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在时域相干层析系统中,太赫兹波的谱密度函数G(ω)与自身的实相干函数G(τ)是一个傅里叶变换对[24],即:
$ G\left( \tau \right)\text{=}\left\langle E\left( t \right)\cdot E\left( t+\tau \right) \right\rangle =\int_{-\infty }^{\infty }{G\left( \omega \right){{\text{e}}^{\text{i}\omega \tau }}\text{d}\omega } $
(1) 式中,ω为角频率; τ为时延; t为时间变量;E(t)为太赫兹波的电场分量。
若在太赫兹波的频谱中引入与深度扫描路径相关的相位Φ0(Φ0=ωτ0,τ0为时延),并考虑色散的影响,(1)式可改写成:
$ \begin{align} & {{G}_{\text{disp}}}\left( \tau +{{\tau }_{0}} \right)=\left\langle E\left( t \right)\cdot E\left( t+\tau \right) \right\rangle \text{=} \\ & \text{ }\int_{-\infty }^{\infty }{G(\omega ){{\text{e}}^{-\text{i}({{\mathit{\Phi }}_{0}}+{{\mathit{\Phi }}_{\text{disp}}}}}){{\text{e}}^{\text{i}\omega \tau }}\text{d}\omega } \\ \end{align} $
(2) 式中,Φdisp为样品的色散相位。
由(2)式可知,Φdisp的引入导致时域的信号包络产生展宽,色散补偿正好可以抵消这种展宽。在时域相干层析系统中,由于未知样品的色散系数和厚度,因此可以从初始设定的评价函数出发,不断地循环调用评价函数,直到达到初始设定的阈值时便可得到最佳补偿结果。
为了获得频域信号Γ″(ω),在所设计的时域相干层析系统中对原始干涉信号I(Δz)的自相关项Γ(Δz)进行希尔伯特变换(Hilbert transform, HT)和快速傅里叶变换(fast Fourier transform, FFT)。然后将Γ″(ω)的相位与已知的2阶和3阶色散做差,再利用快速傅里叶逆变换(inverse fast Fourier transform, IFFT)便可得到色散补偿后的干涉信号。在色散补偿过程中,色散评价函数的设计是核心。通过不断调整每次循环的2阶和3阶的色散补偿系数,可以使色散补偿达到最佳效果。所设计的时域相干层析系统中,迭代补偿算法的流程图如图 2所示。
基于太赫兹相干层析成像系统色散补偿的研究
Study on dispersion compensation based on terahertz coherent tomographic imaging system
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摘要: 为了提高太赫兹相干层析成像系统的纵向分辨率,采用色散补偿方法解决了宽带光源做成像光源带来的色散问题。基于评价函数的迭代补偿算法对降噪滤波后的原始信号进行色散补偿,得到了较为清晰的3维重构图像。结果表明,在迭代补偿算法的计算中,无需预知样品的厚度信息以及色散特性,即可获得高精度的3维重构图像,纵向分辨率高达100μm。该研究在高精度的材料无损探测领域具有较大的研究价值和广泛的应用前景。Abstract: In order to improve the longitudinal resolution of a terahertz coherence tomographic imaging system, the dispersion compensation method was used to solve the dispersion problem caused by the broadband light source. The iterative compensation algorithm based on the evaluation function was used to compensate the dispersion of the original signal after noise reduction, and clear 3-D reconstructed images were obtained. The results show that 3-D high precision reconstruction images can be obtained without predicting the thickness information and dispersion characteristics of the sample. The longitudinal resolution is up to 100μm. This research has great research value and wide application prospect in the field of high precision nondestructive detection.
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[1] JOHNSON J L, DORNEY T D, MITTLEMAN D M. Background-free THz imaging using interferometric tomography[C]//Ultrafast Phenomena Ⅻ. Springer Series in Chemical Physics. Berlin, Heidelberg: Springer, 2001, 66: 262-264. [2] JI W, SUN Sh F, WANG S, et al. Analysis of noise model of optical coherence tomography image in logarithmic domain[J]. Laser Technology, 2014, 38(6): 848-853(in Chinese). [3] HU X Y, LIU L, LU Zh G. Application of optical coherence tomography[J]. Laser Technology, 1998, 22(6): 339-342(in Chinese). [4] TEARNEY G J, BREZINSKI M E, BOUMA B E, et al. In vivo endoscopic optical biopsy with optical coherence tomography[J]. Science, 1997, 276(5321): 2037-2039. doi: 10.1126/science.276.5321.2037 [5] YASUNO Y, ENDO T, MAKITA S, et al. Three-dimensional line-field Fourier domain optical coherence tomography for in vivo dermatological investigation[J]. Journal of Biomedical Optics, 2006, 11(1): 014014. doi: 10.1117/1.2166628 [6] ENDO T, YASUNO Y, MAKITA S, et al. Profilometry with line-field Fourier-domain interferometry[J]. Optics Express, 2005, 13(3): 695-701. doi: 10.1364/OPEX.13.000695 [7] BU P, WANG X Ch, OSAMI S. Fourier-domain optical coherence tomography based on sinusoidal phase modulation[J]. Acta Optica Sinica, 2007, 27(8): 1470-1474(in Chinese). [8] DUAN L, HE Y H, ZHU R, et al. Development of a spectrum domain 3-D optical coherence tomography system[J]. Chinese Journal of Lasers, 2009, 36(10): 2528-2533(in Chinese). doi: 10.3788/JCL [9] YANG L, WANG C, DING Zh H, et al. Image reconstruction in dioptric media for spectral domain optical coherence tomography[J]. Chinese Journal of Lasers, 2011, 38(5): 0504001(in Chinese). doi: 10.3788/CJL [10] WU T, DING Z, WANG K, et al. Swept source optical coherence tomography based on non-uniform discrete Fourier transform[J]. Chinese Optics Letters, 2009, 7(10): 941-944. doi: 10.3788/COL [11] QIN Y W. Study on optical coherence tomography detection of ZnO film[J]. Laser Technology, 2014, 38(6): 845-847(in Chinese). [12] SHI W, LI J Ch, HAN J, et al. Ball bearing measurement based on white-light interferometry technique[J]. Laser Technology, 2014, 38(5): 623-626(in Chinese). [13] CHEN Y P. Research of spectral-domain optical coherence tomography under white light irradiation[J]. Laser Technology, 2014, 38(3): 372-374(in Chinese). [14] QIN Y W. Study on micro-electromechanical system measurement using optical coherence tomography[J]. Laser Technology, 2013, 37(5): 664-667(in Chinese). [15] QIN Y W. Film thickness measurement based on optical coherence tomography[J]. Laser Technology, 2012, 36(5): 662-664(in Chinese). [16] MA B, SUI Q M, XU J. Design and application of new OCT scheme[J]. Microcomputer Information, 2008, 24(21):270-271(in Chinese). [17] HITZENBERGER C K, DREXLER W, BAUMGARTNER A, et al. Dispersion effects in partial coherence interferometry[J]. Proceedings of the SPIE, 1997, 2981:29-36. doi: 10.1117/12.274319 [18] HITZENBERGER C K, BAUMGARTNER A, FERCHER A. Dispersion induced multiple signal peak splitting in partial coherence interferometry[J]. Optics Communications, 1998, 154(4): 179-185. doi: 10.1016/S0030-4018(98)00280-6 [19] BAUMGARTNER A, HITZENBERGER C K, DREXLER W, et al. Resolution enhancement of partial coherence interferometry by dispersion compensation[J].Proceedings of the SPIE, 1997, 3192:162-170. doi: 10.1117/12.297837 [20] TUMLINSON A R, HOFER B, WINKLER A M, et al. Inherent homogenous media dispersion compensation in frequency domain optical coherence tomography by accurate k-sampling[J]. Applied Optics, 2008, 47(5): 687-693. doi: 10.1364/AO.47.000687 [21] GONG Q, JIANG J, WANG R K, et al. Dispersion compensation methods for ultrahigh-resolution optical coherence tomography[J]. Proceedings of the SPIE, 2006, 6047: 60471S. doi: 10.1117/12.710898 [22] DREXLER W, MORGNER U, GHANTA R K, et al. Ultrahigh-resolution ophthalmic optical coherence tomography[J]. Nature Medicine, 2001, 7(4): 502-507. doi: 10.1038/86589 [23] BOUMA B, TEARNEY G J, BOPPART S A, et al. High-resolution optical coherence tomographic imaging using a mode-locked Ti:Al2O3 laser source[J]. Optics Letters, 1995, 20(13):1486-1488. doi: 10.1364/OL.20.001486 [24] HITZENBERGER C K, BAUMGARTNER A, DREXLER W, et al. Dispersion effects in partial coherence interferometry: Implications for intraocular ranging[J]. Journal of Biomedical Optics, 1999, 4(1): 144-151. doi: 10.1117/1.429900 [25] CHEN Y, LI X. Dispersion management up to the third order for real-time optical coherence tomography involving a phase or frequency modulator[J]. Optics Express, 2004, 12(24): 5968-5978. doi: 10.1364/OPEX.12.005968 [26] FERCHER A F, HITZENBERGER C K, STICKER M, et al. Numerical dispersion compensation for partial coherence interferometry and optical coherence tomography[J]. Optics Express, 2001, 9(12): 610-615. doi: 10.1364/OE.9.000610 [27] TAO T, LIAO R, LV J. A new method to compensate dispersion in optical coherence tomography[J]. Optical Instruments, 2006, 28(5): 22-26(in Chinese). [28] MARKS D L, OLDENBURG A L, REYNOLDS J J, et al. Autofocus algorithm for dispersion correction in optical coherence tomography[J]. Applied Optics, 2003, 42(16): 3038-3046. doi: 10.1364/AO.42.003038 [29] TAO T. Dispersion effection in optical coherence tomography thoery[D]. Hangzhou: Zhejiang University, 2006: 62-66(in Chinese). [30] DREXLER W, MORGNER U, KÄRTNER F, et al. In vivo ultrahigh-resolution optical coherence tomography[J]. Optics Letters, 1999, 24(17): 1221-1223. doi: 10.1364/OL.24.001221 [31] BORN M, WOLF E. Principles of optics: electromagnetic theory of propagation, interference and diffraction of light[M]. 7th ed. Cambridge, UK: Cambridge University Press, 2000: 417-429. [32] HUANG Y X, YAO J Q, LING F R, et al. Terahertz imaging technology based on coherent tomography[J]. Laser & Infrared, 2015, 45(10): 1261-1265(in Chinese).