Description of Electromagnetic Radiation Propagating in a Four-Dimensional Space-Time with a Fluctuating Metric Tensor

The effect of spectral broadening of electromagnetic radiation propagating in a four-dimensional space-time with fluctuations in the space curvature caused by relict gravitational radiation is predicted. It is demonstrated that distortion of spectral line profiles of electromagnetic radiation caused by the fluctuating metric is at the level of resolution of the available spectral instrumentation.     

影响光谱辐亮度标定因素的分析

在利用漫反射板标定光谱辐亮度的实验装置中,标准灯完全可以近似为点光源。通过对进入光谱仪的辐射通量的数值积分,可以对应求出仪器的实测光谱辐亮度的计算公式。计算表明,实测光谱辐亮度并不等于漫反射板表面的光谱辐亮度,而有一个随标定条件变化的微小差值。通过计算,对光谱仪与漫反射板的距离、光谱仪入射挟缝的光轴与漫反射板法线的夹角以及标准灯与漫反射板的距离等影响光谱仪标定的因素进行分析,得到了最佳的标定条件。讨论了理想标定情况、光谱仪视场趋于一点的极限情况和采用实际的双向反射分布函数已知的漫反射板标定时,实测光谱辐亮度的计算情况。     

Accumulated programming of a complex spectral grating

A complex spectral grating is accumulated by repeated application of a pair of low-power optical programming pulses to a short-term persistent inhomogeneously broadened transition in Tm:YAG at 4.5 K and then probed to investigate the buildup dynamics. The necessary frequency stability is obtained by locking a cw Ti:sapphire laser to a regenerating transient spectral hole in the same transition. Grating accumulation is demonstrated for both a periodic spectral grating, representing a true-time delay, and a complex spectral grating, permitting correlation-based pattern recognition. This work is a step toward demonstrating an optical coherent transient continuously programmed continuous processor.     

Demonstration of a high-speed nonscanning imaging spectrometer

We report results from a field demonstration of a nonscanning high-speed imaging spectrometer [computed-tomography imaging spectrometer (CTIS)] capable of simultaneously recording spatial and spectral information about a rapidly changing scene. High-speed spectral imaging was demonstrated by collection of spectral and spatial snapshots of a missile in flight. This instrument is based on computed-tomography concepts and operates in the visible spectrum (430-710nm). Raw image data were recorded at video frame rate (30frames / s) and an integration time of 2ms. An iterative reconstruction of the spatial and spectral scene information from each raw image took 10s. We present representative missile spectral signatures from the missile firing. The accuracy of the high-speed spectrometer is demonstrated by comparison of extended-source static-scene spectra acquired by a nonimaging reference spectrometer with spectra acquired by use of CTIS imaging of the same static scenes.     

Navier–Stokes spectral solver in a sphere

The paper presents the first implementation of a primitive variable spectral method for calculating viscous flows inside a sphere. A variational formulation of the Navier–Stokes equations is adopted using a fractional-step time discretization with the classical second-order backward difference scheme combined with explicit extrapolation of the nonlinear term. The resulting scalar and vector elliptic equations are solved by means of the direct spectral solvers developed recently by the authors. The spectral matrices for radial operators are characterized by a minimal sparsity – diagonal stiffness and tridiagonal mass matrix. Closed-form expressions of their nonzero elements are provided here for the first time, showing that the condition number of the relevant matrices grows as the second power of the truncation order. A new spectral elliptic solver for the velocity unknown in spherical coordinates is also described that includes implicitly the Coriolis force in a rotating frame, but requires a minimal coupling between the modal velocity components in the Fourier space. The numerical tests confirm that the proposed method achieves spectral accuracy and ensures infinite differentiability to all orders of the numerical solution, by construction. These results indicate that the new primitive variable spectral solver is an effective alternative to the spectral method recently proposed by Kida and Nakayama, where the velocity field is represented in terms of poloidal and toroidal functions.     

Spectral anomalies due to temporal correlation in a white-light interferometer

We present what we believe to be the first experimental demonstration of anomalous spectral behavior such as spectral shifts and spectral switches due to temporal correlation around the intensity minima in a white-light interferometer. Unusual behavior in the number of spectral fringes, measured within the source bandwidth, as a function of path delay between the interfering beams is also reported. Experimental observations match well with the spectra calculated by using the interference law in the spectral domain.     

Internal distribution of radiation absorption in a semitransparent spherical particle

The internal distribution of spectral radiation absorption in a semitransparent spherical particle irradiated uniformly and isotropically is determined by the ray tracing method, and the detailed computation formulae for the internal spectral radiation absorption are deduced. The computed results show that the peak of internal volumetric spectral radiation absorption may locate at the interior shell of the particle. The dimensionless volumetric spectral radiation absorption is higher near the center for weakly absorbing or small spheres, but the dimensionless volumetric spectral radiation absorption is higher near the surface for strongly absorbing or large spheres. The corresponding physical interpretations of the internal spectral absorption distribution are given.     

Electron spectral density in a disordered system of hard sphere scatterers

The spectral density of an electron propagating in a disordered system of hard sphere scatterers is studied by use of a self-consistent approximation for the self-energy. The spectral density at fixed wavenumber is found to be a single-peaked function of energy. The approximation yields a sharp wavenumber-dependent band edge. For large wavenumbers the spectral density is well approximated by a Lorentzian, but for small wavenumbers it is dominated by a characteristic square root singularity at the band edge.     

Calculation of the spectral response of an arrayed-waveguide gating demultiplexer with a wide-angle beam propagation method in a cylindrical coordinate system

The spectral response of an arrayed-waveguide grating (AWG) demultiplexer is calculated by simulating the field propagation in the output section of an AWG with a wide-angle beam propagation method (BPM) in a cylindrical coordinate system. As in a practical design of an AWG demultiplexer, each output waveguide consists of two straight sections connected by a bending section. The spectral response obtained by the present algorithm is more accurate than those obtained with two popular approximate methods, namely, the conventional overlapped integral method and the standard BPM for radially straight and infinitely long output waveguides. With the present algorithm, the dependence of the spectral response on the parameters of the output section is analyzed. The channel crosstalk and the 3 dB passband width of the spectral response depend mainly on the length of the first straight section, the end separation and the angular separation of the output waveguides. The bending section results in an asymmetrical spectral response with remarkable sidelobes which can be reduced by increasing the bending radius.     

Formation of a sequence of femtosecond optical pulses with a terahertz repetition rate

We have shown experimentally that, upon interference of two frequency-modulated femtosecond pulses, a sequence of ultrashort subpulses is formed to which a quasi-discrete spectral continuum corresponds in the spectral range. We have shown that it is possible to control the duration and repetition rate of subpulses in the sequence by varying the delay between the interfering pulses.     

Construction of spatial-coherence spectral filters using a Fourier-achromat system

Theoretical investigations made not long ago regarding the construction of spatial-coherence spectral filters (SCSFs) are rendered into experiments by designing and fabricating a Fourier-achromat experimental setup analyzed in the theoretical studies. It is shown that the phenomenon of spectral shift due to spatial coherence also known as the Wolf effect can be exploited to make special types of low-pass and band-pass spectral filters with special spectral characteristics that are not shown by the conventional filters. A Fourier-achromat is employed to construct the SCSFs. The experimental results within the experimental limitations and measurement uncertainty agree well with the theory. These filters might find applications in (i) astronomy (in the search of particular spectral lines) (ii) developing spectrum-selective optical interconnects or (iii) in cryptography.     

Phase-sensitive scattering of a continuous wave on a soliton

Using cross-correlation frequency-resolved optical gating, we observe the phase-sensitive resonance in the interaction of a soliton with a continuous wave in a photonic crystal fiber. This interaction strongly depends on the difference in the phase velocities of the orthogonally polarized fiber modes and leads to generation of a new spectral peak. The spectral and temporal structure of this signal is revealed in our measurements, which are supported by analytical theory and numerical simulations.     

Far-field spectral intensity characteristics of a time-dependent gaussian pulse from a circular mask with a linear circular apertures array

The spectral characteristics of a time-dependent Gaussian pulse from a circular mask with a circular apertures array in the far field are theoretically investigated and presented with numerical results. A rigorous treatment, with the Fresnel diffraction integral, to the spectral changes of a time-dependent Gaussian-shaped pulse passing through a circular mask with a linear circular apertures array is given and some properties of the spectral intensity under different situations are provided. Also an accurate and concrete criteria judging the condition for the red shift, blue shift of spectral intensity distributions is obtained as well.     

Power spectral density of self-mixing signals from a flowing Brownian motion system

An analytical expression for the power spectral density of the self-mixing signals from a flowing Brownian motion system irradiated by a focused Gaussian field is derived from the time autocorrelation function of the signals. The power spectral density is composed of two Voigt functions. An improved series summation method (SSM) is proposed to calculate the Voigt function. The characteristics of the power spectral density are analyzed according to the numerical results. The power spectral density can hopefully be used for measuring the flow velocity and the particle size.     

色散介质中啁啾高斯脉冲的时间和光谱特性

对啁啾高斯脉冲光束在色散介质中的时间和光谱特性作了研究.给出了远场光谱蓝移和脉冲展宽的解析式.结果表明：通过选择适当的啁啾参量,啁啾高斯脉冲光束沿轴上传输色散长度时,脉冲可恢复到初始值.随啁啾参量增加,谱线宽度展宽,轴上光谱蓝移增加,在远场蓝移趋于一渐近值.随啁啾参量增加,离轴光谱红移增加.     

一种新型静态成像光谱系统的研究

静态干涉系统具有稳定性好、抗干扰能力强的优势,但其缺点是光谱分辨率低并且光谱测试范围不易调整。针对静态干涉型成像光谱系统光谱分辨率低且不可调等问题,设计了一种新型静态成像光谱系统。系统由光束整形模块、新型静态干涉调制模块以及成像模块构成。光束整形模块将入射光缩束并整形为平行光,进而保证入射干涉具后可以获得较好的干涉效果;新型静态干涉调制模块对入射光进行相干处理。在双折射干涉结构的基础上进行了改进,在不改变原有静态干涉具尺寸的基础上提高了系统的光谱分辨率,并实现了光谱分辨率的静态调制;成像模块完成对目标区域二维可见光图像的采集。系统核心部件由两组光轴相互正交的Wollaston棱镜作为分光器件,在两棱镜间放置电光调制模块,实现光程的静态扫描。分析了新型静态成像光谱系统的工作原理,给出了入射角、折射角等主要参数的函数表达式,并构建了系统的数据模型。通过绘制系统光线追迹图的方式,得到了该系统横向剪切量的函数方程,并对影响横向剪切量的各个参数进行了分析与讨论。通过仿真计算了改变结构角、晶体厚度以及调制度等参数对横向剪切量的影响程度,并定量计算了两个参数对系统光谱分辨率的影响程度。由仿真分析结果可知,增大结构角与加宽调制晶体厚度都可以为系统提供更大的光程差。故通过电光调制的方式实现横向剪切量的静态扫描是可行的,可以实现静态光谱图像的获取。在实验中对660 nm激光进行了测试。新型静态干涉模块采用孔径20 mm×20 mm,厚度10 mm的两块光轴相反的Wollaston棱镜与厚度10 mm的电光调制晶体构成。当调制度分别是0.000 2和0.000 6时,成像模块采集得到干涉条纹具有明显差异。当调制度增大时,其干涉条纹密度增大,说明采用越大的调制度,系统对应的光谱静态扫描能力越强,对光谱分辨率的控制越好。由此可见,本静态成像光谱系统在控制电光晶体调制度的条件下具有光谱分辨率可调的特性,验证了系统的可行性。     

Narrow-wavelength-spread spectral combining laser with a reflector for a double pass with a single grating

We proposed a novel wavelength-spread compression technique for spectral beam combining of a diode laser array. A reflector, which is parallel to the grating, is introduced to achieve a double pass with a single grating.This facilitated the reduction of the wavelength spread by half and doubled the number of combined elements in the gain range of the diode laser. We achieved a power of 26.1 W under continuous wave operation using a19 element single bar with a wavelength spread of 6.3 nm, which is nearly half of the original wavelength spread of 14.2 nm, demonstrating the double-compressed spectrum capability of this structure. The spectral beam combining efficiency was 63.7%. The grating efficiency and reflector reflectance were both over 95%; hence, the efficiency loss of the double-pass grating with a reflector is acceptable. In contrast to double-grating methods,the proposed method introduces a reflector that efficiently uses the single grating and shows significant potential for a more efficient spectral beam combining of diode laser arrays.     

Flat-topped spectrum obtained from partially coherent hollow Gaussian Schell-model beams passing through a slit

The anomalous spectral behaviors of partially coherent polychromatic hollow Gaussian Schell-model beams (HGSMBs) diffracted by a slit are investigated. Besides spectral switches, asymmetrical dual-peak spectral split and multi-peak spectral split are observed. In the vicinity of the positions where spectral switches occur, flat-topped spectrum can be obtained, the bandwidth of which is adjustable by changing the transverse coherence width and the observation position. The flat-topped spectrum has potential application for creating probe beams in optical metrology systems and generating broadband optical sources in fiber optic sensor interface systems.     

Spectral shift of a stochastic electromagnetic Gaussian Schell-model beam in a Gaussian cavity

Spectral changes of a stochastic electromagnetic Gaussian Schell-model (EGSM) beam interacting with a Gaussian cavity are investigated. It is shown that the spectral shift is mainly determined by the degree of polarization of the initial beam and by the parameters of the cavity. Generically the blue shift occurs at on-axis points, while the red shift can occur at off-axis points. The condition under which the relative spectral shift is independent of the degree of polarization of the initial beam is analyzed.     

Four-wave parametric conversion of femtosecond laser pulse in a filament induced in a solid target

Nonlinear interaction of two femtosecond laser pulses in a filament, induced by one of them inside a fused silica plate, leads to generation of the new spectral components. These spectral components reach hundreds of nanometers bandwidth. Their spatial and spectral properties can be explained by four-wave parametric coupling in the filament. The energy measurements indicate the high efficiency of this process.