Spectrally Efficient 3D Optical CDMA Using Coherent Spatial Phase–Time Coding/Decoding

We demonstrate a WDM compatible optical CDMA system incorporating 3D spectral phase–time encoding/decoding. We provide coding and decoding using binary [0, π] phase chips for six users at 6 Gb/s, with a single coded signal separated with an acceptable bit-error rate ≤10 ^?9. The coding and decoding method is based on 3D coding of tightly spaced phase-locked laser lines that is compatible with conventional WDM networking. In optical CDMA systems, we propose to provide encoding and decoding done by converting Hadamard codes (used for conventional CDMA system) to the phase codes. We report that duo-binary modulation format is the best with adequate bandwidth compression. We confirm that better simulation results are reached in terms of the Q factor and bit error rate.     

Expansion velocities of 0.5 ps KrF excimer laser induced plasma by Doppler-shift analysis of pump and probe measurements

High gradient laser plasma is formed by focused KrF laser pulses (248.3 nm, 450 fs, 10¹³ W/cm²) on liquids (water, styrene) and solids (silicon, aluminum, and polyimide). The hydrodynamic expansion of the plasma was studied by measuring the blue Doppler-shift of reflected probe pulses which was produced by a delayed dye laser (496.6 nm, 450 fs). The Doppler-shift corresponds to the velocity of the reflecting surface of the plasma which is defined by the critical electron density. Expansion is investigated as a function of delay time and laser intensity. The reflecting surface of the plasma accelerates over 1–2 ps after the onset of the ablating laser pulse. With increasing intensity up to 2×10¹³ W/cm² the maximum average velocities are monotonously increasing up to 1–2×10⁵ m/s. PACS 52.38.Kd; 52.50.Jm, 52.70.Kz     

Velocity encoding and velocity compensation for multi-spoke RF excitation

PurposeTo investigate velocity encoded and velocity compensated variants of multi-spoke RF pulses that can be used for flip-angle homogenization at ultra-high fields (UHF). Attention is paid to the velocity encoding for each individual spoke pulse and to displacement artifacts that arise in Fourier transform imaging in the presence of flow.Theory and methodsA gradient waveform design for multi-spoke excitation providing an algorithm for minimal TE was proposed that allows two different encodings. Such schemes were compared to an encoding approach that applies an established scheme to multi-spoke excitations. The impact on image quality and quantitative velocity maps was evaluated in phantoms using single- and two-spoke excitations. Additional validation measurements were obtained in-vivo at 7 T.ResultsPhantom experiments showed that keeping the first gradient moment constant for all k-space lines eliminates any displacements in phase-encoding and slice-selection direction for all spoke pulses but leads to artifacts for non-zero velocity components along readout direction. Introducing variable but well-defined first gradient moments in the phase-encoding direction creates displacements along the velocity vector and thus minimizes velocity-induced geometrical distortions. Phase-resolved mean volume flow in the ascending and descending aorta obtained from two-spoke excitation showed excellent agreement with single-spoke excitation over the cardiac cycle (mean difference 0.8 ± 16.2 ml/s).ConclusionsThe use of single- and multi-spoke RF pulses for flow quantification at 7 T with controlled displacement artifacts has been successfully demonstrated. The presented techniques form the basis for correct velocity quantification and compensation not only for conventional but also for multi-spoke RF pulses allowing in-plane B₁⁺ homogenization using parallel transmission at UHF.     

Line shape considerations in ultrafast 2D NMR

We have recently proposed and demonstrated an approach that enables the acquisition of 2D nuclear magnetic resonance (NMR) spectra within a single scan. The approach is based on spatially encoding the spins' evolution along the indirect domain with the aid of a magnetic field gradient, and subsequently decoding this information numerous times over the course of the signal acquisition while spins are subject to a train of gradient echoes. The present paper discusses further considerations pertaining the 2D line shapes arising from this new way of collecting NMR data. Specific issues that are hereby addressed include (i) the effects introduced by fast relaxation onto the spatial encoding process, particularly the line widths and line shapes that will then arise in the frequency domain; (ii) approaches capable of correcting for the mixed-phase kernels resulting in these fast-relaxation cases, corresponding in essence to spatially encoded analogs of the TPPI and hypercomplex time-domain acquisition procedures; (iii) the enveloping characteristics imposed by the use of discrete excitation pulses on the attainable spectral widths along the indirect domain; and (iv) an analysis of the signal-to-noise characteristics of the methodology, with experimental corroborations of theoretical predictions and an illustration of the method's capabilities to analyze protein solutions in the mM-range concentration.     

Laser imprinting of 3D structures in gel-based titanium oxide organic-inorganic hybrids

First results on femtosecond laser 3D-microstructuring are reported in a novel class of organic-inorganic hybrid materials based on titanium oxide gels. Transparent optical-grade polished hybrid samples demonstrate strong photosensitivity assigned to electrons trapped as Ti³⁺ centers. Two different regimes of the microstructuring are observed: reversible and irreversible. In tight-focusing conditions using “on-the-fly” technique the single-pulse microstructuring is achieved with nanojoule laser pulses. The process thresholds have been studied on the surface and into the bulk of the material with irradiation by 1, 2, 10, and 100 laser pulses. A reduction of the damage threshold fluence by a factor of 2.5 is observed when increasing exposure from 1 pulse (1.2 J/cm²) to 100 pulses (0.5 J/cm²). PACS 42.62.-b; 40.70.-a; 78.66.Sq     

Picosecond laser ablation of thin copper films

The ablation process of thin copper films on fused silica by picosecond laser pulses is investigated. The ablation area is characterized using optical and scanning electron microscopy. The single-shot ablation threshold fluence for 40 ps laser pulses at 1053 nm has been determinated toF _thres = 172 mJ/cm². The ablation rate per pulse is measured as a function of intensity in the range of 5 × 10⁹ to 2 × 10¹¹ W/cm² and changes from 80 to 250 nm with increasing intensity. The experimental ablation rate per pulse is compared to heat-flow calculations based on the two-temperature model for ultrafast laser heating. Possible applications of picosecond laser radiation for microstructuring of different materials are discussed.     

注入半导体激光器混沌调制性能与内部相位键控编码方法研究

研究注入半导体激光混沌调制解调及其性能特性，提出外部光注入半导体激光器混沌内部相位编码方法. 小信号调制分析了系统混沌调制响应函数和解调响应函数以及解调因子，系统的调制解调响应函数以及解调因子在60MHz有一个峰值，系统在高频调制时，响应函数和解调因子明显下降. 理论推导出混沌调制解调方程，给出了小信号调制时的信息信号的解，数值分析出系统在低频有一个调制解调峰值，高频调制解调效果下降，但系统仍可以进行10⁹Hz频率调制解调. 数值模拟并证明了系统在较大的参数范围内都能实现同步，模拟了混沌调制速率200Mb/s和调制频率1.4GHz的保密通信应用. 还通过键控调制注入光的相位相移对激光混沌态进行了编码，通过同步与非同步进行混沌解码，数值模拟了具有调制速率10Mb/s的混沌相位键控编码通信应用. 关键词： 混沌 同步 激光器 相位     

Tailoring Chirped MoirÉ Fiber Bragg Gratings for Wavelength-Division-Multiplexing and Optical Code-Division Multiple-Access Applications

The design and fabrication of chirped Moiré fiber Bragg gratings (CMGs) are presented, which can be used in either (1) transmission as passband filters for providing wavelength selectivity in wavelength-division-multiplexed (WDM) systems or (2) reflection as encoding/decoding elements to decompose short broadband pulses in both wavelength and time in order to implement an optical code-division multiple-access (OCDMA) system. In transmission, the fabricated CMGs have single or multiple flattened passbands (< 0.5 dB ripple) and exhibit > 12 dB isolation and near constant in-band group delay. It is shown that these filters do not produce any measurable dispersion-induced power penalties when used to provide wavelength selectivity in 2.5 Gbit/s systems. It is also demonstrated how CMGs can be used in reflection to encode/decode short pulses from a wavelength-tunable mode-locked Er-doped fiber laser.     

The generation,detection and measurement of laser-induced carbon plasma ions and their implantation effects on brass substrate

The generation, detection and measurement of laser-induced carbon plasma ions and their implantation effects on brass substrate have been investigated. Thomson parabola technique was employed to measure the energy and flux of carbon ions. The magnetic field of strength 80?mT was applied on the graphite plasma plume to provide an appropriate trajectory to the generated ions. The energy of carbon ions is 678?KeV for laser fluence of 5.1?J/cm² which was kept constant for all exposures. The flux of ions varies from 32?×?10¹¹ to 72?×?10¹⁴?ions/cm² for varying numbers of laser pulses from 3000 to 12,000. In order to explore the ion irradiation effects on brass, four brass substrates were irradiated by carbon ions of different flux. Scanning electron microscope (SEM) and X-ray diffractometer (XRD) are used to analyze the surface morphology and crystallographic structure of ion-implanted brass, respectively. SEM analysis reveals the formation and growth of nano-/micro-sized cavities, pores and pits for the various ion flux for varying numbers of laser pulses from 3000 to 12,000. By increasing ion flux by increasing the number of pulses up to 9000 shots, the dendritic structures initiate to grow along with cavities and pores. At the maximum ion flux for 12,000 shots, the unequiaxed dendritic structures become distinct and the distance between the dendrites is decreased, whereas cavities, pores and pits are completely finished. The XRD analysis reveals that a new phase of ZnC (0012) is formed in the brass substrate after ion implantation. Universal tensile testing machine and Vickers microhardness tester are used to explore the yield stress, ultimate tensile strength and microhardness of ion-implanted brass substrate. The mechanical properties monotonically increase by increasing the ion flux. Variations in mechanical properties are correlated with surface and structural modifications of brass.     

Comparison of the Effects of Different <Superscript>19</Superscript>F <Emphasis Type="Italic">π</Emphasis> Pulses on the Sensitivity and Phaseability of the <Superscript>19</Superscript>F-<Superscript>13</Superscript>C HSQC

The ¹⁹F-¹³C heteronuclear single quantum coherence (HSQC) experiment is vital for the structural elucidation of polyfluorinated organic species, yet its sensitivity and phaseability are limited by difficulties in uniform excitation of the widely disperse ¹⁹F spectral window. Adiabatic pulses of different types have previously been employed to generate effective π pulses for inversion and refocussing, but a systematic comparison of various adiabatic and other inversion pulses has not been published. In this work, it was observed that the use of a broadband inversion pulse (BIP) during the t ₁ evolution period resulted in properly phaseable spectra for experiments optimized to detect ¹ J _CF, in contrast to CHIRP or WURST adiabatic pulses. For the INEPT and reverse-INEPT transfer segments of the HSQC, optimal sensitivity for resonances distant from the transmitter frequency was afforded by optimized universal rotation (BURBOP) or CHIRP pulses. In HSQC experiments with delays optimized for two-bond correlations, only the use of BURBOP pulses in INEPT and reverse-INEPT sequences afforded spectra cleanly phaseable across the F ₂ and F ₁ spectral windows. This observation is supported by off-resonance pulsed field gradient spin-echo experiments.     

原子非序列双电离的多次返回碰撞电离机理分析

本文采用三维半经典再散射模型研究了He原子在高光强(1.5×10¹⁵ W/cm²)、少周波激光脉冲作用下的非序列双电离问题,重点分析了沿激光电场极化方向的动量关联谱.发现两个电子沿相反方向发射的比例明显比中等光强区(7×10¹⁴ W/cm²)和低光强区(2.5×10¹⁴ W/cm²),以及同等光强的长脉冲情形都偏高, 同时V形结构也更加明显.通过轨道"回溯"分析, 进一步深入 关键词： 强场 非序列双电离 再散射     

多量子比特核磁共振体系的实验操控技术

随着量子信息与量子计算科学的发展,量子信息处理器被广泛地用于量子计算、量子模拟、量子度量等方面的研究.为了能在实验上实现这些日益复杂的方案,将量子计算机的潜能转化成现实,需要不断提高可操控的量子体系比特位数,实现更复杂的量子操控.核磁共振自旋体系作为一个优秀的量子实验测试平台,提供了丰富而又精密的量子操控手段.近几年来在此平台上进行了不少的多量子比特实验,发展并积累了一系列的多量子比特实验技术.本文首先阐述了核磁共振体系多量子比特实验中的实验困难,然后结合7量子比特标记赝纯态制备以及其他有关实验,对多比特实验过程中应用到的实验技术进行介绍.最后对核磁共振体系多量子比特实验技术方向的进一步研究进行了总结和展望.     

Compact high gain double-pass optical parametric chirped pulse amplifier

A novel double-pass noncollinear optical parametric chirped pulse amplifier based on an Yb³⁺-doped passively mode-locked fiber laser has been demonstrated in this paper. The signal was double-pass amplified in a single nonlinear crystal by a long pump pulse, and the signal and pump pulses of each pass were completely phase matched in the plane of the maximum effective nonlinearity. Net saturated gain of 2×10⁶ was achieved and the superfluorescence was suppressed by increasing the overlap time between the signal and pump pulses.     

Fast imaging with the MMME sequence

The multiple-modulation-multiple-echo sequence, previously used for rapid measurement of diffusion, is extended to a method for single shot imaging. Removing the gradient switching requirement during the application of RF pulses by a constant frequency encoding gradient can shorten experiment time for ultrafast imaging. However, having the gradient on during the pulses gives rise to echo shape variations from off-resonance effects, which make the image reconstruction difficult. In this paper, we propose a simple method to deconvolve the echo shape variation from the true one-dimensional image. This method is extended to two-dimensional imaging by adding phase encoding gradients between echoes during the acquisition period to phase encode each echo separately. Slice selection is achieved by a frequency selective pulse at the beginning of the sequence. Imaging speed is mainly limited by the phase encoding gradients' switching times and echo overlap when echo spacing is very short. This technique can produce a single-shot image of sub-millimeter resolution in 5 ms.     

基于双边信息的分布式视频压缩感知模型研究

针对传统视频编码技术计算量大和复杂度高的缺点,提出一种基于双边信息的分布式视频压缩感知算法。该算法将压缩感知技术与分布式视频编码技术相结合,把视频序列分为Key帧和CS帧,Key帧运用传统的帧内编码和解码,CS帧编码端运用压缩感知编码,解码端运用视频块内与视频块间的双边信息和梯度投影算法进行优化重构。通过双边信息的运动估计和压缩编码器的设计,实现基于双边信息的分布式视频压缩感知模型的构建。仿真结果表明该模型既可以实现高效编码,又可以实现复杂度由编码端向解码端转移,在较低的采样率下,提高视频的压缩能力和传输速度。     

Effect of the pulse leading-edge time on the dielectric strength of a vacuum gap

Optimal regimes for electrode conditioning in a vacuum by applying voltage pulses with different waveforms are considered. For nanosecond pulses with a constant duration (t _p = const), the impulse dielectric strength for an oblique voltage wave is shown to be more than four times higher than for a rectangular pulse with an infinitely short leading-edge duration. The dependences of the dielectric strength on the conditioning pulse duration in the range 10^?10 < t _p < 10^?3 s for pulses with different rise rates are obtained. The dielectric strength increases from 2 × 10⁷ V/m for microsecond pulses to 10¹⁰ V/m for subnanosecond pulses.     

Laser-induced ion acceleration at ultra-high laser intensities

Ion acceleration by ultrashort laser pulses of very high intensities of the order 10²²?W/cm² is studied by two-dimensional Particle-In-Cell simulations. We show that laser normal incidence is preferred for such high intensities. For linearly polarized laser radiation, higher maximum proton/ion energies are achieved than for circular polarization. For linear polarization, the transition from the target normal sheath acceleration to the acceleration on the target front side by the radiation pressure is analyzed in detail. The transition intensity is increasing with the target thickness. The radiation pressure dominated regime leads to considerably higher number of accelerated protons and thus to a higher acceleration efficiency.     

Measurement of One-Dimensional Spatially Resolved Self-Diffusion Coefficients and Longitudinal Relaxation Times with a Single B1 Gradient

A very simple experimental setup, involving a single coil for generating a radiofrequency field gradient, enables one to determine the self-diffusion coefficient and the longitudinal relaxation time along the gradient axis. This is accomplished by a two-dimensional experiment involving three gradient pulses. The first part of the sequence includes two pulses of identical duration separated by an evolution interval which, by proper phase cycling, encodes longitudinal magnetization according to translational diffusion. The last pulse is incremented for purposes of spatial encoding.     

The effect of mechanical relaxation on ultra-fast charge pulses in flexible epoxy resin nanocomposites

Previously we have reported the existence of small-amplitude charge pulses in crosslinked Polyethylene (XLPE) and epoxy resin with a mobility several orders of magnitude higher than that found for the incoherent charge transport relevant to the steady state current. Here the relationship of this phenomenon to mechanical relaxation in the material is investigated by using a series of epoxy resin nanocomposites based on a resin that has its flexibility increased above that of the fully cured glassy epoxy network by the addition of a suitable flexibilizing chemical. Differential Scanning Calorimetry (DSC) measurements show that the stiffness of the nanocomposite is progressively increased as the nanoparticle concentration increases. Pulsed Electro-Acoustic (PEA) measurements reveal that both positive and negative fast charge pulses exist in the unfilled epoxy at 45 and 70°C under a field of 10?kV/mm with mobility 5×10^?10 to 9×10^?10 m²?V^?1?s^?1, amplitude between 2×10^?5 and 3.6×10^?5 C?m^?2 and repetition rates between 8 and 12?s^?1. These values are reduced progressively as the nanoparticle concentration is increased from 0% in the unfilled epoxy. A???-mode mechanical relaxation is identified in the loss modulus by Dynamical Mechanical Analysis (DMA), whose activation energy moves to higher values with increasing nanoparticle concentration. It is shown that the repetition rates of both positive and negative pulses have similar values and are correlated with the ??-mode activation energy; a similar correlation is found for the activation energy of the mobility of positive pulses. The correlation of the activation energy of the mobility of negative pulses and that of the ??-mode is weaker although both show a progressive increase with nanoparticle concentration. The modification of the fast charge pulse properties by the mechanical stiffness of the epoxy nanocomposite is discussed in terms of the theory presented previously for their formation and transport.     

Blind encoding into qudits

We consider the problem of encoding classical information into unknown qudit states belonging to any basis, of a maximal set of mutually unbiased bases, by one party and then decoding by another party who has perfect knowledge of the basis. Working with qudits of prime dimensions, we point out a no-go theorem that forbids ‘shift’ operations on arbitrary unknown states. We then provide the necessary conditions for reliable encoding/decoding.