具有高斯横向分布的环形染料腔中带模型下的光学双稳性

利用带模型理论研究了环形腔染料激光器中的光学双稳性，使用了真实的高斯横模腔场分布．并将所得到的结果分别与使用平面波近似下的带模型结果以及单横模近似下的二能级模型结果进行了比较．     

Galerkin solution for the thin circular iris in aTE11-mode circular waveguide

An integral equation for the transverse electric (TE) field in the aperture of a concentric circular iris in a transverse plane of a circular waveguide is approximately solved using Galerkin's method. The aperture field is represented by a finite sum of normal TE and TM (transverse magnetic) circular waveguide modes that fit the circular aperture. The numerical convergence of the Galerkin solution is demonstrated using the resultant aperture field distributions and equivalent shunt susceptance for the case of dominant TE₁₁-mode excitation. The resultant aperture electric field distribution closely resembles that of the TE₁₁ aperture mode alone, except for edge contribution behavior at the edge of the iris. A resonant or capacitive iris is possible over a restricted range of frequencies     

Measurement of the electromagnetic field intensity in the focal region of a wide-angle spherical reflector

In this communication an experimental study has been made of the intensity distribution in the focal region of a wide-angle spherical reflector illuminated by a linearly polarized plane wave of wavelength 12.5 mm. Intensity distributions with respect to the transverse field components were measured along the axis and in several transverse planes. The corresponding distributions for the longitudinal component were measured along directions parallel to the axis and along the same transverse directions. The peak of the longitudinal component was found to occur at a distance of about three wavelengths towards the reflector from the position of the peak of the transverse component. Experimental results have been found to compare well with the available theoretical calculations.     

Mode selection in GaAs injection lasers resulting from fresnel reflection

In several recent papers, attention has been given to the Fresnel reflectivity associated with cleaved facets of the laser. The small dimensions of the heterostructure waveguide give rise to a considerable angular spread in the energy incident on the facet. As a result, the mode reflectivity is not given simply by the Fresnel equation, which is only valid for an infinite plane wave. Rather, it is a properly weighted average over the plane-wave distribution of the mode. The differences in mode reflectivity with respect to TE and TM modes, as well as the variations with mode number, have been offered as a possible explanation for the predominant appearance of TE modes and preference for higher order modes in the large optical-cavity (LOC) geometry. However, the considerations to date have ignored the finite extent of the field in the junction plane. This situation is rectified in this paper. It is shown that the splitting in the mode reflectivity values between TE (electric field in the junction plane) and TM (electric field perpendicular to the junction plane) modes is reduced, and under certain conditions TM modes can be favored. In particular, it is shown that if a TE mode is oscillating, then there is a preference for a lowest order mode in the plane of the junction and a highest order mode in the transverse plane. Conversely, if for some reason a TM mode is oscillating, then the preference is for highest order modes in the plane of the junction and lowest order modes in the transverse plane.     

MRI平面梯度磁场模型的研究

建立了MRI(磁共振成像)平面梯度磁场的纵向和横向梯度模型,并对其进行分析计算,得到均匀的MRI平面梯度磁场.重点分析了由两组不同线圈匝数的"麦克斯韦对"绕组组成的最高到7阶的纵向梯度模型和相距2z0的12个串联供电K匝线圈的矩形绕组组成的横向梯度模型,在此基础上对模型的基本参数进一步优化,得到了更大线性偏差＜1 %的高线性度梯度场.分析表明:模型中的3、5、7阶对磁场梯度的线性均有影响,弱化高阶奇数导数对磁感应强度的影响可以得到更加均匀的线性梯度场.结果表明:应用相同的优化方法,纵向梯度模型的优化效果优于横向梯度模型.     

Electromagnetic Wave Propagation in Lossy Ferrites

The square of the complex transverse propagation constant in a lossy, magnetized ferrite is found to be described approximately by a circle in the complex plane when the magnetic field is varied. A graphical method for obtaining approximate values for the transverse propagation constant when the wave number in the direction of the applied field is given and real is derived here. This method is used to find The power absorbed from an incident plane wave by A semi-infinite ferrite as a function of the magnetizing field amplitude.     

Reflection from a periodically perforated plane using a subsectional current approximation

The scattering from a zero thickness plane having finite sheet resistance and perforated periodically with apertures is calculated for arbitrary plane wave illumination. The surface current density within the unit cell is approximated by a finite number of current elements having rooftop spatial dependence. The transverse electric field is expressed in terms of the current, and the electric field boundary condition is satisfied in an integral sense over the conductor, generating a finite dimension matrix equation whose solution is the current density. Since the conductor shape is defined through the locations of subsectional current elements, arbitrary shaped apertures can be handled. The reflection coefficient and current distribution are calculated for square apertures in both perfectly conducting and resistive sheets, and for cross-shaped apertures. Finite resistivity is shown to cause the magnitude of the transverse magnetic (TM) reflection coefficient to decrease more rapidly and its phase to decrease less rapidly, as the angle of incidence approaches glancing. Through detailed plots of the current density, the current crowding around the apertures is made clearly evident.     

Propagation properties of a cylindrically polarized vector beam

A general expression for the electric field of a cylindrically polarized vector beam propagating in free space is derived on the basis of the exact fully vectorial solution of Maxwell equations in transverse Fourier space,which indicates that a cylindrical polarization can be regarded as the combination of radial and azimuthal polarizations, and the electric field retains cylindrical symmetry under the propagation. The simulation results denote that the longitudinal electric field depends on the ratio of the waist width to wavelength and the angle between the electrical vector and the radial direction; in particular, when this angle is 24.5°, a flattop intensity distribution is obtained at the plane z=0.     

Scattering from periodic array of grounded parallel strips, TEincidence

The field distribution between strips in a periodic array of parallel strips was modeled. The electric field integral equation (EFIE) together with method of moments (MoM) was employed to solve the problem. The incident plane wave was assumed to be transverse electric (TE). The effect of the incident angle on the field between strips was investigated. For comparison, an array of the same configuration but containing a finite number of parallel strips was also modeled using the EFIE and solved via MoM. Good agreement was found even when the number of strips was small. The results are of interest for mobile radio     

Certain features of the effect of diffraction on the acoustooptic interaction in an elastically anisotropic medium

With the use of the distribution of the ultrasonic-beam intensity calculated with allowance for elastic anisotropy, it is shown that the transverse divergence of acoustic waves affects the diffraction efficiency of an acoustooptic cell. The calculations are in agreement with measurements of the acoustic-field distribution in the acoustic lines of crystalline- and fused-quartz acoustooptic devices. The distribution is measured in the plane perpendicular to the diffraction plane.     

Design of wavelength-switching erbium-doped fiber lasers with a multimode fiber Bragg grating using spatial-mode excitation and selection techniques

A multimode fiber Bragg grating (MMFBG)-based wavelength-switching erbium-doped fiber laser and its application in displacement sensing is proposed and demonstrated. According to the spatial-mode distribution in the multimode fiber (MMF), the modes reflected by the MMFBG at different wavelengths build a wavelength-dependent mode field on the transverse plane of the MMF. By varying spatial-launching position of the single-mode fiber against the MMFBG to achieve different mode group excitation and selection, single-wavelength lasing at 31 discrete wavelengths has been realized over a wide wavelength range.     

Iterative Solutions of Waveguide Discontinuity Problems

The method of overlapping regions, together with Schwarz's technique, is applied to waveguide discontinuity problems to illustrate its potential and basic advantages and disadvantages over other methods. The method reasonably corrects an arbitrary initial assumption of field distribution in the plane of discontinuity to the final value in a small number of iterations. The advantages are illustrated for a waveguide bend and dumbbell shaped waveguide as examples of transverse and longitudinal discontinuities, respectively. Numerical results for the case where only the electric field is parallel to the sharp edge discontinuity are presented and compared with available data, while extension to the case where only the magnetic field is parallel to the edge is discussed.     

Resonant modes of GaAs junction lasers

The resonant modes of GaAs junction lasers are obtained from a proposed model and compared with experimental results. Theoretical results are based on an assumed laser medium whose dielectric constant varies both along and perpendicular to the junction plane. The frequency separations of the transverse modes are found to be in very good agreement with presented high-resolution spectral measurements of stripe-geometry laser radiation. Furthermore, the theoretical field distributions are also in good agreement with observed transverse field variations reported previously. The laser output spectrum usually shows a number of "satellites" located adjacent to each longitudinal (Fabry-Perot) resonance. It is shown that each satellite represents the frequency of a transverse resonance having a different mode number along the junction plane and a corresponding Hermite-Gaussian intensity profile along the plane. Theory shows that the frequency separation of two adjacent satellites is related to the falloff rate of the dielectric constant along the junction plane. This focusing is measured from the intensity profile of a far-field pattern. From this information, the theoretical frequency separation is calculated and found to agree well with the measured separation of 6.4 GHz (0.15 Å).     

激光束的横模分析

报道激光器输出光斑的横模组分的分析方法。由谐振腔理论计算各阶单横模的光强分布,籍助扫描干涉仪测量激光模谱,可以得到激光束包含的横模序数及这些横模的强度值。     

Approximate formulas for crossed-field potential-minimum parameters

A parabolic potential distribution is assumed in order to study the static properties of the crossed-field potential minimum. The half-Maxwellian normal velocity distribution function and the full-Maxwellian transverse velocity distribution function are integrated over the initial velocity plane with the proper boundary conditions to find an expression for the ratio of beam current to emission current. For conditions satisfying most practical cases, this current-sorting expression is reduced to a very simple form. Using the current-sorting equation together with the expression which relates the potential minimum position to the minimum potential for zero magnetic field, an approximate method of determining the potential-minimum parameters is devised. The values obtained in this manner agree closely with the results obtained by other more exact (and more complicated) numerical methods.     

Characteristic mode analysis of a slot in a conducting planeseparating different media

The authors analyze a slot in a conducting plane separating different media when the slot is illuminated by a transverse electric and a transverse magnetic plane wave. The general characteristic mode theory for different media is used for the solution of this problem. The characteristic modes, which are computed using the method of moments, provide numerical results for the characteristic currents, the equivalent magnetic currents, and the field pattern for a slot whose width is a function of the wavelength. Analytical expressions for the narrow slot are also given     

An inverse methodology for high-frequency RF coil design for MRI with de-emphasized B1 fields.

An inverse methodology for the design of biologically loaded radio-frequency (RF) coils for magnetic resonance imaging applications is described. Free space time-harmonic electromagnetic Green's functions and de-emphasized B1 target fields are used to calculate the current density on the coil cylinder. In theory, with the B1 field de-emphasized in the middle of the RF transverse plane, the calculated current distribution can generate an internal magnetic field that can reduce the central overemphasis effect caused by field/tissue interactions at high frequencies. The current distribution of a head coil operating at 4 T (170 MHz) is calculated using an inverse methodology with de-emphasized B1 target fields. An in-house finite-difference time-domain routine is employed to evaluate B1 field and signal intensity inside a homogenous cylindrical phantom and then a complete human head model. A comparison with a conventional RF birdcage coil is carried out and demonstrates that this method can help in decreasing the normal bright region caused by field/tissue interactions in head images at 170 MHz and higher field strengths.     

高功率横流CO2激光器的锥面反射镜谐振腔

根据针-板放电5kW横流CO2激光器的结构特点,设计并实验研究了直角锥面镜作为全反射镜的谐振腔。在菲涅耳-基尔霍夫理论的基础上,引入柯林斯公式并考虑增益影响,推导了锥面谐振腔的衍射积分方程,并进行了相应的模拟计算。研究结果表明,在高功率横流CO2激光器中锥面反射镜与平面输出镜相配合使用组成稳定腔,获得了TEM30的低阶模输出;由于锥面反射镜受加工精度的影响中心残留有加工盲孔,导致TEMmn(m=0,n≠0)的模损耗较大不易耦合输出;而镜面中心场分布很弱的其他低阶模因损耗小,则会优先输出。     

Full-wave analysis of traveling-wave electrodes with finitethickness for electro-optic modulators by the method of lines

In order to optimize broadband electrooptic modulators with traveling-wave electrodes, detailed information about the propagation constant, the field distribution, and the wave impedance of the modulating field is needed. To get accurate results for these characteristic values, the numerically efficient method of lines (MoL) is used. The MoL takes advantage of the planar structure by discretizing the wave equation in the directions parallel to the metallization plane, whereas an analytical formulation is retained for the transverse direction. Thus, not only the hybrid character of the modulating field but also the finite conductor thickness and anisotropy of the substrate are taken into account. Results for asymmetric coplanar stripline electrodes with typical dimensions on SiO₂ buffered LiNbO₃ are presented     

On a new type of surface acoustoelectric waves in hexagonal crystals

It is shown that, in the basal plane of crystals with hexagonal symmetry, purely longitudinal surface acoustoelectric waves can propagate. In contrast to well-known Gulyaev-Blustein transverse waves, these longitudinal waves are localized in the surface layer, whose thickness is on the order of the wavelength. The field of mechanical displacements and the potential field of the wave are phase shifted by ??/2. The electricfield vector of the wave is circularly polarized in the sagittal plane.