Spectral domain second-harmonic optical coherence tomography

Optical coherence tomography (OCT) provides micrometer-scale structural imaging by coherent detection of backscattered light. Molecular contrast in OCT has been demonstrated using transient absorption, coherent anti-Stokes Raman scattering, and second-harmonic (SH) generation. The sensitivity of molecular contrast signals can be enhanced by use of Fourier domain techniques. We have constructed a spectrometer-based Fourier domain SH-OCT system for simultaneous acquisition of the fundamental and SH signals. We report a >30 dB increase in SH sensitivity over a similar time domain SH-OCT system and demonstrate contrast between cartilage and bone using collagen as the contrast agent.     

Spectral calibration in spectral domain optical coherence tomography

A novel spectral calibration method is developed for spectral domain optical coherence tomography system. The method is based on two measurements of interference spectra from two reference mirror positions. It removes the influence of dispersion mismatch, and hence accurately determines the spectral distribution on the line-scan charge-coupled device (CCD) for sequent precise interpolation. High quality imaging can be realized with this method. Elimination of the degradation effect caused by dispersion mismatch is verified experimentally, and improved two-dimensional (2D) imaging of fresh orange pulp based on the proposed spectral calibration method is demonstrated.     

Efficient excitation of Raman coherence by a gradient force

The vibrational molecular excitation by strong laser fields have been studied. Starting with the molecular Hamiltonian, we derive the vibrational excitation rate due to a gradient of laser field and show that the rate can be two to three orders of magnitude larger than the regular Raman rates. The developed theory contributes to understanding of interaction of powerful laser pulses with molecular systems, and it might be used in developing quantitative coherent Raman spectroscopy. Copyright © 2013 John Wiley & Sons, Ltd.     

Spectral transformations of wideband fields and their coherence functions

Transformations are examined that allow the field and its coherence function to be related to the spectral-angular amplitude density and mean-power flux density of a distributed source, respectively.Kharkov Aviation Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 36, No. 11, pp. 1061–1063, November, 1993.     

Calculation of coherence pathway selection and cogwheel cycles

The selection of proper coherence pathways is a very important aspect of the design of NMR pulse sequences. This article describes a C++ program for the calculation of coherence pathway selection via phase cycles, including a module to calculate cogwheel cycles. Cogwheel phase cycles shorter than the original ones [M.H. Levitt et al., J. Magn. Reson. 155 (2002) 300] are derived and experimentally tested for the MQMAS experiment for 3/2 spins. Some other cogwheel cycles are derived for the MQNQMAS, the STMAS experiment, and a PFG diffusion pulse sequence. This program is publicly available through our website http://www.nyu.edu/projects/jerschow with additional documentation and examples.     

Spectral shaping for non-Gaussian source spectra in optical coherence tomography

We present a digital spectral shaping technique to reduce the sidelobes (ringing) of the axial point-spread function in optical coherence tomography for non-Gaussian-shaped source spectra. The spectra of two superluminescent diodes were combined to generate a spectrum with significant modulation. Images of onion cells demonstrate the improved image quality in a turbid biological sample. A quantitative analysis of the accompanying penalty in signal-to-noise ratio is given.     

Spectral accuracy in fast Ewald-based methods for particle simulations

A spectrally accurate fast method for electrostatic calculations under periodic boundary conditions is presented. We follow the established framework of FFT-based Ewald summation, but obtain a method with an important decoupling of errors: it is shown, for the proposed method, that the error due to frequency domain truncation can be separated from the approximation error added by the fast method. This has the significance that the truncation of the underlying Ewald sum prescribes the size of the grid used in the FFT-based fast method, which clearly is the minimal grid. Both errors are of exponential-squared order, and the latter can be controlled independently of the grid size. We compare numerically to the established SPME method by Essmann et al. and see that the memory required can be reduced by orders of magnitude. We also benchmark efficiency (i.e. error as a function of computing time) against the SPME method, which indicates that our method is competitive. Analytical error estimates are proven and used to select parameters with a great degree of reliability and ease.     

Spectral measurement of absorption by spectroscopic frequency-domain optical coherence tomography

A new method of measurement that essentially combines Fourier-domain optical coherence tomography with spectroscopy is introduced. By use of a windowed Fourier transform it is possible to obtain, in addition to the object structure, spectroscopic information such as the absorption properties of materials. The feasibility of this new method for performing depth-resolved spectroscopy is demonstrated with a glass filter plate. The results are compared with theoretically calculated spectra by use of the well-known spectral characteristics of the light source and the filter plate.     

Spectral flow and vortex dynamics in a temperature gradient

In the mixed state of superconductors the spectral flow of fermion zero modes in the vortex core couples the motion of vortices to that of the normal fluid. This gives rise to a heat current perpendicular to the direction of vortex motion and therefore to longitudinal thermomagnetic effects like the thermopower and the Peltier effect. Analysis of vortex motion in a temperature gradient on this basis yields excellent agreement with experimental results.     

信息散度与梯度角正切相结合的光谱区分方法

提出了将光谱信息散度和光谱梯度角正切相结合的光谱区分方法(SID×tan(SGAπ/2)),克服了现有光谱区分方法难以同时兼顾光谱整体形状和局部特征的不足.利用仿真光谱作为输入源,根据时空联合调制干涉成像光谱仪的干涉图获取原理和光谱复原算法,模拟了其在不同最大掺杂比下对失真光谱的复原,并采用不同区分方法分别比较了复原光...     

Intermolecular double-quantum coherence imaging without coherence selection gradients and its application in in vivo MRI

In the COSY Revamped with Asymmetric Z-gradient Echo Detection (CRAZED) experiments, magnetization is modulated by the distant dipolar field (DDF) generated by coherence selection gradient (CSG) commonly in sinusoidal wave-form and results in detectable intermolecular multiple-quantum coherence (iMQC) signal. IMQCs have some attractive features, but their intrinsic weak signal intensity prevents their widespread applications. In this paper, a new phase cycling scheme was applied to obtain intermolecular double-quantum coherence (iDQC) signal. It is found that DDF can arise from nonspherical sample geometry or background inhomogeneous field in the absence of CSGs, which is more efficient than that created from CSGs. The experimental results show that the resulting DDF can refocus the ± iDQC signals simultaneously and thus enhance the signal intensity to about two folds of that from the conventional CRAZED sequence. Theoretical prediction and experiments give coincident results.     

Slice selection in solids by localized double-quantum coherence transfer

Double-quantum heteronuclear coherence transfer in solids shows a strong spatial dependence when performed in the presence of a magnetic field gradient. This is a direct consequence of the off-resonance sensitivity of the coherence transfer process and represents a new principle for localized NMR spectroscopy of quadrupole nuclei in solids. Since the slice-selective excitation is achieved simultaneously to the cross-polarization, the suggested pulse sequences avoid the use of shaped pulses, the application of which is problematic in solids. In the present work, the localization efficiency of this new slice-selection principle was analyzed in dependence on the experimental parameters for a spin system consisting of abundant spin-1/2 and rare spin-1 nuclei. The resulting slice profiles and the calculated dependences of the slice thickness for the basic coherence transfer procedures are discussed on the example of¹H?²H in monodeuterated benzene. The proposed method opens the possibility of volume-selective investigations of the structure and dynamics of materials using the well-established methodology of deuteron-NMR spectroscopy.     

Spectral domain optical coherence tomography imaging with an integrated optics spectrometer

We designed and fabricated an arrayed-waveguide grating (AWG) in silicon oxynitride as a spectrometer for spectral domain optical coherence tomography (SD-OCT). The AWG has a footprint of only 3.0 cm × 2.5 cm, operates at a center wavelength of 1300 nm, and has 78 nm free spectral range. OCT measurements are performed that demonstrate imaging up to a maximum depth of 1 mm with an axial resolution of 19 μm, both in agreement with the AWG design parameters. Using the AWG spectrometer combined with a fiber-based SD-OCT system, we demonstrate cross-sectional OCT imaging of a multilayered scattering phantom.     

Spectral interferometric optical coherence tomography with nonlinear beta-barium borate time gating

A high-speed, all optical coherence tomography system was designed and constructed. This tomography system employs spectral interferometry and optical Fourier transformation to reduce the number of mechanical scanning dimensions required for multidimensional profilometry. The system also employs a time gate comprising a beta -barium borate crystal driven by a femtosecond laser pulse to improve measurement time. This system has 43-mum depth resolution and 150-fs temporal resolution and is capable of taking 1000 cross-sectional image frames per second.     

用于干涉测量的短相干固体激光器光谱纵模特性

对固体激光器的光谱纵模特性与复相干度的关系进行了理论分析,推导出了在任意纵模数下的光程差与复相干度的关系式。通过比较窄谱宽激光器与谱宽较宽的测量用普通固体激光器在不同纵模数下的复相干度特性,发现固体激光器谱宽较宽,复相干度衰减较快,不同纵模数下的衰减速度和周期不同,纵模数越多复相干度震荡衰减越快,不会出现周期性的高相干区域。根据理论分析搭建了一台用于干涉测量的短相干多纵模固体激光器,并对其光谱纵模特性和相干长度进行了测量,实验结果与计算结果一致。     

Spectral shaping to improve the point spread function in optical coherence tomography

We demonstrate inhibition of the sidelobes of the axial point spread function in optical coherence tomography by shaping the power spectrum of the light source with a remaining power of 4.54 mW. A broadband amplified spontaneous emission source radiating at 1565 +/- 40 nm is employed in a free-space optical coherence tomography system. The axial point spread functions before and after optical spectral shaping are presented. Results show that spectral shaping of the source can inhibit sidelobes of the point spread function up to 12.9 dB, with an associated small increase of 2.2 dB in noise floor in the far field. The effect of spectral shaping on axial resolution is demonstrated according to three metrics. Image quality improvement is also illustrated with optical coherence tomography images of an onion before and after spectral shaping.     

Crystal growth by dynamic gradient reversal techniques - II. General comments on selection

The difference of the admittable time between growth time and dissolution time (delay time) within one period has been deduced. The general course of the growth parameters with time in gradient reversal methods is given. The general influence of experimental parameters on the selection effect is discussed.     

温度梯度对晶粒生长行为影响的相场模拟

利用相场法建立了一个可应用于研究温度梯度影响下的晶粒生长行为的二维模型,模拟了多晶材料退火过程中由温度梯度引起的非均匀晶粒生长和定向晶粒生长行为.结果表明:退火过程中,在静态温度梯度的影响下,体系的晶粒呈现不均匀生长,且从晶粒生长指数来看,不同程度地偏离了正常晶粒生长;在动态温度梯度的影响下,体系内部常出现柱状晶粒生长,柱状晶粒前端持续生长至温度最高位置;柱状晶粒生长与动态热源的移动速率密切相关,只有当动态热源的移动速率处于最小和最大晶粒生长速率之间时,柱状晶粒才会出现. 关键词： 晶粒生长 相场法 晶界迁移率 局部退火     

Spectral coherence and the radiant intensity from statistically homogeneous and isotropic planar sources

We calculate the cross-spectral density and the spatial-frequency spectrum of planar statistically homogeneous and isotropic sources for a variety of prescribed angular distributions of radiant intensity. We discuss various possible definitions for the corresponding spectral coherence area.