CN100401974C - A method and system for realizing axial super-resolution of optical coherence tomography - Google Patents

Abstract

The invention discloses a method and system for realizing optical coherence tomography (OCT) axial super-resolution. A pupil filter is inserted between the collimating mirror and the detection objective lens of the optical coherence tomography sample arm, so that the main lobe width of the axial response function is narrowed within the coherence gate, thereby realizing the axial ultra-thin optical coherence tomography distinguish. At the same time, the sidelobe caused by the super-resolution pupil filter is suppressed by the coherence gate and does not make an effective contribution to the OCT coherent imaging, thereby ensuring that the imaging contrast is not reduced due to axial super-resolution. The invention is an economical and simple novel super-resolution technology, which can promote the miniaturization and commercialization of the OCT system.

Description

A method and system for realizing axial super-resolution of optical coherence tomography

technical field

The invention relates to optical coherence tomography (OCT) technology and optical super-resolution technology, in particular to a method and system for realizing axial super-resolution of optical coherence tomography by using a pupil filter.

Background technique

Optical Coherence Tomography (OCT) is an emerging optical imaging technology. Compared with traditional clinical imaging methods, it has high resolution, fast imaging speed, no radiation damage, moderate price, and compact structure. It is an important potential tool for basic medical research and clinical diagnostic applications.

Resolution is an important indicator for the development of OCT technology. The axial resolution of OCT is jointly determined by the bandwidth of the light source and the focusing conditions of the probe beam. Under the condition of weak focusing of the probe beam, the width of the main lobe of the corresponding axial response function is much larger than the width of the coherence gate, and the axial resolution at this time is mainly determined by the coherence gate. The wider the source bandwidth, the narrower the coherence gate width. The narrower the coherence gate width, the higher the axial resolution. Therefore, the main ways to improve the axial resolution of OCT include short-pulse laser technology, nonlinear supercontinuum generation technology, combined light source spectrum synthesis technology, etc. However, these broadband light source technologies have the following limitations: 1) The achromatic requirements for optical components such as imaging lenses are greatly increased; 2) Ultra-broadband high-efficiency single-mode fiber devices are not easy to obtain; 3) Ultrashort pulse lasers are expensive and inconvenient to operate. There are certain difficulties in miniaturization and commercialization. Therefore, using broadband light source technology to improve the axial resolution of OCT is often expensive, the system is complex, and there is a bottleneck in the device, it is difficult to continue to improve on the existing basis. Spectral shaping technology can also improve the axial resolution of OCT to a certain extent, such as suppressing the intermediate spectral components to flatten the spectral shape, achieving relative spectral broadening, and then improving the axial resolution of OCT.

Under the condition of strong focusing of the probe beam, the main lobe width of the axial response function is close to that of the coherence gate. Both the source bandwidth and the main lobe width of the axial response function are directly related to the axial resolution of OCT. In this case, compressing the main lobe width of the axial response function is another way to improve the axial resolution of OCT.

Contents of the invention

The object of the present invention is to provide a method and system for realizing axial super-resolution of optical coherence tomography, which reduces the main lobe width of the axial response function to a coherence gate by inserting a suitable form of pupil filter. In this way, the axial super-resolution of OCT is realized; at the same time, the side lobe caused by the super-resolution pupil filter is suppressed by the coherence gate, which does not make an effective contribution to the OCT coherent imaging, thereby ensuring that the imaging contrast is not affected by the axial super-resolution while falling.

The purpose of the present invention is achieved through the following technical solutions:

1. A method for realizing axial superresolution of optical coherence tomography:

A pupil filter is inserted between the collimating mirror and the detection objective lens of the optical coherence tomography sample arm, so that the main lobe width of the axial response function is narrowed within the coherence gate, thereby realizing the axial ultra-thin optical coherence tomography Resolution; the specific steps are as follows:

1) The light emitted from one port of the fiber coupler is first collimated by the collimator, then passed through the pupil filter, and then focused on the sample by the detection objective lens. At this time, the axial main lobe width of the illumination point spread function depends on the pupil The effect of the filter is compressed;

2) The reflected light and scattered light returned from the sample are collected by the detection objective lens, pass through the pupil filter and the collimating mirror again, and then return to the fiber coupler to meet and interfere with the reference light from the reference arm; at this time, the receiving point The spread function is consistent with the spread function of the illumination point, and the main lobe width of the axial response function determined by the two is smaller than the width of the coherence gate.

The pupil filter is composed of N concentric rings, wherein N is a natural number greater than 2, and the specific value is determined according to the resolution requirements of different optical systems,

为光瞳滤波器的光瞳函数，

为光瞳函数的极坐标；其中k_i表示第i个圆环的振幅透过率，0≤k_i≤1；

表示第i个圆环的位相延迟，

r_i-1和r_i表示第i环归一化内外半径；当只有参数k_i变化时，此表达式对应于振幅型光瞳滤波器；当只有参数

变化时，此表达式对应于位相型光瞳滤波器；当参数k_i和

都变化时，此表达式对应于复振幅型光瞳滤波器。

is the pupil function of the pupil filter,

is the polar coordinate of the pupil function; where _ki represents the amplitude transmittance of the i-th ring, _0≤ki ≤1;

Indicates the phase delay of the i-th ring,

r _i-1 and r _i represent the normalized inner and outer radius of the i-th ring; when only the parameter k _i changes, this expression corresponds to the amplitude pupil filter; when only the parameter

When changing, this expression corresponds to the phase pupil filter; when the parameters _ki and

When both vary, this expression corresponds to a complex-amplitude pupil filter.

2. A system for realizing axial super-resolution of optical coherence tomography:

It includes broadband light source, 2×2 fiber optic coupler, detector, data acquisition card, computer, reference arm and sample arm composed of collimating mirror, detection objective lens and sample in turn. A pupil filter is inserted between the collimating mirror of the sample arm and the detection objective lens. The objective collects, passes again through a pupil filter and collimating mirror, and returns to the fiber coupler.

The pupil filter is an amplitude-type pupil filter or a phase-type pupil filter, and the pupil filter is composed of N concentric rings, wherein N is a natural number greater than 2, and the specific value is determined according to the resolution of different optical systems. Rate requirements are determined.

The principle of the invention is: after the collimated light beam passes through the pupil filter, it is focused by the detection objective lens, the light distribution near the focus is changed by the pupil filter, and the corresponding axial response function also changes accordingly. With an appropriate form of pupil filter, the main lobe width of the OCT axial response function can be narrowed within the coherence gate. At this time, the axial resolution of OCT is no longer determined by the bandwidth of the light source, but by the width of the main lobe of the axial response function. An increase in the axial resolution of OCT is achieved in this way. At the same time, the sidelobe of the axial response function is suppressed by the coherence gate and does not participate in coherent imaging, thus ensuring that the imaging contrast is not reduced due to axial super-resolution.

Compared with background technology, the present invention has following advantages:

1) The method is simple and easy to implement with low cost. It is only necessary to insert a pupil filter between the collimator mirror and the detection objective lens of the conventional OCT sample arm to narrow the main lobe width of the axial response function to within the coherence gate. The pupil filter is made by mature binary optical technology and thin film technology, which can achieve high precision and low cost. This low-cost method for improving the axial resolution of OCT avoids the defects of high cost, complex system, and difficult device selection that exist in broadband light source technology.

2) The side lobes produced by the super-resolution pupil filter often lead to a decrease in image contrast, which is the biggest obstacle to the implementation of optical super-resolution in traditional optical systems. However, the influence of side lobes on imaging can be eliminated in OCT, and the inherent coherence gate of OCT can completely suppress side lobes from participating in coherent imaging, which is an important advantage of implementing optical super-resolution in OCT.

3) Preliminary experiments show that the axial resolution of OCT can be improved by at least 10% by the above method.

The invention is an economical and simple new super-resolution technology, which can promote the miniaturization and commercialization of the OCT system.

Description of drawings

Figure 1 is a schematic diagram of the sample arm structure of OCT axial super-resolution;

Fig. 2 is a schematic diagram of an amplitude pupil filter for optical axial super-resolution (taking N=3 as an example);

Fig. 3 is a schematic diagram of a phase-type pupil filter for optical axial super-resolution (taking N=3 as an example);

Fig. 4 is a schematic diagram of the system of the OCT axial super-resolution method.

In the figure: 1. Collimating mirror, 2. Pupil filter, 3. Detection objective lens, 4. Sample, 5. Broadband light source, 6. 2×2 fiber optic coupler, 7. Detector, 8. Data acquisition card, 9. Computer, 10. Reference arm.

Detailed ways

Below in conjunction with accompanying drawing and embodiment the present invention will be further described:

Figure 1 is a schematic diagram of the sample arm structure for implementing OCT axial super-resolution, in which 1 is a collimating mirror, 2 is a pupil filter, 3 is a detection objective lens, and 4 is a sample. The sample 4 is located near the focal point of the detection objective 3 . The collimating mirror 1, the pupil filter 2 and the detection objective lens 3 are placed coaxially. The effective aperture of the pupil filter 2 is equal to the full aperture of the detection objective 3 and normalized to 1. The edge portion outside the effective aperture of the pupil filter 2 does not participate in imaging due to the limitation of the aperture of the detection objective lens 3 and can be used for fixing the pupil filter 2 .

As shown in Figure 1, the light emitted from one port of the 2×2 fiber coupler is first collimated by the collimating mirror 1, then passes through the pupil filter 2, and then focused on the sample 4 by the detection objective lens 3. The reflected light and scattered light returned from the sample 4 are collected by the detection objective lens 3, pass through the pupil filter 2 and the collimating mirror 1 again, and then meet and interfere with the reference light from the reference arm at the fiber coupler. In view of the consistency of the illumination optical path and the receiving optical path in the detection arm, the corresponding illumination point spread function h _in and receiving point spread function h _out are also consistent, and are determined by the following formula:

为光瞳滤波器的光瞳函数，其定义式为：

p(ρ)和

分别表示光瞳滤波器的振幅分布和位相分布，(

)为光瞳函数的极坐标，u为轴向光学坐标，由式u＝(2π/λ)zsin²α＝kzsin²α决定，z代表实际的轴向坐标，k＝2π/λ为光源中心波长对应的波数，sinα表示探测物镜的数值孔径。照明点扩散函数和接收点扩散函数共同决定了OCT的轴向响应函数：

is the pupil function of the pupil filter, and its definition is:

p(ρ) and

Denote the amplitude distribution and phase distribution of the pupil filter, respectively, (

) is the polar coordinate of the pupil function, u is the axial optical coordinate, determined by the formula u=(2π/λ)zsin ² α=kzsin ² α, z represents the actual axial coordinate, k=2π/λ is the center of the light source The wavenumber corresponding to the wavelength, sinα represents the numerical aperture of the detection objective lens. The illumination point spread function and the receiving point spread function jointly determine the axial response function of OCT:

I＝|h _in ×h _out | ² (2)

When the pupil function of the pupil filter 2 takes different forms, the axial response function changes accordingly. When an appropriate form of the pupil function is adopted, the main lobe width of the axial response function shrinks within the coherence gate Δl of the OCT, thereby realizing the improvement of the axial resolution of the OCT.

The improvement of the OCT axial resolution of the present invention is measured by comparing the width of the main lobe of the axial response function with the width of the coherence gate. For an OCT light source with a Gaussian spectral distribution, the coherence gate width is:

$\mathrm{<span\; class="notranslate">\; \&Delta;l</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; ln</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}\frac{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; \&Delta;\&lambda;</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

Among them, λ is the center wavelength of the light source, and Δλ is the bandwidth of the light source.

的表达式为：Pupil filter 2 among the present invention is made up of ring structure of N zone, and its normalized pupil function

The expression is:

表示第i个圆环的位相延迟，

r_i-1和r_i表示第i环归一化内外半径。当只有参数k_i或者

变化时，此表达式分别对应于振幅型光瞳滤波器和位相型光瞳滤波器。作为实施例，图2示意了三区振幅型光瞳滤波器，其光瞳函数的振幅分布为：Where _ki represents the amplitude transmittance of the i-th ring, 0≤ki _≤1 .

Indicates the phase delay of the i-th ring,

r _i-1 and r _i represent the normalized inner and outer radius of the i-th ring. When only parameter _ki or

When changed, this expression corresponds to the amplitude-type pupil filter and the phase-type pupil filter, respectively. As an example, Fig. 2 illustrates a three-zone amplitude pupil filter, and the amplitude distribution of its pupil function is:

$\mathrm{<span\; class="notranslate">\; p</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&rho;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\left\{\begin{array}{cc}\mathrm{<span\; class="notranslate">\; 1</span>}& \mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; \&le;</span>\mathrm{<span\; class="notranslate">\; \&rho;</span>}<span\; class="notranslate">\; \&le;</span>\mathrm{<span\; class="notranslate">\; 0.392</span>}\\ \mathrm{<span\; class="notranslate">\; 0</span>}& \mathrm{<span\; class="notranslate">\; 0.392</span>}<span\; class="notranslate">\; <</span>\mathrm{<span\; class="notranslate">\; \&rho;</span>}<span\; class="notranslate">\; \&le;</span>\mathrm{<span\; class="notranslate">\; 0.92</span>}\\ \mathrm{<span\; class="notranslate">\; 1</span>}& \mathrm{<span\; class="notranslate">\; 0.92</span>}<span\; class="notranslate">\; <</span>\mathrm{<span\; class="notranslate">\; \&rho;</span>}<span\; class="notranslate">\; \&le;</span>\mathrm{<span\; class="notranslate">\; 1</span>}\end{array}\right.<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

Figure 3 shows a three-zone phase pupil filter, and the phase distribution of the pupil function is:

Figure 4 is a schematic diagram of the system implementing the OCT axial super-resolution method. Including broadband light source 5 (BWC-SLD of B&WTek, Inc., center wavelength 1310nm, bandwidth 65nm), 2×2 fiber coupler 6 (WBC type of Futong Showa Company, center wavelength 1310nm, bandwidth 80nm), detector 7 , a data acquisition card 8, a computer 9, a reference arm 10 and a sample arm composed of a collimating mirror 1, a pupil filter 2, a detection objective lens 3 and a sample 4 in sequence. A pupil filter 2 is inserted between the collimator mirror 1 of the sample arm and the detection objective lens 3 optical path. Theoretical and preliminary experiments show that the axial resolution of OCT can be increased by at least 10% by using a pupil filter with pupil functions such as formulas (5) and (6).

Claims (4)

1. method that realizes axial super resolution in tomography of optical coherent, it is characterized in that: at the collimating mirror of optical coherent chromatographic imaging sample arm with survey between the object lens and insert iris filter, the main lobe width of axial response function is narrowed down within the relevant door, thereby realize the axial super resolution of optical coherent chromatographic imaging; Its concrete steps are as follows:

1) light that sends from port of fiber coupler is collimated by collimating mirror earlier, and then by iris filter, by surveying object lens focusing in sample, the axial main lobe width of the lighting point spread function of this moment obtains compression because of the effect of iris filter again;

2) reflected light that returns from sample and scattered light are collected via surveying object lens, once more by iris filter and collimating mirror, return fiber coupler then, converge with reference light from reference arm and interfere; The acceptance point spread function of this moment is consistent with the lighting point spread function, by the main lobe width of the axial response function of the two common decision less than relevant gate-width degree.

2. a kind of method that realizes axial super resolution in tomography of optical coherent according to claim 1, it is characterized in that: described iris filter is made of N donut, wherein N is the natural number greater than 2, and occurrence is determined according to the resolution requirement of different optical system

Be the pupil function of iris filter,

Polar coordinate for pupil function; K wherein _iThe amplitude transmittance of representing i annulus, 0≤k _i≤ 1;

The bit phase delay of representing i annulus,

r _I-1And r _iRepresent outer radius in the normalization of i ring; When having only parameter k _iDuring variation, this expression formula is corresponding to the amplitude type iris filter; When having only parameter

During variation, this expression formula is corresponding to the phase-type iris filter; As parameter k _iWith

When all changing, this expression formula is corresponding to complex amplitude type iris filter.

3. a system that realizes axial super resolution in tomography of optical coherent comprises wideband light source (5), 2 * 2 fiber couplers (6), detector (7), data collecting card (8), computer (9), reference arm (10) and the sample arm of being made up of collimating mirror (1), detection object lens (3) and sample (4) successively; It is characterized in that: insert iris filter (2) between collimating mirror of sample arm (1) and detection object lens (3) light path, the light that comes out from collimating mirror (1) is by behind the iris filter (2), focus on sample (4) by surveying object lens (3), collect via surveying object lens (3) from reflected light and scattered light that sample (4) returns, by iris filter (2) and collimating mirror (1), return fiber coupler (6) then once more.

4. a kind of system that realizes axial super resolution in tomography of optical coherent according to claim 3, it is characterized in that: described iris filter (2) is amplitude type iris filter or phase-type iris filter, iris filter is made of N donut, wherein N is the natural number greater than 2, and occurrence is determined according to the resolution requirement of different optical system.

Images