Low thermal resistance high-speed top-emitting 980-nm VCSELs

Increasing copper plated heatsink radii from 0 to 4 /spl mu/m greater than the mesa in vertical-cavity surface-emitting lasers (VCSELs) reduced the measured thermal resistance for a range of device sizes to values 50% lower than previously reported over a range of device sizes. For a 9-/spl mu/m diameter oxide aperture, the larger heatsink increases output power and bandwidth by 131% and 40%, respectively. The lasers exhibit a 3-dB modulation frequency bandwidth up to 9.8 GHz at 10.5 kA/cm/sup 2/. The functional dependence of thermal resistance on oxide aperture diameter indicates the importance of lateral heat flow to mesa sidewalls.     

Thermal resistance of VCSELs bonded to integrated circuits

The thermal resistance of vertical-cavity surface-emitting lasers (VCSELs) flip chip bonded to GaAs substrates and CMOS integrated circuits has been measured. The measurements on GaAs show that if the bonding is done properly, the bonding does not add significantly to the thermal resistance. However, the SiO₂ under the CMOS bonding pad can double the thermal resistance unless measures are taken to improve the thermal conductance of these layers. Finite element simulations indicate that the thermal resistance of bonded VCSELs increases rapidly as the solder bond size and the aperture size decrease below ~10 μm     

垂直腔面发射激光器的热学特性

通过求泊松方程、电流密度方程、载流子扩散方程以及有源层结压降方程自洽解的方法，计算了垂直腔面发射激光器（VCSEL）的电势分布，进而求解热传导方程，得到VCSEL的温度分布．详细分析了注入电流密度小于或等于阈值电流密度时，晶片键合结构垂直腔面发射激光器的键合界面阻抗、氧化层限制孔径、外加电压以及分布布拉格反射镜的热导率的大小对VCSEL内部温度分布的影响．     

垂直腔面发射激光器的热学特性

通过求泊松方程、电流密度方程、载流子扩散方程以及有源层结压降方程自洽解的方法,计算了垂直腔面发射激光器(VCSEL)的电势分布,进而求解热传导方程,得到VCSEL的温度分布.详细分析了注入电流密度小于或等于阈值电流密度时,晶片键合结构垂直腔面发射激光器的键合界面阻抗、氧化层限制孔径、外加电压以及分布布拉格反射镜的热导率的大小对VCSEL内部温度分布的影响.     

Fabrication and Characteristics of high-speed oxide-confined VCSELs using InGaAsP-InGaP strain-compensated MQWs

This paper presents the fabrication and characteristics of high-performance 850-nm InGaAsP-InGaP strain-compensated multiple-quantum-well (MQW) vertical-cavity surface-emitting lasers (VCSELs). The InGaAsP-InGaP MQW's composition was optimized through theoretical calculations, and the growth condition was optimized using photoluminescence. These VCSELs exhibit superior performance with characteristics threshold currents /spl sim/0.4 mA and slope efficiencies /spl sim/0.6 mW/mA. The threshold current change with temperature is less than 0.2 mA, and the slope efficiency drops less than /spl sim/30% when the substrate temperature is raised from room temperature to 85/spl deg/C. A high modulation bandwidth of 14.5 GHz and a modulation current efficiency factor of 11.6 GHz/(mA)/sup 1/2/ are demonstrated. The authors have accumulated life test data up to 1000 h at 70/spl deg/C/8 mA.     

Thermal modeling of large-area VCSELs under optical pumping

An analytical approach is proposed to calculate the stead-state three-dimensional temperature distribution in multilayered structures, particularly when containing periodic stacks. We apply this model to the case of vertical-cavity surface-emitting lasers under optical pumping and find an excellent agreement with experimental results.     

Thermal impedance of VCSELs with AlOx-GaAs DBRs

The authors have measured the thermal impedance of vertical-cavity surface-emitting lasers (VCSELs) with oxide/GaAs DBRs and shown that it is comparable to that of VCSELs with all-semiconductor DBRs. A VCSEL with an 8-μm oxide aperture shows a thermal impedance of 2.8°C/mW. By varying the aperture size, the thermal conductance of the material below the active area is 0.255 W/cm°C. These results demonstrate that the oxide is not a major barrier to heat transport out of the active region     

Comparison of simulation and measurement of dynamic fiber-coupling effects for high-speed multimode VCSELs

Static and dynamic measurements are performed with GaAs oxide-confined vertical-cavity surface-emitting lasers (VCSELs), using multimode fibers with a core diameter of 50 and 62.5 /spl mu/m and different numerical apertures (NAs). They show that a small NA can have a severe impact on the eye opening and thus also on the bit-error rate. The measurements are analyzed with a spatiotemporal two-dimensional (2-D) multimode VCSEL model. The required parameter extraction for the model is verified with small- and large-signal measurements. The analysis shows that the change of the eye opening can be explained by the interaction between the mode- and the current-injection profile, carrier diffusion, and intermodal gain compression (IGC). IGC increases differences in the modal power distribution caused by the interaction between the mode profiles and the current-injection profile. Carrier diffusion is able to compensate these increased differences of the modal power distribution. Its impact, however, on dynamic changes caused by IGC is moderate.     

Simplified Model of the Dynamic Thermal Tuning Behavior of VCSELs

In this letter, a simplified thermal model is used to derive a closed-form expression for the frequency dependency of the current-to-wavelength tuning coefficient (FM response) of vertical-cavity surface-emitting lasers (VCSELs). It explains the experimentally observed square root law and shows very good agreement with measured FM responses of two different VCSEL types. It yields significantly improved accuracy compared to the usually assumed first-order low-pass model.     

Crosstalk and transient analyses of high-speed interconnects andpackages

A multiconductor interconnect is modeled using resistors and linear-dependent current and voltage sources. The analysis of a high-speed circuit including lossy interconnection buses is then reduced to simulation of the circuit together with the equivalent circuits of the interconnects. The authors present a new method for the crosstalk and transient analysis of lossy interconnects with arbitrary termination circuits. In order to analyze an interconnect containing N signal conductors, they derive closed-form formulas to determine its transfer functions, and they apply the inverse Fourier transform to obtain its time-domain pulse response functions. Two types of equivalent circuit models can be formulated once the pulse response functions of the interconnect are found. The circuit schematics of the models depend on the number of the signal conductors, irrespective of the physical parameters of the interconnect. These models are compatible with standard circuit simulation tools since they consist of linear resistive networks and linear-dependent sources only. Two example circuits are studied to examine the accuracy and efficiency of the method     

Design and evaluation of fundamental-mode and polarization-stabilized VCSELs with a subwavelength surface grating

We demonstrate 850-nm oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with a locally etched subwavelength surface grating that are single-mode and polarization stable from threshold up to thermal roll-over, reaching /spl sim/4 mW of output power. The side-mode suppression ratio (SMSR) is >30 dB and the orthogonal polarization suppression ratio (OPSR) is /spl sim/20 dB. Moreover, no distortion of the far-field beam profile is observed as a result of the surface grating. Our numerical calculations show that a carefully designed VCSEL can have a high simultaneous mode and polarization selectivity without a significant increase in loss for the favored fundamental mode with polarization state perpendicular to the grating lines. This indicates characteristics such as threshold current and resonance frequency will not be notably degraded. The calculations also show a low sensitivity to variations in grating etch depth and duty cycle, which relaxes fabrication tolerances. In our experimental parametric study, where the oxide aperture diameter, surface grating diameter, and grating duty cycle were varied, the combined mode and polarization selection was investigated. For an optimum combination of oxide aperture and surface grating diameters of 4.5 and 2.5 /spl mu/m, respectively, the device is found to be single-mode and polarization stable for a broad range of grating duty cycles, from 55% to 75%, with only a small variation in other laser performances, which is in line with theory.     

Thermal resistance of side by side multi-chip package: Thermal mode analysis

The thermal mode analysis is used in this paper to optimize the thermal management with optimal locations and chip sizes for multi-chip package. The average thermal resistance is defined and analyzed. The spreading thermal resistance can be expanded into Fourier series so that the thermal modes can be established. For the infinite thermal modes, only a few terms are needed to be considered due to the rapid convergence of solution. The optimal locations and chip sizes can then be determined by using the first few modes to reduce the thermal resistance as minimal as possible. The optimal locations have the cosine wave property so that the wave nodes might be the suitable sites. On the other hand, the optimal chip sizes have the cardinal sine property which decays monotonously. For given optimal locations, the optimal chip sizes are determined by certain modes. These special modes can be used to analyze the range of optimal locations and chip sizes.     

Fabrication of high-speed InP/InGaAs/InP DHBT with a new self-aligned metallization technique for reduced base resistance

A new self-aligned emitter–base metallization (SAEBM) technique with wet etch is developed for high-speed heterojunction bipolar transistors (HBTs) by reducing extrinsic base resistance. After mesa etch of the base layer using a photo-resist mask, the base and emitter metals are evaporated simultaneously to reduce the emitter–base gap (S_EB) and base gap resistance (R_GAP). The InP/InGaAs/InP double heterojunction bipolar transistor (DHBT) fabricated using the technique has a reduced R_GAP, from 16.48 Ω to 4.62 Ω comparing with the DHBT fabricated by conventional self-aligned base metallization (SABM) process. Furthermore, we adopt a novel collector undercut technique using selective etching nature of InP and InGaAs to reduce collector–base capacitance (C_CB). Due to the reduced R_GAP, the maximum oscillation frequency (f_max) for a 0.5 μm-emitter HBT is improved from 205 GHz to 295 GHz, while the cutoff frequency (f_T) is maintained at around 300 GHz.     

Thermal characterisation of power devices during transient operation

An experimental infrared method for the thermal characterisation of semiconductor devices during fast transient operation, in the range from a few microseconds and up to some milliseconds, is presented. The features which make it suitable and convenient, particularly for use with power electronics applications are pointed out; its time and space resolution are illustrated by means of properly chosen examples. The considered solution qualifies as a very versatile and powerful tool in many diverse lines of investigation.     

高速光耦瞬态共模抑制性能的电路仿真方法

为了解决高速光电耦合器的瞬态共模抑制不能准确测试的问题,研究了光耦的耦合机理和瞬态共模抑制的测试原理。利用仿真技术对电路功能及瞬态共模抑制评估电路的耦合参数(包括耦合电阻与耦合电容)进行仿真和验证,分析了电路中的不同器件参数对仿真结果的影响,并且对得到的输出波形的数据进行读取与分析。以HCPL-2611为例,使用大电阻和小电容并联的模型模拟实际中的耦合情况,通过仿真分析确定了耦合电容的大小,验证了高速光耦瞬态共模抑制仿真方法的有效性。     

Thermal resistance of light-emitting diodes

A detailed analysis of the heat flow in a light-emitting diode is carried out in the present paper. In the thermal model of a light-emitting diode, the heat flow from the active region throughout the area between it and the top contact, the nonuniform heat flux density distribution in the active region due to the current-spreading effect as well as the temperature dependence of the thermal conductivity of the semiconductor material, are taken into account. The solutions of the thermal conduction equation for a light-emitting diode are obtained for both the steady-state condition and the transient-state condition. The heat-spreading in the heat-sink is analyzed. The effect of LED construction parameters on its thermal resistance is illustrated in numerous figures.     

Experimental evaluation of an array technique for zenith to horizon coverage

An experimental study of a new array concept with application to providing low cost efficient antennas with hemispherical coverage for aircraft to satellite communication links is described. The combined array-surface wave antenna consists of 64 waveguide elements scanned conventionally except at endfire. At endfire the array is shorted to become a corrugated surface-wave antenna and excited by an eight-element feed to provide a directional beam near the horizon. The array is rotated to give hemispherical coverage.     

Evaluation of a processing technique for transient data

A data processing technique, namely Prony's method, is evaluated. The use of the algorithm for EMP and other transient problems is illustrated and the relationship between the waveform parameterization and the singularity expansion method (SEM) is described. The impulse response of a synthesizer network is obtianed using Prony processing of the output of the double exponential excited circuit. Difficulties which arise in this type of processing such as rank deficiency, aliasing, and noise effects are considered and methods for alleviation such as filtering and rank-overspecification are introduced and evaluated. The overall status of waveform parameterization as applied to SEM is considered and suggestions for future research presented.     

瞬态成像模式下高帧频CMOS图像传感器性能研究

高帧频CMOS图像传感器具有集成度高、帧频高、功耗低、抗干扰抗辐照能力强等特性,在科学实验中应用广泛。为提高外同步触发瞬态成像模式下的成像性能,首先介绍了基于高帧频CIS(5T像素,超大快门)的瞬态成像系统构成及其工作模式;从像素结构出发,对该款CIS在不同工作模式下的成像性能进行了理论分析;搭建了基于EMVA1288的标准化测试平台,对瞬态工作模式下的多项关键性能指标进行了测试,并与稳态工作模式下的性能进行了对比。分析结果表明:与稳态工作模式相比,瞬态成像模式下图像传感器具有更大的暗本底和固定模式噪声,但传感器的时序噪声、光响应非均匀性优于稳态工作模式,具有更高的信噪比和动态范围,与理论分析基本吻合。测试分析结果可用于指导科学成像系统设计与性能优化。     

基于噪声平衡抑制的近红外单光子高速探测方法

针对现有基于尖峰噪声电容平衡方法的单光子探测技术工作频率较低的问题,设计了利用高速二极管平衡的InGaAs/InP雪崩光电二极管信号提取方案.配合高速信号处理电路,实现了工作频率最高达700 MHz的近红外单光子探测器.雪崩二极管制冷至218 K,在脉宽小于1 ns、重复频率700 MHz的门脉冲触发下,当探测效率为10%时,暗计数率为7×10~(-6)/pulse,后脉冲率为7.4%.该方案提供了一种重复频率连续可调宽带高速单光子探测方法,结构简单且性能良好.