Analysis of Diffuser Shift Margin in Spatial Spread-Spectrum Holographic Multiplexing

We present a novel numerical model for analyzing shift margin in spatial spread-spectrum (SSS) multiplexing, in which the spatial phase of signal beams, not in reference beams, is modulated and demodulated by a random diffuser to multiplex holograms. The shift margin is determined not by the Bragg effect of a thick hologram, but mainly by the phase correlation property of the diffuser used. In our model, we simulate signal data patterns and a smooth rough surface of a diffuser using oversampling and zero-padding, which allows us to cover crosstalk calculations and direct bit-errors calculations, with a fine shift resolution and a wide diffusion angle range. Calculation results revealed that the practical shift margin that ensures a signal to noise ratio more than 3.0 and a bit-error rate less than 10⁻² is in the range of 0.3ω_eff−0.5ω_eff, where ω_eff is the effective autocorrelation length of the diffuser transmission function. This model will be helpful in designing the optimum random diffuser profile for SSS multiplexing.     

Correlation processing capabilities for a homodyne-radar signal with continuous frequency-modulated radiation

Possible use of optimal correlation-filtering processing of reflected signal in a homodyne radar is considered for the case of periodic frequency modulation of a continuous probing signal. Theoretical basis, mathematical simulation results, and experimental evidence are given and radar features and parameters, the main problems, and methods of their solution are discussed. Possibility of creating simple radars of millimeter wavelength range with a signal base of up to 10⁹–10¹⁰ and resolution of the order of centimeters is presented.     

Lasers as Light Sources for Analytical Atomic Spectrometry

Abstract

Lasers have advantages compared to conventional light sources, which include high power, a monochromatic emission profile, stability, and rapid tuning across an atomic line. These advantages have resulted in superior analytical figures of merit and methods of background correction compared to conventional light sources. The most widely used lasers for atomic spectrometry include dye laser systems, optical parametric oscillator systems, and diode lasers. Three principal techniques employ lasers as light sources. Laser‐excited atomic fluorescence spectrometry (LEAFS) involves the use of laser light to excite atoms that emit fluorescence and serves as the analytical signal. Laser‐enhanced ionization (LEI) involves laser excitation of atoms to an excited state energy level at which collisional ionization occurs at a higher rate than from the ground state. Diode laser atomic absorption spectrometry (DLAAS) employs a DL as a source to excite atoms in an atom cell from the ground state to an excited state. The analytical signal is involves the ratio of the incident and transmitted beams. Recent applications of these techniques are discussed, including practical applications, hyphenated techniques employing laser‐induced plasmas, and work to characterize fundamental spectroscopic parameters.     

Using a grating analyser for SEMSANS investigations in the very small angle range

Spin-echo modulation small-angle neutron scattering (SEMSANS) is based on the detection of spatial beam modulation, which is induced by triangular spin echo precession regions and subsequent spin analyses. In order to detect such signal and exploit it for small angle scattering investigations neutron detection with sub-millimeter spatial resolution is required. Here an approach is reported where instead of a position sensitive detector an absorption grating is used to analyze the beam modulation stepwise. The spin-echo length scan in this case is performed by varying the sample-to-detector distance. The real space correlation functions of reference sample structures in the range 10² nm, i.e. giving rise to small-angle scattering in the very small-angle range, are recorded and analyzed successfully.     

On the quest of production of superheavy nuclei in reactions of 48Ca with the heaviest cctinide targets

The sequence of radioactive decays of an unknown isotope produced in a rare fusion reaction to known lighter isotopes is used to identify mass and atomic number of the mother isotope, which has been separated before from the bulk of other reaction products by an in-flight recoil separator. By this technique the elements 107 to 112 were produced by single atom decay-chain analysis. Such a correlation technique reaches its limit by the occurrence of accidental sequences and it collapses beyond a maximum possible correlation time, at which a true event cannot be distinguished anymore from a random event. ⁴⁸Ca-induced fusion reactions with actinides are discussed. In 1983 at GSI, Darmstadt and LBL, Berkeley, ⁴⁸Ca/²⁴⁸Cm-experiments (II) were performed, which are compared to recent ⁴⁸Ca-experiments at FLNR-Dubna (I) irradiating ²⁴⁴Pu, ²⁴²Pu, and ²³⁸U. In these experiments production of isotopes of superheavy elements 112 and 114 is claimed. Our analysis of accidental sequences in ⁴⁸Ca-induced reactions is presented, which is at variance with the published analysis from FLNR-Dubna. We find that the maximum correlation time using continuous beams at today existing separation systems is not in the one-hour regime, but in the few-minute regime. The five spontaneous fission events observed in the FLNR experiments are preceded by signals in the (1–16)-minute range. These times are shown to be longer than the maximum possible correlation times. The preceding signals are decoupled from the spontaneous fission signal and carry no information on the spontaneous fission events observed. Moreover, random probabilities of 0.2 to 0.6 for the signals preceding the fission events indicate that the correlations are of random origin. The evidence to have discovered element 114 in the reported experiments is classified “very weak”. Received: 13 October 1999     

Backscattering in stratified time-dependent random media—CW and narrow-band pulse propagation

Abstract

We consider backscattering in a random stratified medium where the wave-speed fluctuations depend on time and on the range coordinate, which is normal to the planes of stratification. For the limit where the correlation time is shorter than the mean scattering time, we first derive radiation-transport equations that govern the range evolution of the spectra of the ensemble-averaged forward-and back-propagating components of the field and their bichromatic coherence functions. The latter are governed by integro-differential equations that account for the broadening of the signal spectra due to the time dependence of the random fluctuations. When the correlation time is longer than both the period of the radiating signal and the travel time of a wavefront across a range correlation length, the spectrum of the radiation due to a monochromatic CW excitation remains confined to a narrow band over extensive ranges. This permits a quasi-monochromatic approximation, whereby the integro-differential equations produce ordinary differential equations that govern the quantities of interest. We use this approximation to track the power flux associated with the propagation of a narrow-band pulse.     

A multi‐MHz single‐shot data acquisition scheme with high dynamic range: pump–probe X‐ray experiments at synchrotrons

The technical implementation of a multi‐MHz data acquisition scheme for laser–X‐ray pump–probe experiments with pulse limited temporal resolution (100 ps) is presented. Such techniques are very attractive to benefit from the high‐repetition rates of X‐ray pulses delivered from advanced synchrotron radiation sources. Exploiting a synchronized 3.9 MHz laser excitation source, experiments in 60‐bunch mode (7.8 MHz) at beamline P01 of the PETRA III storage ring are performed. Hereby molecular systems in liquid solutions are excited by the pulsed laser source and the total X‐ray fluorescence yield (TFY) from the sample is recorded using silicon avalanche photodiode detectors (APDs). The subsequent digitizer card samples the APD signal traces in 0.5 ns steps with 12‐bit resolution. These traces are then processed to deliver an integrated value for each recorded single X‐ray pulse intensity and sorted into bins according to whether the laser excited the sample or not. For each subgroup the recorded single‐shot values are averaged over ～10⁷ pulses to deliver a mean TFY value with its standard error for each data point, e.g. at a given X‐ray probe energy. The sensitivity reaches down to the shot‐noise limit, and signal‐to‐noise ratios approaching 1000 are achievable in only a few seconds collection time per data point. The dynamic range covers 100 photons pulse^?1 and is only technically limited by the utilized APD.     

Augmentation of the step‐by‐step Energy‐Scanning EXAFS beamline BL‐09 to continuous‐scan EXAFS mode at INDUS‐2 SRS

An innovative scheme to carry out continuous‐scan X‐ray absorption spectroscopy (XAS) measurements similar to quick‐EXAFS mode at the Energy‐Scanning EXAFS beamline BL‐09 at INDUS‐2 synchrotron source (Indore, India), which is generally operated in step‐by‐step scanning mode, is presented. The continuous XAS mode has been implemented by adopting a continuous‐scan scheme of the double‐crystal monochromator and on‐the‐fly measurement of incident and transmitted intensities. This enabled a high signal‐to‐noise ratio to be maintained and the acquisition time was reduced to a few seconds from tens of minutes or hours. The quality of the spectra (signal‐to‐noise level, resolution and energy calibration) was checked by measuring and analysing XAS spectra of standard metal foils. To demonstrate the energy range covered in a single scan, a continuous‐mode XAS spectrum of copper nickel alloy covering both Cu and Ni K‐edges was recorded. The implementation of continuous‐scan XAS mode at BL‐09 would expand the use of this beamline in in situ time‐resolved XAS studies of various important systems of current technological importance. The feasibility of employing this mode of measurement for time‐resolved probing of reaction kinetics has been demonstrated by in situ XAS measurement on the growth of Ag nanoparticles from a solution phase.     

Maximum-entropy deconvolution applied to electron energy-loss spectroscopy

In the present paper, we investigate the performance of maximum-entropy deconvolution, in removing the instrumental response function from electron energy-loss spectra. To this end we make use of spectra acquired from the carbon K-edge in graphite for a range of signal-to-noise ratios. The zero-loss peak is used as the instrumental profile. The resolution improvement obtained through the application of the deconvolution algorithm as a function of the signal-to-noise ratio is well described by a logarithmic dependency. The claimed resolution improvement is further substantiated by demonstrating the consistency between improvement obtained for the width of the instrumental response function, the width of the π¹ peak and the splitting of the σ¹ peaks for a range of signal-to-noise ratios.     

A correlation study of solar activity index and amplitude of VLF trans-equatorial propagation

Diurnal and seasonal behavior of amplitude of NWC 22.3 kHz signal transmitted from Australia (22⁰49/S, 114⁰23/E), and received at Kolkata (22⁰34/N, 88⁰24/E), India have been investigated The signal amplitude is remarkably low in October. The variations of daily maximum and daily minimum of signal amplitude have been analyzed. The signal on the present propagation path showed non-correlation with A _p index, but showed a moderate but negative correlation with 10.7 cm solar flux.     

The diffusive motion of silver ions in α-AgI: Results from quasielastic neutron scattering

The diffusive motion of silver ions in σ-AgI at 250°C has been studied by quasielastic cold neutron scattering. Spectra were taken in the range of wavevector transfer 0.5 < Q < 2.2Å^?1 for elastic scattering. The quasielastic line shapes contain a narrow and a broad component. They are compared to model calculations allowing for the superposition of two kinds of motion on two different time scales, a local random motion and a translational motion of the jump-diffusion type. The model closely fits the data. The local random motion takes place on a time scale of the order of 10^?12 s, with amplitudes of the order of 1 Å. It is probably caused by rapid fluctuations of the local potentials due to the diffusive motion of the other cations. The translational motion results in a mean displacement of the silver ion over a distance of the order of a lattice constant (5 Å) with a correlation time of the order of 10^?11s. This correlation time is composed of a residence time and a time-of-flight, which are both of comparable magnitude.     

Channel reduction schemes for fiber-optic angle encoders

Three fiber-optic sensor systems designed to remotely determine angular position that achieve high angular resolution with fewer channels than comparable Gray code encoders are described. Two of the Systems use a sheet polarizer affixed to a “codewheel” and the third system uses a two-channel digital shaft encoder style “codewheel.”

The first polarization scheme uses four optical channels, two of which are analog and two digital. The ratio of the two analog channel intensities yields tan²θ. The fourfold quadrant ambiguity is resolved by the two digital channels that are transected by two semicircular masks on the polarizer codewheel (Ref. U.S. Patent No. 4,577,414, 25 Mar 1986). The second polarization scheme again uses quadrant ambiguity masks but employs only one analog channel that simulates a polarization vector that oscillates through 90°. The oscillating vector is produced by the superposition of two sine-wave-modulated beams at the polarizer codewheel. The modulations of the two beams have a phase difference that is created by time delaying one of the beams in a fiber delay loop. The phase difference between the generated composite signal and a reference signal then determines the angle of the codewheel. We have demonstrated experimentally that this type of split analog-digital scheme has a resolution equivalent to a 10-bit digital system (i.e., ±0.35°) independent of codewheel diameter.

The serial digital shaft encoder scheme uses only two digital channels and a codewheel that has two concentric masks with 48 equally spaced windows offset with respect to each other by one-half window width. At 0° there is a unique mask that initializes an up/down decoder chip (Hewlett-Packard HCTL-2000). This system has a resolution better than 7 bits using a 5-cm diameter code wheel and 1-mm Selfoc^TM lenses.

The supporting electro-optical systems including sources, fibers, lenses, mirrors, couplers, WDMs, polarizers, detectors, and signal processing for all schemes are described and the relative merits of each are compared.     

A partitioned density functional theory of freezing: application to soft spheres

A non-perturbative density functional theory (DFT) for inhomogeneous fluids is developed by partitioning the functional into short range (‘entropic’) and long range (‘energetic’) contributions. The short range part is treated using standard weighted density functional techniques and the long range contribution is evaluated exactly. This method, which is a generalization of a method due to Likos, C., and Senatore, G., 1995, J. Phys.: Condens. Matter, 7, 6797, does not require the use of a reference system. Results are presented for the calculation of the crystal-/fluid phase coexistence for systems interacting with inverse-power potentials of the form r ^-n , where n = 4, 6 and 12. These results show that this non-perturbative DFT is capable of predicting the freezing of long range inverse power systems (n = 4, 6) into a body-centredcubic lattice. Improvements over earlier methods also are noted in the current results for the solid structure as measured by the Lindemann ratio.     

Optical characterization methods for solid-state image sensors

The accurate measurement of the optoelectronic properties of imaging sensors is of utmost importance for their appropriate use in various modern application fields, such as in metrology, quality control, environmental monitoring, medicine or for automotive applications. Key sensor parameters include spatial resolution, uniformity, sensitivity, linearity, signal to noise ratio and dynamic range. Today high-end optical systems mostly rely on charge coupled device (CCD) image sensors. Continuous progresses in CMOS submicron technology and the advent of ‘active pixel sensor’ (APS) imagers have however led to a wealth of novel line and area array imaging devices with added functionalities (eg. on-chip control and read-out electronics) or performance optimized for specific tasks (eg. a dynamic range in excess of 120 dB). The optimal use of CMOS image sensing technology nevertheless depends strongly on the absolute and accurate optoelectronic characterization of these devices. Modern measurement techniques for a reliable, traceable, precise and absolute measurement of the most relevant parameters of CCD and CMOS imaging sensors are described and discussed in the present paper, with examples based on recent state-of-the art CMOS imagers.     

Fourier-wavelet regularized deconvolution (ForWaRD) for lidar systems based on TEA-CO2 laser

Lidar is an efficient tool for remotely measuring physical quantities or detecting targets. To improve the range resolution in long pulse lidars, such as lidar systems based on TEA-CO₂ lasers, deconvolution methods were used by previous investigators. Deconvolution is a noise sensitive process. In order to avoid noise amplification during the deconvolution process, the Fourier-wavelet regularized deconvolution method is used to deconvolve and denoise the back-scattered lidar signal simultaneously. This method is applied to lidar systems based on the TEA-CO₂ laser and the results are compared to nitrogen tail clipping method. Numerical simulation shows, in comparison to the clipping nitrogen tail technique, by using the ForWaRD method; the range resolution and working distance of the lidar is improved and the clipping tail apparatus is also eliminated.     

Improved signal processing in pulsed and cw spin-flip Raman laser spectrometers

Double beam optical techniques which allow direct spectroscopic measurements using both pulsed and cw tunable spin-flip Raman lasers have been developed. The systems provide a high resolution spectrometer (spectral linewidths 0.03 cm^-1 pulsed and 0.003 cm^-1 cw) in the range 5.3–6μm. Line frequency measurements are accurate to 0.01 cm^-1 and intensity measurements to a few percent. Results of molecular spectroscopy provide a comparison of the relative performance of each system.     

Computer emulation of the method for comparing the isotopic composition of CO<Subscript>2</Subscript> in gas mixtures based on laser spectral analysis at the 2.05-<Emphasis Type="Italic">µ</Emphasis>m line

An algorithm for processing spectral data in order to determine the isotopic composition of gas mixtures is proposed and simulated using emulated transmission spectra of CO₂ in exhaled air tests. It is shown that the ratio of the optical-density spectra of the analyzed and reference gas mixtures is a contrast spectrum with resonant features and a fixed depth of their amplitude modulation. Based on the simulation results several spectral ranges are found to be optimal for analyzing the relative ¹²CO₂ and ¹³CO₂ contents in exhaled air. The effect of random noise on the isotopic analysis result is estimated, and the correlation between the optical density of analyzed gas media, level of random noise, and sensitivity of isotopic analysis is determined. The use of the algorithm proposed reduces the effect of such noises as interference at optical elements, absorption in open atmosphere, drift of the laser pulse envelope, and disbalance of spectral channels. This algorithm is valid for comparative isotopic analysis of any other gas molecules with similar spectral properties.     

Wigner Surmise for Domain Systems

In random matrix theory the spacing distribution functions p ⁽ⁿ⁾(s) are well fitted by the Wigner surmise and its generalizations. In this approximation the spacing functions are completely described by the behavior of the exact functions in the limits s→0 and s→∞. Most non equilibrium systems do not have analytical solutions for the spacing distribution and correlation functions. Because of that, we explore the possibility to use the Wigner surmise approximation in these systems. We found that this approximation provides a first approach to the statistical behavior of complex systems, in particular we use it to find an analytical approximation to the nearest neighbor distribution of the annihilation random walk.     

Experimental verification of enhanced photon bunching from a saturable absorber

Preliminary results of measurements of the second order correlation function g⁽²⁾ (0) of light transmitted through a saturable absorber are presented.     

脉冲激光探测的最大探测距离的研究

脉冲激光探测的最大探测距离与发射信号功率有关,根据系统发射端瞬时功率的形式,通过计算得出了脉冲激光平均发射功率和最大瞬时功率分别与驱动电流的脉冲上升沿时间、脉冲宽度、脉冲重复周期的关系.结合传输衰减、弱信号检测和信号预处理过程中的信号功率变化,得出了最大探测距离与激励电流波形参数、光电探测器参数、信号预处理电路噪声、信...