Broadband scattering by tapered nanoantennas

We realize a new design of tapered Yagi–Uda nanoantennas and experimentally study their far‐field light scattering. We show that in comparison with untapered structures the tapered nanoantennas exhibit broadband resonances featuring a distinct line shape. Our observations are supported by numerical calculations, which further reveal the tapered nanoantenna's wavelength‐dependent optical near‐field confinement. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Broadband transmission enhancement of acoustic waves through tapered metallic gratings

The phenomenon of the broadband transmission enhancement of acoustic waves is important for its prospective applications in various devices. To achieve broadband transmission enhancement of acoustic waves, we investigate the acoustic transmission in a metallic grating device with linearly tapered slits through eigen-mode matching theory (EMMT) and finite element (FE) simulation. The tapering provides a gradual filling fraction variation from the entrance to the exit of the slits, destroying the Fabry–Perot resonance and thus leading to broadband and wide-angle enhanced transmission in the audible regime. In addition, acoustic waves are strongly localized and enhanced at the slit exits, in contrast with straight slits.     

Broadband filter using cladding profile linearly tapered long-period waveguide grating

A method to increase the bandwidth of long-period waveguide gratings (LPWGs) is proposed using linearly tapered cladding profile and large coupling constant. Compared to conventional uniform LPWG, the simulated 20-dB bandwidth of the designed chirped LPWG is enlarged by 14 times, from 0.9 nm to 12.8 nm. Besides, a chirped LPWG with the new structure is also fabricated, and bandwidth broadening effect is observed.     

High-power pulsed-current-mode operation of an overdriven tapered amplifier

We experimentally investigate the performance of a commercial tapered amplifier diode operating in a pulsed-current mode with a peak current that is significantly higher than the specified maximum continuous current. For a tapered amplifier rated at 500 mW of continuous power, we demonstrate 2.6 W of peak optical output power with 15 mW of injection light for 200 micros, 7 A current pulses. Different failure mechanisms for the tapered amplifier, including thermal and optical damage, are identified under these conditions.     

High power single transverse mode operation of a tapered large-mode-area fiber laser

To get high output power with good beam quality, a tapered section is introduced to large-mode-area (LMA) Yb-doped fiber laser. Output characteristics of the fiber laser without tapered section and with tapered section are compared experimentally. When the launched pump power is 119.1 W, 77.9 W with M² 3.08 and 56.4 W with M² 1.14 can be obtained, respectively. The corresponding slope efficiencies are 71.8% and 54.1%, respectively. Although output power of the tapered fiber laser has 30.6% penalty, brightness of it is as much as 5.28 times of the fiber laser without tapered section. Moreover, spectra of them are measured. It is found that tapered section makes lasing wavelength of the fiber laser shorter.     

Theory and experiment of ultrahigh-gain gyrotron traveling waveamplifier

Physics and technology issues of importance to the high-gain gyrotron traveling wave amplifier (gyro-TWT) are investigated in theory and experiment. The gyro-TWT is known to be highly susceptible to spurious oscillations, especially in high gain operations. In the current study, oscillations of various origins are classified and characterized with detailed theoretical modeling. They are shown to be intricately connected to the interplay between the absolute/convective instabilities, circuit losses, and reflective feedback. Knowledge of these processes leads to the concept of an ultra high gain scheme which employs distributed wall losses for the suppression of spurious oscillations. A proof-of-principle Ka-band gyro-TWT experiment stable at zero drive has produced 93 kW saturated peak power at 26.5% efficiency and 70 dB gain, with a 3 dB saturated output power bandwidth of 3 GHz. The saturated gain is more than 30 dB beyond that previously achieved     

Broadband second harmonic generation in a tapered isotropic semiconductor slab using total internal reflection quasi phase matching

This paper analytically describes the concept of enhancing the bandwidth of second-harmonic generation in the mid-infrared region in an isotropic tapered semiconductor slab configuration. In this slab geometry, the lengths between successive reflection points due to total internal reflection of the incident laser radiation increase when it propagates through the slab. A computer aided simulation has been carried out to determine the possibility of generating broadband second harmonic intensity for broadband fundamental laser radiations as they are allowed to undergo total internal reflection inside the tapered isotropic semiconductor crystal slab made of either gallium arsenide (GaAs) or zinc selenide (ZnSe). The simulated results indicate wide 3 dB bandwidths of 187 nm and 196 nm in a 30 mm long tapered slab of GaAs and ZnSe respectively. The conversion efficiency, after considering the absorption and reflection losses, is quite satisfactory (≈ 1%). The effects of variations in temperature, incident angle of the fundamental laser radiations at the air-slab interface, length and tapering angle of the semiconductor slab have been studied for generated second harmonic radiations in both GaAs and ZnSe crystals. Optimising these parameters a wider broadband frequency converter with appreciable conversion efficiency can be designed.     

Ka波段低温低噪声放大器的设计

介绍了Ka波段低温低噪声放大器的设计和试验结果。在物理温度20K的环境下,在4GHz频率范围内,噪声温度小于27K,增益大于18.5d B。     

0.4 THz回旋行波管的设计与模拟

利用三维粒子模拟仿真软件,对共焦波导结构的0.4 THz回旋行波管进行了注波互作用的模拟研究。以共焦波导的冷腔色散以及衍射损耗入手,最终选定HE06模作为互作用工作模式。为了对寄生振荡进行抑制,在互作用结构中引入了截断。在模拟过程中,通过调节束压、束流、工作磁场、电子束横纵速度比,得到了最优的工作参数。最终,在束压34 kV、磁场14.25、束流2 A、横纵速度比0.75的工作参数下,当输入信号为1 W时,得到了最高2.76 kW的功率输出,增益超过34 dB,3 dB带宽为8 GHz,效率达到4%。     

Ku/Ka波段双通带频率选择表面设计研究

为了实现双通带频率选择表面(FSS)在较厚介质基底、 较大频带间隔和入射角度下的工程应用, 设计制备了一种性能优良的Ku/Ka波段双频FSS结构. 利用FSS栅瓣图分析了FSS具有稳定滤波特性的条件. 应用矢量模式匹配法计算了基于分形技术和复合图形技术的FSS的传输特性. 根据单元谐振模式和FSS传输特性归纳了厚介质基底、较远双通带FSS的设计原则, 最终优化出一种由方环复合“Y”环单元组成的FSS结构. 结果表明: 该结构在6.7 mm厚介质基底上0°-45°扫描范围内, 在Ku/Ka波段具有稳定的双频传输特性, 透过率均优于75%. 这为设计基底厚度较大、频带间隔较远、入射角度要求较高的双带FSS结构提供了理论参考与实验依据. 关键词： 频率选择表面 双通带 分形结构FSS 复合结构FSS     

Broadband square-pulse operation of a passively mode-locked fiber laser for fiber Bragg grating interrogation

A long-cavity, passively mode-locked erbium fiber laser operated in the square-pulse regime is demonstrated as a useful light source for interrogating fiber Bragg grating arrays. Output pulses with 4-W peak-power, 10-ns pulse widths, and bandwidths greater than 60nm were used successfully to interrogate 2% fiber Bragg gratings.     

宽带Ka波段平面反射阵列天线设计

平面反射阵列天线的发展受到了带宽与功率容量两方面限制。为此,本文首先基于多谐振技术提出了一种新型平面反射阵列单元结构,相比于传统单元,所提出单元结构具有功率容量高、剖面低且相移曲线线性度好的特点。其次利用所提出单元,通过优化阵面特性在Ka波段设计了包含20×20个单元的平面反射阵列天线。最后利用电磁仿真软件进行模拟计算,结果显示在中心频点35 GHz处,天线峰值增益为27.58 dBi,口径效率为52.33%,副瓣小于?16.08 dB,并且在30.41～39.64 GHz频率范围内（相对带宽26.37%）天线增益跌落小于3 dB,并且所设计平面反射阵列天线最大功率容量可以达到 13.99 MW,功率密度为218.54 W/mm2。     

Ka频段低温限幅放大器设计

通过对核心低温器件进行低温参数测试的方式进行选型和设计,制作了一款MMIC集成结构的Ka频段低温限幅放大器,测试结果表明,在物理环境温度20 K下,工作频带32 GHz~38 GHz范围内,其噪声温度T_e≤73 K,增益≥16.8 dB,限幅耐功率电平≥2 W,能够有效提升接收机系统灵敏度和抗烧毁能力。     

Ku/Ka波段双通带频率选择表面雷达罩设计研究

为了满足现代通信设备多频带及集成化要求,基于耦合机理和谐振机理,在实心半波壁雷达罩和A 夹层雷达罩等效平板基底上设计了一种由容性表面（内嵌谐振单元）-感性表面-容性表面（内嵌谐振单元）-等效基底级联而成的Ku/Ka波段双通带频率选择表面结构. 根据FSS的物理结构建立了等效电路模型,分析了滤波机理,利用全波分析软件计算了两种FSS雷达罩的传输特性. 该结构基于容性表面与感性表面的耦合作用在Ku波段形成具有微型化特性的第一通带,基于两个容性表面内嵌的方环单元谐振在Ka波段形成第二通带,两通带透过率分别为89%,94.7%（实心半波壁FSS雷达罩）、88.2%/93.7%（A夹层FSS雷达罩）,在0°–45°扫描范围内,两通带传输特性稳定. 在半波壁雷达罩的等效平板上制作了Ku/Ka双通带实验样件,利用自由空间法测试其传输特性,在制作误差允许的范围内,测试结果与仿真结果基本一致. 为研究通带间隔较宽的多频FSS 提供了理论和实验依据. 关键词： 雷达罩 频率选择表面(FSS) 双通带     

S波段锥形磁绝缘线振荡器长脉冲实验研究

 对一种由弱变和强变结构组成的锥形磁绝缘线振荡器进行了长脉冲实验研究。在二极管电压350 kV左右、电流约25 kA、脉宽约128 ns的条件下,获得了500 MW、脉宽约90 ns的高功率微波输出。对波形中出现的拐点进行了分析,研究表明：二极管电压波形好坏会对微波源的性能造成严重的影响,较好的电压波形是实现长脉冲运行的关键。同时对采用不锈钢平面负载、不锈钢丝网状负载以及石墨负载电子收集极进行了实验研究,研究表明采用石墨收集极可以输出较好的微波波形。     

S波段锥形磁绝缘线振荡器长脉冲实验研究

对一种由弱变和强变结构组成的锥形磁绝缘线振荡器进行了长脉冲实验研究。在二极管电压350 kV左右、电流约25 kA、脉宽约128 ns的条件下,获得了500 MW、脉宽约90 ns的高功率微波输出。对波形中出现的拐点进行了分析,研究表明：二极管电压波形好坏会对微波源的性能造成严重的影响,较好的电压波形是实现长脉冲运行的关键。同时对采用不锈钢平面负载、不锈钢丝网状负载以及石墨负载电子收集极进行了实验研究,研究表明采用石墨收集极可以输出较好的微波波形。     

Gaussian-like tapered grating quarter wave-shifted DFB semiconductor lasers for high-power single-mode operation

Received: 27 July 1998/Revised version: 25 September 1998     

Analytic theory of a tapered gyrotron resonator

An analytic theory has been derived for determining the eigenfrequencies, RF-field distribution and Q of the TE_mpq modes of a gyrotron resonator consisting of a circular cylinder joined to a slowly tapered section. Explicit results are obtained for a linear taper. The cavity modes are found to have an RF-field distribution which is useful for prebunching the electron beam and enhancing efficiency. For high Q cavities, the cavity Q depends on axial mode number q as q^–2, which is important for mode discrimination. Proper selection of taper length is found to reduce the Q of high q modes, also aiding in mode discrimination. The present approach may be applied to other forms of weakly irregular cavities, such as cavities with nonlinear tapers.Work supported by U.S.D.O.E. Contract DE-AC02-78ET-51013Supported by U.S. Department of Energy     

The development of a simplified cross-bar mixer operating at Ka-band

Many receivers operating at millimetre wavelengths utilise a frequency downconverter as their first stage. A rugged, low-conversion loss and simplified cross-bar mixer operating at the atmospheric window centered at 33 GHz was designed, developed and tested as a forerunner to mixers operating at atmospheric windows centred at higher frequencies. The cross-bar mixer exhibits a conversion loss of less than 5 dB over a bandwidth of 2 GHz. Design and optimisation procedures of the mixer are outlined.