WO2010071294A1

WO2010071294A1 - Photonic bandgap fibers for high efficiency coherent anti-stokes raman scattering endoscope

Info

Publication number: WO2010071294A1
Application number: PCT/KR2009/005464
Authority: WO
Inventors: Jee Yong Lee; Eun Seong Lee; Dae Won Moon; Dong Il Yeom
Original assignee: Korea Research Institute Of Standards And Science; Ajou University Industry-Academic Cooperation Foundation
Priority date: 2008-12-19
Filing date: 2009-09-24
Publication date: 2010-06-24
Also published as: KR101050369B1; KR20100071233A

Abstract

Provided isa design of a novel photonic bandgap fiber structure for Coherent Anti-Stokes Raman Scattering (CARS) endoscope system. The fiber proposed herein is a solid-core dual- cladding photonic bandgap fiber, which guides a beam only in a specific wavelength (the wave length of pump and Stokes beams) by a photonic bandgap effect. Also, the fiber has a wide core area capable of inhibiting a non-linearity of the guided pump and Stokes beams and shows, at the same time, a low dispersion property in an operation region by the bandgap waveguide effect. Furthermore, the proposed fiber includes a dual cladding fiber structure with a high numerical aperture (> 0.5).

Description

PHOTONIC BANDGAP FIBERS FOR HIGH EFFICIENCY COHERENT ANTI-STOKES RAMAN SCATTERING ENDOSCOPE

The present invention relates to a design for an optical waveguide (optical fiber) which is a core element constituting a Coherent Anti-Stokes Raman Scattering (CARS) endoscope, and more particularly, to a design and a simulation for a novel photonic bandgap fiber capable of transferring pump and Stokes beam for generating a CARS signal without distortion and collecting the generated CARS signal with high efficiency.

With a conventional optical microscope, it is difficult to obtain morphological and chemical images for a transparent bio sample (cell or tissue) and various structures in the cell, in which linear properties of light for an object to be observed and background substances are nearly the same. This is because an optical contrast such as linear absorption and reflection of light is not large in the bio sample. To solve such problem, studies have been actively made to detect characteristics and behaviors of an interior of a cell by dying a fluorescent indicator on a sample and then obtaining a fluorescence distribution of light irradiated onto the sample. However, a fluorescent substance changes characteristics of a bio sample itself, or its fluorescent property is deteriorated with lapse of time. Therefore, it is difficult to obtain a distribution image of a whole sample.

Recently, a method, such as two photon emission fluorescence, a second or third harmonic generation and a Raman scattering spectroscopy, for obtaining a cell image by detecting a unique property of a substance itself without a fluorescent indicator has come into the spotlight. In particular, the Raman scattering spectroscopy has an advantage of easy selection of a laser light source and simple operation thereof since a light source of a single wavelength can be used regardless of a molecular vibrational frequency. However, since the intensity of a Raman scattering signal is extremely weak, it takes much time to obtain microscopic images. For this reason, there is a limitation in observing dynamic characteristics of cells in a living sample. A Coherent Anti-Stokes Raman Scattering (CARS) microscope designed to overcome such limitation uses a principle of four-wave mixing in which one CARS signal beam is generated by interaction of three incident laser beams in a sample using a Raman non-linear effect of light.

Fig. 1 illustrates the principle of a CARS spectroscopy using a Raman shift. When two laser beams (pump beam and Stokes beam) having a frequency difference corresponding to a Raman shift of a specific molecule in a bio sample to be tested are incident, a beat waveform corresponding to the differential frequency is generated. This waveform forcibly induces a forced harmonic oscillation coherent thereto. If a third laser beam (a probe beam) is incident onto molecules that vibrate at the same phase, an anti-Stokes Raman signal is induced in a section of a wavelength shorter than that of the pump beam by the interaction of this third laser beam interacts with the forced harmonic oscillation. The generated signal becomes a coherent light source having an agreed phase and a specific orientation. This non-linear optical signal is precisely mapped on a sample space at a high speed, thereby obtaining CARS microscopic images.

The CARS microscope based on the above described principle has a very high sensitivity as compared to the existing Raman scattering microscope and thus allows high speed obtaining of images. Since the intensity of the non-linear signal is produced by a third-order non-linear effect of the incident light, the generated CARS signal is proportional to a cube of an intensity of an applied electric field. Therefore, a pulse laser withhigh peak power is generally used to maximize the non-linear effect of light. The CARS spectroscopy is a noninvasive microscopy capable of preventing the sample from being thermally damaged by a laser beam since no laser energy remains in the tested sample after laser interaction, and is capable of obtaining three dimensional images for an interior of a sample with a high spatial resolution through focusing and scanning of light. The CARS microscope has come into the spotlight in a field of a non-linear bio imaging due to these advantages thereof.

It is generally preferred to implement the bio imaging technique based on the CARS not through a microscope but through an endoscope to observe tissue images of an organism in real time or to actively apply the CARS-based bio imaging technique to medical and pharmaceutical fields such as clinical diagnosis. Optical images through the endoscope have been only successfully obtained in a conventional microscope or other recent emerging technologies including optical coherent tomography, two photon emission fluorescence and second harmonic generation imaging.

Fig. 2 is a schematic view describing the configuration of a CARS endoscope. In the schematic, the optical fiber is key element for the endoscope that transfers the high-power pump and Stokes laser pulses without any significant distortion of the signal and collects the generated CARS signal with high efficiency. However, a normal single mode optical fiber, which operates in a band of 800 to 1000 nm or an operational range of the CARS microscope, may cause non-linear distortion (such as a spectral broadening due to the self-phase modulation) of high-power pulse laser beams (pump and Stokes beams) during the propagation because of the limited size of a core mode (6 ㎛ or less in diameter). In addition, the linear property such as dispersion - i.e., propagation with different group velocities depending on the wavelength - of the silica optical fibermake the traveling pulse broader, thereby lowing the peak power and, at the same time, causing chirping which spatially separates positions of respective wavelengths. The distortionof the pump and Stokes beams resulted from such linear and non-linear characteristics of the optical fiber may considerably lower an efficiency of generating a CARS signal. Also, a conventional single-mode fiber possessing a low numerical aperture is not suitable for the efficient collection of the back-scattered CARS signal generally exhibiting wide scattered angle. Therefore, a novel type of the optical fiber waveguide has been required to implement a CARS endoscope with high sensitivity and efficiency.

A photonic crystal fiber, which has been experimentally proposed for the first in the late 1990s, has attracted considerableattention since it can easily engineer the waveguide properties such as a dispersion and mode-filed size by proper design of the lateral silica/air-hole structure in an interior of the fiber. In particular, the photonic crystal fiber can operates in a single mode throughout entire wavelengths, guide a wave through a core having a low refractive index by a photonic bandgap effect and implement non-linearity at both extremes. For this reason, the photonic crystal fiber has come into the spotlight in fields including a high power optical fiber laser, implementation of a photonic bandgap fiber, soliton propagation, pulse compression, and generation of broadband light source. There have been the efforts to employ the single mode fiber or the photonic crystal fiber in the CARS endoscope system.However, a photonic crystal fiber which remarkably overcomes the limitations in the aforementioned single mode fiber has not yet been proposed and only transfer properties of laser beams for the CARS endoscope have been partially investigated using a conventional photonic crystal fiber. Therefore, a design for a novel photonic crystal fiber is urgently required to implement a CARS endoscope with high efficiency.

An object of the present invention is to propose, for the first, use of a novel photonic bandgap fiber as a photonic crystal fiber for implementing an effective CARS endoscope.

Another object of the present invention is to guide pump and Stokes beams without loss by a photonic bandgap effect using the photonic bandgap fiber proposed in the present invention.

Further another object of the present invention is to design a waveguide which maintains a fiber core mode with an area 3 to 5 times larger than that of the fiber core mode of a conventional single mode fiber and has almost no dispersion characteristic of a group velocity, thereby maximally reducing linear and non-linear distortions of pulses which may be generated when using a conventional normal single mode fiber.

Still another object of the present invention is to prevent a CARS signal incident to a cladding from being reflected by the bandgap effect by intentionally excluding a center wavelength of a light source, in which the CARS signal is generated, out of a photonic bandgap region. Furthermore, yet another object of the present invention is to provide CARS endoscopic imaging apparatus capable of collecting a back-scattered CARS signal with high efficiency by effectively collecting the CARS signal using an optical fiber having a high numerical aperture when the generated CARS signal is scattered in all directions.

To achieve the above objects, the present invention provides dual-cladding photonic bandgap fibers for a high efficiency Coherent Anti-Stokes Raman Scattering (CARS) endoscope, which include a core positioned at a center of the photonic bandgap fiber and transferring the Stokes and pump beams to the sample, and an inner clad having a periodically structured circular medium having a refractive index higher than a refractive index of the core. By this configuration, the Stokes beam and the pump beam can be transferred through the core without loss by a photonic bandgap effect. Also, the photonic bandgap fibers further include an outer clad formed around the inner clad in a shape of an annular frame and made of an air layer, and a supporting external silica surrounding and supporting an outer circumference of the outer clad, so that the reflected CARS signal can be transferred to the photodetector by total-reflection. Here, the core is formed in such way that one or three circular medium positioned at a center of the inner clad structure is removed.

At this time, the core is made of silica having a refractive index of 1.45, and the circular medium included in the inner clad is made of Ge-doped silica having a high refractive index of 1.47 to 1.7. Also, the circular medium of the inner clad forms a shape in that a triangular lattice is arranged rotation-symmetrically at an angle of 60 with a predetermined distance around the core, and a distance (∧) between the circular media is 7 to 10 ㎛ and a ratio (d/∧) of a diameter (d) of the circular medium to the distance (∧) is 0.2 to 0.45.

This configuration forms a discontinuous photonic bandgap in a wavelength region of 600 to 1100 nm. Therefore, the Stokes and pump beams are guided throughthe fiber core by a photonic bandgap effect and the reproduced and reflected CARS signal is present outside of the bandgap region and is thus transferred through the inner clad region by a total-reflection.

As described above, the present invention proposes a novel optical fiber structure required to implement a CARS microscope to an endoscope. Using the dual-cladding solid-core photonic bandgap fiber, a high power laser pulse (pump and Stokes beams) used in a CARS endoscope is guided by the photonic bandgap effect. At this time, non-linear and dispersion distortions according to pulse traveling is minimized by a unique property of the bandgap waveguiding and the pump and Stokes beams traveling through the optical fiber are thus arrived at a sample with a property similar to that of the initial laser pulse, thereby capable of generating a stable CARS signal with high efficiency. Also, reflection upon incidence into the cladding is reduced by laying the generated epi CARS signal outside the photonic bandgap structure and the signal is efficiently collected by the introduction of the dual-cladding structure, thereby capable of considerably enhancing an efficiency of the CARS endoscope.

The above and other objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic view illustrating a principle of generation of a CARS signal at an imaginary energy level (dotted line) and a vibration energy level of a sample molecule (solid line);

Fig. 2 is a schematic view illustrating the configuration of a CARS endoscope using an optical fiber;

Figs. 3 and 4 are a schematic view, in which (a) illustrates pump and Stokes beams and a CARS signal traveling through a photonic bandgap fiber, and (b) illustrates transmission properties of the photonic bandgap fiber according to a wavelength thereof;

Fig. 5illustrates an optimized design of the photonic bandgap fiber structure based on a computer simulation and distribution properties of the pump, Stokes and CARS beams traveling through the photonic bandgap fiber; and

Fig. 6 illustrates the result of the computer simulation for the transmission and dispersion properties of the photonic bandgap fiber in accordance with an embodiment of the present invention.

[Description of main elements]

100: optical fiber 110: core

120: inner clad 121: circular medium

130: outer clad 140: supporting external silica

The present invention proposes a design of a novel photonic bandgap fiber structure for Coherent Anti-Stokes Raman Scattering (CARS) endoscope system. The fiber proposed herein is a solid-core dual-cladding photonic bandgap fiber, which guides a beam only in a specific wavelength (the wave length of pump and Stokes beams) by a photonic bandgap effect. Also, the fiber has a large core area capable of inhibiting a non-linearity of the guided pump and Stokes beams and shows, at the same time, a low dispersion property in an operation region by the bandgap waveguide effect. Furthermore, the proposed fiber includes a dual cladding fiber structure with a high numerical aperture (> 0.5).

The solid-core photonic bandgap fiber designed as described above has an advantage of selectively guiding only a beam with a desired wavelength (pump and Stokes beams) due to a discontinuous bandgap effect. In particular, the photonic bandgap effect has, unlike a conventional waveguide, a considerable advantage of maximally inhibiting a Group Velocity Dispersion (GVD) of a beam traveling in a source of the pump and Stokes beams. Also, the proposed optical fiber maintains a spatial single mode in an operation region although it has a core mode size several times larger than that of the conventional single mode fiber. These characteristics inhibit linear/non-linear distortions of a high power pump and Stokes laser pulse used in a CARS imaging, thereby capable of preventing lowering an efficiency of CARS signal generation due to travel through the waveguide. A center wavelength of the CARS signal generated in the proposed optical fiber is present outside of the bandgap by the optimized design, which allows efficient collection of the CARS signal by the dual cladding structure having a high numerical aperture. Furthermore, since the proposed solid-core bandgap fiber has no air-portion in the inner cladding region of the fiber as compared to the conventional air-core bandgap fiber, this proposed bandgap fiber is physically stable when a distal end of the fiber is coupled to other optical device, and prevents lowering in waveguiding performance of the optical fiber. Consequently, it is possible to implement a CARS endoscope system with high sensitivity and high quality using the photonic bandgap fiber having such properties.

Hereinafter, a photonic bandgap fiber according to the present invention and a CARS endoscope having the same will be described in detail with reference to accompanying drawings.

Fig. 2 illustrates the configuration of a CARS endoscope. The CARS endoscope includes a light source 1 for generating a Stokes beam having a frequency band and a pump beam for causing a sample to be excited; an optical fiber 100 for simultaneously transferring the two signals generated from the light source 1 without distortion and collecting a back-scattered CARS signal (epi-CARS signal) generated from the sample; a scanner 2 for focusing the two beam transferred through the optical fiber 100 on the sample and spatially scanning the sample; a band pass filter 3 for filtering only the epi-CARS signal detected through the optical fiber; and a photodetector 4 having a photomultiplier tube (PMT) amplifying and detecting the epi-CARS signal.

The light source 1 includes a Stokes beam, a pump beam and a probe beam, and a pulse laser with a peak power of several to tens kW is generally used as the light source for the effective generation of the CARS signal from the sample.

Fig. 3illustrates properties of the fiber waveguide constituting the CARS endoscope system and a traveling direction of a pulse. As described above, to effectively implement the CARS endoscope system using a pulse laser with a high peak power, a non-linearity and a dispersion property of the fiber used as a waveguide are necessarily considered at the same time.

A silica medium consisting the optical fiber has relatively small value of Kerr non-linear coefficient n ₂ (=2 X 10 ^-20 m ² /W). However, a non-linear figure of merit which indicates a non-linear efficiency of the system shows a value increased by 10⁵or more in the optical fiber as compared to a bulk medium, which accumulates an undesired non-linear effect. The non-linear property of a beam can generally be defined by the following non-linear coefficient (γ):

Here, n ₂denotes a non-linear constant of a medium, and A _effdenotes a sectional size (or effective area) of a beam traveling through the waveguide at a given wavelength (lambda λ). From the non-linear coefficient defined as described above, it can be appreciated that it is possible to control the non-linear property of a beam traveling through a waveguide with the effective area A _eff of the beam. Therefore, under the same condition, a waveguide with small A _eff shows a large non-linear property, but a waveguide with a large sectional area of a beam show a relatively small non-linearity. To apply an optical fiber to a CARS endoscope, it is advantageous for the waveguide to have A _eff as large as possible since it is required that the minimum nonlinear effect occurs in the optical fiber which transfers pump and Stokes beams and the maximum non-linear effect occurs in the sample.

GVD in an optical fiber means a phenomenon that a laser pulse including various wavelengths gets broader while traveling through the optical fiber since the speed of the light propagating along the fiber varies as a wavelengthdue to the different group index(n_g) of the core-mode with wavelength. The total GVD consists of material dispersion and waveguide dispersion in the silica optical fiber. The dispersion property of the optical fiber is expressed by a value of D (ps/nm/km). A normal silica fiber has a zero GVD at near 1310 nm and therefore, exhibits a large normal GVD (D=-100~-200 ps/nm/km) in a region around 800 to 1100 nm in which the CARS endoscope operates. Thismay cause a serious distortion in traveling of an ultrashort wave pulse. Studies for implementing zero GVD in the operation region by controlling the waveguide structure have been tried in a field of a non-linear optics. To this end, however, the core size of a photonic crystal fiber should be reduced which considerably reduces the aforementioned value of A _eff resulting in the increase of the nonlinear coefficient γ. Accordingly, the dispersion control in the optical waveguide operating with total internal reflection may cause serious non-linear distortion, which does not meet the objects of the present invention.

To overcome the aforementioned non-linearity and the dispersion property at the same time in the optical fiber used in the CARS endoscope, the present invention proposes the use of a photonic bandgap fiber. In the photonic bandgap fiber, a beam is guided not by a total internal reflection but by a bandgap effect where the zero-GVD can be realized around of center of the discrete bandgaps while maintaining the relatively large Aeff.

Fig. 4 illustrates a transmission property of a photonic bandgap fiber for implementing a CARS endoscope. As illustrated, when implementing an optical fiber having a discrete photonic bandgap transmission property in an operation region, a design is made so that pump and Stokes beams are present inside the photonic bandgap and are guided by the bandgap effect, but the CARS signal is present outside the photonic bandgap and is guided by the total reflection by the outer cladding. A mode size of a beam traveling by the bandgap effect is determined by a structure of a core and a cladding (bandgap) surrounding the core. In the structure to be proposed, the core mode has a area 3 to 5 times larger than that of the core mode of a conventional signal mode fiber, which does not causes a considerable non-linearity effect on an optical fiber within a short length (<2 m) used in an endoscope.

One of important characteristics of a waveguide by the bandgap effect is that a beam shows a zero GVD around the center of each bandgap regardless of the dispersion property of a waveguiding meterial, and varies toward a large GVD as it approaches to a boundary of the bandgap. Therefore, by properly control the position of each bandgap, it is possible to make traveling pump and Stokes beams having a zero GVD or induce soliton (a wave in which an energy is not easily dispersed and scattering is hardly generated by non-linear interaction of boundary around a medium and the wave) propagation of a beam. This has an advantage capable of implementing the soliton propagation of a beam which prevents linear distortion of a pulse due to the dispersion and overcomes the non-linearity effect as well.

Meanwhile, the generated CARS signal is designed so as to bepresent outside of the bandgap as illustrated, and therefore the CARS signal back-scattered with various angles is not reflected by the bandgap structure of the cladding but can be transmitted through the inner cladding. The beam incident onto the inner cladding is guided through the dual cladding structure having a large numerical aperture, which allows efficient collection of the CARS signal.

Fig. 3 illustrates a sectional structure of the optical fiber proposed by the present invention. This optical fiber includes three large portions, i.e., ① a center core, ② an inner photonic bandgap cladding, and ③an outer air cladding. The outer air cladding has an annular structure and the annular structure is supported by a thin silica structure connected with external silica 140, which providesa large difference in a refractive index from the internal structure. When considering a mean refractive index of an internal silica portion, a numerical aperture of the guided beamis a large value of 0.5 or more, which functions to efficiently collect and guide the epi CARS signal scattered from the sample.

The fiber core 110 consists of a silica medium (n₁: ~ 1.45) and surrounded by the photonic bandgap inner cladding structure 120includes a circular medium 121 with a high refractive index (n_h: 1.47 ~ 1.7)in the silica backgroud. The structure of the inner clad 120 illustrated in Fig. 5is a triangular lattice and has a shape in that circular structures with a refractive index of n_hand a diameter of d is arranged rotation-symmetrically at an angle of 60° inside the silica and a distance between the circular mediums 121 is ∧. In the cladding structure, one hole at the center is removed to form the core 110 of the photonic crystal fiber. The bandgap effect of the inner clad 120 is determined by the diameter d and the refractive index difference n_h-n₁, and the property of the core mode is determined by the distance ∧ between the circular mediums 121 and the diameter d of the circular medium 121.

In the present invention, a design is made so that ∧ is 7 to 10 mm, d/ ∧ is 2.2 to 0.45 and n_his 1.47 to 1.7. This allows the beam to generate a discontinuous bandgap in the operation region (700 to 1500 nm). The generated bandgap allows, as illustrated in the lower portion of Fig. 5, the pump beam and the Stokes beam to travel through the core 110 and the CARS beam to travel through the inner clad 120. At this time, the core 110 through which the pump beam and the Stokes beam travel has a relatively large effective area A _eff (100 to 200 ㎛²) to maintain a small non-linear property.

Therefore, the structure illustrated in Fig. 5is the optimized waveguide structure which allows the pump and Stokes beams to travel without the problem of non-linear and dispersion distortions by the photonicbandgap effect and can efficiently collect the CARS beam by the inner clad 120.

Fig. 6 illustrates the bandgap effect and the dispersion property of the beam traveling through the structure in FIG. 4 through a computer simulation. The values used in the computer simulation are d=3 ㎛, ∧=7 ㎛, n_h=1.64. In an optimally designed optical fiber, it can be expected thatthe wavelength (~ 817 nm) of the pump beam and the wavelength (~ 660 nm) of the Stoke beam are present inside the bandgap, i.e., in the core 110, and the wavelength of the CARS signal is present outside the bandgap to thereby travel not by the bandgap effect but through the inner clad 120. The figure shows the effective refractive index of the lowermost order mode traveling the each bandgap, through which it is possible to estimate the dispersion property of the beam in the core mode traveling through the bandgap. The GVD calculated by the computer simulation shows the value of ±30 ps/km/nm for the pump and Stokes beams and this can expectedly be reduced through a more precise design.

Claims

Photonic bandgap fibers for a high efficiency Coherent Anti-Stokes Raman Scattering (CARS) endoscope, which transfers a beam with overlapped Stokes and pump beams to a sample and transfers a CARS signal reflected from the sample to a photodetector, wherein the photonic bandgap fiber comprises:

a core positioned at a center of the photonic bandgap fiber and transferring the Stokes and pump beams to the sample;

an inner clad formed outside the core and having a periodically structured circular medium formed in a longitudinal direction of the fiber and having a refractive index higher than a refractive index of the core, so as to transfer the Stokes beam and the pump beam through the core without loss by a photonic bandgap effect and total-reflect the reflected CARS signal to the photodetector;

an outer clad formed around the inner clad in a shape of an annular frame and made of an air layer; and

a supporting thin silica connected with external silica surrounding and supporting an outer circumference of the outer clad.
The photonic bandgap fibers as set forth in claim 1, wherein the core is made of silica, and the circular medium included in the inner clad is made of other composites having a refractive index higher than that of the core.
The photonic bandgap fibers as set forth in claim 2, wherein the circular medium of the inner clad forms a shape in that a triangular lattice is arranged rotation-symmetrically at an angle of 60° with a predetermined distance around the core, and a distance (∧) between the circular media is 7 to 10 ㎛ and a ratio (d/∧) of a diameter (d) of the circular medium to the distance (∧) is 0.2 to 0.45.
The photonic bandgap fibers as set forth in claim 3, wherein the refractive index (n_h) of the circular medium included in the inner clad is 1.47 to 1.7, and the reflected CARS signal is transferred by a discontinuous photonic bandgap effect in a region in which a wavelength of a beam traveling the inner clad is 600 to 1100 nm.
The photonic bandgap fibers as set forth in claim 1, wherein the core is formed in such way that one or three circular medium positioned at a center of the inner clad structure is removed.