射频已调波同步技术在单频网中的应用探讨

本文首先分析同步广播单频网的常见技术问题,阐述传统基带同步方式与射频同步方式的不同特点,随后对射频已调波技术理论做简要介绍,最后对基于射频已调波方式的同步广播单频组网方法进行探讨.     

Terahertz Photonics: Optoelectronic Techniques for Generation and Detection of Terahertz Waves

Optoelectronic techniques for generation and detection of terahertz (THz) signals have been reviewed. The operation principles of THz photomixer sources and THz parametric sources have been studied and recent developments in these areas have been presented. The performances of developed THz optoelectronic sources have been discussed and compared.      

Terahertz Emission in Asymmetric Quantum Wells by Frequency Mixing of Midinfrared Waves

Generation of terahertz (THz), and sub-THz, coherent waves has been demonstrated at room temperature in GaAs-AlGaAs asymmetric quantum wells by mixing two CO₂ laser beams. Two overlapping regimes were studied: the double resonance regime where three states are involved and the optical rectification which relies on two states only. The measured conversion efficiency of ~10^-7 W^-1 is in reasonable agreement with theoretical predictions. In addition, this technique of THz generation provides a model system to study the emission profile from an ensemble of radiating dipoles. For instance, backward emission and strong diffraction effects have been observed     

Terahertz Frequency Combs

We review recent work on THz frequency combs generated by femtosecond lasers. An overview about the underlying theory and application examples for the analysis of THz signals are given. Using different optoelectronic detection methods, spatially resolved measurements of frequency, phase, and amplitude of GHz and THz emitters are presented. The measurement setup allows for high-precision measurements of the frequency (accuracy of 9 · 10^{? 14}within a 10 s measurement interval), relative amplitude (standard deviation of the mean 0.1 %) and phase (standard deviation of the mean 0.2°). Moreover, the accuracy and invasiveness of different optoelectronic techniques are compared. The large spectral coverage provided by THz frequency combs is demonstrated by simultaneously measuring signals at 16 GHz and 28 THz.     

Scattering of Terahertz Waves by Snow

Journal of Infrared, Millimeter, and Terahertz Waves - The applications of terahertz (THz) wireless communication require studies on link performance in all kinds of atmospheric conditions,...     

Direct Measurements of Terahertz Meta-atoms with Near-Field Emission of Terahertz Waves

We present the direct measurements of terahertz meta-atoms, an elementary unit of metamaterials, by using locally generated terahertz waves in the near-field region. In contrast to a conventional far-field terahertz spectroscopy or imaging, our technique features the localized emission of coherent terahertz pulses on a sub-wavelength scale, which has a potential for visualizing details of dynamics of each meta-atom. The obtained data show the near-field coupling among the meta-atoms and the impact of the electric field distribution from the excited meta-atom to neighbor meta-atoms. The observable LC resonance response is enhanced with an increase of numbers of meta-atoms. Furthermore, our approach also has a potential for visualizing the individual mode of meta-atom at different terahertz irradiation spots. These data can help us to understand the important role of the meta-atom in metamaterials and develop the novel terahertz components and devices such as active terahertz metamaterial and compact, high-sensitive bio-sensor devices.     

基于Wigner-Hough变换的线性调频信号检测性能

根据Wigner-Hough变换思想和检测理论,分析了线性调频信号的检测性能,给出了虚警概率和检测概率的数学表达式,该表达式是Hough变换的积累单元数和检测门限的函数,然后对概率密度函数进行了合理近似,并给出了Hough变换的积累单元数计算公式,最后仿真分析了基于Wigner-Hough变换对线性调频信号相干积累相对于傅立叶变换的优越性.     

单点光学终点检测系统的研究

为了使CMP抛光精度更为精确,根据光学干涉原理设计了单点光学终点检测装置,给出了整套检测系统的简图和处理流程.在理想条件下,仿真单点光学检测的相干相位及反射率迹线,得出反射率迹线与抛光掉的透明层存在着函数关系,在实际条件核实了二者之间映射情况.采用九点测量后确认此函数条件下的单点光学终点检测具备高度的精确性,在此基础上推测了抛光头吸附晶圆以设定频率摆动情况下,光学传感器采集与处理数据的方法和过程.实际工艺操作证明,在综合运动条件下,其精度也达到要求.     

Coherent Generation and Detection of Continuous Terahertz Waves Using Two Photomixers Driven by Laser Diodes

We report on a completely coherent, tunable, continuous-wave THz system where, for the first-time, both the transmitter and receiver are log-periodic-antenna coupled LTG-GaAs based finger-photomixers. This compact room-temperature system exhibits signal-to-noise ratios (> 10³) comparable to or better than what is reported in the literature, and has potential applications in high-resolution spectroscopy and imaging. For the purpose of demonstration, we also present results of spectroscopy measurements carried out on fused silica, which are in agreement with previously published THz-TDS measurements.     

基于帧格式的调频遥测信号检测方法分析

针对航天器测控特点,低信噪比信号检测是实现航天测控系统目标快速捕获的关键技术。为了解决调频遥测信号低信噪比检测难题,介绍了采用正交矢量瞬时测频方法的调频遥测信号解调方法,分析了调频遥测信号帧格式,在此基础上提出了基于帧格式的调频遥测信号匹配滤波检测法。仿真结果表明所提方法能实现在低信噪比条件下调频遥测信号的高概率检测。     

Subjective Tests of a Companded, Frequency Modulated, Single Channel Per Carrier Communication System

Subjective tests were performed to evaluate components of an FM-SCPC communications system including a syllabic compandor, a VOX circuit, and a shallow emphasis network. The subjective improvements in the signal-to-noise ratio were determined for these components separately and in combination. Tests of the syllabic compandor were conducted at four talker levels and at two values ofC/kTfor the SCPC equipment. Further listening tests were conducted to evaluate the advantage produced by (a) the VOX alone, (b) the VOX operating with the compandor, and (c) the VOX, compandor and a shallow emphasis network. Group distributions of compandor advantages are presented for use in designing SCPC systems.     

A Review on Thin-film Sensing with Terahertz Waves

In the past two decades, the development and steady improvement of terahertz technology has motivated a wide range of scientific studies designed to discover and develop terahertz applications. Terahertz sensing is one such application, and its continued maturation is virtually guaranteed by the unique properties that materials exhibit in the terahertz frequency range. Thin-film sensing is one branch of this effort that has enjoyed diverse development in the last decade. Deeply subwavelength sample thicknesses impose great difficulties to conventional terahertz spectroscopy, yet sensing those samples is essential for a large number of applications. In this article we review terahertz thin-film sensing, summarizing the motivation, challenges, and state-of-the-art approaches based predominately on terahertz time-domain spectroscopy.     

利用谐振原理进行单频点检测及频谱分析

在使用FFT进行频谱分析的时候，自身的一些缺点难以克服。由于谐振器件有着一些优良的特性，本文尝试用谐振原理进行单频点检测及频谱分析，推导出谐振子的设计和仿真公式。并用实验进行了验证，初步论证了其可行性。     

Polarization-independent Metamaterial Absorber for Terahertz Frequency

We present the design, fabrication, as well as the characterization of the large incident angle and polarization independent terahertz (THz) metamaterial absorber. The designed absorber consists of metallic cross structure and a metallic ground plane separated with a dielectric as a spacer, this structure shows a strong resonance at terahertz frequency. The influences of the structural parameters (including period, cross width and spacer thickness) on the absorption are discussed. An optimized structure is fabricated and characterized, and a good agreement between simulation and experimental result is obtained. The metamaterial absorbers are potential candidates as absorbing elements for terahertz imaging and other fields of terahertz technology.     

Electrically Controlled Liquid Crystal Phase Grating for Terahertz Waves

This work demonstrates the feasibility of electrically controlled liquid-crystal-based phase grating for manipulating the terahertz (THz) waves. This device can be utilized as a THz beam splitter and the beam splitting ratio of the zeroth- to the first-order diffractions can be tuned from 10 : 1 to 1 : 1.      

Multipath Effects on Phase Measurements with Continuous Terahertz Waves

We evaluate the effect of multipath waves on terahertz phase measurements due to multiple reflections between the transmitter antenna and the sample. We show that the phase shift introduced by the sample will be biased by a value which depends on the sample position in the terahertz path. We show how to remove the bias and use the technique in the measurement of the index of refraction of Mylar at 194.4 GHz.     

基于相位重合点检测技术的测频方法的改进

在测频中利用检测到的相位重合点来触发测量闸门能够有效地消除传统方法中±1字的误差,大大提高了测量精度,但在实际线路中由于不能准确捕捉相位重合点,导致实际测量精度与理论精度有一定的差距,该文对此提出一些改进措施,使用闸门信号与被测信号和标频信号都同步,在一定程度上提高了测量精度.     

Art Painting Diagnostic Before Restoration with Terahertz and Millimeter Waves

Art painting diagnostic is commonly performed using electromagnetic waves at wavelengths from terahertz to X-ray. These former techniques are essential in conservation and art history research, but they could be also very useful for restoring artwork. While most studies use time domain imaging technique, in this study, a painting has been investigated using both time domain imaging (TDI) and frequency-modulated continuous wave (FMCW) system in the millimeter frequency range. By applying these systems to a painting of the eighteenth century, we detect and analyze the structure of some defects. This study underlines the differences between FMCW and TDI. We present the advantages and disadvantages of each technique on a real artwork.     

Electro-optically Induced and Manipulated Terahertz Waves from Fe-doped InGaAs Surfaces

We demonstrate the presence of dual simultaneous nonlinear mechanisms: field-induced optical rectification (FIOR) and field-induced surge current (FISC) for the generation of terahertz (THz) pulses from p-type and n-type Fe:In_0.53Ga_0.47As surfaces upon excitation with femtosecond laser pulses centered at 800 nm wavelength. Experimental investigations of the dependence of the generated THz waves on the incident angular optical polarization, optical irradiance, and the direction and magnitude of applied electric DC fields give confirming results to the proposed THz generation mechanisms. Applying external DC electric fields in the plane of the incident optical field shows efficient capability in manipulating the direction and phase of the generated THz waves, and controlling the refractive index of Fe:In_0.53Ga_0.47As material in the THz range, in addition to enhancing the emitted THz power up to two orders of magnitude. The fast and reliable response of Fe:In_0.53Ga_0.47As to the changes in the direction and magnitude of the optical and electrical fields suggests its use in amplitude and phase modulators, and ultrafast optoelectronic systems.     

Terahertz Microchip for Illicit Drug Detection

To accurately detect minute amounts of substances without disturbing the surroundings of target molecules has been a major goal in bioanalytical technology. Here by integrating an optoelectronic terahertz (THz) microsource into a glass-substrated microchip within the near-field distance, we demonstrate a compact, label-free, noninvasive, and sensitive microbiosensing system with low-power consumption. The demonstrated THz microchip allows us to locally specify various illicit drug powders with weights of nanograms, with a promising future for rapid identification of the static status or even the dynamics of various biomolecules