Intermodulation in multiple fixed beam distributed amplifier phased array antenna systems

In the recent development of phased array/distributed amplifier systems for application to mobile satellite or thin route communication systems, the intermodulation (IM) generated by the element amplifiers is spatially dispersed in such a way that some of the IM radiated energy falls outside the intended beam area. In addition, some of the IM falling inside the beam area has frequencies different from the carriers intended for that area. It is known that for fixed beam systems with frequency reuse, an average IM noise reduction of several dB can be realized. In the specific case under investigation, of four beams, nine frequencies and 12 carriers (1-33 frequency reuse factor), an IM improvement of about 2-0 dB is realized. The improvement in carrier-to-IM-noise ratio can be applied to increase system capacity, or for given capacity it can be used to reduce RF power amplifier back-off, which can be translated into a reduction of spacecraft prime power requirement. The considerable advantage gained from the multiple beam distributed amplifier approach makes it worthwhile to explore the workings of such a system and other benefits it may offer. One of the benefits is a common RF power pool for all carriers and beams, and the other is the spatial dispersion of intermodulation (IM). A satellite multiple beam pattern and the spatial dispersion of IM products are shown in Figure 1.     

A power-sharing multiple-beam mobile satellite in Ka band

This paper presents a new power-sharing multiple-beam mobile satellite system concept in the Ka band. A wide allocated bandwidth and a large amount of frequency reuse based on hundreds to thousands of small spot beams will allow us to draw a drastically new mobile satellite systems concept in the Ka band. At first, requirements for beam size on the surface of the Earth for various signal transmissions are considered. Based on these requirements, Ka band geostationary systems with 3.5 and 10 m satellite antennas are shown. If the number of beams is hundreds to thousands, it is not appropriate to assume a fixed power transmitter for each beam because the traffic in each beam is not uniform or static. In order to cope with this multiple-beam-varying traffic problem, this paper proposes a new type offset reflector antenna fed through an equal phase-shift active array. The proposed active array consists of hundreds to thousands of equal phase-shift elements. Features and simulated performances of the proposed transmitting antenna are presented. Preliminary experimental results from a 2.1 m reflector fed through 332 hollow elements are also shown. Since each beam commonly utilizes all active array elements, power sharing among beams is possible, allowing traffic variation among beams without loss of power efficiency     

Propagation and excitation of perturbations in electron beams with velocity shear

A theoretical investigation of the influence of a spatially varying drift velocity on the perturbations in magnetically focused electron beams is presented. The wave propagation on such beams, focused by infinite magnetic fields, is studied under the small-wave assumption. The dispersion relation for slow waves is derived and solved for different transversal boundary conditions. Two sets of infinitely many propagating modes are found in a beam with small velocity shear. In a beam with sufficiently large velocity shear only two eigenfunctions exist. Since it is not possible to match the two eigenfunctions on arbitrary longitudinal boundary conditions, additional solutions (which cannot be written in form of a plane wave) must exist. The excitation of perturbations by ideal grids is solved by introducing the Laplace transform analysis. Additional solutions are ascertained which lead to a spatial decay of the perturbations according to a power law z^-α. This damping, arising from the spatially varying drift velocity, is similar to the Landau damping in electron beams with velocity distribution. Such damping effects are of great practical importance in conjunction with noise reduction in traveling-wave tubes.     

Novel down link rain fade mitigation technique for Ka‐band multibeam systems

Rain fades at Ka‐Band degrades the link quality and performance significantly. Several rain fade mitigation techniques for Ka‐band satellite systems are being investigated to improve the channel capacity. Methods such as power control and adaptive waveform techniques have been proposed for use in the uplink as they are capable of straightforward implementation. A novel down link power control technique for multi‐beam Ka‐band system has been proposed in this paper. It is based on the use of multi‐port amplifier, which is commonly used for dynamic power sharing of the beams depending upon the traffic. Payload architecture for multi‐beam coverage using multi‐port amplifiers has been designed for the proposed technique. The simulation results to compensate for the rain fade attenuation of one beam by sharing the unused power from other beams have been presented Copyright © 2006 John Wiley & Sons, Ltd.     

阵列合成激光束在湍流大气中传输的研究进展

多路激光束合成是获得高功率和高能量激光输出的有效途径,因此,多年来光束合成方法和技术得到了广泛的关注和深入研究。近年来,随着空间光通信和激光传播工程等应用需求的发展,合成光束在湍流大气中的传播过程及其规律研究也逐渐成为光传播研究领域的热点问题。针对多种典型激光阵列合成光束,首先对比分析了广义Huygens-Fresnel原理、Rytov微扰近似以及随机相位屏数值模拟等方法在激光阵列合成光束大气传输研究中的应用;其次,阐述了合成光束传播应用研究中的桶中功率、衍射极限倍数 、 因子、光束传输因子等光束质量评价因子的评价作用;最后,根据阵列合成激光束的研究现状和成果,提出了应进一步深入研究的问题。     

Gaussian beam mode analysis of the coupling of power between horn antennas

Gaussian beam mode analysis can be applied in an elegant way to study the coupling of power between two horn antennas. Coupling efficiencies are evaluated for a number of horn combinations and the results presented in a straightforward form useful in the design of submillimetre-wave interferometers and astronomical receiver systems. We show that there is a marked variation in efficiency (up to 30%) depending on the coupling optics, even for beams with well matched profiles.     

类 ? 型同频率高功率微波功率合成器的设计

利用基于波导结构的功率合成器来合成高功率微波,是提高窄带高功率微波源输出能力的一个有效方式。本文 设计了一种特殊的类? 型高功率微波合成器,该合成器可以用来合成X 波段同一个频率下的两束高功率微波。文中给出 了该合成器的设计方法及仿真结果,并且将该合成器结构与新型同轴双电子束高功率微波源[1]结合进行了粒子模拟,结果 表明,当加载的二极管电压为674kV,导引磁场为0.8T,内电子束电流为6.6kA,外电子束电流为14.3kA 时,该同轴双 电子束高功率微波源输出的两路微波经功率合成器合成以后输出了频率为9.74GHz,功率高达3.5GW 的微波。     

Mutual Injection-Locking and Coherent Combining of Two Individual Fiber Lasers

In this paper, mutual injection-locking and coherent combining are demonstrated with two individual erbium-doped fiber lasers that were coupled by two fiber splitters. Mutual injection-locking theory of two lasers is analyzed. In the free-running state, the far-field beam profile is a simple intensity superposition, as expected of two incoherent beams. Under mutual injection-locking, interference fringes with high contrast ratio are obtained, and the two fiber lasers lase at the same wavelength with a stable output power. We have found that the two fiber lasers are always in out-of-phase mode, which is consistent with theoretical analysis. Coherent beam combining by mutual injection-locking is realized without the need for length or amplitude control. This method can be easily scaled to combine more beams.     

The traveling-wave phototube part II: Experimental analysis

This paper reports a detailed experimental investigation of the traveling-wave phototube (TWP) as a broad-band demodulator of microwave-modulated light. As Part II of a theoretical and experimental analysis of the interaction between a density-modulated photoelectron beam and a slow-wave circuit, it examines the output power, bandwidth and voltage modewidth of the traveling-wave phototube as a function of photocurrent, beam voltage and interaction length. Three types of experiments are described: 1) direct demodulation of light, amplitude-modulated at 3 Gc by a cavity-type modulator, 2) measurement of the microwave shot noise from photoelectron beams initiated by light from both coherent and incoherent sources and 3) comparison of the previous two measurements with the microwave shot noise of a thermionic electron beam. Comparisons of the results of these experiments with the predictions of the theory of Part I are drawn, showing good agreement. In particular, a number of new effects which are predicted by the detailed analysis of Part I, and which had not been previously observed or predicted are reported.     

用于冷原子干涉仪的拉曼激光的注入锁定

原子干涉仪通常采取拉曼脉冲序列对原子波包进行相干操作,高功率的拉曼光可让更多的原子参与速度敏感型受激拉曼跃迁,有利于实现信噪比高的原子干涉仪条纹。研制高功率的拉曼激光器对冷原子干涉仪的实验研究具有重要意义,文章报道了基于注入锁定技术实现的高功率拉曼激光器的实验结果。主激光器经过1.5 GHz声光调制器产生1级衍射光作为拉曼光的种子光,注入到两个从激光器后锁定,并实现功率放大。两个拉曼光的频率相差3.0 GHz,频率调谐范围为200 MHz。     

双波长高功率半导体激光器波长耦合技术

介绍了非相干耦合技术中波长耦合原理及关键技术,根据波长需要设计耦合器件,自行设计了光学系统对光束进行扩束聚焦,通过实验将808nm和980nm两半导体激光迭阵光束通过此技术进行合束,最终实现更高功率输出,耦合效率为80%,光斑大小为2mm×2mm,可满足将半导体激光器直接应用于熔覆、焊接等场合。     

球差透镜对超高斯光束光束质量影响的研究

根据束宽的传输方程,采用双曲线拟合的方法,计算了超高斯光束通过球差透镜后的M2因子。同时还研究有球差的超高斯光束在透镜几何焦面和实际焦面上的桶中功率(PIB)。以M2因子和PIB作为光束质量评价的参数,进行了详细的数值计算来说明球差对超高斯光束光束质量的影响,并与有球差的高斯光束做了比较。得出了一些有实际应用意义的结果。     

Characterization of an optoelectronic terahertz beam system

The performance of an optoelectronic terahertz (THz) beam system is described. The transmitter operation is based on the repetitive, subpicosecond laser excitation of a Hertzian dipole antenna embedded in a charged coplanar line. With this transmitter electromagnetic beams of 1/2 cycle THz pulses at a repetition rate of 100 MHz are produced. The associated optoelectronic receiver is gated in synchronism with the excitation of the transmitter by subpicosecond pulses from the same laser source. With this receiver, the 10-nW beams of THz pulses were observed with a signal-to-noise ratio greater than 10000:1. Several sources contributing to the noise of the receiver are discussed, together with ways to reduce them. With an integration time of 125 ms, a signal-to-noise ratio of 1 is obtained for a THz beam with an average power of 10^-16 W. The receiver operates in the sampling mode and has a time resolution of 0.5 ps     

强激光的计算模拟:平顶高斯光束模型

基于广义惠更斯－菲涅耳衍射积分,将平顶高斯光束作为一个整体光束,对其基本性质和变换特性作了统一研究。推导出了平顶高斯光束通过有线性增益（损耗）的近轴ABCD光学系统、无光阑和有光阑ABCD光学系统后解析形式的传输方程。对平顶高斯光束的相似变换以及与超高斯光束的一致性也作了分析。所得结果对强激光的模拟提供了一个有用的计算模型。     

指数响应的强非局域介质中正交偏振双厄米高斯光束传输特性

运用变分法研究了具有e指数响应的强非局域介质中两束正交偏振、中心重合的厄米高斯光束的传输特性,得到了光束各参量的演化方程和一个临界功率Pc。当两光束以临界功率入射时,可以形成厄米高斯型空间光孤子;当两光束以总功率2Pc入射,但两光束入射功率不相等时,两光束可以形成厄米高斯型呼吸子。通过数值模拟研究发现,低阶光束时,可以得到近似孤子解;当光束阶数大于3阶时,则得不到孤子解。比较变分解与数值解,结果表明在光束阶数小于3阶时,变分解较好的反映了具有e指数响应的强非局域介质中两束正交偏振、中心重合的厄米高斯光束的传输特性。     

Beamforming for Downlink Multiuser MIMO Time‐Varying Channels Based on Generalized Eigenvector Perturbation

A beam design method based on signal‐to‐leakage‐plus‐noise ratio (SLNR) has been recently proposed as an effective scheme for multiuser multiple‐input multiple‐output downlink channels. It is shown that its solution, which maximizes the SLNR at a transmitter, can be simply obtained by the generalized eigenvectors corresponding to the dominant generalized eigenvalues of a pair of covariance matrices of a desired signal and interference leakage plus noise. Under time‐varying channels, however, generalized eigendecomposition is required at each time step to design the optimal beam, and its level of complexity is too high to implement in practical systems. To overcome this problem, a predictive beam design method updating the beams according to channel variation is proposed. To this end, the perturbed generalized eigenvectors, which can be obtained by a perturbation theory without any iteration, are used. The performance of the method in terms of SLNR is analyzed and verified using numerical results.     

A review of Brillouin-enhanced four-wave mixing

When a Brillouin-shifted signal interacts with a two strong counterpropagating pump beams in a Brillouin active medium, a conjugate field will grow exponentially in time until it depletes the power of the pump beams and hence produce very high reflectivities (>10⁶). The technique can be used to conjugate very weak signals (<10^-13 J), or alternatively a large fraction of the pump power can be transferred into the conjugate beam. A review of the technique is presented and aspects such as the exponential growth rate, the minimum signal requirement, the beam quality, and the threshold for parasitic oscillation between the four-wave mixing cell and a reflecting surface are discussed     

The stimulated Raman scattering threshold for anondiffraction-limited pump beam

The threshold power of stimulated Raman scattering (SRS) has been calculated for various nondiffraction-limited pump beams using a numerical model. These calculations were performed for the following pump beam profiles: Gaussian-Hermite, Gaussian-Laguerre, Gaussian-Schell-model (GSM), and several superpositions of Gaussian-Hermite modes. It is shown that the threshold power, for a tightly focused beam, depends on the M² factor, which is a dimensionless factor that compares the divergence of a given beam with the divergence of a (diffraction-limited) Gaussian beam, related to the pump beam quality. The influence of the exact pump beam profile on the threshold is small. It is shown that the measured SRS thresholds in methane for two pump beams, one with an M² of 1.12 and the other with M²=8.7, correspond with the calculated thresholds for two GSM beams with the same M²     

基于凹面光栅的光纤激光谱组束系统设计

光纤激光谱组束技术是实现大功率、高质量光纤激光输出的有效手段。在传统的外腔谱组束系统中,离轴阵元的效率下降限制了其组束潜力;在基于PTR布拉格光栅的谱组束系统中,随着组束阵元数目的增加,系统复杂程度、体积和重量随之增大,限制了其应用范围。针对当前谱组束系统中存在的诸多问题,采用凹面光栅作为色散元件构建新的谱组束系统。凹面光栅同时具有衍射分光和聚焦的功能,应用于谱组束系统中不仅可以解决离轴阵元效率下降问题,而且降低系统的复杂程度,有利于输出稳定性的提高。     

光束的几何排布对多重径向阵列光束传输特性的影响

研究了阵列几何排布对多重径向阵列相干和非相干合成光束桶中功率（PIB）的影响。在较小的桶内阵列相干合成光束的PIB值随P增大而增大;非相干合成光束的PIB值与几何排布无关。