Dynamic nonoptogalvanic signal polarity and magnitude in prebreakdown gas discharges

Nitrogen laser pulse irradiation of prebreakdown discharges in Ne and Ar result in pulse responses strikingly similar to those reported for dynamic optogalvanic signals. For the latter, response polarity depends primarily upon atomic transition. Here, it depends primarily upon bias. Nevertheless, analysis of the results points to similar internal processes within the gas concerning metastable generation and destruction. Photoionization-assisted electron heating is an additional photon process relevant here which changes relative populations of excited states and plays a key role in reversing prebreakdown signal response polarity. It can also explain the mechanism of reported electron temperature increase in optogalvanic experiments which cannot be explained on the basis of atomic transitions.     

The statistical distribution of switch-on delay in single-modesemiconductor lasers I: equilibrium off-condition

The theory of stochastic processes as applied to photon emission and absorption events is used to calculate the distribution of delay in switch-on from a sub-threshold condition in directly modulated semiconductor lasers down to a probability of 10^-10. This involves the derivation of the relative probability distribution of photon number in the laser late enough in the switch-on process such that deterministic relations can be applied thereafter. This distribution, assumed constant in some treatments, is found to change only a little from its initial form, which is a negative binomial, From this one deduces a delay distribution whose width is proportional to the period of the switch-on transient, relatively independently of the precise starting point, but which can be narrowed by injection of additional spontaneous emission. Experiment satisfactorily supports the theory     

Performance index based on the correlation between the responses of two systems

In the analytical design theory of linear control Systems, an integral-square-error performance index for transient signals and a mean-square-error performance index for stochastic signals are commonly used. It is possible to define a new performance index based on the correlation between the responses of two systems. The response of one of the systems can be regarded as the desired response. This system can be termed the reference system.     

The statistical distribution of switch-on delay in single-modesemiconductor lasers II: pattern effects

For part I see, p. 2006, 1995. The theory of stochastic processes as applied to photon emission and absorption events is used to calculate the distribution of switch-on delay in directly modulated semiconductor lasers down to a probability of 10^-10. In this second paper, the influence of a previous light pulse on the delay of a given pulse is investigated. The probability distribution of photon number in the dying remnants of the first pulse is examined under the relevant transient conditions of gain and loss, and is found to evolve in relative width and shape in a way which depends on only one parameter. This parameter, which consists of an integral over time of the reciprocal of the average photon number, is obtained from a solution of the deterministic rate equations and is used to deduce the delay distribution of the subsequent pulse. The results, although sensitively dependent on the conditions, are supported by experiment     

矩阵随机过程在短程无线信道空时特性建模中的应用

使用矩阵随机过程对短程无线信道的空时特性进行研究。以随机矩阵的列向量描述信道的仰角自由度,行向量描述信道的水平角自由度。使用自由布朗桥过程产生的样本描述无线信道中的多径分量的传播轨迹,在一定的映射规则下可以得到反映无线信道空时特性冲激响应的传统模型和矩阵随机过程模型。对无线信道的空时特性进行了仿真分析,仿真结果表明,自由布朗桥过程所描述的完全随机的约束环境中,多径分量的波达方向在仰角和水平角维度上显示出一定的局部偏好性。     

An algorithm for detecting a change in a stochastic process

The problem of detecting a change from one given stationary and ergodic stochastic process to another such process is considered. It is assumed that both stochastic processes are processes with memory and that they are mutually independent. A sequential test is proposed and analyzed. It is proved that the proposed test is asymptotically optimal in a mathematically precise sense.     

Wavelet decomposition of harmonizable random processes

The discrete wavelet decomposition of second-order harmonizable random processes is considered. The deterministic wavelet decomposition of a complex exponential function is examined, where its pointwise and bounded convergence to the function is proved. This result is then used for establishing the stochastic wavelet decomposition of harmonizable processes. The similarities and differences between the wavelet decompositions of general harmonizable processes and a subclass of processes having no spectral mass at zero frequency, e.g., those that are wide-sense stationary and have continuous power spectral densities, are also investigated. The relationships between the harmonization of a process and that of its wavelet decomposition are examined. Finally, certain linear operations such as addition, differentiation, and linear filtering on stochastic wavelet decompositions are considered. It is shown that certain linear operations can be performed term by term with the decomposition     

Efficient fully 3-D iterative SPECT reconstruction with Monte Carlo-based scatter compensation

Quantitative accuracy of single photon emission computed tomography (SPECT) images is highly dependent on the photon scatter model used for image reconstruction. Monte Carlo simulation (MCS) is the most general method for detailed modeling of scatter, but to date, fully three-dimensional (3-D) MCS-based statistical SPECT reconstruction approaches have not been realized, due to prohibitively long computation times and excessive computer memory requirements. MCS-based reconstruction has previously been restricted to two-dimensional approaches that are vastly inferior to fully 3-D reconstruction. Instead of MCS, scatter calculations based on simplified but less accurate models are sometimes incorporated in fully 3-D SPECT reconstruction algorithms. We developed a computationally efficient fully 3-D MCS-based reconstruction architecture by combining the following methods: 1) a dual matrix ordered subset (DM-OS) reconstruction algorithm to accelerate the reconstruction and avoid massive transition matrix precalculation and storage; 2) a stochastic photon transport calculation in MCS is combined with an analytic detector modeling step to reduce noise in the Monte Carlo (MC)-based reprojection after only a small number of photon histories have been tracked; and 3) the number of photon histories simulated is reduced by an order of magnitude in early iterations, or photon histories calculated in an early iteration are reused. For a 64 x 64 x 64 image array, the reconstruction time required for ten DM-OS iterations is approximately 30 min on a dual processor (AMD 1.4 GHz) PC, in which case the stochastic nature of MCS modeling is found to have a negligible effect on noise in reconstructions. Since MCS can calculate photon transport for any clinically used photon energy and patient attenuation distribution, the proposed methodology is expected to be useful for obtaining highly accurate quantitative SPECT images within clinically acceptable computation times.     

Modeling of non-line-of-sight ultraviolet scattering channels for communication

A stochastic non-line-of-sight (NLOS) ultraviolet (UV) communication channel model is developed using a Monte Carlo simulation method based on photon tracing. The expected channel impulse response is obtained by computing photon arrival probabilities and associated propagation delay at the receiver. This method captures the multiple scattering effects of UV signal propagation in the atmosphere, and relaxes the assumptions of single scattering theory. The proposed model has a clear advantage in reliable prediction of NLOS path loss, as validated by outdoor experiments at small to medium elevation angles. A Gamma function is shown to agree well with the predicted impulse response, and this provides a simple means to determine the channel bandwidth. The developed model is employed to study the characteristics of NLOS UV scattering channels, including path loss and channel bandwidth, for a variety of scattering conditions, source wavelength, transmitter and receiver optical pointing geometries, and range.     

半导体相移分布反馈激光器模式及调制相应特性的研究--牛顿-矢量方法的应用

给出了适于分析DFB激光器稳态特性的数值模型和分析振幅及频率调制响应特性的解析模型.研究了3相移DFB激光器的调制响应特性,并提出了一种能够快速精确得到DFB激光器多个模式解的新方法--矢量牛顿法.该方法将稳定的矢量法与精确的牛顿法结合,保证了求解质量.实践表明该方法非常适合于求解高度非线性方程的多解问题.用此方法,研究了3相移及简单DFB激光器的纵向光子浓度分布,纵模及调制响应特性.结果表明,3相移DFB具有与简单的DFB激光器同样好的调制响应特性,相移的引入在一定程度上抑制了纵向空间烧孔效应,并且有利于DFB激光器的单模输出.     

半导体相移分布反馈激光器模式及调制相应特性的研究--牛顿-矢量方法的应用

给出了适于分析DFB激光器稳态特性的数值模型和分析振幅及频率调制响应特性的解析模型.研究了3相移DFB激光器的调制响应特性,并提出了一种能够快速精确得到DFB激光器多个模式解的新方法--矢量牛顿法.该方法将稳定的矢量法与精确的牛顿法结合,保证了求解质量.实践表明该方法非常适合于求解高度非线性方程的多解问题.用此方法,研究了3相移及简单DFB激光器的纵向光子浓度分布,纵模及调制响应特性.结果表明,3相移DFB具有与简单的DFB激光器同样好的调制响应特性,相移的引入在一定程度上抑制了纵向空间烧孔效应,并且有利于DFB激光器的单模输出.     

Rate-equation analysis of output efficiency and modulation rate ofphotonic-crystal light-emitting diodes

The performance characteristics of photonic-crystal light-emitting diodes (LEDs) are analyzed, taking into account the effects of both nonradiative recombination and photon reabsorption processes using multimode rate equations. It is shown that, in the presence of strong photon reabsorption, the optimum output efficiency and modulation rates are achieved when the width of the photon density-of-state distribution function is comparable to the width of the spontaneous emission lineshape of the active material. On the other hand, when photon reabsorption is weak, it becomes beneficial to construct high-Q cavities. Based on this analysis, the characteristics of different photonic crystal LED configurations are discussed     

无序介质的非相干背散射

讨论两种散射,导出相应的分布函数,运用蒙特卡罗法模拟光子在无序介质中的随机行走过程,得到非相干背散射空间分布及反射率。计算结果表明：1、散射不同,非相干背散射空间分布和反射率不同,非相干背散射反射率随光入射角增大而增大;2、非相干背散射反射率随无序介质吸收系数增大而减小,通过测量非相干背散射反射率可得吸收系数,进一步可得每次散射光子被微粒吸收的概率;3、非相干背散射反射率随光子平均自由程增大而减小,通过测量非相干背散射反射率可得平均自由程,这比检测相干背散射来得到平均自由程要容易。最后,讨论了背散射对无序激光器的影响。     

Internal photon lines associated with the density matrix operator

Previous diagrammatic analyses of the density matrix are extended to incorporate quantum-electrodynamic processes fully by inclusion of the photon propagator and its complex conjugate. It is shown that eight fundamental time-ordered diagrams are necessary to describe radiative processes completely. For each of these, inclusion of the photon propagator permits, in addition to polarization, a direct calculation of electric field expectation values. Electric field correlations associated with the fundamental diagrams lead to a diagrammatic (quantum-electrodynamic) interpretation and derivation of the spontaneous emission limited linewidth associated with simulated emission as given by the Townes-Schawlow line narrowing formula. Stimulated Raman scattering, two-wave mixing, and parametric amplification are illustrative of the extension of these diagrammatic techniques to nonlinear optical phenomena     

Schottky contacts to high-resistivity epitaxial GaAs layers for detectors of particles and X- or γ-ray photons

The electrical characteristics of Ti and Pt Schottky contacts to epitaxial n-GaAs layers with the charge-carrier concentration <10¹² cm^?3 for detectors of particles and X- or ??-ray photons are studied. It is shown that it is preferable to use a diffusion-based theory of charge transport in calculations of the parameters of Schottky contacts to thick high-resistivity lightly compensated GaAs layers. The calculated barrier heights were 0.84 and 0.87 eV for the Ti and Pt contacts, respectively. The fabricated samples of the surface-barrier detectors featured a linear response in the studied range of energies from 6 to 140 keV for ??-ray photons and from 4 to 8 MeV for ?? particles; the efficiency of charge collection was close to 100% and the energy resolution was high at room temperature.     

Photon point process for traveling-wave laser amplifiers

The authors determine the evolution of the photon point process of a light beam as it passes through a traveling-wave laser amplifier (TWA). In particular, when coherent light is presented to the input of the amplifier, the output photon statistics are characterized by a marked-Poisson (MP) point process, which has a noncentral-negative-binomial (NNB) output photon-number distribution (PND). Using this distribution we calculate the probability of error (PE) for an ON-OFF keying (OOK) direct-detection photon-counting communication system, and show that the results differ somewhat from those obtained when the Gaussian-PND approximation is used. It is shown that receiver performance is optimized by filtering the amplifier output. Analysis of the point process is of interest because it permits the time response of the amplifier to be determined; this, in turn, allows the effects of intersymbol interference to be calculated     

旋转轮胎中压电悬臂梁离心距优化

针对压电悬臂梁在低频区域内响应较差的问题,该文基于自调节随机共振提出了一种在旋转轮胎中优化悬臂梁末端磁铁离心距的方法。首先分析推导出了末端磁铁的动力学方程,然后以Kramers逃逸速率为基础,建立了自调节随机共振频率与旋转频率的优化函数,得到最优离心距匹配机理,最后运用最优离心距匹配机理得到最优离心距,并比较了不同离心距下压电悬臂梁的位移、速度和电压响应。结果表明,当离心距达到优化值时,在某一频率范围内,随机共振频率随着旋转频率可持续产生共振现象,其有效响应频带扩展至30~50 rad/s,拓宽频带比约40%,有效提升了能量俘获性能。     

Motion detection and stochastic resonance in noisy environments

Several motion detection schemes are considered and their responses to noisy signals investigated. The schemes include the Reichardt correlation detector, shunting inhibition and the Horridge template model. These schemes are directionally selective and independent of the direction of change in contrast. They function by using spatial information and comparing it at successive time intervals. A rudimentary noise analysis is performed on the Reichardt and inhibition detectors to compare their natural robustness against noise. Using these detectors, stochastic resonance (SR) is applied, which is characterised by an improvement in response when noise is added to the input signal. It is found that the performance of the detectors degrades with the addition of noise. Employing Stocks' suprathreshold SR, an improvement can be gained when considering a network of detectors. Furthermore, when using an incorrect threshold setting for the template model, SR can be displayed.     

Behavior of graded-gap detectors of ionizing radiation under irradiation with alpha particles

The effect of irradiation with α particles on the current and optical responses of graded-gap Al_xGa_1?xAs/GaAs detectors of α particles and X-ray photons is studied. It is established that a reduction in both the current and optical response is caused by an increase in the rate of nonradiative recombination as the dose of α-particle radiation increases.