On pulse compression in dispersive media

A comparison between three methods used for the synthesis of suitable signals which result in pulse compression, when transmitted through dispersive media, is presented. These are the equalization of group-time delay method, the space-time rays method, and the matched-signal method. The three methods are shown to be equivalent if the signals are restricted to be frequency-modulated with uniform envelopes. A generalized expression for the requited frequency modulation law for pulse compression in an inhomogeneous dispersive medium is obtained. Considerations for lossy dispersive media and additional optimization of the signal envelope are also discussed.     

FM pulses in stratified isotropic plasma

A study of signals excited by FM pulses in an infinite inhomogeneous cold isotropic plasma is presented. The medium is assumed to vary arbitrarily with position, though sufficiently slowly with respect to all wavelengths contained in the source spectrum. For simplicity only one spatial dimension is considered, but the development may be extended to include more space variables. After deducing integral representations that are evaluated asymptotically, the theory is applied to a particular plasma inhomogeneity characterized by an exponential variation of plasma frequency. Space-time rays associated with an impulsive source are shown to justify the use of time-inversion in establishing the required phase of an FM pulse capable of producing an optimally compressed signal at a particular space-time point. Since ordinary asymptotic methods fail in the neighborhood of such a focal point, a theory that in a prior study described signals in the focal region of a homogeneous plasma is applied to signals in a plasma with exponentially varying plasma frequency. Computed results based on this theory display the character of optimally compressed signals in focal regions.     

Solution near the wavefront of transient fields in inhomogeneous dispersive media

Uniform asymptotic representations, valid in the vicinity of the wavefront, are derived for transient fields propagating in an inhomogeneous dispersive medium. When the space-time ray equations for these fields can be solved explicitly, the uniform representations yield explicit expressions in the space-time variables; otherwise, the representations involve a parameter which can, however, be eliminated under more restrictive conditions. The results are applied to plane pulse propagation in a plane stratified plasma with exponential variation of electron density, for which the space-time ray equations can be solved in closed form; in this case, the asymptotic representation is found to yield the known exact solution.     

Ray-pulse matrices: a rational treatment for dispersive opticalsystems

A new formalism to describe beam propagation in paraxial optical systems with dispersive elements, including both spatial and temporal variations in the propagating signal, is presented. This formalism makes use of 4×4 ray-pulse matrices which take account of dispersive effects up to quadratic phases in both spatial coordinates (as in the usual paraxial ABCD matrix approach) and in the temporal domain. How to use these matrices to write a space-time integral analogous to a generalized Huygens integral is shown, and propagation laws for Gaussian ray pulses which are space- and time-varying analogs of the conventional results for Gaussian beams are derived. The formalism should be useful for analyzing dispersive optical systems such as prism beam expanders, femtosecond pulse compression systems, and dispersive mode-locked laser cavities     

Phase-space asymptotic analysis of wave propagation in homogeneous dispersive and dissipative media

Arising naturally in the study of one-dimensional pulse propagation in homogeneous dispersive and/or dissipative media are certain classes of oscillatory and/or diffusion integrals which encompass the canonical diffraction catastrophe integrals due originally to Thorn and Arnold. This is especially evident within the framework of a phase-space asymptotic analysis. Depending on the order of approximation of the exact solution beyond the Liouville or "first-order quasiparticle" limit, one recognizes caustic-like structures smoothed over by hyperdiffusion. Asymptotic series for these structures, which essentially define new basic functions, have been derived, but will not be presented here. Only the salient features of these structures will be reviewed briefly, and they will be illustrated by means of several simple canonical problems. Also, their applicability to other physical areas (e.g., wave propagation in deterministically and/or randomly varying channels, diffraction, etc.) will be pointed out.     

Propagation of Large Bandwidth Microwave Signals in Water

Large bandwidth microwave signals propagating in dispersive media can result in pulses decaying according to a non-exponential law. In particular, large bandwidth signals in the microwave band, propagating in media that can be described by the Debye model (for example fresh water), decays as the square root of the inverse of the propagation distance, instead of exponentially. Although it is a direct consequence of well-known theory of propagation in dispersive media, this result is a bit surprising and its experimental evidence has required a careful set-up.      

Compression of transmitted pulses in plasmas

The compression of frequency-modulated RF pulses is discussed when the dispersive property of a lossless, isotropic, and homogeneous plasma is used to enhance the resolution of transmitted pulses at the receiver. Using the basic concept of group velocity, an expression is obtained for the carrier phase variation of a rectangular envelope pulse so that the pulse collapses on itself at the receiver. Unlike the chirp radar pulse, the frequency variation is not linear and the optimum detection of transmitted pulses depends on both the plasma frequency and the distance between the transmitter and the receiver. The method of convolution was used to obtain numerical results, which indicate that the formulation given results in compression for short propagation distances.     

Study of ultrawide-band transmission in the extremely high frequency (EHF) band

The growing demand for broad-band wireless communication links and the lack of wide frequency bands within the conventional spectrum, causes us to seek bandwidth in the higher microwave and millimeter-wave spectrum at extremely high frequencies (EHF) above 30 GHz. One of the principal challenges in realizing modern wireless communication links in the EHF band are phenomena occuring during electromagnetic wave propagation through the atmosphere. A space-frequency approach for analyzing wireless communication channels operating in the EHF band is presented. Propagation of the electromagnetic radiation is studied in the frequency domain, enabling consideration of ultrawide-band modulated signals. The theory is employed for the analysis of a communication channel operating at EHF which utilizes pulse amplitude modulated signals. The atmospheric absorptive and dispersive effects on pulse propagation delay, pulse width and distortion are discussed. The theory and model are demonstrated in a study of ultrashort-pulse transmission at 60 GHz.     

太赫兹波在介质中传播的FDTD分析

时域有限差分法是电磁场领域中应用最为频繁的数值方法之一,它可以有效地处理复杂媒质中颗粒对电磁脉冲信号的散射问题,以及超宽带电磁脉冲信号在色散媒质中的传播问题。几乎所有介质,无论其在太赫兹（THz）波段是否有吸收峰,对太赫兹波的传播都有影响。研究太赫兹波在介质中的传播情况,对优化太赫兹系统,分析与设计太赫兹应用技术有重要意义。本文模拟了太赫兹波在散射介质中的传播,说明了太赫兹波在内弹道干涉测速应用中的可能性;模拟了太赫兹脉冲在色散介质中的传播情况,为太赫兹时域光谱系统中晶体选择提供依据。     

FD－TD法分析频变煤质中基带脉冲波传播和散射

利用ＦＤ－ＴＤ法，分别计算了频变媒质中基带脉冲淡的传播和浅层地下目标的时域电磁散射问题。通过Ｚ－变换技术，推出了媒质的介电常数εｒ＜（ω）和导磁率μｒ（ω）同时用Ｄｅｂｙｅ方程表示时ＦＤ－ＴＤ法的迭代公式，并给出了相应的吸收边界条件。通过将计算结果与其它结果相比较，证实了ＦＤ－ＴＤ法分析有托媒质中电磁场问题的有效性，并比较了吸收边界条件的吸收性能。对基带脉冲波在不同频变媒质中的传播特性进行了讨论。分别计算并给出了位于频变媒质中典型目标的时域散射波形和波形堆积图。     

Cross-Ambiguity Function for a Linear FM Pulse Compression Radar

Three-dimensional computer plots of the cross-ambiguity function are developed in order to illustrate the discrimination against improperly coded signals in a pulse compression matched filter. The filter response envelope amplitude is plotted as a function of time and frequency offset for input signals mismatched in FM rate, pulsewidth, and timedomain amplitude weighting. Illustrative examples of the unnormalized cross-ambiguity diagram reveal the signal-to-interference ratio improvement of this filter in the presence of signals with no FM or reverse slope FM. The effect of Hamming weighting on the waveform response is also shown.     

Complex space-time rays and their application to pulse propagation in lossy dispersive media

Asymptotic transient field solutions of the form A(r,t) exp [iS (r, t)], where S is a rapidly and A a slowly varying function of space and time, may be analyzed in terms of wave packets with central frequency ω =-∂S/∂t and central wavenumber k = ∇S. When the (dispersive) medium is lossless, stationary, and homogeneous, wave packets with constant real ω and k move along straightline trajectories called space-time rays. In the presence of dissipation and (or) when the input signal has an exponential amplitude dependence, S is complex. The corresponding wave packets with constant complex ω and k move along complex space-time rays, i.e., along trajectories defined in a complex (r, t) coordinate space. The properties of complex space-time rays and of the fields propagating along them, and their relation to physical fields observed on real (r, t) coordinates, are illustrated for a plane pulse with Gaussian envelope and frequency swept carrier, launched into a lossy environment. Tracking of spatial and temporal maxima is performed by ray techniques, and a paraxial ray regime is defined that permits discussion of a signal velocity. Special attention is given to ray focusing and the associated phenomena of pulse compression. It is shown how a complex input frequency profile can be synthesized so as to achieve optimum compression at a real space-time observation point in a lossy medium. The general results are applied in detail to a cold dissipative plasma, and a representative set of numerical calculations is included.     

Transients in dispersive media, part I: Theory

The asymptotic evaluation of complex frequency integrals arising in the theory of radiation from transient sources in dispersive media is reviewed and extended to accommodate phenomena observed at distant observation points at any observation time. Helpful graphical methods are emphasized and illustrated for a plasma medium discussed in detail in Part II of this paper. Results are interpreted in terms of space-time rays whose properties are reviewed, and these rays, together with the (k, omega) dispersion surfaces for the medium, are utilized for the study of reflection and refraction processes at an interface.     

脉冲压缩技术的教学探索与实践

针对脉冲压缩技术推导繁杂、学生理解抽象、课堂与工程实践联系脱节等问题，本文从被噪声淹没的两个回波信号的处理出发，引出线性调频信号的匹配滤波分析，通过将时域分析的定量解析结果与频域近似的定性分析结果统一起来，共同得出脉压性能的分析结论。实践表明，该教学方法可帮助学生在课程学习中达到事半功倍的效果。     

脉冲压缩原理及FPGA实现

为解决雷达作用距离和距离分辨力的问题,分析了线性调频脉冲压缩的原理及工程实现方法,并利用Matlab软件对加权前后的线性调频信号脉冲压缩波形进行对比。简述了分布式（DA）算法的基本原理,给出一种基于FPGA分布式算法的时域脉冲压缩实现结构,利用QuartusⅡ软件完成脉冲压缩处理模块设计以及波形仿真。通过分析可以得出基于分布式算法实现的脉冲压缩可以减少资源利用率,大大节省硬件资源。     

激光脉冲在克尔介质中传播的近似描述

将克尔介质等效为时域二次折射率介质的基础上，考虑折射率系数随传播变化的情况，得到了激光脉冲在克尔介质中传播时近似的解析描述。     

The IMCON Pulse Compression Filter and Its Applications

The IMCON is a reflection-mode dispersive delay line capable of high performance in large time-bandwidth product pulse compression systems. As developed in this paper, the unique characteristics of the IMCON are obtained by reflection from a double grating array that is applied to the surface of a strip. Current models of the device have center frequencies in the 4-30-MHz range with bandwidth up to 15 MHz, dispersion to 320 /spl mu/s time sidelobes on the order of --40 dB (with equalization), and other spurious signals at least 70 dB below the compressed output. The characteristics of IMCON operation are developed from a consideration of the device's transfer function. In particular, the IMCON'S high linearity and low sensitivity to fabrication and propagation problems are shown to be due to a unique error rejection effect. By comparison, the error rejection characteristics of single grating and dispersive transducer devices are found to be inferior to the IMCON. Data derived from operating pulse compression systems are utilized to demonstrate the low time sidelobe and high time-bandwidth capability of the IMCON.