蒸发波导环境中雷达二维海杂波仿真

提出了一种大气波导环境下的K分布雷达海杂波仿真方法,仿真了二维海杂波序列.该方法采用射线描迹和抛物方程模拟蒸发波导中电磁波的传播过程,结合雷达方程计算K分布参数,利用球不变随机过程仿真产生K分布海杂波.该二维序列一维为时间,另一维为距离,时间为相关时间,频谱为高斯谱;相邻距离单元之间独立,K分布参数在距离上慢变化,可以认为距离上符合独立同分布假设.文中的二维海杂波序列可应用于雷达海杂波信号模拟中,作为海杂波反演大气波导中的杂波信号,亦可用以分析雷达在不同波导环境中的超视距探测性能.     

类傅里叶变换的多分量信号分离重构

针对多分量信号重构的问题,该文提出了一种新颖的类傅里叶变换方法,并对其基本性质进行了分析。采用该方法将频域上混叠但在时频二维频谱图上不重叠的多分量信号变换到类傅里叶变换域,使之在频谱上不产生混叠,从而达到信号分离重构的目的。与分数傅里叶域最优滤波的方法进行的对比分析说明,类傅里叶变换方法的适用范围更宽。文中对非线性的多分量调幅信号进行了仿真计算,得到了满意的结果。表明该方法在信号检测和分析方面具有应用价值。     

基于分数阶傅里叶变换的线性 调频信号二维波达方向估计

 本文提出了一种基于分数阶傅里叶变换(Fractional Fourier Transform,FRFT)的多线性调频(Linear Frequency Modulation,LFM)信号二维波达方向(Direction of Arrival,DOA)估计方法.该方法利用FRFT对LFM信号的能量聚集特性,构造出一种新的分数阶傅里叶域的阵列信号数据模型,并利用MUSIC算法实现对多个LFM信号的二维DOA估计.仿真实验验证了算法的有效性.     

基于中心对称局部二值模式的合成伪装语音检测方法

针对基于局部二值模式的伪装语音检测方法的合成语音检测准确度较低的情况，提出了一种基于中心对称局部二值模式的伪装语音检测方法。该方法通过短时傅里叶变换得到语音信号的语谱图，再利用中心对称局部二值模式提取语谱图的纹理特征，并用该纹理特征训练随机森林分类器，从而实现真伪语音的判别。该方法综合考虑语谱图中像素点的数值大小和位置关系，包含了更加全面的纹理信息，并将特征维度降低至16维，有利于减少计算量。实验结果表明，在ASVspoof 2019数据集上，与传统的基于局部二值模式的伪装语音检测方法相比，所提方法将合成伪装语音的串联检测代价函数（t-DCF）降低了16.98%，检测速度提高了89.73%。     

具有平移和尺度不变性的自适应小波变换

本文提出了一种具有平移和尺度不变性的自适应小波分解新方法,该方法利用信号的一阶、二阶矩及正交小波尺度函数,先对信号进行自适应小波"重整".然后再对重整后的信号进行普通小波变换.本文证明这种自适应小波变换是平移和尺度不变的,并给出了计算自适应小波变换系数(称为小波不变矩)的一种有效算法.对二维数字信号(图像)的实验证实了我们的结论.     

基于互模糊函数的快速时差频差联合估计

针对无源定位中的时差频差联合估计问题,利用内积公式推导出了基于互模糊函数的时差频差联合估计公式,并提出了相应的快速实现算法。其中通过适当变换互模糊函数计算公式,将传统的二维时差频差搜索算法变为对信号内积进行快速傅里叶变换,然后对变换后的数据进行二维搜索,大大提高了时差频差联合估计的速度,降低了运算量。通过重构离散傅里叶变换项,并进行近似,进一步降低了运算量。仿真结果说明,两种算法都能得出准确的结果,改进的算法计算时间更少。     

二维正交子波变换的VLSI并行计算

本文提出一个二维离散正交子波变换的ＶＬＳＩ并行结构，该结构将二维输入信号分解成不重叠的若干行组，从而使每组中的所有行被并行处理，而不同组的行的处理、不同级上的计算，以至不同信号的计算可以在此结构上流水线地进行。     

二维Tchebichef矩正反变换的快速算法

本文提出了一种二维Tchebichef矩正反变换的快速算法。在正变换中,使用Chebichef递推公式推导了一维Tchebichef矩正变换的快速算法,并将其推广至二维Tchebichef矩正变换的快速计算。在反变换中,使用Clenshaw递推公式我们推导了一维Tchebichef矩反变换的快速算法,并将其推广至二维Tchebichef正交矩反变换的计算。与以迭代方式计算Tchebichef多项式进而计算二维Tchebichef矩正反变换的方法相比,本文算法有效地减少了算术运算的次数,提高了计算速度。实验结果表明了该方法的有效性。     

基于降维处理的MUSIC二维DOA估计算法

针对 MUSIC(多重信号分类法)估计方法实现二维 DOA(波达方向)估计的计算量大且遍历搜索耗时的问题,给出了一种基于降维处理的 MUSIC 算法。该算法无需进行二维谱峰值搜索。该算法利用二次优化方法将二维 DOA 估计分解为一维 DOA 估计,先通过一维 MUSIC 估计获得信号与 x 轴夹角,再利用最小二乘算法估计获得信号与 y 轴夹角。最后利用角度关系式得到信号的二维 DOA 估计值。该算法的复杂和搜索范围都大大降低,仿真表明,该算法具有较好的角度估计效果。     

一种基于FFT的多谱勒扩频信号捕获方法

在通信测控系统中短的信号捕获时间是非常重要的。提出了一种基于快速傅里叶变换(FFT)的多谱勒直接序列扩频信号的快速捕获方法,利用频域循环移位特性和FFT变换特点计算出本地伪码序列与输入信号序列的相位差,检测出输入信号多普勒频移。从理论上对该技术进行了研究,分析了捕获方法的基本原理,推导了二维相关计算的数学表达式。用Matlab/Simulink工具进行了仿真,检测出多谱勒频移和伪码相位,进一步验证了该算法捕获的正确性。     

Two-dimensional mode size transformation by Δn-controlledpolymer waveguides

We report a new type of polymer waveguide for two-dimensional (2-D) mode size transformation in which Δn (refractive index difference between core and cladding) is properly controlled along the waveguide axis while the core cross section is kept uniform. The beam diameter in the Δn-controlled waveguide where the core diameter is 3.7 μm and Δn changes from 0.0027 to 0.0009 toward one end in the 5 mm-gradient Δn, region, is about twice as large as that in a normal waveguide where Δn is 0.0027. The optical loss accompanied by mode size transformation is calculated to be less than 2.5% based on beam propagation methods (BPM's)     

A theoretical study of a Ti-diffused Er:LiNbO3 waveguidelaser

A detailed theoretical procedure of determining the mode sizes of fundamental mode field distribution in a coaxially pumped Ti-diffused Er:LiNbO₃ waveguide laser has been described. The mode sizes, effective refractive indexes, effective pump area, and coupling efficiency as a function of the initial Ti stripe width W were numerically calculated for the waveguide with 95- and 100-nm initial Ti stripe thicknesses, respectively. The results indicated that the threshold pump power is severely affected by the stripe width, while slope efficiency is hardly changed as W; both show little difference between 95- and 100-nm stripe thickness. In addition, the stimulated emission cross section of Er³⁺ in Er:LiNbO₃ channel waveguide versus the wavelength were calculated directly from its fluorescence spectra using the β-τ method. Subsequently, threshold pump power and slope efficiency were evaluated     

Calculation of the fundamental mode sizes in optical channelwaveguides using Gaussian quadrature

A fast numerical method using Gaussian quadrature, which takes only seconds on a microcomputer, is presented for calculating the fundamental mode sizes in optical channel waveguides. Variational expressions for the square of the propagation constant, β² , of the TE- and TM-like modes are derived using the vector wave equations. For channel waveguides with gradual refractive index distributions, these expressions approach the variational expression obtained using the scalar wave equation. To show the usefulness of the numerical technique the authors present the results for titanium indiffused lithium niobate channel waveguide which are commonly used in integrated optical circuits. Since these waveguides have gradual refractive index distributions, both types of expressions give the same results; however, it takes less time to compute the mode sizes when using the variational expression obtained from the scalar wave equation. The authors find the calculated mode sizes are in good agreement with published measurements. From the comparison process, best fit parameters are obtained, which give mode sizes close to the values published in the literature. For one special case the authors are able to obtain an analytical variational expression and they use it to test the accuracy of the numerical method. They find that the values of β² given by both methods agree to six significant figures     

Ultralow internal optical loss in separate-confinement quantum-well laser heterostructures

Internal optical loss in separate-confinement laser heterostructures with an ultrawide (>1 smm) waveguide has been studied theoretically and experimentally. It is found that an asymmetric position of the active region in an ultrawide waveguide reduces the optical confinement factor for higher-order modes and raises the threshold electron density for these modes by 10–20%. It is shown that broadening the waveguide to above 1 μm results in a reduction of the internal optical loss only in asymmetric separate-confinement laser heterostructures. The calculated internal optical loss reaches ∼0.2 cm⁻¹ (for λ≈1.08 μm) in an asymmetric waveguide 4 μm thick. The minimum internal optical loss has a fundamental limitation, which is determined by the loss from scattering on free carriers at the transparency carrier density in the active region. An internal optical loss of 0.34 cm⁻¹ was attained in asymmetric separate-confinement laser heterostructures with an ultrawide (1.7 μm) waveguide, produced by MOCVD. Lasing in the fundamental transverse mode has been obtained owing to the significant difference in the threshold densities for the fundamental mode and higher-order modes. The record-breaking CW output optical power of 16 W and wallplug efficiency of 72% is obtained in 100-μm aperture lasers with a Fabry-Perot cavity length of ∼3 mm on the basis of the heterostructures produced. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 12, 2004, pp. 1477–1486. Original Russian Text Copyright ? 2004 by Slipchenko, Vinokurov, Pikhtin, Sokolova, Stankevich, Tarasov, Alferov.     

Long-wavelength (9.5-11.5 μm) microdisk quantum-cascade lasers

Quantum-cascade whispering-gallery-mode disk lasers emitting at 9.5-μm and 11.5-μm wavelength are reported. Taking advantage of the high-quality resonator (Q≈200), the threshold current density of disk lasers emitting at 9.5 μm is reduced below the value of the corresponding ridge waveguide geometry (J_th,disk=2.39 kA.cm ^-2 versus J_th,ridge=3.0 kA.cm^-2). Additionally, the increase in wavelength compared to previously reported disk lasers at 5.0 μm is a significant step toward the microcavity regime (by an effective scaling factor of 2.5, comparing identical disk sizes), disk diameters from 125 μm down to 20 μm are used to study the approach to the microcavity regime by size reduction. Far-field pattern measurements identify scattering from the pedestal as an important outcoupling mechanism for microdisk lasers. An excellent agreement between the measured and calculated free spectral range of the whispering gallery modes allows us to estimate the beta factor of the microdisks, resulting in β≈0.05 for a 20-μm diameter disk. A two-level rate equation model is evaluated for the quantum-cascade disk laser as a tool for a direct measurement of β. Nevertheless, the actual measurement is at present blurred by luminescence (light-emitting diode) from the disk center accompanied by an unbalanced carrier distribution between the whispering gallery laser and the center light-emitting diode     

All-optical switching with low-peak power in microfabricated AlGaAswaveguides

We report all-optical switching with low-peak power in a microfabricated AlGaAs waveguide operating at 1.6 μm. We show that by using a 1-μm long microfabricated strongly-guided waveguide with 0.8 μm by 0.9 μm mode cross-sectional area, switching is achieved with an average power of 1.2 mW for 82-MHz mode-locked 430 fs pulses. The estimated peak pump power and pulse energy inside the microfabricated waveguide were ~30 W and ~14.6 pJ, respectively, which is 5-10 times lower than the values needed with conventional waveguides. In terms of a practicality index defined via switching power times waveguide length, this waveguide has around the best value     

Compact mode expanders using resonant coupling between a taperedactive region and an underlying coupling waveguide

A novel technique for enhanced laser-fiber coupling, based on resonant power coupling between a tapered active waveguide and an underlying coupling waveguide, is presented. Spot-sizes are transformed from 2.0×1.1 μm in the active region to 6.0×3.1 μm in the coupling waveguide, over a length of 200 μm, with a mode transformation loss of only 0.36 dB. Butt-coupling efficiencies of 55% (2.6 dB loss) are estimated to standard cleaved single-mode fibers at 1.55 μm. The proposed device requires a single epitaxial growth and conventional processing techniques, making it amenable for low-cost manufacturing     

Analysis of the power coupling from a waveguide hyperthermiaapplicator into a three-layered tissue model

The power deposition from a rectangular-aperture flanged waveguide into a three-layered stratified tissue medium is analyzed theoretically. The fields inside the tissue layers are expressed in terms of Fourier integrals satisfying the corresponding wave equations, while the fields inside the waveguide are expanded in terms of the guided and evanescent normal modes. An integral equation is derived on the aperture plane of the flanged waveguide by applying the continuity of the tangential electric and magnetic fields. This integral equation is solved by expressing the unknown electric field in terms of the waveguide mode fields and by applying a Galerkin procedure. The electromagnetic fields inside the tissue medium are then determined and patterns of the deposited power at frequencies of 432 MHz and 144 MHz for apertures of 5.6×2.8 cm² and 16.5×8.3 cm², respectively, are computed and presented     

Mode excitation from sources in two-dimensional EBG waveguides using the array scanning method

An efficient semianalytical algorithm for the evaluation of the field and modal excitation by a line source in a two-dimensional electromagnetic bandgap (EBG) waveguide is presented. The method allows for an accurate and efficient calculation of the near field from the source inside the EBG waveguide, as well as the amplitude of the EBG waveguide mode that is excited. The same method can be applied to a wide variety of structures, as well as other types of sources and discontinuities.     

Propagation-mode-controlled fabrication of self-written waveguide in photosensitive polyimide for single-mode operation

We have developed a new fabrication method of single-mode self-written waveguide by controlling the propagation mode in an optical fiber. This method is very appropriate for repeatable fabrication of the single-mode self-written waveguide. Since a Gaussian-like near-field pattern is required for the fabrication of a tiny and uniform waveguide core, the propagation mode in a conventional optical communication fiber was controlled by coupling with an optical fiber having 3-/spl mu/m core, which shows a single-mode operation at visible wavelength region. Single-mode propagation at optical communication wavelength was confirmed for the fabricated self-written waveguide. The evaluated core diameter of the self-written waveguide was /spl sim/9.5 /spl mu/m.