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.     

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.     

Two‐beam multiplexed CARS based on a broadband oscillator

A two‐beam multiplexed coherent anti‐Stokes Raman scattering (CARS) microscopy setup is demonstrated by using a broadband (BB) Ti:sapphire oscillator without using any specialty fibres. A well‐defined spectral structure of the source leads to a delay‐sensitive CARS measurement in two‐colour CARS and also provides an efficient means of obtaining three‐colour CARS signals combined with the dispersion compensation of the BB pulse. Our result implies that the background suppression is limited by the onset of the spurious signals caused by the different CARS process, qualitatively differing from what is typically observed in the CARS microscopy. Copyright © 2010 John Wiley & Sons, Ltd.     

可调空间分辨USEDCARS技术研究

相干反斯托克斯拉曼散射（CARS）技术是一种非常重要的燃烧诊断技术,该技术具有非常强的抗干扰能力和非常高的测量精度。但空间分辨力不足会使CARS技术产生很强的空间平均效应,引起成CARS光谱畸变,进而造成CARS光谱分析困难,无法通过CARS光谱反演燃烧场参数。针对非稳腔空间增强探测CARS（USEDCARS）技术存在的空间分辨不足以及空间分辨力不易改变的特点,分析了影响USEDCARS技术测量空间分辨力的各种因素,采用一组轴棱锥对USEDCARS系统中的泵浦激光进行环状光束整形,并通过调节轴棱锥之间的距离获得了不同直径的环状光束,在此基础上,建立了空间分辨可调USEDCARS诊断系统。开展了空间分辨力分析实验,获得了CARS信号强度随空间位置的分布数据,以CARS信号总强度95%包含的空间区域代表CARS的纵向空间分辨力,以此计算得到了CARS系统空间分辨力为1.7～6.5 mm连续可调。其中,高分辨力情况,达到了现有BOXCARS技术的空间分辨力。利用所建立的空间分辨可调USEDCARS诊断系统测量了酒精/空气预混火焰温度参数,获得了不同空间分辨条件下的CARS光谱。空间分辨力为1.7 mm时,获得了高质量CARS光谱,通过光谱拟合给出了所测火焰的温度信息。分辨力分别为4.9和6.5 mm时获得了较强的CARS信号,但存在光谱畸变。结果显示,空间分辨力对CARS信号的强度和空间平均效应有很大地影响,提高测量的空间分辨力可以有效消除空间平均效应,获得准确的CARS光谱,增强光谱拟合精度,同时空间分辨可调的特性使该系统能够更好地适应不同实验条件下的诊断工作。     

Selective excitation and suppression of coherent anti-Stokes Raman scattering by shaping femtosecond pulses

Femtosecond coherent anti-Stokes Raman scattering (CARS) suffers from poor selectivity between neighbouring Raman levels due to the large bandwidth of the femtosecond pulses. This paper provides a new method to realize the selective excitation and suppression of femtosecond CARS by manipulating both the probe and pump (or Stokes) spectra. These theoretical results indicate that the CARS signals between neighbouring Raman levels are differentiated from their indistinguishable femtosecond CARS spectra by tailoring the probe spectrum, and then their selective excitation and suppression can be realized by supplementally manipulating the pump (or Stokes) spectrum with the $\pi $ spectral phase step.     

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.     

燃烧流场线CARS测温技术研究

常规CARS采用凸透镜聚焦多束激光于空间一点,在满足相位匹配条件下产生携带该点温度信息的CARS信号。常规CARS一次只能测量一个空间点的温度,难以满足燃烧流场深入研究需要。为了提高CARS测量能力,使得CARS在一次测量中获得更多信息,提出了线CARS测量方法。线CARS测量方法在常规CARS基础上采用柱面凸透镜替换普通凸透镜,使得聚焦位置由焦点变为焦线。由于焦线上的点大部分满足相位匹配关系,因此可以同时获得多点CARS信号。后续光路同样采用柱面凸透镜替换普通凸透镜,通过光谱仪和ICCD相机将CARS信号传输至计算机,解析出聚焦线上CARS信号对应的温度信息,实现CARS测量能力由“点”到“线”的提升。基于平面火焰炉的燃烧实验结果表明：线CARS可以一次有效测量200个空间点的温度信息,空间测量长度约3.6 mm,空间分辨率约18 μm,测量结果相对不确定度优于7%,在保持测量精度的同时有效丰富了单次测量信息。     

基于光强传输方程相位成像的宽场相干反斯托克斯拉曼散射显微背景抑制

相干反斯托克斯拉曼散射(CARS)显微能够对样品的特殊化学组分进行选择性成像,无需荧光标记,在生物医学领域被广泛应用.然而,传统的CARS图像往往存在非共振背景信号.本文将基于光强传输方程的单光束相位成像技术用于CARS显微成像,来抑制CARS的非共振背景信号.该方法通过记录样品在三个相邻平面上的CARS图像,然后利用光强传输方程获取CARS光场的相位分布,最后利用共振CARS信号和非共振背景信号在相位上的差异,实现了对背景噪声的抑制.该方法无需参考光,通过三次测量可完成CARS的背景噪声抑制,具有良好的应用前景.     

Comparison of the Raman detection limit for IRS- and CARS-spectroscopy at excitation near molecular one-photon resonance

The nonlinear Raman methods IRS and CARS are compared according to the signal detectability at excitation of molecules under the condition of one-photon resonance. At one photon resonance it is the background contribution resulting from the scattering molecules themselves that determines the maximum attainable signal to noise ratio. These contributions acting for IRS and CARS respectively are compared. The essential difference between IRS and CARS results from a 3rd order saturation contribution to IRS, which may mask the IRS Raman signal near exact resonance while it does not contribute to CARS. This gives to CARS the preference before IRS at resonance excitation. The situation for IRS with respect to background is similar to that of spontaneous Raman scattering, where the resonance fluorescence — corresponding to the saturation contribution at IRS — masks the resonance Raman signal.     

CARS thermometry in high temperature gradients

CARS is an effective non-intrusive technique for measuring gas temperature in combustion environments. In regions of high temperature gradient, however, the CARS signal is complicated by contributions from gas at different temperature. This paper examines theoretically the uncertainty associated with CARS thermometry in steep temperature gradients. In addition, the work compares the temperature predicted from CARS with the adiabatic mixed temperature of the gas resident in the measurement volume. This comparison helps indicate the maximum sample volume size allowed for accurate temperature measurements.     

CARS difference spectroscopy

Summary In this paper we present a vibrational-spectroscopy technique which combines the advantages of coherent anti-Stokes Raman spectroscopy (CARS) and linear Raman difference spectroscopy. The method which we call CARS difference spectroscopy can be applied for the study of small frequency shifts and/or bandwidth changes in the CARS spectra of liquid mixtures and solutions. First we develop the theory necessary for the interpretation of experimental data obtained from CARS difference measurements of mixtures of two Raman-active liquids and present some model calculations for benzene-toluene mixtures of different concentrations. Then the experimental arrangement used for CARS difference measurements as well as some examples of recorded spectra are described. We show that it is possible to observe the effects of dilution on CARS spectra with high accuracy by applying the discussed technique. Paper presented at the “XI European CARS Workshop”, Florence, Italy, 23–25 March, 1992.     

相干反斯托克斯拉曼散射显微镜的成像特性

对自身不发荧光且不便于荧光标记的化学或生物学样品，集相干反斯托克斯拉曼散射与激光共焦扫描显微镜于一身的相干反斯托克斯拉曼散射显微镜是一种好的选择。因为相干反斯托克斯拉曼散射是一种非线性过程，相干反斯托克斯拉曼散射显微镜的显微成像特性与一般的共焦显微镜非常不同。首先计算了焦点附近相干反斯托克斯拉曼散射激发场的偏振分布，然后，利用格林函数方法，得到了以赫兹偶极子为源的波动方程的精确解，发现对于不同的成像配置和样品形状，像场的相干反斯托克斯拉曼散射场分布非常不同，因此传统的显微镜成像表征方式（如点扩展函数）将不再能描述相干反斯托克斯拉曼散射显微镜的成像特性。     

Phase‐locking of two independent degenerate coherent anti‐Stokes Raman scattering processes: concept,proposed all‐optical implementation,and potential applications

A novel approach toward phase‐locking of two independently produced yet energetically degenerate coherent anti‐Stokes Raman scattering (CARS) processes is put forward. The proposed all‐optical implementation involves a modified Mach–Zehnder interferometer, which is utilized to transfer phase coherence from three totally uncorrelated laser beams into two degenerate CARS beams that are produced in two distinct Raman active samples. Such a CARS interferometer based on coherent phase transport allows explicit measurement and control of phase differences between the two phase‐locked degenerate CARS processes, and hence may find applications in pertinent research fields such as CARS spectroscopy (tomography) as well as quantum information processing and transfer. Copyright © 2011 John Wiley & Sons, Ltd.     

Chirped coherent anti-Stokes Raman scattering as a high-spectral- and spatial-resolution microscopy

Coherent anti-Stokes Raman scattering (CARS) microscopy is a promising tool for chemically selective imaging based on molecular vibrations. While CARS is currently used as a biological imaging tool, many variations are still being developed, perhaps the most important being multiplex CARS microscopy. Multiplex CARS has the advantage of comparing images based on different molecular vibrations without changing the excitation wavelengths. Here we demonstrate both high-spectral- and spatial-resolution multiplex CARS imaging of polymer films, using a simple scheme for chirped CARS with a spectral bandwidth of 300 cm(-1).     

高阶相干喇曼散射光谱研究

本文报道了利用YAG激光系统CARS装置研究方解石的高阶相干喇曼散射(CRS)的实验结果,观察到1—6阶相干反斯托克斯喇曼散射谱(CARS)和1—2阶的相干斯托克斯喇曼散射谱(CSRS),讨论了影响高阶CRS产生的因素及有关的问题。 关键词：     

Characterization of three-color CARS in a two-pulse broadband CARS spectrum

We demonstrate that a broadband coherent anti-Stokes Raman scattering (CARS) spectrum generated with a typical two-pulse scheme contains two distinct, significant signals: '2-color' CARS, where the pump and probe are provided by a narrowband pulse and the continuum pulse constitutes the Stokes light, and '3-color' CARS, where the pump and Stokes are provided by two different frequency components in the continuum pulse and the narrowband pulse serves as the probe. The CARS spectra from the two different mechanisms show distinct characteristics in Raman shift range, laser power dependence, and chirping dependence. We discuss the potential for a 3-color CARS signal to cover the fingerprint region with reduced photodamage of live cells. Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.     

双光子荧光与相干反斯托克斯拉曼散射显微成像技术的实验研究

双光子荧光与相干反斯托克斯拉曼散射同属于三阶非线性效应,二者之间的差异与联系是一个值得研究的问题.本文基于自行搭建的超连续谱近红外宽带相干反斯托克斯拉曼散射显微成像系统进行光谱成像,同时通过理论与实验对比分析了双光子荧光与相干反斯托克斯拉曼散射图像存在差异的原因.结果表明,具有亚微米以上横向分辨率的相干反斯托克斯拉曼散射成像系统,可以使用较大尺寸的荧光珠进行双光子荧光成像,通过解卷积得到双光子荧光成像的系统分辨率,并将它近似等效于相干反斯托克斯拉曼散射成像系统的当下分辨率.如果需要得到相干反斯托克斯拉曼散射成像系准确的分辨率结果,就必须使用尺寸比相干反斯托克斯拉曼散射成像系统实际分辨率小的球形样品进行实验测量.     

Narrow-band coherent anti-stokes Raman signals from broad-band pulses

By tailoring the phase of a 100 femtosecond probe pulse we are able to obtain a narrow-band coherent anti-Stokes Raman spectroscopy (CARS) resonant signal with a width of less than 15 cm(-1), which is an order of magnitude narrower than the CARS signal from a transform limited pulse. Thus, by measuring the spectrum of the CARS signal we are able to obtain a high-resolution energy level diagram of the probed sample in spite of the broad femtosecond pulse spectrum.     

High-sensitivity coherent anti-Stokes Raman scattering microscopy with two tightly synchronized picosecond lasers

We demonstrate a significant improvement in signal-to-noise ratio in coherent anti-Stokes Raman scattering (CARS) spectroscopy/microscopy, using two highly synchronized picosecond Ti:sapphire lasers. A temporal jitter between the pulse trains from the two independent commercial lasers is reduced from a few picoseconds to ~21 fs , maintained over several hours. The tight synchronization brings the fluctuation of the CARS signal down to the shot-noise limit, leading to enhanced CARS vibrational images of living cells and polymer beads.     

Ultrabroadband (>2000 cm-1) multiplex coherent anti-Stokes Raman scattering spectroscopy using a subnanosecond supercontinuum light source

We have developed ultrabroadband (>2000 cm(-1)) multiplex coherent anti-Stokes Raman scattering (CARS) spectroscopy using a subnanosecond (sub-ns) microchip laser source. A photonic crystal fiber specifically designed for sub-ns supercontinuum (SC) generation has been used for obtaining ultrabroadband Stokes radiation, which enables us to achieve simultaneous vibrational excitation in the range from 800 to 3000 cm(-1). We have successfully obtained multiplex CARS spectra for several molecular liquids. Since the CARS system using the sub-ns SC is simple and compact, it can be easily applied to ultrabroadband multiplex CARS microspectroscopy.