紧聚焦条件下相干反斯托克斯拉曼散射信号场的矢量分析

在相干反斯托克斯拉曼散射(coherent anti-Stokes Raman scattering,CARS)显微镜中,共线传输的紧聚焦高斯光束激发具有不同形状和尺寸的待测样品所产生的CARS信号场的空间分布决定了整体系统的结构特点.建立了紧聚焦条件下球形样品产生CARS信号场的理论模型.利用矢量波动方程分析了紧聚焦条件下线偏振的高斯光束的光场强度和相位分布.利用格林函数求解该模型中CARS信号场的矢量波动方程,模拟计算得到了不同直径球形样品的远场CARS信号场的空间分布.理论分析和模拟计算结果表明,对于小体积的球形样品,前向和背向传输的CARS信号场强度接近,因此采用大数值孔径物镜背向探测方式即可获得高对比度图像.对于大体积球形样品,CARS信号场的强度大幅增强,且发射方向主要集中在前向的一定立体角内.因此,采用小数值孔径物镜即可有效收集前向传输的CARS信号.     

相干反斯托克斯拉曼散射显微成像技术研究

基于全量子理论对相干反斯托克斯拉曼散射(CARS)过程进行了分析, 在此基础上搭建了单频CARS显微成像系统, 获得了不同尺寸聚苯乙烯微球高对比度的CARS显微图像. 为了标定成像系统的空间分辨率, 采用逐点扫描方式对直径为110 nm聚苯乙烯微球成像, 从而重构出系统的点扩展函数. 结果表明: 该CARS显微成像系统的横向空间分辨率约为600 nm, 而由阿贝衍射极限决定的理论空间分辨率约为300 nm. 分析了导致分辨率降低的原因, 并提出了解决方案. 为实现纳米分辨的CARS显微成像打下了坚实的基础.     

宽带相干反斯托克斯光谱显微成像技术的实验研究

由于该成像系统采用的超连续谱光源,可以满足所观测样品本源分子在0～4 000 cm-1内的所有拉曼振动活性模式同时共振增强,在探测光的作用下,同时产生宽带相干反斯托克斯拉曼散射信号。然后,根据不同的化学键进行光谱图像重构,可以获得反映不同化学键在样品中的分布的图像。对110 nm的纯的聚苯乙烯珠所形成的具有一定厚度的薄膜,通过改变探测激光与超连续谱脉冲之间的时间重合度,测量形成的相干反斯托克斯拉曼散射信号的时间分布迹线图,通过其中的1 000 cm-1的化学键强度信号进行指数衰减曲线拟合,得出具体的退相时间,与文献中已报道的三色CARS的退相时间相比,判断是否属于三色CARS。为了检验系统在实际生物学成像中存在的问题,我们开展了活体小鼠组织生物学应用成像实验,对记录的数据在2 940 cm-1的CARS信号进行图像重构,获得CH化学键在组织中的分布,然后,对重构图像直接使用小波变换的去噪方式进行图像去噪,去噪后的图像具有比较清晰的轮廓,结果表明,对于对比度比较强的CARS共振信号,直接使用小波变换的去噪方式就可以获得比较好的图像效果。但是,对于信噪比比较差的共振信号,使用这种处理方法是不合适的,需要使用别的方法,先获取好的信号对比度,再根据感兴趣的化学键进行图像重构,然后,再经过小波变换对图像去噪,图像不仅会变得清晰平滑,而且,具有较好的视觉感官效果。     

激光光强扰动对相干场成像降质影响理论研究

激光相干场成像系统发射多束激光,经大气传输对远程目标成像,大气湍流引起的激光束光强扰动是影响成像质量的一个关键因素.本文从湍流引起的激光束光强扰动对回波解调信号的影响关系入手,建立了激光回波光强扰动因子对相位闭合系数和成像频谱分量的降质传函理论模型;基于三光束激光相干场成像系统仿真验证了理论模型的有效性.研究表明激光相干场成像频谱分量和成像像质主要受三光束相位闭合求解算法中第二光束光强扰动影响.该研究揭示了激光回波光强扰动对成像像质的影响机理,对于分析大气湍流等引起的光强扰动降质效应和合理分配多光束光强稳定性以提高成像质量具有理论指导意义.     

幅值和相位配准技术及其在光学相干层析血流成像中的应用

频域光学相干层析系统中扫描机构定位精度、 机械抖动及样品移动会造成A扫描信号幅值和相位发生波动, 影响生物组织成像质量. 利用最小灰度差匹配、Lorentzian曲线极值拟合和谱域光程差补偿等方法对A 扫描信号进行幅值配准. 通过对A扫描信号相位分布特征的匹配实现相位差检测与配准. 通过求已配准的A 扫描复信号之差, 消除静态组织对血流成像的影响. 进行了人眼扫描实验, 有效提取了视网膜三维血流图像. 实验结果表明, 提出的幅值及相位配准方法大大减小了系统扫描精度、人眼跳动等因素对生物组织在体成像质量的影响. 快速、精确的相位配准方法也可广泛应用在多普勒OCT、相位显微等与相位分辨有关的光学成像领域. 关键词： 频域光学相干层析 配准 血流成像 相位分布特征     

扫描相干衍射成像背景噪声分离算法研究

X射线扫描相干衍射成像(ptychography)是一种新型的无透镜成像方法,摆脱了传统透镜成像中聚焦元件对分辨率的限制,使理论分辨率只受到X射线波长和探测器数值孔径的限制.然而实验测量中的噪声限制了该方法对成像质量的改善,甚至最终导致图像重建失败.在研究了ptychography现有的相位恢复迭代算法后,本文提出了一种新型的图像重建迭代算法.该算法利用ptychography数据的高冗余性,通过梯度下降最小化技术,在重建样品和探针图像的同时还完成了背景噪声的同步迭代重建,实现了信号和噪声的盲分离功能.通过仿真模拟和实验数据重建,将该方法与传统的迭代算法进行了对比,结果表明新算法能够较好地实现信噪分离,显著提升ptychography的成像质量.     

二溴甲烷相干反斯托克斯拉曼光谱（CARS）的选择激发

相干反斯托克斯拉曼光谱（CARS）由于信号强度符合无损伤探测等特点，已经日益在生物显微成像、燃烧诊断等领域得到广泛应用。而影响CARS进一步应用的障碍在于非共振信号以及所要激发振动模式周围的能级的干扰。因此要实现对某一振动能级精确激发，或提高拉曼光谱的分辨率，对振动模式的选择激发是非常必要的。     

相干X射线衍射成像的数字模拟研究

相位重建是实现X 射线相干衍射成像的关键, 它利用远场采集的样品傅里叶相干衍射花样、结合过采样理论,再采用迭代算法复原样品的相位信息. 文中采用数字模拟的方法, 利用小尺寸二维非周期性图形作为物场, 研究了过采样比对重构结果的影响, 研究发现, 迭代次数为1000 次时最佳过采样比的范围是3—7 之间. 利用噪声模拟方法, 研究了噪声对相位重建的影响, 找到了完成相位重建的噪声限是信噪比不能低于10. 分析了重构结果中孪生像以及随机平移的产生原因, 并给出了相应的解决办法, 结果表明, 此方法可有效地提高重构图像的质量. 关键词： 相干X射线衍射成像 过采样 相位重建算法 显微成像     

逆合成孔径成像激光雷达实包络成像算法

逆合成孔径激光成像雷达受激光调制技术以及回波相位信息易受大气湍流破坏的限制,采用常规的相位相干积累类方法得到目标二维高分辨图像很困难.针对这一情况,提出了一种基于逆Radon变换的实包络成像算法.利用回波距离脉冲压缩后的实包络信息,实现方位向的非相干积累,最终得到二维高分辨图像.通过该算法,成像系统可以使用非相干激光信号,在脉冲重复频率较低且存在大气湍流的情况下,也可以获得高质量的成像结果.仿真实验验证了此算法的有效性和优越性.     

基于狭缝扫描的表面等离子体共振成像

利用表面等离子体共振技术进行电介质样品成像研究.采用高数值孔径显微物镜作为耦合元件,632.8nm He-Ne激光会聚激发金膜产生表面等离子体共振,通过狭缝光阑限制光束入射角,对金膜上的氮化硅光栅进行成像.反射光由放置在样品像方共轭面上的CCD摄像机接收,获得样品的表面等离子体共振像.通过扫描移动狭缝,得到入射角从44°至54°的扫描样品图像,从图像中提取样品各点的表面等离子体共振曲线,由计算机重构出样品的表面等离子体共振角谱灰度图.     

Image formation in CARS and SRS: effect of an inhomogeneous nonresonant background medium

We investigate the role of a spatially inhomogenous nonresonant background medium on several Raman-based imaging modalities. In particular, we consider a small resonant bead submerged in a spatially heterogeneous nonresonant χ(3) background. Using detailed 3D electrodynamic simulations, we compare coherent anti-Stokes Raman scattering (CARS), frequency-modulated CARS, amplitude-modulated stimulated Raman scattering (SRS), and frequency-modulated SRS. We find that only FM-SRS is background-free.     

Interferometric polarization coherent anti-Stokes Raman scattering (IP-CARS) microscopy

We report a novel interferometry-based polarization coherent anti-Stokes Raman scattering (IP-CARS) implementation for effectively suppressing the nonresonant background while significantly amplifying the resonant signal for vibrational imaging. By modulating the phase difference between the two interference CARS signals generated from the same sample and measuring the peak-to-peak intensity of the periodically modulated interference CARS signal, the IP-CARS technique yields a sixfold improvement in the signal-to-background ratio compared with conventional CARS while providing an approximately 20-fold amplification of the resonant CARS signal compared with conventional polarization CARS. We demonstrate this method by imaging 4.69 microm polystyrene beads and unstained human epithelial cells immersed in water.     

Coherent anti-stokes raman scattering spectral interferometry: determination of the real and imaginary components of nonlinear susceptibility chi(3) for vibrational microscopy

We demonstrate coherent anti-Stokes Raman scattering (CARS) heterodyne spectral interferometry for retrieval of the real and imaginary components of the third-order nonlinear susceptibility (chi(3)) of molecular vibrations. Extraction of the imaginary component of chi(3) allows a straightforward reconstruction of the vibrationally resonant signal that is completely free of the electronic nonresonant background and resembles the spontaneous Raman spectrum. Heterodyne detection offers potential for signal amplification and enhanced sensitivity for CARS microscopy.     

Fourier-transform coherent anti-Stokes Raman scattering microscopy

We report a novel Fourier-transform-based implementation of coherent anti-Stokes Raman scattering (CARS) microscopy. The method employs a single femtosecond laser source and a Michelson interferometer to create two pulse replicas that are fed into a scanning multiphoton microscope. By varying the time delay between the pulses, we time-resolve the CARS signal, permitting easy removal of the nonresonant background while providing high resolution, spectrally resolved images of CARS modes over the laser bandwidth (approximately 1500 cm(-1)). We demonstrate the method by imaging polystyrene beads in solvent.     

Raman-resonance-enhanced composite nonlinearity of air-guided modes in hollow photonic-crystal fibers

Coherent anti-Stokes Raman scattering (CARS) is used to measure relations between the resonant (Raman) and nonresonant (Kerr-type) optical nonlinearities of air-guided modes in a hollow-core photonic-crystal fiber (PCF). We demonstrate that, due to its interference nature, CARS provides a convenient tool for measuring the contribution of the fiber cladding to the total nonlinearity sensed by air-guided modes in hollow PCFs. On a Raman resonance with molecular vibrations in the gas that fills the fiber core, a two-color laser field is shown to induce optical nonlinearities that are several orders of magnitude higher than the nonresonant Kerr-type nonlinearities typical of air-guided PCF modes.     

Heterodyne coherent anti-Stokes Raman scattering (CARS) imaging

We have achieved rapid nonlinear vibrational imaging free of nonresonant background with heterodyne coherent anti-Stokes Raman scattering (CARS) interferometric microscopy. This technique completely separates the real and imaginary responses of nonlinear susceptibility chi(3) and yields a signal that is linear in the concentration of vibrational modes. We show that heterodyne CARS microscopy permits the detection of weak vibrational resonances that are otherwise overshadowed by the strong interference of the nonresonant background.     

Development of rotational CARS for combustion diagnostics using a polarization approach

Rotational Coherent anti-Stokes Raman spectroscopy (CARS) has the last decades been developed into a useful tool for thermometry and concentration measurements in combustion. In this paper, we present a novel polarization approach of the technique, which will enhance its potential and widen the range of conditions at which it can be utilized. The theory of the polarization approach is described in detail. It is shown that by specific arrangement of the polarizations of the laser beams, total suppression of the non-resonant background signal can be obtained, and thus by probing only the resonant CARS signal the diagnostic utility of the technique increases. The main benefit of the approach is in situations where the non-resonant background signal is relatively high in comparison with the resonant signal. The high potential of polarization rotational CARS for thermometry is demonstrated in some illustrative examples, for example, nitrogen thermometry on the fuel side of diffusion flames, and carbon monoxide thermometry in the product gas of ethylene/oxygen/argon-flames.     

Time-resolved coherent anti-Stokes Raman scattering with a femtosecond soliton output of a photonic-crystal fiber

We demonstrate time-resolved coherent anti-Stokes Raman scattering (CARS) by using a frequency-tunable femtosecond soliton output of a silica photonic-crystal fiber (PCF) as a Stokes field. This approach allows quantum beats originating from two close Raman modes to be resolved in the time-domain CARS response. The nonresonant CARS background is efficiently suppressed by introducing a delay time between the probe pulse and the pump-Stokes pulse dyad, suggesting a convenient fiber-optic format for the Stokes source in time-resolved CARS and allowing sensitivity improvement in PCF-based CARS spectroscopes and microscopes.     

Ultrabroadband background-free coherent anti-Stokes Raman scattering microscopy based on a compact Er:fiber laser system

We demonstrate a scheme for efficient coherent anti-Stokes Raman scattering (CARS) microscopy free of nonresonant background. Our method is based on a compact Er:fiber laser source. Impulsive excitation of molecular resonances is achieved by an 11 fs pulse at 1210 nm. Broadband excitation gives access to molecular resonances from 0 cm(-1) up to 4000 cm(-1). Time-delayed narrowband probing at 775 nm enables sensitive and high-speed spectral detection of the CARS signal free of nonresonant background with a resolution of 10 cm(-1).