Machine/drive circuit interactions in small variable-reluctancestepping and brushless DC motor systems

The authors investigate the torque, losses, and efficiency of small brushless DC and stepping-motor systems, and explain how performance is influenced by excitation mode, drive circuit, and machine parameters. Quantitative deductions are made for a three-phase reluctance-type machine based on computed steady-state results. For a given machine and drive circuit, maximum torque is obtained with an excitation mode that allows each phase to be turned on for half of the complete excitation cycle. However, if maximum efficiency is the aim, phase excitation must occur for a shorter period, e.g. by exciting only one phase at a time. When making comparisons between drive circuits, the most important measure of drive-circuit capability is the circuit power available at low speeds. Drive circuits producing rapid current decay at phase turn-off benefit torque production in the two-phase-on excitation mode, but are detrimental with the one-phase-on mode. At high speed the pull-out torque depends on the unsaturated inductance parameters of the machine. For maximum torque these parameters must be correctly proportioned, their values being dependent on the excitation mode     

Large-volume pulsed-RF excited waveguide CO2 lasers

Scalable pulsed-RF excitation of a sealed-off non-flowing CO₂ laser in a configuration compatible with high transverse and high longitudinal mode purity and compatible with very compact laser designs is described. The variation of maximum pulse-repetition frequency (PRF) with discharge transverse dimensions and the effects of electric field uniformity are described. Kinetics code calculations and laser design studies indicate that the results are scalable to the 10-J, 50-Hz level     

Nonuniform Layer Model of a Millimeter-Wave Phase Shifter

The electromagnetic wave propagation of millimeter waves in dielectric waveguides with thin surface plasma layers is characterized. The phase and attenuation of a 94-GHz wave are computed for various surface plasma layer thicknesses as a function of earner density levels. The electron/hole pairs generated in the vicinity of the dielectric waveguide surface by photo excitation are assumed to have an exponential profile due to either carrier diffusion or the exponential absorption of the optical field. Field computations made for a uniform plasma layer are compared with those of the nonuniform plasma to illustrate the effects of the exponential tails of the carrier profiles on both the phase and attenuation of the millimeter wave. The thin plasma layers slightly affect the field profile of the transverse electric modes (fields polarized parallel to the plasma layer). The transverse magnetic fields are highly distorted at plasma densities greater than 10/sup 16/ cm/sup -3/.     

Mode competition in free-electron masers with oversized planar Bragg resonators

The nonlinear dynamics of free-electron masers with planar Bragg resonators is studied. Here, in contrast to previous studies devoted to this class of self-oscillators, the transverse field structure is not fixed and consists of a set of modes of a regular planar waveguide. A criterion for the admissible oversize (the ratio of the gap between the plates forming the resonator to the wavelength) such that the stationary oscillation regime is stable under the variation of the electron beam parameters is obtained. It is demonstrated that, in the case of a large oversize, the synchronism mismatch variation due to the instability of the electron beam parameters should result in jumping of the oscillation frequency. It is found that different oscillation frequencies correspond to excitation of mode bunches with different transverse indices of partial waves. The region of synchronism mismatch corresponding to the excitation of the surface mode whose field localizes close to the corrugated surface is found. It is shown that, unlike regions of excitation of volume modes, this region is weakly sensitive to the gap between the plates.     

Excitation of luminescence in porous silicon with adsorbed ozone molecules

A new effect of the excitation of luminescence in porous silicon during adsorption of ozone from the gaseous phase was investigated. The signals of ozone-induced luminescence and photoluminescence decay with time of ozone exposure in a strictly correlated way; simultaneously, an oxide-phase growth is observed in porous silicon. A linear relationship was found between the luminescence intensity and the amount of oxide phase formed in the presence of ozone. Correlated shifts in the spectra of ozone-induced luminescence and photoluminescence are observed if the porosity of silicon varies. A mechanism for this effect is proposed. According to this mechanism, in the case of the dissociative adsorption of ozone, the exothermic reaction of oxidation of backbonds of a silicon atom takes place on the surface of nanocrystallites. Energy released is spent for the excitation of electron spectrum of silicon crystallites. The radiative relaxation in the case of ozone excitation proceeds similarly to that of the photon excitation of luminescence in porous silicon.     

压电式微固体模态陀螺谐振频率自动跟踪电路

压电式微固体模态陀螺振子通过交变电压激振、传感电极感应出电荷。当激励电压频率为某阶振动模态谐振频率时,感应电荷达到最大值。设计了谐振频率自动跟踪电路,使陀螺稳定工作在谐振模态。使用现场可编程门阵列(FPGA)控制直接数字频率合成器(DDS)产生频率精确可调的激励电压,驱动陀螺振子振动。检测谐振点对应的激励电压和感应信号间的相位差,作为反馈信号调节激励电压频率。实验结果表明,当相位差锁定区域处在98.48°~100.27°时,振子感应电极输出信号最大,振子处于谐振状态,实现了振子谐振频率的跟踪锁定。该系统可用于以谐振器为核心器件的振子工作模态锁定与跟踪。     

Scattering optimization method for the design of compact modeconverters for waveguides

A method for the design of compact and efficient mode converters for waveguides is presented. The required mode converter is modeled in the form of a scatterer placed in the waveguide and then its surface profile is optimized for maximum excitation of power in the required mode. Design examples are provided which show that such converters can achieve efficiencies of above 98%, while keeping the length of the converter less than that of the conventional mode converters     

Hybrid modes, substrate leakage, and losses of slotline atmillimeter-wave frequencies

Guided, hybrid propagation in a slotline at microwave and millimeter-wave frequencies is discussed. The analysis is carried out rigorously in the space domain, involving the variational solution of a singular integral equation for an E, field tangential to the slot. Data are obtained for field distributions, dispersion, and characteristic impedance, and are compared with data available in the literature. Results are reported on power lost by the propagation mode in the conductors and in the dielectric substrate. The analysis highlights the onset of leakage into a sufficiently thick substrate due to the excitation of a TM (transverse magnetic) surface wave. This sets a high-frequency limit for lossless operation     

横向场激励的BT-切石英传感器研究

研究了BT-切石英横向场激励(LFE)的耦合特性、声波相速度及其传感器在液体中的敏感性能.理论计算表明,当横向场方向与晶轴x方向垂直时,b模式的LFE耦合系数达到最大值6.68%,a和c模式的LFE耦合系数为0,此时激发的声波相速度也达到最大值5 079 m/s.实验发现,器件在液体中,具有较高的机械及电特性灵敏度.结果表明,在b-模式LFE耦合系数为0的方向上,器件实际上工作在伪横向场激励模式.     

Fundamental properties and optimization of broadside radiation from uniform leaky-wave antennas

In this paper, radiation at broadside is studied for a general class of leaky-wave antennas (LWAs) comprised of a grounded slab covered with a partially reflecting surface, on the basis of a simple transverse equivalent network model of the structure. The analysis of the one-dimensional (1-D) version of such a LWA excited by a line source shows that a central role in establishing the features of broadside radiation is played by the condition that the phase and attenuation constants of the leaky mode responsible for radiation are equal. When this happens, a beam with a single peak at broadside is on the verge of splitting into two distinct peaks, and maximum power density is radiated at broadside. Design formulas to achieve such an optimized condition, as well as approximate expressions for the frequency bandwidth and pattern beamwidth of the antenna and for the leaky-wave phase and attenuation constants are derived, both in the absence and in the presence of losses; in addition, an optimal-beamwidth condition (which gives the narrowest broadside beam) is derived. Finally, all the results are extended to the practical case of a 2-D LWA excited by a horizontal dipole.     

Inelastic light scattering from electronic excitations in deep-etched quantum dots and wires

Resonant Raman spectroscopy of modulation-doped GaAs/AlGaAs multiple quantum well dots and wires is reported. Deep etching with a SiCl₄ reactive ion etching process achieved an excellent aspect ratio (>10:1) and low surface damage for dots and wires of sizes in the range 60–250 nm. A rich spectrum of single particle excitations was observed at Raman shifts in the range 1–35 meV for both dots and wires. Sharp resonances were found for the Raman intensities. The electronic scattering in wires exhibits distinct polarization properties in agreement with theoretical predictions and the spin density excitation energies are in reasonable agreement with Hartree approximation calculations. The dispersion of the intrasubband plasmon collective mode in 60 nm wires has been determined. The excitations in dots show a systematic shift to higher energy with decreasing dot diameter consistent with increased confinement. Magneto-Raman scattering from dot samples was also investigated at magnetic fields up to 12 T and the excitation spectra show level splitting, level crossing and mode softening with increasing magnetic field.     

Single high-order transverse mode surface-emitting laser with controlled far-field pattern

Single transverse mode vertical-cavity surface-emitting lasers (VCSELs) with high output power are important for high-speed data links, optical recording, laser processing, and so on. We demonstrate a single high-order transverse mode VCSEL with narrow trenches formed on its top surface by using a focused ion beam. The formation of four straight cross trenches resulted in the selection of a stable LP/sub 41/ mode oscillation. A large far-field angle of the LP/sub 41/ mode, which causes a low coupling efficiency even with a multimode fiber, was drastically reduced by loading a phase shift layer. A single-lobe far-field pattern with low scattering loss was realized by loading a spatial phase shift of SiO/sub 2/. We evaluated a coupling efficiency with a single-mode fiber including its alignment tolerance. The controlled far-field pattern enables lens-free direct coupling with a single-mode fiber even for large active-area VCSELs.     

Transverse mode controlled in inGaAsP inP lasers at 1.5 µm range with buffer-layer loaded piano-convex waveguide (BL-PCW) structures

A modified plano-convex waveguide structure is studied analytically and successfully applied to InGaAsP/InP lasers in the1.5- 1.6 mum wavelength region to realize stable transverse mode operation. The structure of these lasers is characterized by a standard buffer layer between an active layer and an upper cladding layer and a waveguide layer of varying thickness between the active layer and the substrate. A theoretical analysis of this structure showed that, for a given channel depth, increases in buffer-layer thickness give rise to larger maximum channel widths of the substrate for fundamental transverse mode operation. It was also shown that the optical confinement factor in the active layer decreases little as the buffer-layer thickness is increased from 0.1 to 0.3 μm. Buffer-layer loaded plano-convex waveguide lasers in the 1.5 μm range were prepared by liquid phase epitaxy and fundamental transverse mode operation up untilI = 2I_{th}was obtained. The dc threshold current was 100-300 mA and the differential quantum efficiency per facet was 10-15 percent. Continuous CW operation for over 2000 h at 25°C has been achieved.     

Three phase excitation of a hollow cathode laser

A new hollow cathode discharge scheme utilizing three phase electrical power at 60 Hz to generate excitation in a sputtered He-Cu laser has been demonstrated. Our results indicate that rectified laser output can be obtained from an unrectified ac power supply using overlapping excitation regions, each excited with a different electric phase. Two preliminary hollow cathode designs are presented, one a tandem and one a transverse configuration. We achieved a rectified or quasi-dc output power of 350 mW. The use of overlapping ac excitation is seen to increase the operating life of a sputtered He-Cu laser and inhibit arc formation in the discharge.     

Selection and modulation of high-order transverse modes invertical-cavity surface-emitting lasers

A theoretical proposal to double the pulse repetition frequency by alternating current modulation of two orthogonal high-order transverse modes of a vertical-cavity surface-emitting laser (VCSEL) is presented. Static and dynamic characteristics of weakly index-guided VCSELs in a multitransverse mode regime are analyzed. An efficient model that takes into account all the transverse modes supported by the waveguide is developed. Mode partition noise in a current modulated VCSEL is studied, taking into account the azimuthal degree of freedom. Different transverse modes can be excited with a probability that is numerically calculated. For high injection currents, modes that are not favored in the steady state can be excited with higher probability. The excitation probability is similar for azimuthally orthogonal modes. This symmetry can be broken by selecting a particular high-order transverse mode by using azimuthal-dependent current profiling. This selection can be achieved over current ranges as wide as ten times the threshold current. Current modulation of this transverse mode is then analyzed. Alternate current modulation of two orthogonal high-order transverse modes is also studied. Alternating modulation of these modes can double the pulse repetition frequency obtained by modulating just one high-order transverse mode, without increasing injected current density levels. This current-induced spatial switching leads to high-frequency beam steering in the laser azimuthal direction     

Polarization selection in birefringent vertical-cavity surfaceemitting lasers

An analysis has been undertaken of the effect of refractive index anisotropies or birefringence on the selection of polarization states in vertical-cavity surface emitting lasers. Attention is focused on the competition between the two orthogonal polarization states of the fundamental transverse mode. It is shown that refractive index differences of 0.005% are adequate for ensuring preferential excitation of a specific polarization state. It is found that at low injection currents both polarizations coexist with the same optical power. The dominant polarization state at higher injection currents is found to depend upon the geometry of the current injection in the active region     

太赫兹二级分布反馈量子级联激光器中的模式竞争与功率特性

结合实验和理论计算研究了太赫兹二级分布反馈量子级联激光器中的模式竞争和功率特性.研究表明,激光器在整个动力学范围内均稳定地工作在横向及纵向的基模.横向基模的产生原因是脊条两侧的吸收边界有效提高了高阶横模的损耗.纵向基模的产生原因主要是谐振腔内基模与高阶纵模的电磁场分布交叠较大,并且频率接近,从而有效避免了增益在空间和频域的烧孔效应.此外,激光器的辐射效率随分布反馈光栅长度的增加而减小,导致只有在特定的光栅长度才能获得最大的输出功率.该工作有助于高性能单模太赫兹激光器及锁相激光器阵列的研制.     

激光脉冲驱动下双腔耦合系统的动力学特性

在外加激光控制下,研究了含有两能级量子比特的双腔耦合系统的动力学行为.基于系统的相互作用哈密顿量,得出了系统量子态激发概率的解析解.通过数值模拟讨论了3种特定情况下系统的动力学特性:外加脉冲与腔场发生近共振、比特与腔模大失谐条件下,腔模的最大激发概率小于0.07,腔模处于非激发状态,比特的激发概率出现瞬时衰减;比特与腔模强耦合条件下,腔模的激发概率近似0.5,腔模处于半激发状态,比特的激发概率出现高频振荡;腔场之间大跳跃条件下,腔模的最大激发概率小于0.07,腔模处于非激发状态,比特的激发概率出现高频振荡.     

Picometer Resolution Wavelength Tracking in the C -Band Using an InP–InGaAsP Dual-Slab Interferometer

A simple III-V semiconductor dual-slab waveguide interferometer is demonstrated which can detect picometer level changes in the C-band laser wavelength. Optical path length imbalance between the two single-slab waveguide modes provides the primary mechanism for detection. The dual-mode output field phase change differences due to input wavelength changes are encoded in shifts in the device output far-field interference pattern. The device detects path length dependent wavelength changes with a phase change difference sensitivity of around 660 mrad/nmldrmm at 1.55 mum and a phase detection floor of plusmn1 mrad. The transverse electric and transverse magnetic polarization responses are approximately equivalent.     

Ferrite Microstrip Phase Shifters with Transverse and Longitudinal Magnetization (Correspondence)

Phase shifts of opposing sign are produced in a finear section of microstrip by transverse and longitudinal magnetization of the ferrite substrate. Nonreciprocal phase shift is also produced by the transverse magnetization. Theoretical calculations of phase shift that account for both the diamagnetic effects and the tensor properties of the ferrite permeability agree well with properly constructed experimental measurements. These measurements use closed magnetic circuits to remove the nonuniform demagnetization effects. A lightweight reciprocal phase shifter has been constructed that utilizes both transverse and longitudinal magnetization at low drive power with closed magnetic circuits to obtain a high figure of merit.