TM模式同轴布喇格结构频率响应研究

基于多波耦合理论,利用Fortran编程软件,以TM模式为工作模式,对工作在太赫兹频段的开正弦槽同轴布喇格反射器的频率响应进行了仿真研究.研究结果表明,在高频高阶TM模式下,竞争模式和所需工作模式带隙之间的带隙重叠可以通过初始相位差的逐渐增大得到明显改善;频响特性曲线的残存旁瓣可通过加窗技术得到有效抑制;加正圆锥或者倒圆锥渐变波纹时,工作模式影响较小,竞争模式会相应地偏离中心频带,对带隙重叠的分离也有一定的影响.研究为同轴布喇格反射器在太赫兹领域以TM模式工作提供了可能性.     

Comparison between Single-mode Treatment and Multi-mode Treatment of a Coaxial Bragg Resonator for a Terahertz Cyclotron Autoresonance Maser Oscillator

As a coaxial Bragg structure has more complex geometry configuration than a cylindrical or a planar Bragg structure, for simplicity of calculation a single-mode treatment is often adopted. However, heterogeneity of a coaxial Bragg structure always gives rise to the multi-mode conversion, which can change the performance of the resonator. Based on the scattering matrix method, detailed comparison between the single-mode treatment and the multi-mode treatment is presented to a coaxial Bragg resonator for a cyclotron autoresonance maser (CARM) oscillator operating in the higher-order mode TE₆₁ at frequency of 0.35 THz. Results indicate that there is a little difference between the results obtained by these two treatments to the upstream reflector due to its shallow ripple amplitude, whereas to the downstream reflector, due to its relatively deep ripple amplitude, an apparent difference occurs at the operating frequency and consequently it gives notable difference to the frequency response and Q-factor of the whole resonator. Optimization shows possible application of a coaxial Bragg resonator with a single higher-order mode operation in constructing a terahertz CARM oscillator.     

Effect of Ripple Taper on Coupling Modes in a Coaxial Bragg Structure

Based on the mode-coupling method, numerical analysis is presented to demonstrate the influence of ripple taper on coupling modes on the frequency response in a coaxial Bragg structure. Results show that the interval between the band-gaps of the competing mode and the desired working mode is narrowed by use of a positive taper, but is expanded if a negative taper is employed, and the influence of the negative taper is more obviously advantage than the positive taper. The residual side-lobes of the frequency response on coupling modes can be effectively suppressed by employing the windowing-function technique. These characteristics of a tapered coaxial Bragg structure are favorable to improvement of the performance as a reflector or a filter in its application, also favorable to the mode selectivity and further weaken the excitation of unwanted spurious modes.     

高阶耦合模倒圆锥双重正弦同轴结构频率响应

为了形成带宽窄、品质因数Q高、选择性好的同轴布喇格反射器,提出一种倒圆锥型双重正弦同轴结构设计。基于耦合模式理论,在普通正弦槽结构的基础上,叠加刻蚀周期较小的辅助正弦分布,在导体内外壁上增加倒锥度,形成倒圆锥型双重正弦同轴布喇格结构。通过FORTRAN软件仿真得出,与普通正弦槽相比,倒圆锥型双重正弦同轴布喇格结构的工作模式和竞争模式的带宽更窄,品质因数Q得到提高,残余旁瓣现象得到抑制。同时,竞争模式的中心谐振频率点远离工作模式,带隙重叠进一步分离,频率选择性得到提高。该结构设计简单,方法合理,可以更好地分离工作模式和竞争模式,构建高品质因数Q、高功率的单一高次模谐振腔。     

Comparitive Study of Coaxial Bragg Resonators

A coaxial Bragg resonator can be performed in four kinds of structures: it is constructed by inserting a regular waveguide between two selective Bragg reflectors (being called the upstream reflector and downstream reflector, respectively), or it consists of only two Bragg reflectors without regular waveguide section between them, where connection of these two reflectors is smooth with phase difference Δ??=?0 or is step-changed with corrugation phase difference Δ? = +π or ?π, respectively. The present paper is devoted to a comparative study for the response of reflectivity and quality factor on frequency in these four kinds of coaxial Bragg resonators, including the influences of the location and corrugation-phase difference of the step-changed connection of the two reflectors. Results show that a coaxial Bragg resonator possesses most attractive properties when the corrugation-phase difference is Δ? = +π at the point of connection and the upstream reflector is longer than the downstream reflector.     

新型坡度同轴布喇格反射器数值模拟比较研究

将一种新型加坡度方式同轴布喇格反射器的多模耦合理论与电磁仿真软件(CST)进行了数值模拟比较研究.结果表明:两者吻合较好,但多模耦合理论方法计算编程容易,运算速度快,参数调整非常方便;具有正圆锥形坡度的同轴布喇格反射器的带宽随着所加坡度角的增大而变窄,具有倒圆锥形坡度的同轴布喇格反射器的带宽随着所加坡度角的增大而变宽;...     

开槽深度和坡度对高频高阶模带宽和中心频移的影响

为对构建太赫兹频率下高阶单模工作的同轴布喇格谐振腔提供理论依据,基于模式耦合理论,比较研究了不同开槽深度和不同开槽坡度形状及坡度角对工作在太赫兹频率高频高阶耦合模式下工作模式和竞争模式带宽、中心频率偏移特性的影响. 结果表明:工作模式和竞争模式的带宽随着开槽深度的加深都变宽,但中心谐振频率点几乎没有偏移,槽深加深,带隙重叠现象会恶化,不利于模式选择;正圆锥形坡度方式时随着坡度角的增加,工作模式的中心频率点没有发生偏移,竞争模式中心频率点靠近工作模式中心频率点,不利于带隙重叠现象的抑制;倒圆锥形坡度方式时,随着坡度角的增大,竞争模式的中心频率点远离工作模式中心频率点,很好地抑制了带隙重叠;正圆锥形坡度和倒圆锥形坡度对带宽的影响都不大. 这些特性有利于拓展同轴布喇格结构作为反射器和滤波器的性能.     

Method for excitation of a high-order spatial mode in the input cavity of gyroklystron

Microwave-power input devices for a multimegawatt pulsed gyroklystron that works in the centimeter-wavelength range using a relativistic electron beam are considered. A device representing a supply waveguide with rectangular cross section that is tangentially interfaced with the side wall of a hollow cylindrical cavity is proposed for the excitation of a high-order rotating spatial mode as the working mode of the input cavity. The efficiency of the device is numerically optimized and measured to be 70% at a low power level using a broadband Bragg reflector for the input variant that provides the excitation of the TE_6,1,1 mode.     

Surface-relief type Bragg grating reflector fabricated after overcladding removal

A novel silica-based Bragg grating reflector on Si substrate is described. The reflector is formed on a core whose overcladding layer is removed by RIE until is is level with the core surface. The reflector which comprises a rectangular relief type grating only 1.6 mm long has the advantages of a highly controllable Bragg wavelength and a high reflectivity of 76% for the TE mode.<>     

Radiation characteristics of coaxial waveguides as a small primary feed

The radiation patterns of coaxial waveguides with a TE11 exciting mode are computed numerically. It is shown that by a proper selection of the coaxial aperture dimensions the symmetry of the copolar patterns can be improved considerably. Coaxial radiators can therefore be designed as a useful small primary feed for paraboloid reflector antennas.     

Suppression of Residual Side-lobes in a Coaxial Bragg Reflector

This note demonstrates the existence and possible suppression of the residual side-lobes in a coaxial Bragg reflector by employing Blackman window distribution, no matter if the phase difference between the outer and inner corrugations is 0, or π/2, or π. Physical explanation is qualitatively given to the effect of the window distribution.     

Optimization of modulation bandwidth in DBR lasers with detunedBragg reflectors

The dynamical behavior of distributed Bragg reflector lasers with detuned Bragg reflectors is investigated theoretically. The model is based on the traveling wave equations, which are solved by expanding the solution in terms of the longitudinal eigenmodes. It is shown that for a drastic enhancement of the modulation bandwidth the interplay of the dominant mode and one side mode has to be enforced. Under these conditions, the modulation bandwidth can be enhanced to more than 70 GHz     

A description and the measured performance of three coaxial beam-rotating antenna prototypes

Many high-power microwave (HPM) sources utilize an azimuthally symmetric output mode, like the TM/sub 01/ circular waveguide or the coaxial TEM modes. If radiated directly, these modes produce a doughnut-shaped radiation pattern, with a boresight . Mode-conversion techniques for transforming the azimuthally symmetric mode to one with a more desirable radiated pattern are possible, but mode conversion is typically undesirable, due to inefficiencies and due to increases in system size and weight. Antenna designs have been explored that will radiate the azimuthally symmetric mode directly, but those considered to date tend to exhibit low gain, and do not radiate a boresight peak (along the longitudinal axis of the source). This article describes the measured performance of three prototype antennas, all of the coaxial beam-rotating antenna (COBRA) class. These accept directly an azimuthally symmetric mode, and radiate a high-gain, circularly polarized beam with a boresight peak. The antennas achieve this capability by varying the electrical length of a path from a focal point to the aperture plane as a function of the azimuthal angle of the aperture. A brief overview of the general theory of COBRA operation is first presented. Next, measured data, characteristic of the input impedance and far-field patterns of three COBRA prototypes, are given. The architectures of COBRA prototypes reviewed in this article include those utilizing (1) a single, stepped paraboloidal reflector; (2) a dual reflector; and (3) a dual reflector with a coaxial feed.     

Design of distributed Bragg reflector structures for transverse-mode discrimination in vertical-cavity surface-emitting lasers

An angular spectrum of plane-wave representation is employed to calculate the discrimination between the fundamental and higher order transverse modes in step-index-guided vertical-cavity surface-emitting lasers. The effect of material composition and number of layer pairs in the distributed Bragg reflectors, as well as mode size and structure, are examined with the goal of optimizing the mode discrimination for better mode stability and higher single-mode power. In particular, it is shown that decreasing the width of the distributed Bragg reflector stopband, by means of controlling the material composition, improves significantly the mode discrimination.     

Nonlinear operation of a planar circular-grating DBR laser

An approximate method for the analysis of the nonlinear operation of a planar circular-grating distributed Bragg reflector laser is presented. The analysis is based upon vector-wave self-consistent coupled-mode equations modified to take into account gain saturation effects. With the help of an energy theorem and threshold field approximation, an approximate formula relating small-signal gain to the output power and laser parameters is derived. The laser characteristics obtained reveal behavior of the optimal coupling strength of the Bragg reflector, which provides maximal power efficiency as a function of the laser parameters. It is also shown that the gain saturation effect provides mode selectivity in the laser structure     

Radiation and matching characteristics of a novel dual-band dielectric loaded coaxial horn

Copolar and crosspolar radiation as well as matching characteristics of a novel dual-band coaxial hybrid-mode feed horn with partial dielectric loading [1] for reflector antennas have been numerically and experimentally investigated. In comparison with dual-band corrugated horns this horn has higher crosspolar level, but provides larger band separation and rather good electrical performances over a wide frequency range (20% and more) within each operational frequency band. Furthermore, the main concept of the coaxial feed system can be easily extended for multi-band cost-effective applications in reflector antennas with polarization diversity.     

Effect of Bragg reflector on the threshold current density in AlGaInP visible laser

The GalnP/AlGalnP layers are known as attractive materials for 600 nm band laser diodes. In this paper, a Bragg reflector between GaAs substrate and n-cladding layer was applied for the reduction in the lasing threshold. In a SCH-MQW structure, a Bragg reflector was composed of alternating λ/4n layers of AlAs and AlGaAs layers, 12.5 pairs. The effect of Bragg reflector on the threshold current and spontaneous emission intensity was appreciable because most of spontaneously emitted photons were reflected from a Bragg reflector and carriers were regenerated in the GalnP active layer.     

Mode stabilization method for superstructure-grating DBR lasers

A wavelength stabilization method for widely tunable superstructure-grating (SSG) distributed Bragg reflector (DBR) laser is described. The output characteristics under tuning are studied theoretically and experimentally. It is found that peak reflectivity states, in which the lasing mode is just aligned with the reflection peaks of both DBR's, are obtained at saddle points in the output characteristics while changing the two SSG-DBR currents. Based on these results, a method for the Bragg frequency control of the two SSG-DBRs is proposed. The feedback control circuit keeps the lasing mode at a peak reflectivity state, and it suppresses mode hopping. Additionally, the oscillation mode is locked to arbitrary reference wavelengths of an optical filter. Stabilization at 200 GHz (1.6 nm)-spaced 16 wavelengths was achieved within the wide tuning range of the SSG-DBR laser. Control was maintained under a laser temperature variation of ±5°C as a result of the Bragg frequency control of the two DBR's     

Cavity-induced antiguiding in a selectively oxidizedvertical-cavity surface-emitting laser

Cavity-induced antiguiding of a lasing mode is demonstrated in a vertical-cavity surface-emitting laser (VCSEL). Selective oxidation of a distributed Bragg reflector (DBR) layer is used to generate a spectrally red-shifted cavity region cladding the lasing mode, with a second selectively oxidized aperture used to form the current confinement to the antiguided mode. Near-field, far-field, and spectral measurements are used to characterize lasing in both the antiguided mode and the antiguiding region     

Dual-wavelength DBR fiber laser

Stable continuous-wave lasing of two longitudinal modes of a distributed Bragg reflector fiber laser is reported. Intensity noise and coupling between the modes was characterized for both 0.8- and 0.2-nm mode separations.