新型内腔太赫兹波参量振荡器的设计与阈值分析(英文)

设计了一种新型的表面辐射内腔太赫兹波参量振荡器,交叉谐振腔结构允许增益介质放置在泵浦光谐振腔内,使用浅表垂直辐射结构能够从铌酸锂晶体中有效耦合输出太赫兹波。推导了在太赫兹波参量振荡器中非共线相位匹配条件下有效参量增益长度的表达式。大的泵浦光尺寸和长的增益介质长度能增加有效参量增益长度。在有效参量增益长度表达式的基础上推导了太赫兹波参量振荡器的阈值表达式,在不同的参数条件下理论分析了谐振腔阈值表达式。大尺寸泵浦光、低频太赫兹波、短谐振腔腔长、斯托克斯光高反射率输出镜、长增益介质能有效降低太赫兹波参量振荡器阈值。文中的阈值分析对太赫兹参量振荡器的实验研究有参考价值。     

Detailed analysis of the effect of a hysteretic quantizer in a multibit, Sigma-Delta modulator

A detailed analysis of the impact of a hysteretic quantizer on a multibit, Sigma-Delta modulator has been carried out in this paper. Both discrete-time and continuous-time modulators have been considered. A qualitative modeling of the hysteretic quantizer based on a hysteretic block followed by an ideal quantizer was proposed. Due to the hysteresis effect, the quantizer output signal is delayed and distorted with respect to the quantizer input signal, where the delay causes a phase-shift independent on the signal frequency. Yet, the effect of the hysteresis depends on the input signal amplitude. This model was validated by using system-level simulations for a second order, 3-bit, discrete-time Sigma-Delta modulator. A linear model for hysteresis was derived by assuming a narrow hysteresis cycle. The quantizer input signal plays a fundamental role in the discussion. In order to include this signal into the linear analysis some approximations are proposed. The quantizer output signal is decomposed by the use of the Fourier series analysis only into the in-phase and quadrature components (with respect to the input signal) whose Fourier series coefficients can be analytically calculated. A quantitative analysis for both a second order, 3-bit, DT and CT Sigma-Delta modulators including a hysteretic quantizer was carried out. For the CT modulator, finite GBW in amplifiers, excess loop delay, and a hysteretic quantizer were considered separately and combined. A good agreement with both system-level simulations and experimental results is found, despite the approximations considered for the quantizer input signal.     

Threshold analysis of vertical-cavity surface-emitting lasers with intracavity contacts

A numerical analysis of vertical-cavity surface-emitting lasers (VCSELs) incorporating intracavity contacts and distributed Bragg reflectors (DBRs) is presented. The model considers polarization dependent reflection at the DBRs, current spreading, and nonuniform carrier density distribution self consistently. Analytic expressions for the current spreading and the corresponding series resistance for VCSELs incorporating intracavity contacts are derived. It is shown that current spreading strongly affects the lateral gain profile, the threshold current density, the transverse mode shape and the transverse mode discrimination through the creation of intracavity optical phase and gain apertures. The series resistance and the depth of the dip in the current density distribution are used as figures-of-merit to provide guidelines for device optimization, as illustrated by means of two examples of long wavelength VCSEL designs.     

Detailed design analysis of erbium-doped fiber amplifiers

When pumping the erbium-doped fiber amplifier at 0.98 and 1.48 mu m, the optimum cutoff wavelength for step profiles with arbitrary numerical aperture is shown to be 0.80 and 0.90 mu m, respectively. The use of a confined erbium profile can improve the gain coefficient up to 45%. The index raising co-dopant is shown to be very significant for the gain coefficient when pumping at 0.98 mu m.<>     

Improved resolution method to study at 3D the conduction phenomena inside GaAs PIN photodiode

GaAs PIN photodiodes are presently receiving much attention for low-noise photoreceivers. In this paper, specific feature of GaAs-based PIN photodiodes are given, in particular on the investigation of the GaAs properties. In order to show the potentialities of the GaAs PIN photodiode, a three-dimensional resolution and simulation are realized. The simulation adopted the drift-diffusion model applied to transport equations for semiconductor P⁺-ν-N⁺ structure under illumination. The generation term is the function of the incidental flow, reflection, and absorption coefficients. We developed subroutines in the simulation program (SIM 3D) attended for the simulation of the transport phenomena inside the PIN structure using the combined Newton-Gummel method, which uses the electrostatics potentials and the normal variables. These programs are implemented in the simulator to simulate the conduction inside the GaAs PIN photodiode. The model is applied on the carriers’ minority at high injection. The electric field and the photocurrent are then calculated. At the end we have characterized electrically the PIN photodiode from the results of the simulation, to show its band-width and then its domain of use.     

One-dimensional analysis of turnoff phenomena for a gate turnoff thyristor

Turnoff phenomena for a one-dimensional gate turnoff thyristor (GTO) are investigated using exact numerical solutions of a full set of semiconductor device equations. It is shown that the time responses of the hole and the electron densities around the center junction J2 are responsible for the dynamic behavior of the GTO. The storage time almost corresponds to the time period required for J2 to come out of saturation. The fall time is the period from the coming out of saturation of J2 to the point when the cathode emitter junction recovers. Time variations in the rates of replenishment and removal of holes in the p-base during the dynamic turnoff process are discussed, and an understanding of the switching mechanism, which is not obtainable in the generally used static transistor analogy, is obtained. Though a one-dimensional model is employed in this paper, it still provides a great deal of insight into the device's operation.     

Detailed structural analysis of semiconductors with X-ray scattering

Some of the most important material systems, GaInN alloys and quantum dot structures create interesting and complex challenges for structural analysis. This paper concentrates on the interpretation of the microstructure of both these materials, the former to assess the defect separation and the latter to obtain the shape and composition of the quantum dots. The methods used are based on mapping the X-ray intensity in reciprocal space and simulating proposed models to achieve good agreement with the experimental results. An indication of the reliability of these methods is presented.The simulation of the 0002 reciprocal space map of a 0 0 0 1 orientated InGaN/GaN sample yielded a range of dimensions of the perfect regions between defects of 60 and 220 μm. This comes from careful fitting of the tails of the scattering parallel to the surface plane. The average composition within an InGaAs quantum dot has been determined to a reliability of ±3% and the dimensions of these buried dots evaluated from simulating the reciprocal space maps using the in-plane scattering geometry.     

Detailed analysis of absorption data for indium nitride

It is shown that the 0.7 eV band gap recently announced for InN is actually due to a sub band gap deep level trap with |s like symmetry. This level had been known in the literature, but was previously misinterpreted as a deep level trap with |p like symmetry. It is also shown that proper interpretation of the absorption data for InN requires that the energy dependence of the refractive index be taken into account.     

Detailed analysis of coherence collapse in semiconductor lasers

Experimental and theoretical studies of coherence collapse in GaAs/AlGaAs laser diodes with weak optical feedback show two distinct routes to chaos. In each case we observe undamped relaxation oscillations, then external cavity mode beating, and finally coherence collapse. When there is frequency locking between the relaxation oscillations and external cavity modes, a period doubling sequence is followed, otherwise the route to chaos is via quasiperiodicity     

Detailed analysis of edge effects in SIMOX-MOS transistors

A comprehensive investigation of edge effects in LOCOS-isolated silicon-on-insulator devices is made by combining the measurements of the static characteristics, charge pumping, and noise. Even when a substrate bias is used to mask the conduction on the island edges, the high-frequency edge effects are still detectable. Appropriate models are proposed to separate the edge contribution from those of the front and back interfaces. It is found that the defect density on the edges is inhomogeneous, increasing vertically from the top to the bottom of the film and laterally from the middle to the end of the channel. Slow traps are identified at the back interface, close to the source/drain junctions     

General intracavity method for laser transition characterization byrelaxation oscillations spectral analysis

Monitoring the transient buildup of laser emission is proposed as a simple method for spectro-temporal intracavity diagnostics of laser transitions and materials. Using this method, we determined the absolute magnitude of both laser emission and absorption cross sections for a line-narrowed tunable Er³⁺-fiber laser     

Detailed structural analysis of GaAs grown on patterned Si

The growth of GaAs on patterned Si substrates is essential for the integration of GaAs and Si devices. Moreover this growth may have to be done in wells to planarize the surfaces of the Si and GaAs devices for their interconnection. In this study, GaAs is grown by MBE on such patterned Si substrates, with window width down to 3 μm, and a complete structural characterization of the material is made by electron microscopy. In every case, SEM observations show a very good definition of the pattern by the contrast of the flat surface of the monocrystalline GaAs compared to the very rough surface of the polycrystalline GaAs. Cross-sectional observations by STEM or TEM on non-etched samples reveal that the quality of the monocrystalline GaAs is at least as good in terms of defect density as that of the standard GaAs on Si. On samples etched with a plasma to produce wells, grains often form against the Si sidewall. Chemical etching with lateral etching avoids contact of the growing GaAs with the Si sidewall and subsequent grain formation. The crystalline quality obtained on etched samples is not as good as on non etched samples. A way of preparing the wells to improve this crystalline quality is proposed.     

Estimation of the intracavity power levels for a hybrid free-electron laser device

A hybrid free-electron laser device (called a GYROFEL), employing axial bunching in a solenoidal field configuration, has been analyzed to determine potential operational power levels. A small signal gain model was utilized to estimate the intracavity electric field strengths attained at equilibrium in such a device. The results indicate that out-coupled power levels approaching 1 MW might be achievable for electron beams near the 1 MeV energy range.     

Initial analysis of SAR from a cell phone inside a vehicle by numerical computation

The purpose of this paper is to analyze the influence of the metallic structures of a realistic car body frame on the specific absorption rate (SAR) produced by a cell phone when a complete human body model is placed at different locations inside the vehicle, and to identify the relevant parameters responsible for these changes. The modeling and analysis of the whole system was conducted by means of computer simulations based on the full wave finite-difference time-domain (FDTD) numerical method. The excitation considered was an 835 MHz lambda/2 dipole located as a hands-free communication device or as a hand-held portable system. We compared the SAR at different planes on the human model, placed inside the vehicle with respect to the free space situation. The presence of the car body frame significantly changes the SAR distributions, especially when the dipole is far from the body. Although the results are not conclusive on this point, this change in SAR distribution is not likely to produce an increase above the limits in current guidelines for partial body exposure, but may be significant for whole-body exposure. The most relevant change found was the change in the impedance of the dipole, affecting the radiated power. A complementary result from the electromagnetic computations performed is the change in the electromagnetic field distribution inside a vehicle when human bodies are present. The whole vehicle model has been optimized to provide accurate results for sources placed inside the vehicle, while keeping low requirements for computer storage and simulation time.     

Fast beam propagation method for the analysis of second-ordernonlinear phenomena

We adopt an implicit scheme to solve both the dispersive/diffractive and the nonlinear step in the beam propagation method (BPM) code. Unlike the Runge-Kutta implementation used in the nonlinear part of the propagation, the totally implicit scheme is considerably fast. Transparent boundary conditions can be also applied to effectively treat radiation problems. To validate the method, numerical examples are given and results compared with analytical solutions     

MOEMS静电旋转结构的Pull-in现象分析

对MOEMS静电旋转结构的Pull鄄in(吸合)现象进行了理论分析,重点研究了平板电极旋转结构和摆动梳齿旋转结构。摆动梳齿旋转结构不出现Pull鄄in现象的原因是电容变化率为恒值;平板电极旋转结构在电极旋转过程中电容变化率持续增大是Pull鄄in现象出现的根本原因,设计平板电极旋转驱动器时,如果需要得到较大的稳态旋转角度,可以通过合适的结构参数减小电容变化率的变化速率。     

An analysis of the effects of cable layout on cable to antennacoupling inside a screened room

An equivalent circuit model is presented for a screened room emission measurement setup. Transfer functions that are used to illustrate the variations in received emission levels produced by changes in interconnecting cable layout when the cable is placed over a conducting bench in the room are developed. The model is based on a coupled transmission line approach and is valid at frequencies up to 50 MHz     

Transient phenomena and control of such phenomena in a generator with a delay

Results of the numerical analysis of transient processes in a mathematical model describing a generator with a time lag and delayed action are presented. Regular and chaotic oscillation modes under chaotically varying initial conditions are considered. The possibility of controlling a generator’s chaotic oscillations via a radio pulse is analyzed in the case when there are one or two attraction basins.     

Characteristics of a grating-external-cavity semiconductor lasercontaining intracavity prism beam expanders

A grating tuned external cavity semiconductor laser incorporating a gradient-index rod lens and a pair of intracavity silicon prism beam expanders was constructed using a configuration aimed at simultaneously achieving the following objectives: the ability to operate at any external cavity longitudinal mode without tuning gaps caused by residual resonances of the laser diode cavity, stable feedback coupling between the diode and external cavities, narrow optical linewidth, and a high degree of external cavity longitudinal sidemode suppression