, in order to maximize the nonlinear effect of light, a high peak power pulse laser is generally used. CARS microscope can obtain high-resolution three-dimensional images of the inside of the sample through the focusing and scanning of light, and it does not leave any laser energy on the measured sample after laser interaction. In principle, it is a non-invasive measurement that can be avoided.

In order to observe tissue images of living organisms in real time or to actively use them in pharmaceutical fields such as clinical diagnosis using CARS-based bio-imaging technology, it is necessary 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 achieved by optical cohrence tomography, single or two photon fluorescence imaging, and secondary harmonics. Endoscopic image acquisition is also performed in fields such as second harmonic generation imaging. Optical coherence tomography, however, is based solely on the difference in the sample's linear index of refraction, and other nonlinear based endoscopes have limited application compared to CARS microscopy. Recently, a CARS endoscope using a general single mode optical fiber has been proposed in principle, but due to the waveguide characteristics of the single mode optical fiber, it has been difficult to implement as an endoscope.

2 shows a schematic diagram for implementing a CARS endoscope. As shown in the figure, an optical waveguide that can efficiently collect the CARS signal generated after waveguide of strong intensity pump and Stokes laser pulse is focused on the sample from the opposite side. However, the general single mode optical fiber operating in the 800 ~ 1000 nm band, which is the operating range of CARS microscope, has the nonlinearity of high power pulsed lasers (pumps and stokes) that run through the waveguide due to the limited size of the core mode (less than 6mm diameter of the core mode). It easily causes distortion, which can greatly reduce the efficiency of CARS signal generation. In addition, CARS endoscope should use backscattered CARS signal due to its structure. In general, it is difficult to collect CARS signal generated using general single mode optical fiber with low backscattered signal intensity and wide scattering angle. There are many. Therefore, a new type of optical waveguide has been required to realize a highly sensitive and efficient CARS endoscope.

The photonic crystal fiber first proposed experimentally 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 along the waveguide length. Great attention has been received. In particular, it has shown great interest in nonlinear optics because it can operate in a single mode or implement bipolar nonlinearity over all wavelengths. However, the photonic crystal fiber which overcomes the limitation of the single mode optical fiber mentioned above in CARS endoscopy has not been proposed yet, and it has only recently been experimentally investigated the laser beam transmission characteristics for CARS endoscope using the existing photonic crystal fiber. . Therefore, it is urgent to design a new photonic crystal fiber for high sensitivity CARS endoscope.

In order to construct an efficient CARS endoscope device, a non-crystalline phase modulation that can occur when using a conventional single mode optical fiber by designing a photonic crystal fiber having an optical fiber core mode of an area of 5 to 10 times larger than that of a general single mode optical fiber. The present invention provides a large-area polarization-maintaining photonic crystal optical fiber for high-performance coherent anti-Stocktox Raman scattering endoscopy that minimizes the nonlinear distortion caused by the effect.

In addition, the present invention maintains the spatial single mode and the polarization state of the light propagating in the optical fiber to prevent the interference between the space mode of the pump light and the Stokes light traveling through the waveguide and to maintain a linearly polarized state along the waveguide Raman scattering to provide a large-area polarization maintaining photonic crystal optical fiber for endoscopy.

In addition, another object of the present invention is a high-performance coherent anti-Stocks Raman to collect the backscattered CARS signal with high sensitivity by efficiently focusing the generated CARS signal when scattered in all directions using a high numerical aperture optical fiber It is to provide a large-area polarization maintaining photonic crystal optical fiber for scattering endoscope.

The large-area polarization-maintaining photonic-crystal optical fiber for high-performance coherent antistock Raman scattering endoscope for achieving the above-mentioned object transmits a beam in which a stokes beam and a pump beam are superimposed to a measurement sample, and reflects the coherent anti-Stocks Raman reflected back from the measurement sample. In the high performance coherent anti-Stokes Raman Scattering (CARS) signal to the photodetector, a high-performance coherent anti-Stokes Raman scattering endoscope large-area polarization maintaining photonic crystal optical fiber 100, made of silica, located in the center and in a triangular shape The core 110 and the silica, and formed on the outside of the core 110, a plurality of air holes 121 in the inner portion to form a hexagonal shape at regular intervals around the core 110 of the optical fiber An inner clad 120 formed to extend in a longitudinal direction, an outer cladding 130 having an annular rim shape surrounding the outer clad 120, and the outer portion It characterized by comprising a clad (130) supporting the external silica 140 for supporting the outer wrap.

In addition, the distance (L) between the air hole 121 formed in the inner clad 120 is 6 ~ 10㎛, the ratio of the diameter (d) and the distance (L) of the air hole 121 is 0.2 ~ 0.3 It is characterized by.

In addition, the core 110 is characterized in that formed by removing the three air holes located in the center in the inner clad 120 structure.

In addition, an oval air hole 111 having a long axis of 0.4 to 1.5 μm and an eccentricity (long axis / short axis) of 1.2 to 2.5 is formed at the center of the core 110.

In addition, the outer clad 130 is characterized in that consisting of an air layer.

The present invention proposes a new photonic crystal optical fiber structure required to implement a CARS microscope with an endoscope. The large-area polarization-maintaining photonic crystal fiber proposed in the present invention can reduce the non-linear distortion of the high-power laser pulses guiding the waveguide compared to the general optical fiber, so that the pump and Stokes light passing through the optical fiber are similar to the initial laser pulse. To reach the sample. At the same time, the photonic crystal fiber operates in a single mode in a wide operating area, thereby reducing the distortion of the signal due to the inter-mode interference effect of the optical fiber. This feature has the effect of safely reaching the initial laser pump, Stokes light, to the sample without spectral distortion caused by nonlinear effects or interference. In addition, by maintaining the polarization state of the pump and Stokes beam as the fiber progresses, the pump and Stokes beam of the same polarization state is incident on the sample to generate a stable CARS signal. In addition, the sensitivity of the endoscope can be further improved by efficiently collecting the generated backscattered CARS signal using a double cladding structure having a high numerical aperture.

In order to achieve the above object, a large-area coherent anti-stock Stock Raman scattering endoscope proposed in the present invention will be described with reference to the accompanying large-area polarization maintaining photonic crystal fiber.

Figure 2 shows the configuration of the Raman endoscope. The CARS Raman 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 and collects backscattered CARS signals (epi-CARS signals) generated from a sample; A scanning device (2) for focusing two signals transmitted from the optical fiber (100) to a sample and spatially scanning them; It consists of a bandpass filter (3) for filtering only the backscattered CARS signal detected through the optical fiber and a photodetector (4) consisting 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, and generally has a high peak power for efficient Raman signal generation of a sample to be measured. Use a pulsed laser. In general, a laser pulse having a peak power of several to several tens of kW is incident on the optical fiber constituting the endoscope to proceed.

Silica medium constituting the optical fiber itself has a relatively small non-linearity, but the non-linear effect of the high intensity light due to the efficient accumulation of the non-linear effect as the light travels along the long length of the optical fiber occurs. In a waveguide such as an optical fiber, the nonlinear characteristic is generally defined by the following nonlinear coefficient (γ).

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). The inverse of the product of the nonlinear coefficient value and the peak power of the advancing light is defined as the nonlinear length L _N as shown in the following equation.

이며 이에 따른 일반적인 단일 모드 광섬유에서의 비선형 계수 g는 1000 nm의 중심파장의 빛에 대하여 약 2~4 /W/km가 된다. If the calculated nonlinear length L _N for a given waveguide and light intensity is sufficiently long compared to the actual length of the waveguide, it can be considered that no nonlinear effect occurs. In general, n ₂ values in silica are approximately

Therefore, the nonlinear coefficient g of a typical single mode optical fiber is about 2 to 4 / W / km for the light having a center wavelength of 1000 nm.

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 the optical fiber to the CARS endoscope, it is advantageous to have a large A _eff as much as possible because the minimum nonlinear phenomenon must occur in the optical fiber carrying the pump and Stokes signal and the maximum nonlinear effect must occur in the sample.

Figure 3 shows a cross-sectional view of the structure of the large-area polarization maintaining photonic crystal fiber proposed in the present invention. Fig. 3 (a) shows the overall structure of the proposed photonic crystal fiber, in which the black part represents silica and the white part represents air. The optical fiber 100 is largely composed of ① core, ② inner cladding, and ③ outer cladding. The core mode of the optical fiber shows characteristics of large area, polarization maintenance, and single space mode, which will be described in detail with reference to FIGS. 3 (b) to (d). The first incident pump, Stokes light, is guided by an inner clad 120 surrounding the core 110 of the fiber center. The outer clad 130 has an annular rim shaped air structure and is supported by a thin support outer silica 140. The outer clad 130 has a large refractive index difference from that of the silica including the core 110 and the inner clad 120. Considering the average refractive index of the silica portion of the inner clad 120, the numerical aperture of light guided by the outer clad 130 is greater than 0.5, which is a backscattered CARS signal scattered from the sample in FIG. It focuses and guides (epi CARS signal) efficiently.

The structures of the inner clad 120 and the core 110 are shown in FIGS. 3 (b) to (d). The internal clad structure shown in FIG. 3 (b) is a triangular lattice commonly used in a general photonic crystal fiber structure. As shown in the drawing, an air hole 121 having a diameter d of 60 d is symmetrically rotated and has an interval of L length. It has a shape that is drilled into. The optical properties of the first cladding are determined by the diameter d and the length L. When three air holes are removed from the cladding of this structure as shown in FIG. 3C, this portion becomes the central core portion of the photonic crystal optical fiber.

In the present invention, L is designed to have a value of 6 ~ 10 mm, d / L is 0.2 ~ 0.3 so that the light proceeds to the center core with three holes removed in the single mode in the operating region (700 nm ~ 1500 nm) To maintain.

At the same time, the light propagating at the above values has a large effective area A _eff (150-300 mm ² ), which results in very small nonlinear coefficient values.

In addition, since the bending loss of light propagating through the same effective area is smaller than that of the case where one hole is removed, the core of the photonic crystal optical fiber having three holes removed can realize a waveguide having a relatively large effective area for the same bending loss. Has the advantage.

The optical fiber core 110 shown in (c) of FIG. 3 has a triangular shape, and this triangular core 110 has essentially no birefringence due to the symmetry of the structure. Therefore, in order to apply birefringence to the optical fiber, a new structure as shown in Fig. 3 (d) is inserted. 3 (d) shows an elliptic air hole 111 inserted into the triangular core structure. In the figure, a and b represent the short axis and the long axis of the elliptical air hole 111, the long axis b has a value in the range of 0.4 to 1.5 mm and the eccentricity ratio b / a has a value of 1.2 to 2.5. The inserted structure induces birefringence in the optical fiber by changing the boundary condition according to the polarization of the light progressing.

As a result, the photonic crystal optical fiber having a double cladding structure as shown in FIG. 3A and a core as shown in FIG. 3D is an optimized structure suitable for pump-free transmission of Stokes signals and efficient collection of CARS signals.

The characteristics of light propagating through the optimized designed photonic crystal fiber described in FIG. 3 are shown through computer simulation in FIG. 4. Figure 4 shows the mode distribution of light propagating through the designed waveguide for various wavelengths. The values used in the computer simulation are d = 1.6 mm, L = 8 mm, b = 1.2 mm and a = 0.48 mm. It can be seen that the designed photonic crystal optical fiber has an effective area of about 200 μm ² or more over various wavelengths. In this structure, the size of the birefringence gradually decreases toward the shorter wavelength, but it is confirmed that about 10 ^-5 or more birefringence is maintained over the operating region.

FIG. 5 illustrates the spectrum and pulse change characteristics of an initial pulse laser that is 2m of the photonic crystal fiber proposed in the present invention. For comparison, the spectral and pulse variations of a typical single mode fiber (A _eff ~ 20 mm ² ) operating near 800 nm are shown. In the simulation, a pulsed laser with a conversion limit of 200 mW average intensity, 76 MHz repetition rate, and 0.4 nm spectral line width was used. In the proposed photonic crystal fiber, the laser pulse shows little change in the spectrum and pulse characteristics after the progress of the length of 2m as shown in the left figure of FIG. The very small spectral characteristic change is due to the minimized nonlinearity of the photonic crystal fiber proposed in the present invention. In addition, the photonic crystal fiber has a relatively large dispersion (group velocity dispersion ~ -100 ps / nm / km at 800 nm) at a given wavelength, but it is confirmed that the pulse change due to dispersion is not large due to the wide pulse width and the short fiber length. Can be. On the other hand, as shown in the figure on the right of Figure 5, a typical single mode optical fiber can be seen that the spectrum is greatly widened by the nonlinear self-phase modulation effect after 2 m length. In addition, it can be seen that the pulse width becomes wider and the intensity decreases due to the interaction between the nonlinear effect of the waveguide and the dispersion characteristic. If the wider spectral width becomes larger than the Raman line width, it will not produce an efficient CARS signal, which causes a severe deterioration of the CARS endoscope.

1 is a schematic diagram illustrating the principle of CARS optical signal generation at an energy level.

2 is a schematic diagram of a CARS endoscope including a CARS photonic crystal optical fiber.

Figure 3 shows (a) a schematic diagram of a new photonic crystal fiber for CARS endoscope implementation; (b) an inner cladding structure of the optical fiber; (c) a core structure with three air holes removed; and (d) a final fiber core structure with an elliptic air hole inserted therein. Figures shown respectively.

4 is a view showing a distribution of light propagating through a photonic crystal optical fiber based on computer simulation.

5 is a graph showing a comparison of light spectrum and pulse characteristics of a photonic crystal optical fiber (left) and a general single mode optical fiber (right) according to the present invention.

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

100: optical fiber 110: core

111: elliptical air hole 120: inner clad

121: air hole 130: outer clad

140: external silica for support

Claims (5)

High-performance coherent half-stock Raman scattering endoscope that delivers the beam overlapped with Stokes and pump light to the measurement sample and transmits Coherent Anti-Stokes Raman Scattering (CARS) signal to the photo detector In the large-area polarization maintaining photonic crystal optical fiber 100, A core 110 made of silica, centered and triangular in shape, An inner clad made of silica and formed on the outside of the core 110, and having a plurality of air holes 121 formed in a hexagonal shape at a predetermined interval around the core 110 at an inner portion thereof and formed long in the longitudinal direction of the optical fiber. 120, An outer clad 130 having an annular rim shape surrounding the outer clad 120; The outer cladding 130 is made up to include an outer support for supporting the silica (140) wrapped around the outside Large area for high-performance coherent anti-stock Raman scattering endoscope, characterized in that formed in the center of the core 110 is an elliptical air hole 111 having a long axis of 0.4 ~ 1.5㎛ and an eccentricity (long axis / short axis) of 1.2 ~ 2.5 Polarization maintaining photonic crystal optical fiber. The method of claim 1, The distance L between the air holes 121 formed in the inner clad 120 is 6 to 10 μm, and the ratio between the diameter d of the air holes 121 and the distance L is 0.2 to 0.3. A large-area polarization-maintaining photonic crystal fiber for high-performance coherent antistock Raman scattering endoscope. The method of claim 1, The core 110 is a large-area polarization maintaining photonic crystal optical fiber for high-performance coherent anti-Stockstock Raman scattering endoscope, characterized in that formed by removing the three air holes located in the center in the inner clad 120 structure. delete The method of claim 1, The outer clad 130 is a large-area polarization maintaining photonic crystal optical fiber for high-performance coherent antistock Raman scattering endoscope, characterized in that consisting of an air layer.

Images