Remote tuning of NMR probe circuits

There are many circumstances in which the probe tuning adjustments cannot be located near the rf NMR coil. These may occur in high-temperature NMR, low-temperature NMR, and in the use of magnets with small diameter access bores. We address here circuitry for connecting a fixed-tuned probe circuit by a transmission line to a remotely located tuning network. In particular, the bandwidth over which the probe may be remotely tuned while keeping the losses in the transmission line acceptably low is considered. The results show that for all resonant circuit geometries (series, parallel, series-parallel), overcoupling of the line to the tuned circuit is key to obtaining a large tuning bandwidth. At equivalent extents of overcoupling, all resonant circuit geometries have nearly equal remote tuning bandwidths. Particularly for the case of low-loss transmission line, the tuning bandwidth can be many times the tuned circuit's bandwidth, f(o)/Q.     

High-speed nanoscale metal-semiconductor-metal photodetectors with terahertz bandwidth

A model for the maximum bandwidth achievable in metal-semiconductor-metal photodetectors is developed and simulated to determine the dimensions required for terahertz bandwidth. The bandwidth is found to exceed 1 THz for devices with line pitch less than 200?nm.     

Velocity magnitude estimation with linear arrays using Doppler bandwidth

The dependence of pulsed wave Doppler bandwidth on parameters typical of linear transducer arrays used in commercial Duplex and color flow mapping systems is investigated experimentally. For a single flow line it is observed that this bandwidth generally depends not only on the scatterer velocity and the beam-to-flow angle, but also on the flow line range and orientation. This is due to the fact that in Duplex and color flow systems the transducer is differently focused in the scan and elevation planes and its aperture and focal lengths are often made to vary, depending on the distance of the flow line from the transducer. It is however experimentally demonstrated that, at points where the ultrasound beamwidths in the scan and elevation planes are both comparable to the sample volume length, the Doppler bandwidth is independent of the beam-to-flow angle. It is also shown that this invariance can be extended to other ranges by appropriately modifying the array aperture. Finally, as an application of this independence, the flow-line velocity magnitude in these beam regions is estimated with better than 5% uncertainty through a simple bandwidth measurement.     

Fine frequency tuning of high power TEA CO2 lasers

A new technique is proposed to fine tune a TEA CO₂ laser within the bandwidth of a single ro-vibrational emission line. A tuning range of ±2 GHz off line center is obtained with an emission bandwidth of about 250 MHz and with a total energy output exceeding 1 joule at line center for the P20 (00°1–10°0) line. This technique can be used for single longitudinal mode operation and has the advantage of being directly scalable to higher power lasers.     

Design of a novel wideband single-mode waveguide in a photonic crystal slab structure

We propose a novel photonic crystal slab waveguide that provides a single-mode band with large bandwidth. The proposed waveguide is obtained by introducing a line defect in a triangular lattice of air holes in a dielectric slab. This line defect consists of holes which are not located in the original lattice points. The plane wave expansion (PWE) method is used to extract the band diagram of guiding modes and based on the results, photonic crystal holes in the defect row and its adjacent rows are modified to maximize the waveguide bandwidth. We show that using the proposed structure, a single-mode bandwidth of 17% can be achieved.     

柔软型高功率脉冲馈线带宽特性分析及实验验证

基于馈线结构,分析了影响馈线带宽特性的两个关键因素谐振和高次模。通过测量馈线的时域阻抗和脉冲频谱范围内的S参数,详细研究了它们的影响并进行了实验验证,测试结果表明:馈线周期性阻抗不均匀经傅里叶变换可分解成两个谐波分量,它们导致电压驻波比曲线出现谐振峰。馈线不连续结构激励出TE11和TE21高次模,它们在传输损耗曲线上引起损耗峰。馈线－3 dB带宽由一次谐波分量产生的谐振峰决定,但仍能匹配脉冲频谱的主瓣宽度。最后,根据测试结果,对馈线后续优化设计提出了相应的改进措施。     

开关振荡器特性阻抗选取对输出波形影响

开关振荡器是宽带高功率电磁脉冲的重要产生方式之一,以开关激励同轴振荡器为例,采用理论计算和电磁仿真为主要手段,从传输线特性阻抗与开关振荡器储能、传输线特性阻抗与天线阻抗关系及传输线特性阻抗和开关导通阻抗关系三个方面进行了研究。研究结果表明:传输线阻抗越小,振荡器储能越高;天线阻抗与传输线阻抗比值越大,输出信号品质因数越大,频谱上能量越集中,带宽越小,能量效率随比值增大呈现先增大后减小趋势;传输线特性阻抗很小时,开关阻抗对输出振荡信号影响增大,此时随特性阻抗减小,输出信号中心频率降低,品质因数减小,频谱上能量分散,带宽较宽。     

UWB monocycle pulse generation using two-photon absorption in a silicon waveguide

We propose and experimentally demonstrate ultrawideband monocycle pulse generation using nondegenerate two-photon absorption in a silicon waveguide. The free-carrier absorption induced pulse tail at the rising edge of inverted probe pulse is largely compensated by the overlapped pump pulse and results in a symmetric negative monocycle pulse. A 143 ps Gaussian monocycle pulse is successfully obtained with a 131.7% fractional 10 dB bandwidth using a 68 ps pulsed pump. The 10 dB bandwidth and center frequency of the RF spectrum for the generated monocycle pulse can be largely tuned using an optical delay line. An operational bandwidth of 30 nm is demonstrated experimentally with stable performance, and larger optical bandwidth is expected.     

Modified Tunneladder Slow-Wave Structures for High-Power Millimeter-Wave TWTs

In this paper, five modified Tunneladder slow-wave structures (SWS), the ridge-loaded stub-supported meander line with circular electron tunnel (CET-RSML), ring-plane frame line (RPFL), the ridge-loaded thick ladder line (RLTLL), the ridge-loaded pole-piece folded-waveguide circuit (RLPFWL), and the double-period Tunneladder line (DPTL) were discussed. The dispersion characteristics and the interaction impedances of the five modified Tunneladder SWS were calculated by simulation and the corresponding analytic models. The calculation results explain that the CET-RSML and the RLTLL hold wider bandwidth in comparison with the Tunneladder, when the volume dimensional lengths are same. When the bandwidth is definite, the interaction impedance of RLTLL can be higher than Tunneladder, but that of CET-RSML can’t. The dispersion curves of the RLPFWL and the DPTL are more linear in comparison with Tunneladder, but the interaction impedances are very low. Supported by the National Natural Science Foundation of China, under Grant No. 60532010.     

非线性传输线高功率实验北大核心CSCD

设计了基于交叉耦合铁氧体非线性传输线高功率射频微波产生系统,系统由脉冲形成线、非线性传输线以及高功率匹配负载(或组合振子辐射天线)组成。由100kV高压电源和高压微波电缆构成单传输线高功率脉冲形成线,形成线输出脉冲幅度35kV,脉冲半宽60ns。高压脉冲经过非线性传输线的脉冲压缩和调制,与高功率匹配负载相连时,实验得到了峰峰值31kV、中心频率308 MHz、3dB带宽为13%的射频振荡脉冲;与组合振子天线相连时,实验得到了中心频率380MHz、3dB带宽为12%的宽谱辐射。实验结果与数值模拟基本吻合。     

Quantum efficiency and signal bandwidth of thallium atomic line filters

We derive a new, simplified method for the computation of trapping effects in coupled three-level atomic systems. The linearity of the Holstein radiation trapping equation allows the reduction of the general rate equation to the solution of the basic two-level Holstein equation and to an algebraic eigenvalue problem. The method is applied to the numerical simulation of the quantum efficiency and the signal bandwidth of thallium atomic line filters. With a pumping scheme that achieves population inversion between the ground state and the metastable state, 90% quantum efficiency and 10 MHz signal bandwidth can be achieved, while in noninverting pumping schemes, even quite high pump intensities result in less than 60% quantum efficiency and 8 MHz signal bandwidth. A tradeoff between quantum efficiency, signal bandwidth and pump power is possible.     

Convergence properties of the accelerated Λ-iteration method for the solution of radiative transfer problems

We investigate the convergence properties and the computational speed of the accelerated Λ-iteration method for the solution of a wide variety of radiative transfer problems. The formal solution of the transfer equation is done using the short characteristics method. As the approximate Λ-operator we use bands of the discretized Λ-operator and derive the formulae for computing operators with arbitrary bandwidth. We define the optimum bandwidth of the approximate Λ-operator as the bandwidth for which the CPU time used for the solution of any particular radiative transfer problems is minimum. The radiative transfer equation is solved for a number of continuum and line transfer problems in spherical symmetry and for static to very rapidly expanding media. We find the optimum bandwidth is around 5–15 for workstation class computers whereas it is around 1–2 for supercomputers.     

An interconnect width and spacing optimization model considering scattering effect

As the feature size of the CMOS integrated circuit continues to shrink, the more and more serious scattering effect has a serious impact on interconnection performance, such as delay and bandwidth. Based on the impact of the scattering effect on latency and bandwidth, this paper first presents the quality-factor model which optimises latency and bandwidth effectively with the consideration of the scattering effect. Then we obtain the analytical model of line width and spacing with application of curve-fitting method. The proposed model has been verified and compared based on the nano-scale CMOS technology. This optimisation model algorithm is simple and can be applied to the interconnection system optimal design of nano-scale integrated circuits.     

Fast acquisition of reflectance spectra using wide spectral lines

We analyze a recently proposed technique for fast acquisition of the two-dimensional (2D) spatial distribution of reflectance spectra to figure out how much its ability of distinguishing almost similar spectra declines with increase of the spectral line bandwidth of the light source. This analysis was carried out using the experimentally measured reflectance spectra of four metameric samples and simulating the system response to an illumination by spectral lines with variable bandwidth. It was shown that the metameric samples are distinguishable even when the bandwidth of illuminating lines is 20–30 nm. A wider bandwidth allows implementation of simultaneous illumination of an object that leads to a diminution of the acquisition time of 2D-multispectral images due to both faster operation in the parallel mode of Light-emitting diodes (LEDs) switching and higher output power.     

Invariance of the Doppler bandwidth with flow displacement in the illuminating field.

It is known that if single frequency continuously transmitted ultrasound or electromagnetic energy is reflected from "straight line flow," defined here as one or more scatters moving with constant velocity along an infinite straight line, the Doppler effect will shift the echo spectrum center frequency from the transmitted value, and broaden its bandwidth. It is proved that if such straight line flow is shifted laterally or in range anywhere in the field, i.e., without change of orientation, its Doppler bandwidth remains unchanged. (The "Doppler bandwidth" is here defined as the frequency difference between the extrema of the echo power spectrum.) The theorem holds true even though the time domain echo changes dramatically with motion of the flow path, and is believed to be valid for electromagnetic as well as ultrasound waves. Its implications with respect to flow measurement, as well as preliminary experimental and computational confirmation, will be discussed.     

Ultranarrow bandwidth tunable atomic filter via quantum interference-induced polarization rotation in Rb vapor

We report the experimental demonstration of an ultranarrow bandwidth atomic filter by optically induced polarization rotation in multilevel electromagnetically induced transparency systems in hot Rb vapor. With a coupling intensity of 2.3 W/cm^2, the filter shows a peak transmission of 33.2% and a bandwidth of 10 MHz. By altering the coupling frequency, a broad tuning range of several Doppler linewidths of the D1 line transitions of STRb atoms can be obtained. The presented atomic filter has useful features of ultranarrow bandwidth, and the operating frequency can be tuned resonance with the atomic transition. Such narrowband tunable atomic filter can be used as an efficient noise rejection tool in classical and quantum optical applications.     

Improved narrow wavelength band blocking filters

A design approach is described to achieve spectral blocking filters of any spectral width and optical density for narrow blocking bands. We give new criterions to find the necessary number of layers from the desired bandwidth and optical density, and give new estimate equations which describe the number of layers required for designing a blocking filter of given bandwidth, high index, and optical density. This approach can be useful for laser line blocking, night vision filters, and many other general applications.     

Tunable delay line with interacting whispering-gallery-mode resonators

We theoretically study a parallel configuration of two interacting whispering-gallery-mode optical resonators and show a narrowband modal structure as a basis for a widely tunable delay line. For the optimum coupling configuration the system can possess an unusually narrow spectral feature with a much narrower bandwidth than the loaded bandwidth of each individual resonator. The effect has a direct analogy with the phenomenon of electromagnetically induced transparency in quantum systems for which the interference of spontaneous emission results in ultranarrow resonances.     

Open-loop electrooptic sampling for real-time analysis and near-field imaging of ultrafast electronic devices

In this paper, we present an open-loop electrooptic sampling system for real-time characterization and near-field scanning of ultrafast electronic devices. The system capabilities such as picosecond time resolution as well as 210 GHz of measurement bandwidth are verified with measurement of a CMOS nonlinear transmission line and an ultimate bandwidth of 230 GHz has been achieved with a post-process algorithm. The noise of the system is quantified and imaging over a broad range of frequencies for an on-chip antenna is demonstrated.     

Engineering of the metamaterial-based cut-band filter

Metamaterial-based cut-band filters are realized by combining either split-ring resonators (SRRs) close to a microstrip line or complementary split-ring resonators under this line. Indeed, their use reduces the size of conventional microwave filters. The study of this paper focuses on the influence of the association of different arrays of SRR especially on the bandwidth of the cut-band filter.