Eigenmode confinement in the dielectrically apertured Fabry-Perot microcavity [VCSEL]

The three-dimensional (3-D) optical confinement of the dielectrically apertured Fabry-Perot microcavity is analyzed by considering electromagnetic coupling between the confined region and parasitic waveguide modes that exist outside the aperture. If the cavity length is close to one-half the emission wavelength and for large enough aperture sizes, all parasitic waveguide modes can be evanescent except one which couples only weakly to the emitting region. If the aperture diameter is too small, loss of optical confinement is predicted, and the optical mode size at which this occurs can be estimated from the cavity length and the relative shifts in the vertical resonances inside and outside the apertured region.     

Eigenmodes of the waveguide formed in a 2D photonic crystal composed of metal cylinders

Eigenmodes of the waveguide in a photonic crystal formed from circular metal cylinders are considered. The method of compensating sources is used to solve a boundary value problem for a guiding channel in a photonic crystal formed by removing an infinite row of cylinders from the crystal. Eigenvectors for amplitudes of compensating sources are found and the dispersion relation for determining the eigenmode propagation constants is derived. The eigenvectors for amplitudes of compensating sources are used to obtain expressions for electromagnetic fields of waveguide eigenmodes. The operating frequency range of the photonic-crystal waveguide is determined. The influence of losses on attenuation of the fundamental waveguide mode is investigated. Methods of compensating sources and eigenmode expansion are used to solve the excitation problem for waveguide eigenmodes. A simple expression is derived for the power carried by the fundamental mode.     

Current spreading and carrier diffusion in long-wavelengthvertical-cavity surface-emitting lasers

Current spreading and carrier diffusion in vertical cavity surface-emitting lasers (VCSELs) critically affect the threshold current density, transverse gain profile, and optical modal characteristics of the devices. We have numerically analyzed these effects for four popular long-wavelength VCSEL structures with wafer-bonded GaAs-AlGaAs Bragg bottom mirrors. The results show that current confinement for p-mirror VCSELs is approximately twice as effective as the corresponding n-mirror VCSEL's although the confinement factor for all practical VCSEL structures studied here is considerably lower than the “ideal” structure. Calculations also show a strong dependence of current confinement on device aperture size and a tradeoff between the optical modal property and the current confinement efficiency for different apertured devices     

A mode-matching technique for mode coupling in a gyrotron cavity

The mode-matching technique (MMT) is used to compute the electromagnetic fields, stored energy, and input admittances of a gyrotron cavity coupled to one or more waveguides. The method is based on matching the cavity and waveguide eigenmodes across the cavity apertures and accommodates cavity walls of finite conductivity. The MMT is used in the gyrotron problem because fields in and near the aperture must be computed accurately, and because the eigenmode decomposition is advantageous for inclusion of an electron beam. Irrotational modes are part of the complete set of orthogonal vectors required to expand an H-field in an open cavity, but were excluded in most gyrotron literature; here, this is corrected. The MMT is numerically implemented for cavities of rectangular and circular cross section. Coupling between different modes in a gyrotron cavity through external and ohmic losses is demonstrated. A coupled (complex) cavity gyrotron design is analyzed using MMT. The energy and modal spectra of the cavity are computed, demonstrating the mode selective properties of the design     

Low-threshold native-oxide confined narrow-stripe folded-cavity surface-emitting InGaAs-GaAs lasers

Narrow-stripe folded-cavity surface-emitting InGaAs-GaAs lasers are demonstrated, AlAs native-oxide layers above and below waveguide region are employed for current and optical confinement to form narrow-stripe InGaAs-GaAs quantum-well lasers. A low-temperature (400/spl deg/C) selective wet-oxidation technique and an ion-beam-etching technique are used to fabricate insulator confined narrow-stripes and internal 45/spl deg/ deflectors, respectively. Continuous-wave threshold currents as low as 4.5 mA and 59% surface-emitting quantum efficiencies are achieved on the devices with 2-/spl mu/m-wide aperture and a 420-/spl mu/m-long cavity.     

Analysis of a cylindrical antenna containing an aperture coupledload

In this paper we present the basis for the analysis of shielded tuning networks coupled to wire antenna elements. The structure analyzed comprises three conducting cylindrical tubes that form an aperture-fed circular coaxial waveguide. Two methods are presented for formulating and solving integral equations for the structure. The first method is based on the mixed potential electric field integral equation enforced on the three cylindrical tubes, and the second is based on aperture theory. An end correction capacitance is used to adjust the reflection coefficient in the eigenmode expressions of the aperture method. The data obtained by the two methods are in close agreement. Differences in actual currents and equivalent currents obtained from solutions are discussed and reconciled     

VCSEL稳态热特性分析

使用数值方法对氧化层限制型垂直腔面发射激光器(VCSEL)内部稳态热场分布进行了计算,结果显示其分布形式取决于DBR区热导率及其与高阻限制层热导率的差异,并指出在限制层孔径变化及外加电极电压变化时对热场分布的影响;器件中温度最高的部分处于中心氧化限制层附近。有源层中温度沿径向的分布情况表明,在氧化限制孔径下方形成明显的温度台阶是导致器件有源层中产生折射率台阶的主要原因。     

Experimental and theoretical analysis of efficient erbium-dopedfiber power amplifiers

The efficiency of Er³⁺-doped fiber power amplifiers (EDFAs) pumped at 980 nm was experimentally investigated and quantum conversion efficiencies (QCE) up to 0.89 were achieved. The experiment was accurately simulated by a computer model using only measured input parameters. The model was further used in an analysis of power amplifiers pumped at 980 and 1480 nm that included waveguide optimization and Er³⁺ confinement. The QCE can be enhanced by increasing the numerical aperture (NA) and confining the Er³⁺ ions to the central region of the core. At pump powers typically used for packaged EDFAs (25-100 mW). QCE can be improved by up to 60% by increasing the NA from 0.15 to 0.25, and confined Er³⁺ doping can provide an improvement of up to 20%. However, NA and Er³⁺ confinement have insignificant effects on the noise figure when both the cutoff wavelength and the fiber length are optimized with respect to QCE     

Multiantenna capacities of waveguide and cavity channels

Multiple-input-multiple-output (MIMO) capacity of waveguide and cavity channels is investigated using the modal expansion technique. Rectangular and circular waveguides and cavities are studied in details. Approximate expressions for the number of modes and for the capacity are given. A MIMO system architecture is suggested for a waveguide channel, which achieves the full capacity by making use of the mode orthogonally (or near orthogonality) using an eigenmode modulator at the Tx end and a spatial correlation receiver at the Rx end. Various practical limitations (e.g., nonideal waveguides and modulators, using discrete sensors instead of continuous, one-dimensional sensors instead of two-dimensional, etc.) and their impact on the capacity are discussed. It is demonstrated that long cavities are equivalent to waveguides in terms of capacity. The concept of spatial capacity is introduced to characterize the limits on the transmission rates that are due to both electromagnetic and information-theoretic considerations, which can be evaluated in a closed form for ideal waveguides and cavities. It follows that the traditional single-mode transmission is optimum in terms of capacity in the small signal-to-noise ratio region only.     

Frequency-Saturation Effects in Transferred Electron Oscillators

Frequency-saturation effects have been observed in the mechanical tuning characteristics of transferred electron (Gunn) oscillators operating in conventional waveguide cavities at frequencies from 12 to 40 GHz. Their existence leads to a restriction in oscillator tuning range and enhances frequency switching and hysteresis effects in the oscillators. Investigation of oscillator behavior in coaxial cavities as well as in waveguide cavities of varying height has shown that frequency saturation is caused by a coaxial resonance along the mounting post, which in the limit of fully reduced-height waveguide becomes the resonance of the diode with the encapsulation itself, or by a frequency limitation imposed by the diode thickness and operating voltage. The transverse coaxial resonance will occur in any waveguide size at a frequency near to if not within the band of interest. Simulation of the encapsulated diode by a localized reduction in mounting-post diameter shows that the frequency of transverse resonance can be increased by moving the encapsulation to a central position in the waveguide cross section. It is suggested that this technique and that of using cavities with a local reduction of height will make an important contribution to improving the performance characteristics of waveguide-mounted oscillators.     

AlInAs氧化物限制1.3μm低阈值边发射激光器

研究制作了一种利用AlInAs氧化物作为限制的1.3μm边发射AlGaInAs多量子阱激光器.有源层上方和下方的AlInAs波导层被氧化作为电流限制层.这种结构提供了良好的侧向电流限制和光场限制.当电流通道为5μm宽时,获得了12.9mA的阈值电流和0.47W/A的斜率效率.与具有相同宽度的脊条的脊波导结构的激光器相比,这种AlInAs氧化物限制的激光器的阈值电流降低了31.7%,斜率效率稍微有所提高.低阈值和高效率的特性表明,氧化AlInAs波导层能够提供良好的侧向电流限制.这种AlInAs氧化物限制的激光器垂直方向的远场半高全宽角为36.1°,而水平方向的是21.6°,表明AlInAs氧化物对侧向光场也有很强的限制能力.     

光阑约束和离轴失调Lohmann系统的分数傅里叶变换

为了研究受硬边光阑约束及失调Lohmann光学系统对分数傅里叶变换的影响,采用了平顶多高斯光束模型模拟硬边光阑,推导了平面光波在受硬边光阑约束和系统透镜离轴时的Lohmann 型分数傅里叶变换光学系统中传播的解析表达式;计算机模拟了受半径为7.696cm的硬边光阑约束和透镜离轴失调量时的输出.结果表明,在有硬边光阑约束和离轴失调时Lohmann两种结构并不等效.     

High-speed GaAs-(GaAl)As DH LED's with output waveguide structure for optical communications

High-speed GaAs-(GaAl)As DH edge-emitting LED's with an output waveguide structure and a reversed p-n junction for current confinement are described. A cutoff frequency of 115 MHz and a radiance of 1100 W/sr cm²at a diode current of 200 mA have been measured. The output power at the end of a fiber pigtail is about 200 µw with a driving current of 200 mA (core diameter of fiber = 60 µm and numerical aperture NA = 0.2). The coupling efficiency between the diode and a single fiber amounts to 10 percent. Factors affecting both the radiative output and the modulation characteristics are analyzed and discussed.     

Propagation of hermite-cosh-gaussian beams passing through ABCD optical system with an annular aperture

By using the expansion of the aperture function into a finte sum of complex Gaussian functions, the corresponding analytical expressions of Hermite-cosh-Gaussian beams passing through annular apertured paraxially and symmetrically optical systems written in terms of ABCD matrix were derived, and they could reduce to the cases with squared aperture. In a similar way, the corresponding analytical expressions of cosh-Gaussian beams through annular apertured ABCD matrix were also given. The method could save more calculation time than that by using the diffraction integral formula directly.     

Very closely spaced quadspot native-oxide confined 780-nmsemiconductor lasers

We describe the fabrication and characteristics of 7-μm spaced quadspot, independently addressable 785-nm native-oxide confined ridge waveguide laser diodes. The devices are fabricated from an Al_0.1 Ga_0.9As-Al_0.4Ga_0.6As-Al_0.5 In_0.5P quantum-well separate confinement heterostructure laser structure. Wet oxidation of the p-Al_0.5In_0.5P cladding layer is used to form a native oxide for not only ridge waveguide confinement, but also electrical insulation to allow electrical connection to laser stripes. These diodes show excellent performance: uniform threshold currents below 8 mA and differential quantum efficiencies over 35%/facet. The diodes show crosstalk less than 5%     

受光阑限制光束衍射场的高阶修正

为了模拟受光阑限制光束衍射场的分布,利用泰勒展开得到了一系列的高阶修正场.随着传输距离z的减少,只需增大到合适的阶数M即可模拟衍射场分布.对于不同的传输距离z,可根据不等式来选择合适的阶数M.以超高斯光束为例,用数值计算例说明理论公式的应用.所得结果对于研究通过光阑衍射光束的传输是有用的.     

受光阑约束的离轴厄米余弦高斯光束的传输

采用将光阑函数表示为复高斯函数叠加的方法,从理论上给出了离轴厄米余弦高斯光束通过受光阑约束的傍轴ABCD轴对称光学系统传输后的解析表达式,并可退化为无光阑约束时的情形。用同样方法分析得到了离轴余弦高斯光束通过受光阑约束的傍轴ABCD轴对称光学系统传输后的解析式。该方法比直接运用衍射积分公式可节省机时,其结果也可直接推广到二维情形。     

Aperture Coupling Between Microstrip and Resonant Cavities

This paper presents a simple analysis for the coupling between microstrip and a cavity through an aperture located in the substrate ground plane. The analysis is based on Wheeler's equivalent-energy concept for small-hole coupling and an approximate parallel-plate waveguide model for the microstrip. The theory appears adequate for most design purposes, and has been used successfully in the design of stabilizing cavities for experimental 12-GHz low-noise FET oscillators.     

Spectral iterative algorithm for RCS computation in electricallylarge or intermediate perfectly conducting cavities

A novel algorithm designed to compute efficiently and accurately the high-frequency electromagnetic scattering from open-ended waveguide cavities is presented. The cavity is converted into a stepped-waveguide model so that the field spectra are propagated, forward and backward, along each waveguide section. As boundary conditions for perfect electric conductors are applied via image theory, they are of local nature and take into account only the first-order interactions between each pair of waveguide sections. Accordingly, additional forward-backward iterations must be performed if multiple interactions are to be taken into account. Finally, the radar cross section due to the interior irradiation is calculated by a Kirchhoff-based aperture integral. Good agreement with method of moments and hybrid modal solutions is found, as well as with experimental data, for two-dimensional and three-dimensional cavities with rectangular cross section     

A circuital approach to evaluating the electromagnetic field on rectangular apertures backed by rectangular cavities

In this paper, the problem of evaluating the electromagnetic field on rectangular apertures backed by rectangular cavities is investigated. The electromagnetic-field distribution is derived by using a circuital model of an aperture and suitable forcing terms introduced into the equations related to the aperture model. The effects of a rectangular cavity on the aperture-field distribution are assessed by considering the rectangular cavity as a load impedance. The impedance value is obtained by modeling the rectangular cavity as a length of rectangular waveguide back-ended by a short. The distribution of the electromagnetic field on the aperture is used as an exciting source to evaluate, through a modal expansion, the electromagnetic field inside the cavity. Numerical simulations are in a good agreement with both other theoretical models and experimental data.