On strong graph bundles

We study strong graph bundles : a concept imported from topology which generalizes both covering graphs and product graphs. Roughly speaking, a strong graph bundle always involves three graphs $E$, $B$ and $F$ and a projection $p:E\to B$ with fiber $F$ (i.e. $p^{?1}\left(x\right)?F$ for all $x\in V\left(B\right)$) such that the preimage of any edge $xy$ of $B$ is trivial (i.e. $p^{?1}\left(xy\right)?K_2?F$). Here we develop a framework to study which subgraphs $S$ of $B$ have trivial preimages (i.e. $p^{?1}\left(S\right)?S?F$) and this allows us to compare and classify several variations of the concept of strong graph bundle. As an application, we show that the clique operator preserves triangular graph bundles (strong graph bundles where preimages of triangles are trivial) thus yielding a new technique for the study of clique divergence of graphs.     

Diffusion tensor encoding schemes optimized for white matter fibers with selected orientations

A method to produce gradient encoding schemes that minimize the noise of diffusion tensor imaging (DTI) indices for selected fiber orientations has been developed. The accuracy of DTI measurements depends on the gradient encoding scheme used. Most current acquisition schemes contain diffusion directions uniformly distributed in 3D space in order to provide equal noise levels for fibers in any orientation. However, when considering specific fiber bundles such as the corticospinal tract (CST) or parts of fiber bundles, the range of fiber orientations of interest may be limited. We hypothesized that, when studying fiber tracts with a limited range of orientations, measuring diffusion in directions that are uniformly distributed in 3D space may be suboptimal for the noise levels of various DTI indices. Therefore, we first used simulations to determine six diffusion directions that minimize the noise of DTI measurements for selected fiber orientations. The resulting optimized set of directions was then tested on the right CST of a healthy human subject, and its performance was compared with that of conventional acquisition strategies. Both the simulations and the experiments on the human subject demonstrated that the new scheme significantly reduced the standard deviation of DTI indices for tensors with primary eigenvectors within a selected range of orientations.     

Fiber optic illumination of a poly(dimethylsiloxane) sheath flow cuvette for diode laser induced fluorescence detection in capillary electrophoresis

A Tee configuration sheath flow cuvette with square cross‐section channels has been produced in PDMS for CE detection. The output of a 1.4 W laser diode operating at 450 nm was focused onto the 300 μm core of a 370 μm od fiber optic whose end was inserted into one arm of the Tee for LIF. The optimal configuration had the fiber optic positioned 500 μm downstream from the intersection and the end of the 35 cm 50 μm id 365 μm od capillary just outside the intersection and in the leg of the Tee, resulting in a 90° configuration. Detection limits of 50 and 3 pM and linear calibrations of at least three orders of magnitude were obtained for Lucifer Yellow and fluorescein, respectively.     

Fiber array based hyperspectral Raman imaging for chemical selective analysis of malaria-infected red blood cells

A new setup for Raman spectroscopic wide-field imaging is presented. It combines the advantages of a fiber array based spectral translator with a tailor-made laser illumination system for high-quality Raman chemical imaging of sensitive biological samples.     

Multiscale spectroscopy using a monolithic liquid core waveguide with laterally attached fiber ports

In conventional absorption spectrometers, the range of accessible concentrations of analytes in aqueous solution is significantly limited by the dynamic range of the measurement system. Here we introduce the concept of multiscale spectroscopy allowing extending that range by orders of magnitude within one single device. The concept relies on using multiple light-sample interaction lengths, boosting the accessible concentration range by a particular extension factor. We experimentally implement our concept by a liquid core waveguide having multiple fiber ports side-wise attached to the waveguide, thus probing the light propagating inside the core at predefined distances from the input. This configuration provides three orders of magnitude of interaction length in one device. To verify the concept we exemplarily determine the concentrations of nitrate and of Rhodamine 6G in water, showing one hundred times improved measurement capabilities. The multiscale spectrometer uses the entire sample volume and allows the simultaneous measurement of fluorescence and attenuance. Due to its integrated design and the extended measurements capabilities, we anticipate application of our device in many application-relevant areas such as water quality analysis or environmental science.     

Achieving full effective microwave absorption in X band by double-layered design of glass fiber epoxy composites containing MWCNTs and Fe3O4 NPs

The glass fiber epoxy composites containing MWCNTs and Fe₃O₄ NPs were manufactured by composites liquid molding process. The microwave absorbing properties of single-layered and double-layered glass fiber/MWCNTs/epoxy and glass fiber/Fe₃O₄ NPs/epoxy composites were evaluated. The reflection loss(RL) were calculated by the measured complex permittivity and permeability using waveguide method by vector network analyzer. Based on the mechanism analysis and deficiency of single-layer absorber, the double-layered composites were fabricated by using matching layer and absorbing layer to enhance the microwave absorption performance, which can be modulated by tailoring the electromagnetic parameters and thicknesses of each layer. The optimized microwave absorbing properties of double-layered composites with minimum RL of −45.7 dB and full X-band effective absorption can be achieved when the total thickness of the matching layer and absorbing layer is 1.8 mm, which can be attributed to synergistic effect of improved impedance matching characteristic and superior microwave attenuation characteristic of the absorbing layer. The combined utilization of dielectric loss and magnetic loss absorbent and their double-layered structure design shows great design flexibility and diversity and can be a promising candidate for designing high performance microwave absorbing materials.     

Fiber optic probes for linear and nonlinear Raman applications – Current trends and future development

This review focuses on fiber optic probes for linear and nonlinear Raman spectroscopy, especially for medical applications. It aims at providing an overview over contemporary technology, recent first clinical trials, and helps identifying future developments necessary to bring the emerging technology to clinical end users. After a short introduction to linear and nonlinear Raman spectroscopic modalities, general design considerations will be discussed and compared to common fiber probe setups. Subsequently, examples for medical applications of fiber optic Raman probes will be given concentrating on probes for linear Raman spectroscopy as these devices are technologically more mature compared to their counterparts based on nonlinear Raman spectroscopy. The review also includes a brief summary of first multimodal fiber optic probes and highlights their benefits for clinical applications. Finally, probes are introduced which employ either nonlinear Raman spectroscopy or surface enhanced spectroscopy.     

高精度干涉仪针孔空间滤波器研制

应用于高重复频率、高功率193 nm准分子激光器会聚光路中的针孔空间滤波器,除了需要考虑它的材料加工难易、厚度、针孔尺寸等因素外,还需考虑材料抗激光损伤特性。本文利用激光损伤理论中一维热流模型对无限厚和有限厚不同金属样品在高峰值功率193 nm 准分子激光器照射下的损伤阈值进行了分析。结果表明：铝材料在厚度为1.2μm处比其它金属的损伤阈值高,可达1.16×1010 W/cm2,且材料易于加工。利用聚焦离子束技术加工了航空铝材料样品,得到了厚度为1.2和1.5μm的针孔空间滤波器样品。扫描电镜观察其具有较好圆度和内壁粗糙度,基本满足剪切干涉仪对针孔空间滤波器的需求。     

一种基于增益调制技术的全光纤化脉冲Yb光纤激光器

以波长为975 nm的半导体激光器作为泵浦源,周期性地脉冲泵浦一个包含Yb掺杂光纤和光纤光栅对的Yb光纤激光器,实现了基于增益调制技术的全光纤化高功率Yb光纤激光器的稳定脉冲输出.在50 kHz重频下,采用20 W的泵浦功率和2.4 μs的泵浦脉冲宽度,获得了1 060 nm波长脉冲宽度仅100 ns的稳定脉冲激光输出,单脉冲激光能量约为20 μJ.以此作为脉冲激光种子进行功率放大,获得了性能稳定的全光纤结构高功率脉冲激光输出,放大后单脉冲能量超过200 μJ,激光放大器斜率效率达到60%.