WO2009109098A1 - 相衬成像方法及设备 - Google Patents

相衬成像方法及设备 Download PDF

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Publication number
WO2009109098A1
WO2009109098A1 PCT/CN2009/000154 CN2009000154W WO2009109098A1 WO 2009109098 A1 WO2009109098 A1 WO 2009109098A1 CN 2009000154 W CN2009000154 W CN 2009000154W WO 2009109098 A1 WO2009109098 A1 WO 2009109098A1
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Prior art keywords
grating
terahertz radiation
diffraction
phase contrast
intensity distribution
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PCT/CN2009/000154
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English (en)
French (fr)
Inventor
康克军
陈志强
张丽
王迎新
赵自然
李元景
刘以农
黄志峰
邢宇翔
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清华大学
同方威视技术股份有限公司
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Application filed by 清华大学, 同方威视技术股份有限公司 filed Critical 清华大学
Priority to US12/920,161 priority Critical patent/US8039803B2/en
Priority to EP09717946A priority patent/EP2256477B1/en
Publication of WO2009109098A1 publication Critical patent/WO2009109098A1/zh

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3581—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation

Definitions

  • the invention belongs to the technical field of Terahertz Imaging, and particularly relates to obtaining phase contrast (phase contrast) information of an object under terahertz radiation by a diffraction grating, thereby enhancing imaging contrast and spatial resolution.
  • Method and equipment Background technique
  • Terahertz (THz, or 10 12 Hz for short) Radiation usually refers to electromagnetic radiation with a frequency between 0.1 and 10 THz, which lies in the far infrared band on the electromagnetic spectrum.
  • terahertz radiation imaging has the following complementary features: Terahertz electromagnetic waves can easily penetrate non-polar or weakly polar dielectric materials, and non-destructive testing can be performed on such materials. Or detect objects hidden by them; terahertz emits low photon energy, does not cause photoionization and damage, and is safer for imaging biological samples; terahertz imaging based on spectral information has the identification of species and composition ability. Therefore, terahertz imaging is a safe and effective non-destructive testing technology, and has broad application prospects in the fields of materials science, biomedicine, and safety inspection.
  • Optics Letters, 1995, 20(16): 1716-1718. discloses an imaging system that simultaneously measures terahertz radiation amplitude and phase information, using a pulse Light source, based on terahertz time-domain spectral analysis (THz-TDS) technology, coherently detects the terahertz pulse time domain waveform, and then obtains the spectral amplitude and phase by Fourier transform, which carries the absorption and refractive index information of the object to terahertz radiation.
  • THz-TDS terahertz time-domain spectral analysis
  • the terahertz time-domain spectrum imaging system has basically been developed on the basis of this.
  • the common feature is that the pump-detection method is complicated and the cost is complicated.
  • phase contrast imaging technology In the field of X-ray imaging, phase contrast imaging technology has been vigorously developed, which broadens the range of X-ray detectable substances to weakly absorbed light element materials and advances the spatial resolution from millimeters to micrometers or even sub- Micron order.
  • X-ray phase contrast imaging technology has developed three different modes to date (4. R. Fitzgerald. Phase-sensitive X-ray imaging. Physics Today, 2000, 53(7): 23-26.), including interferometry Diffraction enhancement method and coaxial-like method, these methods have certain reference significance for phase contrast imaging of terahertz radiation.
  • Document 5 Takeda, et al. X-ray phase imaging with single phase grating.
  • Japanese Journal of Applied Physics, 2007, 46(3): L89-L91. discloses a single-phase grating for X-rays.
  • Experimental system for phase contrast imaging When a phase grating is placed after a weakly absorbing object and irradiated with partially coherent X-rays, the periodic intensity pattern due to the Talbot effect in the grating diffraction field will be deformed, and this intensity distribution is recorded by a CCD (Charge Coupled Device) detector. Then, using the phase restoration algorithm, a phase contrast image of the object can be obtained.
  • This method requires high coherence of the light source and resolution of the CCD, but the system structure is very simple.
  • the wavelength of terahertz radiation is much longer, the grating period is on the order of millimeters, so that the resolution of the detector is only on this order, and the grating of this size is easy to manufacture, and the continuous terahertz source is Coherence is also easy to guarantee, so it is feasible to perform terahertz radiation phase contrast imaging with a single grating.
  • the present invention provides a terahertz radiation phase contrast imaging method and apparatus, which obtains phase change information of terahertz radiation interacting with an object by means of a diffraction grating, thereby generating a phase lining of the object. Degree image.
  • a method for phase contrast imaging using a diffraction grating comprising the steps of: illuminating an object with coherent terahertz radiation to interact with an object; and illuminating the diffraction grating with terahertz radiation after interaction with the object;
  • the diffraction grating is translated along the wave vector direction of the diffraction grating to measure the intensity distribution of the terahertz radiation after the interaction with the object and the grating in the diffraction field for different grating positions; and to restore the phase contrast image of the object according to the light intensity distribution.
  • the step of illuminating the diffraction grating with terahertz radiation after the action of the object comprises: illuminating the diffraction grating with terahertz radiation passing through the object.
  • the step of illuminating the diffraction grating with terahertz radiation after the action of the object comprises: illuminating the diffraction grating with terahertz radiation reflected by the object
  • the diffraction grating is translated within a period of one cycle along the wave vector direction of the diffraction grating, so that for each particular grating position, a plane measuring the terahertz of the diffraction fringe in the grating diffraction field is selected. Radiation intensity distribution.
  • the terahertz radiation intensity distribution is measured point by point in a grid scan manner using a single point detector.
  • the terahertz radiation intensity distribution is measured directly using an area array detector.
  • the step of recovering the phase contrast image of the object according to the light intensity distribution comprises: recovering the phase distribution of the grating incident field in a plane perpendicular to the incident direction according to the periodic intensity map and the diffraction characteristic of the grating; The phase distribution constructs a phase contrast image of the object.
  • a phase contrast imaging apparatus comprising: a terahertz radiation emitter that generates terahertz radiation, illuminates an object to interact with an object; a diffraction grating, terahertz radiation after interaction with the object The diffraction grating; the terahertz radiation detector measures the intensity distribution of the terahertz radiation after the action of the object and the grating in the diffraction field for different grating positions; the data acquisition and processing system recovers the object according to the light intensity distribution Contrast image.
  • the phase contrast imaging apparatus further comprises: a collimating portion disposed on an output side of the terahertz radiation emitter for collimating the terahertz radiation into a parallel beam.
  • the collimating portion is a terahertz lens or a parabolic mirror.
  • the phase contrast imaging apparatus further comprises: translating means for averaging the gratings at equal intervals in the wave vector direction.
  • the translating means translates the diffraction grating within a period of one cycle along the wave vector direction of the diffraction grating, so that for each particular grating position, the terahertz radiation detector selects a plane having a higher contrast ratio of the diffraction fringes in the grating diffraction field. Measure the terahertz radiation intensity distribution.
  • the terahertz radiation detector is a single point detector for measuring the terahertz radiation intensity distribution point by point in a grid scanning manner.
  • the terahertz radiation detector is an area array detector for directly measuring the terahertz radiation intensity distribution.
  • the data acquisition and processing system recovers the phase distribution of the grating incident field in a plane perpendicular to the incident direction based on the periodic intensity map and the diffraction characteristics of the grating; and constructs a phase contrast image of the object based on the phase distribution.
  • the present invention Due to the adoption of the above method and structure, the present invention has the following advantages over the prior art: 1) The present invention implements phase contrast imaging on a continuous wave terahertz system, and enhanced contrast for imaging of weakly absorbing objects And raise the spatial resolution.
  • the invention extracts phase change information of interaction between terahertz radiation and an object by means of a diffraction grating,
  • the system is compact and easy to operate.
  • FIG. 1 is a schematic diagram of the structure of apparatus for terahertz radiation phase contrast imaging using a diffraction grating in accordance with an embodiment of the present invention
  • Figure 2 is a layout diagram illustrating the apparatus of the embodiment of the present invention in a reflective mode
  • FIG. 3 is a detailed structural diagram of a diffraction grating in an apparatus according to an embodiment of the present invention.
  • Fig. 4 shows the total transmittance curve and the associated diffraction efficiency curve when the total thickness of the grating is 360 ⁇ m and the groove depth is varied from 0 to 200 ⁇ m.
  • FIG. 1 is a schematic structural view (side view) of an apparatus for performing transmission terahertz radiation phase contrast imaging using a diffraction grating according to an embodiment of the present invention.
  • the apparatus of the embodiment of the present invention comprises a continuous wave terahertz radiation emitter 1, a terahertz beam collimator lens 2, a transmission diffraction grating 3, a single-point continuous wave terahertz radiation detector 4, a one-dimensional sum Two-dimensional translation devices 51, 52, and a computer-based data collection and processing system 6.
  • the data acquisition and processing system 6 includes a computer, intensity image acquisition software, and phase contrast image generation software, etc., which controls the coordination of the entire device, and simultaneously reads the detector output signal to obtain the intensity distribution after the parallel beam terahertz radiation interacts with the object and the grating. The image is then restored to the phase contrast image of the object according to the phase restoration algorithm.
  • the terahertz radiation emitter 1 uses a continuous wave radiation source, such as a back wave tube (BWO), a terahertz parametric oscillator, a terahertz laser, etc.; because the terahertz wave emitted from the light source needs to be expanded to perform two-dimensional imaging, The power of the light source should be high enough.
  • a continuous wave radiation source such as a back wave tube (BWO), a terahertz parametric oscillator, a terahertz laser, etc.
  • the terahertz radiation detector 4 can be single-point type, such as a thermal radiation meter (Bolometer Pyroelectric Detector ⁇ Golay Cells, etc., or a face array, such as a focal plane array (Microbolometer Focal-Plane). Array Camera) and so on.
  • the diffraction grating 3 has two types of transmission and reflection.
  • the grating material is selected from materials having good transmittance for terahertz radiation, such as high resistance silicon, high density polyethylene (HDPE), and the like.
  • materials with high reflectivity for terahertz radiation, such as metals should be chosen.
  • the beam collimating lens 2 is a terahertz lens which collimates terahertz radiation into parallel beams to illuminate the object and the grating, which makes the analysis of the grating diffraction process and the solution of the phase contrast image simple and easy.
  • Parabolic mirrors are also available for this type of device.
  • the continuous wave terahertz radiation emitter 1 produces a continuous wave of terahertz radiation which is a divergent beam 11, which is collimated by the lens 2 into a parallel beam 12 and then incident on the weakly absorbing sample 71.
  • the phase of the terahertz radiation passing through the sample will change.
  • the phase shift information will appear in the diffraction field of the grating.
  • Select a detection plane 8 parallel to the grating face in which the grating diffraction fringes should have a high contrast ratio, which can be selected by the calculation of the raster simulation software, using a single-point detector 4 for point-by-point scanning
  • the method measures the intensity of terahertz radiation in this plane.
  • the intensity signal measured by the single point detector 4 is transmitted to the data acquisition and processing system 6, while the data collection and processing system 6 controls the two-dimensional translation device 52 to cause the single point detector 4 to move in the xy plane for scanning measurements.
  • the periodic terahertz radiation intensity distribution in plane 8 is finally obtained.
  • the apparatus can also operate in a reflective mode.
  • a terahertz beam collimated by the lens 2 is incident on the surface of the object 72, and the single-point detector 4 measures the reflected wave transmitted through the grating.
  • Periodic intensity distribution Relative to the placement of a reference plane (such as a metal mirror) at the object 72, the phase of the terahertz wave reflected by the object and the reference plane is deviated due to the difference in the contour of the surface of the object and the complex refractive index. Will be expressed in the grating diffraction field. Reflection is commonly used to image objects that are opaque to terahertz radiation.
  • Figure 3 shows the detailed structure of a diffraction grating used in the apparatus according to the invention.
  • a front view 31 and a top view 32 thereof are shown in FIG.
  • the grating is designed with the known frequency of the terahertz radiation source, including material, period, duty cycle, groove depth, thickness and other parameters, so that its diffraction characteristics meet certain requirements, such as suitable diffraction angle, high enough diffraction efficiency.
  • the grating can be placed in the near or far field after the object.
  • the grating is fabricated by etching a rectangular groove having a depth / ⁇ , a width w, and a period on a silicon wafer substrate having a thickness of ⁇ .
  • the above dimensions are set mainly in consideration of factors such as diffraction efficiency, energy distribution between diffraction orders, and diffraction angle.
  • the thickness of the silicon wafer is not strictly required, and it is easy to process; the period determines the number of diffraction orders and the diffraction angle.
  • the period should satisfy i > 300 / , and considering that the incident beam is finite width in practice, the fringe region is distributed within a limited range.
  • Figure 4 shows , /z is a total transmittance curve (331) and 0 (332), ⁇ 1 (333), ⁇ 2 (334) diffraction efficiency curves when changing between 0 and 200. It can be seen that in order to achieve a sufficiently high total transmittance and a non-zero order ( ⁇ 1 order) diffraction efficiency, it is suitable to select a groove depth of about 52 or 183 m.
  • phase information recovery method will be described below by taking a transmissive phase contrast imaging system as an example.
  • the intensity of the terahertz radiation passing through it does not change and therefore cannot be directly imaged.
  • the phase change of the terahertz radiation caused by the object will be reflected in the intensity variation of the grating diffraction field, which in turn can be captured by conventional detectors. If the diffraction characteristics of the grating are known, the phase shift of the incident field can be deduced by the diffraction field intensity distribution.
  • the method of solving the phase shift quantity needs to rely on the grating diffraction theory.
  • the wavelength I of the terahertz radiation is much smaller than the grating period (such as l/d ⁇ 0.1)
  • the scalar theory is applicable; when L and d are close, it should be used.
  • Vector theory The former is an example, assuming that a monochromatic plane wave of wavelength I is incident on the grating (the definition of the coordinate system is as shown in the figure).
  • I(x, y,z) ⁇ a m (z) exp (irnKx) ( 1 ) is the wave vector of the grating
  • the wave vector direction is translated at equal intervals, and the light intensity distribution in the detection plane at each grating position is collected, and then the phase shift amount can be obtained by the data acquisition and processing system 6 combined with the formula (2), and the phase contrast of the object is further drawn. image.
  • the translating device 51 translates the grating within a period of one cycle along the grating wave vector direction, and for each specific grating position, selects a plane with a high contrast ratio of the diffraction fringes in the grating diffraction field, using a single point type
  • the detector 4 measures the terahertz radiation intensity distribution point by point in a grid scan manner or directly measures the two-dimensional signal using an area array detector.
  • the data acquisition and processing system 6 recovers the phase distribution of the grating incident field in a plane perpendicular to the incident direction from the measured periodic intensity map and the diffraction characteristics of the grating, that is, the phase shift of the terahertz radiation caused by the corresponding object. , thereby constructing a phase contrast image of the object.

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Description

相衬成像方法及设备 技术领域
本发明属于太赫兹成像 （Terahertz Imaging) 技术领域， 特别涉及一种通过衍射 光栅获得物体在太赫兹辐射照射下的相位衬度 （Phase Contrast, 相衬） 信息， 从而增 强成像对比度和空间分辨率的方法及设备。 背景技术
太赫兹 （Terahertz, 简称 THz, 即 1012Hz) 辐射通常是指频率在 0.1〜10THz之 间的电磁辐射， 在电磁波谱上位于远红外波段。 相对于传统的可见光、 X射线等成像 技术， 利用太赫兹辐射迸行成像具有如下的互补特征： 太赫兹电磁波容易穿透非极性 或弱极性的电介质材料， 可以对这类材料进行无损检测或者探测被它们隐藏起来的物 体； 太赫兹辐射光子能量低， 不会引起光致电离和损害， 对生物样品成像更为安全； 基于测量光谱信息的太赫兹谱成像技术具有鉴别物质种类和成分的能力。 因而， 太赫 兹成像是一种安全、 有效的无损检测技术， 在材料科学、 生物医学、 安全检查等领域 有着广阔的应用前景。
传统意义的太赫兹成像 ( 1. T. S. Hartwick, et al. Far infrared imagery. Applied Optics, 1976, 15(8): 1919-1922. ) 是利用物质对太赫兹辐射的吸收衰减特性， 反映了太 赫兹辐射与物体相互作用之后的强度分布信息， 即振幅的变化。 对于弱吸收物质， 太 赫兹辐射的衰减程度将很小， 图像对比度较差， 难以分辨物体的内部结构。 然而， 介 质折射率的变化会引起电磁波的相位改变 （相移）， 并且相移程度要比衰减程度大， 检测相移信息更为有效； 如果能够捕捉这种信息， 得到物体的相位衬度图像， 对于弱 吸收物质的成像， 对比度和分辨率将有明显改善 （2. 席再军等. 二维透射式 Terahertz 波时域谱成像研究.光子学报, 1^)6, 35(8): 1171-1174)。文献 3(B. B. Hu and M. C. Nuss. Imaging with terahertz waves. Optics Letters, 1995, 20(16): 1716-1718. ) 披露了一种同时 测量太赫兹辐射振幅和相位信息的成像系统， 该系统利用脉冲式光源， 基于太赫兹时 域谱分析 （THz-TDS ) 技术相干探测太赫兹脉冲时域波形， 然后通过傅立叶变换获得 频谱幅度和相位， 它们携带了物体对太赫兹辐射的吸收和折射率信息。 近年来的太赫 兹时域谱成像系统基本都是以此为基础发展起来的， 其共同特点是所釆取的泵浦一探 测方式使得系统结构复杂、 成本较髙。 在 X射线成像领域， 相位衬度成像技术巳得到了蓬勃的发展， 它将 X射线可检 测的物质范围拓宽到弱吸收的轻元素物质， 并且把空间分辨率由毫米量级推进到微米 甚至亚微米量级。 X射线相衬成像技术至今已发展出三种不同的模式（4. R. Fitzgerald. Phase-sensitive X-ray imaging. Physics Today, 2000, 53(7): 23-26. ) ,包括干涉测量法、衍 射增强法和类同轴法， 这些方法对于太赫兹辐射的相衬成像具有一定的借鉴意义。 文 献 5 ( Υ· Takeda, et al. X-ray phase imaging with single phase grating. Japanese Journal of Applied Physics, 2007, 46(3): L89-L91. )披露了一种采用单相位光栅实现 X射线相衬成 像的实验系统。 当在弱吸收物体之后放置一相位光栅并用部分相干的 X射线照射时， 光栅衍射场内由于 Talbot效应引起的周期性强度图案将发生变形，通过 CCD (电荷耦 合器件）探测器记录这种强度分布，然后利用相位复原算法即可得到物体的相衬图像。 该方法对光源的相干性和 CCD的分辨率要求较高， 但是系统结构非常简单。 相对而 言， 太赫兹辐射的波长要长得多， 光栅周期在毫米量级， 从而探测器的分辨率也只需 在这个量级，并且这种尺寸的光栅制作容易，连续式太赫兹光源的相干性亦容易保证， 因此釆用单光栅的方式进行太赫兹辐射相衬成像是可行的。 发明内容
为了克服现有技术中存在的不足,本发明提出了一种太赫兹辐射相衬成像方法及 设备， 它借助衍射光栅获得太赫兹辐射与物体相互作用后的相位变化信息， 从而生成 物体的相位衬度图像。
在本发明的一个方面，提出了一种利用衍射光栅进行相衬成像的方法，包括步骤- 用相干太赫兹辐射照射物体以与物体相互作用； 使得与物体作用后的太赫兹辐射照射 衍射光栅； 沿衍射光栅波矢方向平移衍射光栅， 以便针对不同的光栅位置， 在衍射场 内测量与物体和光栅作用之后的太赫兹辐射的光强分布； 以及根据光强分布恢复出物 体的相衬图像。
优选地， 使得与物体作用后的太赫兹辐射照射衍射光栅的步骤包括：用穿过物体 后的太赫兹辐射照射衍射光栅。
优选地，得与物体作用后的太赫兹辐射照射衍射光栅的步骤包括：用经物体反射 的太赫兹辐射照射衍射光栅
优选地，沿衍射光栅波矢方向在一个周期的距离之内平移衍射光栅， 以便对于每 一个特定的光栅位置， 选择光栅衍射场内衍射条纹明暗对比度较高的平面测量太赫兹 辐射强度分布。
优选地， 用单点式探测器以格栅扫描方式逐点测量太赫兹辐射强度分布。
优选地， 用面阵式探测器直接测量太赫兹辐射强度分布。
优选地，所述根据光强分布恢复出物体的相衬图像的步骤包括：根据周期性强度 图和光栅的衍射特性， 恢复出光栅入射场在垂直于入射方向的平面内的相位分布； 以 及根据相位分布构建出物体的相衬图像。
在本发明的另一方面， 提出了一种相衬成像设备， 包括： 太赫兹辐射发射器， 产 生太赫兹辐射， 照射物体以与物体相互作用； 衍射光栅， 与物体作用后的太赫兹辐射 照射该衍射光栅； 太赫兹辐射探测器， 针对不同的光栅位置， 在衍射场内测量与物体 和光栅作用之后的太赫兹辐射的光强分布； 数据采集和处理系统， 根据光强分布恢复 出物体的相衬图像。
优选地，该相衬成像设备还包括:设置在太赫兹辐射发射器的输出侧的准直部分， 用于把太赫兹辐射准直成平行光束。
优选地， 所述准直部分是太赫兹透镜或者抛物面镜。
优选地， 该相衬成像设备还包括： 平移装置， 用于将光栅沿波矢方向以等间距平 移。
优选地，平移装置沿衍射光栅波矢方向在一个周期的距离之内平移衍射光栅， 以 便对于每一个特定的光栅位置， 太赫兹辐射探测器选择光栅衍射场内衍射条紋明暗对 比度较高的平面测量太赫兹辐射强度分布。
优选地，所述太赫兹辐射探测器是单点式探测器，用于以格栅扫描方式逐点测量 太赫兹辐射强度分布。
优选地，所述太赫兹辐射探测器是面阵式探测器，用于直接测量太赫兹辐射强度 分布。
优选地，数据采集和处理系统根据周期性强度图和光栅的衍射特性，恢复出光栅 入射场在垂直于入射方向的平面内的相位分布； 以及根据相位分布构建出物体的相衬 图像。
由于采用了上述的方法和结构， 本发明与现有技术相比具有如下几方面的优势： 1 ) 本发明在连续波太赫兹系统上实现了相衬成像， 对于弱吸收物体的成像能够 增强对比度和提髙空间分辨率。
2) 本发明借助衍射光栅提取太赫兹辐射与物体相互作用后的相位变化信息， 系 统结构紧凑、 操作简单。
3 ) 连续波太赫兹辐射的强度测量不需要对时域波形进行扫描， 因此本发明能够 以较快的测量速度获取相位分布信息。 附图说明
从下面结合附图的详细描述中， 本发明的上述特征和优点将更加明显， 其中： 图 1是根据本发明实施例的利用衍射光栅进行太赫兹辐射相衬成像的设备结构示 意图；
图 2是说明本发明实施例的设备在反射模式下的布置图；
图 3是根据本发明实施例的设备中衍射光栅的详细结构示意图；
图 4示出了光栅总厚度为 360μιη、 槽深在 0〜200μιη之间变化时的总透射率曲线 以及相关的衍射效率曲线。 具体实施方式
下面， 参考附图详细说明本发明的优选实施方式。 为了清楚和简明， 包含在这里 的已知的功能和结构的详细描述将被省略， 否则它们将使本发明的主题不清楚。
图 1是根据本发明实施例的利用衍射光栅进行透射式太赫兹辐射相衬成像的设备 结构示意图 （侧视图）。
如图 1所示， 本发明实施例的设备包括连续波太赫兹辐射发射器 1， 太赫兹光束 准直透镜 2， 透射型衍射光栅 3， 单点式连续波太赫兹辐射探测器 4， 一维和二维平移 装置 51、 52， 以及基于计算机的数搪釆集和处理系统 6。 数据采集与处理系统 6包括 计算机、 强度图像采集软件和相衬图像生成软件等， 它控制整个设备协调工作， 同时 读取探测器输出信号得到平行束太赫兹辐射与物体和光栅作用之后的强度分布图像， 然后根据相位复原算法恢复物体的相衬图像。
太赫兹辐射发射器 1釆用连续波辐射源， 比如返波管（BWO )、 太赫兹参量振荡 器、 太赫兹激光器等等； 因为从光源发射的太赫兹波需经扩束做二维成像， 光源的功 率要足够高。
太赫兹辐射探测器 4可以为单点式的， 如热辐射测量仪（Bolometer 焦热电探 测器 （Pyroelectric Detector ) ^ Golay Cells 等， 也可以是面阵式的， 如焦平面阵列 (Microbolometer Focal-Plane Array Camera) 等。 衍射光栅 3具有透射和反射两种类型，对于透射型，光栅材料选取对太赫兹辐射 具有良好透过率的材料， 比如高阻硅、 高密度聚乙烯 （HDPE) 等。 对于反射型， 应 该选择对太赫兹辐射具有高反射率的材料， 比如金属。
光束准直透镜 2是太赫兹透镜，它可以把太赫兹辐射准直成平行光束来照射物体 和光栅， 这使得光栅衍射过程的分析和求解相衬图像简单易行。 这类器件还可以采用 抛物面镜。
连续波太赫兹辐射发射器 1产生的连续波太赫兹辐射为发散光束 11，经透镜 2准 直为平行束 12， 然后入射到弱吸收样品 71上。 透过样品的太赫兹辐射相位将发生改 变， 经过衍射光栅 3之后， 相移信息会在光栅衍射场内表现出来。 选择平行于光栅面 的探测平面 8 (在该平面内， 光栅衍射条紋应该具有高的明暗对比度， 这可以通过光 栅模拟软件的计算来选择）， 利用单点式探测器 4 以逐点扫描的方式测量该平面内的 太赫兹辐射强度。 单点式探测器 4测量的强度信号传输到数据采集与处理系统 6， 同 时数据釆集与处理系统 6控制二维平移装置 52， 使得单点式探测器 4在 x-y平面内运 动进行扫描测量， 最终获得平面 8内的周期性太赫兹辐射强度分布。
上面描述的是该设备工作在透射模式下的过程。 根据本发明实施例的设备也可以 工作在反射方式下， 如图 2所示， 经透镜 2准直的太赫兹光束入射到物体 72表面， 单点式探测器 4测量反射波透过光栅后的周期性强度分布。 相对于在物体 72处放置 一参考平面 （如金属反射镜） 时， 由于物体表面轮廓起伏和复折射率的不同， 经物体 和参考平面反射的太赫兹波相位存在偏差， 这一相位变化信息同样会在光栅衍射场内 表现出来。 反射方式通常用于对太赫兹辐射不透明的物体成像。
图 3示出了根据本发明的设备中使用的衍射光栅的详细结构。 图 3中给出了其正 视图 31及俯视图 32。在太赫兹辐射源频率己知的情况下设计光栅， 包括材料、 周期、 占空比、 槽深、 厚度等参数， 使得其衍射特性满足一定的要求， 如合适的衍射角、 足 够高的衍射效率。 如上所述， 光栅可以置于物体之后的近场或者远场区。 该光栅的制 作方法是在厚度为^的硅片基底上刻蚀出深度/ ί、 宽度 w、周期为 的矩形槽。上述 尺寸的设定主要考虑衍射效率、 各衍射级次之间的能量分配以及衍射角度等因素。 其 中，硅片厚度 没有严格要求，便于加工即可；周期 决定衍射级次的数量和衍射角， 为了产生具有周期性强度分布的衍射场， 必须存在非零级衍射； 占空比通常选为 0.5， 即 W = c /2。 确定上述参数之后， 槽深 将决定各级次的衍射效率。 假设波长为 300 m (频率 1ΤΗζ)、 ΤΕ偏振的单色平面波正入射到光栅上， 首先若要产生非零级 衍射， 根据光栅方程可知周期应满足 i > 300 / ， 此外考虑到实际当中入射光束是 有限宽度的， 条纹区分布在有限范围之内， 为了获得面积更大的条纹区域， ±1级的衍 射角应尽量小， 或者应具有尽量多的衍射级次。例如， d = 2mm， 此时最高级次为 6， +1级衍射角为 8.6° 。基于以上参数选择，计算不同槽深情况下各级次的衍射效率， 图 4给出了
Figure imgf000008_0001
、 /z在 0— 200 之间变化时的总透射率曲线（331 )以及 0 级 （332)、 ±1级 （333 )、 ±2级 （334)衍射效率曲线。 可以看出， 为了实现足够高的 总透射率和非零级（±1级）衍射效率，选择槽深为 52 或者 183 m左右是合适的。
下面以透射式相衬成像系统为例说明相位信息恢复方法。
对于无吸收的相位物体， 太赫兹辐射穿过其后的强度没有变化， 因此不能直接成 像。 然而， 在物体后面放置一光栅时， 物体引起的太赫兹辐射相位改变将反映在光栅 衍射场的强度变化上， 这种强度信息进而可以通过常规的探测器捕捉到。 如果已知光 栅的衍射特性， 由衍射场强度分布可以反推出入射场的相移量。
相移量的求解方法需要借助光栅衍射理论， 当太赫兹辐射波长 I比光栅周期 小 很多时 （如 l/d < 0.1 )， 标量理论是适用的； 当 L与 d比较接近时， 则应该采用矢量 理论。 以前者为例， 假设波长为 I的单色平面波正入射到光栅上 （坐标系的定义如图
1所示）， 透射场的光强分布为
I(x, y,z) = ^ am (z) exp (irnKx) ( 1 ) 是光栅的波矢，
Figure imgf000008_0002
是光栅传输函数的傅立叶展开系数。 当光栅之前存在一相位物体时， 太赫兹辐射穿 过物体将发生相位移动 （对于反射式的情况， 太赫兹辐射经物体反射后相位分布发生 变化）， 假设相移量分布为 Φ(Χ, : ) , 此时光栅透射场的强度分布变为：
I(x,y,z) « - ζφ(χ, (2)
Figure imgf000008_0003
m 其中 ， = 3φ(χ， 。 在一个周期的距离之内， 通过平移装置 51将光栅沿其
2π dx
波矢方向以等间距平移， 采集各个光栅位置下的探测平面内的光强分布， 然后由数据 采集与处理系统 6联合公式 （2) 即可求出相移量， 进一步绘出物体的相衬图像。
例如， 平移装置 51沿光栅波矢方向在一个周期的距离之内平移光栅， 对于每一 个特定的光栅位置， 选择光栅衍射场内衍射条纹明暗对比度较高的平面， 采用单点式 探测器 4以栅格扫描的方式逐点测量太赫兹辐射强度分布或者利用面阵探测器直接测 量该二维信号。
然后，数据采集与处理系统 6由测得的周期性强度图和光栅的衍射特性，恢复出 光栅入射场在垂直于入射方向的平面内的相位分布， 即对应物体引起的太赫兹辐射的 相位移动， 从而构建出物体的相衬图像。
上面的描述仅用于实现本发明的实施方式， 本领域的技术人员应该理解， 在不脱 离本发明的范围的任何修改或局部替换， 均应该属于本发明的权利要求来限定的范 围， 因此， 本发明的保护范围应该以权利要求书的保护范围为准。

Claims

权利要求
1、 一种利用衍射光栅进行相衬成像的方法， 包括步骤- 用太赫兹辐射照射物体以与物体相互作用；
使得与物体作用后的太赫兹辐射照射衍射光栅；
沿衍射光栅波矢方向平移衍射光栅， 以便针对不同的光栅位置，在衍射场内测量 与物体和光栅作用之后的太赫兹辐射的光强分布； 以及
根据光强分布恢复出物体的相衬图像。
2、 如权利要求 1所述的方法， 其中使得与物体作用后的太赫兹辐射照射衍射光 栅的步骤包括- 用穿过物体后的太赫兹辐射照射衍射光栅。
3、 如权利要求 1所述的方法， 其中使得与物体作用后的太赫兹辐射照射衍射光 栅的步骤包括：
用经物体反射的太赫兹辐射照射衍射光栅。
4、 如权利要求 1所述的方法， 其中， 沿衍射光栅波矢方向在一个周期的距离之 内平移衍射光栅， 以便对于每一个特定的光栅位置， 选择光栅衍射场内衍射条紋明暗 对比度较高的平面测量太赫兹辐射强度分布。
5、 如权利要求 4所述的方法， 其中， 用单点式探测器以格栅扫描方式逐点测量 太赫兹辐射强度分布。
6、 如权利要求 4所述的方法， 其中， 用面阵式探测器直接测量太赫兹辐射强度 分布。
7、 如权利要求 1所述的方法， 其中所述根据光强分布恢复出物体的相衬图像的 步骤包括：
根据周期性强度图和光栅的衍射特性，恢复出光栅入射场在垂直于入射方向的平 面内的相位分布； 以及
根据相位分布构建出物体的相衬图像。
8、 一种相衬成像设备， 包括- 太赫兹辐射发射器， 产生太赫兹辐射， 照射物体以与物体相互作用； 衍射光栅， 与物体作用后的太赫兹辐射照射该衍射光栅；
太赫兹辐射探测器，针对不同的光栅位置，在衍射场内测量与物体和光栅作用之 后的太赫兹辐射的光强分布；
数据釆集和处理系统， 根据光强分布恢复出物体的相衬图像。
9、 如权利要求 8所述的相衬成像设备， 还包括：
设置在太赫兹辐射发射器的输出侧的准直部分，用于把太赫兹辐射准直成平行光 束。
10、如权利要求 9所述的相衬成像设备，所述准直部分是太赫兹透镜或者抛物面 镜。 '
11、 如权利要求 8所述的相衬成像设备， 还包括：
平移装置， 用于将光栅沿波矢方向以等间距平移。
12、 如权利要求 11所述的相衬成像设备， 其中平移装置沿衍射光栅波矢方向在 一个周期的距离之内平移衍射光栅， 以便对于每一个特定的光栅位置， 太赫兹辐射探 测器选择光栅衍射场内衍射条纹明暗对比度较高的平面测量太赫兹辐射强度分布。
13、 如权利要求 12所述的相衬成像设备， 其中所述太赫兹辐射探测器是单点式 探测器， 用于以格栅扫描方式逐点测量太赫兹辐射强度分布。
14、 如权利要求 12所述的相衬成像设备， 其中所述太赫兹辐射探测器是面阵式 探测器， 用于直接测量太赫兹辐射强度分布。
15、如权利要求 8所述的相衬成像设备，其中数据釆集和处理系统根据周期性强 度图和光栅的衍射特性， 恢复出光栅入射场在垂直于入射方向的平面内的相位分布； 以及根据相位分布构建出物体的相衬图像。
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EP2256477B1 (en) 2012-08-01
US20110024624A1 (en) 2011-02-03
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